BBC Focus

UNDERSTAND

THE ENDOF THE UNIVERSE

the whole thing – getting bigger. And

theeffect isdramatic. As the Universe

isabout 13.8billion years old,the

furthest we couldever see islightthat

has taken 13.8 billion years to reach

us.(Inpractice, it’s morelike13.5

billion years, as theUniversewas

opaquebefore that.)Ye t themost

distantpotentially visible objects are

thoughttobeabout 45billionlight-

years away,because oftheexpansion

ofthe cosmos.

Without dark energy, General

Relativity models predict different

finaloutcomesdepending on the

curvature ofspace-time. Ifspace-time

isflat, as is generallythought, we

wouldexpectsomekindofBigFreeze

outcome,but withother types of

curvature ofspace-time, gravity could

overcome the expansion and produce

a collapsing Universe. Add dark

Inonetweet...

The Universe is dying, pulled apart by dark energy.

Don’t send flowers – theywon’t exist at the end and

there are billions of years to go.

2 expansion: 73.2kilometres per

secondper megaparsec (a megaparsec

equals 3.26 million light-years).

The ‘extrapolation into the future’

partistrickier.Wecan’texperiment

withaUniverseandtryoutdifferent

scenarios. We onlyknow whathas

happenedsofar. There’s nothing to say

thatthingswillcontinueinthefuture

thewaytheyhave in thepast–it’sjust

anassumption. It’s a bit like trying to

predict the stock market. On the whole

there are clear trends, but it’s always

possible that we’ll get caught out in a

crash. Perhaps most doubtful is the

use ofGeneralRelativity. Although

Einstein’s theory has proved hugely

effective in predicting theeffects of

gravity, it doesn’t work at the level of

quantum particles – thephysics

required in most end-of-the-Universe

scenarios. Also, using thetheory to

model the Universe requires vast

simplifications, making themodel

significantly different from reality.

Which theory is the most popular

among cosmologists today?

Itdependswho you ask!Theproblem

with theories like the Big Crunch and

theBigBounceisthatmodelsofthe

Universe suggest that suchprocesses

wouldrun out ofsteam, unabletokeep

recycling unless there was some

externalinput. Thebest supported

version ofthe Big Bouncedependson

something called ‘ekpyrotic theory’, a

conceptbasedon an unproven

advancedversion ofstring theory – an

attempttocombinequantum theory

and General Relativity. According to

thispicture, our Universe is a four-

dimensional‘brane’(three ofspace,

one oftime),floating in a space-time

continuum, andthe Big Bounce occurs

when two such branes collide,

providing that externalinput.

Va r ia nt s oftheBigFreeze,or‘heat

death’, i n whicheverything runs out of

energy and stars finally stop forming

in around 100 billion billion years,

were most popular among

cosmologistsfor along time. Now,

though, the Big Rip is probably the

best supported theory, because dark

energy – the entity that’s speeding up

PHOTO: SCIENCE PHOTO LIBRAR

Y

The Dark Energy Camera in Chile is used by cosmologist s to probe the expansion of the Universe

the expansion of the cosmos – seems

tobedrivenbythesizeofthe

Universe, so thebigger it gets, themore

powerfultheeffect. It’s an eternal

feedback loop. Based on this, our

Universehas atleast 20billion years to

go,but this is very speculative–we

don’tknow,for instance, ifdarkenergy

will continue to have t he sa me effect.

What’s dark energy?

Though it sounds like the latest comic

bookvillain,darkenergy is whatever

itisthat’s causing the acceleration of

the expansion ofthe Universe. The

‘dark’bit doesn’tmean it’sevil and

brooding, just that wedon’tknow

what it is.

Thinking about an expanding

Universe isn’t easy because we’re used

tostuffexpandingwithinthe

Universe. This is the Universe itself –