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CALTECH AY 21 - Inside inflation

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From issue 2593 of New Scientist magazine, 03 March 2007, page 33-37 Inside inflation: after the big bang Peter Coles MASSACHUSETTS, 1981. A young physicist comes up with what seems to be an absurd idea: the universe went through a period of ultra-fast expansion just after the big bang. Alan Guth cannot prove that this "inflation" actually happened nor can he suggest a compelling physical reason why it should have, but the idea seems nevertheless to solve several major problems in cosmology. Fast forward to today. Guth is a professor at the Massachusetts Institute of Technology and inflation is now well established as an essential component of cosmology. But should it be? There is little direct evidence that inflation actually took place. Observations of the cosmic microwave background - "fossil" radiation from the big bang - are consistent with the idea that inflation took place, but that doesn't mean it actually happened. What's more, we still don't know what would have caused it if it did. So how confident can we be that inflation is really a part of the universe's history? A quarter of a century ago, our understanding of the universe was much less precise than it is today. In those days it was a domain in which theoretical speculation ruled over measurement and observation. Technology simply wasn't up to the task of performing large-scale galaxy surveys or detecting the all-important details in the cosmic microwave background (see "Shadow of the big bang"). The lack of stringent experimental constraints made cosmology a theorists' paradise in which many imaginative and esoteric ideas blossomed. Not all survived to be included in the standard model of cosmology, but inflation has proved to be one of the hardiest, and indeed most beautiful, flowers in the cosmological garden. Although some of the concepts involved had been formulated in the 1970s by Russian physicist Alexei Starobinsky, it was Guth's 1981 paper that first crystallised the picture of the inflationary universe. At this time cosmologists didn't know that the universe was as flat as we now believe it is, but it was still a puzzle why it was even anywhere near flat. After all, the great breakthrough of Einstein's general theory of relativity was the realisation that space could be curved. Of all the possible initial conditions, isn't it very improbable that our universe should be flat? What's more, the distribution of stuff in our universe is also astonishingly smooth. Although it contains galaxies that cluster into immense chains more than a 100 million light years long, on scales of billions of light years it is almost uniform. This also seems surprising. Why is the celestial tablecloth so immaculately ironed? Guth grappled with these questions and realised that they could be resolved rather elegantly if only the force of gravity could be persuaded to change from pull to push for a very short time just after the big bang. The expansion of the universe would then speed up rather than slow down. The universe could then inflate by an enormous factor (1030 or more) in a fraction of a millisecond. Even if it were initially curved and wrinkled, all memory of this messy starting configuration would be wiped out. The present-day universe would be very flat and very smooth no matter how it had started out. So how could this bizarre period of anti-gravity be possible? Guth hit upon a simple physical mechanism by which inflation might just work in practice. It relied on the fact that in the extreme conditions just after the big bang, matter would not adhere to the classical laws describing gases and liquids but must instead bedescribed by quantum field theory. The simplest type of quantum field is called a scalar field; such objects are associated with particles that have no spin, the quantum parallel of angular momentum. Modern particle theory involves many scalar fields that are not observed in low-energy interactions, but which may well dominate affairs at the extreme energies of the primordial fireball. Just as classical fluids can undergo a "phase transition" if they are heated or cooled, such as the transition from steam to liquid water, a similar thing happens with scalar fields: their configuration is expected to change as the universe expands and cools. Phase transitions do not happen instantaneously, however, and sometimes a bubble of the substance involved can get trapped in an uncomfortable state in between where it was and where it wants to be, like a bubble of gas trapped in a liquid. Guth realised that if a scalar field got stuck in such a false state, it could free up energy - in a form known as vacuum energy - to drive a small piece of the universe into accelerated expansion. In the process, the tiny bubble can inflate to the size of the entire universe. We don't know which scalar field of the many that may theoretically exist is responsible for generating inflation, but whatever it is, it is now dubbed the inflaton. This mechanism is an echo of a much earlier idea introduced to the world of cosmology by Einstein in 1916, although he didn't use the term vacuum energy. He called it a cosmological constant, and also considered it to be a modification of the law of gravity rather than something arising from quantum fields. Nevertheless, Einstein's idea was incorporated by Dutch mathematician Willem de Sitter into a theoretical model of an accelerating universe. This is essentially the same mathematics that is used in modern inflationary cosmology. The connection between scalar fields and the cosmological constant may also eventually explain why our universe's expansion seems to be accelerating now - something that has been attributed to a mysterious force called dark energy. However, that would require a scalar field with a much lower energy than that required to drive inflation. Perhaps dark energy is some kind of shadow of the inflaton. Guth wasn't the sole creator of inflation. About the same time, many others including Andy Albrecht, Paul Steinhardt, Andrei Linde and Starobinsky, produced different and in some cases more compelling variations on the basic theme. It was almost as if it was an idea whose time had come. Within just a few years


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