The Universe is expanding at 74.2 km/sec/Mpc

By Phil Plait | May 7, 2009 11:11 am

How far away are things?

That is perhaps the most basic question in astronomy, and in some ways the most aggravating. For nearby stuff — and by that I mean everything from the Moon out to stars about 1000 light years away — we can measure distances directly. Bouncing radar pulses off of planets gives us their distance, and in some cases we’ve sent probes to them so their distances are extremely well known. For nearby stars we can use parallax, which is using the motion of the Earth around the Sun to see how that affects how we see the position of the stars in the sky.

But for distant galaxies, getting their 10-20 is a lot harder. That’s why objects like the spiral galaxy NGC 3021 are so useful:

Hubble’s view of NGC 3021. Click to get to much more cromulently embiggened images.
Both images credit: NASA, ESA, and A. Riess (STScI/JHU)

OOoo. Ahhhhhh.

That pretty li’l thing is a cosmic bootstrap device. It and other galaxies like it have been under the scrutiny of my old friend Adam Riess, who has some big questions for them. Like, how fast is the Universe expanding?

And NGC 3021 may just have the answer.

That particular galaxy has two extremely useful properties. One is that it contains a type of variable star called a Cepheid variable, which are stars that literally pulsate in size and brightness. The time it takes to change in brightness is directly related to its absolute brightness: the more luminous the star is, the longer it takes to pulse. If you measure its precise pulse period, you find the star’s true brightness. Compare that to how bright it appears in a telescope, and voila! You get the distance.

Cepheids in NGC 3021
Cepheids in NGC 3021.

Cepheid measurements yield pretty good distances for nearby galaxies, but after a certain distance they are too faint to see. We need a better rung on this distance ladder… and we have one. Supernovae.

Type I supernovae are a kind of exploding star that we think (well, we’re actually pretty sure) all explode in such a way that their absolute brightness can be determined, so, like Cepheids, their true distance can be found. And we can see them out for hundreds of millions of light years, which is really really far away. This makes them incredibly powerful beacons for astronomers.

The cool thing is, NGC 3021 and others like it have Cepheids in them, and are also known to have hosted Type 1 supernovae! Over ten years ago, a Type I went off in NGC 3021, and very precise measurements of it were made, including how far away it was. So for galaxies like NGC 3021 we have two methods of measuring distances which can be tied together in a single galaxy that can be observed with a single telescope, like Hubble. That means that uncertainties in the distance measurements using the two systems can be hammered away, and we get more reliable measurements.

And since we can see supernovae out to such fantastic distances, that means we can accurately measure the expansion of the Universe. Using supernovae to measure the distances of remote galaxies can be compared to the distances we get for those using the redshift, the Doppler-like shift in the starlight coming from those galaxies. So again, we’re tying together different ways of measuring distances, allowing us to refine just how this old cosmos of ours is, and how quickly it’s expanding.

Adam Riess and his team observed quite a few galaxies in this way, and figured just how fast the Universe is growing to unprecedented accuracy. His result: 74.2 ±3.6 kilometers/second/megaparsec. That means for every megaparsec (about 3 million light years) you go out, the Universe is expanding 74.2 km/sec faster. So a galaxy 10 Mpc away would be moving away from us at 742 km/sec. Adam’s measurement jibes well with other measurements, so there is reason to be confident in his results.

By knowing this number accurately, all we have to do is measure how fast the galaxy is moving away from us — a very easy measurement to make — and we can find its distance. Of course, it’s more complicated than that, but that’s the basic idea.

But by nailing down all these numbers, we can in turn nail down such things as how much dark energy is in the Universe, and maybe even rule out some theories as to what this mysterious stuff is. It’s pushing on the fabric of space and time, making the Universe swell faster every second of every day, and we have no clue what it really is. Well, that’s unfair: we have lots of clues, but we don’t know what’s causing it. Observations of NGC 3021 and other galaxies like it will help us unravel some of these mysteries, which are among the biggest in science today.

Who would have thought that all this could happen just by figuring out how bright some stars are?

Oh yeah, scientists did. That’s what they do.

CATEGORIZED UNDER: Astronomy, Pretty pictures, Science

Comments (118)

  1. Tim G

    That expansion rate is the inverse of 13.2 billion years. Does this mean that if the universe had always been expanding at the current rate (which it certainly doesn’t), then the projected age of the universe would be 13.2 billion years?

  2. Anon

    The Hubble Constant! Nice, we’ve used a similar figure in Cosmology papers recently.

    Interesting for me was that if you get this number in a simpler unit, per seconds (seconds to the -1), and then take the inverse of that figure, you get an approximation to the age of the universe.

  3. @BA “Type I supernovae are a kind of exploding star that we think (well, we’re actually pretty sure) all explode in such a way that their absolute brightness can be determined”

    Actually Phil, there are 3 different subtypes of Type I supernovae: A, B, C. It is only subtype A that can be used as cosmic “standard candles”. Why? Because they involve a white dwarf orbiting a companion star in a close orbit and stealing matter from it. When the white dwarf exceeds Chandrasekhar’s limit (~ 1.4 solar masses) it explodes. It is the stability of Chandrasekhar’s limit that gives Type Ia their stability in terms of peak brightness (absolute magnitude of -19.3).

    Type Ib supernovae are thought to be core-collapse supernovae involving a single star (similar to Type II) only the hydrogen envelope of the star has been ejected into space leaving the outer helium envelope intact (they have strong helium spectral lines). Type Ic supernovae have both the hydrogen outer envelope and the helium envelope under that stripped away so that they have neither hydrogen nor helium spectral lines. Both Type Ib and Type Ic are somewhat dimmer than Type Ia supernovae.

    Type Ia supernovae have a strong spectral absorption line at 615 nanometers caused by singly ionized silicon. If you look at their spectra Type Ia’s stand out like a sore thumb.

  4. In case anyone is interested there have been 9 supernovae observed in our Milky Way galaxy during the last 2,000 years. Four of them were Type Ia supernovae and the other five were Type II supernovae. The brightest one, SN 1006, was a Type Ia (Type Ia’s tend to be brighter than Type II’s) and it had a peak apparent magnitude of -9 (some sources say -7.5) which was much brighter than Venus.

    Supernova: SN 185
    Date: Dec. 7, 185 CE
    Constellation: Centaurus
    Peak Apparent Magnitude: -2
    Distance: 8,200 light-years
    Supernova type: Ia
    Supernova remnant: RCW 86
    Remained visible for 8 months

    Supernova: SN 386
    Date: 386 CE
    Constellation: Sagittarius
    Distance: 14,300 light-years
    Supernova type: II
    Supernova remnant: G11.2-0.3 (pulsar)
    Remained visible for 3 months

    Supernova: SN 393
    Date: 393 CE
    Constellation: Scorpius
    Peak Apparent Magnitude: -3
    Distance: 4,200 light-years
    Supernova type: II
    Supernova remnant: G347.3-0.5 (neutron star)
    Remained visible for 7 months

    Supernova: SN 1006
    Date: Apr 30, 1006 CE
    Constellation: Lupus
    Peak Apparent Magnitude: -9 (the brightest supernova ever observed)
    Distance: 7,200 light-years
    Supernova type: Ia
    Supernova remnant: PKS 1459-41
    Remained visible for 21 months

    Supernova: SN 1054
    Date: Jul. 4, 1054 CE
    Constellation: Taurus
    Peak Apparent Magnitude: -6
    Distance: 6,500 light-years
    Supernova type: II
    Supernova remnant: Crab Nebula (pulsar)
    Remained visible for 653 days (visible for 23 days during daylight)

    Supernova: SN 1181
    Date: Aug. 6, 1181 CE
    Constellation: Cassiopeia
    Peak Apparent Magnitude: -1
    Distance: 10,000 light-years
    Supernova type: II
    Supernova remnant: 3C 58 (pulsar PSR J0205+6449, 65 ms period)
    Remained visible for 185 days

    Supernova: SN 1408
    Date: 1408 CE
    Constellation: Cygnus
    Distance: 8,200 light-years
    Supernova type: II
    Supernova remnant: CTB 80 (pulsar PSR B1951+32, 39.5 ms period)

    Supernova: SN 1572 (Tycho’s new star)
    Date: Nov. 6, 1572 CE
    Constellation: Cassiopeia
    Peak Apparent Magnitude: -4
    Distance: 7,500 light-years
    Supernova type: Ia
    Supernova remnant: 3C 10
    Remained visible for ~16 months (visible for ~2 weeks in daylight)

    Supernova: SN 1604 (Kepler’s new star)
    Date: Oct. 9, 1604 CE
    Constellation: Ophiuchus
    Peak Apparent Magnitude: -2.5
    Distance: 20,000 light-years
    Supernova type: Ia
    Supernova remnant: 3C 358
    Remained visible for 18 months

  5. Sir Eccles

    Surely, the real question is:

    If one can do the Kessel Run in 11 parsecs, will the chicken ever reach the other side of the road?

  6. This result implies that the expansion rate is a clean linear function of distance. Wasn’t the whole point to the “dark energy” construct to put a name next to observations that the Hubble Constant varies with distance?

  7. mike

    TimG, no.

    That age comes from integrating the Friedmann equation backwards, probably assuming a flat universe. If Omega were 1 (it isn’t…), the result would be 2/3H, not 1/H. It’s a bit older than that with Omega ~0.3 (but not a lot), and a bit more so with Omega_Lambda ~0.7. It’s not constant expansion. The mass term slows expansion down, and the energy term speeds it up.

