Trans-cosmic flow broadens our horizon

By Phil Plait | September 25, 2008 11:08 am

In one of the weirder astronomy press releases I’ve seen in a while, it appears that material literally outside the visible Universe is tugging on material that we can see.

A cluster showing traces of an unseen tug.
Hubble image of the Bullet Cluster, which appears to be getting yanked by material from The Other Side. Click to embiggen.

What does this mean? First off, let’s take a sec and talk about the visible Universe. If you go outside and look around, you don’t see the whole Earth. You only see a small fraction of its surface, because the Earth is a curved ball. The solid planet itself blocks your view. The farthest you can see is out to the horizon, where the curving Earth dips everything else below your view (well, except for tall objects like buildings and ships at sea, but we can ignore them for this analogy).

The Universe is the same way. The fabric of space is expanding, with the cosmos getting bigger every day. This has an odd effect: objects farther away appear to be moving more rapidly away from us. Eventually, an object can be so far away that the space between us is effectively expanding faster than the speed of light! This does’t violate any physical laws, because nothing material is actually moving at transluminal speeds; it’s just that there is more space itself between us and that object all the time.

This effect naturally provides us with a cosmic horizon. Any object "moving away" from us faster than light can’t be seen by us; the photons it emits can’t keep up with the expansion of space. They lose energy and fall away from view (like a slow walker on a fast treadmill… or better yet, an ant walking along a rubber band that is being stretched). So, to us, an object far enough away is invisible, beyond the Universal horizon.

Weird, huh? Yeah, as usual, things get even weirder.

Now imagine a third object, say a cluster of galaxies, that lies between us and the one that is beyond our horizon. To the cluster, the object may still be visible, because it’s closer, and therefore not receding as rapidly. It’s like an island just over the horizon to you as you look seaward from the beach; to you the island is invisible, but to someone a few kilometers out to sea in a yacht the island is still visible.

That cluster can still be affected by the more distant object, pulled by its gravity, for example. To us, farther away, we don’t see that distant object, but to the cluster it’s sitting right there and still, literally, a force with which to be reckoned.

If you want to go away for a moment and take some Tylenol, I understand. I feel a bit headachy myself just writing this.

The thing is, astronomers now think they’ve detected this force! Clusters of galaxies are filled with extremely hot gas, or plasma, heated by things like the galaxies’ motion in the cluster. As light from objects farther away passes through this gas, it gets affected by it, and we can measure that change. This is called the Sunyaev-Zel’dovich effect, and it’s too weak to measure well in individual clusters, but by looking at literally hundreds of clusters, the effect adds up and can be seen. [Edited to add: the photons that are being affected are not from the matter beyond the horizon, but from the cosmic microwave background, the relic radiation from a very early time in the Universe, but still in our visible Universe. Sorry I wasn't clear about that in the original post.]

The total force is fairly big, in fact. Clusters seen in the direction of the constellations Centaurus and Vela appear to have an additional 3 million kilometers per hour added to their usual velocity! That means that some very large clump of matter — probably a cluster of galaxy clusters, called a supercluster — lies in that direction, over the horizon to us but very much visible to clusters we can see.

Imagine! It’s a sobering reminder that the Universe itself is literally bigger than we can see, with the majority of it forever beyond our ken.

And if your quota of weird isn’t yet sated, then ponder this: the expansion is accelerating. That means that objects we can see today, so distant they linger on our current horizon, will eventually fall away from view as the accelerating expansion beats out the velocity of the light they emit. They will literally move beyond the horizon and become invisible. In a sense, it’s as if the visible Universe is shrinking, the horizon getting closer to us every day. The physical Universe is getting bigger, but almost paradoxically what we see of it gets smaller. Someday, billions of years from now, only the closest of objects will remain visible.

Everything else will have sailed below the horizon. So we better take a look around while we still can.

[Incidentally, I cover this topic in more detail in my book Death from the Skies!, coming out in October, but already available for pre-order.]

Comments (143)

Links to this Post

  1. OK. Go. « Communion Of Dreams | September 25, 2008
  1. wright

    Wow… Just… wow.

    Can’t remember who it was that said, “The universe is not only stranger than we imagine, but stranger than we CAN imagine,” but they were onto something.

    Of course the literally unimaginable will remain forever beyond our conceptual grasp, but here is a magnificent example of nature being much, MUCH stranger (and BIGGER) than it appears on the surface. This is a great time to be alive, when such discoveries are being made.

    Even if I do get a headache trying to get this particular theory to fit between my ears.

  2. Andy

    Hmm, can one ask the question: How much of the universe is currently visible?

  3. IVAN3MAN

    Dr. Phil Plait:

    Eventually, an object can be so far away that the space between us is effectively expanding faster than the speed of light! This does’t violate any physical laws, because nothing material is actually moving at transluminal speeds; it’s just that there is more space itself between us and that object all the time.

    Which means that Warp Drive is possible, then?

  4. Non-Elite

    You botched it Mr. Elite …

    The researchers think all the matter in our whole universe is moving at the same speed towards Centaurus and Vela. And it is all moving at the same speed, so it can’t be a simple cluster of galaxies beyond our horizon. It’s BIG news Mr. Elite.

  5. Cheyenne

    And this entire universe started off in a space that was smaller than a proton and expanded from that in the Big Bang? At an energy that is something like 10 to the gabillion jillion uptillion degrees? Cosmology and Astronomy are so freaking interesting- but very complicated!

    Very neat article Phil!

  6. DeiRenDopa

    @wright: J.B.S. Haldane (“My own suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose”) or Eddington (“Not only is the universe stranger than we imagine, it is stranger than we can imagine”).

  7. At first site, I was dazzled by it, and thought that it could have been a 2 d-brane interaction in a Multiverse.
    However, it’s still an incredible discovery. Can’t wait for more data!!!

  8. Trykt

    I don’t quite comprehend how space itself could be moving faster than the speed of light, but not really. Or is it?

    This leads me to a question my high school physics teacher could never answer satisfactorily. If you’re in a car going 50 mph and you pass a car going in the exact opposite direction also going 50mph, the observed effect is the same as if you were stationary and observing a car passing you at 100mph.

    If you change those figures from 50mph to light speed (or near-light speed, or basically anything over half light speed) what is the observed effect? Even though neither object is going faster than the speed of light the observation would be an accumulated value larger than the speed of light, so how is that observed? Can it even be observed?

  9. Todd W.

    Has it ever been observed that something really, really far away that we see one day is just not there the next, with no trace? If so, what were the explanations that have been offered before? Does this change those explanations?

  10. DeiRenDopa

    @Cheyenne: perhaps it’s more accurate to say that the two most powerful physical theories we have today, General Relativity and quantum mechanics, are so horribly inconsistent in the Planck regime that we cannot, reliably, sketch a history of the universe back beyond such a state?

  11. DeiRenDopa

    @Trykt: that ‘rule’ of velocity addition assumes Newtonian physics; in special relativity (SR) velocities do not add that way.

    In the LHC, for example, imagine riding on one of the protons, while a friend rides on another, going the other way round; imagine you do not collide when the two beams you are in come together. As you leave the ‘pass each other’ point, I see each of receding from the other at 0.999 (plus however many more 9s I need) c; you see your friend receding at a somewhat greater speed, but certainly not at 1.99 (etc) c!

    For expansion of the universe, the explanation has to do with General Relativity, not SR.

    There’s an excellent article by Lineweaver and Davis explaining this, as well as more detailed, technical, paper … in a nutshell, it has to do with how you perceive distance, in an expanding universe (or, if you prefer, how you define it).

  12. So…. it’s like a “Great Attractor” on cosmological scale, if true?

  13. But Mr. BA, this doesn’t even make sense to me. If the universe is old enough for cluster A to send gravity waves to cluster B which sends light waves to Earth, it must also be old enough for cluster A to send light waves directly to Earth. It just seems like a case of the triangle inequality. In other words, if we can see the effects of cluster A on cluster B, that implies that we can also see cluster A. What am I missing?

    I also believe you’ve made a mistake here:
    “Any object “moving away” from us faster than light can’t be seen by us; the photons it emits can’t keep up with the expansion of space.”
    My understanding is that even galaxies beyond the Hubble distance (the distance at the rate of expansion is the speed of light) can be visible. This is because the Hubble distance recedes at the speed of light (ignoring the acceleration of expansion).

