Dark Energy FAQ

By Sean Carroll | October 4, 2011 3:19 pm

In honor of the Nobel Prize, here are some questions that are frequently asked about dark energy, or should be.

What is dark energy?

It’s what makes the universe accelerate, if indeed there is a “thing” that does that. (See below.)

So I guess I should be asking… what does it mean to say the universe is “accelerating”?

First, the universe is expanding: as shown by Hubble, distant galaxies are moving away from us with velocities that are roughly proportional to their distance. “Acceleration” means that if you measure the velocity of one such galaxy, and come back a billion years later and measure it again, the recession velocity will be larger. Galaxies are moving away from us at an accelerating rate.

But that’s so down-to-Earth and concrete. Isn’t there a more abstract and scientific-sounding way of putting it?

The relative distance between far-flung galaxies can be summed up in a single quantity called the “scale factor,” often written a(t) or R(t). The scale factor is basically the “size” of the universe, although it’s not really the size because the universe might be infinitely big — more accurately, it’s the relative size of space from moment to moment. The expansion of the universe is the fact that the scale factor is increasing with time. The acceleration of the universe is the fact that it’s increasing at an increasing rate — the second derivative is positive, in calculus-speak.

Does that mean the Hubble constant, which measures the expansion rate, is increasing?

No. The Hubble “constant” (or Hubble “parameter,” if you want to acknowledge that it changes with time) characterizes the expansion rate, but it’s not simply the derivative of the scale factor: it’s the derivative divided by the scale factor itself. Why? Because then it’s a physically measurable quantity, not something we can change by switching conventions. The Hubble constant is basically the answer to the question “how quickly does the scale factor of the universe expand by some multiplicative factor?”

If the universe is decelerating, the Hubble constant is decreasing. If the Hubble constant is increasing, the universe is accelerating. But there’s an intermediate regime in which the universe is accelerating but the Hubble constant is decreasing — and that’s exactly where we think we are. The velocity of individual galaxies is increasing, but it takes longer and longer for the universe to double in size.

Said yet another way: Hubble’s Law relates the velocity v of a galaxy to its distance d via v = H d. The velocity can increase even if the Hubble parameter is decreasing, as long as it’s decreasing more slowly than the distance is increasing.

Did the astronomers really wait a billion years and measure the velocity of galaxies again?

No. You measure the velocity of galaxies that are very far away. Because light travels at a fixed speed (one light year per year), you are looking into the past. Reconstructing the history of how the velocities were different in the past reveals that the universe is accelerating.

How do you measure the distance to galaxies so far away?

It’s not easy. The most robust method is to use a “standard candle” — some object that is bright enough to see from great distance, and whose intrinsic brightness is known ahead of time. Then you can figure out the distance simply by measuring how bright it actually looks: dimmer = further away.

Sadly, there are no standard candles.

Then what did they do?

Fortunately we have the next best thing: standardizable candles. A specific type of supernova, Type Ia, are very bright and approximately-but-not-quite the same brightness. Happily, in the 1990’s Mark Phillips discovered a remarkable relationship between intrinsic brightness and the length of time it takes for a supernova to decline after reaching peak brightness. Therefore, if we measure the brightness as it declines over time, we can correct for this difference, constructing a universal measure of brightness that can be used to determine distances.

Why are Type Ia supernovae standardizable candles?

We’re not completely sure — mostly it’s an empirical relationship. But we have a good idea: we think that SNIa are white dwarf stars that have been accreting matter from outside until they hit the Chandrasekhar Limit and explode. Since that limit is basically the same number everywhere in the universe, it’s not completely surprising that the supernovae have similar brightnesses. The deviations are presumably due to differences in composition.

But how do you know when a supernova is going to happen?

You don’t. They are rare, maybe once per century in a typical galaxy. So what you do is look at many, many galaxies with wide-field cameras. In particular you compare an image of the sky taken at one moment to another taken a few weeks later — “a few weeks” being roughly the time between new Moons (when the sky is darkest), and coincidentally about the time it takes a supernova to flare up in brightness. Then you use computers to compare the images and look for new bright spots. Then you go back and examine those bright spots closely to try to check whether they are indeed Type Ia supernovae. Obviously this is very hard and wouldn’t even be conceivable if it weren’t for a number of relatively recent technological advances — CCD cameras as well as giant telescopes. These days we can go out and be confident that we’ll harvest supernovae by the dozens — but when Perlmutter and his group started out, that was very far from obvious.

And what did they find when they did this?

Most (almost all) astronomers expected them to find that the universe was decelerating — galaxies pull on each other with their gravitational fields, which should slow the whole thing down. (Actually many astronomers just thought they would fail completely, but that’s another story.) But what they actually found was that the distant supernovae were dimmer than expected — a sign that they are farther away than we predicted, which means the universe has been accelerating.

Why did cosmologists accept this result so quickly?

Even before the 1998 announcements, it was clear that something funny was going on with the universe. There seemed to be evidence that the age of the universe was younger than the age of its oldest stars. There wasn’t as much total matter as theorists predicted. And there was less structure on large scales than people expected. The discovery of dark energy solved all of these problems at once. It made everything snap into place. So people were still rightfully cautious, but once this one startling observation was made, the universe suddenly made a lot more sense.

How do we know the supernovae not dimmer because something is obscuring them, or just because things were different in the far past?

That’s the right question to ask, and one reason the two supernova teams worked so hard on their analysis. You can never be 100% sure, but you can gain more and more confidence. For example, astronomers have long known that obscuring material tends to scatter blue light more easily than red, leading to “reddening” of stars that sit behind clouds of gas and dust. You can look for reddening, and in the case of these supernovae it doesn’t appear to be important. More crucially, by now we have a lot of independent lines of evidence that reach the same conclusion, so it looks like the original supernova results were solid.

There’s really independent evidence for dark energy?

Oh yes. One simple argument is “subtraction”: the cosmic microwave background measures the total amount of energy (including matter) in the universe. Local measures of galaxies and clusters measure the total amount of matter. The latter turns out to be about 27% of the former, leaving 73% or so in the form of some invisible stuff that is not matter: “dark energy.” That’s the right amount to explain the acceleration of the universe. Other lines of evidence come from baryon acoustic oscillations (ripples in large-scale structure whose size helps measure the expansion history of the universe) and the evolution of structure as the universe expands.

Okay, so: what is dark energy?

Glad you asked! Dark energy has three crucial properties. First, it’s dark: we don’t see it, and as far as we can observe it doesn’t interact with matter at all. (Maybe it does, but beneath our ability to currently detect.) Second, it’s smoothly distributed: it doesn’t fall into galaxies and clusters, or we would have found it by studying the dynamics of those objects. Third, it’s persistent: the density of dark energy (amount of energy per cubic light-year) remains approximately constant as the universe expands. It doesn’t dilute away like matter does.

These last two properties (smooth and persistent) are why we call it “energy” rather than “matter.” Dark energy doesn’t seem to act like particles, which have local dynamics and dilute away as the universe expands. Dark energy is something else.

That’s a nice general story. What might dark energy specifically be?

The leading candidate is the simplest one: “vacuum energy,” or the “cosmological constant.” Since we know that dark energy is pretty smooth and fairly persistent, the first guess is that it’s perfectly smooth and exactly persistent. That’s vacuum energy: a fixed amount of energy attached to every tiny region of space, unchanging from place to place or time to time. About one hundred-millionth of an erg per cubic centimeter, if you want to know the numbers.

