Brief History of the Universe

Cosmology is the branch of science dealing with the large scale structure of the Universe. It is the study of the origin, evolution, history, composition, and dynamics of the Universe. We aim to understand the Universe as a whole and various physical processes happening in different parts (space) and epochs (times). Mankind is always keen to learn about the dynamics of the Universe. Ancient philosophers have applied themselves to understand what the Universe is? Where do we come from? Is the Universe infinite? How did the Universe begin? What is the fate of the Universe? But, their understanding of the Universe was somewhat different. They believed that humans were special, the Earth being the center of the Universe and the Sun revolving around it. This belief that humans are the most important entity in the Universe is known as Anthropocentrism. With the passage of time and advancement in tools, our understanding of the previously known Universe changed. Scientists discarded ancient beliefs and developed new theories and mathematical formulations based on new observations. In the 16th century, Nicolaus Copernicus said that the Sun is at the center of our planetary system, and the Earth revolves around the Sun like others planets. In the early 17th century, Galileo Galilei developed the telescope and used it to explore the sky. In the same decade, Johannes Kepler gave the laws of planetary motion based on observations. Sir Isaac Newton proposed the theory of gravitation, which explained planetary motion and gave theoretical ground to Kepler's conjectures. He treated gravity as a force of attraction caused by the masses of the particles. His theory has huge implications for modern science. In 1915, Albert Einstein came up with a new theory of gravitation known as the General Theory of Relativity. This theory describes gravity based on the geometry of the space-time fabric distorted by massive objects. In particular, the curvature of space-time is directly related to the energy content present. The general theory of relativity is superior to Newton's theory as it explained several phenomena left unexplained by Newtonian theory. Newton's theory of gravitation is a limiting case (weak field, stationary field, and nonrelativistic particles) of Einstein's general theory of relativity. In cosmology, gravitation plays a major role as huge masses are involved in the picture. Other forces of nature, weak and strong, don't seem important at large scales. The electromagnetic interaction is also of no importance, even though it is a long-range force, because the major constituents of the Universe are believed to be electrically neutral. So among the four fundamental forces of nature, gravitational force plays the dominant role in the evolution of the Universe. Attempts to understand the structure of the Universe through Newton's theory of gravitation did not make much progress as it involved instantaneous propagation of gravitational information, which fails when applied over large distances. As a special theory of relativity limits the maximum speed of information propagation, it is better to use the relativistic theory of gravitation, Einstein's General Theory of Relativity. Modern science began by discovering that Earth is not at the center of the Universe. In fact, neither the solar system nor our galaxy or any group of galaxies occupy any especially favored position in the cosmos. The modern cosmological theory is built on the cosmological principle. The cosmological principle states that the Universe is spatially homogeneous and isotropic at a large enough scale (order of 100 Mpc), i.e., there is no preferred position and direction in the Universe. However, it does not apply to the Universe in detail, but a smeared-out universe averaged over large enough distances to include many clusters of galaxies. Unlike ancient people, modern physicists believe in Antianthropocentrism, i.e., no position is more special than the other in the Universe. There are two approaches, the Newtonian approach and the general relativistic approach, to derive the equations that govern the evolution of the Universe called Friedmann equations. One important goal of cosmology is to estimate the unknown parameters in these Friedmann equations, which tell a lot about the cosmos. This is done by "listening" to our messengers: Photons coming from far past and narrating the situation of the Universe at the time of their generation. We set up huge telescopes to collect these photons carrying enormous amounts of information and analyze useful data. Our current knowledge of cosmic evolution is that Universe started from a "singularity" called the big bang. We have almost no knowledge about Universe now besides the fact that it was highly dense and hot. The Universe started expanding and cooling. Various elementary particles were synthesized at this time. Particle physicists aim to understand the reactions in this high-energy era. Baryons and leptons formed for the first time in our Universe. Protons and neutrons combine to form some light elements via nucleosynthesis. All these particles were coupled with each other making a particle soup. Photons were trapped in this soup scattering with charged particles. With time, as Universe expanded and cooled, Universe energetically favored the synthesis of neutral atoms, i.e., electrons and nuclei combined to form neutral atoms, and the Universe became neutral. This era is called recombination. Photons get decoupled from matter soup as there are no charged particles to scatter photons. This era is called photon decoupling and last scattering. Photons are now free to move all around the Universe and form cosmic microwave background (CMB); these radiations contain huge amounts of information about the Universe at very early times. There was no luminous object at this time, hence this era is called the dark ages. There were small perturbations in the matter density of the Universe at times of photon decoupling. The Universe further expanded, and these perturbations grew with time, resulting in large-scale structures in the Universe at later times. This era when the first luminous objects (stars or galaxies) formed is called cosmic dawn. The formation of large scale structures happened when Universe shifted from radiation dominated to a matter-dominated era. The radiations from these luminous objects again ionized the neutral atoms presented in the Universe, known as the reionization era. The Universe kept on expanding and diluting, and then the Universe entered the modern era where dark energy dominated the energy content of the Universe. The nature of dark energy is still a hot topic of research. What we know about dark energy is that it is responsible for the acceleration of the expanding Universe, which was observed by the "High-Z Supernova Search Team" in 1998 and the "Supernova Cosmology Project" in 1999. Cosmological constant Λ (proposed by Einstein initially for a steady-state solution of the Universe) is believed to be a potential candidate for dark energy and is consistent with various observations. Presently, there is very little radiation content present in the Universe, and most of the energy content of the Universe is due to matter (mainly dark matter) and dark energy. Dark matter is believed to be nonrelativistic and sometimes called cold dark matter. The model representing the flat Universe with dark energy as the cosmological constant and dark matter as any nonrelativistic matter is dubbed as ΛCDM model. It is also called the standard model of cosmology. This model is fairly consistent with various observations. However, it faces some theoretical problems and hence is also a hot research topic.

The Universe is full of mysteries, and there are a plethora of unsolved problems which researchers are working on with collective efforts from all around the globe. The advancements in observational instruments and computational capabilities have revolutionized studying such a large and complex 'system,' the Universe. Various ongoing and upcoming surveys like DESI, LSST, EUCLID, and SKA are aimed to cover large volumes of the order of several Gpc3 in the sky and provide us with unprecedented cosmological information. The powerful computational techniques and machine learning implementations in cosmology helped us understand the Universe more accurately. As a result, cosmology is now evolved as a 'precision science' instead of a philosophical subject for ancient thinkers. Theories are being tested against the observational data collected from CMB, LSS, supernovae, quasars, HI 21cm line, BAO, ages of galaxies, gravitational waves, Lyman-alpha forest, etc. The theoretical background is discussed in the tab 'Theoretical Cosmology.' You can find classic resources to dig deep into the subject in the More tab.