  14. Bjoern

    @Trykt:
    “I don’t quite comprehend how space itself could be moving faster than the speed of light, but not really.”

    You misunderstood. It is not “space itself” that is moving. Space is expanding, which means that the distances between the objects in it (galaxies, galaxy clusters etc.) increase. And that increase in distance can be larger than the speed of light.

    “If you change those figures from 50mph to light speed (or near-light speed, or basically anything over half light speed) what is the observed effect? Even though neither object is going faster than the speed of light the observation would be an accumulated value larger than the speed of light, so how is that observed? Can it even be observed?”

    Well, that depends. ;-)

    Let’s take two objects (say, space ships) which *as seen by the same observer* move at 60% of light speed in different directions. Then that same observer will see that the distance between the two space ships increases at 120% of light speed. (Note that the observer does *not* see an object which moves with 120% of light speed – he sees only that the *distance* between the two increases with that speed. Hence no rule of Special Relativity is violated here.)

    However, if you are sitting in one of those space ships and looking at the other, you will *not* see that the other ship moves away from you at 120% of light speed. The actual speed you will observe would be about 88% of light speed. (That you are only observing 88% and not 120% is essentially due to time dilation.)

    (if you are interested: for the calculation, you have to use the formula for addition of velocities of Special Relativity. If one object moves with the speed v1 in one direction and the other object with the speed v2 in the other direction, the relative velocity of one with respect to the other will be observed to be: vrel = (v1 + v2)/(1 + v1 v2 / c^2) )

  15. How can the effect that you describe be differentiated from a chunk of “dark matter” that lies beyond the bullet cluster, but if that chunk of “dark matter” was normal, light emitting matter, would be considered part of the visible universe?

    It’s probably obvious that I don’t know what the current thinking of dark matter and dark energy are.

    Dave Regan

  16. Bjoern

    @miller:
    “But Mr. BA, this doesn’t even make sense to me. If the universe is old enough for cluster A to send gravity waves to cluster B which sends light waves to Earth, it must also be old enough for cluster A to send light waves directly to Earth. It just seems like a case of the triangle inequality. In other words, if we can see the effects of cluster A on cluster B, that implies that we can also see cluster A. What am I missing?”

    What you are missing is that “cluster A exerts a gravitational force on cluster B” is not the same as “cluster A sends gravity waves to cluster B”. The gravity is “already there”, it does not need to be “sent” there somehow. Only *changes* in a gravitational field propagate at the speed of light (such changes essentially *are* gravity waves).

  17. @DeiRenDopa: True. The Large Hadron Collider to the rescue. We should find low-mass (relatively) sparticles which will give a marvelous welcome into M Theory (Super String Theory) on to the greatest unification in the history of mankind. Just couple more months! Awesome!!!

  18. Non-Elite, did you read the links I provided, or did you just decide to attack me anonymously as quickly as you could? Specifically, this part:

    “WMAP data released in 2006 support the idea that our universe experienced inflation. Kashlinsky and his team suggest that their clusters are responding to the gravitational attraction of matter that was pushed far beyond the observable universe by inflation. “This measurement may give us a way to explore the state of the cosmos before inflation occurred,” he says.”

  19. Tyler Durden

    “Someday, billions of years from now, only the closest of objects will remain visible. ”

    ——————

    ‘Look, in the sky! The stars! Oh my God.. they’re going *out*.

    Should I be worried that at any moment Daleks may be snatching the Earth from its orbit and moving it to the Medusa Cascade?

  20. @IVAN3MAN: Certainly yes, but we need to confirm Calabi–Yau manifold (or other type) dimensions, which will be tweaked to change compressions in the local space-time. Lets hope we find black holes and/or missing energy from gravitons at LHC.

  21. theinquisitor

    Is there any potential means to determine the actual size of the universe beyond the horizon? Could it actually be infinite?

  22. Non-Elite

    Elitist,

    Did you miss this part?

    “What’s more, this motion is constant out to at least a billion light-years. “Because the dark flow already extends so far, it likely extends across the visible universe,” Kashlinsky says.

    The finding flies in the face of predictions from standard cosmological models, which describe such motions as decreasing at ever greater distances.”

    Did you also miss the links to the actual paper at the bottom of the news release?

    Can you explain how a source of gravity beyond the observable universe can cause all the matter in the whole observable universe (or just a billion LY accross) to move uniformly toward it?

    This pretty much upsets all of cosmology Mr. Elitist.

  23. Mike

    Once again, if Halton Arp is right about redshift, these anomalies can easily be resolved without resorting to outlandish ideas.

  24. justcorbly

    Ok. Suppose we had a Magic Warp Drive that could move us anywhere in the universe instantly.

    Suppose we measured the distance to the very edge of our visible universe, and then moved that distance.

    Having arrived, would we be able to see a distance of X into previously invisible portions of the universe? I.e., there’s no edge, and we just see a portion of the universe whose size is determined by the speed of light and the scale and speed of expanding space?

    Or, is the size of our visible portion constant and the matter in it diminishes as space expands?

  25. When I think about accelerating universal expansion before going to sleep, or while I pretend to watch stupid reality shows with my lady, or while I eat chips and salsa, it almost makes me depressed.

    Anyone else?

  26. Joseph

    Seriously depressed. Once the rate of expansion exceeds the force of gravity, it’s pretty much over. And let’s not talk about the nuclear forces! Which reminds me…

    “Someday, billions of years from now, only the closest of objects will remain visible.”

    Is it really just billions of years? I was under the impression that the universe had at least hundreds of billions (if not trillions!) of years left.

  27. I’m rather confused by Bjoern’s comment.

    I’m not sure what you mean by ‘the force was already there’. The effect of the gravitational force also travels at the speed of light, no? If object A beyond our horizon is affecting object B within our horizon and the light from object B has reached us, then we should also be able to feel the force from object A. But for us to feel the force from an object, we must have been in causal contact with it at some point.

    Is the point therefore that we are seeing effects from pre-inflation when we were in causal contact with objects which are now outside our horizon?

  28. DeiRenDopa

    @Mike: can you please explain how (“if Halton Arp is right about redshift, these anomalies can easily be resolved without resorting to outlandish ideas”)?

    IIRC, I asked you something like this the last time you made such a comment, and you didn’t answer.

  29. @justcorbly: “Or, is the size of our visible portion constant and the matter in it diminishes as space expands?” Yes, being on Earth that is almost true, but the size of visible portion depends on the ability of light to reach us over a specific period of time before space-times expansion gets faster than speed of light.

    @Dave Mosher: It doesn’t make me depressed, actually it makes me feel good to know we have larger and larger universe by the second (about 156 billion light years and counting). And in the future we will be able to go anywhere in the universe, and might have a new way of seeing the universe than with extremely slow electromagnetism (speed compared with the size of our universe).

  30. Bjoern,
    Thanks for the answer! I have two more questions though.

    First, your answer seems to imply that the gravitational fields were already there from the initial moment of expansion. Is that true?

    Second, I quickly looked up the SZ effect, and Wikipedia seems to describe something entirely different than what the BA describes. According to Wikipedia, the SZ effect is when the CMB travels through hot electrons, and becomes distorted in measurable ways. I gather that these electrons are otherwise invisible not because they’re outside of the observable horizon, but because they’re simply not bright enough. How does this relate to the BA’s description?

  31. IVAN3MAN

    Dr. Phil Plait:

    Imagine! It’s a sobering reminder that the Universe itself is literally bigger than we can see, with the majority of it forever beyond our ken.

    Kenny McCormick

  32. Eric H.

    @Miller & DeiRenDopa:

    I am no cosmologist, so you may be able to dismiss my understanding of the matter completely, as there is a good chance that I am completely wrong, but here is how I grasp the idea.

    There are objects A, B and C. the space between A and C is expanding too quickly for light to travel directly between the two. However, if you set up a relay station at object B light could be re-directed from object C to Object A through object B. In theory then object A could see object C if object B was able to reflect all the light between the two. In this case, there is no mirror and nothing to reflect the light so there is no way that we could see the galaxy that is beyond the horizon. The residual effects of gravity are, however, being “reflected” to us via the galaxy that is within our “horizon”.