Is vacuum energy really the same as the cosmological constant?

Yes. Don’t believe claims to the contrary. When Einstein first invented the idea, he didn’t think of it as “energy,” he thought of it as a modification of the way spacetime curvature interacted with energy. But it turns out to be precisely the same thing. (If someone doesn’t want to believe this, ask them how they would observationally distinguish the two.)

Doesn’t vacuum energy come from quantum fluctuations?

Not exactly. There are many different things that can contribute to the energy of empty space, and some of them are completely classical (nothing to do with quantum fluctuations). But in addition to whatever classical contribution the vacuum energy has, there are also quantum fluctuations on top of that. These fluctuation are very large, and that leads to the cosmological constant problem.

What is the cosmological constant problem?

If all we knew was classical mechanics, the cosmological constant would just be a number — there’s no reason for it to be big or small, positive or negative. We would just measure it and be done.

But the world isn’t classical, it’s quantum. In quantum field theory we expect that classical quantities receive “quantum corrections.” In the case of the vacuum energy, these corrections come in the form of the energy of virtual particles fluctuating in the vacuum of empty space.

We can add up the amount of energy we expect in these vacuum fluctuations, and the answer is: an infinite amount. That’s obviously wrong, but we suspect that we’re overcounting. In particular, that rough calculation includes fluctuations at all sizes, including wavelengths smaller than the Planck distance at which spacetime probably loses its conceptual validity. If instead we only include wavelengths that are at the Planck length or longer, we get a specific estimate for the value of the cosmological constant.

The answer is: 10120 times what we actually observe. That discrepancy is the cosmological constant problem.

Why is the cosmological constant so small?

Nobody knows. Before the supernovae came along, many physicists assumed there was some secret symmetry or dynamical mechanism that set the cosmological constant to precisely zero, since we certainly knew it was much smaller than our estimates would indicate. Now we are faced with both explaining why it’s small, and why it’s not quite zero. And for good measure: the coincidence problem, which is why the dark energy density is the same order of magnitude as the matter density.

Here’s how bad things are: right now, the best theoretical explanation for the value of the cosmological constant is the anthropic principle. If we live in a multiverse, where different regions have very different values of the vacuum energy, one can plausibly argue that life can only exist (to make observations and win Nobel Prizes) in regions where the vacuum energy is much smaller than the estimate. If it were larger and positive, galaxies (and even atoms) would be ripped apart; if it were larger and negative, the universe would quickly recollapse. Indeed, we can roughly estimate what typical observers should measure in such a situation; the answer is pretty close to the observed value. Steven Weinberg actually made this prediction in 1988, long before the acceleration of the universe was discovered. He didn’t push it too hard, though; more like “if this is how things work out, this is what we should expect to see…” There are many problems with this calculation, especially when you start talking about “typical observers,” even if you’re willing to believe there might be a multiverse. (I’m very happy to contemplate the multiverse, but much more skeptical that we can currently make a reasonable prediction for observable quantities within that framework.)

What we would really like is a simple formula that predicts the cosmological constant once and for all as a function of other measured constants of nature. We don’t have that yet, but we’re trying. Proposed scenarios make use of quantum gravity, extra dimensions, wormholes, supersymmetry, nonlocality, and other interesting but speculative ideas. Nothing has really caught on as yet.

Has the course of progress in string theory ever been affected by an experimental result?

Yes: the acceleration of the universe. Previously, string theorists (like everyone else) assumed that the right thing to do was to explain a universe with zero vacuum energy. Once there was a real chance that the vacuum energy is not zero, they asked whether that was easy to accommodate within string theory. The answer is: it’s not that hard. The problem is that if you can find one solution, you can find an absurdly large number of solutions. That’s the string theory landscape, which seems to kill the hopes for one unique solution that would explain the real world. That would have been nice, but science has to take what nature has to offer.

What’s the coincidence problem?

Matter dilutes away as the universe expands, while the dark energy density remains more or less constant. Therefore, the relative density of dark energy and matter changes considerably over time. In the past, there was a lot more matter (and radiation); in the future, dark energy will completely dominate. But today, they are approximately equal, by cosmological standards. (When two numbers could differ by a factor of 10100 or much more, a factor of three or so counts as “equal.”) Why are we so lucky to be born at a time when dark energy is large enough to be discoverable, but small enough that it’s a Nobel-worthy effort to do so? Either this is just a coincidence (which might be true), or there is something special about the epoch in which we live. That’s one of the reasons people are willing to take anthropic arguments seriously. We’re talking about a preposterous universe here.

If the dark energy has a constant density, but space expands, doesn’t that mean energy isn’t conserved?

Yes. That’s fine.

What’s the difference between “dark energy” and “vacuum energy”?

“Dark energy” is the general phenomenon of smooth, persistent stuff that makes the universe accelerate; “vacuum energy” is a specific candidate for dark energy, namely one that is absolutely smooth and utterly constant.

So there are other candidates for dark energy?

Yes. All you need is something that is pretty darn smooth and persistent. It turns out that most things like to dilute away, so finding persistent energy sources isn’t that easy. The simplest and best idea is quintessence, which is just a scalar field that fills the universe and changes very slowly as time passes.

Is the quintessence idea very natural?

Not really. An original hope was that, by considering something dynamical and changing rather than a plain fixed constant energy, you could come up with some clever explanation for why the dark energy was so small, and maybe even explain the coincidence problem. Neither of those hopes has really panned out.

Instead, you’ve added new problems. According to quantum field theory, scalar fields like to be heavy; but to be quintessence, a scalar field would have to be enormously light, maybe 10-30 times the mass of the lightest neutrino. (But not zero!) That’s one new problem you’ve introduced, and another is that a light scalar field should interact with ordinary matter. Even if that interaction is pretty feeble, it should still be large enough to detect — and it hasn’t been detected. Of course, that’s an opportunity as well as a problem — maybe better experiments will actually find a “quintessence force,” and we’ll understand dark energy once and for all.

How else can we test the quintessence idea?

The most direct way is to do the supernova thing again, but do it better. More generally: map the expansion of the universe so precisely that we can tell whether the density of dark energy is changing with time. This is generally cast as an attempt to measure the dark energy equation-of-state parameter w. If w is exactly minus one, the dark energy is exactly constant — vacuum energy. If w is slightly greater than -1, the energy density is gradually declining; if it’s slightly less (e.g. -1.1), the dark energy density is actually growing with time. That’s dangerous for all sorts of theoretical reasons, but we should keep our eyes peeled.

What is w?

It’s called the “equation-of-state parameter” because it relates the pressure p of dark energy to its energy density ρ, via w = p/ρ. Of course nobody measures the pressure of dark energy, so it’s a slightly silly definition, but it’s an accident of history. What really matters is how the dark energy evolves with time, but in general relativity that’s directly related to the equation-of-state parameter.

Does that mean that dark energy has negative pressure?

Yes indeed. Negative pressure is what happens when a substance pulls rather than pushes — like an over-extended spring that pulls on either end. It’s often called “tension.” This is why I advocated smooth tension as a better name than “dark energy,” but I came in too late.

Why does dark energy make the universe accelerate?

