91av

WOULD YOU like to see the beginning of time? It can be arranged. Astronomers
now believe there is a way to look back further than anyone thought possible, to
a time before stars and galaxies, before atoms and molecules, before even
protons and neutrons existed. You will be looking at the origin of everything,
the detonation of the explosion that became our Universe. You will know how it
all began. And all you need is a pair of sunglasses.

Just looking at the stars means seeing into the past, because light takes
time to travel from a distant star to our eyes. So if you look far enough out
into space, maybe 15 billion light years, surely you must reach back to the
birth of the Universe?

Well, not quite. In their quest for the big bang, astronomers hit a brick
wall. Long before the days of galaxies, stars and planets, the newly born
Universe was a hot soup of electrons and atomic nuclei that was completely
opaque. Only some 500,000 years after the beginning did it cool down enough to
become transparent. So however powerful your telescope, you can’t see anything
that happened during the first 500,000 years.

Astronomers aren’t satisfied with this. They want to look back to the almost
unimaginable time of 10^-38 seconds after time zero—that’s a hundredth of
a billionth of a billionth of a billionth of a billionth of a second—to
find out what really happened during the most momentous episode in history.

Big bang theory says that the Universe started out in a very hot, dense
state, and has been expanding ever since. It can explain a lot: the chemical
make-up of the Universe, for example, can be calculated from conditions in the
first three minutes of existence. But the big bang has problems.

According to Einstein’s theory of general relativity, matter and energy tend
to curve space-time. But on large scales our Universe turns out to have just the
right amount of matter and energy to make it flat. Big bang theory allows any
degree of curvature but it can’t explain why our Universe has become flat. Also,
there’s nothing in the big bang model to say that widely separated regions in
our Universe should look similar, yet galaxies cluster in much the same numbers
and patterns on one side of the Universe as on another. Finally, there must have
been some small density fluctuations in the early Universe to gradually gather
matter together by their gravity—or else there would be no galaxies today,
just a smooth tenuous gas filling space. But again the theory doesn’t explain
why these fluctuations occurred.

To fix these puzzling problems, an idea called inflation was proposed in 1980
by Alan Guth, then at the Massachusetts Institute of Technology. He assumed
there was a brief burst of acceleration that took space and stretched it out in
all directions. The patch of space that we now call the observable Universe
would have started at subatomic size and blown up to a few centimetres across in
a fraction of a second.

Then it doesn’t matter whether space started out curved or not, because any
initial curvature will be flattened by this expansion. The observable Universe
would be like your immediate surroundings on Earth, seemingly very flat, even
though it is curved overall. What’s more, it would have started out so tiny that
any big differences in temperature or density, say, would have rapidly evened
out. Finally, inflation could explain the small density variations in the early
Universe: they started out as tiny quantum fluctuations in the energy fields
filling space, and got magnified by inflation.

Without any justification, this would seem like a desperate remedy for the
ills of the big bang, but there are theories of how it might have happened. They
generally agree that a pervasive energy field blew space apart and then
vanished, leaving behind a sea of subatomic particles. But what was behind it?
The trouble is that there is a wide range of different inflation theories, from
the weird to the downright exotic, based on different ideas of how the
fundamental forces of nature become unified at high energies.

They all predict different time and energy scales for inflation, so in
principle we could tell them apart. But how in practice? It’s like trying to
identify a thief when all you’ve got is a movie of the robbed house that
was filmed 500,000 years after the burglary. But perhaps the thief left
footprints?

If so, the thief’s boots are gravitational waves. According to Einstein’s
theory of general relativity, these travelling warps in space-time are produced
when large masses are strongly accelerated. Although they have not been detected
directly, most scientists are convinced they exist.

During inflation, powerful gravitational waves should have been wrenched into
existence. Just like the density fluctuations that got blown up into
galaxy seeds, other kinds of wrinkles in space-time were inflated into
travelling waves.