  33. Eric H.

    Woops, @ DeiRenDopa was supposed to be @Jonathan

  34. justcorbly

    Thanks, Umair.

    Anyone up for a lay explanation of why space is expanding and what, if anything, is making it do that?

  35. Confused

    Assume 2 objects, A and B that start at the same spot (the location of the big bang) and these two objects travel in opposite directions at an arbitrary .6 light-years per year. At year 2 an event occurs at object A causing a ‘beam of light’ to travel away from object A at the speed of light (I think this is obvious, but I’m not a physicist). At the 2 year point, objects A and B are 2.4 light-years apart. The light from object A’s event would (after extrapolating the distances travelled by object B and the light from object A’s event) reach object B at year 8, six years after the event that occurred when the objects were only 2.4 light-years apart. But, at that point object A is 9.6 light-years away from object B.

    Is this, in a simplistic way, what we’re talking about?

    We can’t “see” the objects that are farther than 13.7 billion light-years away … yet? Someday the light from those objects, however faint, will reach earth?

  36. Pieter Kok

    Very cool! And very well-explaind, I might add.

    Todd W: At the horizon even the shortest wavelength is redshifted to zero wavelength. That means that before we can see the “stars go out” they have already red-shifted to unobservable wavelengths.

  37. Pieter Kok

    justcorbly: It is supposed to be dark energy, but nobody really knows what that is.

  38. DeiRenDopa

    @miller: there are two SZ effects, a “thermal” one and a “kinematic” one (there are actually more, but these are the only two we need to consider for now).

    What Wikipedia covers is (I think, I didn’t check, and anyway there is likely to be more than one W webpage on it) the thermal effect; what’s under discussion here is the kinematic SZE, which is what you get if the scattering medium (the hot electrons in the cluster gas) is moving wrt the CMB, in a bulk sense (“Hubble flow”).

    In any case, the hot electrons are indeed ‘visible’; they are what emits the x-rays Kashlinsky et al. analysed (it’s a little more complicated than this, but not too much more).

  39. Uh… the cosmic horizon we are “seeing” is caused by the fact that our universe is so young… If the claim in the original post was correct, we wouldn’t be seeing the cosmic background radiation because it would have already redshifted to infinity. Right? The moment when most distant visible galaxies start to disappear from our local universe is still billions of years in the future.

    Which of course doesn’t mean that there could already be galaxies which are receding superluminal speeds, i.e. they’re will never be visible from the Earth.

  40. Jose

    @Mike
    Please explain why Halton Arp’s ideas don’t qualify as outlandish?

  41. JohnW

    If the radiation from the CMB “becomes distorted in measurable ways” when it passes through hot electrons, how do they measure the distortion? If the radiation is passing through the clusters to reach us, how do we know what it looks like without the distortion?

  42. Bryan

    I’m confused… aren’t objects outside of the Observable Universe causally disconnected from us? I thought the definition of the Observable Universe was that there was a horizon outside of which there hasn’t been enough time in the universe’s history for photons, or anything, to have reached us from there, thus it cannot have ever interacted with us? Or is that changed in the inflationary model, or was my definition outdated when we discovered that dark energy is speeding up the expansion of the universe, or something like that? Because if it isn’t, I’m confused as to how photons can’t have had time to reach us, but can affect something along the way, and then the light of that interaction DOES have time to reach us… You seem to describe a different horizon where objects can be moved outside of the visible universe simply by the expansion of space itself.

  43. Elmar_M

    Oich, my head spinns and spinns.
    I am not on the latest when it comes to cosmology, so the current state of the art says that the universe will expand until it “cools off” and simply ends (gee I remember when I was a kid the “expansion and collapse cycle” was still the state of the art). How certain is this now compared to the expansion collapse theory.
    I actually found the e and c theory less depressing, since at some point the universe would simply undo itself (from what I understand time would go backwards during the imagined collapse phase of a universe) and we would all come back to life and live our life backwards. It was almost beatiful from a philosophical POV. The cool off and die scenario is a little more depressing, at least to me. Well, we will never know, maybe the universe will look completely different tomorrow ;)
    Umair, its funny but I was thinking of colliding Branes too, when I started reading the headline and the first few lines of the article. From what I understand, the existance of Branes would at least partially confirm String Theory, right?
    Or am I mixing this up now? Geez its hard to keep track of all that when your day job is something completely unrelated…

  44. !AstralProjectile

    (Someone has to say it)
    So the Tibetian monks will write down the 9,000,000,000th Name of god just as the Big Rip occurs. Coincidence or casuality?

  45. Bjoern

    @miller and Jonathan:
    “First, your answer seems to imply that the gravitational fields were already there from the initial moment of expansion. Is that true?”

    Yes, that’s essentially true (one only should keep in mind that according to General Relativity, something like a “gravitational field” doesn’t actually exist – in reality, it’s the curvature of spacetime!). I hope this also helps Jonathan somehow?

    WRT to your question about the SZ effect: the description you gave from Wikipedia is right, AFAIK (I am no cosmologist…). I think the BA meant simply that the SZ effect was somehow used to measure the velocities of the galaxies, and in this way it was discovered that a huge force is acting upon it. If you go to the actual press release, you’ll find this explanation (below an image of the CMBR):
    “Hot gas in moving galaxy clusters (white spots) shifts the temperature of cosmic microwaves. Hundreds of distant clusters seem to be moving toward one patch of sky (purple ellipse). ”
    In the text, that’s explained in more detail.

    @Non-Elite:
    I wonder if *you* have read the actual paper which you mentioned? I’ve looked at it, and the conclusions at which they arrive are essentially the ones which the BA outlined above.
    They don’t talk about a supercluster of galaxies directly, right – but they also say the results can be explained if a large amount of matter lies there beyond our horizon.

  46. Elmar_M

    I doubt there will be a Tibet then ;)
    Monks, maybe, but highly unlikely…

  47. Bjoern

    @Pieter Kok and justcorbly:
    It is *not* Dark Energy which is causing the expansion. Dark Energy causes the *acceleration* of the expansion (I hope you see the difference?).

    The actual cause of the expansion was essentially the Big Bang itself – that was the time when everything started to fly away from everything other. ;-) What caused the Big Bang, you ask now? Well, we don’t know… (so far, I hope).

  48. Elmar_M

    Oh yeah Bjoern, that was the other beautiful thing about the expansion and collapse theory: we had an actual idea what caused the big bang!
    Yeah, guess the universe is never as simple as you imagine (or want) it to be (gee that sounded really good, gotta take that down somewhere ;) ).

  49. @Eric H.
    The part you missed was that we are not seeing object B in the present. We are seeing what object B was like in the past, because it takes time for its light to travel to us. In the past, object B’s observable universe was smaller.

    @DeiRenDopa
    I may have missed the details (I was looking at Wikipedia after all), but my main point was that the BA seems to be describing something entirely different. The BA describes the SZ effect as a way to observe things past our horizon, and Wikipedia describes it as a way to observe things that are between us and the CMB. Which is it?

  50. @Bjeorn

    Ah! So we used the kinematic SZ effect to observe movements which are caused by spacetime curvature which was already there at the time of the Big Bang. Thanks!

  51. @justcorbly: It is dark energy, but “how” and “what kind of” dark-energy-functions are present is not known at all. I suspect it has do with d-brane (universe) interactions with energy fluctuations in the Multiverse. So basically our understanding of everything rests with size of the Lord of the Rings (largest colliders). That’s why LHC is so important, it is our road to the future, otherwise we will be in dark from where technological process stops to the level of our understanding of universe.

  52. DeiRenDopa

    @JohnW: there are a couple of very nice explanations on the web, by cosmologists (one from the theory, the other from an observational perspective).

    Unfortunately, by the time my comment with links clears moderation, it’ll be so far from yours that you’ll probably miss it :-(

    One is SZA at Chicago – SZE Primer, the other The Sunyaev-Zel’dovich Effect at a nedwww.ipac.caltech site.

    But very briefly, the undistorted CMB can be found where (rich) clusters (of galaxies) aren’t … those rich clusters have the gravitationally-bound soup of hot electrons and are detected by their x-ray emission.