Because it’s persistent. Einstein says that energy causes spacetime to curve. In the case of the universe, that curvature comes in two forms: the curvature of space itself (as opposed to spacetime), and the expansion of the universe. We’ve measured the curvature of space, and it’s essentially zero. So the persistent energy leads to a persistent expansion rate. In particular, the Hubble parameter is close to constant, and if you remember Hubble’s Law from way up top (v = H d) you’ll realize that if H is approximately constant, v will be increasing because the distance is increasing. Thus: acceleration.

Is negative pressure is like tension, why doesn’t it pull things together rather than pushing them apart?

Sometimes you will hear something along the lines of “dark energy makes the universe accelerate because it has negative pressure.” This is strictly speaking true, but a bit ass-backwards; it gives the illusion of understanding rather than actual understanding. You are told “the force of gravity depends on the density plus three times the pressure, so if the pressure is equal and opposite to the density, gravity is repulsive.” Seems sensible, except that nobody will explain to you why gravity depends on the density plus three times the pressure. And it’s not really the “force of gravity” that depends on that; it’s the local expansion of space.

The “why doesn’t tension pull things together?” question is a perfectly valid one. The answer is: because dark energy doesn’t actually push or pull on anything. It doesn’t interact directly with ordinary matter, for one thing; for another, it’s equally distributed through space, so any pulling it did from one direction would be exactly balanced by an opposite pull from the other. It’s the indirect effect of dark energy, through gravity rather than through direct interaction, that makes the universe accelerate.

The real reason dark energy causes the universe to accelerate is because it’s persistent.

Is dark energy like antigravity?

No. Dark energy is not “antigravity,” it’s just gravity. Imagine a world with zero dark energy, except for two blobs full of dark energy. Those two blobs will not repel each other, they will attract. But inside those blobs, the dark energy will push space to expand. That’s just the miracle of non-Euclidean geometry.

Is it a new repulsive force?

No. It’s just a new (or at least different) kind of source for an old force — gravity. No new forces of nature are involved.

What’s the difference between dark energy and dark matter?

Completely different. Dark matter is some kind of particle, just one we haven’t discovered yet. We know it’s there because we’ve observed its gravitational influence in a variety of settings (galaxies, clusters, large-scale structure, microwave background radiation). It’s about 23% of the universe. But it’s basically good old-fashioned “matter,” just matter that we can’t directly detect (yet). It clusters under the influence of gravity, and dilutes away as the universe expands. Dark energy, meanwhile, doesn’t cluster, nor does it dilute away. It’s not made of particles, it’s some different kind of thing entirely.

Is it possible that there is no dark energy, just a modification of gravity on cosmological scales?

It’s possible, sure. There are at least two popular approaches to this idea: f(R) gravity , which Mark and I helped develop, and DGP gravity, by Dvali, Gabadadze, and Porati. The former is a directly phenomenological approach where you simply change the Einstein field equation by messing with the action in four dimensions, while the latter uses extra dimensions that only become visible at large distances. Both models face problems — not necessarily insurmountable, but serious — with new degrees of freedom and attendant instabilities.

Modified gravity is certainly worth taking seriously (but I would say that). Still, like quintessence, it raises more problems than it solves, at least at the moment. My personal likelihoods: cosmological constant = 0.9, dynamical dark energy = 0.09, modified gravity = 0.01. Feel free to disagree.

What does dark energy imply about the future of the universe?

That depends on what the dark energy is. If it’s a true cosmological constant that lasts forever, the universe will continue to expand, cool off, and empty out. Eventually there will be nothing left but essentially empty space.

The cosmological constant could be constant at the moment, but temporary; that is, there could be a future phase transition in which the vacuum energy decreases. Then the universe could conceivably recollapse.

If the dark energy is dynamical, any possibility is still open. If it’s dynamical and increasing (w less than -1 and staying that way), we could even get a Big Rip.

What’s next?

We would love to understand dark energy (or modified gravity) through better cosmological observations. That means measuring the equation-of-state parameter, as well as improving observations of gravity in galaxies and clusters to compare with different models. Fortunately, while the U.S. is gradually retreating from ambitious new science projects, the European Space Agency is moving forward with a satellite to measure dark energy. There are a number of ongoing ground-based efforts, of course, and the Large Synoptic Survey Telescope should do a great job once it goes online.

But the answer might be boring — the dark energy is just a simple cosmological constant. That’s just one number; what are you going to do about it? In that case we need better theories, obviously, but also input from less direct empirical sources — particle accelerators, fifth-force searches, tests of gravity, anything that would give some insight into how spacetime and quantum field theory fit together at a basic level.

The great thing about science is that the answers aren’t in the back of the book; we have to solve the problems ourselves. This is a big one.

CATEGORIZED UNDER: Science, Top Posts
  • Douglas Watts

    “… but science has to take what nature has to offer.”

    Well played, Sean !!!

  • http://sites.google.com/site/pietrhitzig/ Pietr Hitzig

    Is it unreasonable to think that these fine scientists were chosen this year to emphasize the importance of the James Webb Space telescope? I hope so.

  • http://twitter.com/#!/msyriac Mathew

    Not to nitpick, but is the answer to Q2 completely accurate?

    Quote:
    ““Acceleration” means that if you measure the velocity of one such galaxy, and come back a billion years later and measure it again, the recession velocity will be larger.”

    Even if the expansion isn’t accelerating, if you measure the velocity of a galaxy and come back a billion years later and measure the velocity of the *same* galaxy (which is what’s implied by the sentence), the recession velocity will be larger, simply because of Hubble’s law v=Hd, since now it’s farther away.

    Didn’t you mean that if you come back a billion years later and measure the velocity of another galaxy at the same *distance* as the original one was at a billion years earlier, you’ll get a larger recession velocity?

  • http://sithacademy.com Sith Master Sean

    Thank you for this informative FAQ. To summarize:

    “Dark Energy is what gives a Sith Master his power. It’s an energy field created by the universal void. It surrounds us and penetrates us. It drives the galaxies apart.”

  • Eluisa

    Atlantis was said to have used dark energy instead of energy from shells as we use. It was said that the use of dark energy can be the answer to clean energy if only humanity finds it and learns how to use it properly. What do you have to say about it?

  • http://olsenjaynelson.com olsenjaynelson

    Thanks for the post. I’m not a physics aficionado, but I have a basic interest in astrophysics etc. What I’ve noticed about physicists on TV documentaries etc, is that when trying to communicate to the idiots like me, their language gets too simple, even simplistic. One thing that annoys me is that they quickly stop talking about their hypotheses in hypothesis-related terminology; instead, they start using simple structures that imply the ontological existence of their hypotheses, this annoys me particularly with their more extreme and speculative hypotheses. The problem is they keep doing this while discussing it, and the lay person can forget that they’re talking about hypotheses and end up treating these things as definite entities. You did a good job at avoiding that.
    Moreover, in researching this a bit more lately, I’m also now convinced by the reasoning and the evidence associated with dark energy and dark matter etc … if it matters whether I am or not. What I mean is that we accept gravity even though we can’t see it etc, so why not this when there’s such powerful and mounting indirect evidence for it? Of course, we should. I know they’re hypotheses, but they’re strong ones. Thanks …

  • Chris

    Great post, but one question I didn’t see addressed. Is there a numerical value for the acceleration of the universe?

  • http://sidudoexisto.blogspot.com Jorge Laris

    Wow. Thank you for all the post.

