HOW do you weigh the Universe? Astronomers have been asking this question for
decades, and using every trick they can think of to get at the answer.
Frustratingly, the results never added up. Different techniques gave different
answers.
Now a new cosmic weight-scale has been pressed into service to try to resolve
the conundrum. It鈥檚 the faint afterglow of the big-bang fireball in which the
Universe was born. This glow can still be seen in every part of the sky. Map its
structure, the idea goes, and you can work out the cosmic mass.
It isn鈥檛 as easy as it sounds. The structure in this afterglow鈥攖he
cosmic microwave background (CMB)鈥攊s very subtle. What鈥檚 more, from the
surface of the Earth the faint features of the CMB are obscured by the dirty
window of our damp, cloudy atmosphere. To get round this, researchers have set
up shop in some of the most arid deserts on the planet: the Atacama plateau in
Chile, for instance, and high on the dry, icy plateau at the South Pole, site of
the telescope built by my research team. Others have suspended their telescopes
from helium-filled balloons and floated them high into the stratosphere, above
most of the water vapour that causes the problems.
Advertisement
This year, all these efforts are finally bearing fruit. Thanks to a flurry of
results published in the past 18 months or so, we finally know what the Universe
weighs. And the answer is great news for theorists. It tallies with their
long-held conviction that the Universe began with a dramatic expansion known as
inflation. However, there鈥檚 bad news too. The new results imply that our
Universe is dominated by strange forms of matter that we can鈥檛 see and don鈥檛
understand.
It was back in 1981 that Alan Guth from the Massachusetts Institute of
Technology first proposed that an episode of energy release that he called
鈥渋nflation鈥� happened in the first minute or so of the Universe鈥檚 existence.
During inflation, the part of the Universe we can see today swelled by a factor
of 1060. Then, so the theory goes, the Universe鈥檚 expansion slowed to a more
normal rate.
Why propose something that sounds so strange? Well, it solves lots of thorny
puzzles in cosmology. In particular, it explains why the Universe seems to be
flat, rather than curved. It鈥檚 hard to picture a three-dimensional universe
being curved, but space in any dimensions can have positive curvature, like a
ball, or negative curvature like a saddle. Whether the Universe is flat or
curved depends on what it weighs鈥攐r more precisely, on its density. If the
density is just right, the Universe will be flat. If it鈥檚 higher than this
critical value, the gravitational pull of the matter forces space to have
positive curvature. If it鈥檚 lower than the critical value, space is negatively
curved.
And here鈥檚 the problem cosmologists faced before the inflation idea appeared.
If you start off with a perfectly flat universe early on, it stays flat forever.
If, on the other hand, space starts off slightly curved, it quickly becomes
dramatically more curved. It鈥檚 almost impossible for a universe to hover close
to flatness for any length of time unless it has no curvature at all. Even in
1981, the signs seemed to be that the density of the Universe was at least
somewhat close to the critical value. So some process early on must have made
the Universe flat.
Inflation fits the bill perfectly. It automatically creates a flat Universe
because it stretches out any wrinkles in the curvature鈥攋ust as blowing up
a balloon flattens out its surface. Inflation fills space with material whose
density has precisely the critical value. So theorists assumed that inflation
must have happened and that the Universe must be at its critical density. In
their view, all that was left to do was confirm this by observation.
The trouble is that for decades optical observations have thrown up results
that fall short of the critical density. In their efforts to inventory all the
matter in the Universe, astronomers have mapped the rotational velocities of
galaxies to see how much matter was holding them together. They have also looked
at clusters of galaxies, and even measured how light is bent by gravity as it
passes massive objects on its way to Earth. Over and again, they measured a
density that was close to, but still crucially shy of, the critical value. There
seemed to be only 30 per cent of the expected matter out there.
That鈥檚 where the microwave background comes in. Imprinted upon it are the
frozen images of a time when the Universe rang with vibrations. These vibrations
are the key to weighing the Universe.
A hundred thousand years after the start of the big bang, conditions were
similar to those inside the Sun today. An almost uniform plasma of electrons and
hydrogen and helium ions filled the entire Universe, all bathed in a brilliant
glow of light鈥攖he blaze of the big bang itself. At this early stage, the
free electrons played a key role. They scattered the photons so that they
careened from free electron to free electron like a relativistic pinball
machine, rendering the Universe opaque.