  53. Brian

    This stuff is why I love this blog.

  54. @ DeiRenDopa: “Unfortunately, by the time my comment with links clears moderation, it’ll be so far from yours that you’ll probably miss it”.

    You can avoid that problem by submitting the link via the “website” box — just like I have done on the “Future of an expanding universe”. :-)

  55. Zippy the Pinhead

    You’re saying the Hubble radius is getting smaller. Is there an estimate how fast that is happening? What if the Hubble radius became as small as a galaxy?

    Could another way of interpreting the expansion of the universe be that the curvature of the universe is changing?

  56. tacitus

    Once again, if Halton Arp is right about redshift, these anomalies can easily be resolved without resorting to outlandish ideas.

    Once again, Halton Arp is stuck in the 1960s and refuses to accept that the field of cosmology has moved and left him and his theories way behind. It happens all the time — there’s no shame in that except to the extent that he’s become too dogmatic to let go of his claims. Deep field surveys from Hubble, Keck, and other great observatories have invalidated his theories for all but those young-Earth creationists who cling to them like a life jacket as they drown in a torrent of new data confirming and reconfirming the basis of Big Bang cosmology.

  57. themadlolscientist

    Waybackinthewayback, Disney(?) made an educational cartoon history flick that included the requisite misconception that everyone thought the earth was flat and Columbus would sail off the edge. It was represented by a choir of monks chanting. The updated version:

    [Gregorian chant]
    Cantor: What is the shape of the universe?
    Choir: Flat.
    Cantor: What happens when you get to the edge?
    Choir: You fall off……
    [/Gregorian chant]

  58. Kullat Nunu, the background radiation is everywhere, permeating the Universe. So it’s all been redshifted, and still inside our horizon. The objects over the horizon are discrete, not everywhere.

    Your question threw me for a second. I have to admit, wrapping your brain around this is decidedly difficult.

  59. So, if space itself is expandind, and distance between objects are growing, doesn’t that mean that even our componing elemental particles are getting further away from each other? In other words, are we expanding?

    If we do expand, then why do we see this space expansion in outer space? Wouldn’t we be “bigger” and thus “larger”, thus relatively approaching any object in space in all 3 directions, compensating the growing distance between us and said object?

    This question leads me to ask myself if space expansion is not the same in different parts of the Universe. For us to see the objects going “away”, our own expansion must be lower than that of the distance between us and those objects.

    Are nuclear/gravitational/electromagnetic forces keeping us from expanding at the same speed?

    If anyone has answers to these questions I’ll be greatly thankfull!

    PS: Good post BTW Phil

  60. Bjoern

    @Elmar_M: I don’t know if the “expansion and collapse-theory” was ever really established (and I don’t understand why you say it explained the cause of the Big Bang – do you mean the preceding Big Crunch?). I learned cosmology in the nineties, and IIRC, at that time, it was far from clear if our universe had enough matter for a re-collapse; it was even fairly certain back then already (and IIRC even decades before) that there is *not* enough matter (although there were already speculations about Dark Matter back then).

    “I am not on the latest when it comes to cosmology, so the current state of the art says that the universe will expand until it “cools off” and simply ends (gee I remember when I was a kid the “expansion and collapse cycle” was still the state of the art). How certain is this now compared to the expansion collapse theory.”

    It is far from certain (but as outlined above, AFAIK, the “e and c theory” was also always far from certain). The fate of the universe depends on how Dark Energy behaves in the future. If its energy density stays constant, we’ll have an accelerated expansion forever, “cooling off” etc. But if its energy density decreases with time, there could be a collapse after all far in the future. We simply don’t know so far (although the evidence so far points more to Dark Energy density staying constant, AFAIK).

    Additionally, there are other ideas like Steinhardt’s cyclic universe, which would lead to something like a new Big Bang even if the expansion goes on forever (if I understood him correctly). etc.

    @Zippy the Pinhead:
    “Could another way of interpreting the expansion of the universe be that the curvature of the universe is changing?”

    If the universe isn’t flat (its curvature is not zero), you are indeed right: expansion of the universe means that its curvature *radius* increases.

  61. JohnW

    Thanks, DeiRenDopa. But I think I’ll just have to chalk this up to “above my pay grade” for now!

  62. Mark Roberts

    “Sunyaev-Zel’dovich effect”

    Stop making things up!
    ;)

  63. Caleb

    Correct me if I’m wrong (no seriously, I’d like someone more knowledgeable than me to answer this), but wouldn’t the horizon of our visible universe act much like an event horizon of a black hole? If so, it’s as if we are surrounded by an event horizon in every direction, like taking the event horizon of a black hole and turning it inside out with us inside of it.

    Now, my understanding is that we can never directly see beyond a redshift of about 1000 (corresponding to about 400,000 years after the Big Bang), since only then did the the universe become transparent to radiation. Now I know that what is explained here is not ‘direct’ observation but more transitive observation, but my assumption was that if we could see behind it, we would just see the universe in its primordial state. This seems to go against that (unless my mind is already too much in a pretzel), since it seems to indicate that what’s beyond redshift 1000 is just more universe (ie: the super galaxy cluster you mention Phil).

    Anyone care to correct, or further enlighten?

  64. See, this is why I’m not quite sold after all these years on the “Dark matter”/”Our universe is missing mass” theory.

    It’s not that it’s not a perfectly viable theory, it’s just that we’re so unsure of what is out beyond the edge of our vision, tugging and pulling with forces either understood by us, or not understood by us yet.

  65. DGKnipfer

    A problem I have with your description Phil is that light traveling from distant objects does not stop even as the object moves away. There’s a Doppler shift yes, but does that shift actually reach a point where light looses all of its momentum relative to us? Even as the expansion accelerates the expansion is limited by relativistic speeds (I presume) so will the expansion reach a point where all light being projected in the direction the universe is expanding from (toward us) be Doppler shifted to a zero speed relative to us? And if so, what’s the Doppler shift on the other side going to look like? Will the expansion of the universe eventually crash into the light speed limit? If so, will that create an expanding sphere of emitted photons on the expanding edge of the universe?

    I’ll take those pain meds now and go back to studying rocks.

  66. Bjoern

    @Nicolas:
    “Are nuclear/gravitational/electromagnetic forces keeping us from expanding at the same speed?”

    Short answer: yes.

    Longer answer: expansion happens only on scales on which the universe looks pretty much homogeneous (the Big Bang theory actually assumes that the universe *is* homogeneous – and all the galaxies, stars, planets etc. are only “small” perturbations…), i. e. on scales of tens of millions of light years. On smaller scales, like “only” some million light years, the universe is anything but homogeneous – and on these scales, there is no expansion. The most prominent example: the Andromeda galaxy, at a distance of about 2.5 million light years, is actually not moving away from us, but towards us.

    @Caleb:
    You confuse things at a greater distance with things in the past. When you talk about objects at a given redshift, you talk not only about things at a certain distance – you automatically also talk about how these things looked in the past! E. g. redshift 1000 does not only correspond to a certain distance, but also to the time 13.7 billion years ago. So, “beyond redshift 1000″ there indeed is not “more universe”, but indeed only the “primordial” (intransparent) state of the universe. But that does not in the least imply that beyond a certain *distance*, there is not “just more universe”.

    Did this help in any way?

  67. Davidlpf

    @DRD, That is one favourite quotes too. Not getting the debate that is going on but the universe is just one whacky place.

  68. Caleb

    @Bjoern
    “Did this help in any way?”

    Yes, but what confuses me was that if beyond redshift 1000 there is only the primordial, why have I heard it explained that what’s acting upon the Bullet Cluster is more galaxies? Wouldn’t what’s acting on the Bullet Cluster (assuming it’s beyond redshift 1000) be the primordial universe since galaxies wouldn’t have formed yet?

  69. @!AstralProjectile: Causality of coincidence conspiracy? LOL.