  • AJKamper

    TERRIFIC post. Answers a number of questions I had, and I’m relatively versed in this stuff for a total layperson (I mean, I’m a lawyer, for crying out loud).

    Mathew, #3:

    Not that I’m an expert, but I don’t think that’s right, is it? Dark energy aside, galaxies aren’t receding faster because they’re further away: rather, galaxies that are further away from us are receding faster than closer galaxies. Isn’t the ratio not absolute but relative?

    By that logic, of course, the Hubble constant is only correct for this point in time–it’s an empirical value, and would gradually grow smaller as galaxies got further away from us without accelerating. (Again, dark energy aside.)

    Or am I missing something here?

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  • Dan

    It’s “not that hard” to get de Sitter vacua in string theory?? :P

  • LTD.Edition

    No Matthew, a specific object having a change in velocity over a period of time is called acceleration, in this case a galaxy. It’s part of how hubbles law was created, but not because of it, nothing is because of a scientific law, but the scientific law is because that is how nature works.

  • Chris Wedgwood

    As I understand it, spacetime is one symmetric 4-dimensional thing, and translations are possible between each of the axes.

    So what is this odd thing of inflation that can expand 3 of the axes, yet leave the fourth one untouched Sean?

  • http://twitter.com/#!/msyriac Mathew

    Right, my bad. I forgot to account for the fact that H changes as the galaxy moves away and so the net effect is that v changes only if the deceleration parameter q is non-zero (which is how an accelerating/decelerating universe is usually defined).

  • http://heldervelez1.tiddlyspace.com/ heldervelez

    Matter is shrinking and we can not detect this fact locally. The measures are made with a shrinking ruler and so we measure a space expansion. The model “A self-similar model of the Universe unveils the nature of dark energy” ( http://vixra.org/pdf/1107.0016v1.pdf ) formally proved that there is no need of DE. The observed universe evolution is modeled only with the Hubble parameter. Why is matter shrinking? Because the electrostatic and gravity field spread away from particles at c speed filling the universe at expenses of the particle contents. There are no free lunches.

  • Jason

    Is it possible that the expansion is not accelerating, but that local space is in an accelerating collapse towards our galaxies black hole?

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  • Dave

    Sean, your answer to
    “Is negative pressure is like tension, why doesn’t it pull things together rather than pushing them apart?”
    isn’t quite right.

    The source for Einstein gravity is the energy density plus 3 times the pressure because the source term is the stress-energy tensor. This is why a relativistic gas exerts a stronger gravitational force than a non-relativistic gas with the same energy. The higher pressure “pushes” on things, but the gravitational force “pulls” more than an equivalent mass of non-relativistic gas.

    The gravitational repulsion effect of this negative pressure can be seen quite clearly if we consider a static cosmic string, which has negative pressure in only 1 direction. For the cosmic string, there is no gravitational force because 1-d negative pressure cancels with the energy density. For a 2-d domain wall, the 2-d negative pressure dominates, and there is a repulsive gravitational force. Of course, in Einstein gravity, you can always interpret the gravitational forces as an effect on space-time, but in the Newtonian interpretation, negative pressure implies repulsive gravity.

    In the 3-d case, the positive vacuum energy (of the false vacuum) can terminate on a domain wall, which is what happens in the original inflation model. The fate of bubbles of true vacuum that appear in the false vacuum depends on the competition between the negative pressure of the false vacuum, which accelerates the bubble walls to nearly the speed of light and the gravitational effect of the negative pressure, which tires to accelerate the volume of space between the bubbles.

    So, I would say that you don’t quite understand the source term in Einstein’s theory of gravity if you don’t understand that pressure produces gravity that acts to in the opposite sense of the physical pressure force.

  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    “By that logic, of course, the Hubble constant is only correct for this point in time–it’s an empirical value, and would gradually grow smaller as galaxies got further away from us without accelerating. (Again, dark energy aside.)”

    Right. In general, the Hubble constant, the density parameter Omega and the normalised cosmological constant lambda (lowercase) change with time. Omega is essentially the density divided by (the square of) the Hubble constant, so it changes because a) the density drops with the expansion and b) the Hubble constant can change with time. Lowercase lambda is essentially Lambda divided by (the square of) the Hubble constant, so it changes only if the Hubble constant changes with time. The Hubble constant is a constant relating distance and velocity, like in y = ax+b, where a and b are constants and x and y are variables. The cosmological constant is actually constant in time (at least in the traditional sense; theoretically, it could perhaps vary, and it is important to look for this, but as of now there is no evidence for such a time-variable Lambda.)

  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    Bring back smooth tension!

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  • Sid

    “The great thing about science is that the answers aren’t in the back of the book.”

    Awesome line!

  • Enda

    Great summary… Really clear explanation…. thanks

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  • martenvandijk

    I am afraid that the only purpose of the elusive “dark energy” is to save the Big Bang Theory which predicts much less matter than we can “see” by measuring gravitational effects.

  • keyfeatures

    What is the physics of an explosion generally? Wouldn’t we normally see accelerating expansion simply due to the geometric reality of an expanding sphere? That is until it ran out of puff.

  • http://terpconnect.umd.edu/~sgralla/ Sam Gralla

    That was a useful post; thank you. One question: what are these purely classical contributions to vacuum energy? I’ve never heard that one before.

  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    GR is a classical theory; GR contains the cosmological constant.

  • cecil kirksey

    Sean:
    Is there not some significant inconsistencies between cosmology and QFT? Foe example. The Higgs field is a scalar field with a non-zero VEV which should act as a CC. But apparently this issue is not considered by either community AFAIK. Why?

  • James

    @29:
    The vev doesn’t necessarily contribute energy, as such. It simply represents the value of the Higgs field in its lowest energy configuration. There’s nothing wrong with setting that energy to zero, with a potential that looks like:
    V(h) ~ (h^2 – v^2)^2,
    where h is the Higgs field and v is the Higgs vev… When h = v, V = 0.

  • http://www.lillibridgepress.com/book/Still_Life dcpetterson

    What if we live in an oscillating universe, where a Big Bang is followed by a Big Crunch? Please bear with me.

    In the expansion phase, the farther back we look in time, the faster the galaxies are moving away from each other. As time goes on,the force of gravity slows them down, so that nearer (more recent) galaxies are moving slower. This is what physicists expected to see, and what cosmologists thought until very recently.

    What would it look like if we enter the contraction phase? We see the really distant galaxies still in the expansion phase, as they were billions of years ago. But the nearer, more recent galaxies are already, like us, in the Crunch phase.

    In this phase, galaxies are accelerating — they are leaving behind the galaxies in the past, moving away from them at an ever-increasing rate as they fall toward the Crunch Point.

    Any galaxy we see from the past has to be moving slower than we are — because we’re father along in time, and therefore, farther along in the Crunch. And any galaxy we see has to be in the past, because it took time for its light to reach us.

    Therefore, if we are already in the Crunch Phase of an oscillating universe, wouldn’t we see exactly what we are seeing? The really distant (far past) galaxies are still in the Expansion phase, and are moving faster as we go farther back in time and farther away in space. But the nearer ones are, like us, in the Crunch, and are moving faster as they are closer to us (father along in the Crunch).

    Perhaps we’re not seeing the effects of any otherwise undetectable Dark Energy. Perhaps we’re seeing the effects of the gravity of galaxies that are ahead of us in the Crunch, ahead of us in time — and therefore, not yet visible to our observations, since, by definition, anything we see is in the past.

