THE NEXT BIG STEPS FOR SCIENCESCIENCEPHOTO^ LIBRARYThe four most popular scenarios
HOW WILL THE UNIVERSE END?
BIG FREEZE
The Universe cools and
runs out of energy as it
expands. Matter particles
drift aimlessly through
space and star formation
ceases, plunging the
cosmos into a
frigid darkness.BIG RIP
With an ever-accelerating
e xpansion, ever y thing in the
Universe (including fundamental
particles) rip themselves apart,
giving off v a s t amount s of light.
In the extreme, space-time
itself disintegrates.BIG BOUNCE
As the shrinking Universe
approaches the singularity,
quantum effects cause the
subatomic particles that
permeate the cosmos to
repel each other. The
collapse reverses and the
s ame Univer se begins to
expand again.BIG CRUNCH
The expansion reverses and
the Universe shrinks down
to an infinitely dense point- a singularity – where all
physic s a s we know it
breaks down, triggering a
new Big Bang.
BIG BANGCOSMIC
EXPANSIONUNIVERSE TODAYhave in the past. Perhaps most
doubtful is the use of General
Relativity, as it doesn’t work at the
level of quantum particles, and using
it to model the Universe requires vast
simplifications, making the model
significantly different from reality.Q
Which theory is the most
popular among cosmologists?A
It depends who you ask! The
problem with theories like the
Big Crunch and the Big Bounce is that
models of the Universe suggest that
such processes would run out of
stea m, unable to keep recycling unless
there was some external input. Thebest supported version of the Big
Bounce depends on something called
‘ekpyrotic theory’, a concept based on
a n unproven adva nced version of
string theory. According to this
picture, our Universe is a four-
dimensional ‘brane’ (three of space,
one of time), f loating in a space-time
continuum and the Big Bounce
occu rs when two such bra nes collide,
providing that external input.
Variants of the Big Freeze, or ‘heat
death’, in which everything runs out
of energy and stars finally stop
forming in around 100 billion billion
yea rs, were most popula r a mong
cosmologists for a long time. Now,
though, the Big Rip is probably the
best supported theory, because ‘dark
energy’ seems to be driven by the size
of the Universe, so the bigger it gets,
the more powerful the effect.QWhat is dark energy?^
A
No one knows exactly what
dark energy is, but it causes the
acceleration of the expansion of the
Universe. Without dark energy,
General Relativity models predict
different final outcomes. It might be a
fundamental property of empty space,
or it might be a new kind of energy
field or fundamental force, filling all
of space but having the opposite
effects to normal energy and matter.
Finally, it might be t hat Einstein’s
t heor y of gravity is incor rect, a nd t hat
a new theory is needed. The person
who solves this mystery will be an
instant Nobel Prize winner.Q
Will another universe be
born after this one dies?A
If either the Big Crunch or Big
Bounce happens, definitely. But
even t he more likely ever-expa nding
options don’t mean the end of
everything. Most cosmologists believe
t hat t his Universe is one of ma ny in a
larger ‘multiverse’, with Big Bangs
happening regularly.5 extrapolating some key aspects of
physics into the future, notably the
General Theory of Relativity. This
theory, Einstein’s masterpiece
describing the relationship between
matter, gravity, space and time, can
be used to model the entire Universe
in a crude fashion. Of all of the factors
involved in predicting the future of
the Universe, the existence of the
accelerating expansion is the most
reliable. The ‘extrapolation into the
future’ part is trickier. We can’t
experiment with a Universe and try
out different scenarios. There’s
nothing to say that things will
continue in the future the way they
“Einstein’s General Theory of Relativity
can be used to model the entire
Universe in a crude fashion”
by BRIAN CLEGG (@brianclegg)
Brian is a science writer and author. His latest
book is Professor Maxwell’s Duplicitous Demon.