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| Illustration showing the expansion of the Universe from the Big Bang to the present day. Credit: Andreus/iStock/ Getty Images Plus |
Scientists have proposed a way that the universe could stop expanding, ending
in a 'Big Crunch' that resets space and time as we know it.
Will the universe end in a bang or a whimper? A pair of theoretical physicists
have proposed a third path: perhaps the universe will never end.
In a study that attempts to define the nature of dark energy — a mysterious
phenomenon thought to be causing the universe to expand faster and faster
every moment — the physicists find that cosmic expansion isn't always a
given. Rather, they write, dark energy may periodically "switch" on and off,
sometimes growing the cosmos, sometimes shrinking it down until the
conditions are right for a new Big Bang to occur — and for a new universe to
be born.
The great escape
Our universe is currently experiencing a phase of runaway expansion: the
cosmos is getting bigger faster with every passing moment. Cosmologists do
not understand the cause of this acceleration, which they name dark energy.
If this acceleration persists, then our universe will eventually expand into
oblivion, with all matter and radiation torn apart.
This wouldn't be the first period of runaway growth. In the earliest moments
of the Big Bang, the energies and densities were so extreme that existing
physics cannot cope — it predicts a singularity, a point of infinite density
where the math breaks down. After that, the universe experienced a period of
incredibly rapid expansion known as inflation, which is also poorly
understood.
Astronomers have long wondered if these two phases of accelerated expansion
— one in the earliest moments of the Big Bang and one in the present epoch —
are connected to each other, and whether an entity that drives both of them
avoids the problem of the big bang singularity.
Dynamical demons
To answer that, a pair of theoretical physicists
published a study
Feb. 7 in the preprint database arXiv which examined a model of the universe
where dark energy has always played a role. Previous research modeled dark
energy "switching on" at various times to drive cosmic expansion, but the
new research proposes a more realistic model that includes matter and
radiation.
They wanted to see if dark energy can avoid a Big Bang singularity, drive
inflation, and accelerate the late universe. To avoid that initial
singularity, the universe can't begin from a point of infinite density.
Instead, the universe we live in would have to be one in an infinite series
of repeated "Big Bounces."
In this scenario, dark energy drives the universe until it reaches a certain
size. But then the dark energy transforms itself, forcing the universe to
contract. The cosmos then suffers a big crunch, but right before reaching a
state of infinite density, dark energy turns around again, driving a period
of incredibly rapid inflation and starting the cycle anew.
A finely tuned mechanism
The researchers found a model of dark energy that performed the trifecta.
But crucially, matter and radiation could not be present in the extremely
early universe, otherwise they spoiled inflation. Instead, matter and
radiation had to appear just after inflation, as a portion of the dark
energy decayed away, flooding the universe with light and matter.
While initially successful, the researchers weren't able to find a generic
class of dark energy models that could always lead to the same results.
Instead, they had to artificially put in a smaller value for the present-day
accelerated expansion than quantum mechanics predicts in order to get the
exact right outcome.
However, this new research does point in a promising direction, providing a
viable platform for further exploring models like this. Humans are not
necessarily destined to live in a cold, empty cosmos, because dark energy
might behave differently in the far future. Only continued research will
uncover our ultimate fate.