Cosmology Primer: Frequently Asked Questions

What is the universe expanding into?
As far as we know, the universe isn't expanding "into" anything. When we say the universe is expanding, we have a very precise operational concept in mind: the amount of space in between distant galaxies is growing. (Individual galaxies are not growing, as they are bound together by gravity.) But the universe is all there is (again, as far as we know), so there's nothing outside into which it could be expanding. This is hard to visualize, since we are used to thinking of objects as being located somewhere in space; but the universe includes all of space.

Are distant galaxies moving faster than the speed of light? Wouldn't that violate relativity?
A profound feature of relativity is that two objects passing by each other cannot have a relative velocity greater than the speed of light. An even more profound feature, one which has received much less publicity, is that the concept of "relative velocity" does not even make sense unless the objects are very close to each other. In Einstein's general theory of relativity (which describes gravity as the curvature of spacetime), there is no way to define the velocity between two widely-separated objects in any strictly correct sense. The "velocity" that cosmologists speak of between distant galaxies is really just a shorthand for the expansion of the universe; it's not that the galaxies are moving, it's that the space between them is expanding. If the distance isn't too great, this expansion looks and feels just like a recession velocity, but when the distance becomes very large that resemblance breaks down. In particular, it's perfectly plausible to have distant galaxies whose "recession velocity" is greater than the speed of light. (We couldn't see such galaxies directly, since light from them would never reach us, but that doesn't mean they aren't there.) The resolution to this paradox is simply that we have taken a convenient analogy too far, and there isn't a well-defined "speed" between us and distant objects.

Does the universe have a center?
No. Our observable universe looks basically the same from the point of view of any observer. We see galaxies moving away from us in all directions, but an astronomer living in any one of those galaxies would also see all the galaxies (including our own) moving away from them. In particular, the Big Bang is not an explosion that happened at some particular point in space; according to the Big Bang model, the entire universe came into existence expanding at every point all at once.

Could we detect the expansion of the universe by trying to measure the expansion of the solar system?
No. Any system that is bound together by internal forces -- whether it is a table, the solar system, or the galaxy -- does not expand along with the universe. (Not just that it only expands slightly; it really doesn't expand at all, or at least not because of the expansion of the universe.) To observe the expansion, we need to study objects that are very distant, not directly bound to us by gravity or anything else.

Is the universe finite or infinite? Will it recollapse or expand forever?
We don't really know in either case. Since the Big Bang happened a finite time ago (about 14 billion years), and since light travels at a finite speed, there is an unbreakable upper limit to how far away we can see in the universe. Up to the limits of the observable universe, what we observe is consistent with a uniform distribution of matter and energy that could easily extend forever. On the other hand, it might eventually turn into something very different, beyond what we can see; indeed, this might arise naturally as a result of inflation (see the really early universe). Similarly, we can straightforwardly extrapolate the current evolution of our universe, dominated by dark energy, to predict a future in which the universe continues to expand for all time (see the dark universe). However, the dark energy might someday change its character into something different, in which case the universe might very well collapse. So, given how little we currently understand about the nature of dark energy, we can't say anything for sure about the ultimate fate of our universe.

Is space flat or curved? I've heard both.
There is an important distinction between "space" and "spacetime," and also a distinction between exact statements and useful approximations. Our universe is a four-dimensional spacetime -- to describe the location of an event, you need to specify three coordinates of space and one of time. According to Einstein, spacetime can be curved, and gravitation is the manifestation of that spacetime curvature. Since there is certainly gravity in the universe, there is no question that the universe is curved. But for cosmological purposes it is useful to model spacetime as a three-dimensional space expanding as a function of time; then the total curvature is a combination of the curvature of space by itself, plus the expansion of the universe. Observations indicate that space by itself is very nearly flat, rather than having an overall positive or negative curvature (see the expanding universe); that is the origin of the statement that we live in a "flat universe." Of course this is only an approximation, since the real world features galaxies and voids in large-scale structure, rather than perfect smoothness; but it's a good approximation. So "space" is (approximately) flat, while "spacetime" is definitely curved.

Is energy conserved in an expanding universe?
This is a tricky question, depending on what you mean by "energy." Usually we ascribe energy to the different components of the universe (radiation, matter, dark energy), not including gravity itself. In that case the total energy, given by adding up the energy density in each component, is certainly not conserved. The most dramatic example occurs with dark energy -- the energy density (energy per unit volume) remains approximately constant, while the volume increases as the universe expands, so the total energy increases. But even ordinary radiation exhibits similar behavior; the number of photons remains constant, while each individual photon loses energy as it redshifts, so the total energy in radiation decreases. (A decrease in energy is just as much a violation of energy conservation as an increase would be.) In a sense, the energy in "stuff" is being transferred to the energy of the gravitational field, as manifested in the expansion of the universe. But there is no exact definition of "the energy of the gravitational field," so this explanation is imperfect. Nevertheless, although energy is not really conserved in an expanding universe, there is a very strict rule that is obeyed by the total energy, which reduces to perfect conservation when the expansion rate goes to zero; the expansion changes the rules, but that doesn't mean that anything goes.

