We all are familiar of the term the universe and the Milky Way galaxy but there was a time billions of years ago when none of these existed. We can’t even imagine it right?

Well this is true and our universe, the galaxies all began with a bang, a big bang.

Scientists believe that the universe began with a bang about 15 millions of years ago. At the very beginning, the universe was just a minute hot ball, many times smaller than an atom. Inside this ball was everything to needed to make the universe.

In the 1930s an American astronomer Edwin P Hubble discovered that light from all distant galaxies is coming from a receding source. This is the only possible explanation of the light spectrum is shifted towards the red end. The real fact is due to Doppler’s effect .in 1948 George Gamov a Russian-born-American physicist in a theory concerned with the origin of the light elements, interpreted the Doppler effect as demonstrating the that the universe was expanding, which simply means that it had originated in a single point billions of years ago. This idea got wide support and British cosmologist Freud Hoyle in a highly critical and satirical manner referred to it as ‘Big Bang’ hypothesis.

There are evidences for the Big Bang theory:

• First of all, we are reasonably certain that the universe had a beginning.
• Second, galaxies appear to be moving away from us at speeds proportional to their distance. This is called "Hubble's Law," named after Edwin Hubble (1889-1953) who discovered this phenomenon in 1929. This observation supports the expansion of the universe and suggests that the universe was once compacted.
• Third, if the universe was initially very, very hot as the Big Bang suggests, 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 (-454.765 degree Fahrenheit, -270.425 degree Celsius) Cosmic Microwave Background radiation (CMB) which pervades the observable universe. This is thought to be the remnant which scientists were looking for. Penzias and Wilson shared in the 1978 Nobel Prize for Physics for their discovery.

Finally, the abundance of the "light elements" Hydrogen and Helium found in the observable universe are thought to support the Big Bang model of origins

A common analogy explains that space itself is expanding, carrying galaxies with it, like raisins in a rising loaf of bread. The graphic scheme above is an artist's concept illustrating the expansion of a portion of a flat Universe.

The Big Bang is the prevailing cosmological theory of the early development of the universe. Cosmologists use the term Big Bang to refer to the idea that the universe was originally extremely hot and dense at some finite time in the past and has since cooled by expanding to the present diluted state and continues to expand today. According to the best available measurements as of 2010^[update], the initial conditions occurred around 13.3 to 13.9 billion years ago.

Georges Lemaitre proposed what became known as the Big Bang theory of the origin of the Universe, although he called it his "hypothesis of the primeval atom". The framework for the model relies on Albert Einstein's general relativity and on simplifying assumptions (such as homogeneity and isotropy of space). The governing equations had been formulated by Alexander Friedmann. After Edwin Hubble discovered in 1929 that the distances to far away galaxies were generally proportional to their red shifts, as suggested by Lemaitre in 1927, this observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point: the farther away, the higher the apparent velocity. If the distance between galaxy clusters is increasing today, everything must have been closer together in the past. This idea has been considered in detail back in time to extreme densities and temperatures, and large particle accelerators have been built to experiment on and test such conditions, resulting in significant confirmation of the theory, but these accelerators have limited capabilities to probe into such high energy regimes. Without any evidence associated with the earliest instant of the expansion, the Big Bang theory cannot and does not provide any explanation for such an initial condition; rather, it describes and explains the general evolution of the Universe since that instant. The observed abundances of the light elements throughout the cosmos closely match the calculated predictions for the formation of these elements from nuclear processes in the rapidly expanding and cooling first minutes of the Universe, as logically and quantitatively detailed according to Big Bang nucleosynthesis.

Fred Hoyle is credited with coining the term Big Bang during a 1949 radio broadcast. It is popularly reported that Hoyle, who favored an alternative "steady state" cosmological model, intended this to be pejorative, but Hoyle explicitly denied this and said it was just a striking

Image meant to highlight the difference between the two models. Hoyle later helped considerably in the effort to understand stellar nucleosynthesis, the nuclear pathway for building certain heavier elements from lighter ones. After the discovery of the cosmic microwave background radiation in 1964, and especially when its spectrum (i.e., the amount of radiation measured at each wavelength) sketched out a blackbody curve, most scientists were fairly convinced by the evidence that some Big Bang scenario must have occurred.

Big Bang

According to the theory of big bang, the universe is thought to have expanded into existence from a single minute point, some 15 million years ago. This theory is now accepted as the standard model for the origin of the universe.

Just after the big bang, the temperature of the universe was roughly 10 billion degrees and was filled with a sea of neutrons, protons, electrons, positrons, photons, and neutrinos. As the universe cooled, the neutrons either decayed into protons and electrons or combined with protons to make deuterium.

