3.6.4  Cosmic Inflation

All Big Bang models, in general, speculate that the Universe was filled with a very high energy density, and huge temperatures and pressures which caused rapid expanding and cooling. According to the cosmic inflation scenario, approximatelyseconds after the singularity, the size of the Universe grew exponentially, which caused density fluctuations, in the otherwise homogeneous and isotropic Universe, that occurred because of the uncertainty principle. Due to the effect of gravity and collisions, these fluctuations were soon amplified into the seeds that would later form the large-scale structure of the Universe, including galaxies and clusters. After this inflation epoch, that lasted untilseconds after the singularity, the Universe continues to expand, but at a less rapid rate. During inflation the temperature drops by a factor of more than 100,000, depending on different models. This relatively low temperature is maintained during the inflationary phase, but when inflation ended the temperature returns to the pre-inflationary temperature, because the large potential energy of the inflation field decays into matter particles and electromagnetic radiation, that dominate the Universe. This is called the radiation phase and the process that led to that is called reheating or thermalization.

Reheating occurred until the Universe obtained the temperatures required for the production of a quark-gluon plasma and other elementary particles. Temperatures were so high that the random motions of particles were at relativistic speeds, and particle-antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point, baryogenesis violated the conservation of baryon number, leading to a very small excess of quarks and leptons over anti-quarks and anti-leptons, of the order of one part in 30 million, which resulted in the predominance of matter over antimatter in the present Universe. A few minutes into the expansion, when the temperature was about one billion kelvin and the density was about that of air. Under these conditions, neutrons combined with protons to form the deuterium and helium nuclei in a process called nucleosynthesis, while most protons remained uncombined as hydrogen nuclei. After about 379,000 years, the electrons and nuclei combined into hydrogen atoms; hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is known as the cosmic microwave background radiation, which was predicted in 1948 and discovered in 1964.

As we noted in section 6.3, inflation theory was introduced Alan Guth in 1979, and was developed further in the early 1980s. Many physicists also believe that inflation explains why the Universe appears to be isotropic, the same in all directions, or why the cosmic microwave background radiation is distributed evenly, why the Universe is flat, and why no magnetic monopoles have been observed as we shall discuss further in sections 9.18 and 9.8.

The basic inflationary paradigm is accepted by most scientists, as some predictions have been confirmed by observation, but other scientists are still dissent from this position. In 2002, three of scientists were recognized for their major contributions in this field: Alan Guth, Andrei Linde (b. 1948), and Paul Steinhardt (b. 1952), who shared the prestigious Dirac Prize “for development of the concept of inflation in cosmology”. In 2012, Alan Guth and Andrei Linde were awarded the Breakthrough Prize in Fundamental Physics for their invention and development of inflationary cosmology.