    As for unprecedented? No. WMAP first year results are about the same (and consistent). But it’s a very nice more-or-less completely independent confirmation.

  8. Doc

    Um, maybe this is a dumb question, but why do Cepheid variables vary?

  9. Bjoern

    @Anon: Even more interesting to me was the realization that expressing the Hubble “constant” in units of per second (or, more sensible, per billion years), you directly get an answer to the question “at what rate does the universe expand”? What would you guess? Do the calculation – it’s only about 7% per billion years!

  10. Tom beat me to it, and added in the observed ones. Nicely done.

    Doc: The variation in luminosity is caused by a cycle of ionization of helium in the star’s atmosphere, followed by expansion and deionization. While ionized, the atmosphere is more opaque to light. This cycle has a period equal to the star’s dynamical time scale, therefore giving information on the mean density of the body as well as its luminosity. (Wiki)

    And then there is the Type I and Type II of Cepheids to mix into it….

  11. “That expansion rate is the inverse of 13.2 billion years. Does this mean that if the universe had always been expanding at the current rate (which it certainly doesn’t), then the projected age of the universe would be 13.2 billion years?”

    Yes. Part of the evidence in favour of a cosmological constant (which makes the universe
    accelerate) is that it makes the universe older than the corresponding “Hubble time”.

    Just a few years ago, misguided journalist and, sadly, a few misguided scientists uttered
    such nonsense as “universe younger than objects it contains”. Actually, they just had a
    too narrow model of the universe, neglecting work by Friedmann done back in the 1920s,
    showing that, with a cosmological constant, the age of the universe can be larger, even
    much larger, than the Hubble time.

  12. One of the strange things about Type Ia supernovae is that they don’t leave behind any compact remnant such as a neutron star or a black hole. It’s just the nebula plus the companion star which goes careening off at high velocity since its gravitational companion has disappeared. It’s amazing to me that the companion star can even survive the supernova explosion, but apparently they can. I’m sure any planets don’t fare so well.

    A few years ago a team of astronomers found the companion star for the Type Ia supernova known as SN 1572 (Tycho Brahe’s supernova). It is known as Tycho G and interestingly enough, it is very similar to our sun only a bit older (spectral class G2IV). Click on my name to read the full journal article talking about the discovery.

    Here are some excerpts:

    “The Search for the Companion Star of Tycho Brahe’s 1572 Supernova

    J. Mendez (University of Barcelona and ING)

    In recent years, type Ia supernovae (SNe Ia) have been used successfully as
    cosmological probes of the Universe (Riess et al., 1998; Perlmutter et al.,
    1999). However, the nature of their progenitors has remained somewhat of a
    mystery. It is widely accepted that they represent the disruption of a
    degenerate object, but there are also numerous progenitor models (see for
    instance Ruiz-Lapuente, Canal, Isern, 1997a, for a review), but most of these have serious theoretical/observational problems or do not appear to produce sufficient numbers to explain the observed frequency of SNe Ia in our Galaxy (~3E-3 per year; Cappellaro & Turatto, 1997).
    .
    .
    .
    The Search for the Companion Star of SN 1572

    Tycho Brahe’s supernova (SN 1572) is one of the only two supernovae observed in our Galaxy that are thought to have been of type Ia as revealed by the light curve (Ruiz-Lapuente, 2004), radio emission (Baldwin et al., 1957) and X-ray spectra (Hughes et al., 1995).

    The field that contained Tycho’s supernova, relatively devoid of background
    stars, is favourable for searching for any surviving companion. With a Galactic latitude b=+1.4, Tycho’s supernova lies 59 parsecs above the Galactic plane. The stars in that direction show a consistent pattern of radial velocities with a mean value of -30 km/sec at 3 kpc. The star most likely to have been the mass donor of SN 1572 has to show a multiple coincidence: being at the distance of SN 1572, showing an unusual motion in comparison to the stars at the same location, having stellar parameters consistent with being struck by the supernova explosion and lying near the remnant centre.

    The distance to SN 1572 inferred from the expansion of the radio shell and by other methods lies around 3 kpc. Such a distance, and the light-curve shape of SN 1572, are consistent with it being a normal type Ia supernova in luminosity, like those commonly found in cosmological searches (Ruiz-Lapuente, 2004).
    .
    .
    .
    The Case of Tycho G

    Tycho G is a subgiant G2IV star located at 0.49 arcmin from the Chandra centre of Tycho SNR. From low resolution spectroscopy, and after dereddening by E(B-CV)=0.60 +/- 0.05 mag, we derive a temperature of Teff = 5750K, a surface gravity log g = 4.0, and solar metallicity from high-resolution spectroscopy. For the spectral type found and being a slightly evolved star (surface gravity not much below the main-sequence value), the mass should be about solar and thus the radius, for the range of surface gravities above, should be 1.0 solar radii, which translates via our photometric data (Tycho G’s apparent V magnitude is 18.71) into a distance d = 2.5-4.0 kpc.

    Tycho G could have been a main-sequence star or a subgiant before the explosion. Main-sequence stars no longer look like ordinary main-sequence stars after the explosion of the supernova, but subgiants with envelopes expanded. Subgiants remain subgiants of lower surface gravity (Marietta et al., 2000; Podsiadlowski, 2003).

    Stars at distances d = 2.0-4.0 kpc in the direction of Tycho SNR move at
    average radial velocity vr = -20 to -40 km/sec (in the Local Standard of
    Rest) with a ~20 km/sec velocity dispersion (Binney & Merrifield, 1998; Dehnen & Binney, 1998). Tycho G moves at -108 km/sec (heliocentric) in the radial direction. In contrast, all other stars with distances compatible with that of SN 1572 have radial velocitites within the velocity dispersion as compared with the average of all stars at the same location in the Galaxy (see Figure 4).
    .
    .
    .
    From detailed proper motion measurements on Hubble Space Telescope WFPC2 images (Ruiz-Lapuente et al., 2003b) it is found that Tycho G has tangential
    velocitites = 1.34 mas/year resulting in a total tangential velocity of 94.27 km/sec. This proper motion programme continues in HST Cycle 13 where
    measurements with smaller error bars will be obtained using both WFPC2 and ACS. The other stars of our sample do not show such coincidence in distance and high tangential velocity. Putting together radial and tangential velocities, we derive a value of 136 km/sec for the modulus of the velocity vector of Tycho G, being a factor of 3 larger than the mean velocity value at 3 kpc.

    This derived velocity lies in the range of expected peculiar velocities of the companion star from the disruption of a white dwarf plus subgiant/main-sequence system. The system would have resembled the recurrent nova U Scorpii, ie. a system made of a white dwarf close to the Chandrasekhar mass (initial mass of the white dwarf 0.8 solar masses) plus a companion of roughly a solar mass (initial mass of the evolved companion 2.0-2.5 solar masses filling its Roche lobe) at the moment of the explosion. The excess velocity corresponds to a period of about 2.7 days (a period of 6 days correspond to an orbital velocity of 90 km/sec approximately). The effective radius of the Roche lobe of the companion just before the explosion would have been 7 solar radii. Given the effective temperature and luminosity of Tycho G, the radius is less than 3 times the solar radius. This smaller radius would be a consequence of mass stripping
    and shock heating by the supernova impact, plus subsequent fast cooling of the outer layers up to the present time.

    Such a high velocity, however, could be explained if Tycho G belongs to the
    Galactic halo population. The lower limit to the metallicity obtained from the spectral fits [M/H]> -0.5, however, excludes this possibility (see Figures 5 and 6). Spectra taken at five different epochs also exclude Tycho G is a single-lined spectroscopic binary.
    .
    .
    .
    Conclusions

    Our search for the binary companion of Tycho’s supernova has excluded giant
    stars. It has also shown the absence of blue or highly luminous objects as
    post-explosion companion stars. One of the stars, Tycho G of our sample, show a high peculiar velocity (both radial and tangential velocities), lies within the distance range for the explosion of SN 1572, and its type, G2IV, fits the post-explosion profile of a type Ia supernova companion whose position in the Hertzsprung-Russell diagram is untypical for a standard subgiant.

    If Tycho G is the companion star of SN1572, its overall characteristics imply that the supernova explosion affected the companion mainly through the
    kinematics. Therefore, a star very similar to the our Sun but of a slightly more evolved type would have been the mass donor that triggered the explosion of type Ia SN1572, connecting the supernova explosion to the family of cataclysmic variables.”

  13. Chris A.

    @Doc:

    Not dumb at all.

    Cepheids possess conditions in their innards that cause the gases to vary their opacity strongly with relatively small changes in temperature and pressure. Big changes in opacity has a big effect on how easily the energy from their cores can reach the surface. When the opacity is low, the photons zip right through. When it’s high, they get absorbed and heat the gas, which in turn drives up the pressure, which causes the outer layers of the star to expand. The expansion cools the gas, causes the opacity to drop until the gas is more or less transparent again, the star contracts, and the cycle repeats.

    There’s a whole family of stars whose exact combination of energy output (luminosity, L) and surface temperature (T) makes them susceptible to this sort of oscillation. Since they occupy a diagonal swath across the L vs. T graph (AKA the Hertzsprung-Russell or H-R diagram), it’s known as the “instability strip.” Other variable stars (including the Mira variables and the RR Lyrae variables) fall along different parts of this strip.

  14. Kingthorin

    Ok so I know we’re not supposed to think of the universe expanding from a single point. But, if we can now measure the speed of expansion wouldn’t we find some sort of “sidedness” or directionality in that stars/galaxies in one direction/region are all moving faster (or slower) vs another direction/region.

