It is hardly surprising that something called dark energy is
hard to study. It is important, though. If it exists at all, it makes up about
three-quarters of the stuff in the universe. And if it does not exist, then
existing theories of physics will have to be useless. The latest
evidence that dark energy really does exist was produced on December 16th by
Alexey Vikhlinin, of the Smithsonian Observatory in Cambridge, Massachusetts,
and his colleagues. They used an orbiting X-ray telescope called Chandra to
study the way that groups of galaxies grow. They discovered that this growth is
restrained in exactly the way that dark-energy theory predicts.
The idea of dark energy was dreamed up ten years ago, to explain why the
expansion of the universe that began with the Big Bang seems to be accelerating,
rather than slowing down. That unexpected finding was the result of studies of
supernovas (超新星) whose apparent brightness (and therefore distance) did not
match the previous theory. Dark energy causes the acceleration by pushing space
itself apart, altering the distances to the supernovas as it does so.
Dark energy is also a convenient explanation for another curiosity.
Geometrically, space is flat. For it to be held that way, rather than getting
more and more curved over time, the amount of matter and energy it contains must
be at a particular, critical density. Without the dark energy needed to explain
the acceleration, the universe would have only a quarter of the necessary
density. It is therefore a relief that Dr Vikhlinin’s results agree with the
theory: the pushing apart of space that dark energy causes makes it harder than
it otherwise would be for galactic clusters(银河星团) to grow. This
result does not bring physicists much closer to understanding what dark energy
actually is. The favored explanation is the so-called universal constant -- an
as-yet-unobserved consequence of the general theory of relativity. But that
theory predicts a force far more powerful than the one actually seen. So the
truth is that physicists are still in the dark. Where did the physicists get the fact that universe expansion is accelerating