  • SteveB

    Thanks Sean,

    This material is naturally very similar to your famous Great Courses lectures. I would recommend that to those who want another way to approach this subject.

    One thing that struck me in those lectures and which was not brought out here is the following. You had mentioned that another way to interpret some of the findings would be if the universe was a constant size, but everything in it, including elementary particles, was shrinking. You then dismissed that idea as silly. While watching the lectures, both my daughter and I actually preferred that seemingly equivalent (mathematically, anyway) viewpoint.

    Today, I wonder if the “amazing shrinking universe” approach would have a different way to deal with infinity problems and actually be useful to cosmologists. I suspect that would make no difference and is more a philosophical question of whether one prefers infinities or one prefers limits approaching zer0.

  • http://madsoftware.blogspot.com Mike

    Amazing FAQ. Not just the content, but the way it was presented. Many scientists today don’t take the time to explain to laymen why they think certain aspects of the their theories are correct. They also don’t explain what sorts of things they don’t know, presumably out of some fear that if they express any lack of certainty, they won’t be believed. As if all laymen are too dense to make good judgments on their own. Scientists need to present their own fallibility as they express their certainty about their ideas. This is what should separate them from pastors, reverends, prophets and preachers. Except that many scientists sound just like religious authorities, claiming that one should believe them because of who they are and how smart they are.

    If there were more of this sort of explanation from Environmental Scientists, I think we’d have fewer folks doubting global warming.

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  • Sesh

    Nice summary. However, I have a few little quibbles, especially regarding the evidence for dark energy.

    Broadly speaking, the cosmological data we have is consistent with the standard cosmological constant model. But that’s not the same as having proven dark energy to exist, and it is important to note that. The supernovae data, and the WMAP data and the BAO data are all essentially telling us one thing: the distances that we measure and the expansion rates that we measure are inconsistent with a homogeneous universe with normal matter and normal gravity (they’re off by a factor of about 2). From this we INFER acceleration, we have not observed acceleration directly (though apparently this may be possible in the future with redshift drift measurements).

    On the other hand, other corroborating, non-geometrical, evidence for dark energy is debatable. For instance two groups combine several datasets and say they see an ISW signal at high significance (>4 sigma), but another group say they see no evidence, and a fourth group have challenged the first two, saying they underestimated their errors. Yet another group saw a significant ISW effect (again at >4 sigma) that was way too big (!) to fit with the standard picture of dark energy (at >3 sigma). The apparent detection of the BAO signal – claimed as another neat bit of evidence for dark energy – has decreased in significance as the amount of data has increased over the years.

    The standard model is also crucially dependent on a few assumptions – which may be well motivated (for example a power-law primordial spectrum which you naturally get from some theories of inflation), but have certainly not been confirmed to be true independent of other assumptions. And personally, some of these assumptions (e.g. the near-perfect homogeneity of the universe), seem to me almost too good to be true. Surely our universe is more complicated than the simplest possible model we could write down!

    I’m not going to argue that the whole cosmological picture is wrong and dark energy doesn’t exist, but let’s face it, cosmology is not particle physics. There are very few 5 sigma results out there, and a lot of the apparently corroborating evidence for dark energy is based on something much less secure than that. Modified gravity and quintessence and all sorts of other theories are certainly exciting, but theorists need to be reminded to keep their feet on the ground sometimes.

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  • Bill C

    @ 26. That has also confused me. If every particle had an equal non-conservative force applied at time =0 from the center from opposite directions (such as an explosion), then at some time in the future the larger mass particles would be decelerating from the centroid faster than the smallest mass particles (due to the sum of all gravity of all particles). Eventually, if you observed all of the other particles from some particle in the middle, all other particles would appear to be accelerating away from you when in reality, all particles (including the one you’re observing from) are decelerating from a common centroid.

  • lunchstealer

    Perhaps obscure question on terminology: “Before the supernovae came along, many physicists assumed there was some secret symmetry or dynamical mechanism that set the cosmological constant to precisely zero…”

    I’ve seen this usage of ‘dynamical’ in several cosmological contexts, where I’d have expected the word ‘dynamic’. What’s the difference between the words “dynamic” and “dynamical”? My understanding of the suffixes suggests that there isn’t a difference, and in some cases it simply flows better in English pronunciation, eg “biological” vs “biologic” and “geological” vs “geologic” although in the second, I tend to prefer ‘geologic’ to ‘geological’ in most usage, such as ‘geologic time scale’ vs ‘geological time scale’. But I’ve almost never seen the ‘dynamical’ formulation, so I wonder if it has some specific nuance that I’m not getting.

    Or are you just being poncy? =]

  • http://terpconnect.umd.edu/~sgralla/ Sam Gralla

    @28, who said “GR is a classical theory; GR contains the cosmological constant.”

    I’m pretty sure that’s not what he meant by a classical contribution to vacuum energy. The standard definition of GR wouldn’t include it anyway. One usually hears “GR plus cosmological constant” if Lambda is included.

    What is a purely classical contribution to vacuum energy?

  • Vishnu Bharadwaja

    How can we say that light travels in constant speed in space as their might be worm holes or dark matter or high gravitation emitting bodies which can change the speed or direction drasticaly

  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    “Broadly speaking, the cosmological data we have is consistent with the standard cosmological constant model. But that’s not the same as having proven dark energy to exist, and it is important to note that. The supernovae data, and the WMAP data and the BAO data are all essentially telling us one thing: the distances that we measure and the expansion rates that we measure are inconsistent with a homogeneous universe with normal matter and normal gravity (they’re off by a factor of about 2). From this we INFER acceleration, we have not observed acceleration directly (though apparently this may be possible in the future with redshift drift measurements).”

    Yes, it is inferred. By the way, the only way to “directly” measure an acceleration is to measure the speed at two separate times. This will never happen. Other methods might be more direct, but they are still inferences. But this doesn’t matter: if the universe is described by GR, then we can determine the entire expansion history of the universe and many other things about it by measuring the relation between apparent magnitude and redshift at one point in time. This is standard cosmological theory which has been around since the 1930s, and even the necessary practical details were filled in decades ago. On the other hand, if it is not described by GR, then you can’t say anything unless you have another theory with which to interpret observations. You can’t assume GR is OK for some things and not for others and still be consistent.

  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    “Therefore, if we are already in the Crunch Phase of an oscillating universe, wouldn’t we see exactly what we are seeing?”

    No. We would see nearby galaxies blue-shifted and not red-shifted. (OK, some really nearby galaxies are blue-shifted, but this is due to peculiar motion, not cosmological blueshift.)

  • Sesh

    Phillip: it is perfectly possible for the universe be described by GR, but not by an FRW metric. Inferring acceleration relies also on the assumption of the FRW metric description being valid, at least on some scales. There are some hints in the data that this may not be consistent, or at least not on the scales that we previously thought it might be.

    The FRW metric is pretty much the simplest possible assumption we could have made. If the real universe is more lumpy or inhomogeneous than this, the inference of acceleration need not hold. In fact it is perfectly possible to build toy models of this type in which there is no acceleration, but the observed relation between redshift and apparent magnitude is perfectly matched (obviously toy models don’t always match all the other data at the same time).

    I’m not saying these ideas are necessarily right, but there are plenty of respectable scientists in the world who work on the effects of inhomogeneities, and the question is certainly not settled one way or the other.