  15. DFarmerTX

    Go, go Google Calc!
    http://www.google.com/search?q=(((74.2(km+per+s))%2F(1+Mpc))*13800000000+light+year)%2Fc

    This means that the accepted edge of the visible universe, that point at which we can see no further, 13.8 billion light years, is about right. Or, does it mean the rate is correct?

    The result of the forumla about is about 1.0.

    Neat!

  16. So… with the diameter of the Earth at 4.1339993 * 10^-16 Megaparsecs…

    The Earth is expanding at 3.067427 * 10^-14 km/s, or 3.067427 * 10^-8 mm/s, or 0.9673439 mm/year?

    Or, has expanded about 3871 km since 4 billion years ago?

    (Anyone want to check my math?)

  17. DrFlimmer

    @ Kingthorin

    Cosmology (and basically all physics) is based on a small principle: Our place in the universe is not special, we are as insignificant as any other place. This means: We should measure that all galaxy fly away from us with a speed only depending on their distance and with a direction that is almost radial. And this is what we measure. Apart from nearby galaxies (like Andromeda) every galaxy is rushing away radially.
    Indeed, this sounds like a special place, but the important point is that we can “transform” ourselves into another galaxy and would see the same picture – every galaxy would move away….

    @ inverse

    I didn’t check your mass, but space expansion only works if there is not enough gravity nearby. So a big mass (like the earth or a galaxy) will easily overshadow the effect of DE.

  18. @BA “But that language is clearly saying the Universe is old, and there is a small amount of uncertainty (actually, only about 120 million years) in the age estimate of the Universe.”

    @Phillip Helbig “Just a few years ago, misguided journalist and, sadly, a few misguided scientists uttered such nonsense as “universe younger than objects it contains”.”

    The alleged accuracy of ~100 million years in the age of the universe is strictly based on cosmological evidence. The other methods of dating the age of the universe (spectra of radioactive isotopes, main-sequence turnoff dating, white dwarf cooling) all yield uncertainties of at least one billion years. So the cosmological age of 13.7 billion years is usually within the uncertainty range of the other methods, but the error bars are usually large enough that it would be possible to support an object older than the cosmological age of the universe. Click on my name to go to a link containing a summary of the various methods for determining the age of the universe. Scroll down to the bottom. It has the following information:


    Dating technique: Cosmological
    Authors: various
    Age: 13.7 +/- 0.2 Gyr

    Dating technique: Radiometric
    Authors: Cowan, et al (1999)
    Object: HD 115444CS
    Age: 14.5 +/- 3.0 Gyr

    Dating technique: Radiometric
    Authors: Wanajo, et al (2002)
    Object: CS 31082-001
    Age: 16 +/- 5 Gyr

    Dating technique: Main-sequence turnoff
    Authors: Gratton, et al (1999)
    Object: Multiple globular clusters
    Age: 12.3 +/- 2.5 Gyr

    Dating technique: Main-sequence turnoff
    Authors: Chaboyer, et al (2001)
    Object: Multiple globular clusters
    Age: 12.0 +/- 1.5 Gyr

    Dating technique: White dwarf cooling
    Authors: Hansen, et al (2004)
    Object: M4
    Age: 12.8 +/- 1.1 Gyr”

    To summarize the other methods:

    Radiometric: universe may be as young as 11 Gyr and as old as 21 Gyr
    Main sequence turnoff: universe may be as young as 9.8 Gyr and as old as 14.8 Gyr
    White dwarf cooling: universe may be as young as 11.7 Gyr and as old as 13.9 Gyr

    All 3 alternative methods have maximum ages of the universe greater than the accepted cosmological age, although the cosmological age is within the error bars.

  19. Ok so I know we’re not supposed to think of the universe expanding from a single point. But, if we can now measure the speed of expansion wouldn’t we find some sort of “sidedness” or directionality in that stars/galaxies in one direction/region are all moving faster (or slower) vs another direction/region.

    No.

    You can show yourself why not too.

    Get a (deflated) balloon. On the deflated balloon, draw little circles with a pen.

    Now blow the balloon up slowly.

    As you watch, relative to any circle, all the other circles will be moving away. In addition, the farther you are from any given circle, the faster you’ll be moving away from the reference circle.

    But you can’t find a “direction” of expansion in the plane of the circles because there isn’t one.

  20. Kingthorin

    “In addition, the farther you are from any given circle, the faster you’ll be moving away from the reference circle.”

    Exactly all the circles at the furthest point (centre of the universe/origin of the Big Bang) from the reference circle (Milk Way) will be moving faster than the close ones.

  21. By the way, is anyone else reminded of this little ditty:
    Whenever life get you down, Mrs. Brown
    And things seem hard or tough
    And people are stupid, obnoxious or daft
    And you feel that you’ve had quite enu-hu-hu-huuuuff

    Just remember that you’re standing on a planet that’s evolving
    And revolving at 900 miles an hour
    That’s orbiting at 19 miles a second, so it’s reckoned
    A sun that is the source of all our power
    The sun and you and me, and all the stars that we can see
    Are moving at a million miles a day
    In an outer spiral arm, at 40,000 miles an hour
    Of the galaxy we call the Milky Way

    Our galaxy itself contains 100 billion stars
    It’s 100,000 light-years side-to-side
    It bulges in the middle, 16,000 light-years thick
    But out by us it’s just 3000 light-years wide
    We’re 30,000 light-years from galactic central point
    We go round every 200 million years
    And our galaxy is only one of millions of billions
    In this amazing and expanding universe

    The universe itself keeps on expanding and expanding
    In all of the directions it can whiz
    As fast as it can go, at the speed of light you know
    Twelve million miles a minute and that’s the fastest speed there is
    So remember, when you’re feeling very small and insecure
    How amazingly unlikely is your birth
    And pray that there’s intelligent life somewhere up in space
    Because there’s bugger all down here on Earth

  22. Bored...

    @inverse: Unfortunately, it doesn’t really work that way. The expansion is not the same for gravitationally bound systems as it is for more “empty” space.

    To put it another way, the expansion mostly affects the distance between galaxies, not between stars or planets.

  23. @DFarmerTX actually, it is a common misconception that the size of the observable universe is given simply by cosmological time (~13.7bil years) times the speed of light. That would be true only in a flat universe. The universe, however, is highly curved on cosmological scale. Due to the expansion of the space metric itself it is possible for the light to travel much further than 13.7 bil. years. It is estimated that the size of observable universe is about 93 billion ly.

    Furthermore for a redshift less than 0.8 a good approximation to the distance is obtained using d=cz/H (z is red shift, c speed of light, H the hubble constant), however, beyond z = 0.8 “it’s more complicated than that” – check wiki’s Distance_measures_(cosmology)

  24. Simon

    There’s a question that’s always bugged me. Whenever a Cepheid is mentioned there is the discussion about the high correlation between periodicity & absolute brightness.

    Whatever no-one ever mentions is the physical model that explains it. We know stars are powered by fusion, great — but what makes a Cepheid a Cepheid? Is it intrinsic to the star (some sort of instability in the fusion reaction, maybe because of it’s chemical makeup) or is it an external influence such as a close orbiting body?

    If anyone has a link to an explanation I’m interested!

  25. So, I know it doesn’t apply within galaxies, but if it did does that mean that Alpha Centari is retreating from us at 99 m/s?

  26. Chris A.

    @Simon:

    This has been answered already. Look back up the thread a bit.

    Short answer: Stellar helium’s opacity varies a lot, making it a sort of “oscillating energy valve” under the right conditions of density, pressure, and temperature. The rate of fusion is constant.

  27. Bjoern

    @Kingthorin: “But, if we can now measure the speed of expansion…”

    We can’t, and we haven’t, and it even makes no sense to talk about the “speed of expansion” in the usual sense of a speed (distance per time). What we can measure, and have measured, is the *rate* of expansion. As Anon and I already pointed out, the Hubble “constant” essentially has units of inverse time, i. e. it tells you how many *percent* the universe expands per time.
    The Hubble constant is *not* a speed!

  28. “The universe, however, is highly curved on cosmological scale. ”

    Actually, current evidence is that the universe is flat, or very close to it.

    Or are you talking about the curvature of spacetime rather than space?

    In either case, it’s too complicated to explain in a followup to a blog post.

    Don’t worry; you’re in good company:

    Georges Lemaitre: Do you think you can really understand the curvature of space?

    Alan Sandage: No.

    Georges Lemaitre: Young man, I think you should change fields.

    (Related by Sandage at the Saas-Fee school back in 1993)

  29. “@Phillip Helbig “Just a few years ago, misguided journalist and, sadly, a few misguided scientists uttered such nonsense as “universe younger than objects it contains”.”

    The alleged accuracy of ~100 million years in the age of the universe is strictly based on cosmological evidence. The other methods of dating the age of the universe (spectra of radioactive isotopes, main-sequence turnoff dating, white dwarf cooling) all yield uncertainties of at least one billion years. So the cosmological age of 13.7 billion years is usually within the uncertainty range of the other methods, but the error bars are usually large enough that it would be possible to support an object older than the cosmological age of the universe. ”

    Back in the early 1990s, it was usually (mis)represented as a conflict between
    the Hubble time and the age of objects in the universe, assuming the Einstein-de Sitter
    model (based on essentially no evidence for this model, which fortunately no one except
    perhaps Rocky Kolb still believes in today). If one looks at the age of the universe, i.e.
    the actual time since the big bang and not just the inverse of the Hubble constant, then
    the conflict is less severe and no-one seriously expects it to be explained by anything other
    than wrong error bars or other statistical or measurement errors. Back then, the
    difference was much too great to be explained away, especially since a really high value
    of the Hubble constant was not ruled out. Bad journalists used this to question the whole
    basis of modern cosmology, and even among more serious scientists it prompted all manner
    of papers on globular-cluster ages etc since we KNEW they couldn’t be that old etc. I
    even heard people say stuff like “Hey, thanks for bringing those globular-cluster ages
    down to 10 billion years. I was really worried there for a minute.”

