Dark Energy FAQ

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 Trodden 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.

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88 Responses to Dark Energy FAQ

  1. keyfeatures says:

    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.

  2. Sam Gralla says:

    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.

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

  4. cecil kirksey says:

    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?

  5. James says:

    @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.

  6. dcpetterson says:

    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.

  7. SteveB says:

    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.

  8. Mike says:

    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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  10. Sesh says:

    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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  12. Bill C says:

    @ 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.

  13. lunchstealer says:

    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? =]

  14. Sam Gralla says:

    @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?

  15. Vishnu Bharadwaja says:

    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

  16. “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.

  17. “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.)

  18. Sesh says:

    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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  20. 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.)

  21. Sesh says:

    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.

  22. BoringOldPositivist says:

    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.)

  23. 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.

  24. Sesh says:

    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.

  25. Iñigo says:

    Very interesting…