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| Beginnings
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BEGINNINGS
by Martin Rees
 
Cosmology is the study of the large scale structure and evolution of the Universe. Where do space, matter, energy, and the forces of nature come from? How did the Universe begin?
70 years ago George Lemaître, a Belgian priest who was also an Massachusetts Institute of Technology (MIT) graduate, pioneered the idea that everything began in a dense state. He called this 'the primeval atom', but that phrase never caught on. Nor did the word 'ylem', introduced by the boisterous Russian/American, George Gamow.
These phrases were usurped by the term 'big bang' a flippant term introduced in the 1950s by Fred Hoyle as a detrimental description for a theory he didn't like. Hoyle favoured the view that our Universe was in a 'steady state', with new atoms being 'created' all the time, forming new galaxies and filling in the gaps between the old galaxies, so that everything looked the same. This idea was popular in England in the 1950s, but the voices of the articulate trio who invented it, Hermann Bondi, Thomas Gold, and Fred Hoyle, never carried across the Atlantic. Nor were their views ideologically acceptable in the Soviet Union.
In the 1940s and 1950s, the most vocal advocates of the Big Bang were George Gamow with collaborators Ralph Alpher and Robert Hermann. Gamow studied the physics of the hot dense beginning attracting little interest at the time, mainly because theories about dense beginning of the Universe (if there was one) seemed inaccessible to observations, and therefore pure speculation. The final demise of the steady state model came in 1965, when Arno Penzias and Robert Wilson detected the afterglow of hot dense material from the beginning of the Universe. This background hiss perplexed the duo who even considered whether bird droppings in their horn antenna were responsible.
They had, in fact, accidentally discovered the fingerprint of the early Universe. They announced their discovery in a famous paper, Excess Antenna Temperature at 4080 Mc/s. Arno Penzias and Robert Wilson were radio astronomers, with expertise in electronics rather than cosmology. Their success stemmed from persistence and their technical skills. It wasn't surprising that it took others to convince them of what their discovery meant. In fact Wilson only realised the consequence of his achievement when he read a report of it in the New York Times.
Intergalactic space isn't completely cold, it is warmed slightly by microwaves with no apparent source, suffusing the entire Universe. Its intensity at different wavelengths, when plotted on a graph, traces out what physicists call a 'black body' or 'thermal curve'. This particular curve is explained when the radiation has come into balance with its environment (as happens deep inside a star, or in a steadily burning furnace). But in 1990 the Cosmic Background Explorer (COBE) provided the evidence for this special spectrum. COBE confirmed beyond reasonable doubt that everything, all the matter that galaxies are now made of, was once a compressed gas, hotter than the Sun's core.
The expansion has cooled and diluted the radiation, and stretched its wavelength. But this primordial heat, the afterglow of creation is still around. It fills the Universe with nowhere else to go! We see it every day, it even causes interference on our TV screens. For its first few minutes, our Universe was far hotter than the centre of the Sun, hotter even than big stars at the end of their lives, and certainly hot enough for nuclear fusion. Fortunately for us, it cooled before there had been enough time to 'process' everything into iron – the most tightly bound nucleus. If it had stayed hot for longer (or if the reactions had happened faster), there would be no nuclear fuel left to power the stars.
The expanding Universe took hundreds of thousands of years to cool to the temperature of the Sun's surface. At that stage, the electrons and ions combined into neutral atoms that no longer scattered the radiation. The Universe became transparent, lifting the 'fog'. The primeval light then shifted into the infrared part of the spectrum, and the Universe literally became dark, until the first stars formed, lighting it again.
The only cosmological input is the expansion rate. How strongly should we believe the hot Big Bang theory? Certainly more strongly now than in earlier decades. It has lived dangerously ever since the 1960s, and survived. Several things could have been discovered during the 1980s or 1990s that would have discredited this hypothesis. The Big Bang theory deserves to be taken at least as seriously as anything geologists or paeleontologists tell us about the early history of our Earth. Our knowledge is just as indirect (and less quantitative). Its survival gives me (and I suspect most cosmologists today) 99% confidence in extrapolating right back to the first few seconds of cosmic history.
So what can we predict about the future fate of the Universe? Friedmann and Lemaître stated that the expansion of the Universe balanced gravitational attraction to avoid a complete collapse. They concluded that this expansion could either continue forever or eventually reverse into a phase of contraction. With the picture of the expanding Universe coming from the Big Bang, one must wonder if this seemingly endless expansion will reverse itself and lead to a process of Universal contraction.
Sir Martin Rees became Astronomer Royal in 1995 and is Professor in the Institute of Astronomy, University of Cambridge.
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