  30. Simon

    @Chris:

    Ooops, thanks. :) And indeed if I followed the Wikipedia references I also would have seen that.

    Which leads me to realise that this must be the first time I’ve heard of a Cepheid since the rise & rise of Wikipedia. I’ve never seen the explanation anywhere other than Wikipedia and this blog.

  31. Stark

    So, out at a real distance of 13,179,555,229 light years the rate of expansion, from our point of view, equals the speed of light…. cool. So, if my thinking is correct we cannot ever see any events which occur today (local time) beyond this distance. As in, a hypernova occurs today, 13.2 billion light years away, and we will never even have a chance of knowing it happened. Whereas same said event occurring at 13billion light years would eventually, in theoretical physics land, be detectable here on Earth (though far far longer than 13 billion years from now).

    Right?

  32. dreikin

    Hm, so that translates to about
    148.377 Planck Lengths/s/picometer
    For some reason, I thought there’d be less planck lengths at first. Interesting.

  33. Davidlpf

    I thought Cepheids luminosity varied because what was going in the
    core not the ionization of the helium. I guess you learn something new everyday.

  34. AliCali

    If I may bring up a point, Dr. Plait says, “…and maybe even rule out some theories as to what this mysterious stuff is.”

    This triggered a thought. The creationists or IDers or whatever they’ll call themselves will say that evolution is “just a theory.” And those in the know will state that “theory” in scientific parlance is much more advanced, has been tested over and over, and is quite solid.

    So when we can rule out some theories, are we really talking about ruling out some hypotheses (which as I understand “hypothesis” to mean a good guess supported by evidence, but not tested enough to beat out the other hypotheses).

    I’m not harping on Dr. Plait’s use of language as I believe I understood its meaning, but it seems very easy to understand why the layperson would not elevate “theory” to the level of scientific parlance when even Dr. Plait would use the word to mean hypothesis.

  35. The interesting thing about Type Ia supernovae is that they leave nothing behind besides the nebula and the companion star. No black hole. No neutron star. It just goes pffttt and then it’s gone. I’m surprised that the companion star isn’t annihilated during the supernova explosion, but apparently it’s not. A few years ago a team of astronomers found the companion star for the Type Ia supernova known as SN 1572 (i.e., Tycho Brahe’s supernova). Click on my name for a link to the main article.

    Some excerpts are as follows:

    “The Search for the Companion Star of Tycho Brahe’s 1572 Supernova

    J. Mendez (University of Barcelona and ING)

    In recent years, type Ia supernovae (SNe Ia) have been used successfully as
    cosmological probes of the Universe (Riess et al., 1998; Perlmutter et al.,
    1999). However, the nature of their progenitors has remained somewhat of a
    mystery. It is widely accepted that they represent the disruption of a
    degenerate object, but there are also numerous progenitor models (see for
    instance Ruiz-Lapuente, Canal, Isern, 1997a, for a review), but most of these have serious theoretical/observational problems or do not appear to produce sufficient numbers to explain the observed frequency of SNe Ia in our Galaxy (~3E-3 per year; Cappellaro & Turatto, 1997).
    .
    .
    .
    The Search for the Companion Star of SN 1572

    Tycho Brahe’s supernova (SN 1572) is one of the only two supernovae observed in our Galaxy that are thought to have been of type Ia as revealed by the light curve (Ruiz-Lapuente, 2004), radio emission (Baldwin et al., 1957) and X-ray spectra (Hughes et al., 1995).
    .
    .
    .
    The Case of Tycho G

    Tycho G is a subgiant G2IV star located at 0.49 arcmin from the Chandra centre of Tycho SNR. From low resolution spectroscopy, and after dereddening … we derive a temperature of Teff = 5750K, … For the spectral type found and being a slightly evolved star (surface gravity not much below the main-sequence value), the mass should be
    about solar… (Tycho G’s apparent V magnitude is 18.71) …

    Tycho G could have been a main-sequence star or a subgiant before the explosion. Main-sequence stars no longer look like ordinary main-sequence stars after the explosion of the supernova, but subgiants with envelopes expanded. Subgiants remain subgiants of lower surface gravity (Marietta et al., 2000; Podsiadlowski, 2003).

    Stars at distances of 2.0-4.0 kpc in the direction of Tycho SNR move at
    average radial velocity 20 to 40 km/sec (in the Local Standard of
    Rest) with a ~20 km/sec velocity dispersion (Binney & Merrifield, 1998; Dehnen & Binney, 1998). Tycho G moves at 108 km/sec (heliocentric) in the radial direction. In contrast, all other stars with distances compatible with that of SN 1572 have radial velocitites within the velocity dispersion as compared with the average of all stars at the same location in the Galaxy.
    .
    .
    .
    Putting together radial and tangential velocities, we derive a value of 136 km/sec for the modulus of the velocity vector of Tycho G, being a factor of 3 larger than the mean velocity value at 3 kpc.
    .
    .
    .
    Conclusions

    Our search for the binary companion of Tycho’s supernova has excluded giant
    stars. It has also shown the absence of blue or highly luminous objects as
    post-explosion companion stars. One of the stars, Tycho G of our sample, show a high peculiar velocity (both radial and tangential velocities), lies within the distance range for the explosion of SN 1572, and its type, G2IV, fits the post-explosion profile of a type Ia supernova companion whose position in the Hertzsprung-Russell diagram is untypical for a standard subgiant.

    If Tycho G is the companion star of SN1572, its overall characteristics imply that the supernova explosion affected the companion mainly through the
    kinematics. Therefore, a star very similar to the our Sun but of a slightly more evolved type would have been the mass donor that triggered the explosion of type Ia SN1572, connecting the supernova explosion to the family of cataclysmic variables.

  36. @ Phil: Funny you should be talking about Cepheids and the cosmic distance ladder. I just posted about them in my blog.

    @ Those asking why Cepheids work: Here’s a snip from a paper I did on them:

    The mechanism that drives pulsation is understood to be a balance between opacity, photosphere temperature, and gravity. When the opacity of the photosphere in a star increases to a critical point, light is not allowed to escape, and thus the temperature of the photosphere rises. The heating causes an expansion of the photosphere, which continues until the excess heat is released and the photosphere cools and shrinks, at which point the cycle starts again. While this is a simplistic model, it is the base upon which all more complex models are based.

    To make a fuller explanation, it should also be noted that the opacity is based on the ionization of various elements within the star. As a star contracts, the temperature increases to the point at which helium (the easiest abundant material in stars to ionize) can be singly ionized, the ionization absorbs the extra energy and the contraction continues since the radiative energy that would normally help to balance the contraction is being absorbed by the ionized helium. Thus, the pressure continues to build until it can only be relieved by an expansion. As the expansion occurs, the temperature decreases resulting in a recombination of the helium. The star then contracts again and the process begins again. But to make things more complex it becomes possible to have several different layers of ionization in which helium can become doubly ionized and even hydrogen can become ionized. These multiple layers of ionization fronts create complex system of expansion and contraction.

  37. Chris A.

    @Simon:
    In my case, the Cepheid mechanism was explained in a “Stellar Atmospheres” course I took in grad school.

  38. “So, out at a real distance of 13,179,555,229 light years the rate of expansion, from our point of view, equals the speed of light…. cool. So, if my thinking is correct we cannot ever see any events which occur today (local time) beyond this distance.”

    It’s not that simple. First, you need to be clear on what you mean by distance. Second,
    you need to be clear on what you mean by velocity. Third, the behaviour of the horizons
    (of which there are several types) depends on the cosmological model.

    Here are some suggestions for further reading:

    http://www.astro.multivax.de:8000/helbig/research/publications/Abstracts/angsiz.html

    Shameless plug. See the appendices for relationships between different distances.

    http://adsabs.harvard.edu/abs/1993ApJ…403…28H

    http://adsabs.harvard.edu/abs/1981csu..book…..H

    The book now has a second edition; I’m familiar with only the first, but that is
    completely adequate here since nothing new happened in this area of theory after
    the first edition.

    Edward Harrison is a personal hero of mine. He’s not very well known, despite
    having written many papers and a very good textbook. Many people have heard
    his name in the “Harrison-Zeldovic spectrum of primordial density fluctuations”, but
    actually this is just a footnote to his enormous body of work on cosmology. He has
    written many things about the fundamental principles of cosmology and also about
    topics which confuse many people, even professional cosmologists.

    I urge all people interested in cosmology, whether amateur or professional, to
    check out Harrison’s work.

  39. Torbjörn Larsson, OM

    I’m pleased that the new result isn’t only still consistent with a (cosmic) constant but constrains it further. (I believe the new results have a “tension” between measurement methods, but as layman I can have the luxury of not being overly concerned. :-) )

    So, out at a real distance of 13,179,555,229 light years the rate of expansion, from our point of view, equals the speed of light

    Didn’t we have this question the other day?

