“Heat death” of infinitely-lived universe
So if the universe always existed it must have existed for an infinite period. If the universe existed for an infinite period, it must have already suffered a “heat death” and be at maximum entropy in thermodynamic equilibrium with no heat, work or other energy available to do any further work. Since it is also a closed system, it cannot acquire heat, work or other energy from its environment.
And yet as is explained below, we observe that the universe is expanding as Edwin Hubble discovered in 1929.
Therefore by the laws of thermodynamics, the universe cannot always have existed and must have begun to exist.
Standard big bang model
In 1929 Edwin Hubble analysed the light of huge distant groups of billions of stars called galaxies to show that, most were receding from Earth at tremendous speeds.
He compared the light of certain giant stars in these galaxies to those of similar stars in our own galaxy whose distances were known. He observed that the light of these receding galaxies were shifted away from their normal wavelengths toward the red long-wavelength part of the spectrum of colour.
This redshift effect proved that the space in the universe was expanding, carrying the galaxies along in it and stretching the wavelengths of light emitted from these galaxies towards the red part of the spectrum of colour.
Hubble’s study also provided evidence of the cosmological principle that the universe is isotropic or looks about the same in all directions, and is homogeneous or is about the same everywhere. Every cluster of galaxies, including our own, is receding away from all others in space as the universe expands so that an observer anywhere in the universe would see about the same thing.
Rewinding back to a few hundred thousand years from the beginning, electrons would have detached from their atoms in temperatures of thousands of degree Fahrenheit. At about a second from the beginning, the nuclei of atoms would have disintegrated into neutrons and protons in temperatures of billions of degrees. Even earlier than that, neutrons and protons would have disintegrated into quarks that would be embedded in a soup of mainly gamma ray radiation.
Moving forward in time again, as the universe rapidly cooled, a small proportion of protons and neutrons would have fused into elements heavier than hydrogen. So a few minutes after the big bang when this fusion had ended, the cooling gas would have consisted of nearly 75 per cent hydrogen and 25 per cent helium with trace elements of deuterium and lithium. This 3:1 hydrogen to helium ratio should, therefore, be present in the universe today.
Spectroscopic studies show that the visible matter in the universe is mostly hydrogen and helium in the 3:1 ratio predicted by the standard big bang theory. The small amounts of heavier elements such as present in the Earth and organisms were derived from the explosion of stars that produce these heavier elements by fusion reactions in their cores.
Before the formation of atoms about 400,000 years after the big bang, light particles called photons would have scattered off the electrons that would later form part of atoms. Once atoms began to form, light would have a clear path through space.