Gravitational waves would have passed unaffected through the primordial
fireball, so they could be carrying a signal from the dawn of time. Some
peculiar instruments are being built to capture these on Earth (see “Catch the
wave”), but we are more likely to detect them by their
footprints—faint marks left on the surface of that primordial
fireball.

Way back then, photons were continually bouncing off the electrons that
roamed the Universe. Eventually, the fireball got cool enough for neutral
hydrogen and helium atoms to form, which are far less effective at scattering
light. At that point, the photons bounced for the last time and headed off into
space.

The light was as dazzling as a trillion Suns, but it has been stretched and
diluted ever since by the expansion of space. All that’s left is a very faint
microwave glow known as the cosmic microwave background (CMB). This primordial
radiation was discovered in 1965.

There are tiny differences in the brightness or temperature of the CMB
between different parts of the sky. These were seen by NASA’s Cosmic Background
Explorer satellite (COBE) in the early 1990s, and have been studied in much more
detail by balloon experiments like Boomerang and MAXIMA
(91av, 16 December 2000, p 26).

Some of this variation traces the slight density fluctuations in the early
Universe, and some of it comes from Doppler shifts produced by moving gas. Giant
sound waves propagated through the blazing hot matter, compressing and diluting
it, and as gravitational waves warp whatever they pass though, they must also
have squeezed and stretched the primeval fireball.

Unfortunately, no matter how accurate the measurements of brightness,
cosmologists will never be able to tell one contribution from another, so it’s
not possible to separate the effect of primordial, inflationary gravitational
waves from the real density enhancements or from the effect of sound waves.

Luckily there is a subtler message in the microwaves: polarisation. Each
microwave photon vibrates in a particular direction—its plane of
polarisation. Usually, each photon is polarised in a different, random
direction, but when ionised matter is moving it imposes an overall direction of
polarisation on the radiation it scatters.

And crucially, gravitational waves generate a distinctive type of
polarisation, unlike the patterns made by density changes and sound waves. If
you measure the polarisation direction for every part of the sky, and draw a
short line in that direction at each particular spot, what you get looks a bit
like a map of wind velocities on Earth. Gravitational waves create swirl-like
patterns, unlike any other phenomenon.

To see these patterns, what we need is the microwave equivalent of Polaroid
sunglasses. A set of parallel wires will stop waves with one polarisation while
letting others pass. By rotating this set-up and measuring how much radiation
leaks through, you can work out the polarisation.

So, have we caught the thief? Not yet. The polarisation will probably be very
weak, so you need highly accurate measurements of the brightness of the
background to get a reliable signal.

A few balloon-borne and ground-based instruments stand a chance. At the South
Pole, DASI (Degree Angular Scale Interferometer) combines signals from 13
telescopes, making it a sensitive and high-resolution instrument. John
Carlstrom, an astrophysicist at the University of Chicago, thinks it might be
the first to catch CMB polarisation. Meanwhile, Fred Lo of the Academia Sinica
in Taipei, Taiwan, is in charge of AMiBA (Array for Microwave Background
Anisotropy), a large, sensitive 19-element interferometer due to be built in
2004, either at Mauna Kea, Hawaii, or in Chile. And two balloon-borne
experiments that hit the headlines last year with their detailed measurements of
the microwave background will fly again later this year: MAXIMA this spring, and
Boomerang in December. Jeff Peterson of Carnegie Mellon University in
Pittsburgh, Pennsylvania, expects one of these instruments to detect CMB
polarisation within the next three years. And the highly sensitive European
Planck satellite, which will be launched in 2007, will almost certainly see
it.

Because gravitational waves contribute only a small part of the total
polarisation, even Planck probably won’t be able to extract the gravitational
inflationary signal. “I‘d bet my money on post-Planck missions,” says
Robert Caldwell of Dartmouth College in Hanover, New Hampshire. It will probably
be another decade or so before the weak polarisation signal from primordial
gravitational waves can be tracked down.