    By the way, I was as sceptical as you about the possibility of directly measuring acceleration, but here is a paper that argues it might be possible in the future with Gaia or CODEX: http://xxx.lanl.gov/abs/0909.4954

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  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    OK, misunderstood your comment. Comments come from a wide range of people (check one recent comment on the Nobel-Prize post on Peter Coles’s blog for a few laughs).

    OK, the averaging problem and work by George Ellis and so on. Wait, that’s too much for a blog comment.

    Apparently Rocky Kolb tried to explain away the cosmological constant with the idea of gross inhomogeneity. Actually, the CMB tells us the universe is pretty homogeneous. I’m not inclined to take claims like this seriously since most are a) ad-hoc explanations to explain away an unwelcome result or b) persistent claims which are the same whatever the data (e.g. the fractal-universe crowd).

    Some people have suggested that small-scale inhomogeneities could affect the m-z relation (check out papers by Ron Kantowski). In fact, this is true but doesn’t affect the conclusions in the papers for which the Nobel Prize was awarded. The main Perlmutter et al. paper looked at this in some detail (citing one of my papers and using some Fortran code I wrote to make one of their figures).

    As you say, it is easy to construct a model to match one observation, but one has to match all of the observations (or, rather, all observations which are correct). What is interesting about modern cosmology are independent tests which lead to the same result. This is quite similar to what happened with Avogadro’s number a bit more than 100 years ago (with an important contribution by Einstein).

    As for redshift drift (e.g. in the arXiv paper you cite), this was suggested by Avi Loeb a while back, pointing out that spectroscopy is now precise enough to allow such a measurement within an astronomer’s lifetime. (I think Kayll Lake also suggested this even earlier. This guy has done some really interesting stuff and is my dark horse for a future Nobel Prize in cosmology.) However, this is still not “directly” measuring the acceleration. (Once at a conference, Geller talked about estimating Omega from galaxy catalogues, and Rindler asked about using the m-z relation, number counts etc to measure Omega. Her reply: “Oh, measure it directly? It’s not deep enough for that.” Conclusion: many people use the word “directly” to mean many different things, but usually, in my view, these things are actually not direct, just different.)

  • Sesh

    Hi Phillip. Yes, I am not (yet) a crank, though it is early days yet.

    I hate to be so pedantic, but again, the CMB tells us that our observed universe is isotropic. We assume this means it is homogeneous. The FRW metric assumption is even stronger – that the universe is locally homogeneous and isotropic (as opposed to merely statistically so). Showing that the CMB is consistent with the standard model also requires assumptions about the nature of inflation, which again is currently speculative.

    Sometimes assumptions can very well be justified because of prior theoretical prejudice. I have no problem with this, so long as it is always acknowledged that it is theoretical prejudice, rather than hard data, that is the basis for the assumption.

    Personally, I think inhomogeneous models are more likely to explain dark energy than modified gravity, but less likely than a cosmological constant. And unlike modifications to gravity or quintessence fields (which may or may not exist) inhomogeneities patently do exist, and studying them is important, even if only to show why their effect is small.

  • BoringOldPositivist

    Is vacuum energy really the same as the cosmological constant?

    Yes. Don’t believe claims to the contrary. When Einstein first invented the idea, he didn’t think of it as “energy,” he thought of it as a modification of the way spacetime curvature interacted with energy. But it turns out to be precisely the same thing. (If someone doesn’t want to believe this, ask them how they would observationally distinguish the two.)

    Is Sean Carroll really conscious, or just an automaton programmed to behave as if he were? These are precisely the same thing. (If someone doesn’t want to believe this, ask them how they would observationally distinguish the two.)

  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    If we observe isotropy, then there is homogeneity, unless we are in a special place.

    Clearly FRW is an approximation; the question is, is it a good enough approximation?

    CMB and inflation? Yes, for details of the power spectrum etc, but not for the observed isotropy.

    As far as I know, no-one has ever demonstrated that any large-scale inhomogeneity leads to appreciable deviation from FRW. Small-scale, yes, but the effects are known and corrected for.

  • Sesh

    The belief that we can’t live in a special place but we can live at a special time (which is what Sean’s “preposterous universe” boils down to) is exactly one of those theoretical prejudices that is better challenged by hard data. Fortunately, there are some people doing this.

    As for the rest of the issues you raise, I could stop to discuss each in detail, but actually – as I’ve just submitted a PhD thesis that deals with exactly this topic – I think I’d rather sit in the sunshine than argue on a blog.

  • http://www.irekia.es Iñigo

    Very interesting…

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  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    “The belief that we can’t live in a special place but we can live at a special time (which is what Sean’s “preposterous universe” boils down to) is exactly one of those theoretical prejudices that is better challenged by hard data. Fortunately, there are some people doing this.”

    I’m very interested in this question myself (look for a paper on the arXiv within the next few weeks), but I don’t see how that living at a special time can provide large-scale inhomogeneity.

    “As for the rest of the issues you raise, I could stop to discuss each in detail, but actually – as I’ve just submitted a PhD thesis that deals with exactly this topic”

    Can you send me a link to it?

    ” – I think I’d rather sit in the sunshine than argue on a blog.”

    Of course, one can do both simultaneously. :-)

  • Brian Too

    Thank you for this FAQ Sean. I think I learned 10x more than I ever knew before about Dark Energy. I didn’t understand it all but I did understand most of it.

    In particular I liked that you laid out the lines of evidence, skimmed through possible answers, and attempted to quantify the uncertainties involved.

    @32. SteveB & @6. olsenjaynelson, I agree on all your major points.

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  • martenvandijk
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  • MichaelFretwell

    I’ve looked far and wide for the pennies that have just dropped.
    Thank you.

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  • http://poeteye.com James Ph. Kotsybar

    SUPERLUMINELLE
    — James Ph. Kotsybar

    The Universe is expanding,
    Faster than the limit of light,
    Beyond common understanding.

    Cosmology is demanding. I
    ts study is by no means slight.
    The Universe is expanding.

    Physics’ heroes, quite outstanding,
    Have applied their full mental might
    Beyond common understanding.

    There’s no point in reprimanding,
    As we gaze out into the night,
    The Universe is expanding.

    The truth of fact is commanding.
    Whatever is has to be right,
    Beyond common understanding.

    Einstein’s physics notwithstanding,
    Much quicker than what we call bright,
    The Universe is expanding
    Beyond common understanding.

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  • Barry J White

    Please anyone, everyone, tell me where I am wrong here. Sean, in November Discover lists
    the four forces: gravity, electromagnetism, the strong and weak nuclear forces. He challenges
    us to find the fifth force. It seems to me that the fifth force is the preprogrammed pattern in
    every dynamic system for its own maturation and regeneration. A living organism or an
    evolving solar system follows a predertmined course. This preprogramming is as much
    a force as the others. By adding gravity to this theorem, the universe is expanding as part of the preprogrammed evolution of the multiverse so that the gravitational pull of other universes on the matter in our universe on the way to its eventual demise and contribution to the rebirth of new universes explains why matter is moving apart at an increasing rate toward the edge of our universe possibly to combine with matter from other universes. At the very least, if gravity is one of stronger of the four known forces, why do we not allow for the tremendous gravitaional pull of the multiverse? A far simpler explanation than the elusive search for dark matter

    O.K. Beat up on me, student of the behavioral sciences that I am.