    It seems a bit messy to calculate. FWIW, Wikipedia puts the current radius of this “edge” of the observable universe at “46.5 billion light-years away”. [ http://en.wikipedia.org/wiki/Observable_universe ]

  40. Cusp

    Megaparsec has a capital M

  41. Torbjörn Larsson, OM

    So when we can rule out some theories, are we really talking about ruling out some hypotheses (which as I understand “hypothesis” to mean a good guess supported by evidence, but not tested enough to beat out the other hypotheses).

    Discussing different uses of terms would be rather fruitless. But ordinary, a hypothesis isn’t a mere “explanation of data” (a possibly untestable model) but is supposed to be a scientific, testable, prediction.

    The idea being that you want to be able to exclude those ideas that can be shown false at all. (Preferably but not necessarily before you start to compete remaining ideas for any measure of “goodness”.) Otherwise you risk (more precisely, are) making stuff up.

    Similarly, a theories is usually a coherent set of hypotheses.

    For example, in this model Newton’s mechanics consists of three testable hypotheses (three laws) which together makes up a theory of classical mechanics (“Newton’s laws of motion”). While “my great great … great grandfather ten times removed was a Swede” is not a testable hypothesis, albeit it is a good guess based on current evidence.

    As “dark energy” formally makes several testable predictions already (drives expansion with w ~ 1, seems to be constant, et cetera) it is IMHO a toss up if you want to call the proposals for hypotheses or theories. (Personally I would go for the later.)

  42. Turing E.

    2.40465615 × 10-^18 Hz? Man, my computer is faster than that. Universe needs to upgrade.

  43. @Jon Voisey “The mechanism that drives pulsation is understood to be a balance between opacity, photosphere temperature, and gravity. When the opacity of the photosphere in a star increases to a critical point, light is not allowed to escape, and thus the temperature of the photosphere rises. The heating causes an expansion of the photosphere, which continues until the excess heat is released and the photosphere cools and shrinks, at which point the cycle starts again. While this is a simplistic model, it is the base upon which all more complex models are based.”

    That explains why the pulsations happen. That doesn’t explain why the pulse period is proportional to average luminosity which is the factor which allows Cepheid variables to be used as “standard candles”. I’m guessing the explanation of that is more complicated. :)

  44. Adam Zeldin

    He is my (wonderful) astronomy/astrophysics professor at JHU!

  45. coolstar

    Bascially, what Phil has done here is to regurgitate (hope it didn’t burn coming up) this post:http://www.physorg.com/news160923906.html . That article is actually much more informative about the techniques used. Neither tells us why a measurement of H now! (don’t know how to do subscripts here) tells us anything about the equation of state of dark matter. To learn that, look at the preprint at http://xxx.lanl.gov/PS_cache/arxiv/pdf/0905/0905.0695v1.pdf.

  46. AliCali

    @ Torbjörn Larsson, OM

    Thanks for the reply. I understand that a hypothesis is testable. I’m not talking about anything that is not testable.

    And the only reason I’m harping on the term “theory” is because of its use in everyday-speak vs. scientific-speak. This seems to matter when you encounter those who don’t understand the Theory of Evolution and say, “It’s just a theory.”

    I’ve read many of Dr. Plait’s blog entries, and I get the impression that, in scientific-speak, a theory is a model that has been tested extensively and is the best known reason or rules for particular characteristics. So to say that the General Theory of Relativity is “just a theory” downgrades the actual status of such a theory, which has been tested and verified so many times.

    So when Dr. Plait says that “…maybe even rule out some theories as to what this mysterious stuff is,” is he talking about theories in the scientific manner, such as used with Relativity, or is he speaking in more everyday language and using the word in a colloquial way (i.e., there are several competing models, and we’re not sure which is the most correct yet, if any)? Or perhaps I’m not understanding the definition of “theory” well enough.

    If Dr. Plait is using the term in a colloquial way, it seems we should conclude that it’s understandable why some people say, “It’s just a theory,” since that’s the common usage of “theory.”

  47. @ Simon:

    Which leads me to realise that this must be the first time I’ve heard of a Cepheid since the rise & rise of Wikipedia. I’ve never seen the explanation anywhere other than Wikipedia and this blog.

    Except in a few thousand astronomy books. Conveniently located at your friendly neighborhood library! :)

    Now get off my lawn, young whipper snapper.

  48. Errr…whilst I go close a few italics….. :(

  49. Larian,
    I think that ditty is from Monty Python – I’d forgotten all about it. Thanks for posting, it’s the only post I understood. Now I’ll get back to sweeping up what’s left of my neurons.

  50. @ Evolving Squid.

    Thank you for the balloon analogy. I can visualise it pretty well, and will have to get hold of a balloon and actually see it. Kingthorin’s question was one that had been bothering me for a while; all the discussion about expansion of the universe, and galaxies flying away “from us”, as it were, sounded fairly geocentric (ok, perhaps Milky Way-centric) – but I’m reassured now, cheers. ^_^

  51. Adam Zeldin

    Let’s see if I’ve learned anything in Professor Riess’ class this semester…

    The deceleration of the universe is due to the shape of the Hubble Diagram on a larger scale. Basically it levels off, showing different values of H for different times in the universe (as we look further and further back in time, via speed of light)… so we know that H used to be smaller…. so the expansion used to be less, so it’s accelerating.

    So… weird value of cosmological constant. Implies “anti-gravity” or some repulsion… dark energy is current hypothesis.

  52. Adam Zeldin

    Or not repulsion, but something driving the universe in its acceleration. Why wouldn’t it just compress back unto itself if it has gravity? That’s the dark business.

  53. CJA

    Hi guys, I get that even if we’re at the edge of the universe then everything is still expanding away from us, but, my question is this: 1) Is everything in the universe expanding away from the earth at the same rate in every direction?, or, 2) are some things expanding away from us slower in one direction than other directions?

    If 1) then aren’t we at the centre of the universe
    if 2) the we aren’t at the edge.

    All a quick search online shows is that everything is expanding, I haven’t found info about rate in every direction, so I came here for the answers.

    Thanks
    CJA – (long time reader)

  54. Cindy

    When I was in grad school in the mid-90′s there were two camps about the value of the Hubble constant. One camp thought it was 50 km/s/Mpc and the other thought it was 100 km/s/Mpc. Most cosmologists/astronomers averaged the two and used 75 since it gave an age roughly consistent with ages determined by main-sequence turn-offs in globular cluster. I remember smiling when the value of 74 km/s/Mpc due to the Type Ia’s came out as most astronomers who used the averaged value wasn’t too far off.

  55. @CJA “1) Is everything in the universe expanding away from the earth at the same rate in every direction?”

    Well, it’s well known that for nearby galaxies Hubble’s Law breaks down. For example, the Andromeda galaxy is approaching us at ~100 km/sec whereas Hubble’s Law says that it should be receding at 58 km/sec. So there is always going to be some statistical jitter which becomes less and less relevant the farther the distance.

    “2) are some things expanding away from us slower in one direction than other directions?”

    That’s actually a very good question. To the best of my knowledge the expansion that has been measured is isotropic – it does not depend on the direction of the galaxy from Earth. The redshift of a galaxy seems to correlate with the distance measured via the Cepheid variables (or Type Ia supernovae) regardless of direction, although I have no idea what precision this can be established to.

  56. Dr Fish

    @Cusp.
    Megaparsec has a capital M

    Yes, but megaparsec has a lower case m. :)
    Actually, the prefix mega- (meaning 10^6) is properly spelled with a lower case, even though an upper case M is used for the abbreviation (as in Mpc).

  57. QUASAR
  58. Jess Tauber

    OK- we have 148.377 Planck lengths per second per picometer thanks to dreikin above- how many Planck lengths per Planck time per Planck length does this translate to (just to keep units consistent)?

  59. Caleb Jones

    Ok, my math may be way off and it’s very unlikely that the 72.4 km/s per megaparsec expansion has been constant throughout the history of the universe, but if it were, wouldn’t that mean that in order for a region of space to be expanding from us at or faster than the speed of light (I’ve always understood that c was only the speed limit of traveling THROUGH space and that space itself can be manipulated beyond that), it would have to be 1.21209889 × 10^10 light years away?

  60. Mount

    Thanks Phil, that was pretty damn cool!

  61. madge

    Oxymoron: Why does the Hubble CONSTANT keep changing? :)

  62. “But by nailing down all these numbers, we can in turn nail down such things as how much dark energy is in the Universe, and maybe even rule out some theories as to what this mysterious stuff is. It’s pushing on the fabric of space and time, making the Universe swell faster every second of every day, and we have no clue what it really is. Well, that’s unfair: we have lots of clues, but we don’t know what’s causing it.”

    It’s amazing and humbling to think about all that we still just don’t know. Science is in its infancy, really, and I can’t think of a more exciting time to be an observer to it. Honestly, I’d rather be here at the beginning, when the wonder is fresh and new, and much of the mystery remains hidden. It makes discovery all the more sweeter. Those guys 2 thousand years in the future are going to take all this for granted – where’s the fun in that? ;-)

  63. Aleksandar

    Lets see if I understood this correctly. The farther the galaxy is the faster it is moving away from us, with speed increasing with distance. But as the farther away it is, it is also farther in the past? Shouldn’t that mean universe expansion is slowing down, not accelerating like modern theories say?
    /SARCASM MODE ON
    I believe they invented dark energy and accelerating expansion just to get to the Big Rip theory, nice little unsurvivable apocalypse maybe just 10 By in future.

