Cosmology Primer: The Dark Universe
Our knowledge of the universe comes from looking at it in various ways -- different wavelengths of light, neutrinos, cosmic rays, and hopefully some day gravitational radiation. But how do we know that we are seeing everything there is? How could we determine whether there were substances in the universe not directly visible in our telescopes?
The answer lies with gravity. Long ago, Galileo determined that every object falls the same way in a gravitational field. Later, Einstein extended this idea: every substance with any kind of energy will create a gravitational field. (Note that mass is a kind of energy, since E=mc2.) So we can, in principle, detect everything in the universe, by mapping out the gravitational field throughout spacetime. To our surprise, there is much more out there than we can see directly. The invisible stuff comes in two forms: dark matter and dark energy.
Dark matter is some kind of particle that we have not yet detected in experiments here on Earth, but nevertheless comprises most of the matter in the universe. The first evidence for its existence came from studying the dynamics of galaxies and clusters of galaxies. The basic point is that something in orbit around a massive object moves more rapidly, the more mass the object has. Fritz Zwicky was the first to put this idea into action, studying the motion of galaxies in the Coma cluster; their motions were too rapid to be accounted for by the visible matter in the galaxies. Later, Vera Rubin looked at matter orbiting at the edges of individual galaxies, and noticed an similar effect -- the rotation speeds of the galaxies did not fall off with distance as they should if the gravitational fields were being caused by the visible matter alone.
These discrepant motions, and more modern methods confirming this behavior with greater precision, are strong evidence for unseen matter in galaxies and clusters. Of course, it is natural to imagine that the extra matter is quite ordinary, but simply invisible and transparent (much like air on a clear day). And indeed, observations in different wavelengths have provided evidence for previously unseen gas in galaxies and clusters. However, we have very good evidence that the substantial majority of dark matter is something exotic, rather than ordinary matter that we can't see. This evidence comes from two independent sources -- the abundance of light elements from primordial nucleosynthesis, and temperature anisotropies in the cosmic microwave background. Both phenomena are described in the page on the early universe. The result is that the abundance of exotic dark matter, some kind of particle that has never been directly observed here on Earth, is about five times the abundance of ordinary matter in the universe.
What we know about dark matter, then, is that it is a new kind of unseen particle that falls readily into galaxies and clusters. The fact that it is dark means that it is neutral, rather than electrically charged; in general, charged particles interact readily with light, and would not be dark. The fact that the dark matter is concentrated in galaxies and clusters indicates that it is slowly moving; particles of this form are referred to as "cold." What could the cold dark matter be made of? We are not at all sure, although numerous theories have been proposed. Two ideas are especially popular: neutralinos and axions. "Neutralinos" are a kind of particle predicted by supersymmetry, a popular (but as yet purely conjectural) theory in particle physics. According to supersymmetry, each kind of known particle has a "superpartner" with a different intrinsic spin; the neutralino is simply a massive, neutral, stable superpartner of one of the known particles, an is a natural dark matter candidate. (Such a particle would interact predominantly through the weak nuclear force, and is therefore an example of a Weakly Interacting Massive Particle, or WIMP.) Axions, on the other hand, are another kind of hypothetical particle, originally postulated to explain certain symmetries of the strong nuclear force. In the case of both neutralinos and axions, active experimental programs are underway to detect these dark matter candidates in the laboratory, either by producing them directly or by observing the effects of ambient particles floating through the Solar System. Finally, it may be possible to detect dark matter particles indirectly, if they annihilate into photons in high-density regions of the universe; the resulting radiation would have a characteristic form that would signal the existence of a new kind of particle.
Of course, there is a kind of known particle which is neutral, stable, and (as we now know) massive -- the neutrino. Neutrinos are abundant in the universe, approximately as abundant as photons. However, they are not good dark matter candidates, because they are not very cold. Even if neutrinos today are moving relatively slowly, in the early universe they were moving near the speed of light. As a result, they would stream freely away from galaxies and clusters, rather than settling into them as dark matter is observed to do. Indeed, a useful way to put an upper limit on the mass of the neutrino is to insist that it not comprise such a significant fraction of the mass of the universe that it would interfere with the evolution of large-scale structure.