  70. Elmar_M

    Bjoern, thank you for the reply. Well I was more talking about the eighties actually. My words when I said “state of the art” were badly chosen, please forgive me. Lets just say that some books on this matter were touting the expanding and collapse theory while some others were actually listing both theories as equally likely.
    Yes I was talking about the big crunch. I cant remember anywhere where I got that from (its been 20 years, so please forgive me), but somewhere I got the idea that the big crunch was theorized to cause a big bang again – > repeating cycle.
    Please forgive my uninformed babbling here.
    I guess I am way to far out of the loop now (not that I ever was that deeply into all this, well as 10 or 13 year old boy can be unless he is Albert Einstein, or the BAs son ;) ).
    Of course my remarks about “the beauty” of either theory are to be understood as more philosophic and purely unscientific (and gee it would be foolish to prefer a theory simply because of it being “more beautiful”, that would be something for religion then and not science ;) ).

  71. Even as the expansion accelerates the expansion is limited by relativistic speeds (I presume)

    No, it’s not, actually. The reason why the light from such distant objects can’t reach us is because they are moving away from us faster than the speed of light, so the light can never cross the space between us and those objects. This can happen because it is space itself that is stretching., not the objects moving through space faster than light.

  72. be the primordial universe since galaxies wouldn’t have formed yet?

    I’m not sure about this. The primordial universe may not have had galaxies in the sense we know them, but it did still have lumps of mass. Those lumps have coalesced into smaller objects now, but they exerted influence before that. A given mass has the same gravitation attraction on a distant object regardless of its density.

  73. This article reminds me of a way out idea I had while trying to explain gravity to my 5 year old. I was using the ‘bowling ball, and marble on a mattress’ demonstration to explain the fabric of time.

    Now I know that theorizing from metaphor has multiple inherent fallacies, nonetheless, it led me to ponder the nature of curved space.

    Everyone here has most likely seen the flat sheet, with different massed objects created a warp in the otherwise smooth surface. The basic assumption is that an object at rest, will stay at rest until other objects interfere with its mass.

    My question is, is it possible that the ‘flat’ surface of space is actually curved? To use the sheet metaphor again, what if the entire cosmos originated at the top of a curved surface (only thinking 2 dimensionally). During the initial expansion of space, the force of gravity creating dimples in the curve was far stronger than the angle of the sheet. As the cosmos expanded, and came closer to the edges, the strength of the energy pulling the edge would be greater than the force of gravity causing dimples in its surface.

    In a model like that, dark energy wouldn’t have to be an external force, but just a natural property of mass. As space expands, one piece of matter would have a smaller effect on each other piece of matter.

  74. Caleb

    @Naked Bunny with a Whip

    “The reason why the light from such distant objects can’t reach us is because they are moving away from us faster than the speed of light, so the light can never cross the space between us and those objects. This can happen because it is space itself that is stretching., not the objects moving through space faster than light.”

    Right, I’ve always understood the speed of light as the “speed limit” to which objects cannot be ACCELERATED to or past. It says nothing about when space itself is warped (ie: wormholes, universe expansion).

  75. That’s my understanding, too, Caleb. And from the point of view of a distant galaxy, it’s the place sitting essentially unmoving in space while we are the ones hurtling away. That’s different from if a galaxy were actually moving through space at relativistic speeds, in which case the universe would look very different for people there than it does to us.

  76. So what this suggests then is that the previous assumed nature of a light cone (http://www.fourmilab.ch/documents/gtpp/) is not quite correct. The objects at the “edges” (or on the surface) of the cone are subject to the gravitational effects of objects which may be within their own light cone, but not in ours which, in effect, gives us information about the universe outside of our own light cone.

    Hmmm… warp speed indeed Mr. Phil. Let’s see how many of these observations we can make in the future, each one telling us a little more about the extended universe until we can see right to the edge.

    Can we learn how to use this info before the expansion of the universe closes it off to us for ever?

    Of course we can – we’re human beings, that’s what we do!

  77. llewelly

    This isn’t half so weird as dark matter. In fact – it’s much more of a ‘interesting, should have expected somethig like that’ sort of phenomena.

  78. dave

    wright (#1):

    Can’t remember who it was that said, “The universe is not only stranger than we imagine, but stranger than we CAN imagine,” but they were onto something.

    The late great Arthur C. Clarke.

  79. Re: light cones

    I suppose the equations describing causal relationships get more complex when you start having to factor expanding space into them, just as they got more involved when scientists started having to deal with Lorentz transformations.

  80. justcorbly

    So, OK. What is space expanding into? What is un-space?

    Nobel Prize to the first one who can prove their answer.

  81. @Percy Leaf
    Not quite. Remember, when we look at a far away galaxy cluster, we are not only looking into the distance, but looking into the past. That cluster’s past light cone is a subset of our present light cone. What’s really going on (and I’m basing this on Bjeorn’s previous comments in this thread) is that the clusters are being affected by the gravity of objects outside of their lightcones. This is possible because the gravitational effects do not need to propagate at the speed of light, they were already there at the beginning of the expansion of the universe.

  82. So, billions of years from now, is this how it all ends, we just drop over the horizon, and blink out of existence?

  83. Only if your a ‘Flat Galaxier’ Michael.

    I plan on sailing my ship Rasanante on a voyage around a circular galaxy to find a shorter journey to Betelgeuse to open up a trade route for Pan Galactic Gargleblasters.

  84. “There are objects A, B and C. the space between A and C is expanding too quickly for light to travel directly between the two. However, if you set up a relay station at object B light could be re-directed from object C to Object A through object B. In theory then object A could see object C if object B was able to reflect all the light between the two. In this case, there is no mirror and nothing to reflect the light so there is no way that we could see the galaxy that is beyond the horizon. The residual effects of gravity are, however, being “reflected” to us via the galaxy that is within our “horizon”.”

    How does that NOT violate special relativity. Hasn’t information travelled from A to C at faster than the speed of light since the existence of A is deduced by its effects on B which is observable from C. If some ET civilization on A sent a EM signal to B and B in turn sent an EM signal to C hasn’t information propagated from A to C at faster than the speed of light?

  85. Celtic_Evolution

    Ahhh… this is why I love this site. I’ve learned more in this thread than I ever could have imagined. Along with JohnW, I am reminded just how much of an amateur I am… but the material has been covered well, and explained thoroughly by the likes of DeiRenDopa (who, because of his stellar work in this thread and others recently, is quickly becoming one of my heroes) and others, and I’m following it pretty well so far. And the conversation has been thought provoking and enlightening. Call me a nerd, but I’m just geeked following this thread. Keep it up, BABloggees…

  86. Click my name to go to a fascinating site that explains Relativity very well – and all in words of four letters or less!

  87. amphiox

    Trykt, this is a long thread so it’s possible someone has already said this, but the thought experiment you outlined is in fact one of the exact same ones that Einstein himself used to formulate special relativity in the first place.

    Which, if you think about it, is pretty cool.

    Tom Marking, I’m no expert on relativity but my understanding was that special relativity does not apply to accelerating (or was it curved?) spacetime reference frames, hence the need to expand the theory to general relativity. And this case is exactly that, an accelerating reference frame.

  88. Joseph

    @justcorbly

    Space isn’t expanding “into” anything, as I understand it. There is no edge that you can cross into “unspace.” In fact, if you went in a straight line long enough, you’d end up back where you started! (I think!)

    @ Michael L

    That’s one theory. Eventually the rate of expansion will overcome gravity, tearing apart galaxies, stars, and planets. Soon after, the strong nuclear force will succumb, tearing apart atoms. I believe this theory (the “Big Rip”) gives us about 50 billion years, so start working on way to stop it, just in case.

  89. Naomi

    “Sunyaev-Zel’dovich effect”

    Close relative of the Blinovitch limitation effect? ;)

    Space is so bloody cool XD

  90. Davidlpf

    From wikipedia
    Often quoted for saying, “My own suspicion is that the universe is not only queerer than we suppose, but queerer than we can suppose.”[7] Haldane is sometimes misquoted as saying, “Not only is the universe stranger than we imagine, it is stranger than we can imagine” which should be attributed to Arthur Stanley Eddington.[8]
    http://en.wikipedia.org/wiki/J._B._S._Haldane#Quotes

  91. justcorbly

    Joseph, whether space is curved or not, the question is still there. Once you start talking about space as something that expands, you define it as something that exists apart from something else. So, the question remains: What is space expanding into?

  92. “I’m no expert on relativity but my understanding was that special relativity does not apply to accelerating (or was it curved?) spacetime reference frames, hence the need to expand the theory to general relativity. And this case is exactly that, an accelerating reference frame.”

