THE NEXT BIG STEPS FOR SCIENCE

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THE END OF

THE UNIVERSE

We know the Universe started with the Big Bang, but how will it end?

With another bang? Or a will it be a rip, a crunch, a freeze or a bounce

instead? Brian Clegg gazes into a cosmological crystal ball

QWill the Universe end soon?

A

No need to pa nic. It won’t end
for many billions of years.
Depending on the scenario, we have
between 20 billion and 100 billion
billion years left to enjoy our cosmos.
The idea that the Universe can’t last
forever is based on the second law of
thermodynamics, which states that
systems have a tendency to degenerate
when left to their own devices.

QHow might the Universe end?

A

This is where we enter the realm
of cosmological speculation.
There are four broad scenarios that
have the most support.
Two of these scenarios involve
the Universe continuing to expand,
getting increasingly thinner and more
dispersed. The most conventional
scenario, the Big Freeze, is simply
the ultimate outcome of standard
thermodynamics. Everything evens
out until there is absolutely nothing
happening in a totally diffuse
Universe. The more dramatic version
incorporates the observation that the
Universe is not just expa nding, but
that the expansion is accelerating.
If this accelerating expansion is
extrapolated to the extreme, we get
the Big Rip, in which all of the matter

in the Universe, from planets and

galaxies to fundamental particles and

space-time itself, is pulled apart as the

expansion heads off to infinity.

By contrast, the other two scenarios

see the expansion of the Universe

eventually reversing. If everything

ends in the Big Crunch, we see a

reversal of ever y t hing we’ve

experienced to date, returning to an

infinitely dense point – a ‘singularity’.

This can then produce a new Big

Bang and a new Universe, giving a

possibility for a cycle of universes.

In the subtly different Big Bounce, the

Universe again reaches a peak size and

begins to contract, but in this instance,

it never gets as far as a singularity

before bouncing and expanding again.

The difference from the Big Crunch

is t hat some aspects of t he ea rlier

Universe can carry over into the next

one. In effect, the Big Crunch generates

a new Universe, whereas the Big

Bounce sees the same Universe

repeatedly expand and contract.

QWhat does it depend on?

A

All these possibilities are

devised by taking the observed

behaviour of the Universe and then 5

The Big Bounce is a scenario in which the Universe will contrac t ,

before bouncing and expanding again to form the same Universe