How the Universe Will End
Scientists think they know
how the universe began, but what happens at the other end
of the space-time continuum was a deep, dark mystery-until
now
By
MICHAEL D. LEMONICK
For those
who live in a city or near one, the night sky isn't much to
look at-just a few scattered stars in a smoggy, washed-out,
light-polluted expanse. In rural Maine, though, or the desert
Southwest or the high mountains of Hawaii, the view is quite
different. Even without a telescope, you can see thousands
of stars twinkling in shades of blue, red and yellow-white,
with the broad Milky Way cutting a ghostly swath from one
horizon to the other. No wonder our ancient ancestors peered
up into the heavens with awe and reverence; it's easy to imagine
gods and mythical heroes inhabiting such a luminous realm.
Yet for
all the magnificence of the visible stars, astronomers know
they are only the first shimmering veil in a cosmos vast beyond
imagination. Armed with ever more powerful telescopes, these
explorers of time and space have learned that the Milky Way
is a huge, whirling pinwheel made of 100 billion or more stars;
that tens of billions of other galaxies lie beyond its edges;
and, most astonishing of all, that these galaxies are rushing
headlong away from one another in the aftermath of an explosive
cataclysm known as the Big Bang.
That event-the
literal birth of time and space some 15 billion years ago-has
been understood, at least in its broadest outlines, since
the 1960s. But in more than a third of a century, the best
minds in astronomy have failed to solve the mystery of what
happens at the other end of time. Will the galaxies continue
to fly apart forever, their glow fading until the cosmos is
cold and dark? Or will the expansion slow to a halt, reverse
direction and send 10 octillion (10 trillion billion) stars
crashing back together in a final, apocalyptic Big Crunch,
the mirror image of the universe's explosive birth? Despite
decades of observations with the most powerful telescopes
at their disposal, astronomers simply haven't been able to
decide.
But a series
of remarkable discoveries announced in quick succession starting
this spring has gone a long way toward settling the question
once and for all. Scientists who were betting on a Big Crunch
liked to quote the poet Robert Frost: "Some say the world
will end in fire,/ some say in ice./ From what I've tasted
of desire/ I hold with those who favor fire." Those in the
other camp preferred T.S. Eliot: "This is the way the world
ends/ Not with a bang but a whimper." Now, using observations
from the Sloan Digital Sky Survey in New Mexico, the orbiting
Hubble Space Telescope, the mammoth Keck Telescope in Hawaii
and sensitive radio detectors in Antarctica, the verdict is
in: T.S. Eliot wins.
For that
reason alone, the latest news from space would be profoundly
significant; understanding where we came from and where we
are headed have been obsessions of thinking humans, probably
for as long as we've walked the earth. But the particulars
of these discoveries shed light on even deeper mysteries of
the cosmos, lending powerful support to radical ideas once
considered speculative at best. For one thing, the new observations
bolster the theory of inflation: the notion that the universe
went through a period of turbocharged expansion before it
was a trillionth of a second old, flying apart (in apparent,
but not actual, contradiction of Albert Einstein's theories
of relativity) faster than the speed of light.
An equally
bizarre implication is that the universe is pervaded with
a strange sort of "antigravity," a concept originally proposed
by and later abandoned by Einstein as the greatest blunder
of his life. This force, which has lately been dubbed "dark
energy," isn't just keeping the expansion from slowing down,
it's making the universe fly apart faster and faster all the
time, like a rocket ship with the throttle wide open.
It gets
stranger still. Not only does dark energy swamp ordinary gravity
but an invisible substance known to scientists as "dark matter"
also seems to outweigh the ordinary stuff of stars, planets
and people by a factor of 10 to 1. "Not only are we not at
the center of the universe," University of California, Santa
Cruz, astrophysical theorist Joel Primack has commented, "we
aren't even made of the same stuff the universe is."
These mind-bending
discoveries raise more questions than they answer. For example,
just because scientists know dark matter is there doesn't
mean they understand what it really is. Same goes for dark
energy. "If you thought the universe was hard to comprehend
before," says University of Chicago astrophysicist Michael
Turner, "then you'd better take some smart pills, because
it's only going to get worse."
ECHO OF
THE BIG BANG
Things seemed
a lot simpler back in 1965 when two astronomers at Bell Labs
in Holmdel, N.J., provided a resounding confirmation of the
Big Bang theory, at the time merely one of several ideas floating
around on how the cosmos began. The discovery happened purely
by accident: Arno Penzias and Robert Wilson were trying to
get an annoying hiss out of a communications antenna, and
after ruling out every other explanation-including the residue
of bird droppings-they decided the hiss was coming from outer
space.
