BBC Focus

PHOTOS: GET T Y, SCIENCE PHOTO LIBRARY X2

2 existedbeforeitarespeculative–

there areplenty ofpossibilities. But

the basics of the last 13.8 billion years

of growth are clearly understood.

So how might the universe end?

This is where we enter therealmof

cosmological speculation. It ispossible

toproduce all kinds of hypothetical

outcomes,but there arefourbroad

scenarios thathave themostsupport.

Two of these scenarios involve the

Universe continuing to expand,

continuously getting thinner and more

dispersed,althoughthefinal

outcomes are very different. The most

conventional,theBigFreeze,issimply

theultimateoutcomeofstandard

thermodynamics. Everything evens

out untilthere is simplynothing

happeninginatotallydiffuse

Universe. Themoredramatic version

incorporates the observation that the

Universe is not just expanding,but

that the expansion isaccelerating. If

this accelerating expansion is

extrapolatedto theextreme,wegetthe

BigRip,inwhichallofthematterin

the Universe,from planets and

galaxies to fundamentalparticles and

space-time itself,ispulledapart as the

expansion heads off to infinity.

By contrast, theother two scenarios

see the expansion of the Universe

eventuallyreversing.Ifeverything

endsintheBigCrunch(sorry about

the corny names, but we’re stuck with

JARGON BUSTER

Brane

Some theories combining

quantum physics and gravity

require there to be at least

10 dimensions. In such

scenarios, objects with fewer

dimensions called branes

(short for membranes) could

float in the multidimensional

environment.

Cosmology

The branch of astronomy

concerned with the origin,

evolution, and eventual fate

of the Universe.

Quantum particles

Small particles such as

electrons and photons of light

obey completely different

physical rules to ordinary

objects. Such ‘quantum

particles’ respond to the

probabilistic rules of

quantum theory.

Singularity

Various physical models

predict that certain

characteristics of cosmic

objects – their density, for

example – will get bigger and

bigger until they become

infinite. When this happens,

it’s called a singularity and

current theory breaks down.

Space-time

Albert Einstein showed that

space and time were not

separate but linked in a

structure known as

space-time. This is

the underlying framework

of the Universe.

thinktheUniverse

isoneofmanyina

largermultiverse

them),weseeareversalofeverything

we’ve experienced to date, returning

to an infinitely densepoint – a

‘singularity’. This can thenproduce a

new Big Bang and a new Universe,

givingapossibilityforacycleof

universes.Inthesubtlydifferent Big

Bounce, the Universe again reaches a

peaksizeandbeginstocontract,butin

this instance, it never gets asfar as a

singularity before bouncing and

expanding again. The difference from

theBigCrunchisthatsomeaspects of

the earlier Universe can carry over

into the next one. In effect, the Big

Crunchgenerates a new Universe,

whereas the Big Bounce sees the

same Universe repeatedly expand

and contract.

What does it depend on?

Allthese possibilities aredevisedby

takingtheobservedbehaviourofthe

Universe andthen extrapolating some

key aspects of physics into the future,

notablythe GeneralTheory of

Relativity. This theory, Einstein’s

masterpiecedescribing the

relationshipbetween matter, gravity,

space and time, can be used to model

the entire Universe in a crude fashion.

Ofallofthefactorsinvolvedin

predicting thefuture ofour Universe,

the existence of the accelerating

expansion is the most reliable. First

discoveredin 1998,we now even

have a figure for the rate of 2

TheBigBounce

isascenarioin

whichthe

Universewill

contract,

before

bouncingand

expanding

againtoform

thesame

Universe

UNDERSTAND

THE ENDOF THE UNIVERSE