A brief History of Time: From Big Bang to Black Holes

Dark energy and the accelerating expansion of the universe
For example, in 1998, our picture of the universe’s future was radically
revised. Two competing teams using the Hubble Space Telescope
independently reached the conclusion that the expansion of our universe is
accelerating. The implications for the fate of space are immediate: the
eventual re-collapse of the universe (Friedmann’s big crunch, pp.50–2) no
longer appears to be an option. Space, it seems, will expand for ever.
Why should space expand at an accelerating rate? The cause has become
known as ‘dark energy’. But this is just a name; it doesn’t tell us anything in
itself. In fact Friedmann’s original picture seemed compelling: either
gravity is strong enough to pull everything back together, and the expansion
decelerates over time; or it isn’t strong enough, and the expansion coasts
along unimpeded. Neither of those scenarios suggested anything about the
expansion actually speeding up.
Einstein’s own work holds part of the answer. At one point, he tried
modifying his theory of general relativity to make the universe eternal and
unchanging – something he was convinced ought to be true – by

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introducing a so-called cosmological constant (p.47) into his equations.
This constant plays the role of an ‘antigravity’ force built into the very
fabric of space-time. That was in 1917, long before the expansion of the
universe was established. Later, Einstein retracted the idea once he realized
that Friedmann’s models neatly explained Edwin Hubble’s observations
(p.10).
The retraction might have been premature. At present, it seems that the
acceleration first spotted in 1998 can, in fact, be explained by Einstein’s
antigravitation. But that’s not the end of it, because the underlying
cosmological constant can be given any value, and therefore can push space
apart at any rate. Simple estimates suggest that the acceleration should whip
the universe apart long before galaxies can form. So why is the strength of
antigravity just as it is?
If the no boundary proposal (p.131) is correct, an infinity of universes
exist in parallel. Each of these universes might well have a different
strength for antigravity, especially if string theory is on the right track to a
complete under - standing of physics. We would then naturally live in one
of the universes with a comfortably small dark energy; the anthropic
principle (p.140) reminds us that, if galaxies had never formed, we would
not be here to discuss the matter.