ON THE night of 5 March last year, the huge telescope of the Cerro Tololo
Inter-American Observatory in Chile intercepted a message from deepest space.
Transmitted a billion years before the Earth was born, its contents have proved
to be of truly cosmic significance.
The message was barely readable after its journey halfway across the
Universe, and an international team of experts laboured for months to decode it.
In January, Saul Perlmutter of the Lawrence Berkeley Laboratory in California
and his colleagues revealed to the world what they believe to be its gist: “The
Universe will never end.”
A month later, a team led by Brian Schmidt of the Mount Stromlo and Siding
Spring Observatories near Canberra in Australia published the decoded contents
of more of these cosmic missives, which arrive as bursts of light from supernova
explosions in far-flung galaxies. The message was the same. Now Chris Kochanek
and his colleagues at the Harvard-Smithsonian Center for Astrophysics in
Cambridge, Massachusetts, are about to publish more evidence, this time from
light that has been bent and sculpted by the gravity of unseen galaxies.
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These three sets of cosmic missives all suggest that instead of collapsing in
on itself in a big crunch, our Universe will go on expanding forever. And that’s
not all. They also hint that the expanding Universe is in the grip of a
mysterious force that is fighting against gravity—a force that pervades
the entire cosmos and springs literally from nothing.
While the details are complex, the basic argument about the ultimate fate of
the Universe is simple. Throw a ball into the air and gravity will slow it down,
stop it, and then make it fall back to Earth. But if you throw the ball really
hard—and tie it to a rocket for good measure—the ball will soar into
space and carry on forever.
The same is true of the Universe. If it contains enough matter, gravity will
eventually slow its expansion, stop it, and reverse it—producing a
cataclysmic big crunch billions of years hence. But if there is too little
matter—or if there is an extra source of “oomph” at work in the
cosmos—then the Universe will expand forever.
So the Universe’s fate depends on how much matter there is. If the density of
matter is great enough, then gravity will bring the cosmic expansion to a
grinding halt. If it is too low, then the Universe will easily outrun gravity.
Cosmologists call the ratio of the actual density of matter in our Universe to
this critical density Omega. And whole armies of astronomers have spent decades
trying to work out if Omega is less than, more than or equal to 1.
But there’s more at stake than the fate of the Universe. Showing that Omega
is exactly 1 would be a major triumph for current theories of how the Universe
began. These claim that just after its birth, the tightly packed Universe
“inflated” by a huge amount, like a colossal balloon. This, in turn, leads to a
prediction: just like the surface of that colossal balloon, space should have
become less tightly curved as the cosmos expanded during inflation, and should
now seem perfectly flat. Einstein’s theory of gravity ties the amount of spatial
curvature to the amount of matter in the Universe—and to get a flat
Universe, his theory shows that Omega must be exactly 1.
With both the fate of the Universe and their theories riding on Omega,
cosmologists have done their best to work out how much matter there is in the
Universe. The obvious way is to tot up everything you can see. Obvious, but
wrong. Studies of the gravitational effects of clusters of galaxies have
revealed that there must be at least 10 times as much mass tied up in invisible
“dark matter” in the Universe as there is in the familiar form of luminous stars
and gas. Yet even when all this dark matter is thrown into the equation, it
still doesn’t make the theorists happy. Despite searching every cosmic nook and
cranny, astronomers have never found anything like the amount needed to make
Omega equal to 1.
Hence the excitement over the supernovae results. Astronomers can use distant
supernovae to measure how much the expansion of the cosmos has slowed since the
big bang, and hence how much gravity—and matter—is out there. When a
star explodes in a supernova at the end of its life, the brightness of the
explosion is determined by tried-and-tested laws of physics. That makes a
supernova an astronomer’s dream come true: a standard “candle” whose brightness
in the night sky is dimmed only by the distance the light has had to travel on
its way to Earth. So by measuring the apparent brightness of distant supernovae,
astronomers can work out how far the light has had to travel on its journey to
Earth.
Boom or bust?
This distance in turn gives a measure of whether the Universe is accelerating
or decelerating. If the Universe has been decelerating, the light from a
supernova will have travelled a shorter distance than if it had been
accelerating. Though the argument is complicated, the basic idea is that the
shorter the distance the light has travelled, the brighter the supernova will
seem in the night sky. “We measure whether the supernovae look brighter or
dimmer than you’d expect for a constant rate of expansion,” says Robert Kirshner
of the Harvard-Smithsonian Center for Astrophysics, and a member of Schmidt’s
team.
