The Big Bang Theory helps explain and predict various phenomena in the Universe that astrophysical observations suggest. Based on this, we will study 2 models that physicists had discovered in the early 20th century but failed to gather enough evidence to explain why this was happening. However, after the advent of the Big Bang model, these models could be easily predicted and a more thorough study of them was also possible.

Let us discuss each of them one-by-one –

**(1)** **Expansion of space**

The expansion of the Universe was discovered in the early 20^{th} century by Edwin Hubble based on astronomical observations of various galaxies. This led Hubble to formulate the well-known Hubble’s law. This is an essential ingredient of the Big Bang theory.

General theory of relativity describes spacetime by a metric (distance between any 2 points in the cartesian coordinate system). These points can be galaxies, stars, or planets and are specified using a coordinate chart or grid that is assumed to cover all of the space-time sheet. According to the cosmological principle, the laws of physics should hold the same in all the sub-grids of the Universe and there is no preferred location of the Universe for a particular event. Therefore, the metric should be homogeneous and isotropic on galactic scales, which gives a preference to Friedmann-Lemaitre-Robertson-Walker metric. This special metric has a scaling factor, which shows how the Universe will change with time. Therefore, this gives a suitable choice choosing the desired coordinate system, called comoving coordinates. In this type of coordinate system, the grid expands along the Universe, and objects that are only because of the expansion of the Universe, remain at points fixed on the grid. While the coordinate distance or comoving distance between them remains constant but the physical distances or cartesian distance between them changes proportionally with the scale factor of the Universe.

According to the Big Bang Model, Big Bang is not the discharge of matter moving outward to fill an empty and infinite universe. Instead, it is the space itself that expands with time everywhere, increasing the physical distance between two objects, moving at a speed faster than the speed of light. Therefore, it is not an explosion of space but rather an expansion of spacetime itself.

This image shows how the mean distance between 2 galaxies changed over time because of the expansion of the Universe.

**(2)** **Horizons**

One of the key features of the Big Bang model is that it predicts the presence of particle horizons. Careful observation of the astrophysical observation and analysis of the expansion of the Universe suggests the age of the Universe to be 13.8 billion years. Ince, the Universe has a finite age, and the fact that light travels at a finite age, there may be events which haven’t had the time to reach us yet. This provides a fundamental limit, or a past horizon on the most distant objects that can be observed.

Similarly, as space is expanding at a speed faster than light causing the distant object to recede away faster than they can be observed, light emitted by us may never reach them. This defines a fundamental limit, or a future horizon, which puts a limit on the events we will be able to the events which we will be able to influence in the future.

Written by-

Sukhjit Singh

He is a student currently pursuing B.Sc. Mathematics honors from SGTB Khalsa College, Delhi University. He aims at developing multiple skills by taking up exciting projects and desires to be a professor in Mathematics.

Edited by-

Shweta Mittal