  64. “So… weird value of cosmological constant. Implies “anti-gravity” or some repulsion… dark energy is current hypothesis.”

    No “weird” value, just a positive value. “Dark energy” is not a hypothesis, just another
    name, and not a very good one. As Sean Carroll pointed out, everything has energy,
    and lots of things are dark. A much better—and cooler-sounding—name would be
    “smooth tension”.

    No offense intended, but it seems to me that some people asking questions in followups
    to blog posts would learn a lot more by reading an introductory cosmology textbook.

  65. Kingthorin

    @ Evolving Squid

    I’m with Raphael, the balloon example is good.

    I realized after my second reply yesterday (1:04pm) I was assuming a stationary or constant reference point. Which was a mistake.

  66. Adam Zeldin

    The expansion is isotropic.

    @ Phillip
    The “weird” value is that its changed. Not that it’s positive.

  67. Adam Zeldin

    I.E., it got bigger.

  68. C Murray

    Click to get to much more cromulently embiggened images

    I am very saddened to see that Discover has a focus on two things.

    1. Science
    2. Words made up by the writers of The Simpsons.

  69. Bjoern

    @Adam Zeldin:
    “… so we know that H used to be smaller…. so the expansion used to be less, so it’s accelerating.”

    No, actually H was not smaller, but larger. That the expansion is accelerating does not (necessarily) imply that H is increasing. What an accelerated expansion means is that the second derivative of the scale parameter is greater than zero, essentially.

    @Aleksandar:
    “Lets see if I understood this correctly. The farther the galaxy is the faster it is moving away from us, with speed increasing with distance. But as the farther away it is, it is also farther in the past? Shouldn’t that mean universe expansion is slowing down, not accelerating like modern theories say?”

    Actually, the farther away a galaxy is *now*, the faster it is moving away from us *now*. If you measured (or calculated – I don’t think that can be measured directly) how fast it was moving away at the time when the light we see now from it was emitted, you’d get other results.

    Additionally, that the expansion of the universe is accelerating does no* (necessarily) mean that the Hubble “constant” is increasing (actually, it is indeed decreasing!). See my reply above to Adam Zeldin.

  70. Gary Ansorge

    Dark matter detected by its gravitational influence on visible matter,,,so we know there’s SOMETHING there,,,but is it a WIMP or,,,

    I predict(who remembers Chriswell now?), since such prediction will have absolutely no effect on my professional credibility, that Dark MAtter will be discovered to be nothing more than the gravitational effect of ordinary matter stuck to a nearby Brane,,,

    No WIMPs or other exotic matter need apply,,,

    GAry 7

  71. Gary Ansorge

    PS:
    Just finished reading Hawkings “Universe in a nut shell” and his statement that “,,,all P-Branes are equal”,,,” just tickled the heck out of me,,,

    Gary 7

  72. Adam Zeldin

    @Bjoern… how is that a different statement?

    From one of Prof. Riess’ lecture power points: “Powerful way to see if expansion rate is slowing, is H0<H1<H2

    H3??

  73. Adam Zeldin

    “Powerful way to see if expansion rate is slowing, is H0<H1<H2<H3? "

    That didn't copy. Is this out of context?

  74. Adam Zeldin

    Powerful way to see if expansion rate is slowing, is H0 < H1 < H2 < H3?

    That is what I'm trying to paste.

  75. Albert Bakker

    CJA said: “1) Is everything in the universe expanding away from the earth at the same rate in every direction?, or, 2) are some things expanding away from us slower in one direction than other directions?
    If 1) then aren’t we at the centre of the universe
    if 2) the we aren’t at the edge.

    In a certain way we are in the centre of the universe, but the uniqueness that it implies derives from the “we” that “are” at a certain place at any moment. Because that “any moment” is a special time, it is always exactly now. And that special time, now, at which point you exist (verb), is always the farthest removed from t = zero, the time when the universe began. There is no special point in space, but there is that special point in time, the very edge of time in a way. You cannot exist outside it, but the universe as a whole does.

    Tom Marking said the expansion that has been measured is isotropic, or in other words there is no preferred direction to where space expands (to), but to my understanding there is some controversy about this. Some people are losing sleep over the “axis of evil” found in WMAP data and other anomalies. And to make this already impenetrable mystery completely incomprehensible for normal human beings, Nasa scientist infer a cause for a preferred direction of distant clusters independent of expansion outside our observable universe. http://www.nasa.gov/centers/goddard/news/topstory/2008/dark_flow.html

    The dark energy business (a better word for it is still the cosmological constant) is to my knowledge best understood not as some mysterious energy with properties opposed to “normal” E=mc^2 kind of energy, but to a feature of the universe as a whole, not the observable universe but the whole of it, the multiverse if you will.

  76. Bjoern

    @Adam Zeldin:

    You have to distinguish between “the expansion rate is getting bigger” and “the expansion is accelerating”. The two are not the same – and, as I have pointed out, in reality (Lambda CDM model), actually the expansion rate H is decreasing, although the expansion is accelerating.

    Do the math yourself: flat universe (Omega = 1), cosmological constant Lambda with density parameter Omega_lambda; since the radiation energy density is negligible nowadays, you can take Omega_radiation = 0, and hence you have Omega_matter = 1 – Omega_lambda. Plug all this into the Friedmann equations and solve for H(t). You’ll arrive at a Cotanges hyperbolicus – a decreasing function. If you then also solve for the scale parameter a(t), you’ll arrive at something like a Sine hyperbolicus (to the power of 2/3, IIRC) – and the second derivative of that a(t) is positive, i. e. acceleration, although H(t) is a decreasing function.

    I have done the calculations in some detail for myself; if you like, I can give you a link to the pdf file.

  77. Albert Bakker

    Some links to clear up some of the “other anomalies” in WMAP data I left unspecified.

    http://blogs.discovermagazine.com/cosmicvariance/2008/11/07/a-special-place-in-the-universe/

    http://blogs.discovermagazine.com/cosmicvariance/2008/07/17/a-new-cmb-anomaly/

    http://blogs.discovermagazine.com/cosmicvariance/2008/06/08/the-lopsided-universe/

    I specifically like the complaint in the last link “It’s no fun being a theoretical cosmologist these days, all the data keeps ruling out your good ideas.”

    If only that sentence would come more often out of the mouths of string-theorists.

  78. Albert Bakker

    No, I retract that idea of a special time. It is wrong.

  79. DaveS

    I’ve got a BASIC and SILLY question. As part of these physical observations of cepheids and supernovas, you use the fact that the measurable cepheid is in the same galaxy, in the vicinity of the supernova. You’re using that fact to correlate luminance independently of red-shift.

    How do you know that a given star, cepheid or supernova, is a member of a particular galaxy, rather than in front or behind it? Of course, you can measure red-shift, but that’s relying on the Hubble principle, which is the rule that you’re using other physical measurements to try to prove.

    Isn’t it then a tautology, using the Hubble principle to prove the Hubble principle?

    Or do you wait long enough to see the galaxy revolve, and see that the stars are gravity/dark-energy coupled in the galaxy?

  80. Bjoern

    @DaveS:
    “How do you know that a given star, cepheid or supernova, is a member of a particular galaxy, rather than in front or behind it?”

    I’m not an astronomer, but I think one would use the following criteria:
    1) Stars are usually associated with galaxies, not floating through space on their own. Hence if the star would be in front or behind the observed galaxy, we should see another galaxy there, too. And especially, if the star would be in our own galaxy, it would be a *lot* brighter!
    2) In some cases, you could perhaps measure the velocity of a star (by measuring its Doppler shift with respect to other stars which are also supposed to lie in the observed galaxy), and thereby determine whether it orbits in the observed galaxy.
    3) Red shift.

    The second criterium doesn’t use the Hubble principle directly, as you implied – it merely uses the fact that objects which have the same redshift should be at about the same distance. Additionally, this is no attempt to “prove the Hubble principle” – it is only an attempt to measure the Hubble “constant”.

    Your proposed criterium, waiting long enough to see the galaxy revolve, would be nice – but unfortunately, that would take several 100 million years of observation time…

  81. Bjoern

    Err, I meant the “third”, not the “second” criterium, obviously.

  82. “How do you know that a given star, cepheid or supernova, is a member of a particular galaxy, rather than in front or behind it?”

    In addition to the reasons given by Bjoern – & a bit of an expansion on his reason 1 – stellar surveys and stellar astrophyiscs hold the key :

    Luminosity & spectral type – if a star is a Cepehid its absolute magnitude can be determined by its period with the period-luminosity law – brighter Cepheids taking longer to go through their cycle of brightening and fading as has been well studied by many historical astrponomers notably Henrietta Leavitt & her survey of Cepheids in the Large Magellanic Cloud.

    So using the Cepehid’s period you can determine the stars absolute magnitude (or how bright a star actually is rather than how bright it appears to us.) and by comparing that with its apparent magnitude (how bright it seems to be from our skies) you can calculate the distance.

    Absolute magnitude con pared with apparent magnitud ecan be useful if somewhat less conclusive in giving us ballpark figures to all spectral clases eg. the brightest A & B type supergiants are all about minus seven to minus nine in apparent magnitude. This extreme brightness makes them visible along way off eg. in other galaxies – and how bright they are in a given galaxy eg. app. mag. of plus nineteen. enables us to estimate approximately how far away they must be. Luminous Blue variables like Eta Carinae have for instance been noted in M33, the Triangulum or “Pinwheel” Galaxy.

