Happy accident

In the year 1965, an important discovery was made, which provided strong evidence that our universe started out about 15 billion years ago in a super hot and dense state. This was the second biggest discovery in cosmology after Hubble’s definitive evidence for the expansion of the universe. Close to the year 1930, Hubble had unambiguous evidence that all galaxies in the universe were moving away from each other with a velocity proportional to their distance from the Earth. So that the farther a galaxy is, the faster it is moving away.

The discovery of the expanding universe resolved many paradoxes inherent in the idea of a static unchanging universe, infinite in extent, favoured by many philosophers and scientists. One of these is the Olber’s paradox, which implies that in an infinite uniform density universe, the night sky illuminated by so many stars would be as bright as the day time sky, which is obviously not true.

A simple observation like that of the night sky being pitch dark rules out an infinite static universe. The expansion of the universe would ‘red shift’ the light more and more from the distant galaxies, with the lower frequencies (and energies) of the red shifted radiation (together with the finite ages of the stars) giving a much lower value for the brightness of the night sky (as compared with the sun dominated daytime sky).

So if the universe is expanding, it follows that in earlier epochs, the matter and radiation would have been concentrated in much smaller volumes. After all, the total amount of matter is conserved. Similar observations hold good for radiation also. The radiation energy density scales as the fourth power of the temperature, so that soon after the expansion started, the temperatures would have been billions of degrees, implying that the universe in this evolutionary picture began in a very hot and dense state. This became known as The Big Bang Model of the universe.

First steps

Physicists like George Gamow (and his students) came up with a precise formula for the temperature of this background radiation as a function of the time since the universe started expanding. The temperature dropped inversely as the square root of the time. The formula implied that one second after the expansion began, the temperature was ten billion degrees, hundred seconds later it was one billion degrees and so on. Gamow even estimated that the present temperature of this background radiation would be a few degrees Kelvin, which would imply that it would be predominantly in the form of microwaves (wavelength being related to the temperature). Following this, not many attempts were there to look for such all pervading radiation.

The accidental discovery of the cosmic microwave background radiation was made by Arno Penzias and Robert Wilson, decisively in 1965 as they experimented with the Holmdel Horn antenna. They were neither astronomers nor scientists, but were communication engineers with Bell Labs. Their job was to reduce background noise to detect faint radio waves bouncing off passive communication satellites. They had to maximise the signal from these orbiting satellites through which TV and other programmes were being relayed.

They suppressed the interference from the heat in the receiver itself by cooling it with liquid Helium to just 4° K above absolute zero. Even with all these efforts, when they reduced their data, they found a low, steady, mysterious noise that persisted in their receiver. This residual noise (corresponding to about 3° K), a hundred times more than expected, was evenly spread all over the sky and was independent of day or night (or seasons), being present in whichever direction the antenna was pointed. They made certain that this radiation detected on a wavelength of 7.35 cm did not come from Earth, the sun or our galaxy.

Even after thoroughly checking their equipment, and getting rid of hindrances, the noise persisted. They were not aware of any radio source that could account for it. When Bernard Burke, astrophysicist at MIT told Penzias about a preprint paper he had seen by Jim Peebles on the possibility of finding radiation left over from The Big Bang and which permeated the universe since the beginning, they began to realise the significance of their discovery.

At this same time, Robert Dicke, David Wilkinson and Peebles at Princeton University were preparing to search for the microwave background in this part of the spectrum. Dicke and his colleagues realised that Penzias and Wilson had indeed discovered the remnant radiation from The Big Bang which they themselves were trying to detect at Princeton. Penzias invited Dicke to Bell Labs to look at the Horn antenna and listen to the background noise. They decided to publish their results side-by-side.

In the first note, Dicke and his associates outlined the importance of discovering the cosmic background radiation as a substantiation of The Big Bang Theory. In the second note by Penzias and Wilson, ‘Measurement of excess antenna temperature at 4080 MHz’, they noted the existence of the residual background noise attributing the possible explanation to that given by Dicke in the companion note.

In 1978, Penzias and Wilson were awarded the Nobel Prize in Physics for their discovery of the cosmic microwave background radiation. It was a purely accidental discovery and remarkably enough, they themselves had no idea at all of the importance of the work. When they finally learned of the work of Dicke and Peebles, predicting a low level background radiation existing throughout they realised what they were dealing with.

Forgotten foundations

Unfortunately in all this excitement, the work of George Gamow from 20 years ago was all but forgotten. Indeed, Gamow wrote a letter to Penzias, drawing attention to all his old work in journals like Nature, Physics Review etc. He noted that even in his popular book Creation of the Universe published in 1952, he had estimated the background temperature to be five degrees. Gamow concluded his letter to Penzias by saying “Thus, you see, the world did not begin with almighty Dicke”. Gamow himself died in 1968, but the cosmic microwave background (CMBR) continues to be an important area of research in cosmology.

The Planck satellite is currently studying in detail the CMBR. Many balloon based instruments in the Antarctic and other places are busy studying several aspects of this radiation. Detailed study of the CMBR gives invaluable information about the very early phase of the universe and the type of structures which initially formed and evolved later into galaxies and huge galaxy clusters. The all important question of the state in which the universe began and evolved is now clearly answered.