So we would expect a “wall of light” called cosmic background radiation released from the early universe to reach us now at near the speed of light. Since the universe is expanding, this light should be redshifted into the microwave region of the spectrum of colour. Since the universe is also isotropic and homogeneous, this microwave glow should be observed from all directions in the sky and with uniform intensity.
In 1965, it was discovered that a nearly uniform glow of microwave radiation was coming from all directions in the sky. Later satellite observations proved that the microwave radiation belonged to the blackbody spectrum of colour, which was predicted for a hot flowing gas of the early universe.
The Big Bang singularity
If the universe is expanding, then rewinding back in time compresses the universe towards an extremely small dense and hot initial state until the volume of the universe reaches zero at the Big Bang singularity.
In the Big Bang singularity, there is neither matter nor energy nor space nor time. In other words, there is nothing and nowhere in and prior to the singularity. The Big Bang created matter, energy, space and time.
The universe, therefore, began to exist with a Big Bang. This is the standard big bang theory of the universe.
Cosmological inflation
However the standard big bang model does not explain the isotropic and homogeneous nature of the universe.
For example, the temperature of the cosmic background radiation from different regions of the early universe is very nearly the same. In the standard big bang model before the formation of atoms, the photons were scattered by the electrons. So, light would have been prevented from heating up the different receding regions of the early universe to the same temperatures.
From the 1980’s, inflation theory developed to explain the isotropic and homogeneous nature of the universe.
At the 10-35 second after the big bang, the rapidly cooling universe became trapped in a “false” or temporary vacuum in which gravity, which is ordinarily an attractive force between each and every body in the universe, repelled regions of the early universe into an extremely fast expansion. This flattened space and allowed the regions already in communication to become spread over vast regions. With cosmological inflation, the universe cools to very low temperatures, and this generates a transition to the true vacuum state releasing vast amounts of energy that reheated the universe that condensed to the universe we have today.
Quantum mechanics predicts tiny rapid fluctuations in local energy in a vacuum. Inflation would expand these microscopic irregularities so greatly allowing the gravitational clustering required to form galaxies. Since the vacuum contains microscopic irregularities that inflation would expand, small temperature fluctuations in cosmic background radiation would be expected. In 1992, the COBE satellite detected small temperature fluctuations in the cosmic background radiation. So there is evidence for inflation theory.
However, inflation does not explain the period before 10-35 second and particle physics has not yet explained the origin of the “false” vacuum.