    Then too, because type Ia supernova all arise from white dwrafs of a specific mass – just on or above the Chandraseker limit of 1.4 solar mass. Thus they all share a common maximum brightness and this too enables us to determine distance via contrasting the magnitude it appears and the actual magnitude.

    Stellar spectroscopy using the star’s light (spectra) and nature of its specteral lines means we can underatstand the sort of star we’re examining and then we can usually tell and whether or not the stars distance fits with the theoretical and spectral models for a particular galaxy also at that distance.

    (Of course other factor also need to be considered such as reddening and obscuring by interstellar dust and whether a star is particularly metal-poor or rich .. but that’s the basic principle.)

    Furthermore, spectral comparisons between the star in question and others in the galaxy in question can matched to see how well they fit. If astar is particularlymetal-poor and so the possible host glaxy then itstrengths the case for tehstar actuallybeing in that galxy – but if the host galxy is metal-rich and the star metal-poor, OTOH, it indicates we may have a problem ..

    That plus the redshift and Doppler shift as mentioned – & certain other indications such as whether a star seems to be interacting with its host galaxy in any way; eg. lighting up nebulae, following a path indicating its being ejected from a nearby cluster or OB Association et cetera … answers the question.

  83. CORRECTION :
    ————————————–

    If a star is particularly metal-poor and so the possible host galaxy then it strengthens the case for that star actually being in that galaxy – but if the host galaxy is metal-rich and the star metal-poor, OTOH, it indicates we may have a problem ..

    —————————————–

    Typos. * Sigh *. :-(

  84. & correcting once again :

    If a star is particularly metal-poor and so *is* the possible host galaxy then it strengthens the case for that star actually being in that galaxy.

    Absolute magnitude *compared* with *apparent magnitude* can be useful, if somewhat less conclusive, in giving us “ballpark” figures on the distances to all spectral classes.

    This extreme brightness makes them visible *a* *long* way off eg. in other galaxies – and how bright they appear to be in a given galaxy enables us to estimate approximately how far away they (& the galaxy they’re part of) must be.

    ..type Ia supernova all arise from white *dwarfs* of a specific mass -

  85. @ Tom Marking :

    A few years ago a team of astronomers found the companion star for the Type Ia supernova known as SN 1572 (Tycho Brahe’s supernova). It is known as Tycho G and interestingly enough, it is very similar to our sun only a bit older (spectral class G2IV). Click on my name to read the full journal article talking about the discovery.

    Thanks – I did read that link – interesting. :-)

    Thanks also for the other supernova data there too.

  86. @ Simon :

    Which leads me to realise that this must be the first time I’ve heard of a Cepheid since the rise & rise of Wikipedia. I’ve never seen the explanation anywhere other than Wikipedia and this blog.

    James Kaler’s excellent books on astronomy eg. ‘The 100 Greatest Stars’ (Copernicus press, 2002.) ceratinly mention them.

    I’m pretty sure bothPatrick Moore & Isaac Asimov have also mentioend Cepehids, supernovae and other astronomical ‘standard candles’ on occassion.

    I’ve seen a number of Cepehid references in astronomy texts and magazines.

    But yeah, Wikipedia is good & handy and generally reliable – or as reliable as the net can be. (Personally I’d take printed texts and articles over online stuff but both have their place.)

  87. @Myself “The interesting thing about Type Ia supernovae is that they leave nothing behind besides the nebula and the companion star. No black hole. No neutron star.”

    That post took about 3 days to appear here. I thought it had gone in the bit bucket. I’m assuming this is caused by the time delay it takes some actual human being to check out any URLs provided and verify that they are legit. Still, sometimes it’s a bit frustrating.

    BTW, I’ve tried to register at Universe Today 3 times and nothing happens. No e-mail ever comes with the registration info. Oh well, I guess they had enough of the EU crowd and they decided to get defensive by requiring registration. Maybe they have me confused with an EUer or something. :)

  88. John Polasek

    Please don’t use gray font.

  89. @Myself “That explains why the pulsations happen. That doesn’t explain why the pulse period is proportional to average luminosity which is the factor which allows Cepheid variables to be used as “standard candles”. I’m guessing the explanation of that is more complicated”

    I’m trying to get my head around how this can be. For Population I Cepheids we have the following relationship:

    L = 300 * P

    P is the period of the Cepheid variable in days
    L is the average luminosity of the Cepheid variable relative to the sun

    So average luminosity is proportional to the period of the variable star. What simple mechanism could explain that fact? I believe I’ve found one. Let’s assume that the Cepheid variable has a constant temperature during its pulsation (this is probably not correct but it helps to understand a simple model). Then, according to the Stefan-Boltzmann law the power emitted per square meter is constant over the period of the variable star since the surface temperature is constant.

    Then, changes in luminosity are strictly caused by changes in the surface area of the Cepheid. We have:

    L = k1 * (R^2)

    where L is the luminosity in suns, k1 is a constant, and R is the radius of the Cepheid. So we have:

    R = sqrt(L / k1) = k2 * sqrt(L)

    .
    .
    .

    To be continued

  90. IVAN3MAN

    @ Tom Marking: “BTW, I’ve tried to register at Universe Today 3 times and nothing happens. No e-mail ever comes with the registration info.”

    DrFlimmer had the same problem, but he said that he eventually succeeded in registering with Universe Today by using a new e-mail account. That might work for you too, Tom.

  91. So we have:

    R = k2 * sqrt(L)

    where R is the current radius of the Cepheid, k2 is a constant, and L is the current luminosity of the Cepheid (assuming an isothermal atmosphere during the pulsation).

    Let’s treat the pulsation as some type of diffusion phenomenon – photons will have to diffuse from the minimum radius up to the maximum radius as the photosphere of the star expands. This can be approximated as a random walk problem as follows:

    D = delta-D * sqrt(N)

    D is the distance from the origin,
    delta-D is the distance of each step (i.e., mean free path of the photon),
    N is the number of steps

    Let’s make one final simplifying assumption which is, let the mean free path of the photons (i.e., delta-D) be a constant. Then we have:

    N = (D / delta-D)^2

    The total elapsed time T = N * (delta-D / c) where c is the speed of light, so we have:

    T = k3 * D^2 where k3 is a constant

    Now, let D = R and we get:

    T = k3 * (k2 * sqrt(L))^2

    T = k4 * L where k4 is a constant

    In other words, the time for the expansion is proportional to the luminosity which means the total period of the Cepheid variable is proportional to the luminosity.

    By making certain simplifying assumptions the proportionality of the period to the luminosity for Cepheid variables falls out of the equations naturally. It’s probably a bit more complicated than this, but this gives you the general gist of where the relationship between luminosity and period comes from for Cepheid variables.

  92. DrFlimmer

    @ Tom Marking: “BTW, I’ve tried to register at Universe Today 3 times and nothing happens. No e-mail ever comes with the registration info.”

    DrFlimmer had the same problem, but he said that he eventually succeeded in registering with Universe Today by using a new e-mail account. That might work for you too, Tom.

    That’s right. I created that account at gmail/googlemail and it worked – never used it again since then ;)

    Btw: Nice calculations, Tom Marking :)

  93. John Polasek

    Your list of supernovae have distances of the order of 10,000 LY. This implies z = 9e-7 or about 1 millionth. Can redshift be measured to this accuracy? Or do they use the luminosity distance?

  94. telson

    http://koti.phnet.fi/elohim/howdideverythingbegin2.html

    Concerning the Big Bang and expansion, it is an issue that we cannot detect with the naked eye or even with a telescope, no matter how much we look. Revolving and rotary movements of the bodies we can see – at least in the near space – but we cannot see expansion.

    Instead, some have thought that the best piece of evidence supporting the Big Bang is red shift, which can be observed in distant stars. It has been thought that when the spectrums of light in distant galaxies and stars move towards the red end of the spectrum, this indicates expansion. Red shift values of these celestial bodies should indicate their escape velocity and distance, so that all bodies are drawing away from us at a velocity proportional to their distance.

    However, using the red shift as evidence for expansion is questionable. It arises, for example, from the following factors:

    The light of all stars is not red shifted. The first problem with the red shift is that the light of all stars is not red shifted. For example, the Andromeda Galaxy and certain other galaxies show blue shifted light, which means that they should be approaching us. (It has been estimated that the Andromeda Galaxy is approaching us at 300 kilometres a second! On the other hand, the escape velocity of the Virgin Constellation should be 1,200 km/s and that of Quasar PKS 2000 as much as 274,000 km/s. Where do these more than a hundredfold differences come from, if everything began at the same point?) These kinds of exceptions indicate that there may be some other explanation to the red shift values than drawing away from us. Maybe the values have nothing to do with their movements.

    The values of adjacent galaxies. Another problem with the red shift is that some adjacent galaxies may have completely different red shift values, even though they are in connection with each other and quite close to each other. If the red shift value could be really used to tell the distance, there is no way these galaxies could be close to each other: instead, they should be far away from each other. This indicates that the red shift must be caused by some other facts, such as internal reactions and radiation of stars, which can also be detected from the Earth.

