In one end-time scenario, the entire
universe—from galaxies down to
atoms— would rip apart at its seams.
become a cold, dark, lonely place where
life could not survive — a Big Freeze.
But dark energy gives the fleeing
rocket some extra oomph, making end-time predictions quite a bit fuzzier.
“A crucial issue is how the dark energy
will behave in time,” says cosmologist
Rocky Kolb of the University of Chicago.
“Until we have some way to grapple with
that, the fate of the universe hangs in the
balance.”
If the strength of dark energy’s extra
push remains forever unchanging, it
could be the cosmological constant — a
term Albert Einstein added to his equa-
tions for general relativity in 1917 and
later dismissed as his “biggest blunder.”
In this case, something like the Big
Freeze would play out. But if dark ener-
gy’s strength decays over time, then a Big
Crunch of sorts remains an option.
If instead dark energy grows stron-
ger, exceeding the repulsive force of
Einstein’s cosmological constant, a
more painful scenario awaits: “In a
finite amount of time, dark energy gets
infinitely dense,” says cosmologist Max
Tegmark of MIT. “First denser than our
galaxy, and our galaxy flies apart. Then
denser than Earth, and that flies apart.
Then denser than atoms, and atoms
fly apart. In a finite time, everything is
ripped apart.”
Figuring out whether the universe
would end with this Big Rip, or a Freeze
or Crunch, requires determining a prop-
erty of dark energy called its equation
of state. That quantity is the ratio of the
pressure exerted by the dark energy to its
density. The most recent findings, based
on data that come from seven years of
mapping the glowing radiation left over
from the Big Bang, suggest that the equa-
tion of state is close to that expected for
the cosmological constant, deviating by
no more than 14 percent.
Expansion of the universe over time
Cosmic Armageddon the discovery of dark energy made the fate of the universe much
more difficult to forecast. scientists typically talk about three possible endings (depicted below),
depending on what this mysterious force actually is and how it behaves over time.
Big Rip
Constant dark
energy
(Big Freeze)
Scale of the universe
Big Crunch
Acceleration
Deceleration
Big Bang
source: cXc/nasa, m. weiss
Time
Present
Future
Beyond the end
But others say that a theoretical breakthrough is necessary. Measuring the
equation of state with enough precision,
they argue, is impossible; a tiny deviation could always linger.
“We don’t just want to measure a
number,” Kolb says. “We want to under-
stand how this crucial piece of physics
fits into the overall fabric of the theory
of nature. And until we do that, I am not
going to be comfortable with any expla-
nation of dark energy.”
Kolb thinks no current proposal
adequately explains dark energy, thus
no proposal decides among a Freeze,
Crunch or Rip scenario.
Of course, the right theory might even
predict that the universe meets its doom
by some other, unknown means. One such
possibility presents itself if the observ-
able universe is just one of many bubble
universes constantly being created and
growing in some larger space. In this
“multiverse” scenario, bubble universes
can collide. If another bubble encroached
on the bubble that people occupy, it would
be bad news, says Anthony Aguirre of the
University of California, Santa Cruz. “ We
would just be sitting around,” he says,
“and this other bubble would smash into
us at the speed of light with some huge
energy and we would die.”
Beyond predicting another possible
end, the multiverse ushers in a new way
of thinking about what an “end” actually
means. “We’d have to be living in a lucky
(for cosmologists) or simple universe for
the part that we see to be telling us about
the whole thing,” Aguirre says.
Imagining the death of the observable
universe as the ultimate end may be just
as naïve as imagining that the destruction
of the Earth, for that matter, means the
end of all life in the galaxy. There might
be much more out there. Even if the
bubble occupied by people bursts, other
universes could live long and prosper. s
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april 23, 2011 | Science newS | 31
