It says that the universe started with an infinitely hot and dense singularity then inflated to the cosmos that we know today.

Much of what we understand today about the Big Bang theory comes from mathematical formulas and models. Today majority of the astronomical community accepts the theory with some theorists have alternative explanations besides the Big Bang, such as eternal inflation or an oscillating universe.

Around 13.7 billion years ago, everything was an infinitesimally small singularity. Then explosive expansion began, faster than the speed of light, a period of cosmic inflation that lasted mere fractions of a second according to physicist Alan Guth’s 1980 theory.

The light of the Big Bang sometimes called the “afterglow” and is commonly known as Cosmic Microwave Background (CMB). First predicted by Ralph Alpher and scientists in 1948 and accidentally found about 20 years later.

After the initial expansion the universe cooled sufficiently to form the subatomic particles and later atoms. These primordial giant clouds elements later forming early stars and galaxies, the descendants of which are visible today.

Besides these primordial elements there is also the gravitational effects of an unknown dark matter surrounding galaxies. This leads to the expanding universe model.

Georges Lemaitre, a Belgian cosmologist and a Catholic priest, in 1927 noted that the expanding universe could be traced back in time. He also theorized that the universe began from a single primordial atom. He first suggested the Big Bang theory in the 1920’s. Later on, Edwin Hubble, an American astronomer and cosmologist, confirmed through analysis of galactic redshifts in 1929 that galaxies are indeed drifting apart, important evidence for an expanding universe.

These two scientists put the fundamental aspects and developed the foundation of the Hubble-Lemaitre law, the law that describes the observation in physical cosmology that galaxies are moving away from the Earth at speeds proportional to their distance. In other words : the farther the galaxies are, the faster those galaxies are moving away from Earth.

For several decades, the scientific community was divided between supporters of the Big Bang and the steady-state model. In 1964, the CMB was discovered that convinced many cosmologists that the steady-state theory was falsified.

The Big Bang is now essentially universally accepted after a wide range of empirical evidence strongly favors the theory due to its comprehensive explanation for a broad range of observed phenomena. The theory depends on two major assumptions : the universality of physical laws and the cosmological principle.

The earliest phases of the Big Bang are subject to much speculation, since astronomical data about them are not available.

One the common misconceptions about the Big Bang model is that it fully explains the origin of the universe but indeed the Big Bang does not describe how energy, time and space were caused but rather it describes the emergence of the present universe. How the initial state of the universe originated is still an open question, but the Big Bang model constrains some of its characteristics.

How will the future of universe will be ? Before observations of dark energy, cosmologists considered two scenarios : First, if the mass density of the universe were greater than the critical density the universe would reach a maximum size and then begin to collapse, becoming denser and hotter again.

Second, if the density in the universe were equal to or below the critical density the expansion would slow down but never stop. Stars would burn out, black holes would be larger and larger and the temperature of the universe would approach absolute zero. The universe would end in a cold, dark state where no organized form of energy could be extracted from it, or known as the heat death.

The Big Bang for regular people.

Source :

• Wikipedia
• Space.com