    So is it the consensus here that an ET outside our observable universe can still communicate with us via the SZ effect? I find that conclusion highly doubtful. I think what is going is more like the following:

    When ET civilization A sent its message outward towards ET civilization B and then onward to C (Homo sapiens civilization) which happens to lie in the same direction, C was within A’s light cone. By the time A’s signal arrived at C the expansion of the universe had proceeded to a point where C is no longer in A’s light cone. So if A chose to send the same message now (whatever that word NOW might mean) it would no longer reach C because C has moved outside of A’s light cone. Even if B mediates the second message can never reach C.

  93. Darth Robo

    Cool! Freaky. But cool!
    :)

  94. @Tacitus: “Once again, Halton Arp is stuck in the 1960s and refuses to accept that the field of cosmology has moved and left him and his theories way behind.”

    Easy there. According to Wikipedia:

    http://en.wikipedia.org/wiki/Halton_Arp

    “His bachelor’s degree was awarded by Harvard (1949), and his Ph.D. from Caltech (1953). Afterwards he became a Fellow of the Carnegie Institution of Washington in 1953, performing research at the Mount Wilson Observatory and Palomar Observatory. Arp became a Research Assistant at Indiana University in 1955 and subsequently in 1957 became a staff member at Palomar Observatory, where he worked for 29 years. In 1983 he joined the staff of the Max Planck Institute for Astrophysics in Germany.”

    Ooo, Ph.D. from prestigious CalTech, Fellow of the prestigious Carnegie Institution, etc., etc. I think he qualifies as one of these “elites” that the BA keeps harping about. Which probably means he must be correct in his theories – after all, he has a Ph.D. from CalTech. Do you? Who are you to question him? (Just playing Devil’s advocate here – it sort of puts the whole “elite” theory down the toilet where it belongs).

    Despite his non-mainstream views about quasars and the Big Bang, he gave us the “Atlas of Peculiar Galaxies” which has definitely been worthwhile.

    BTW, I loved your book “The Annals of Imperial Rome” published c. 110 CE.

  95. Bob

    I started reading this post bright and alert, sitting in my chair at my desk in the correct upright position.

    By the time I finished reading through the comments section, I was curled up in a ball under my desk, sucking my thumb and gibbering softly.

  96. Gavin Polhemus

    Phil,

    From the abstract of the “results and implications” preprint:

    “This flow may be indicative of the tilt exerted across the entire current horizon by far-away pre-inflationary inhomogeneities.”

    This is not the attraction of a far off super-cluster, it is (if the paper is correct) left over from early inflation or before.

    This velocity cannot be caused by super-cluster beyond the horizon. If we are observing some far off cluster (call it A), then we are seeing A’s motion as it was a very long time ago. In order for A’s motion to be caused by something even more distant (call it B), then B must have been within A’s horizon a long time ago. This means light from B had to be able to reach A in time to cause the motion that we see. For use to see the motion light has to be able to get to us. Therefore, light has to be able to get from B to A and continue on to us, so B is also in our horizon too.

    Anything that has an effect which we observe is in our horizon. This includes indirect effects.

    The exception is effects that predate inflation. Inflation pushed stuff out of our horizon, so that stuff could have affected us before it got pushed out.

  97. MarkH

    Ah ha! so it’s not the Earth that is flat, it’s the universe. All of the galaxies are flowing out from the center and drifting towards the “edge” of the universe like leaves flowing down a river. As the galaxies move closer to the “edge” they pick up speed, much like the leaves as they move closer to the edge of a waterfall.

    This HAS to be it…. waahhahahahahaha!!!

    OK I’m better now.

    The REAL universe will never cease to amaze. :)

  98. Elmar_M

    Yeah Bob, it is quite fascinating is it not?! I mean it can make your head spin trying to understand all this and then the consequences of it make your head spin even more!
    When people talk about “the shape” of the universe and the way it expands, I just have to turn off my brain because I am to much of a 3d thinker. The problem is that the universe is not 3d, at least from what I understand, so you can not give it a shape. Well shapes are my dayjob so if I can not give something shape this is quite scary, indeed.

  99. Daniel

    So logically… the universe is a rotating Globe…its not flat! ;) lol I KID!!!

  100. Galvin: Hmmm, I’ll have to reread that part. If the clusters on this side of the horizon were given a velocity earlier on in the age of the Universe, they would still retain that now, even if the matter were no beyond our local horizon. I think that’s so, at least; this gets pretty hairy.

    However, a non-homogenous distribution in the inflationary stage would make sense as well. But I’m not sure that’s very different than the explanation I’m given; it’s just describing it differently.

  101. Chip

    @MarkH – There’s no evidence for a “center” to the universe. Its more like everywhere is the center.

    What is interesting is that a cluster of galaxies completely beyond our view or detection has gravity that effects a closer group of galaxies that we can see. Are we detecting the gravity of the closer group only as it is effected by the further group? Or could the unseen group effect us directly?
    And its all long ago in the past.

  102. Anwyn

    I don’t get this, gravity waves supposedly moves at the speed of light, then what happens if some big object almost in the near edge of the visible universe affect us gravitationally, and the space between us start to expand faster than the speed of light, would we lost the “gravity waves” from them since the speed of expansion between us is faster than light and the gravitationally waves aren’t keeping up with that rate of expansion?

  103. “Anything that has an effect which we observe is in our horizon. This includes indirect effects.”

    Yeah, that’s kind of what I suspected. On the journey of the photon or graviton from A to B to C both B and C have to be within A’s light cone when the photon/graviton starts out. So there is no faster than light communication. It kind of kills the interest in the story for me now – it’s pretty much ho hum.

  104. Gavin Polhemus

    Phil,

    A pre-inflationary inhomogeneity would have been a tiny fluctuation over a microscopic region of space which inflation stretched to be far more vast than the observable universe. This could reveal something about early inflation or even what preceded inflation.

    The supercluster explanation gives the impression that something outside of our horizon can have an effect that we can see. This is wrong. If light cannot get to us from the supercluster, then no other signal of its existence, direct or indirect, can reach us.

    I wish I could draw a Penrose diagram here, but we aren’t really set up for that, are we?

  105. I saw an article about this on I09 and understood it to be much more science fiction-y and mysterious. Thanks Phil, I always appreciate your grounding effect.

  106. Buzz Parsec

    Nicolás – Brooklyn is *not* expanding!

  107. Buzz Parsec

    I think the so-called “over the horizon super cluster A” is actually a super cluster that formed at the site of a pre-inflationary higher-mass fluctuation. In other words, there is a super cluster there “now” (for some value of “now”) but what’s actually affecting the motion of the distant clusters in the Centaurus/Vela direction (point B in the middle) is the residual curvature in space at location B from the pre-inflationary fluctuation, when the universe was extremely small, and super cluster A (or its eventual location) was extremely close to the Centaurus/Vela cluster’s at point B. Now we at point C can see the clusters at point B, but due to inflation, point A is way over our horizon. Now, where did I leave that aspirin bottle?

  108. Pouria

    Ok, I have to ask, not really related to this article, but somewhat still:

    Gravity, the cluster between us and the horizon is being affected by the objects outside of our horizon, but aren’t we aswell affected by that gravitational force, albeit extremly faint?

    Ok, if I refrase and uhm, go on a tangent. One thing I’ve been thinking of lately, and mostly due to not having any real science education other than highschool/college (gymnasiet in sweden) physics etc.
    Lets there is only one object in space, and suddenly another object appears/gets created, with say the same mass, 2 million lightyears away, it would take the light from that object 2 million years to reach the first one (and the first object should be seen instantly at the new object) right?
    But what will happen with the gravitational force, will it take 2 million years for the gravity of the two, to start pulling at eachother, or will they start affecting eachother instantly?
    Would it be the same if both objects were to “appear” at the same time?

    Maybe a better question would be, does gravity “travel”. I don’t think so, but can anyone verify this?

    /P

  109. Pouria

    I see some have mentioned things like Pre-inflationary higher-mass fluctuation… uhm, leymansterms? =) simply but the objects gravitational force still resides at point B (the cluster) and thus affecting it?

