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Cosmic Snapshots

We saw in the last article that the earlier phases of the universe would have been very hot and filled with particles of high energy. This has led to an influx of ideas from high energy particle physics that has enriched our understanding of cosmic evolution. One such idea which shot into prominence in the Eighties is that of the “inflationary universe”. This has helped cosmologists to look at the universe in an entirely new light.

To understand the importance of this, it is first necessary to outline a key difficulty in the usual models.

In an expanding universe, physical processes cannot act as quickly as in a static one. Suppose we keep some gas inside a container and maintain different temperatures in its left and right halves. Within a very short time, the hot gas will mix with the cold and a common temperature will be reached inside the container. But if the walls of the container were expanding, then the equality of temperature cannot be reached easily. Regions near the centre of the container where the hot and cold gases are close to each other will attain equal temperature after some time and slowly the entire amount of gas will try to attain a constant temperature. Whether this constancy can ever be achieved in an expanding container depends on two factors: one, the rate at which gas molecules interact with each other and exchange energy; and two, the rate at which the gas is expanding. By changing one of the two factors, we can prevent the hot and cold portions from getting mixed.

It turns out that in the radiation-dominated universe, the expansion rate is such that no significant mixing can take place. In other words, if regions of the universe had different temperatures to start with, they will never come to equilibrium and reach the same temperature. Observations, however, show that the temperature in the early universe within the region that one can observe was the same at all spatial locations. In conventional cosmological models, this is a mystery.

The inflationary models of the universe make the universe expand much more rapidly than the conventional models. The idea is to inflate a small patch of the universe with uniform temperature to produce our entire observed universe. Since the physical conditions were uniform within the small patch initially, it is easy to account for the large-scale uniformity of the universe in the inflationary models.

What drives the universe into such a phase of rapid expansion? Normally it is the energy density of matter that causes the universe to expand. As the universe expands, this energy density decreases and, consequently, the expansion rate also slows down. During inflation the energy density remains constant, thereby maintaining a constant rate of expansion even though the volume of the region is increasing. In order to achieve this, the inflationary phase of the universe should have negative pressure. For normal systems that we are accustomed to (like a balloon filled with gas), pressure is a positive quantity. If the balloon expands, the energy content inside it will decrease. But for a system with negative pressure, the energy content will increase if the system expands. It turns out that certain models of particle physics use quantum fields which have negative pressure in the above sense. What inflationary models do is arrange matters such that these quantum fields dominate the expansion of the universe for a short span of time in the very early universe.

The inflationary models have another significant fallout for our universe. The present day universe contains several inhomogeneous structures like galaxies, clusters, etc. It is believed that these structures have originated from the growth of small inhomogeneities present in the early universe. In order to develop a complete theory of structure formation, one has to understand how these small inhomogeneities came into being in the first place. This problem is not adequately answered in conventional cosmology. In fact, the expansion rate of a radiation-dominated universe is not conducive to producing inhomogeneities of the correct type. Inflation of the universe helps in this regard as well. The quantum fields which were driving the inflation will also contain small fluctuations in the energy density in a natural manner. These small fluctuations can grow and form the seeds for structures like galaxies. Given any particle physics model which can produce inflation, one can also compute the kind of fluctuation that would have been generated.

How can we test such models? Rather, how can we be sure that the universe ever underwent an inflationary phase? Since the inflation is supposed to have occurred at a very early phase of the universe (at about 10-35 seconds after the Big Bang), it is not possible to test these models by direct observations. The typical energy of particles in the universe during inflation is far higher than the energies achieved in the laboratory; hence it is also not possible to verify the particle physics models leading to inflation directly in the laboratory.

However, some indirect verification is possible. Each inflationary model predicts a particular spectrum of fluctuations for the inhomogeneities of the universe. The small temperature anisotropies (anisotropy means having properties that differ according to the direction of measurement) in the microwave background radiation contains information about these fluctuations. By comparing these observations with predictions for inflationary models, one can validate or rule out these models. The current observational data on the microwave background fluctuations is consistent with the inflationary models, giving a boost to this idea. Many theoreticians thus believe that inflation is a good thing and is here to stay!

Source: The Telegraph (Kolkata, India)