3.3. The Composition of the universe
Thirty years ago the universe seemed much simpler. We only had knowledge of ordinary matter, and even the fact that most of the ordinary matter did not reside in stars was still to be discovered. Today, we have a much more complete (and correspondingly much more complicated) accounting, with five components: ordinary matter, massive neutrinos, cold dark matter, dark energy, and photons, cf Figure 10. And now even the "ordinary" matter is not simple - the bulk of it is dark, and in a form yet to be firmly identified.
The leading candidates for the CDM particle are the axion and the neutralino, two hypothetical elementary particles. If they exist, these particles would have been produced in the earliest moments of the universe, and survived in sufficient numbers to account for the dark matter. Both are new forms of stable matter, predicted to exist by theories that attempt to unify the forces of Nature. But they have wildly different masses: a trillion times smaller than that of the electron for the axion, and a hundred times larger than the mass of the proton for the neutralino.
If the cold dark matter hypothesis is correct, then the halo of our own Milky Way should be awash in axions or neutralinos (or some other slowly moving particle). While the interactions of axions and neutralinos with ordinary matter are very weak (and can be neglected in almost all circumstances) specialized detectors have already been built to confirm (or rule out) their presence. In addition, the neutralino, the lightest of a new class of particles predicted by superstring theory, has two other signatures of its existence. It can be produced at a particle accelerator, given sufficient energy. Alternatively, high-energy neutrinos produced by the annihilation of the few neutralinos that are captured by the Sun could be detected; or positrons and/or gamma rays produced by neutralinos annihilating in the halo might be found. With the efforts underway, evidence that axions or neutralinos comprise the cold dark matter could be forthcoming in the next decade (Sadoulet, 1999, Griest and Kamionkowski, 2000).
Our state of understanding of the origin of the various components comprising the universe varies widely. If inflation is correct, then photons arose from the decay of the potential energy of the scalar field that drove inflation. The existence of quark-based matter that we are made of involves three elements: the action of microscopic forces that do not conserve the net number of quarks (baryon-number violation) and break the symmetry between particles and antiparticles (referred to as CP violation), and a departure from thermal equilibrium. These three conditions, first spelled out by Russian dissident and physicist Andrei Sakharov in 1967, are necessary for the universe to develop a slight excess of baryons over antibaryons. When the universe was around 10-5 sec old, the bulk of the baryons and antibaryons annihilated, leaving the few residual baryons for every 10 billion photons that now constitute the ordinary matter we see today. The details of "baryogenesis" - which may even involve neutrinos - are not fully understood. Critical in this regard is a better understanding of CP violation and neutrinos.
In some ways the emergence of neutrinos and cold dark matter from the post-inflation thermal bath is better understood. At very early times, when temperatures were extremely high, a kind of particle democracy existed, with roughly equal numbers of all particle types. As the temperature dropped below the point where a given species could be still be produced in pairs (i.e., where the thermal energy kT is less than its rest mass energy mc2), the numbers of those particles and antiparticles decreased rapidly through mutual annihilation. Because of their small masses and their small annihilation cross sections, neutrinos never annihilate, leaving them as abundant today as CMB photons. Once their masses are known, their contribution to the mass density follows directly. From what we know about neutrino masses, their contribution is at most comparable to that of ordinary matter, too small to account for the bulk of the dark matter. Nonetheless, neutrinos validate the basic idea of dark matter in the form of something other than baryons and do play a small role in the formation of structure.
The neutralino story is more complicated. Cosmic neutralinos do annihilate significantly; however, once their abundance falls to about 1 per billion photons, they become so rarified that they cease annihilating. Their relic abundance, determined by their annihilation cross section, turns out to be in the right range to account for the dark matter.
Dark energy is the largest and most perplexing component of the universe. The simplest possibility, that it is the energy associated with quantum vacuum fluctuations, suffers from the fact that calculating how much "quantum nothingness weighs" has eluded theorists for more than five decades and naive estimates are at leats 55 orders-of-magnitude too large! This suggests to some that ultimately it will be shown that "even quantum nothingness weighs nothing," because that outcome seems more likely than finding a means to reduce the naive estimate by precisely 55 orders-of-magnitude or more. Yet, if the dark energy is not quantum vacuum energy, what is it? A host of possibilities have been suggested, from a mild version of inflation involving an extremely light scalar field to the influence of new physics occurring in the extra spatial dimensions, as predicted by superstrings. However, the small magnitude of the dark energy is not the only problem. Another is trying to understand why at this point in time, the dark energy is only just beginning to dominate the expansion. It seems an odd coincidence, and one that so far defies explanation.
Finally, what can we say about the destiny of the universe? In the simple universe containing only matter, the destiny of the universe is linked to its geometry: uncurved and negatively curved universes expand forever; and positively curved universes recollapse. While we have determined that we live in an uncurved universe, adding dark energy to the mix severs the link between geometry and destiny. Depending upon the nature of the dark energy, a flat universe can continue accelerating forever (if the dark energy is quantum vacuum energy), or it can slow down or even recollapse if the dark energy dissipates with time (Krauss and Turner, 1999).