Quantum Theory Cannot Hurt You_ A Guide to the Universe - Marcus Chown [71]
One of the pluses of inflation is that it provides a possible explanation of the origins of structures such as galaxies in today’s Universe. For such structures to have formed, there must have been some kind of unevenness in the Universe at a very early stage. That primordial roughness could have been caused by so-called quantum fluctuations. Basically, the laws of microscopic physics cause extremely small regions of space and matter to jiggle about restlessly like water in a boiling saucepan. Such fluctuations in the density of matter were minuscule—smaller even than present-day atoms. However, the phenomenal expansion of space caused by inflation would actually have enhanced them, blowing them up to noticeable size. Bizarrely, the largest structures in today’s Universe—great clusters of galaxies—may have been spawned by “seeds” smaller than atoms!
Inflation, however, predicts something about our Universe that does not seem to accord with the facts. Currently, the Universe is expanding. However, the gravity of all the matter in the Universe is acting to brake the expansion. There are two main possibilities. One is that the Universe contains sufficient matter to eventually slow and reverse its expansion, causing the Universe to collapse back down to a Big Crunch, a sort of mirror image of the Big Bang in which the Universe was born. The other is that it contains insufficient matter and goes on expanding forever. Inflation predicts that the Universe should be balanced on the knife edge between these two possibilities. It will continue expanding, but slowing down all the time, and finally running out of steam only in the infinite future. For this to happen, the Universe must have what is known as the critical mass. The problem is that, even when all the matter in the Universe—visible matter and dark matter—is added up, it amounts to only about a third of the critical mass. Inflation, it would seem, is a nonstarter. Well, that’s how it seemed—until a sensational discovery was made in 1998.
DARK ENERGY
Two teams were observing “supernovas”—exploding stars—in distant galaxies. One team was led by American Saul Perlmutter and the other by Australians Nick Suntzeff and Brian Schmidt. Supernovas are exploding stars that often outshine their parent galaxy and so can be seen at great distances out into the Universe. The kind the two teams of astronomers were looking at were known as Type Ia supernovas. They have the property that, when they detonate, they always shine with the same peak luminosity. So if you see one that is fainter than another, you know it is farther away.
What the astronomers saw, however, was that the ones that were farther away were fainter than they ought to be, taking into account their distance from Earth. The only way to explain what they were seeing was that the Universe’s expansion had speeded up since the stars exploded, pushing them farther away than expected and making them appear fainter.
It was a bombshell dropped into the world of science. The sole force affecting the galaxies ought to be their mutual gravitational pull. That should be braking the expansion, not speeding it up.
The only thing that could be accelerating things was space itself. Contrary to all expectations, it could not be empty. It must contain some kind of weird stuff unknown to science— “dark energy”—that was exerting a kind of cosmic repulsion on the Universe, countering gravity and driving the galaxies apart.
Physicists are totally at sea when it comes to understanding dark energy. Their best theory—quantum mechanics—predicts an energy associated with empty space that is 1 followed by 123 zeroes bigger than Perlmutter observed! Nobel laureate Steven Weinberg has described this as “the worst