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  • James T. Dwyer

    Sean – Excellent posting & discussion. Kudos also to Philip Helbig (hi!)

    In very broad, simple strokes:

    Since the universe has been expanding for a very long time and, I presume, the local effects of gravitation have increased the universal ‘clumpiness’ of matter (galaxies merge, etc.), it seems that it should be primarily the size of voids that is generally increasing.

    Since any effect of gravitation that opposes expansion should be diminishing within the voids, might not the expansion of spacetime within the voids be accelerating – simply because of the diminishing long range effects of gravitation (as distances between clumps increase)?

  • http://galvanizedjazz.com/physics.html Art Hovey

    Descriptions of the accelerating expansion found in popular journals such as Scientific American, Physics Today, and the above set of FAQ all state that distant supernovas with very large red-shift appear to be farther away than they would be if the expansion rate were constant or if there were a deceleration due to gravitation.

    I think this is something that a simple-minded high school physics teacher should be able to understand, but I am having trouble seeing how that observation leads to that conclusion.

    If one car or galaxy travels away from me with constant speed and another accelerates away from me (starting at the same time but with any smaller speed) then at some time the two objects will have equal speeds away from me and therefore equal red-shifts. But the accelerating object will have a smaller average speed than the first one. Doesn’t that mean that it must be closer to me at the moment when the speeds are equal?

  • James T. Dwyer

    Art Hovey – I’m never studied physics, but during the past couple of years I did study the original research reports for which the Nobel prize was awarded. As I understand:

    – The more accurate (presumed) consistent peak emission luminosity of type Ia supernovae were used to estimate their distance based on their observed apparent luminosity.

    – These ‘new’ SNe distance estimations were compared with traditional distance estimates derived from standard cosmological models based on the redshift of their host galaxies’ light emissions.

    – The more distant ‘high-z’ group of SNe observed (3-5 billion light years away) were determined to be more distant than the standard cosmological models had predicted. The nearer group of SNe observed did not exhibit conflicting distance estimates – the distances predicted by standard cosmological models were in general agreement with the SNe based distance estimates.

    – To adjust the cosmological models’ estimates to match the SNe distance estimates for the ‘high-z’ SNe group, the researchers employed the models’ cosmological constant parameter and changed the normally positive deceleration parameter to a negative value, indicating ‘negative deceleration’, or acceleration.

    To this innocent bystander, it would seem that it is the the more ancient light emissions of distant galaxies that indicate the acceleration of expansion – not the more recently emitted light from nearer galaxies. I have been assured that this is just an artifact of the complex procedures employed by physicists in their analysis.

    As I understand, the necessary observations of initial type Ia SNe peak emissions have not been made for more distant galaxies, so the relationship between distance and acceleration of expansion for the ‘edge’ or periphery of the observed universe has not been established – the ‘acceleration’ of expansion has only been determined from SNe that are 3-5 billion light years away, in the ‘middle’ of the observed universe.

    I’m sure I just don’t properly understand the findings within the full context of current cosmological understanding…

  • James T. Dwyer

    As I understand, there are still several uncertainties presumed in the analysis of type Ia supernovae luminosity.

    One such uncertainty is the effect of metalicity on type Ia SNe peak emission luminosity, used to precisely derive distance in the studies that concluded the universe is accelerating.

    Metalicity is the measure of heavier elements contained within the universe: it has generally increased as the universe developed, with each new generation of massive main-sequence stars.

    It is currently presumed to have no effect, but the metalicity of more distant type Ia SNe would be different from that of nearer type Ia SNe and its effect on their peak emission luminosity has not yet been determined. Please see: http://blogs.nature.com/news/2011/08/bright_supernova_one_of_the_ne.html#more

  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    @66: It’s not that simple. The redshift (actually 1+z where z is the redshift) gives us the ratio of the size of the universe now to when the light was emitted. To convert this into a distance (and there are many in cosmology, depending on how they are defined (and which are all the same when the redshift is negligible), one needs to know how the universe has expanded since the light was emitted and what it’s geometry is. Both of these are determined by the cosmological parameters. So what one does is measure the luminosity distance (from the apparent magnitude and the (presumed known or calculable) absolute magnitude and then compare it to predictions for various combinations of the cosmological parameters. In particular, the distance as a function of redshift is different for different combinations of parameters, so one determines the parameters by finding the best-fit curve to match the observations.

    Since the Hubble constant, i.e. present rate of expansion, is the same in both cases, you need to think of a universe which is accelerating now as having expanded slower in the past than a comparison universe. Let’s compare the two for observing an object at a redshift of 1. That means that, in both cases, the light was emitted when the universe was half the size it is now. But because the universe which is accelerating now was expanding more slowly in the past, the object has to be farther away, since otherwise the light would have reached us too soon.

    You might be getting confused by the (wrong) assumption that equal speeds implies equal redshifts. Equating redshift with speed like this can only be done at very small redshifts.

  • Pete Veslocki

    Sorry guys, this is from a layman’s point of view:

    It’s a matter of perspective.

    On the large scale obviously the universe is larger than we can ever observe.

    And if it were rotating, which it probably is, as everything does, then all the matter in the universe will eventually be pulled outward at an accelerating rate forming an immensely thinning outer shell by centrifugal force. Which is what I think we are observing.

    On the small scale as matter decomposes and as the universe expands the distance between atoms increases and density goes down eventually forming a predictably loosely defined apparently steady state of really decreasing gaseous like pressure independent of it’s composition ( no pun intended ). Hence the uniformity and persistence observation.

    And as these intervening levels of ” vacuums ” being virtually nothing continuing to become even less nothing ” they ” are even less likely to interact with anything and become more and more undetectable. Which is what we are also observing.

    The shoe is beginning to fit.

    We need to do two things:

    1 . Measure the density of empty space around us as far out as we can and see if it does
    decrease at an increasing rate implying it is accelerating dispersion breaking the bonds
    of gravity between atoms.

    2. Try to vector back all the paths these outer galaxies have traveled to try and locate a
    central point everything may have emanated from to see if there is any uniform
    evidence of rotation about that point.

    3. Then predict future positions of the outer galaxies and vacuum densities as a possible
    way of explaining both the reason the universe is expanding and the mystery of what
    dark matter is and what appears to be dark energy.

    pete veslocki

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  • James T. Dwyer

    70. Pete Veslocki – It seems to me you’re envisioning a universe still composed of plasma. I don’t think there’s any evidence for any spacetime expansion within galaxies, for example. In fact, galaxies seem to have been generally merging for most of the universe’s existence, and are expected to continue to do so – the Milky Way is generally expected to merge with the Andromeda galaxy in the next 3-4 billion years.

    Since the general effect of gravitation is to localize matter and the general effect of universal expansion (as observed) is to expand intergalactic spacetime, universal expansion may not produce any decomposition of matter.

    Extrapolating existing trends, one could project the future state of the universe to be ever more isolated galaxies of increasing size, or ‘island universes’, since an observer in each may not be able to detect the others’ existence.

    However, this is also just the speculations of a lay person.

  • Bruce Cox

    I tried to stay with this piece to the end, but my almost uncontrolled laughter at a point interfered. I got as far as “the coincidence problem”…. after numerous other “Jabberwocky” questions and probable, possible, maybe, but we’re not sure answers, as if a group of physicists and cosmologists got together and made all this up. No disrespect intended, but the maze of questions and “around unending corners” answers eventually seemed hilarious.

