THE EARLY UNIVERSE

Cosmology Primer:The Early Universe

We have no direct knowledge of what the universe was like before the Big-Bang nucleosynthesis era, when the universe was between a few seconds and a few minutes old . It is worth emphasizing that any ideas we have about earlier times are only that ideas. Nevertheless, just as we can successfully extrapolate the laws of physics from the present day back to the time of nucleosynthesis, we may also extrapolate these laws even further back, to construct a picture of what the very early universe may have been like.

In a universe dominated by matter and radiation (as opposed to dark energy), the mutual gravitational pull of all the particles tends to slow down the expansion rate as the universe expands. When the universe was smaller and more dense, it therefore follows that the expansion rate was much larger than it is today. Indeed, as we extrapolate the universe further back in time, we reach a point where the density, temperature, and expansion rate were all infinitely large. This point is a singularity, which we refer to as the Big Bang (although that term is also used for the entire cosmological model that includes the later universe as well). At the Big Bang, our knowledge of what happens gives out; the fact that physical quantities become infinite is a sign that we don't know what is going on. Presumably, in the real world there is no singularity; instead, something happens that cannot be described by physics as we currently understand it.

Just because we don't understand the Big Bang itself doesn't mean we can't usefully talk about the period immediately afterwards, when the universe was in a hot, dense, rapidly expanding state. In the absence of a sensible theory of the origin of the universe, cosmologists ask what initial conditions are necessary to explain the observed features of our universe today. But in fact we want more than that; we would like to believe that these initial conditions are somehow natural, rather than arbitrarily finely-tuned. This desire may or may not be accommodated by reality, but has led to a great deal of interesting speculation about the very early universe.

One puzzle we have about the universe is the apparent dominance of matter over antimatter. Every type of elementary particle (electrons, protons, neutrons, and so on) has a corresponding type of antiparticle (positrons, antiprotons, antineutrons) of equal mass and opposite electric charge. But what we observe in the universe is overwhelmingly matter and not antimatter, which we know because matter and antimatter tend to explosively annihilate when they come into contact with each other. If other galaxies, for example, were made of antimatter, there would be regions in between where particles would intermix, giving rise to high-energy radiation that has not been detected. It is possible that this asymmetry between matter and antimatter is simply a feature of the initial conditions of the universe, but it would seem more satisfying if we could explain how it arose dynamically as the universe evolved. Such a hypothetical process is known as "baryogenesis," since the observed imbalance between matter and antimatter is actually an imbalance between baryons (protons and neutrons) and antibaryons. There are numerous models of baryogenesis, many of which may be testable at upcoming particle accelerators; to date, however, no single model has proven so successful that it has been accepted as a standard picture.

overdense regions collapse to form stars and galaxies. But the finite speed of light makes the situation even more surprising. The CMB shows us what the universe was like 370,000 years after the Big Bang. But when we observe widely separated parts of the CMB, we are seeing regions of the universe that were much more than 370,000 light-years apart at that time. In other words, there was not enough time for any signal to travel from one region to another. So how do these separated regions know that they should be at the same temperature? This conundrum is known as the "horizon problem," since the finite distance light can travel since the Big Bang defines an horizon around each point, and the horizons of distant parts of the microwave sky do not overlap.

Unlike the puzzle of the baryon asymmetry, the horizon problem does have a popular solution -- the idea of inflation. One way of thinking of inflation is to simply imagine that the very early universe went through a period where it was temporarily dominated by an extremely large amount of dark energy, which then suddenly decayed into ordinary matter and radiation. This inflationary dark energy caused the universe to accelerate at a fantastic rate, taking nearby points and moving them very far apart -- so that the widely-separated regions we observe in the CMB were actually quite nearby and in contact early on. This elegant solution to the horizon problem comes along with extra benefits. For one thing, the process of inflation takes any initial curvature of space and diminishes it to near zero, explaining the observed flatness of the universe. For another, inflation wipes out any particles that may have existed before inflation began, which is useful if we don't observe those particles today. An example is provided by "magnetic monopoles," which some theories of particle physics predict should exist in copious amounts, even though none has ever been detected. The desire to get rid of magnetic monopoles was actually the original motivation that led to the invention of inflation by Alan Guth in 1980.

Inflation has another unanticipated benefit: it provides a possible origin of the primordial density perturbations that lead to temperature anisotropies in the CMB and ultimately grow into the large-scale structure we observe today. This phenomenon arises from considering inflation in the context of quantum mechanics. Modern physicists understand that classical mechanics, in which particles have definite positions and velocities, is only an approximation to quantum mechanics, in which such quantities are subject to a certain irreducible uncertainty. The same thing holds true for the density of an expanding universe. Thus, while inflation does its best to make the universe absolutely uniform, quantum mechanics prohibits it from doing so; there is always a small amount of fluctuation in the amount of energy from place to place that no amount of inflation can erase. Indeed, we can use the rules of quantum mechanics to predict what kinds of fluctuations should arise from inflation. The result is a set of perturbations of approximately equal strength at all distance scales. As mentioned in the page on the early universe, these are precisely the kind of fluctuations needed to explain what we observe in anisotropies of the CMB. This doesn't mean that inflation is necessarily correct, but certainly provides some evidence in its favor.

But there remains a great deal that we don't understand about inflation. In particular, while the general framework remains attractive, no specific model of inflation has become popular. In other words, we don't know exactly what this mysterious dark energy was that dominated the universe at very early times, nor how it converted into ordinary matter and radiation. A possible clue could come from another prediction of inflation: gravitational waves. Just as quantum mechanics predicts irreducible fluctuations in the density of matter during inflation, it also predicts fluctuations in the gravitational field, which manifest themselves as gravitational waves. These waves can lead to a specific unmistakable signature in the polarization of the microwave background. Unfortunately, we don't know for sure how strong these gravitational waves will be, and they might be so weak as to be undetectable. But cosmologists are planning experiments to look for them, and if they are detected it will be a great triumph for inflation.

In a sense, inflation hides from view anything that came before it. Nevertheless, we are still curious about the very origin of the universe, and the conditions that gave rise to inflation (if indeed it happened). Presumably any sensible description of this epoch will involve quantum gravity (the long sought-after reconciliation of quantum mechanics with Einstein's general relativity), and perhaps require an understanding of more esoteric physics such as superstring theory.