'Runaway universe' may collapse in 10 billion years, new studies predict (Forwarded)
Andrew Yee
Stanford, California
CONTACT: Mark Shwartz, News Service
(650) 723-9296; e-mail: mshw...@stanford.edu
9/13/02
'Runaway universe' may collapse in 10 billion years, new
studies predict
By Mark Shwartz
The recent discovery that the universe is expanding at an
ever-increasing rate has led many astronomers to forecast a
dark and lonely future for our galaxy. According to some
predictions, the rapidly accelerating universe will cause
all galaxies to run away from each other until they are no
longer visible. In this widely accepted scenario, our own
Milky Way will become an isolated island adrift in a sea of
totally black space 150 billion years from now.
But two new studies by Stanford University cosmologists
suggest that it may be time to rethink this popular view of
a "runaway universe." Instead of expanding exponentially,
our cosmos may be in danger of collapsing in a "mere" 10 to
20 billion years, according to the Stanford team.
"The standard vision at the moment is that the universe is
speeding up," said physics Professor Andrei Linde, "so we
were surprised to find that a collapse could happen within
such a short amount of time."
Linde and his wife, Renata Kallosh -- also a professor of
physics at Stanford -- have authored two companion studies
that raise the possibility of a cosmic "big crunch." Both
papers are available on the physics research website,
www.arXiv.org .
"We tried our best to come up with a good theory that
explains the acceleration of the universe, but ours is just
a model," Linde noted. "It's just part of the answer."
If the Linde-Kallosh model is correct, then the universe,
which appears to accelerating now, will begin to slow down
and contract.
"The universe may be doomed to collapse and disappear," Linde
said. "Everything we see now, and at a much larger distance
that we cannot see, will collapse into a point smaller than
a proton. Locally, it will be the same as if you were inside
a black hole. You will just discontinue your existence."
Einstein's "blunder"
The fate of the cosmos has been hotly debated for decades.
In the early 20th century, Albert Einstein, along with most
physicists, believed that the universe was static -- even
though the equations he developed for his general theory
of relativity in 1917 suggested that space itself was
either contracting or expanding.
To ensure that his new theory was consistent with nature,
Einstein invented the "cosmological constant": an arbitrary
mathematical term he inserted into his equations to
guarantee a static universe -- at least on paper.
To Einstein, the cosmological constant may have represented
some kind of invisible energy that exists in the vacuum of
empty space -- a force strong enough to repel the
gravitational force exerted by matter. Without this
mysterious vacuum energy opposing gravity, the universe
eventually would crash in on itself, according to general
relativity theory.
But observations by astronomer Edwin Hubble and others in
the 1920s proved that distant galaxies are not stationary
but are, in fact, moving away from one another. Since the
universe was expanding, Einstein no longer needed an
antigravity factor in his equations, so he rejected the
cosmological constant as irrelevant.
"First Einstein introduced the cosmological constant in his
equations, then he said that this was the biggest blunder
of his life," Linde observed. "But I recently heard that,
apparently, he still liked the idea and discussed it many
years later -- and continued writing equations that included
it."
Dark energy
Fast-forward to 1998, when two independent teams of
astronomers discovered that not only is the universe expanding,
it is doing so at an ever-faster pace. Their findings were
based on observations of supernovae -- exploding stars that
emit extraordinarily bright light.
A supernova is a rare event, but new telescopes equipped with
sophisticated electronic sensors allowed the research teams
to track dozens of stellar explosions in the sky. What they
saw astonished the world of astronomy: The supernovae, it
turned out, actually were speeding up at a rate that
outpaced the predicted gravitational pull of matter.
What force could be strong enough to overcome gravity and
cause the universe to accelerate? Perhaps Einstein was right
all along -- maybe there is some kind of vacuum energy in
space. Einstein called it the cosmological constant, and 80
years later, astronomers would give this invisible force a
new name -- dark energy.
"The supernova experiments four years ago confirmed a simple
picture of the universe where approximately 30 percent of it
is made of matter and 70 percent is made of dark energy --
whatever it is," Linde observed.
Overnight, a concept that Einstein had rejected was now
considered the dominant force in the universe.
"The cosmological constant remains one of the biggest mysteries
of modern physics," Linde pointed out.
Negative energy
Current predictions that dark energy will continue to
overwhelm gravity and produce a runaway universe are based
on the assumption that the total density of dark energy
in the universe is greater than zero and will remain so
forever.