Meanwhile, throughout the Universe matter was gradually gathering around the
areas of slightly higher density that were eventually to become the galaxies and
clusters that we see in the Universe today. Pulled by gravity, matter fell
towards these slightly denser regions. But, bombarded by the scattering photons,
it was forced out again. In and out the plasma bounced, never fully collapsing,
but never quite pulling out of these gravitational hot spots. The material of
the early Universe quivered like a shaken bowl of jelly.
Then, 300,000 years after the big bang, the slowly falling temperature of the
Universe reached 4500 kelvin. Electrons no longer had enough energy to resist
being captured by nuclei. Atoms formed, and because photons had no more free
electrons to scatter off, the Universe became transparent. But the photons did
not disappear, they simply continued in whatever direction their last scattering
sent them. Some of these photons happened to scatter in our direction and we can
still detect them today. They make up the CMB and they have been travelling
unimpeded towards us for almost 12 billion years.
Imprinted on this afterglow should be an image of the compressed and rarefied
regions frozen at age 300,000 years, showing up as bright and dim regions on the
sky. Measure that pattern, the idea goes, and you learn the density of the
Universe.
Here鈥檚 how it works. Different-sized regions had different periods of
oscillation鈥攖he smaller the region, the faster it oscillated. For instance
the largest patches had not even completed their first 鈥渂ounce鈥� when the
Universe became transparent, and the smallest patches had been through several
cycles. It鈥檚 the regions that were exactly halfway through their first
oscillation cycle when the free electrons disappeared that should show up most
strongly in the microwave background. 鈥淗alfway through a cycle鈥� describes the
point at which the material was at its maximum compression, giving the strongest
contrast against the sky. Theorists have worked out exactly how big such regions
would have been 300,000 years after the big bang. Knowing how the Universe has
expanded, they can also work out how big the same regions should appear on the
sky today.
Here鈥檚 where the connection with the Universe鈥檚 weight comes in. Those
regions of compression look bigger to us than they would if the Universe were
low-density. That鈥檚 because matter exerts a gravitational pull on light, curving
its trajectory. As the microwave background photons travelled towards us, their
paths were bent by the matter in the Universe. The more matter there is in the
Universe, the more the light paths are bent and the bigger the regions will
appear on the sky. So to weigh the Universe, all you have to do is calculate how
big those oscillating regions must have been, see how big they actually look in
the microwave background, and work out how much matter is needed to create that
distortion in the image (see 鈥淕ood vibrations鈥�).
During the 1990s a series of CMB observations began to show that the sky did
indeed contain the signature of those ancient wobbles. But for most of the early
microwave telescopes, the images were too smeared-out to resolve the individual
bright and dim patches.
Then in 1998, my telescope at the South Pole鈥攃alled Viper鈥攁nd the
Mobile Anisotropy Telescope in the Atacama Desert each mapped out a few square
degrees of sky with much higher resolution. In both sets of results, the
half-cycle regions seemed to be present. But the observations covered very
little sky and it was hard to tell if the structure they were finding was truly
representative.
Then in April and May this year results from two balloon-borne telescopes,
Boomerang and MAXIMA, were reported. Launched from the McMurdo Station on Ross
Island, Antarctica, the Boomerang telescope spent 10 days riding the polar
stratospheric vortex in a long arc about the South Pole. By the time it
returned, it had mapped a whopping 400 square degrees鈥攁round one per cent
of the sky鈥攚hich is plenty enough to see whether the results are
representative. In addition, even though the MAXIMA telescope only had a
one-night flight from Palestine, Texas, the team succeeded in mapping 100 square
degrees. In the data from each of these two experiments the half-cycle regions
stand out strongly (see Graph).
Between them, the two projects have enough
data to make an accurate determination of the density of the Universe. (Convert
this to a weight by considering the volume of the visible Universe and you get
100 trillion trillion trillion trillion tonnes, give or take a few kilograms.)
The measured density of the Universe matches the critical value to within about
6 per cent. It looks as though the balloon projects have nailed it: the Universe
is flat, and the theorists and their ideas about inflation seem to be right.
So is cosmology now all figured out? Far from it. Our cosmological models are
full of gaps. For one thing, there is the discrepancy between the results from
optical observers and the microwave background telescopes. It鈥檚 not necessarily
a conflict鈥攖hey may both be right. The optical observations focus on
concentrations of matter such as stars and galaxies. In contrast, the cosmic
background reveals the average density not just of matter, but of energy too.
