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MACROSCOPE

Modern Cosmology: Science or Folktale?

Current cosmological theory rests on a disturbingly small number of independent observations

Michael J. Disney

A Short History of Cosmology

Almost a century ago, Einstein's general theory of relativity posited that matter and energy could bend spacetime. This idea was philosophically attractive because it removed the need to worry about cosmic boundaries if the universe closed back on itself.

Unfortunately (or so Einstein then thought), general relativity implied that the universe would have to either collapse or expand. So in 1921 he found room in his theory for a new free parameter, the so-called "cosmological constant," an arbitrary antigravity term that would put a stop to all that. Ironically, the observers who were examining faint nebulae (distant galaxies) at the time discovered that their spectra were dramatically redshifted—hinting that on its largest scale the universe was expanding after all.

In 1965 Arno Penzias and Robert Wilson stumbled accidentally onto the cosmic background radiation, a microwave whisper arriving from all directions of the sky. As cosmologists interpret it now, they were observing optical radiation emitted by the gas of the universe when it was hot (3,000 degrees Celsius), opaque and relatively young (300,000 years old), redshifted through the enormous factor of a thousand by subsequent cosmic expansion. They were looking into the past with a vengeance and seeing the remnants of what astronomer Fred Hoyle dismissively called the "Big Bang." From then on, the expanding universe was accepted, usually without question, as a natural explanation for the microwave background.

At the same time, astrophysicists sought to understand the origin of the elements. It seemed that most had formed from the fusion of pristine hydrogen inside stars and then had been expelled into general circulation when those stars exploded as supernovae. However, some of the lighter elements, in particular helium, deuterium and lithium, would have had to form much earlier, during the first minutes of the Big Bang. The theory of Big Bang nucleosynthesis did a fair job of predicting the relative amounts of most of these substances, lending more support to the notion of an expanding universe.

Robert Dicke meanwhile noticed a worrying paradox in the Big Bang model: Opposite sides of the cosmos look very much the same, even though they had never been sufficiently close to equilibrate—indeed they had never been sufficiently close for any kind of information (which is limited to the speed of light) to travel between them. This difficulty was virtually unadmitted until 1981, when Alan Guth suggested a vague conceptual solution called "inflation": a slow start to expansion, followed by a rapid acceleration. The necessary causal contacts could then have taken place when the universe was young but not yet flying apart too fast. If inflation actually happened, sufficient stretching during that period of rapid acceleration would have lowered the local curvature today so that it would look flat to the observer, even if it wasn't so on a much larger scale (just as the Earth looks flat to someone with a limited horizon).

At about this time in Holland, Albert Bosma discovered that spiral galaxies are spinning far too rapidly to be held together by the mutual gravitational tugs of their observable contents. Astronomers concluded that there had to be far more dark than ordinary, visible matter around to keep galaxies (and galaxy clusters) together. Most cosmologists welcomed the possibility of such dark matter, because it might be lumpy enough to get the galaxies formed in the early universe—another serious problem for theorists. The apparent uniformity of the cosmic background radiation had cosmologists struggling to figure out how the present uneven structure of galaxies and clusters evolved out of such a smooth beginning.

They thus posited the existence of primordial "seeds" of unknown origin, which somehow survived the early, hot era when radiation would tear material things apart. Cosmologists argued that these seeds would grow over time, finally collapsing into the galaxies seen today. A type of dark matter that ignored radiation ("cold dark matter") would be the ideal stuff for such seeds. It could condense into lumps, thereafter dragging the much lesser amounts of ordinary matter in afterwards, matter that would eventually light up as stars. By the 1980s the theoreticians' universe was entirely dominated by such invisible material.

Meanwhile, observations of distant supernovae in the late 1990s told an astonishing, almost shocking, story. The results suggested that the expansion, far from being slowed by gravitation, as was expected, had instead accelerated. Moreover, this acceleration had started only in comparatively recent times (7 billion years ago). The physics responsible for this seeming acceleration is entirely unknown and goes under the deliberately inscrutable name "dark energy," which may or may not have something to do with Einstein's cosmological constant.




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