The Big Bang theory originated in 1929 when Edwin Hubble discovered that everything in the universe is moving away from everything else. Hubble explained this by theorizing that the universe was expanding in every direction.

Stages in Development of Universe:

1. Initially all matter forming the universe existed at one place in the form of a “tiny ball” (singular atom) with an unimaginably small volume, infinite temperature and infinite density.
2. At the Big Bang the “tiny ball” exploded violently. This led to a huge expansion. The event of big bang is believed to have taken place approximately 13.7 billion years ago.
3. The expansion continues even to the present day. As it expanded, some energy wasconverted into matter. There was particularly rapid expansion within fractions of a second after the bang. Thereafter, the expansion has slowed down. Within first three
   minutes from the Big Bang event, the first atom began to form.
4. Within 300,000 years from the Big Bang, temperature dropped to 4,500 Kelvin and gave rise to atomic matter. The universe became transparent.

Evidences which Support Big Bang Theory:

1. According to Hubble’s Law, galaxies are moving away from us at speeds proportional to their distance. This phenomenon was discovered by Edwin Hubble (1929). This observation supports expansion of the universe and also suggests that the universe
   was once compacted at a point.
2. The phenomenon of Red Shift also supports the theory. As the distance between earth and galaxy increases, the light appears to shift towards the red end of the spectrum. Red colour has the highest wavelength among visible spectrum. Further
   away the galaxy, greater the red shift.
3. As the universe was initially very hot, we should be able to find some remnant of this heat. In 1965, Radio-astronomers Arno Penzias and Robert Wilson discovered a 2.725 degree Kelvin (-270.425 degree Celsius) Cosmic Microwave Background Radiation (CMBR) which pervades the observable universe. Both of them shared Nobel Prize for Physics for their discovery in 1978.
4. Light elements like Hydrogen and Helium found in abundance in the observable universe also support the Big Bang theory.

Steady State Theory:

1. In 1940s, Sir Fred Hoyle and others developed an alternative mathematical model of the universe that did not start in a massive expansion as in Big Bang Theory.
2. According to them, matter is continuously created at a rate that keeps the average density of the universe same as it expands.
3. Though the Steady State theorists’ ideas are largely discredited today, their research pushed the Big Bang supporters to back up their theory with evidence.

The Big Bang Missions to Study

1. Wilkinson Microware Anisotropy Probe (WMAP) : It was the second mission to investigate cosmic background radiation (WMAP).
   • It surveyed the entire sky with much higher resolution than COBE, measuring temperature differences in microwave radiation that is nearly uniformly distributed.
   • By combining this evidence scientists have concluded that the Universe is “flat,” which means that the geometry of space satisfies the rules of Euclidean geometry on cosmological scales (e.g., parallel lines never meet, the ratio of circle circumference to diameter is pi, etc.
2. Planck: The European Space Agency launched Planck, a third mission with significant participation from NASA in 2009.
   • Planck is creating the most precise maps of microwave background radiation to date.
   • It measures the fluctuations of the temperature of the CMB with an accuracy set by fundamental astrophysical limits.

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