    Because of the same matter some researchers deny the importance of the red shift. They say or doubt it having anything to do with expansion. In fact, the whole Big Bang theory is then without its most important evidence:

    I do not want to imply that everyone is of the same opinion regarding the interpretation of the red shift. We do not actually observe the galaxies rushing away from us; the only issue that is sure is that their spectrums have moved towards red. Famous astronomers doubt whether the red shift has anything to do with the Doppler shifts or with the expansion of space. Halton Arp of the Hale Observatory has emphasized that groups of galaxies can be found in space where some galaxies have quite different red shifts; if these groups are really composed of galaxies that are close to each other, they could hardly move at very different velocities. Furthermore, Maarten Schmidt noticed in 1963 that certain kinds of objects resembling stars had enormously high red shifts, up to more than 300 per cent! If these “quasars” are at the distances that can be deducted from their red shifts, they must radiate an extremely large amount of energy in order to continue being so bright. It is also very difficult to measure the correlation between velocity and distance when the objects are really far away. (Steven Weinberg, Kolme ensimmäistä minuuttia / The Three First Minutes, p. 40)

  95. Bendik Hansen

    Very interesting blog and comments.

    I’ve recently been bothered by the question: How in Hell can everything in the universe expand away from everything else in every direction, and even at an increased rate? In my logic, there would have to be a center where something moved slower, or clashed together.

    The inflation theory worked that out, but that leaves me with another question (it may be be that I take this balloon analogy a bit too biblically): Is he supposing that the universe is only a thin layer, like the rubber of an inflated balloon? (“Thin” like in 13,7 billion LY or something).

    And does he suggest that spacetime is curved like the outside of a globe to fit this inflated universe? Or are there simply “walls” that mark the outside and inside of the balloon which light (or anything else) cannot pass through?
    If the former: The univerzal horizon doesn’t make any sense, because light would be curved to reach us no matter where it came from.
    If the latter: That’d be weird, wouldn’t it? ^^ That would imply that light can only pass through the matter of which the universe consists and that outside that it just…Idunno.

    One last thing: If my first question is answered with a “yes”, then shouldn’t we be able to look further in one direction than the other (i.e. to the outside/inside of the balloon)? We’re not very likely to be placed in the exast middle of those 13 billion LY of which it consist.

    I’ve no experience with cosmology or anything related, so I’m sorry if this just seem like a load of rubbish to you (try to avoid the worst banter if you reply, thanks). I WILL purchase an introductionary book to cosmology, but my list of books to get is long, and I can’t have this on my mind forever ^^

  96. Bendik Hansen

    Correction: is he suggesting that the universe is only a thin layer shaped like a sphere (or something similar), like the rubber of an inflated balloon?

  97. Oz

    The thought we were past the whole universe being flat…

  98. bruce

    pardon my denseness, but given that the radius size of the observable universe at 14gpc, won’t that make the ‘expansion rate’ at 10,388,000 km per sec? about 34x faster than the speed of light?

  99. I think near of “104. bruce”:

    If the universe expand in any radius at light speed: How can accelerate? At what speed expand the diameter (radius x 2)?

    I have write a blog against Universe expansion, with proofs, doubts and hypothesis in http://bigbangno.wordpress.com

    Sorry if my note disturb here.

  100. The Captain

    I don’t understand all I know about astrophysics. Having said that, isn’t it true that when one measures the Doppler shift of the light emanating from a star, the result represents the relative speed at the time the light left the star? My point is that if we are measuring red shift from stars that are very far away with respect to distance, they are also very distant temporally as well. Would it be incorrect to assume (I know) that under the laws of high school physics (just about the limit of my understanding) objects would be travelling faster more or less shortly after the initiating event (Big Bang)? Measuring 13.7 billion year old red shift doesn’t tell us anything about how much the rate of expansion has changed in the eons since.

  101. Bob Smith

    I thought the universe was expanding at an accelerating rate, rather than a linear acceleration? If it is expanding at an accelerating rate, and you assume the rate of increasing acceleration is constant (rather than a constant rate of linear expansion), how long ago was the rate of increasing accleration zero? (which I would argue, would be a better estimate of the age of the universe)

    Simple example: A universe 100 miles in diameter at year x is increasing it’s acceleration at a rate of 10 mph/yr/yr, with a present acceleration of 100 mph/yr/yr, so in 1 year it will be acceleration at a rate of 110mph/yr/yr, and one year ago it was accelerating at a rate of 90 mph/hr/yr, meaning that less than 10 years ago, this presently 100 mile wide universe was at ~10 mile diameter, with an increasing acceleration rate of zero (all too simple an example, but I hope it makes my point if the preview paragraph didn’t)

  102. Les Hendrickson

    Duh! The universe is accelerating because it is surrounded by a vacuum. Get it?

  103. Nick

    So is it possible that the universe has such a large time frame that we are still in the process of the initial acceleration of the big bang?

  104. booboo

    All this expansion stuff is a hoax !! There was no BB !! The red shift does not indicate an expanding universe because between the earth and all the rest there is hydrogen gas which absorbs some of the radiation, hence lowering the frequency !! google for no big bang. Astronimers are only seeking job security and funding for bigger toys. All this knowledge will never benefit humanity for the next zillion years. Ban cosmology now !!!!!

  105. swanand

    the universe is expanding because there may be energy expanding space because after big bang so many other things were formed like galaxies planets etc big bang happened from a single particle and there may be some energy which is leading to expansion of space…..

  106. A few years ago, there was an interesting article that showed how one could observe eclipsing binary stars in other galaxies to determine distance geometrically. It went something like this: 1) watch for orbit, 2) figure out the orbit size, 3) measure max separation angle), 4) do the math for the distance. However, i’ve not heard of any followup. Does anyone use this method? Is there some problem with it?

  107. ajollynerd

    @Tom Marking: Interestingly, if you average each of the methods, then average all of those results, you end up w/ 13.7 Gyr. (not sure if that’s how the 13.7 number was reached originally, but it is interesting nonetheless; at least, I find it interesting)

  108. mannon

    @booboo There is a lot of gas and dust in our galaxy, but not that much between galaxies (with exceptions). So there are several reasons why this would not explain red shift. First of all simply absorbing some radiation would dim the light from distant galaxies, but it would not cause a red shift. We determine the red or blue shift independently of luminosity by looking for the tell tale absorption and emission lines of known elements in the spectra of visible objects. Basically there are patterns and we know where on the spectrum these patterns should be so when they are too to one direction or the other we know that the light has been shifted.

    On top of that even if hydrogen gas caused a red shift nearly all the gas between us and distant galaxies is contained within our own galaxy, with a whole lot of nothing between us and most other galaxies we can observe. Our galaxy is obviously not spherical, but we know the shape of our galaxy and the approximate distribution of gas and dust within it, and the red shift does not obey that shape. Instead more distant galaxies tend to show a higher red shift. This implies that the distance it-self is the important factor and not what part of our galaxy the light came through.

    @telson It’s really not all that mysterious, especially for Andromeda. Just because the Universe is expanding that does not mean that all of the galaxies within it are static like the points on the balloon. That’s merely a thought experiment to help visualize expansion. We know very well that galaxies are not static as in-fact pretty much nothing in the Universe is static. Everything is moving. Everything has some intrinsic velocity and is being nudged and pulled in various directions by gravity. Galaxies clump together and even collide. All of these motions impart their own red and blue shifts. The importance of expansion is that it adds an additional red shift on top of all these other shifts, and that shift grows over distance while all the other shifts do not. That means for relatively close galaxies we observe quite a bit of “noise” in the shifts from all of the galaxies moving through space under their own momentum and the gravitational influences upon them. But at extreme distances those shifts are minuscule, even negligible, compared to the red shift from expansion. In fact beyond certain distances there are no blue shifts at all… anywhere, because expansion red shift drowns them all out. So at cosmic distances the expansion of space dwarfs the movement through space and this is consistent with our observations.

  109. Ghost72

    I had an idea that since the universe is expanding, time would be relational to the point in space relative to another point in space. In other words; if I didn’t move, my location in space in an hour would be completely different than it once was. ( the earth was rotating, the earth was revolving around the sun, the sun was revolving in our solar system, the universe was expanding)

    Then I read this article and thought … oh never-mind.

  110. malectric

    I see a revolution in perception and interpretation of observations coming (from someone much brighter than me). Some things about “observed” expansion rates bother me enormously. For example, I often see it said that such-and-such a galaxy is x LY away from us and that its redshift implies that it is moving away from us at such and such a rate. Here are problems I have with some issues:
    It is all very well to say that some particular galaxy is some particular distance from us but, isn’t it the case that our current perception/observation of its distance is actually not current at all? As in the light we now see was emitted a very long time ago (according to our up-to-date clocks)? So how current are such observations?
    Another problem I can’t fathom is why, when we look back in time (i.e. at increasing “distances”) why does the universe not appear to get hotter as we approach the visible horizon? After all we are told that at some point it was extremely hot. I know about the opacity argument but as we approach that point should we not being seeing a much higher temperature than that in our locality?
    On what grounds can one argue that the expansion rate is increasing? It would seem to me that if this is based on observing objects at ever greater distances that we are kidding ourselves because we are not looking at all at the present situation but at a situation which is receding into the past, the further out we look. To me, the place to look for the actual expansion rate is in our locality as this is the most up-to-date we can get in observing anything.
    I would welcome any enlightenment.

  111. malectric

    Please forgive typos in what I just submitted. They should be obvious.

  112. I looking for a women that use panty hose and open toe sneakers. from dallas tx

NEW ON DISCOVER
OPEN
CITIZEN SCIENCE
ADVERTISEMENT

Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

ADVERTISEMENT

See More

ADVERTISEMENT
Collapse bottom bar
+

Login to your Account

X
E-mail address:
Password:
Remember me
Forgot your password?
No problem. Click here to have it e-mailed to you.

Not Registered Yet?

Register now for FREE. Registration only takes a few minutes to complete. Register now »