This seems obvious at first glance, since logic dictates
that the density of dark energy has to be a positive number.
After all, how could the universe be filled with "negative
energy"?
But in the strange world of quantum physics and elementary
particle theory, everyday logic doesn't always apply.
"During the last year, physicists came to the realization
that it is very difficult to understand the origin of
positive dark energy in the most advanced versions of
elementary particle theory -- such as string theory and
extended supergravity," Linde said.
"We have found that some of the best attempts to describe
dark energy predict that it will gradually become negative,
which will cause the universe to become unstable, then
collapse," he added. "People who studied general relativity
many years ago were aware of this, but to them, this was
an academic possibility. It was weird to think about
negative vacuum energy seriously. Now we have some reasons
to believe it."
The Linde-Kallosh model produced another surprising result:
The cosmos will collapse in 10 to 20 billion years -- a
timeframe comparable with the age of the universe, which
is estimated to be about 14 billion years old.
"This was really strange," Linde recalled. "Physicists
have known that dark energy could become negative and the
universe could collapse sometime in the very distant
future, perhaps in a trillion years, but now we see that
we might be, not in the beginning, but in the middle of
the life cycle of our universe."
The good news, wrote Linde and Kallosh, is that "we still
have a lot of time to find out whether this is going to
happen."
Cosmic bubbles
Linde is quick to acknowledge that the collapsing universe
scenario is not the final word on the fate of the cosmos.
"Astronomy is a science once known for its continuous errors,"
he quipped."There was even a joke: 'Astrophysicists are
always in error but never in doubt.' We are just in the very
beginning of our investigation of this issue, and it would
be incorrect to interpret our results as a reliable doomsday
prediction. In any case, our model teaches us an interesting
lesson: Even the most abstract theories of elementary
particles may end up having great importance in helping us
understand the fate of the universe and the fate of humanity."
Direct observation of space with state-of-the-art telescopes,
satellites and other instruments will answer many unresolved
questions, he added. "We're entering the era of precision
cosmology, where we really can get a lot of data, and these
data become more precise. Perhaps 10 years, 20 years, 30
years, I don't know, but this is the timescale in which we
will get a map of the universe with all its observable parts.
So things that were a matter of speculation will gradually
become better and better established."
Linde helped pioneer inflationary cosmology -- the theory
that the universe began not with a fiery big bang but with
an extraordinarily rapid expansion (inflation) of space in
a vacuum-like state. According to inflationary theory, what
we call the universe is just a minute fraction of a much
larger cosmos.
"The universe actually looks, not like a bubble, but like a
bubble producing new bubbles," Linde explained. "We live in
a tiny part of one bubble, and we look around and say,
'This is our universe.'"
If our bubble collapses into a point, a new bubble is likely
to inflate somewhere else -- possibly giving rise to an
entirely new form of life, Linde said.
"Our part of the universe may die, but the universe as a
whole, in a sense, is immortal -- it just changes its
properties," he concluded. "People want to understand
their place in the universe, how it was created and how
it all will end -- if at all. That is something that I
would be happy to know the answer to and would pay my
taxpayer money for. After all, it was never easy to look
into the future, but it is possible to do so, and we
should not miss our chance."
Graduate student Sergey Prokushkin and Marina Shmakova, a
research associate at the Stanford Linear Accelerator
Center, also contributed to the studies. Research was
supported with grants from the National Science Foundation,
the Templeton Foundation, the U.S. Department of Energy and
the Stanford Graduate Fellowships program.
-30-
EDITORS: A photo of Professors Linde and Kallosh is available
at
http://newsphotos.stanford.edu (slug: "linde/kallosh")
Supernova images are available at
http://www.nasa.gov
The study, "Supergravity, Dark Energy and the Fate of the
Universe," can be downloaded at
http://www.arxiv.org/abs/hep-th/0208156
"M-theory, Cosmological Constant and Anthropic Principle" can
be downloaded at
http://www.arxiv.org/abs/hep-th/0208157
Relevant Web URLs:
* http://physics.stanford.edu/linde/
* http://physics.stanford.edu/kallosh/
* http://snap.lbl.gov/brochure/index.html
* http://www.biols.susx.ac.uk/home/John_Gribbin/
* http://imagine.gsfc.nasa.gov/docs/science/know_l1/supernovae.html
--
Andrew Yee
ay...@nova.astro.utoronto.ca