It is this line of argument that the supernova teams have been exploiting to
“decode” the cosmic messages. On that March night last year, the 4-metre
telescope at Cerro Tololo detected the light of a supernova about 10 million
times fainter than the eye can see—about halfway to the edge of the
visible Universe.
But the message was very surprising. Instead of deceleration, Perlmutter and
his colleagues reported in Nature that they had picked up what appeared
to be a slight acceleration in the Universe’s expansion rate. Not only was there
not nearly enough matter in the Universe to be slowing it down to a gradual
halt, as you would expect if Omega equals 1, but there seemed to be some
additional force speeding it up. This could hardly be worse news for current
theories of the very early Universe, which predict that the Universe is
flat—so Omega must be precisely 1—and that its expansion cannot be
accelerating.
Hopes that Perlmutter and his colleagues must been led astray by a small,
unrepresentative set of supernovae evaporated at the annual meeting of the
American Astronomical Society in January. “We now have analysed 40 of the over
65 supernovae that we have discovered,” says Perlmutter. “And these still point
to a value of Omega a lot less than 1.”
A few weeks later, Schmidt’s group revealed more bad news: their study of 14
distant supernovae—including the most distant ever seen—gave the
same answer. Clearly, something is missing from existing theories, and the main
candidate is a bizarre quantum theoretical force that pervades the whole of
space. Theorists call it the cosmological constant, or simply “Lambda”. Created
by the fleeting presence of virtual particles that constantly appear and
disappear, Lambda has just the right properties to provide the “oomph” needed to
make the Universe accelerate. Better still, when combined with Omega it can also
ensure that space stays flat, as current theories demand. All that is required
is that Omega plus Lambda equals 1.
It sounds perfect, yet most theorists have to be dragged kicking and
screaming towards Lambda. One reason is that they share Einstein’s distaste for
adding yet another unknown to the equations of cosmology (he described his own
toying with Lambda as the “biggest blunder of my life”). Also, all attempts to
calculate Lambda using quantum theory give ludicrously high, even infinite
answers.
Despite their qualms, however, cosmologists may have to learn to live with
Lambda. Some have already started to measure it using a phenomenon called
gravitational lensing. When light from a distant galaxy passes another on its
way to Earth, the intervening galaxy’s gravity bends its rays. This creates
multiple images of the distant galaxy, which can be detected by Earth-bound
telescopes. Using Einstein’s theory of gravity, it is possible to estimate how
many of these images should be visible in the night sky.
If Lambda is real, then its anti-gravitational influence will stretch the
volume of space, boosting the chances of a light-bending galaxy lying between us
and a distant quasar. Thus you would expect more lensing events with Lambda than
without. Kochanek and his colleagues have been using this effect to put some
limits on the size of the strange cosmic force. In 1994, Kochanek’s team
announced that a survey of quasars—incredibly bright, young galaxies at
the edge of the visible Universe—had revealed six whose images seemed to
have been gravitationally lensed. Comparing this figure to what you’d expect
with no Lambda, the researchers came up with a rough upper limit on Lambda of
0.7.
Since then, Kochanek and his colleagues have been trying to improve both the
size and reliability of their data set. One worry has been that astronomers may
be missing a lot of quasars: “Surveys often pick them out on the basis of their
colour, but a lensed quasar is contaminated by light from the lens galaxy, and
by the effect of dust.”
The flat Universe
To check for these effects, Kochanek and his team have been trawling through
catalogues of radio-emitting galaxies whose emissions have been bent into
multiple sources—which show up as arc-like smears of light—by
intervening galaxies. “Radio waves are unaffected by dust absorption because
their wavelength is huge compared to the size of the dust particles,” explains
Kochanek.
The results—about to be published in The Astrophysical Journal
—suggest that surveys of lensed quasars are pretty reliable: the radio
galaxy results give the same upper limit on Lambda of about 0.7, and thus a
lower limit on Omega of around 0.3. And both these figures are bang in line with
the supernova teams’ results, provided Lambda exists, and together with Omega
tots up to 1.
