Page 19 - All About Space 68 - 2017 UK

Mirror universe

and fossils are formed,” Koslowski says. So they – can be in multiple states at once, for example, being
looked at how information about the previous in many places simultaneously. We only observe
locations of the particles was stored in their system them in a definite state or location once we make
– that's where the Janus point emerged. They found a measurement. A mathematical entity, called the
regions with a shared past, but two distinct futures. wavefunction, describes the probability that we'll find
If their model is an accurate description of reality, the electron in each of the possible states. Hartle and
our universe could be one of those regions. There'd Hawking's original work calculated the wavefunction
then be another universe out there where time runs of the young universe, giving us a list of possible
in the opposite direction, because their arrow of time states. But which one are we in?
is flipped with respect to ours. Their arrow would “We know from cosmology that quantum
still create disorder from order, but events in our past fluctuations in our universe were small in the
would be in their future. This idea may sound eerie, beginning,” says Hartle. We see them as tiny
but physicists have considered something similar temperature variations in the Cosmic Microwave
before. Back in the early 1980s, Jim Hartle teamed Background – the afterglow of the Big Bang. As the
up with Stephen Hawking to publish details of what universe expanded, these fluctuations became the
has since been called the Hartle-Hawking state. They seeds around which matter gathered to form the
imagined our universe as a single, quantum particle. structure of the modern cosmos. In more recent work
The rules of quantum physics are downright weird. with Thomas Hertog, Hartle looked at the quantum
They famously say that particles – such as electrons states of the universe in which fluctuations were
small. One such type of universe is a bouncing one –
Evidence of a multiverse could
be hidden in radiation left over it grows, then collapses, before rebounding outwards
from the Big Bang again. Hartle and Hertog showed that under this
scenario the arrow of time flows in two opposite
directions during the bounce.
If our universe is indeed the result of a bounce,
that would mean there's another region of space out
there where the time flows in the opposite direction.
Sometimes this other place is called a mirror
universe, but Hartle cautions against the term. “It
suggests that one half is a reflection of the other, that
the same things happen in each half,” he says. “We
mean there are two halves, but different things can
happen in different halves.”

© Tobias Roetsch, Rebekka Hearl; Alamy, Fermilab

If the infinite multiverse theory is correct
there would be a possibility of a universe
in which time flows backwards

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