    Oh, and btw, if in my post, say the first object (A), was the size of the sun, and the second object (B), that appears later, is huge, many milliontimes that mass, with extreme gravitational force. Would object A feel the thug of object B at the same time that Object B appears to object A?

    Sorry, I just can’t get my head around gravity traveling…

    /P

  110. quasidog

    I love anything to do with the ‘expansion of space’. The whole idea caves my head in but I still love thinking about it. What confuses me the most is how space can be said to expand, so that 2 points are moving away from each other, but that space is said to be nothing also. It’s almost like space itself is something, not an ether, but something similar. It seems to have substance, rather than being nothing at all. I understand about zero point energy and dark energy and space-time etc, but it still amazes me to think about what other properties it may have. What is space ‘really’ made of ?

  111. Jon D

    One thing that has been bugging me since I first read about the limits on the visible universe a couple of years ago is this:

    At what level is the fabric of space expanding? Is it expanding between clusters of galaxies and not between individual galaxies? or between individual galaxies and not between stars?

    The author I first heard about it from (cant remember who it was unfortunately) used the analogy of an inflating balloon with dots drawn on its surface to represent points in space which expand away from eachother as the rubber is stretched. Unfortunately this analogy has a problem, since you would also need to imagine dots floating about in the space inside the balloon and they’re also all expanding away from eachother as the ballon is inflated. This is why I have my question above: What are those dots? galaxy clusters? galaxies? stars? all matter in the universe?

  112. Unspeakably Violent Jack

    The balloon analogy shows the expansion of a 2 dimensional universe, so there wouldn’t be any “inside” (at least as far as the 2D inhabitants were aware) and the surface _is_ space. & so that’s how you could set off in what looked like a straight line & eventually end up where you started.

    I guess that for a 3D universe you’d need a 4D balloon. My brain hurts.

  113. redx

    Jon D: You’re imagining it wrong. There are no dots inside the balloon. That doesn’t work, the expansion has a center in those terms(the middle of the balloon). The dots are all on the surface of the balloon, all expanding away from each other. They are also expanding away from the center of the balloon, but that is in a direction you can’t point to living in the 2 dimensional plane.

    The dots are all the matter in the universe.

  114. Bjoern

    @Caleb:
    I’m a bit late, I hope you’ll read this anyway… ;-)

    “Yes, but what confuses me was that if beyond redshift 1000 there is only the primordial, why have I heard it explained that what’s acting upon the Bullet Cluster is more galaxies? Wouldn’t what’s acting on the Bullet Cluster (assuming it’s beyond redshift 1000) be the primordial universe since galaxies wouldn’t have formed yet?”

    You are still confusing “higher redshift” with “larger distances”. What we *see* beyond the Bullet Cluster is at higher redshift *and* at larger distances. What is *acting* on the Bullet Cluster is *only* at larger distances, but *not* at higher redshift (but actually at the *same* redshift as that Cluster).

    The light from that stuff *as it was at the time at which we see the Bullet Cluster* hasn’t reached us yet. What we *now* see beyond the Bullet Cluster is how that stuff looked at an *earlier* time.

    Got it now?

  115. yy2bggggs

    Gavin:

    “A pre-inflationary inhomogeneity would have been a tiny fluctuation over a microscopic region of space which inflation stretched to be far more vast than the observable universe. This could reveal something about early inflation or even what preceded inflation.”

    From the Cosmology FAQ (link to specific FAQ page):
    http://www.astro.ucla.edu/~wright/infpoint.html

    Answering the question: “How can the Universe be infinite if it was all concentrated into a point at the Big Bang”

    Response: “The Universe was not concentrated into a point at the time of the Big Bang. But the observable Universe was concentrated into a point.”

    There are explanations and diagrams… it’s worth reading the entire response (and broader still, browsing the entire FAQ).

  116. Jon D

    Unspeakably Violent Jack and redx: Thanking you muchly for shedding some light on that. Was wondering why the author hadnt included the ‘inside dots’

    So would the expansion explanation go something like this: the space between everything that isnt being held together strongly enough by gravity or intermolecular forces is expanding between everything else that isnt being held together strongly enough by gravity or intermolecular forces?

    now my head hurts too!

  117. animal

    I’m not sold on the “Anything that has an effect which we observe is in our horizon. This includes indirect effects.”

    Imagine us at A, with B right on the edge of our visible horizon and then C right next to B, for example (—A–B)-C [() = horizon from A, A = us, BC = entities and - is space]

    If you’re thinking C starts to act on B this second, and then adding the time+distance together from C->B->A then we will always be too “late” to observe the changes as per your example.

    Rather if you imagine that C has always acted on B, and we can observe B which then means we must be observing the modified B. We won’t just suddenly be able to observe C as well though, since its too far away. Therefore we must be able to witness an unobservable entity acting on an observable entity.

  118. Jon D

    Bah I didnt think that sentence through properly..
    The space between everything that isnt being held together strongly enough by gravity or intermolecular forces is expanding?

  119. Jon D: Yep, you got it. Right now, the expansion isn’t sufficient to overcome nearby gravitation pulls and atomic forces, in much the same way that the Earth’s gravity isn’t sufficient to overcome the electromagnetic forces that are keeping me from falling through the floor.

    As the expansion accelerates, though, it will eventually be able to overcome other forces, ripping everything in the universe apart. Or so I’ve heard. Perhaps Phil’s latest book covers that scenario, but I’m not sure it fits the theme.

  120. Pouria: Changes in gravitational effects are believed to propagate at the speed of light as waves, just like other forces. To my knowledge, nobody has yet succeeded in measuring gravity waves. It’s a difficult feat because gravity is so weak relative to other forces.

  121. Bjoern

    @Tom Marking:
    “I think he [Arp] qualifies as one of these “elites” that the BA keeps harping about. Which probably means he must be correct in his theories – after all, he has a Ph.D. from CalTech. Do you? Who are you to question him? (Just playing Devil’s advocate here – it sort of puts the whole “elite” theory down the toilet where it belongs).”

    I wouldn’t say so. You only have to consider that what matters is not what *individual* “elites” say. What matters is what the *consensus* of the “elites” is.

  122. Bjoern

    @Elmar_M:
    “Yes I was talking about the big crunch. I cant remember anywhere where I got that from (its been 20 years, so please forgive me), but somewhere I got the idea that the big crunch was theorized to cause a big bang again – > repeating cycle.”

    I remember that it was speculated that following each Big Crunch, there will be a new Big Bang. But AFAIK, no one ever explained how a Big Crunch could *cause* a Big Bang.

    “Please forgive my uninformed babbling here.”

    Don’t underestimate yourself – I’d say that you know more about cosmology than a whole lot of the commenters here…

    “Of course my remarks about “the beauty” of either theory are to be understood as more philosophic and purely unscientific…”

    I understand you – philosophically, I’d also prefer the “e and c theory”. That’s why I like Steinhardt’s cyclic universe idea so much… :-)

  123. madge

    My brain hurts trying to get my head round this. I think it could be a TWO dippy-egg-and-toast-soldiers problem Watson. Fascinating stuff. I LOVE SCIENCE ! :)

  124. Okay, my brain hurts. Maybe it’s the fact that I didn’t get to bed until 2:30 this morning. :-)

    So, light can’t get to us from the “over the horizon” object because the space between us is expanding at 1.5c. But, if light can get from the “beyond the horizon” object to the “intermediary” object (where space is expanding at 0.75c), and light can get from the intermediary object to us (also at an expansion of 0.75c), why can’t light from the “beyond the horizon” object get reflected off of the intermediary object toward us?

    What if radiation from the distant object were to cause the intermediary object to “glow”, creating new photons?

  125. Todd W.

    Going out on a limb here, but if, before the Big Bang, the observable universe was basically a pinpoint, as one commenter noted, then that idea, combined with the ever-nearing (shrinking) visible horizon, perhaps we are headed for another Big Bang, without “space” ever collapsing on itself according to the e and c idea.

    So, the visible range shrinks and shrinks and shrinks until it gets to a point at which it is so small, it explodes into a vast expanse for extremely tiny beings. To put it figuratively, a person becomes a galaxy to the microorganisms inside it.

