This article first appeared in light2015blog.org and is written by Claudia Mignone
The Universe has not always been the sparkling mix of stars and galaxies we live in and observe today. In fact, in its first few minutes, it was an extremely hot and dense jumble of light and particles that has been expanding and cooling down ever since. Likewise, our understanding of the Universe and its evolution has changed dramatically over time, with the current picture of cosmic history emerging from an exciting progression of theoretical and experimental investigations throughout the past century. In particular, this year we celebrate the anniversary of two milestones in the build-up of modern cosmology: the general theory of relativity, presented by Einstein in November 1915, which allowed scientists to study the effect of gravity on cosmic scales, and the discovery of the most ancient light in the history of the Universe, the cosmic microwave background (CMB) radiation, announced in May 1965, exactly fifty years ago.
A chance discovery
The CMB is a relic of the light that filled the early cosmos, almost 14 billion years ago. It can still be observed today across the entire sky, but not with the naked eye, as it shines at much longer wavelengths than those of visible light, in the domain of microwaves. Finding the CMB was crucial to settling the debate that was going on in the 1950s and early 1960s among cosmologists. At the time, the community was mainly divided in two factions: on one side, those who supported an expanding Universe that started hot and dense and then cooled down; on the other, those who believed in a stationary Universe, whose properties are the same not only when averaged on large spacial scales, but also over time. Interestingly, the name “big bang” that is now universally used to refer to the former theory was coined by Fred Hoyle, one of the scientists who devised the latter. The decisive event in this scientific dispute, the discovery of the CMB, happened by pure chance.
It was 1964, when Arno Penzias and Robert Wilson, two radio astronomers working for the Bell Telephone Laboratories in Holmdel, New Jersey (USA), first detected this curious signal. They were scanning the sky with an antenna that had been built, a few years before, to perform satellite communications experiments at microwave wavelengths. Penzias and Wilson were planning to use this very sensitive antenna for radio astronomy, to observe long-wavelength light emitted by celestial sources in our Galaxy. However, anywhere they pointed in the sky, they seemed to detect a slightly higher signal than expected: the sky appeared to be filled with a uniform background “noise” corresponding to a temperature of about 3 degrees Kelvin (K). Penzias and Wilson considered all possible sources of this nuisance to their data, but without success.
From the stars to the Universe
What the two Bell Laboratories astronomers did not know at the time was that, about fifteen years before and for completely different reasons, three physicists had contemplated the existence of just such a background. These are George Gamow, Ralph Alpher and Robert Herman, who were studying the origin of chemical elements in the Universe around 1948 (1). The nuclear reactions that take place in the interior of stars, turning hydrogen into helium and releasing copious amounts of energy, had been discovered only a decade earlier, and the three physicists were looking at them in detail, trying to explain the abundance of the different chemical elements that are observed in the present Universe. In a series of papers published between 1948 and 1949, they suggested that the nuclei of elements heavier than hydrogen might have formed in the early Universe, as long as it had been a sufficiently hot and dense place in the first couple of minutes of its existence.
As a side remark, in one of these papers, Alpher and Herman noted that, had the Universe been so hot early on – about one billion degrees – to allow these nuclear fusion processes, then a relic of this primordial heat must have survived until present time. Due to cosmic expansion, however, the hot light that pervaded the ancient Universe would have become increasingly colder, reaching a temperature only a few degrees above absolute zero, between 1 and 5 K (2).
It all comes together
Following similar lines of thought, a group of cosmologists at Princeton University were investigating this “big bang” origin of the Universe in the early 1960s. Their scientific leader, Robert Dicke, had developed a sensitive type of microwave receiver during the second world war and, at the end of the conflict, he had tried to use it to seek cosmic microwaves in the sky. He later set on this search again, together with Peter Roll and David Wilkinson, who built a new receiver specially to look for the CMB, and with James Peebles, a theoretical physicist who was studying the cosmological consequences of finding such a signal. However, by 1965 the Princeton group had not detected anything. But as the news about their search spread, eventually making their way to the Bell Laboratories, Penzias and Wilson realised the significance of the mysterious signal of 3 K they had found across the sky only a few months before.
Finally, the search was over. The discovery of the CMB was announced by two papers, published in the May 1965 issue of the Astrophysical Journal: one by Dicke, Peebles, Roll and Wilkinson, outlining the implications of a microwave background for the history of the Universe; the other by Penzias and Wilson, describing their experimental detection (3).
Finding the CMB represented a triumph for the “big bang” description of the Universe: the fossil microwave light, observed to have roughly the same temperature across the sky, could not be easily explained in the alternative model, the steady-state cosmos. In 1978, Penzias and Wilson would be awarded the Nobel Prize in Physics for their discovery (4).
But that was not the end: in fact, the detection of the CMB was only the beginning of the era of observational cosmology, setting into motion a long series of increasingly accurate experiments and ever finer theoretical calculations. About fifty years later, this body of research has led to our current cosmological view, consisting of a Universe that is not only expanding from the early hot big bang state but also appears to be dominated by two mysterious components, the dark matter and the dark energy.
Notes
1 – In fact, the first published paper on this series is by R. A. Alpher, H. Bethe and G. Gamow, “The Origin of Chemical Elements”, Physical Review, Vol. 73, Issue 7, Pages 803-804. The addition of Hans Bethe to the author list was a famous pun by Gamow, who wanted the sequence of author names to resemble the first three letters of the Greek alphabet: alpha, beta, gamma. Bethe is one of the physicists who discovered the nuclear reactions that power the stars, but he was apparently unaware that his name had been added by Gamow to this paper.
2 – R. A. Alpher & R. C. Herman, “Remarks on the Evolution of the Expanding Universe”, 1949, Physical Review, Vol. 75, N. 7
3 – R. H. Dicke, P. J. E. Peebles, P. G. Roll and D. T. Wilkinson, “Cosmic black-body radiation”, 1965, Astrophysical Journal, Vol. 142, Page 414; A. A. Penzias & R. W. Wilson, “A measurement of excess antenna temperature at 4080 Mc/s”, 1965, Astrophysical Journal, Vol. 142, Page 419
4 – http://www.nobelprize.org/nobel_prizes/physics/laureates/1978/
Claudia Mignone is an astrophysicist and science writer with a passion for science and telling stories about it.
