THE BIG BANG THEORY

               What is the Big Bang Theory?

The Big Bang Theory is a cosmological model of the observable universe that describes how the universe began from a tiny, densest point.

According to this theory, the Universe expanded from an extremely hot, tiny and extremely dense state. Since then, it has begun to expand, and its density and temperature have decreased. All the matter and energy in the universe, and even space itself, came out of this explosion.

The Big Bang is the best model used by astronomers to describe the creation of matter, space, and time 13.7 billion years ago.

Georges Lemaître first proposed the Big Bang Theory in the 1920s, when he theorized that the universe originated from a single “Primeval Atom”.

Astronomer Sir Fred Hoyle first coined the term "Big Bang" during a series of radio lectures on Astronomy in the 1950s. However, he never accepted the Big Bang Theory because of his belief in the "Steady-State Theory" of the universe.

.                                           Pic credit- Gravitational waves generated by cosmic inflation are the farthest signal back in time humanity can conceive of potentially detecting, which originate from the end of cosmic inflation and the very beginning of the hot Big Bang. Image credit: National Science Foundation (NASA, JPL, Keck Foundation, Moore Foundation, related) — Funded BICEP2 Program; modifications by E. Siegel.

                   AFTER THE BIG BANG

What happened to the universe the moment after the Big Bang?

The universe was originated with the Big Bang as an extremely hot, dense point. When the universe was just 10^-34 of a second or so old, which is equivalent to a hundredth of a billionth of a trillionth of a trillionth of a second in age — it experienced an incredible burst of expansion known as Inflation, wherein the space itself expanded faster than the speed of light.

The inflationary era lasted from 10^−36 seconds after the conjectured Big Bang singularity to some time between 10^−33 and 10^−32 seconds after the singularity. After the inflationary period, the universe continued to expand, but at a slower rate.

As space expanded, the universe cooled down and matter formed. A second after the Big Bang, the universe was filled with neutrons, protons, electrons, anti-electrons, photons and neutrinos.

During the first three minutes of the universe, the light elements were born in a process called Big Bang nucleosynthesis. Temperatures cooled from 100 nonillion (1032) Kelvin to 1 billion (109) Kelvin, and protons and neutrons collided to produce deuterium, an isotope of hydrogen. Most of the deuterium combined to produce helium, and a trace amount of lithium were also generated.

For the first 380,000 years or so, the universe was essentially too hot. The generated heat smashed the atoms into a high-density plasma with sufficient force, which is an opaque mixture of proton, neutron and electron that scattered light.

About 380,000 years after the Big Bang, matter cooled enough to form atoms in the recombination era, resulting in a transparent electrically neutral gas.

Roughly 400 million years after the Big Bang, the universe began to emerge from the cosmic dark ages during the epoch of reionization. During this time, which lasted more than a half-billion year, matter began to condense into gas clouds, forming the first stars and galaxies, and its high-energy ultraviolet light ionized and destroyed most of the neutral hydrogen. Galaxies were collected into groups, clusters, and super-clusters.

Although the expansion of the universe gradually slowed down as the matter in the universe pulled on itself through gravity, about 5 or 6 billion years after the Big Bang, a force called Dark Energy began to accelerate the expansion of the universe again, a phenomenon that continues today. After the universe has been more than 9 billion years ago (~4 billion years ago), the acceleration of this expansion caused by dark energy began.

Approximately, 9 billion years after the Big Bang, our solar system was formed.

     EVIDENCES OF THE BIG BANG THEORY

What evidence is there to support the Big Bang theory?

Some major scientific discoveries that strongly support the Big Bang theory:

1) Redshift of Galaxies-

The redshift of distant galaxies means that the Universe is probably expanding. The distant galaxies (or galaxy clusters) are systematically moving away from us at increasing speeds. As a result of General Relativity, this means that space itself is expanding, carrying the galaxies with it.

2) The discovery of the Cosmic Microwave Background Radiation (CMBR)

According to the Big Bang Theory, the Universe was initially extremely hot and dense. As it expanded, it cooled down. The cosmic microwave background radiation (CMBR) observed by Arnold Penzias and Robert Wilson is the thermal radiation left over from the Big Bang. Today, CMBR is very cold due to the expansion and cooling of the universe. It is only 2.725 Kelvin (-270.4 °C), which is only 2.725 °C above than absolute zero.

3) Mixture of Elements

According to the Big Bang Theory, the temperature of the early universe was so high that fusion reactions could take place. This led to the formation of light elements: Hydrogen, Deuterium, Helium (two isotopes), Lithium and trace amounts of Beryllium.

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                FACTS ABOUT THE BIG BANG

1) The spacecraft WMAP determined the age of the universe to be 13.7 billion years.

Various predictions and calculations indicated that the age of the universe is approximately 14.5 billion years. Very precise imaging of the background radiation was used to obtain a more accurate number of 13.7 billion years.

2) The standard Big Bang Theory did not work well for the very early universe.

In the 1980s, physicists Alan Guth and Andrei Linde developed the Theory of Inflation, which took us back further and explained the irregularities in the background radiation.

3) Planck Time is 10^-43 seconds after the Big Bang.

It is believed that all the forces of the universe (gravity, strong nuclear, weak nuclear and electromagnetic) were combined into a single force prior to this moment in time. At the Planck Time, gravity is considered to have been separated from other forces.

4) The Inflationary theory tells that the universe expanded rapidly.

Between 10^-36 seconds to 10^-32 seconds after the Big Bang, the universe briefly expanded at multiple times the speed of light. Since then, the rate of expansion has been much slower. It also explains the observed ratio of hydrogen to helium.

5) The elementary particles formed in the first 3 minutes after the Big Bang.

The first particles formed were the quarks and as the universe expanded and cooled, they combined to form protons, neutrons and other particles.

6) The early universe was opaque.

In the early universe, light could not travel very far before being scattered by electrons. Between 240,000 and 300,000 years after the Big Bang, most electrons were captured by positive nuclei and light could travel farther-the universe became transparent.

7) The irregularities in the primitive universe formed galaxies.

These irregularities were amplified by gravity and formed huge gas clouds, which evolved into galaxies.

8) 90% of the universe may be composed of dark matter and dark energy.

There are certain things in our universe that cannot be explained without the existence of "dark matter" and "dark energy".

           THE ACCELERATING UNIVERSE

The Universe Today: What It All Looks Like Now…

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The accelerating universe is the observation that the expansion of the universe is such that the velocity at which a distant galaxy is receding from the observer is continuously increasing with time. The accelerated expansion was discovered during 1998, by two independent projects, the Supernova Cosmology Project and the High-Z Supernova Search Team, both of which used distant type Ia supernovae to measure the acceleration.

The accelerated expansion of the universe is considered to have started since the universe entered the era of dark energy dominance about 4 billion years ago.

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 The rest of the universe seems to be composed of a mysterious invisible matter called dark matter (25%) and a force that repels gravity called dark energy (70%).

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SUCHISMITA