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Mirror universe

of Technology on how an arrow of time emerges beginning of time in the multiverse. “If you create a
out of this inflationary scenario. They are still model in which inflation goes infinitely far back into
working on the finer details, but they've shared the the past, then you end up with a twin universe with
basic premise. Like Koslowski's team, their model an opposite direction of time,” Aguirre says.
tracks a group of particles let loose, this time in an As with all these versions of a time reversed
inflationary universe. Following these particles, they universe, communication and observation are off
found arrows of time arising spontaneously, and limits. So if we can never see or talk to these places,
crucially, in opposite directions. So, in some of the what's the point in studying them? “We're trying to
other universes that make up the vast multiverse, find basic theory for the origins of our universe,” says
the arrow of time points the other way. Yet the same Hartle. The best theory is the one that most matches
rules about communication apply – we'd be in their what we see in our universe. Several of the versions
past and they in ours. of a reversed arrow of time, including Hartle's and
So, there are many rifts on the general idea of Koslowksi's, have cropped up when looking for a
time running backwards in another universe. But theory of quantum gravity. If a consequence of
what would these other universes be like? Could finding this theory predicts other universes where
stars, planets and people emerge there too? Maybe. time runs the other way then so be it.
“In principle is it possible to get stars and planets Aguirre doesn't think they will ever be any more
with the arrow of time pointing the other way,” says than a curiosity. “I don't honestly see an experimental
Along with Jim Hartle,
Koslowski. The fact that the laws of physics work way of proving that they're there,” he says. Yet is still Hawking modelled the
the same forwards and backwards might mean that amazing to think that all the events we remember universe as a single
a reversed direction of time wouldn't be a big deal. and those we read about in history books could well quantum particle.
“A rightward spinning electron moving forwards in be in somebody else's future.
time is the same as a leftwards spinning anti-electron
moving backwards in time,” says Anthony Aguirre
from the University of California, Santa Cruz. “ A rightward spinning electron moving
Aguirre has also been exploring this area. As
far back as 2003, he published a paper on eternal forwards in time is the same as a leftwards
inflation with Steven Gratton, now at the Institute
for Astronomy at The University of Cambridge. spinning anti-electron moving backwards
They were looking into the possibility that it has
been going on forever, and that there was no overall in time” Anthony Aguirre

Welcome to
the multiverse Perpetual expansion
In universes like our own,
an expansive force called
dark energy seems to have
Big Crunch grown stronger over time,
Universes that fall back on overwhelming the attraction
themselves end in a similar of gravity to produce an
hot, dense state to the Big ever-growing cosmos.
Bang itself. Some cosmologists
speculate that this kind of
'Big Crunch' could give rise to
another universe in turn.

Perpetual slowdown
If the forces of expansion
and contraction are
Big Bang precisely-balanced, a
Universes begin with a Big universe may expand ever
Bang that may be triggered more slowly, but will never
by a chance fluctuation in come to a halt.
the quantum foam, or a
collision between branes.

Raw material
Closed universe The background fabric of
The development of an individual the multiverse would be Short-lived universe Long-lived universe
universe is determined by beyond time and space, Some universes may be so massive With less matter and mass,
fundamental properties such perhaps in the form of that the expanding force of dark a universe might be able to
as the strength of gravity and many-dimensional 'branes', energy is never able to take hold, expand to a huge size and exist
the amount of matter contained or as an all-pervading so they rapidly collapse back onto for trillions of years before
within it. themselves, ending in a 'Big Crunch'. eventually collapsing back.
'quantum foam'.

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