Approximately 400,000 years after the Big Bang, the infant universe was cooled enough to allow the electrons to join up with nuclei to create the first atoms. The bouncing photons suddenly travelled in straight lines and the universe became transparent. Those photons, the afterglow of the big bang was formally known as cosmic background radiation. This is what we can observe today with the Wilkinson Microwave Anisotropy Probe (WMAP).

The Universe’s baby picture WMAP's map of the temperature of the microwave background radiation shows tiny variations (of few micro degrees) in the 3K background. Hot spots show as red, cold spots as dark blue.

Big Bang

Artist's depiction of the WMAP satellite gathering data to help scientists understand the Big Bang

During the 1930s other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including the Milne model, the oscillatory Universe (originally suggested by Friedmann, but advocated by Albert Einstein and Richard Tolman) and Fritz Zwicky's tired light hypothesis.

After World War II, two distinct possibilities emerged. One was Fred Hoyle's steady state model, whereby new matter would be created as the Universe seemed to expand. In this model, the Universe is roughly the same at any point in time. The other was Lemaitre’s Big Bang theory, advocated and developed by George Gamow, who introduced big bang nucleosynthesis (BBN) and whose associates, Ralph Alpher and Robert Herman, predicted the cosmic microwave background radiation (CMB). For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio source counts, began to favor the latter. The discovery and confirmation of the cosmic microwave background radiation in 1964 secured the Big Bang as the best theory of the origin and evolution of the cosmos. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding the physics of the Universe at earlier and earlier times, and reconciling observations with the basic theory.

Huge strides in Big Bang cosmology have been made since the late 1990s as a result of major advances in telescope technology as well as the analysis of copious data from satellites such as COBE, the Hubble Space Telescope and WMAP. Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the Universe appears to be accelerating.

The Hubble Ultra Deep Field showcases galaxies from an ancient era when the Universe was younger, denser, and warmer according to the Big Bang theory.

Assumptions

The Big Bang theory depends on two major assumptions: the universality of physical laws, and the Cosmological Principle. The cosmological principle states that on large scales the Universe is homogeneous and isotropic.

These ideas were initially taken as postulates, but today there are efforts to test each of them. For example, the first assumption has been tested by observations showing that largest possible deviation of the fine structure constant over much of the age of the Universe is of order 10⁻⁵. Also, General Relativity has passed stringent tests on the scale of the solar system and binary stars while extrapolation to cosmological scales has been validated by the empirical successes of various aspects of the Big Bang theory.

If the large-scale Universe appears isotropic as viewed from Earth, the cosmological principle can be derived from the simpler Copernican Principle, which states that there is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10⁻⁵ via observations of the CMB. The Universe has been measured to be homogeneous on the largest scales at the 10% level.

Big Bang Theory - The Premise
The Big Bang theory is an effort to explain what happened at the very beginning of our universe. Discoveries in astronomy and physics have shown beyond a reasonable doubt that our universe did in fact have a beginning. Prior to that moment there was nothing; during and after that moment there was something: our universe. The big bang theory is an effort to explain what happened during and after that moment.

Big Bang Theory - Common Misconceptions
There are many misconceptions surrounding the Big Bang theory.

For example, we tend to imagine a giant explosion. Experts however say that there was no explosion; there was (and continues to be) an expansion. Rather than imagining a balloon popping and releasing its contents, imagine a balloon expanding: an infinitesimally small balloon expanding to the size of our current universe.

Another misconception is that we tend to image the singularity as a little fireball appearing somewhere in space.

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Big Bang Theory - The only plausible theory?
Is the standard Big Bang theory the only model consistent with these evidences? No, it's just the most popular one. Internationally renowned Astrophysicist George F. R. Ellis explains: "People need to be aware that there is a range of models that could explain the observations….For instance, I can construct you a spherically symmetrical universe with Earth at its center, and you cannot disprove it based on observations….You can only exclude it on philosophical grounds. In my view there is absolutely nothing wrong in that. What I want to bring into the open is the fact that we are using philosophical criteria in choosing our models. A lot of cosmology tries to hide that.
In 2003, Physicist Robert Gentry proposed an attractive alternative to the standard theory, an alternative which also accounts for the evidences listed above. Dr. Gentry claims that the standard Big Bang model is founded upon a faulty paradigm (the Friedmann-lemaitre expanding-space-time paradigm) which he claims is inconsistent with the empirical data. He chooses instead to base his model on Einstein's static-space-time paradigm which he claims is the "genuine cosmic Rosetta." Gentry have published several papers outlining what he considers to be serious flaws in the standard Big Bang model. Other high-profile dissenters include Nobel laureate Dr. Hannes Alfvén, Professor Geoffrey Burbidge, Dr. Halton Arp, and the renowned British astronomer Sir Fred Hoyle, who is accredited with first coining the term "the Big Bang" during a BBC radio broadcast in 1950.

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