Cosmology Primer: Frequently Asked Questions
- What is the universe expanding into?
- Are distant galaxies moving faster than the speed of light? Wouldn't that violate relativity?
- Does the universe have a center?
- Could we detect the expansion of the universe by trying to measure the expansion of the solar system?
- Is the universe finite or infinite? Will it recollapse or expand forever?
- Is space flat or curved? I've heard both.
- Is energy conserved in an expanding universe?
- What is the difference between dark matter and dark energy?
- Will we ever be able to detect dark matter or dark energy directly?
- Isn't "dark energy" just like the older concept of the "ether"?
- How do you know that dark matter isn't just ordinary matter that we can't see?
- Could the inferred existence of dark matter and dark energy be due to a modified behavior of gravity?
- Is inflation testable?
- What came before the Big Bang?
- Is our universe the only one, or are there others?
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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