3.2. The Expansion
Hubble's discovery of the expansion of the universe changed our cosmic perspective forever - we now know that we live in an evolving universe with a big-bang beginning. Its interpretation within Einstein's theory of gravity was a striking confirmation of the dynamical nature of space and time - the expansion is due to the stretching of space itself. However, within the framework of general relativity there is no answer to the most basic question - what happened just before the big bang to get the expansion going? According to general relativity, the big bang was the singular creation of matter, energy, space and time itself. If correct, then there was no "before" before the big bang - a neat and tidy, if not entirely satisfying, answer.
Since Einstein's theory does not incorporate quantum mechanics, there is reason to believe that it is not complete or fully applicable around the time of the big bang. Answering the "before the big bang" question most likely still depends upon marrying quantum mechanics to gravity, and applying that merged theory to the earliest moments of the universe. Currently, superstring theory is the most promising idea for making such a connection, and we will return later to some of the speculations based upon it.
Even within general relativity there are conceptual questions about the beginning. Not all big-bang models necessarily lead to a universe like ours. Unless the initial conditions were "just so" (see, e.g., Hogan, 2000), the universe might well have recollapsed long ago; or it might have gone into a coasting phase of indeterminate duration; or the expansion need not necessarily have been the same in all directions (isotropic), or there might not necessarily have been large-scale regularity in the distribution of matter (homogeneity), as we observe in the universe today.
Apparently a special beginning is required. There are two ways to read this: The first is that the state of the universe today accurately pins down the initial conditions, a point of view advocated by a few (Penrose, 1979). The second is to look for a way to get around the special conditions, a road that led to the idea of cosmic inflation.
In 1980, Alan Guth pointed out that an early, brief period of very rapid (indeed exponential) expansion, now called inflation, could change the cosmic landscape dramatically (Guth, 1980). He had in mind a scenario wherein the universe got trapped in a "false-vacuum state" during a cosmological phase transition associated with the breaking of symmetry between the strong, weak and electromagnetic forces. Although this specific idea does not seem to work, it nevertheless led to a paradigm for inflation based on the potential energy associated with an as-of-yet hypothetical scalar field called the inflaton (Linde, 1983, Albrecht and Steinhardt, 1982). (If it exists, this field is distantly related to the Higgs scalar field that particle physicists believe explains why particles have mass.)
According to inflation, small bits of the universe underwent a burst of expansion when the universe was extremely young (t << 10-2sec). This expansion flattened the local geometry in the same way inflating a balloon makes a fixed region look flatter and smoother as the balloon inflates (in the case of the universe the blow-up factor exceeded a factor of 1040!). The conversion of the scalar-field potential energy into particles and photons accounts for the tremendous heat content of the big bang and initiates the early, radiation-dominated era. During the conversion of scalar field energy, quantum fluctuations in the inflaton field on subatomic scales, stretched to astrophysical size by the tremendous expansion, became the seed density inhomogeneities. Moreover, quantum fluctuations in the metric of spacetime itself lead to a predicted spectrum of long-wavelength gravitational waves.
Thus, cosmic inflation explains the smoothness of the universe, the heat of the big bang, and it predicts a flat universe with characteristic seed irregularities in the matter distribution and a spectrum of gravity waves. It also says that all we can see arose from "our big bang," one of many bursts of inflationary expansion that took place early on.
As of yet there is no single, agreed-upon model of inflation. However, all models are based upon the paradigm described above and make three firm predictions: (i) a spatially flat universe; (ii) a nearly scale-invariant distribution of density perturbations; and (iii) a nearly scale-invariant spectrum of gravitational waves.
In the early 1980s when inflation was gaining sway with cosmological theorists, its prediction of a flat universe looked to be its downfall. The best measurements indicated that matter contributed only about 10% of the critical density, although the uncertainties were quite large given that the amount of dark matter was still largely undefined. But as we saw above, recent determinations indicate that the universe is almost certainly flat, with dark matter contributing one third of the critical density and dark energy the other two thirds.
The seed inhomogeneities of inflation were impressed upon the universe very early on. This results in a kind of synchronized motion of irregularities on different length scales, and leads to a characteristic pattern of "acoustic peaks" in the multipole power spectrum of the CMB anisotropy. Measurements now show a clear pattern of acoustic peaks, cf. Figure 9. Their relative heights also indicate that the seed perturbations are consistent with being almost scale-invariant, again as inflation predicts.
Inflation has passed its first round of tests. Over the next decade additional observations, especially those coming from measurements of the CMB, will test inflation more decisively and may shed light on how the inflaton field fits into the larger picture of particle physics.
The spectrum of gravitational waves extending from wavelengths of kilometers to billions of light years are certainly beyond the reach of the current generation of Earth-based gravity-wave detectors. There is some hope that gravity-wave-induced polarization signatures in the cosmic microwave background can be detected with a new generation of experiments. If so, not only would the third prediction of inflation be tested, but also the time when inflation took place would be identified.
While inflation is making a good case for being included in the standard cosmological model, it does not address the biggest question, how the Universe began. Superstring theory shows promise of shedding more light on how the Universe began. With superstring's prediction that there are more than three spatial dimensions, it opens new dimensions in cosmology, both figuratively and literally. While there have yet to be successes or even compelling predictions, superstring theory has evoked intriguing cosmological ideas. The "brane world" idea holds that our universe is actually a three-dimensional "sheet" in a much higher dimensional space where gravity exists in the full spacetime, but other forces and particles are confined to only three dimensions. From this paradigm has come the idea that the big bang might have been the collision of two such sheets, and even further that such collisions happen repeatedly, adding new life to the old oscillating universe scenario. Such ideas are not yet well developed enough to be testable, but their creativity speaks to the vitality of cosmology. And it should be remembered that about twenty years ago inflation seemed radical and untestable!