The Big Bang Theory is supported by strong empirical evidence that makes it universally accepted. However, the scientific community was divided between the Big Bang and its rival, a steady-state model for much of the 20th century. The characteristics of the initial state of the universe in extreme density and temperature can be calculated using the laws of physics. Big bang theory offers an explanation for the initial expansion of the universe and observes phenomena such as the abundance of light elements, the CMB, large-scale structure, and Hubbles law. Hubbles law in physical cosmology detects that galaxies move away from Earth at velocities proportional to their distance or in other words, the further they are the faster they move away from Earth. It is confirmed by scientists that the abundance of helium is a key prediction of the big bang. It was cool enough during the first second of expansion for the remaining matter to merge into protons and neutrons, the particles of atoms nuclei. In the first three minutes, protons and neutrons amassed into hydrogen and helium nuclei. Hydrogen was 75 percent of the matter of early universe and helium was 25 percent, a key factor leading to the big bang. Scientists believe that the expansion began with every fragment of energy jammed into an extremely tiny point. The universe is still expanding at an accelerating pace. The source of acceleration is thought to be propelled by a force called dark energy that repels gravity. It covers 68 percent of the universes total matter and energy. But dark energy is still ambiguous to explain. The first star of the universe unleashed light capable enough to once again strip electrons from neutral atoms, a key chapter of the universe called reionization.
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The Big Bang theory is a cosmological model of the observable universe from the earliest known periods through its subsequent large-scale evolution.[1][2][3] The model describes how the universe expanded from an initial state of extremely high density and high temperature,[4] and offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, and large-scale structure.
A model of the expanding universe opening up from the viewer's left, facing the viewer in a 3/4 pose.
Timeline of the metric expansion of space, where space, including hypothetical non-observable portions of the universe, is represented at each time by the circular sections. On the left, the dramatic expansion occurs in the inflationary epoch; and at the center, the expansion accelerates (artist's concept; not to scale).
Crucially, the theory is compatible with Hubble-Lemaître law – the observation that the farther away galaxies are, the faster they are moving away from Earth. Extrapolating this cosmic expansion backwards in time using the known laws of physics, the theory describes a high density state preceded by a singularity in which space and time lose meaning.[5] There is no evidence of any phenomena prior to the singularity. Detailed measurements of the expansion rate of the universe place the Big Bang at around 13.8 billion years ago, which is thus considered the age of the universe.[6]
After its initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles, and later atoms. Giant clouds of these primordial elements – mostly hydrogen, with some helium and lithium – later coalesced through gravity, forming early stars and galaxies, the descendants of which are visible today. Besides these primordial building materials, astronomers observe the gravitational effects of an unknown dark matter surrounding galaxies. Most of the gravitational potential in the universe seems to be in this form, and the Big Bang theory and various observations indicate that this gravitational potential is not made of baryonic matter, such as normal atoms. Measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, an observation attributed to dark energy's existence.[7]
Georges Lemaître first noted in 1927 that an expanding universe could be traced back in time to an originating single point, which he called the "primeval atom". For several decades, the scientific community was divided between supporters of the Big Bang and the rival steady-state model, but a wide range of empirical evidence has strongly favored the Big Bang, which is now universally accepted.[8]
