The third plank of evidence: cosmic radiation

The discovery of the expansion of the universe and the subsequent estimate of the age of the universe (once the distance scale was sorted out) was a key step in the development of the Big Bang model. Another step was the correct prediction of the Hydrogen and Helium content of the universe from Big Bang nucleosynthesis by Gamow (see post below). However, it was a third piece of evidence that was to prove the most convincing of all – the discovery of radiation left over from the hot young universe, now known as the cosmic microwave background (CMB).

Two of Gamov’s students, Ralph Alpher and Robert Herman, continued with his interest in the early universe. In particular, they calculated that as the early universe expanded and cooled, atoms would form after about 100,000 years (a process known as recombination). At this time, radiation left over from the cataclysmic origin of the universe that had been continually scattered by elementary particles would no longer be scattered. The two young scientists postulated that the universe would become transparent to radiation from this time onwards, and that this radiation might even be observable in today’s universe (like a cosmic fossil).  They calculated that it would be isotropic, homogeneous, of black body spectrum and extremely low temperature . Most intriguingly, the radiation would be Doppler shifted (by billions of years of universe expansion) from the extreme high-energy part of the electromagnetic spectrum all the way down to microwave frequencies, the least energetic part of the energy spectrum.

Sadly, no-one paid much attention to this prediction. (At the time, the Big Bang estimate of the age of the universe was way off and Gamov’s initial work on the nucleosynthesis of the heavier elements was also wrong). However, in 1965, Penzias and Wilson, two engineers at Bell Lab, detected an unexpected background noise in data they obtained with the world’s most sensitive radiotelescope. Having spent a year trying to eliminate it, they concluded that the source was extra-galactic and contacted the theoretician Bob Dicke at Princeton. Dicke was amazed. The Princeton group had been working on the theory of cosmic background radiation and drawing up plans for the construction of an experiment to search for it – now the Bell Lab  astronomers already had the  data  (“Boys, we’ve been scooped!”). The two groups published their findings, experiment and theoretical explanation side by side, in a famous issue of the Astrophysics Journal.

The world of science was stunned. This was convincing evidence indeed for the Big Bang model (there is no alternate explanation for the spectrum of the CMB) and the debate was effectively over. Penzias and Wilson were awarded the Nobel Prize and Dicke became instantly famous. (As a bonus, the redshift of the CMB was also extremely convincing evidence that space is indeed expanding, as the radiation isn’t going anywhere).

Penzias and Wilson with their giant antenna in the backgound

There is a sad postscript, however. Neither group was aware of the work of Alpher and Herman twenty years earlier and it was a long time before the work of the Gamow group was acknowledged. Ironic that the Nobel went to two astronomers who knew little of cosmology and that the great work of the theoreticians who predicted the CMB went unsung until recently.

Today, much of modern cosmology is concerned with the study of the CMB with ever more precision, using more and more sopisticated space telescopes. In particular, the COBE satellite  showed the blackbody spectrum of the radiation in precise detail in 1992, while the WMAP mission gave us priceless information on perturbations in the spectrum in 2006. More on this later….