What is the difference between dark matter and dark energy?
Dark matter behaves much like a collection of ordinary matter made of particles, except that it's dark. In particular, dense regions of dark matter tend to become even more dense, as the mutual gravitational force of the matter pulls it together. For this reason, we suspect that the dark matter is some sort of new, massive particle, just one we haven't yet discovered in the laboratory (yet). Dark energy, on the other hand, doesn't act anything like particles: it doesn't cluster together, nor does it dilute as the universe expands. Its density remains constant (so far as we can tell) throughout space and time. So whatever the dark energy is, it's something different than dark matter.

Will we ever be able to detect dark matter or dark energy directly?
Hopefully. Different candidates for what the dark matter particles are lead to different strategies for detecting them, either directly in laboratories here on Earth or indirectly through high-energy particles from space. But numerous efforts are being undertaken, and we might find dark matter in the near future. Dark energy is an even longer shot; if it is a strictly constant vacuum energy, we could never detect it directly, while a dynamical field could conceivably be detected. Probably we will have to content ourselves with understanding dark energy indirectly, through its gravitational effects on the expansion of the universe. See the page on the dark universe.

Isn't "dark energy" just like the older concept of the "ether"?
No; in fact, it's just the opposite. The ether was supposed to be an invisible substance that determined the rest frame of the universe. It was expected by theorists, but eventually abandoned when experimenters could not find any evidence for it (and Einstein figured out that it wasn't necessary). Dark energy, meanwhile, was not at all expected by most working cosmologists; we need it to explain observed facts, like the acceleration of the universe and the mismatch between matter and total energy. And the dark energy appears the same to all observers, so there's no sense in which it determines a rest frame.

How do you know that dark matter isn't just ordinary matter that we can't see?
We can measure the total amount of matter through the gravitational field it creates, both in galaxies and in clusters of galaxies. But we can separately measure the amount of ordinary matter by less direct means. The traditional method is to study the abundances of light elements (hydrogen, deuterium, helium, and lithium) in the early universe. These elements are produced by primordial nucleosynthesis (see the early universe), and the amount of ordinary matter in the universe directly affects the relative amounts of different elements that we predict. Observations of these elements are consistent with ordinary matter comprising only 5% of the total energy of the universe, whereas the total amount of matter is closer to 30%, with dark matter making up the difference. Confirmation of this result comes from temperature fluctuations in the cosmic microwave background; the precise pattern of these fluctuations depends on the ratio of ordinary matter to dark matter in a way which matches the requirements of primordial nucleosynthesis. So there are strong (and independent) reasons to believe that the dark matter is something new, not just ordinary matter that is somehow hiding.

Could the inferred existence of dark matter and dark energy be due to a modified behavior of gravity?
It's possible, and in fact there are scientists working hard on just this scenario -- doing away with dark matter and/or dark energy, and instead invoking a new law of gravity on very large scales. There are a few obstacles to this idea, though, and two are worth stressing. One is that the dark matter/dark energy paradigm does an extremely good job of explaining the data, and in a wide variety of apparently disconnected circumstances. The other is that our current theory of gravity -- Einstein's general theory of relativity -- is both conceptually compelling and experimentally very well tested. It's hard to come up with a new theory that fits the data nearly as well as the conventional model of dark matter and dark energy in the framework of general relativity (but that's no reason not to keep trying).

Is inflation testable?
Yes and no. Inflation makes quite strong predictions, including a geometrically flat universe (already verified by measurements of the cosmic microwave background) and a particular set of primordial perturbations, both fluctuations in the matter density and in gravitational waves. The fluctuations in the matter density seem consistent with the predictions of inflation, while looking for gravitational waves from inflation is a major goal of experimenters. However, there are two caveats. First, there are many different models of inflation, and they give somewhat different predictions, so it's possible to wriggle out of almost any definitive statement. And second, the predictions of inflation may also be predictions of some alternative model which has not yet been thought of. We will never prove inflation beyond any possible doubt; we will only gain increasing (or decreasing) confidence in the inflationary paradigm, as developments in both theory and experiment either remain consistent with inflation or make it seem less likely.

What came before the Big Bang?
The strictly correct answer is: nobody knows, and nobody even knows if the question makes sense. According to general relativity, Einstein's theory of gravity and our best understanding of what governs the early universe, there is no such thing as "before the Big Bang" -- it is the point at which space and time come into existence. However, it is also a "singular" point, at which our theories break down. It is possible that some future reconciliation of general relativity with quantum mechanics will help us understand the origin of the Big Bang, just as it is possible that we may come to believe that the universe had an interesting history even before what we now call the Bang. Both possibilities are being actively pursued by cosmologists.

Is our universe the only one, or are there others?
Hopefully you won't be disappointed if we say that we don't know. There are different kinds of "other universes" that one could reasonably imagine -- other regions of space that are very far away and look very different, or regions that are separated from our own by extra dimension of space, or different branches of the quantum-mechanical wavefunction of the universe. These are all profound ideas which we won't discuss in detail here. Suffice it to say that these kinds of other universes are perfectly plausible, and are sometimes even predicted by ambitious theories of fundamental physics. However, it is hard to see how we could test their existence experimentally. So we don't know one way or the other, but speculations along these lines play an important role in the attempt to construct a unified framework of physics and cosmology; perhaps in the future we will be able to be more definite.

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