So the challenges are great. But then, the stakes are high. From the
polarisation data, it should be possible to deduce the strength of the
gravitational waves. This, in turn, tells you when inflation took place.
The earlier inflation started off, the more violent the expansion, and so the
stronger the gravitational waves are expected to be.

And if we know when it happened, we might find out why. It was once thought
that inflation took place 10^-38 to 10^-35
seconds from time zero, according to
Caldwell, when an original cosmic superforce split into three independent
forces: the strong and weak nuclear forces, and the electromagnetic force. This
force splitting happens when space goes through a phase transition, like the
transition from liquid water to ice. The energy released by the phase transition
is what drives inflation.

But there is no definitive theory of how the three forces come to be
unified—only a slew of tentative “grand unified theories”. So there’s
plenty of uncertainty as to when and why the forces split in the early Universe.
Some theories say there are extra, hidden dimensions to space. If these extra
dimensions are as large as a fair fraction of a millimetre, inflation might have
occurred as late as 10^-14 seconds after time zero. “But,” says Caldwell, “this
is a rock bottom latest time.”

Finding the gravitational wave spectrum would knock out many of the dozens of
inflation models thought up by creative theorists. “Right now,” says Bram
Achterberg of Utrecht University in the Netherlands, “no single model seems to
be more plausible than any other, but from the gravitational wave spectrum it
would be possible to disprove some of them.”

So studying the gravitational wave spectrum would be a powerful way of
probing extremely high energies, and examining theories that describe the true
fundamental structure of matter. Physicists may never be able to build
accelerators powerful enough to reach the high energies experienced at the start
of the Universe, so tracing inflation may be the only way to do it.

Then again, we might find a spectrum that can’t be explained by inflation at
all. The theorists would have to start from scratch. Nobody really knows what
was going on in that first split second, which is why a signal from the big bang
would be so important.

It’s not a sure thing, however. “If inflation took place at a low energy
scale, the gravitational wave signal will be too small to be detected,” says
Peterson. This would not disprove inflation, but it would extinguish any hope of
looking beyond the microwave background to the beginning of time.

GRAVITATIONAL waves from the beginning of time may be constantly passing
through the Earth, stretching and flexing everything they meet. But how can we
detect them?

A pair of huge devices called LIGO (Laser Interferometer Gravitational Wave
Observatory) is being built in Louisiana and in Washington State. Each one will
use laser beams to monitor distances inside two 4-kilometre-long tunnels, which
should vary when a gravity wave passes through. LIGO is the star among the new
generation of gravitational wave detectors—but it’s not sensitive to the
very-low-frequency waves that are expected to come from the big bang. Moreover,
the amplitude of these waves is probably too small to be measured by LIGO, even
if it were sensitive to the frequency. The planned space-based detector called
LISA (Laser Interferometer Space Antenna), due to be launched in 2010, samples
lower-frequency waves, but it too will probably not be sensitive enough to pick
up the feeble primordial waves.

Then again, although most theories do not predict high-frequency,
short-wavelength gravitational waves, very little is really known about what
happened less than a quadrillionth of a quadrillionth of a second after time
zero. There’s at least one model, by Massimo Giovannini of Tufts University in
Medford, Massachusetts, that has large amounts of gravitational waves with
frequencies of many gigahertz coming from a slower expansion phase, after
inflation.

At the University of Birmingham, Mike Cruise has built a prototype detector
for these high-frequency waves. A gravitational wave can induce a tiny rotation
of the plane in which an electromagnetic wave moves. So Cruise and his
colleagues are pumping microwaves around a circular copper wave guide to try to
amplify this effect to measurable proportions. “It’s still many orders of
magnitude less sensitive than what we ultimately need,” he says, “but in the
next couple of years, we hope to make real technical progress.”

It seems unlikely that we will see primordial gravitational waves on Earth
for some time, however. Looking for their influence on the cosmic microwave
background is probably the best bet.