Energy exerts a gravitational pull on the paths of CMB photons just as matter
does. And the latest idea is that the missing component of the Universe鈥檚 weight
comes from some type of dark energy
(快猫短视频, 11 April 1998).
Still, nobody knows for sure what this energy is, or why it has the value it
does.
Then there鈥檚 the problem that optical observers can鈥檛 explain the nature of
all the matter they measure. They know that some of it is just ordinary stuff
like stars and planets. But they also require at least five times as much exotic
鈥渄ark鈥� matter as ordinary matter to explain the way that galaxies rotate, and to
explain the fast orbits of galaxies within clusters. Could the Universe really
have two mysterious ingredients, dark matter and dark energy?
There are also many open issues within inflation theory. Even if current
observations point to an inflationary episode in the history of the Universe,
they don鈥檛 tell us how inflation occurred or at what temperature. So far, we
don鈥檛 have a hint as to what sent the big bang booming.
Some cosmologists are so dissatisfied with all these mysterious ingredients
they prefer to question the laws of gravity. Stacy McGaugh of the University of
Maryland has recently shown that we can understand our Universe without the need
for exotic dark matter if we accept that gravity might be slightly higher at low
accelerations than Newton or Einstein predict. However, even with a modified
theory of gravity, McGaugh needs some kind of dark energy to explain the cosmic
observations. It looks as though it will be some time yet before the Universe
gives up all its secrets.
IN THE hot, early Universe, matter tried to collapse into regions of higher
density, where the gravitational pull was stronger. But the pressure of photons
left over from the big bang pushed the matter outwards again. In and out it
bounced, in a series of well-defined oscillations.
When the Universe had cooled enough to become transparent, the photons
trapped in the hot plasma were suddenly free to travel through space. Frozen
into this microwave background鈥攖he faint afterglow of the big
bang鈥攊s the pattern of oscillations that existed when the Universe reached
the critical temperature. Researchers can measure the imprint of these
oscillations in the microwave background. Small regions oscillated more quickly
than large ones, so different sized regions were at different points in their
鈥渂ounces鈥� when the imprints were frozen in. Because the cycles tended to die
down in amplitude after the initial collapse and rebound, a region which we see
at the maximum compression of its first bounce will show up more strongly than
one at the same point of its second cycle.
Hidden in images of the microwave background are different frequencies
corresponding to oscillations by different-sized regions. Researchers use
mathematical techniques to filter out these frequencies. The result of this
process is called a power spectrum, a plot displaying the amount of each
frequency present. Searching for a particular oscillation amounts to searching
for a peak in the power spectrum.
Results published earlier this year from two experiments鈥擝oomerang and
MAXIMA鈥攕how a strong first peak in the power spectrum
(see Graph).
This corresponds to regions that had gone through half a cycle when the imprint
was frozen into the microwave background. Because they were caught at their
maximum compression, and hence their maximum density contrast, they are the
easiest to see. By measuring the frequency of this peak, researchers can work
out how big the half-cycle regions now look to us on the sky. They look bigger
than they would in a low-density universe, because the light has been bent on
its way to us by the gravitational pull of intervening material. That shifts the
peak to a lower frequency. Thus the position of the peak indicates how much
material there is in the Universe, and hence how much it weighs.FIG-mg22694201.JPG
In the future it should be possible to confirm this result, and learn much
more, by looking for more peaks in the spectrum. To do this, NASA has built the
Microwave Anisotropy Probe. Scheduled for launch in the middle of next year, MAP
will produce a detailed image of the entire sky. Hopefully, it will spot the
second peak in the spectrum, corresponding to regions that had gone through one
entire cycle and then overshot slightly. If so, that will help pin down the
nature of the dark matter in the Universe. It may also see the third
peak鈥攄ue to regions that were at maximum compression on their second
oscillation. This would improve our measurement of the density of the Universe
and fill in details of what the conditions were when the vibrations started.
Meanwhile, the telescopes at less lofty altitudes continue the quest. Another
Antarctic balloon launch, carrying the 鈥淭op Hat Telescope鈥�, is planned for
January. Two new interferometer telescopes, DASI at the South Pole and CBI at
Atacama, have been collecting data and should weigh in soon with their results.
And a new receiver, ACBAR, is being installed on Viper which will give it a
resolution three times greater than MAP.
Further down the road, ESA plans to launch the Planck satellite in 2007.
Planck will image the entire sky with a sensitivity better than MAP, and with
twice the resolution.