    Totally off the wall, but interesting thought, I think. :)

  126. I notice a lot of people saying how much they love science even though it apparently causes them pain. Sounds like other relationships I’ve been in and enjoyed immensely.

  127. Celtic_Evolution

    @ NBWAW

    I notice a lot of people saying how much they love science even though it apparently causes them pain. Sounds like other relationships I’ve been in and enjoyed immensely.

    I sort of assumed that already, given your screen name… ;)

  128. Jon D

    Naked Bunny: much obliged! thats been nagging at me for years! love the feeling when stuff comes together

  129. Bryan

    “…and the universe extends, to a place that never ends, which is maybe just inside a little jar…” – Yakko, Animaniacs

  130. andyo

    Just one thing, Phil.

    The physical Universe is getting bigger, but almost paradoxically what we see of it gets smaller. Someday, billions of years from now, only the closest of objects will remain visible.

    Everything else will have sailed below the horizon. So we better take a look around while we still can.

    I hope that by “the closest of objects” you meant “the closest of galaxies”, since I don’t think anything in this galaxy, and probably closer galaxies in the cluster, are going anywhere any time “soon”, since they’re already bound by gravity. “We” will still see everything pretty much the same, the stars at night, etc, unless we look at the sky with powerful telescopes.

  131. Tod

    @ BA: My gods and little fishes. This is a very interesting concept and after nearly a day thinking about, I’m finally seeing the light (pun intended). Your analogy about the island below the horizon is a good one, one that most of us will understand. The larger and more difficult concept is *how* volume is being created between us the observers and the distant (non-visible) ends of the Universe. It’s like bubbles of gas popping into existence when you boil water, except the volume of water increases in this scenario.

    Phil, my hat’s off to you yet again for the lucid explanation of a complex (and interesting) topic.

  132. andyo

    @ Tod,

    I haven’t read thoroughly every comment, but anyway I’m not sure what you mean by volume being created.

    Volume is not being created, it’s just that the light coming from something beyond this “horizon” will never reach us because the space in-between it and us is expanding</i faster than light itself. Note that in this context, nothing is moving through space faster than light even relative to other things, so it does not violate relativity because of this.

    Think about a balloon and you mark some dots on it (which would be galaxies). As you inflate it, all the dots grow apart, each at a faster speed the farther it is from other points.

    I took some time to realize it too. Not even my Cosmology “101″ professor knew how to answer when I asked him if the space expanded so much so rapidly right after the Big Bang, shouldn’t stuff have moved away at far higher speeds than light speed, hence violating Relativity. Well, he was just a philosopher who had all but memorized Hawking’s A Brief History of Time (and not even the newer, updated 1998 edition!). I got my answer from other books and/or articles (a flaw in Hawking’s otherwise great writing is that he doesn’t anticipate much about possible tangential questions like this the reader might have, something that, say, Brian Greene does masterfully – I enjoyed his books much more than Hawking’s).

  133. andyo

    Sorry, I messed up my tags, the second paragraph should have read like this:

    Volume is not being created, it’s just that the light coming from something beyond this “horizon” will never reach us because the space in-between it and us is expanding faster than light itself. Note that in this context, nothing is moving through space faster than light even relative to other things, so it does not violate relativity because of this.

  134. “Therefore we must be able to witness an unobservable entity acting on an observable entity.”

    I think the hangup a lot of people are having is distinguishing current action versus lingering effects left over from previous action. The earliest time that the unobservable entity can affect the observable entity is their separation distance divided by the speed of light. The earliest time that we can see the effects on the observable entity is our separation distance from it divided by the speed of light. It therefore does not make sense that the unobservable entity can propagate effects to us via the observable entity at faster than the speed of light. But it might make sense that earlier in the universe’s history the currently unobservable entity caused effects in the observable entity which remain until now. Therefore the whole “propagating effects across the horizon” meme is fundamentally flawed.

  135. Gavin Polhemus

    I wrote an email to Phil with my concerns about this post. Although I’m not sure how the equations will look here, and I’m not sure if anyone is still reading this thread, I’m going to post it here as well. Even though I think there are some problems with this post, I love the Bad Astronomy Blog. Here it goes…

    1. The horizon is not the place where objects are receding faster than the speed of light. Objects which are receding faster than the speed of light can be seen. Here is an example using the usual balloon analogy for an expanding, positively curved universe. Inflate a ballon so that the radius of curvature is increasing at the speed of light, r = ct. The surface distance between flat-landers living at opposite points on the surface of the ballon is increasing at pi c, greater than the speed of light. However, light will be able to travel all the way around the balloon in a finite time. The angular speed of light is c/r = 1/t so the angle, theta, covered by light between t1 and t2 is

    theta = ln( t2 / t1 )

    Light departing one pole at time t1 will reach the other pole, and angle pi away, at time t2

    t2 = t1 e^pi

    Although the opposite pole is moving away faster that the speed of light, it can be seen at redshift

    z = e^pi – 1.

    So the question isn’t whether the object is receding faster than the speed of light, the question is whether we can see it or not. They are different questions, and answering the question of whether we can see something actually requires some calculus. I wish it was as simple as looking at how fast something is receding, but it’s not.

    2. We cannot see a cluster being pulled by the gravity of a supercluster which is beyond the horizon. Every day we can see farther out into the universe because every day light is arriving from more distant places. Clusters that we can see today would have been beyond the edge of the observable universe billions of years ago. This is, of course, happening for everybody. When we look at that distant cluster, we are seeing the way it looked a long time ago. Back then that distant cluster’s observable universe wasn’t very big. In fact, everything that was in its observable universe back then is in our much larger observable universe now. If a supercluster was close enough to pull on the cluster back then, then the supercluster is close enough for us to see today.

    3. The horizon isn’t getting closer. It is currently getting farther away (as described above) but eventually it will settle at a fixed distance. Here’s the balloon example again, but with exponential growth,

    r = (c / H) e^(Ht)

    The ballon has a radius of curvature equal to the Hubble radius at t=0. In this case the angular speed of light is

    v = H e^(-Ht)

    so the angle that light can cover between t1 and t2 is

    theta = e^(-H t1) – e^(-H t2)

    Consider something that is a distance c / H away at time t1. Its angular distance is

    theta = c / ( H r ) = e^(H t1)

    We can see we will have to wait for t2 -> infinity to see something at this distance. Anything farther away we will never see. In this case the horizon is exactly where things are moving away at the speed of light, but this only works because H is constant in this case.

    The current horizon is much closer than c/H, but it is getting bigger. It will gradually approach the value c/H in the distant future.

  136. IVAN3MAN

    Gavin Polhemus:

    [...] and I’m not sure if anyone is still reading this thread, [...]

    I am. :-)

  137. Res

    Map the visible flow (speed, direction, whatever), and you will have a rudimentary map of mass beyond the visible universe.

  138. Bjoern

    @Gavin Polhemus:
    Nice calculations. ;-)

    But I don’t understand entirely what you wrote in your point 2, especially this:
    “In fact, everything that was in its observable universe back then is in our much larger observable universe now.”

    How did you arrive at that conclusion?

  139. Gavin Polhemus

    A cluster is in our observable universe today if we can see it today, i.e. light can get from the cluster (at some time in the past) to us today. Likewise, the alleged supercluster was in the cluster’s observable universe at that past time if light could get from the supercluster to the cluster by that time. So if light can get to the cluster by that time, and get from the cluster to us by today, then clearly light can get from the supercluster to us today. So the supercluster is in our observable universe today if it was in the cluster’s observable universe back then.

    I use light, but gravity is the same. The gravity is not “already there.” The gravitational fields (or better the space-time metric) does not reveal anything about what is beyond the horizon. The universe could have been created with all sorts of clumps and voids, as well all sorts of bends and twists in the space-time (creating a crazy mess of initial gravitational fields) but it would just be noise until there there has been time for the field to bring us information about other masses out there. This observation can be views as evidence for some of this noise on wavelengths larger than our observable universe, suggesting that the noise is left over from early inflation or from before inflation. It does not signal the presence of a supercluster over the horizon.

  140. Bjoern

    @Gavin Polhemus:
    Thanks, much clearer now! :-)

  141. Thomas Mackiewicz

    …counting the seconds before the religious fanatics suppose that the massive force is god.

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 »