As if dark matter weren't exotic enough, dark energy is even more mysterious. We know very little about dark energy, other than two characteristic features: it is spread uniformly throughout space, and maintains an approximately constant density as the universe expands. It must be (nearly) uniform throughout space, since otherwise it would clump into galaxies and clusters and affect local motions just as dark matter does. Instead, the dark energy only affects the overall curvature of spacetime. How, then, do we know that dark energy exists? Its affects on spacetime curvature show up in two ways -- it makes the universe accelerate, and it contributes (along with matter) to the curvature of space alone.
According to Einstein, the rate of expansion depends on the average energy density of the universe. To measure the expansion rate, we use standard candles, objects whose intrinsic brightness is known. The further away a standard candle is, the dimmer it will appear, allowing us to accurately determine its distance. We want standard candles that are very bright, so they can be observed at cosmological distances; good candidates are provided by supernova explosions, which can rival the entire light output of the galaxy they are in. It turns out that supernovae come in various forms, of different brightness; but a certain kind, the Type Ia supernovae, have a brightness which depends directly on how fast they explode and fade away. Observations of Type Ia supernovae at high redshifts by two groups (the Supernova Cosmology Project and the High-Redshift Supernova Team) in 1998 provided the first direct evidence that the universe is accelerating rather than slowing down.
Why does dark energy make the universe accelerate? Because, unlike matter and radiation, it does not dilute away as the universe expands -- the density of dark energy remains close to constant. Therefore, according to Einstein, the Hubble parameter remains close to constant. But remember that the apparent velocity of a galaxy is given by the Hubble parameter times the distance, as explained in the page on the expanding universe. Since the distance to any given galaxy is increasing, a nearly-constant Hubble parameter implies that the apparent velocity will also be increasing -- in other words, the galaxies are accelerating away from us.
The other piece of evidence for dark energy comes from the curvature of space, as measured through temperature fluctuations of the Cosmic Microwave Background. As described in the page on the expanding universe, the curvature of spacetime can be thought of as a combination of the curvature of space, and the expansion of space through time. Through observations of temperature fluctuations in the CMB (described in the page on the early universe), we can measure the overall curvature of space, and find that it is close to zero. Meanwhile, the total amount of matter in the universe (both ordinary and dark) falls well short of what is needed to explain the flatness of space. It turns out that the amount of dark energy required to explain the acceleration of the universe is just right to explain the fact that space is flat. We therefore seem to have a complete inventory of the constituents of our contemporary universe:
- five percent ordinary matter,
- twenty-five percent dark matter,
- seventy percent dark energy.
We don't know what the dark energy is. Since observations constrain its density to be close to uniform throughout space and approximately constant in time, the simplest candidate would be something that is exactly constant in space and time. Such a substance is called vacuum energy -- an energy density that is inherent in empty space itself, unchanging from point to point in the universe. But once we admit the possibility of vacuum energy, we can go back and estimate how large such energy should be. The result, according to our best understanding of quantum field theory, is 10120 times the observed amount -- a fantastically large factor, and a sure sign that there is something we don't understand about vacuum energy. This discrepancy is known as the "cosmological constant problem," since the vacuum energy turns out to be equivalent to Einstein's old idea of a cosmological constant. In addition to this problem, there is the puzzle of why the dark energy and the matter density are relatively close to each other (only differing by a factor of two or three. After all, matter dilutes away as the universe expands, while dark energy remains essentially constant, so their relative abundance changes dramatically; we seem to be born lucky, in an era when both matter and dark energy play an important role in the universe.
In an effort to address these puzzles, cosmologists are trying to learn all they can about dark energy. In particular, they would like to know whether it really is absolutely constant, or merely changing very slowly. If dark energy is dynamical, rather than a strictly-constant vacuum energy, its origin in fundamental physics would be completely different. It is even conceivable that the acceleration and flatness of the universe are not due to dark energy at all, but rather to a breakdown of Einstein's theory of general relativity on cosmological scales. All of these possibilities are in play, and future observations will help us decide between them once and for all.
Next: The Early Universe