So the theorists might yet be able to breathe again, with the combination of
Lambda and Omega ensuring that although the Universe will expand forever, it is
at least flat. Happily, there is one more independent line of evidence to
suggest that the grand design of the Universe is indeed neat and tidy. An
international team led by Matthias Bartelmann of the Max Planck Institute for
Astrophysics at Garching, Germany, has developed what appears to be an
extraordinarily sensitive gauge for measuring the combined effect of Lambda and
Omega.
It exploits their joint influence on the birth of galaxy clusters in the
early Universe. Mathematical models show that the formation of such clusters
depends very sensitively on the value of Omega. Roughly speaking, the higher
Omega is, the longer it takes for clusters to form. This means that a high-Omega
Universe will have relatively few clusters around which gravity can bend and
smear the images of distant galaxies into arcs— cutting the number of
expected lenses.
But Lambda also has a role in determining how many of these lensing events we
see today. The bigger Lambda is, the less compact clusters will tend to be, and
the fewer arcs we should see. So by counting the number of lenses and comparing
it with theory, they hoped to get a handle on the values of Omega and Lambda.
A constant problem?
The team used a supercomputer to estimate just how many lens effects should
be visible in universes with different combinations of Omega and Lambda. Because
of the massive amount of computing demanded by each simulation, Bartelmann and
his colleagues could only look at three combinations. But to their delight, he
and his colleagues found that each produced a radically different number of lens
effects—suggesting that they had found a very sensitive gauge of Omega and
Lambda.
For example, setting Lambda to 0 and Omega to 1 led to a universe featuring
just a few dozen gravitational lens effects over the whole sky. But turning down
Omega to 0.3 boosted the expected number of lens effects by a factor of 70, to
2400.
So what fits best with our Universe? “Nobody has scanned the entire sky for
arcs, so we can only extrapolate how many arcs there actually are in the whole
sky,” says Bartelmann. “But that suggests a figure of around 1500 to 2400. And
the only one of our models which comes close to giving that is a universe with
Lambda at zero and Omega around 0.3.”
On the face of it, this seems like bad news for all concerned—a small
Omega, and no Lambda to top it up to 1. But Bartelmann stresses that the precise
figures should not be taken too literally, because the massive computational
problems of doing the simulations forced them to look at only three combinations
of Omega and Lambda. “If you want to save Lambda,” says Bartelmann, “I guess you
would have to make Omega still somewhat smaller, say around 0.2 and Lambda
around 0.8.”
The end of everything
The sensitivity of the simulations to different values of Omega and Lambda
has impressed other researchers. However, says Bartelmann, there could still be
sources of error—in particular if the lensing effect of each galaxy making
up the clusters is ignored: “They can make the clusters more efficient lenses
than in our model, and they would allow a larger value of Omega, but nothing
like 1—the effect certainly cannot bridge that gap.”
So the take-home message looks the same as that now emerging from the
supernova and quasar surveys: the Universe is going to expand forever, and it
may yet prove to be flat. Certainly the idea of the big crunch seems to have
gone for good, but the exact values of Lambda and Omega, and the fate of the
cosmologists’ theories, are still up for grabs. These values may finally be
nailed early in the next century, with the launch of NASA’s Microwave Anisotropy
Probe (MAP) and the European Space Agency’s PLANCK missions. These will use the
heat left over from the big bang to try yet another way of measuring Omega and
Lambda, which may lay the question to rest for good
(“Genesis to Exodus”, żěè¶ĚĘÓƵ, 19 October 1996, p 30).
But even if the Universe lives forever, its inhabitants will not be so lucky.
A mere thousand billion years from now, all the stars will have used up their
fuel and fizzled out. There will still be occasional flashes in the perpetual
night: the death throes of stars so large that they have collapsed in on
themselves to form black holes. Even these will eventually evaporate in a blast
of radiation. For the next 10122 years, this Hawking radiation will be the only
show in town. By then even the most massive black holes will evaporate, leaving
the Universe with nothing to do for an unimaginable 10 to the power of 1026
years. Quantum theory then predicts that atoms of iron—the most stable of
all elements—will undergo “tunnelling” and disappear into tiny black
holes, which will themselves end in a final fizz of Hawking radiation. In the
beginning there may have been light, but in the end, it seems, there will be
nothing but darkness.
