Astronomers think they have
spotted a thread of pure energy streaking through our galaxy.Is this the
first evidence for string theory,asks Marcus
Chown
IF YOU consider them separately, these two observations are
hardly going to set the scientific world on fire. But together they add up
to a spectacular possibility. In a tiny region of sky, astronomers have seen
a dozen galaxies that appear as a curious sequence of double images. They
have also observed a quasar whose brightness oscillates in an unexpected
way. What could cause these odd phenomena? The only explanation that covers
both is pretty mind-bending: "superstrings" of
pure energy that can stretch millions of lights years across the universe.
Is this the first experimental evidence for string theory? The theory is
our best hope of understanding how the universe works at its most fundamental
level. It suggests that the basic constituents of matter are impossibly narrow
threads of concentrated energy. The various different
ways these superstrings can vibrate correspond to different
fundamental particles, such as the
up-quark and the muon-neutrino. The idea is well on the way to becoming a
"theory of everything", uniting the laws of physics
to explain how all matter and energy behave.
Visible
strings
One of the strangest features of string theory is that it requires many more
dimensions than we can see: the only way the vibration modes of the superstrings
can be sufficiently diverse to create all particles is if the superstrings
vibrate in a space-time of 10 dimensions. Of course,
we appear to live in a universe with only four
dimensions - three of space and one of time - so string theorists have
postulated that the extra dimensions are "rolled up" much smaller than the
dimensions of an atom. However, until now no one had seen evidence to support
string theory and many scientists dismiss its ideas as untestable conjectures.
But are they about to be proved wrong? The answer lies with the
big bang that kicked our universe into existence.
String theory suggests that our universe maybe a three-dimensional island
or "brane" moving through 10-dimensional space, and that the big bang might
have been caused by a collision between two such branes (New Scientist, 16
March 2002, p26).
"Superstrings are well on the way to becoming a 'theory
of everything', uniting the laws of physics to explain how all matter and
energy behave"
This kind of collision would release a tremendous amount of
energy, which would create a plethora of different kinds of stringy object.
One type is the fundamental superstrings. Another is strange objects called
Dirichlet or "D" branes that exist within each brane and as connections between
branes, but intersect with only one dimension of our universe. As a result,
they look to us like one-dimensional superstrings. But these are not necessarily
the tiny strings we associate with fundamental particles: they can be of
all sizes right up to astronomical dimensions. "Contrary to what we used
to think, fundamental strings need not be ultra-tiny," says Tom Kibble of
Imperial College London. And the bigger strings can be big enough to leave
a visible mark on our universe. That's because a string distorts the space
around it in a unique way. We are used to objects with mass or energy distorting
the space around them, rather like a person's weight distorting the flat
surface of a trampoline. This distortion of space is the origin of every
object's gravitational attraction. However, a string is somewhat different
from a normal object. All its energy is held on a one-dimensional line, not
spread through space, and this concentrated energy distorts the space around
it into a conical shape, with the string as its axis. If there were a string
between us and a distant galaxy, it would distort the light of the galaxy
so that it could take two possible routes to the Earth. The result would
be two identical images of the galaxy only a few arc-seconds apart in the
sky (an arc-second is roughly the angle a small coin would make when seen
from 2 kilometres away). And this is exactly what an Italian-Russian group
claims to have found last year. The team, led by Mikhail Sazhin of Capodimonte
Astronomical Observatory in Naples and the Sternberg Astronomical Institute
in Moscow, christened the image pair Capodimonte-Sternberg Lens Candidate
1, or CSL-1. It consists of two apparently identical elliptical galaxies
roughly 10 billion light years from Earth and a mere 2 arc-seconds apart
(Monthly Notices of the Royal Astronomical Society, vol 343, p353). Seeing
two identical galaxies is nothing new: it also arises from a phenomenon known
as gravitational lensing (New Scientist, 13 November, p42). This occurs
when light from a distant galaxy passes close to another galaxy on its way
to Earth. The mass of the intervening galaxy distorts the path of the light,
producing multiple images of the distant galaxy. But gravitational lensing
tends to manifest itself as an odd number of images that differ in brightness,
often greatly. In the case of CSL-1, no intervening galaxy or cluster of
galaxies is visible, there are just two images, and they are of equal brightness.
So gravitational lensing doesn't seem to offer an explanation. "It looks
like the signature of a string to me," says Kibble. Tanmay Vachaspati of
Case Western Reserve University in Ohio is similarly optimistic. When he
first noticed Sazhin's paper, he and his student Dragan Huterer tried to
come up with reasons why a string could not be responsible. One of the first
things that occurred to them was gravitational lensing, and they soon realised
this hypothesis could easily be tested. The way galaxies are randomly distributed
throughout the universe means that if you look at a patch of sky, gravitational
lensing should be a rare phenomenon. If there's a string around, however,
double images will be a lot more common. "A string should create other double
images of galaxies in the neighbourhood - far more than would be expected
by random chance," he says. "A simple follow-up observation should be enough
to resolve the issue." Sazhin and his colleagues have now made just such
an observation in a "field" 16 arc-minutes square centred on CSL-1, they
found in other double images. Between nine and 200 would be expected for
a string, they say, but just two would be expected by chance from the
gravitational lensing of intervening galaxies. "This already sounds very
exciting," Vachaspati says.
