In the early 1920s, the American astronomer Edwin Hubble made a remarkable discovery that changed our view of the Universe forever. He found that distant galaxies are all speeding away from each other, which led him to conclude that the Universe is expanding. Remarkably, George Lemaitre, a Belgian physicist, had already predicted such an expansion from Einstein’s general relativity. Lemaitre’s work (together with that of the Russian physicist Alexander Friedman), bolstered by Hubble’s observations, led to the inference that the Universe had expanded continually from a state of an infinite energy density a finite time ago!

This catastrophic moment of the Universe's birth, now called the Big Bang, is calculated to have happened around 14 billion years ago. This hasn't been easy to accept — that the Universe was born a finite time ago, as it raises the uncomfortable question of what was happening before (it came into existence).

Indeed, although Lemaitre arrived at the Big Bang origin of the Universe from Einstein’s theory, Einstein famously remarked to Lemaitre in 1927: "*Vos calculs sont corrects, mais votre physique est abominable* (Your calculations are correct, but your physics is atrocious)". Way into the second half of the 20th century, astronomers were still divided into two groups — those who believed in the Big Bang model and those who believed in what was called a Steady State model, in which the Universe existed for ever. It took another discovery in 1965 by astronomers Penzias and Wilson, of a Universal relic of the Big Bang, permeating all space in the form of a cosmic microwave background (CMB) radiation, to turn the scales in favour of the Big Bang model.

Mathematically, doubts about the Big Bang origin, technically called the 'initial singularity', still persisted. Lemaitre et al had deduced a Big Bang Universe from Einstein’s theory assuming a high degree of symmetry. What if the Universe was asymmetric in the beginning? As Einstein’s equations are one of the most formidable sets of equations ever written down, is it possible that other solutions, with a high degree of asymmetry, behaved differently and avoided the initial singularity?

Stephen Hawking and Roger Penrose from Cambridge University nailed the answer by proving a series of mathematical results, called singularity theorems, drawing on the profound insights of an Indian cosmologist, Amal Kumar Raychaudhuri. These results stated that, provided that the matter and energy pervading the Universe satisfied some reasonable conditions, the initial singularity is inevitable — howsoever asymmetric the Universe was in the past.

Did Hawking and Penrose’s work, then, conclusively prove the Big Bang origin of the Universe?

It turns out that the history of the Big Bang model is full of twists and turns.

It was soon discovered that there were some serious problems with the Big Bang model. These problems particularly came to the fore when it was found that the CMB radiation mentioned earlier, was inexplicably uniform wherever one looked (to 1 part in 100,000).

Now why is that a problem? The point is that, at every instant of time, there is only a certain part of the Universe that we are able to see, whose boundary is called the horizon. Light from regions very far away simply has not had time to reach us, because the Universe has a finite age. If the galaxies were not speeding away, as time goes, we would see more and more of the Universe, because light from those galaxies would have enough time to reach us. It turns out that the expansion of the Universe in the Big Bang model is too gentle to change this conclusion.

Thus, we see more of the Universe than before, in particular compared to the time when the microwave radiation was created (from some colliding matter particles). Why is this a problem? Basically, the microwave radiation is expected to be of a uniform nature when it was created by particles within the horizon at that time. Our current horizon is the size of many, many horizons of that time, hence the microwave radiation has no reason to be uniform across such vast distances!