The Universe Might Last Forever, Astronomers Say, but Life Might
Not
January 1, 2002
By DENNIS OVERBYE
In the decades that astronomers have debated the fate of
the expanding universe - whether it will all end one day in
a big crunch, or whether the galaxies will sail apart
forever - aficionados of eternal expansion have always been
braced by its seemingly endless possibilities for
development and evolution. As the Yale cosmologist Dr.
Beatrice Tinsley once wrote, "I think I am tied to the idea
of expanding forever."
Life and intelligence could sustain themselves indefinitely
in such a universe, even as the stars winked out and the
galaxies were all swallowed by black holes, Dr. Freeman
Dyson, a physicist at the Institute for Advanced Study,
argued in a landmark paper in 1979. "If my view of the
future is correct," he wrote, "it means that the world of
physics and astronomy is also inexhaustible; no matter how
far we go into the future, there will always be new things
happening, new information coming in, new worlds to
explore, a constantly expanding domain of life,
consciousness, and memory."
Now, however, even Dr. Dyson admits that all bets are off.
If recent astronomical observations are correct, the future
of life and the universe will be far bleaker.
In the last four years astronomers have reported evidence
that the expansion of the universe is not just continuing
but is speeding up, under the influence of a mysterious
"dark energy," an antigravity that seems to be embedded in
space itself. If that is true and the universe goes on
accelerating, astronomers say, rather than coasting gently
into the night, distant galaxies will eventually be moving
apart so quickly that they cannot communicate with one
another. In effect, it would be like living in the middle
of a black hole that kept getting emptier and colder.
In such a universe, some physicists say, the usual methods
of formulating physics may not all apply. Instead of new
worlds coming into view, old ones would constantly be
disappearing over the horizon, lost from view forever.
Cosmological knowledge would be fragmented, with different
observers doomed to seeing different pieces of the puzzle
and no single observer able to know the fate of the whole
universe or arrive at a theory of physics that was more
than approximate.
"There would be a lot of things about the universe that we
simply couldn't predict," said Dr. Thomas Banks, a
physicist at the University of California at Santa Cruz.
And perhaps most important, starved finally of the energy
even to complete a thought or a computation, the domain of
life and intelligence would not expand, but constrict and
eventually vanish like a dwindling echo into the silence of
eternity. "I find the fate of a universe that is
accelerating forever not very appealing," said Dr. Edward
Witten, a theorist at the Institute for Advanced Study.
That is an understatement, in the view of Dr. Lawrence M.
Krauss, an astrophysicist at Case Western Reserve
University in Cleveland, who along with his colleague Dr.
Glenn D. Starkman has recently tried to limn the
possibilities of the far future. An accelerating universe
"would be the worst possible universe, both for the quality
and quantity of life," Dr. Krauss said, adding: "All our
knowledge, civilization and culture are destined to be
forgotten. There's no long-term future."
Einstein's Last Laugh
Until about four years ago, an overwhelming preponderance of astronomers
subscribed to the view that the cosmic expansion was probably slowing down
because of the collective gravity of the galaxies and
everything else in the universe, the way a handful of
stones tossed in the air gradually slow their ascent. The
only question was whether the universe had enough
gravitational oomph to stop expanding and bring itself back
together in a "big crunch," or whether the galaxies would
sail ever more slowly outward forever.
It was to measure that rate of slowing of this outward
flight, and thus find the long- sought and elusive answer
to the cosmic question, that two teams of astronomers
started competing projects in the 1990's using distant
exploding stars, supernovas, as cosmic beacons.
In 1998 the two teams announced that instead of the
expected slowing, the galaxies actually seem to have
speeded up over the last five or six billion years, as if
some "dark energy" was pushing them outward.
"It's definitely the strangest experimental finding since
I've been in physics," Dr. Witten said. "People find it
difficult to accept. I've stopped expecting that the
finding will be proved wrong, but it's an extremely
uncomfortable result."
To astronomers this dark energy bears a haunting
resemblance to an idea that Albert Einstein had back in
1917 and then abandoned, later calling it his biggest
blunder. In that year he inserted a mathematical fudge
factor that came to be known as the cosmological constant
into his equations of general relativity in order to
stabilize the universe against collapse; Einstein's
constant acted as a kind of cosmic repulsion to balance the
gravitational pull of the galaxies on one another.
Einstein gave up the cosmological constant after the
American astronomer Edwin Hubble discovered that the
universe was expanding and thus did not need stabilizing.
But his fudge factor refused to die. It gained a new
identity with the advent of quantum mechanics, the
bizarre-sounding rules that govern the subatomic realm.
According to those rules, empty space is not empty, but
rather foaming with energy. Inserted into Einstein's
equations, this energy would act like a cosmological
constant, and try to blow the universe apart.
