We saw in the last episode that physical conditions in our universe change because of its expansion. By tracing the expansion of the universe back in time, one can try to understand the physical conditions that would have existed in the past. Such an exercise leads to a very fascinating picture about the early universe.

The most important effect of expansion is the decrease in the density of matter and radiation present in the universe. In the past, the universe would have been denser in its content. Today, the matter density in the universe is about one hydrogen atom per cubic metre of space. However, when the universe was 10 times smaller, the radiation energy would have been 10,000 times higher. This is because the wave length of the photons also changes with the expansion of the universe and would have been shorter at earlier times. Lower wave lengths correspond to higher energies for photons and this leads to an additional increase in the energy of the radiation as compared to that of matter. Today, the radiation energy density is about 1,000 times smaller than the energy density of matter. But since radiation energy increases faster than matter energy density, it would have caught up with the latter in the past and the early universe would have been dominated by radiation.

The temperature of the radiation also goes up as we go into the past. When the universe was 1,000 times smaller, it would have also been 1,000 times hotter, with a temperature of about 2,730 Kelvin (2,500 oC). The typical temperature of the universe at any given time can be obtained from theoretical considerations. It is estimated that when the universe was about one second old, its temperature was about 10,000 million Kelvin. The typical energy of a particle in the universe at this epoch would have been about a million electron volts (denoted as “MeV”) which is roughly the energy involved in many nuclear processes.

What was the material content of the universe at this time? It turns out that the universe at this time was a hot soup of elementary particles – protons, neutrons, electrons, positrons, photons and neutrinos. Among these particles, protons, neutrons and electrons are the basic constituents of normal matter and are familiar to us. Positrons are identical to electrons except that they carry positive electric charge (while electrons carry negative electric charge). Neutrinos are particles like electrons with a very tiny mass and no electric charge and they interact only very weakly with the rest of the matter. For all practical purposes, we can ignore the existence of these neutrinos in our discussion.

There is a simple reason for the existence of positrons in the hot universe. The mass of the electron (or positron) is equivalent to an energy of about 0.5 million electron volts. When the universe was one second old, the energy of, say, a typical photon was about 1 MeV. Since energy and mass are inter-convertible, electron-positron pairs can be produced out of energetic photons at these high temperatures. As the universe expanded and cooled, this process became ineffective and electron-positron pairs ceased to be produced. The existing electron-positron pairs, however, annihilated each other producing more radiation. When the universe cooled to about 1,000 million Kelvin, most of the positrons disappeared through their annihilation with electrons. The resulting universe contained protons, neutrons, electrons and photons. Among these particles protons are positively charged and electrons are negatively charged. They exist in equal numbers maintaining the overall charge neutrality of matter.

The temperature of the universe was still too hot to form neutral atoms. However, it was just about cool enough to form atomic nuclei. The simplest atomic nucleus, of course, is hydrogen which is just a single proton. Combining a proton and a neutron, one can form an “isotope” of hydrogen called deuterium; by combining two deuterium nuclei, one can obtain a helium nucleus with two protons and two neutrons and so on. As the universe cooled, it would have been possible — in principle — to form all these nuclei. In practice, however, it was not possible to synthesise these atomic nuclei in large quantities, mainly because the expansion of the universe prevented a sufficient number of protons and neutrons from getting together. Detailed calculations show that only helium was produced in significant quantities. When the universe was about three minutes old, it would have contained about 75 per cent (by weight) of hydrogen, about 25 per cent of helium and small traces of deuterium and other heavier elements.

As time went on, the universe continued to expand and cool. Curiously enough, nothing of importance occurred until the universe was about 700,000 years old! By this time, the temperature of the universe would have fallen sufficiently low to form atomic systems. The electrons combine with hydrogen and helium nuclei forming a neutral gas of atomic hydrogen and helium. As long as free charged particles like electrons or protons existed, photons would have undergone repeated scattering and hence would have remained coupled to matter. The formation of neutral atoms makes the radiation decouple from the matter and propagate freely. These photons continue to exist in the universe, forming a thermal background all around us. Such a background was discovered by A.A. Penzias and R.W. Wilson in 1965 for which they were awarded the Nobel Prize. This background radiation is considered to be a definitive piece of evidence in favour of the cosmological scenario outlined above and, in the last two decades, this radiation has given us fascinating clues about the universe.

T. Padmanabhan is an astrophysicist at the Inter University Centre for Astronomy and Astrophysics, Pune