  • James T. Dwyer

    69. Phillip Helbig – I think there were some transcription errors as you recorded your thoughts in responding to Art Hovey. I think your explanation would be very interesting and hope you can restate or clarify. You stated:

    “To convert this into a distance (and there are many in cosmology, depending on how they are defined (and which are all the same when the redshift is negligible), one needs to know how the universe has expanded since the light was emitted and what it’s geometry is. Both of these are determined by the cosmological parameters.”

    BTW, the statement “one needs to know how the universe has expanded since the light was emitted and what it’s geometry is” would be more correctly stated as something like: ‘models must presume…’ since it is not definitively known how the universe has expanded and what it’s geometry is – thus the requirement to evaluate model results, correct?

    Likewise the statement “Both of these are determined by the cosmological parameters” would be more correctly stated as ‘Both of these are specified by the cosmological parameters’, correct?

    Thanks in advance.

  • John Motz

    What is ment by Comoving objects in space.

  • Skeet

    What if the speed if light is not constant? How would that change the apparent acceleration of distant galaxies? If light behaves like a particle, maybe it does slow down but only at very long distances, perhaps that could explain some of the shift.

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  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    @74: Please prefix to all of my blog comments: “Assuming the laws of physics as we know them are correct and, in particular, that the large-scale structure of the universe is described by General Relativity to at least a very good approximation.”

  • James T. Dwyer

    78. Phillip Helbig – Fine, thanks, but that still doesn’t clarify statements made with uneven numbers of parenthesis:
    “To convert this into a distance (and there are many in cosmology, depending on how they are defined (and which are all the same when the redshift is negligible), one needs to know how the universe has expanded since the light was emitted and what it’s geometry is. Both of these are determined by the cosmological parameters.”

    Likewise, for those who are interested in clearly representing reality, the parameter values supplied to cosmological models do not determine the reality of ” how the universe has expanded since the light was emitted and what it’s geometry is” but specify the current assumptions of the analyst.

    By the way, the proposed acceleration of universal expansion is not consistent with either the laws of physics or general relativity – isn’t that correct? Otherwise, what law of physics specifies that the entropy produced by the dispersal of universal mass/energy might somehow transform into accelerating expansion? This fundamental violation would seem to require far more supporting evidence than has currently been amassed.

  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    OK, add a second closing parenthesis immediately after the first one (or before it!).

    Again, assuming GR describes the universe, then the parameters determine the expansion history and geometry.

    Why isn’t it consistent? Where do you get the idea that the dispersal is the cause of accelerating expansion? Yes, both happen, but that doesn’t mean one causes the other. You seem to be making unwarranted assumptions. :-|

  • James T. Dwyer

    OK, forget the parens.

    The parameters specified determine the expansion history and geometry represented to an analytical model – they do not determine anything in the physical universe. They may or may not represent any current consensus of how the universe has expanded or what its geometry is. I’m just saying that you are attributing causation in your terminology when it is more correct to always refer to the fact that a model is merely an evaluation tool.

    The Nobel prize winning researchers changed the parameter used to specify the deceleration rate of universal expansion to a negative value in order to fit the models to their interpretation of their observations. Strictly speaking, those cosmological models did not include any normal effect of entropy – only ‘negative deceleration’.

    So again, what law of physics provides for an acceleration of universal expansion, proposed to have begun about 3-5 billion years ago following ~8-10 billion years of decelerating expansion? How is that transition to an accelerating universe consistent with the established laws of physics?

    As I understand the only established law of physics applicable to any change in the rate of universal expansion is the second law of thermodynamics. But then, I know little about these things. Please explain!

  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    I give up. I can’t provide a cosmology course in a blog comment. The laws of physics are the usual ones, in particular General Relativity. I suggest you read some standard cosmology textbooks.

    Of course, what parameter I choose does not cause the universe to change its behaviour, but you are playing word games.

  • James T. Dwyer

    82. Phillip Helbig – What specific law or laws of physics provides for an acceleration of universal expansion? Is this too difficult a question for a Dark Energy FAQ article?

  • martenvandijk

    I am told by Professor Rodenbach that abundant quantities of Dark Ale also provide for an accelerated expansion.

  • Pingback: Ciencia Kanija » FAQ sobre energía oscura()

  • http://www.astro.multivax.de:8000/helbig/helbig.html Phillip Helbig

    @82: For the last time, General Relativity with a cosmological constant. This has been known for almost 100 years.

  • James T. Dwyer

    86. Phillip Helbig – Again, cosmological model parameters do not physically produce the characteristics of universal expansion. The second law of thermodynamics provides for the physical effects that result in the deceleration of universal expansion. Einstein’s cosmological constant parameter was simply a non-physical, unspecified ‘fudge factor’ to produce a constant universe model result. What physical phenomena (like entropy) does it represent? The answer is none.

  • Rick Bliss

    I sent an explanation of the origin of the universe, where it come from, its effective age before the big bang, why the universe is expanding at an increasing rate i.e. dark energy, etc to Discover. All this has a simple explanation. It is not mysterious at all. By the way, dark energy is a mis-nomer. The expansion does not require energy at all. Think about this: what does space push on and how does distance comes about? The theory continues to advance as I learn more about the details and ramifications…. Let me say this in closing … it shows why it is highly unlikely that the multiverse can exist. This universe is it and it took a long time (basically from minus eternity) to get here! Well, OK, time requires some explanation….

  • http://www.facebook.com/bruce.ing.9 Bruce Ing

    1) So, dark matter means that thing are attracting each other with more mass than we see and dark energy means that things are repelling each other more than they should be (alternatively that gravity isn’t acting as strongly as it should).
    Couldn’t these be two parts of the same coin, we’ve made some errors or there is something we haven’t observed yet that will evenually say, “Oh, that makes sense, they cancel each other out!” ?
    2) Another question I have, is where are we in the universe? That is, where is the center of the big bang in relation to us?
    – If we are on one side going 0.3 the speed of light and a galaxy is on the other side going 0.31 the speed of light then we seem to be going apart at 0.61 the speed of light and you’d say we’re going apart faster than we should.
    – If we are on the same side and we’re going 0.3 and you look at a galaxy going 0.31 then we’re going apart at 0.01 the speed of light and you might say we’re going apart too slow.
    No where have I heard someone say where we are in the universe in relation to the centre!
    3) Why does everyone assume things should be homogenious from the big bang? If there’s an explosion, the material farther out usually goes faster and farther. The material closer to the explosion usually goes slower or may not leave the epicentre at all. Why is that not a valid theory for the way the universe expanded? How come it has to be so homogenious?
    Wouldn’t an explosion theory, maybe with the outer material pushing back in during the initial explosion (while material was still dense and in contact with each other) explain how structures were formed and why things aren’t homogenous?
    Wouldn’t there have been ripple effects through the initial explosion causing waves and troughs of denser and less dense material? (Granted, it wasn’t “matter” yet when it was initially propogating.)

  • http://www.facebook.com/bruce.ing.9 Bruce Ing

    What if the formation of the universe, the Big Bang, cause a huge implosion creating a super massive black hole at the centre of our universe? Wouldn’t that cause matter closer to the centre to travel slower and material farther out would travel faster, because the matter in the universe isn’t spread out evenly as we assume?

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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .

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