Unbeknownst
to the duo, physicists at nearby Princeton University were
about to turn their own antenna on the heavens to look for
that same signal. Astronomers had known since the 1920s that
the galaxies were flying apart. But theorists had belatedly
realized a key implication: the whole cosmos must at one point
have been much smaller and hotter. About 300,000 years after
the instant of the Big Bang, the entire visible universe would
have been a cloud of hot, incredibly dense gas, not much bigger
than the Milky Way is now, glowing white hot like a blast
furnace or the surface of a star. Because this cosmic glow
had no place to go, it must still be there, albeit so attenuated
that it took the form of feeble microwaves. Penzias and Wilson
later won the Nobel Prize for the accidental discovery of
this radio hiss from the dawn of time.
The discovery
of the cosmic-microwave background radiation convinced scientists
that the universe really had sprung from an initial Big Bang
some 15 billion years ago. They immediately set out to learn
more. For one thing, they began trying to probe this cosmic
afterglow for subtle variations in intensity. It's clear throughout
ordinary telescopes that matter isn't spread evenly through
the modern universe. Galaxies tend to huddle relatively close
to one another, dozens or even hundreds of them in clumps
known as clusters and superclusters. In between, there is
essentially nothing at all.
That lumpiness,
reasoned theorists, must have evolved from some original lumpiness
in the primordial cloud of matter that gave rise to the background
radiation. Slightly denser knots of matter within the cloud-forerunners
of today's superclusters should have been slightly hotter
than average. So scientists began looking for subtle hot spots.
FIRE OR
ICE?
Others,
meanwhile, attacked a different aspect of the problem. As
the universe expands, the combined gravity from all the matter
within it tends to slow that expansion, much as the earth's
gravity tries to pull a rising rocket back to the ground.
If the pull is strong enough, the expansion will stop and
reverse itself; if not, the cosmos will go on getting bigger,
literally forever. Which is it? One way to find out is to
weigh the cosmos-to add up all the stars and all the galaxies,
calculate their gravity and compare that with the expansion
rate of the universe. If the cosmos is moving at escape velocity,
no Big Crunch.
Trouble
is, nobody could figure out how much matter there actually
was. The stars and galaxies were easy; you could see them.
But it was noted as early as the 1930s that something lurked
out there besides the glowing stars and gases that astronomers
could see. Galaxies in clusters were orbiting one another
too fast; they should, by rights, be flying off into space
like untethered children flung from a fast-twirling merry-go-round.
Individual galaxies were spinning about their centers too
quickly too; they should long since have flown apart. The
only possibility: some form of invisible dark matter was holding
things together, and while you could infer the mass of dark
matter in and around galaxies, nobody knew if it also filled
the dark voids of space, where its effects would not be detectable.
So astrophysicists
tried another approach: determine whether the expansion was
slowing down, and by how much. That's what Brian Schmidt,
a young astronomer at the Mount Stromlo Observatory in Australia,
set out to do in 1995. Along with a team of colleagues, he
wanted to measure the cosmic slowdown, known formally as the
"deceleration parameter." The idea was straightforward: look
at the nearby universe and measure how fast it is expanding.
Then do the same for the distant universe, whose light is
just now reaching us, having been emitted when the cosmos
was young. Then compare the two.
Schmidt's
group and a rival team led by Saul Perlmutter, of Lawrence
Berkeley Laboratory in California, used very similar techniques
to make the measurements. They looked for a kind of explosion
called a Type Ia supernova, occurring when an aging star destroys
itself in a gigantic thermonuclear blast. Type Ia's are so
bright that they can be seen all the way across the universe
and are uniform enough to have their distance from Earth accurately
calculated. That's key: since the whole universe is expanding
at a given rate at any one time, more distant galaxies are
flying away from us faster than nearby ones. So Schmidt's
and Perlmutter's teams simply measured the distance to these
supernovas (deduced from their brightness) and their speed
of recession (deduced by the reddening of their light, a phenomenon
affecting all moving bodies, known to physicists as the Doppler
shift). Combining these two pieces of information gave them
the expansion rate, both now and in the past.
DARK ENERGY
By 1998
both teams knew something very weird was happening. The cosmic
expansion should have been slowing down a lot or a little,
depending on whether it contained a lot of matter or a little-an
effect that should have shown up as distant supernovas, looking
brighter than you would expect compared with closer ones.
But, in fact, they were dimmer-as if the expansion was speeding
up. "I kept running the numbers through the computer," recalls
Adam Riess, a Space Telescope Science Institute astronomer
analyzing the data from Schmidt's group, "and the answers
made no sense. I was sure there was a bug in the program."
Perlmutter's group, meanwhile, spent the better part of the
year trying to figure out what could be producing its own
crazy results.