According to astronomers the recently discovered dark
energy now accounts for about two-thirds of the mass of the
universe. But is this Einstein's old fudge factor, the
cosmological constant, come home to roost - in which case
the universe will accelerate eternally? Or is the presumed
acceleration only temporary, driven by one of the many
mysterious force fields, dubbed quintessence, allowed by
various theories of high energy physics?
Or is the acceleration even real?
"It's important to find
out if the cosmological constant is really constant," said
Dr. Witten.
Because the repulsive force resides in space itself, as the
universe grows, the push from dark energy grows as well.
"If dark energy is the cosmological constant then it is a
property of the vacuum that will always be with us, getting
more powerful as the universe gets bigger and the universe
will expand forever," explained Dr. Adam Riess of the Space
Telescope Science Institute in Baltimore. But if the dark
energy is some form of quintessence, "then there may be
more such fields which arise in the future, possibly of the
opposite sign, and then all bets are off for the future of
the universe."
Dr. Krauss said, "The good news is that we can't prove that
this is the worst of all possible universes."
The Long Goodbye
It might seem strange or presumptuous
for astronomers to try to describe events all the way to
the end of time when physicists are still groping for a
"theory of everything." But to Dr. Krauss, this is
testimony to the power of ordinary physics. "We can still
put ultimate limits on things without even knowing the
ultimate theory," he said. "We can put limits on things
based on ordinary physics."
Dr. Dyson said his venture into eschatology was inspired
partly by a 1977 paper on the future of an ever expanding
universe by Dr. J. N. Islam, now at the University of
Chittagong in Bangladesh, in The Quarterly Journal of the
Royal Astronomical Society. Dr. Dyson was also motivated,
he wrote in his paper, to provide a counterpoint to a
famously dour statement by Dr. Steven Weinberg, who wrote
in his book "The First Three Minutes," "The more the
universe seems comprehensible, the more it also seems
pointless."
Dr. Dyson wrote, "If Weinberg is speaking for the 20th
century, I prefer the 18th."
If the present trend of acceleration continues this is the
forecast:
In about two billion years Earth will become uninhabitable
as a gradually warming Sun produces a runaway greenhouse
effect. In five billion years the Sun will swell up and
die, burning the Earth to a crisp in the process. At about
the same time the Milky Way will collide with its twin the
Andromeda galaxy, now about two million light- years away
and closing fast, spewing stars, gas and planets across
intergalactic space.
Any civilization that managed to survive these events would
face a future of increasing ignorance and darkness as the
accelerating cosmic expansion rushes most of the universe
away from us. "Our ability to know about the universe will
decrease with time," said Dr. Krauss. "The longer you wait,
the less you see, the opposite of what we always thought."
As he explains it, the disappearance of the universe is a
gradual process. The faster a galaxy flies away from us,
the dimmer and dimmer it will appear, as its light is
"redshifted" to lower frequencies and energies, the way a
police siren sounds lower when it is receding. When it
reaches the speed of light, the galaxy will appear to
"freeze," like a dancer caught in midair in a photograph,
in accordance to Einstein's theory of relativity, and we
will never see it get older, said Dr. Abraham Loeb, an
astronomer at Harvard. Rather it will simply seem dimmer.
The farther away an object is in the sky, he said, the
younger it will appear as it fades out of sight. "There is
a finite amount of information we can collect from the
universe," Dr. Loeb said.
About 150 billion years from now almost all of the galaxies
in the universe will be receding fast enough to be
invisible from the Milky Way. The exceptions will be
galaxies that are gravitationally bound to the cloud of
galaxies, known as the Local Group, to which the Milky Way
belongs. Within this cloud, life would look much the same
at first. There would be galaxies in the sky. "When you
look at the night the stars will still be there," said Dr.
Krauss. "To the astronomer who wants to see beyond, the sky
will be sadly empty. Lovers won't be disturbed - scientists
will be."
But about 100 trillion years from now, when the
interstellar gas and dust from which new stars condense is
finally used up, new stars will cease to be born. From that
time on, the sky will grow darker and darker. The galaxies
themselves, astronomers say, will collapse in black holes
within about 1030 years.
But even a black hole is not forever, as Dr. Stephen
Hawking, the Cambridge University physicist and
best-selling author, showed in path-breaking calculations
back in 1973. Applying the principles of quantum mechanics
to these dread-sounding objects, Dr. Hawking discovered
that a black hole's surface, its so-called event horizon,
would fluctuate and exude energy in the form of random
bursts of particles and radiation, growing hotter and
hotter until the black hole eventually exploded and
vanished.
Black holes the mass of the sun would take 1064 years to
explode. For black holes the mass of a galaxy those
fireworks would light up space-time 1098 years from now.
Against the Fall of Night
Will there be anything or
anyone around to see these quantum fireworks?
