History of Cosmology

Cosmology is the branch of Astrophysics that deals with studies of the large-scale structure of the universe. For Observational purposes, it requires data on the most remote objects while theoretically it demands the largest possible extrapolations of the basic laws of physics. Despite these severe restraints, cosmology has of late emerged as a very important branch of science where predictions can be made and tested.

When did modern cosmology begin? Indeed, one should go back to Isaac Newton. Newton attempted to construct the model of a homogeneous and isotropic but static universe and he realized that it is unstable. Later attempts within the Newtonian framework, before relativity came on the horizon, were by C. Neumann and H. Seeliger during 1895–1896.

The advent of the General Theory of Relativity in 1915 offered a possible resolution of the conflicts which were beginning to surface between the Newtonian laws of dynamics and gravitation and Special Relativity. General relativity linked the phenomenon of gravitation to the geometry of space and time. Only 2 years after he proposed the theory, Albert Einstein made a bold attempt to apply it to construct a model of the entire universe.

Like Newton, Einstein also found that a static model was not permitted by his 1915  equations of relativity and introduced the so-called Cosmological constant, λ, which implied (in the Newtonian approximation) a repulsive force that varied directly with distance. The static model that emerged required the universe to be closed. Einstein felt that the emergence of such a model was a demonstration of a unique and consistent relationship between space-time geometry and the matter contents of the universe.

However, a paper by William De Sitter in the same year demonstrated that the model was not unique. De Sitter found a model universe which was empty but expanding. Although it was considered esoteric at the time, this model has played a key role in cosmology on a number of later occasions. Meanwhile, understanding of the universe on the observational front was also growing. In 1924 Edwin Hubble had established, through the use of Cepheid variable stars, that the Andromeda Nebula is so far away that it has to be extragalactic. Indeed in the next few years the existence of extragalactic nebulae as galaxies in their own right began to be established.

However, it was the announcement of the velocity - distance relation of these nebulae by Hubble in 1929 that turned the tide in favor of these models. For, after a careful analysis of data on nebular redshifts, Hubble arrived at what is today known as ‘Hubble’s Law’, namely that the radial velocity of a typical galaxy away from us is proportional to its distance from us. Thus, soon after Hubble’s law became accepted, Einstein saw that a static model was unrealistic and abandoned the cosmological constant as the ‘greatest blunder’ in his life. There were others, however, who thought otherwise and, even today, this constant continues to feature in cosmological literature. Cosmology today is a field that has some of the best minds in the world working together to solve the endless mysteries of the Universe.

Questions to Ponder

• How does Cosmology benefit mankind?
• Who discovered each planet?