In the end,
both teams adopted Sherlock Holmes' attitude: once you have
eliminated the impossible, whatever is left, no matter how
improbable, has got to be true. The universe was indeed speeding
up, suggesting that some sort of powerful antigravity force
was at work, forcing the galaxies to fly apart even as ordinary
gravity was trying to draw them together. "It helped a lot,"
says Riess, "that Saul's group was getting the same answer
we were. When you have a strange result, you like to have
company." Both groups announced their findings almost simultaneously,
and the accelerating universe was named Discovery of the Year
for 1998 by Science magazine.
For all
its seeming strangeness, antigravity did have a history, one
dating back to Einstein's 1916 theory of general relativity.
The theory's equations suggest that the universe must be either
expanding or contracting; it couldn't simply sit there. Yet
the astronomers of the day, armed with relatively feeble telescopes,
insisted that it was doing just that. Grumbling about having
to mar the elegance of his beloved mathematics, Einstein added
an extra term to the equations of relativity. Called the cosmological
constant, it amounted to a force that opposed gravity and
propped up the universe.
A decade
later, though, Edwin Hubble discovered that the universe was
expanding after all. Einstein immediately and with great relief
discarded the cosmological constant, declaring it to be the
biggest blunder of his life. (If he had stuck to his guns,
he might have nabbed another Nobel.)
Even so,
the idea of a cosmological constant wasn't entirely dead.
The equations of quantum physics independently suggested that
the seemingly empty vacuum of space should be seething with
a form of energy that would act just like Einstein's disowned
antigravity. Problem was, this force would have been so powerful
that it would have blown the universe apart before atoms could
form, let alone galaxies-which it clearly did not. "The value
particle physicists predict for the cosmological constant,"
admits Chicago's Turner, "is the most embarrassing number
in physics."
Aside from
that detail, the Einstein connection made the idea of dark
energy, or antigravity, seem somewhat less nutty when Schmidt
and Perlmutter weighed in. Of course, some astrophysicists
had lingering doubts. Maybe the observers didn't really have
the supernovas' brightness right; perhaps the light from faraway
stellar explosions was dimmed by some sort of dust. The unique
properties of a cosmological constant, moreover, would make
the universe slow down early on, then accelerate. That's because
dark energy grows as a function of space. There wasn't much
space in the young, small universe, so back then the braking
force of gravity would have reigned supreme. More recently,
the force of gravity fell off as the distance between galaxies
grew and that same increase made for more dark energy. Nobody
had probed deeply enough to find out what was really going
on in the distant past.
Or rather,
nobody had got enough data. Back in 1997, astronomers Mark
Phillips of the Space Telescope Science Institute and Ron
Gilliland of the Carnegie Institute of Washington had used
the Hubble to spot a distant supernova designated SN 1997ff
and, with the help of Peter Nugent, a Lawrence Berkeley astronomer
on Perlmutter's team, had determined its speed of recession
from Earth. Nugent couldn't figure out the distance, though:
determining the brightness of a Type Ia calls for not just
one but several measurements, spread over time.
On the rival
team, Riess knew of the discovery, but he learned soon afterward
that other Hubble photos had also caught the supernova, completely
by chance. So one day last summer, he recalls, "I called up
Peter and began fishing around for information. I guess I
wasn't especially cagey. He said almost right away, ÔAre you
asking about 1997ff?'"
Rather than
try to scoop each other, the friendly rivals decided to cooperate-and
soon realized they had stumbled onto something truly astonishing.
The new supernova, some 50% closer to the beginning of the
universe than any supernova known before, was far brighter
than had been predicted. That neatly eliminated the idea of
dust, since a more distant star should have been even more
dust-dimmed than nearer ones. But the level of brightness
also signaled that this supernova was shining when the expansion
of the cosmos was still slowing down. "Usually," says Riess,
"we see weird things and try to make our models of the universe
fit. This time we put up a hoop for the observations to jump
through in advance, and they did-which makes it a lot more
convincing."
MORE>>
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June 25, 2001 | No. 25
COVER
STORY
How
It All Ends
If you are still trying to wrap your mind around how the universe began-with
that Big Bang that created everything out of nothing-wait until you find
out what is coming at the other end of the space-time continuum
TRAVELERS
ADVISORY...
PACIFIC
BEAT: Aboriginal leader accused; coral corralled...
PACIFIC
OSERVED: Fraser vs. Wake...
THE
ARTS
TELEVISION: Stealthy product placements are
making ads the stars of the show...
CINEMA: Shrek's adventures in animation
MUSIC: Another hot album by Air
BOOKS: Un-endearing Indira Gandhi
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