Written by 
In 1927, Belgian physicist Georges Lemaitre had arrived at a startling realisation about the Universe. Using Einstein’s then cutting-edge theory of General Relativity, Lemaitre was able to work out that the farther a galaxy is in the Universe, the faster it will recede i.e. distant galaxies would appear to be moving away faster than their closer counterparts. Thus Lemaitre’s results implied that at some point in the distant past all the galaxies in the Universe were closely packed together. This meant that the Universe, according to Lemaitre, had a beginning. Right about the same time, American astronomer Edwin Hubble had managed to successfully measure the speeds and distances to several nearby galaxies using the world’s largest telescope at the time, the 100-inch Hooker telescope at Mount Wilson Observatory in the US. Hubble’s measurements yielded a simple relationship between the recession speed of a galaxy and its distance. Now known as the Hubble-Lemaitre law, it states that the further a galaxy is, the faster it appears to be moving away from us.
Hubble’s meticulous measurements therefore confirmed what Lemaitre had figured out on purely theoretical grounds – that the Universe is indeed expanding. What this means is that space itself between galaxies is constantly being stretched as a result of which distant galaxies appear to be moving away from us. Think of dots on an inflating balloon. As the balloon expands, the dots themselves would move further away from each other, not because of any motion of their own, but because of the stretching of the balloon. Despite the Universe’s expansion proven beyond a shadow of a doubt, there remained throughout the twentieth century a good dose of skepticism regarding the prospect of a cosmic beginning as hinted at by Lemaitre. This moment zero was ridiculed by the famous British astronomer Fred Hoyle who jokingly called it “The Big Bang”. To Hoyle the Universe having a beginning seemed more in line with religious tenets, which was unacceptable to a modern physicist such as himself.
Interestingly the debate surrounding the eternity of the Cosmos has been raging for at least a millenia if not longer. Famous theologian and mystic Ghazali was highly critical of the doctrine of the eternity of the Cosmos that was held by philosophers in the Islamic tradition. While philosophers such as Ibn-e Rushd saw no contradiction between the Cosmos being eternal and core Islamic beliefs.
This issue remained contentious at best and purely metaphysical for many years until the advent of modern science and its unprecedented tools of observation. In 1948 American physicists Robert Herman and Ralph Alpher considered the physical implications of the Universe starting out from a hot and extremely dense state that was thought to have emerged in the immediate aftermath of the Big Bang. They found to their surprise that if this Big Bang did occur then one should expect to observe leftover radiation in the Universe in all directions. This remnant radiation, they argued, was the result of the Universe transitioning from being opaque to transparent. In simpler terms, initially due to the extreme density and temperature of the Universe, light would continuously bounce around from particle to particle without any end in sight. However with time as the Cosmos expands and cools down gas molecules begin to form, as a result of which light begins to freely propagate in all directions.
Astronomers at the time were not very optimistic about this “leftover” radiation ever being discovered. However in 1964, radio astronomers Arno Penzias and Robert Wilson whilst working at Bell Labs in upstate New York serendipitously picked up a strange hiss whilst taking measurements with the facility’s antenna. The hiss they found was ubiquitous regardless of direction and appeared at a wavelength of 3mm. This baffled Penzias and Wilson who were unaware that they had accidentally discovered the elusive leftover radiation predicted by Alpher and Herman. Both Penzias and Wilson were awarded the Nobel prize in physics for their discovery of the Cosmic Microwave Background (CMB), as this radiation came to be called.
The CMB’s discovery proved definitively that the Universe did indeed start out with a Big Bang after which it began to expand and cool, thus settling the age-old debate regarding the Cosmos being eternal or not. Interestingly Fred Hoyle despite the existence of the CMB remained a staunch critic of the Big Bang theory till his death in 2001. The CMB is remarkably homogeneous, with a temperature of 3 degrees Kelvin (-270 degrees Celsius!) with very miniscule variations, about a part in a million. However according to cosmologists the tiny fluctuations in the CMB are thought to have played a seminal role in formation of galaxies in the early Universe. The CMB has been adequately mapped with high precision since its initial discovery and has allowed astronomers to derive profound insights regarding the very nature of our Universe.
By analysing the fluctuations in the CMB we now know that about 70% of the Universe is composed of something known as Dark Energy, a force that is responsible for the accelerating expansion of the Universe. Another 25% consists of Dark Matter, which is the missing matter comprising most of a galaxy’s mass. The remaining 5% consists of visible matter, which includes galaxies, stars, gas, and us. Basically everything that we can observe and measure directly.