Good vibrations
It's particularly exciting because CSL-1 is not the only observational evidence
for a string: there is also the curious case of the double quasar known as
Q0957+561A,B, the first confirmed case of a gravitationally lensed object,
observed by the Jodrell Bank telescope
in the UK, in 1979. The two images are formed by the gravity of a galaxy
that the light of the quasar so that it follows two distinct paths to Earth.
The paths are different lengths and so the light takes a different time
to travel along each one As a result,outbursts in one image are mimicked
by identical outbursts in the other image 417days later This year, a
team from the US and Ukraine, led by Rudolph Schild of the
Harvard-Smithsonian center for Astrophysics, noticed some peculiar anomalies.
Four times between September 1994 and July 1995, the two images of Q0957+561A,B
brightened and faded by about 4 per cent but without any time delay. Each
oscillation in brightness lasted about 100 days, and they were not repeated.
The only way such a synchronous change in brightness could occur would be
if the cause was not the quasar itself but rather an object between the quasar
and the Earth. Schild and his colleagues claim that the idea that best fits
the bill is an oscillating loop of string. These oscillations would occasionally
cause the string to encroach on the two light paths from the quasar, altering
the images we see. The string also appears to be moving across our line of
sight at about 70 per cent of the speed of light - which is why it affected
the quasar for only a limited time.
Return of the cosmic string
WE ALREADY know there cannot be an enormous number of giant
superstrings out there~ Thats because they share many characteristics with
"cosmic strings"1 concentrated threads of energy that physicists once believed
to be scattered throughout space. In the l980s, cosmologists were greatly
interested in such structures: they were thought to be defects in space and
time, formed by abrupt misalignments in the misalignments in
the fundamental fields of nature when the universe cooled in the aftermath
of the big bang,and locked forever in the weave of the fundamental fields
threading the universe. |
"We are left with the conclusion that we are very lucky to have a string on our doorstep"
To oscillate once every 100 days or so, the loop has to be very small in astronomical terms - roughly 1011 kilometres. It also has to subtend an angle at the Earth substantially smaller than the separation of the images or it would create a spiky variation in the quasar's brightness rather than the smooth, periodic variation observed. The combination of these two conditions implies that the string is shockingly close to us - in our own galaxy, within about 10,000 light years of the sun. So is it pure coincidence that a stringy relic of the big bang has ended up in our neighbourhood, or are these things scattered liberally throughout the universe? Strings would also emit gravitational waves and these should distort the space between them and us, introducing fluctuations in the time light takes to reach us and therefore in the observed timings of pulses. Though Kibble points out that there are a number of uncertainties in this calculation, the fact we do not see such an effect suggests a limit on how many strings there are between us and known pulsars. So we are left with the conclusion that we are very lucky to have a string on our doorstep. Scientifically speaking, that's not a very satisfying conclusion. Indeed, the whole question of string observation is still riddled with uncertainty and many researchers are wary of rushing to conclusions about Sazhin's observations. "I think it is too early to get excited," says Edmund Copeland of the University of Sussex in the UK. "There may be other possible explanations. Until the unique string aspects are confirmed, I think we should remain a little cautious:'
It is always possible, for example, that the fluctuations in the brightness of Q0957+561A,B looked the same entirely by chance. Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics still favours the possibility that we have just seen a set of identical twin galaxies. "CSL-1 is most likely just a pair of galaxies that happened to be close together on the sky," he says. "We know of many close pairs of galaxies in the local universe, including our own Milky Way and Andromeda." Such a coincidence would be disappointing, Vachaspati says. "I am hoping nature won't have played such a trick on us." What everyone needs now is more evidence. To prove that each galaxy pair is a lensed, double image of a single galaxy, it will be necessary to measure the spectra of both objects and show them to be the same. Another angle of attack would be to find more candidates like CSL-1 and Q0957+561A,B. But the best approach might be to look for gravitational waves. Strings would produce gravitational waves because they get kinked as they meet each other in space. As two straight strings cross, for example, they can emerge from the meeting as two V-shaped strings. Every time strings cross, they can become more kinked, and to shake off a kink they emit a shockwave, cracking like a whip. This shockwave travels at almost the speed of light, and should produce an intense burst of gravitational waves. As first pointed out by Thibault Damour of the Institut des Hautes Études Scientifiques in Paris and Alex Vilenkin of Tufts University in Massachusetts, "cusp" signals could be spotted by the VIRGO or LIGO gravitational wave detectors. "The signals are very distinctive," says Joe Polchinski of the University of California at Santa Barbara. "If they exist, they could be picked up in the next few years.
According to Polchinski, if strings are discovered it will take at least a decade to measure the signals precisely enough to deduce their properties. This, he says, may enable us to pin down their origins. It could be another source of disappointment: the observed strings could have nothing to do with string theory, but be low-energy versions of the cosmic strings that were once thought to have seeded the universe's structure (see "Return of the cosmic string", above). Nevertheless, it's an exciting prospect. String theory is big on imagination-stirring concepts, such as vibrating threads of energy that inhabit a multidimensional reality, and awesome collisions that create new universes. It may be that these elusive and fantastical strings have finally shown themselves.