Dr. Dyson argued in his 1979 paper that life and
intelligence could survive the desert of darkness and cold
in a universe that was expanding infinitely but ever more
slowly by adopting ever slower and cooler forms of
existence. Intelligence, could reside, for example, in the
pattern of electrically charged dust grains in an
interstellar cloud, a situation described in the 1957
science fiction novel "The Black Cloud," by the British
astronomer Sir Fred Hoyle, who died in August.
As an organism like the black cloud cooled, he argued, it
would think more slowly, but it would always metabolize
energy even more slowly, so its appetite would always be
less than its output. In fact, Dr. Dyson concluded, by
making the amount of energy expended per thought smaller
and smaller the cloud could have an infinite number of
thoughts while consuming only a finite amount of energy.
But there was a hitch. Even just thinking requires energy
and generates heat, which is why computers have fans. Dr.
Dyson suggested that creatures would have to stop thinking
and hibernate periodically to radiate away their heat.
In an accelerating universe, however, there is an
additional source of heat that cannot be gotten rid of. The
same calculations that predict black holes should explode
also predict that in an accelerating universe space should
be filled with so-called Hawking radiation. In effect, the
horizon - the farthest distance we can see - looks
mathematically like the surface of a black hole. The amount
of this radiation is expected to be incredibly small -
corresponding to a fraction of a billionth of a billionth
of a billionth of a degree above absolute zero, but that is
enough to doom sentient life.
"The Hawking radiation kills us because it gives a minimum
temperature below which you cannot cool anything," said Dr.
Krauss. Once an organism cools to that temperature, he
explained, it would dissipate energy at some fixed rate.
"Since there is a finite total energy, this means a finite
lifetime."
Infinity on Trial
Although Dr. Dyson agrees with this gloomy view of life in
an accelerating universe, he and Dr. Krauss and Dr.
Starkman are still arguing about whether life is also
doomed in a universe that is not accelerating, but just
expanding and getting slower and colder.
Quantum theory, the Case Western authors point out, limits
how finely the energy for new thoughts can be shaved. The
theory decrees that energy is emitted and absorbed in tiny
indivisible lumps called "quanta." Any computation must
spend at least this much energy out of a limited supply.
Each new thought is a step down an energy ladder with a
finite number of steps. "So you can only have a finite
number of thoughts," said Dr. Krauss.
"If you want to stare at your navel and not think any new
thoughts, you won't dissipate energy, " he explained. But
that would be a boring way to spend eternity. If life is to
involve more than the eternal reshuffling of the same data,
he and Dr. Starkman say, it cannot be eternal.
Dr. Dyson, however, says this argument applies only to
so-called digital life, in which there is a fixed number of
quantum states. Creatures like the black cloud, which could
grow along with the universe, he said, would have an
increasing number of quantum states, and so there would
always be more rungs of the ladder to step down. So the
bottom need never be reached and life and thought could go
on indefinitely.
But nobody knows whether such a life form can exist, said
Dr. Krauss.
Compared with the sight of the World Trade Center towers
collapsing or the plight of a sick child, this future
extinction may seem a remote concern. Dr. Allan Sandage, an
astronomer at Carnegie Observatories in Pasadena, Calif.,
who has spent his life investigating the expansion and fate
of the universe, said: "Life on this earth is going to
vanish in 4.5 billion years. I wouldn't get hung up on the
fact that the lights are all going out in 30 billion
years."
Dr. Dyson said he was still an optimist. It is too soon to
start panicking, he counseled in an e-mail message. The
observations could be wrong.
"At present all possibilities are open," he wrote. "The
recent observations are important, not because they answer
the big questions about the history of the universe, but
because they give us new tools with which to explore the
history."
Even in an accelerating universe, Dr. Dyson said, humans or
their descendants might one day be able to rearrange the
galaxies and save more of them from disappearing. Another
glimmer of hope comes from the deadly and chilling Hawking
radiation itself, said Dr. Raphael Bousso, from the
Institute of Theoretical Physics at the University of
California at Santa Barbara. Since that radiation is
produced by unpredictable quantum fluctuations, he pointed
out, if you wait long enough anything can appear in it,
even a new universe. "Sooner or later one of those quantum
fluctuations will look like a Big Bang," he said.
In that case there is the possibility of a future, if not
for us, at least for something or somebody. In the fullness
of time, after all, physics teaches that the improbable and
even the seemingly impossible can become the inevitable.
Nature is not done with us yet, nor, as Dr. Dyson
indicates, are we necessarily done with nature.
We all die, and it is up to us to decide who and what to
love, but, as Dr. Weinberg pointed out in a recent article
in The New York Review of Books, there is a certain
nobility in that prospect.
"Though aware that there is nothing in the universe that
suggests any purpose for humanity," he wrote, "one way that
we can find a purpose is to study the universe by the
methods of science, without consoling ourselves with fairy
tales about its future, or about our own."
|