IT IS not every day that you watch the Universe being born. But five years
ago, researchers in the US published pictures of the early stages of the big
bang, just 10-34 seconds or so after it all started. Now others are getting in
on the act, and in July two international teams of physicists described their
own privileged views of the Universe’s birth pangs.
Their pictures are not the product of fancy computer trickery. The universes
are tangible enough, but scaled down: they sit in beakers on a laboratory bench
top. Strange though it may seem, there are some things you can find out about
unimaginably early events in the birth of the Universe by peering into a bucket
of liquid. Mind you, the liquids are far from ordinary. Some experiments used
“superfluid” helium, a bizarre liquid that exists at temperatures just a whisker
above absolute zero. Superfluid helium does weird things, like climbing up the
walls of containers and escaping. Others used liquid crystals, molecules that
exist in a strange state that is partway between ordinary liquids and
solids.
So what have these tubs to do with the behaviour of the early Universe? It
all hinges on the baffling question of why galaxies are distributed so unevenly
across the sky, clustering together in filaments separated by giant voids. Back
in the 1970s, Thomas Kibble, a theoretical physicist at Imperial College,
London, suggested a possible explanation. Perhaps, he said, when matter first
condensed shortly after the big bang it gathered around long, thin “cracks” in
the fabric of space-time, known rather whimsically as cosmic strings.
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Theorists believe that cosmic strings are the leftovers from a process that
separated the four fundamental forces of nature—gravity, the
electromagnetic force and the strong and weak nuclear forces—that we see
today. In the beginning, within the first split second after the big bang, there
was simply a single “primeval” force. Then the individual forces began budding
off in a series of abrupt transformations. Gravity separated first, followed
between 10-35 and 10-34 seconds after the big bang by the strong force.
Hot little Universe
It was during this second defection, in a Universe about a billion billion
times the temperature of the inside of a star and much, much smaller than a
single hydrogen atom, that cosmic strings may have been created.
Frozen into the fabric of the Universe while it was still unimaginably small,
the strings would now be stretched out into a vast, ethereal web across billions
of light years. But twenty years after Kibble suggested that cosmic strings
might be the seeds of the large-scale structure of the Universe, they remain
just an idea. No one knows if it is even possible to see them directly in deep
space. How, then, can we know if Kibble’s idea holds water?
One way is to try to recreate the processes that could have formed cosmic
strings. Unfortunately, these involve such furiously energetic conditions that
not even the most advanced high-energy physics labs can even get close. The
alternative is to look for a system that shares mathematical characteristics
with the early Universe, even if it is physically very different.
Enter superfluid helium and liquid crystals. Both can undergo abrupt changes,
called phase transitions, which transform the liquids into a different state.
When this happens, “flaws” can be created that are mathematically equivalent to
cosmic strings. In superfluids, liquid crystals and the early Universe, these
phase transitions convert the system from a symmetrical state to one that is
less symmetrical—a process called symmetry breaking.
Imagine a dinner table on which glasses are set on either side of each plate.
The glass on the left and that on the right are the same distance from the
plate, and this symmetry means that no guest is sure which to take. Then someone
makes an arbitrary choice—they take the glass on the right, say. This
means that everyone else must reach to their right, and the left-right symmetry
is broken. The choice could just as easily have gone the other way, but once it
is made somewhere, the whole table is switched to either a left-handed or
right-handed state.
Cosmic strings, then, are the debris left after a symmetry-breaking
transition like this, which split the strong, weak and electromagnetic forces
and left the strong force different from the other two (the weak and
electromagnetic forces split from one another at a later stage). Cosmic strings
could be formed if the transition did not happen smoothly throughout the
Universe. Left to its own devices, a phase transition happens everywhere at
once. But because the choices made when symmetry is broken can be arbitrary,
they can vary from place to place.
Back to the dinner table, it’s as if one guest reaches for the glass on their
left at the same instant someone else reaches for the glass on their
right—somewhere there is going to be an unlucky guest who gets no glass at
all and a lucky one who gets two. In the language of phase transitions, these
anomalous guests are called defects. Some kinds of defect can be smoothed out
with a little local reshuffling: a guest with two glasses can pass one down the
table to a bereft guest nearby.
But for the early Universe, and for liquid crystals and superfluids, there
are types of defect that can’t be put right without everything going back and
starting again. Imagine a symmetric state in which a crowd of people can choose
to look in any direction. In the symmetry-broken state the people would all be
looking the same way (like the diners who have all reached to their right). Now
imagine two crowds meeting, one in which people are all looking in one direction
and the other in which they are all looking somewhere else. Each represents an
area in which symmetry has been broken in a different direction.
Where the crowds meet there are defects. But at the boundary between any two
crowds, it is fairly easy to patch up the defects if the people at the edge
shuffle around so that the direction of their gaze changes smoothly from that of
one crowd to the other. This is not as “perfect” a situation as one where all
the people look in the same direction, but at least there are no abrupt
defects.
If three crowds meet, however, there is a point at the centre where one
unfortunate person has the impossible task of trying to look in the same
direction as every neighbour, even though their gazes rotate smoothly from one
direction to the other through a full circle. This person is like a single
leftover from the initial symmetric phase, doomed to remain undecided while all
around the lines of sight circulate in a vortex. Defects of this kind, which are
impossible to remove without everyone reorienting themselves, are called
topological defects.
In three dimensions, symmetry breaking can lead to several kinds of
topological defect. One kind is where the field lines—the gazes of the
crowd—radiate out in all directions from a single point. These are called
monopoles, and they are predicted by many cosmological theories. An example is a
magnetic monopole, which is a magnet with a south pole (say) but no north. You
can’t create magnets like this—if you chop a bar magnet in half, you
simply create two new poles at the cleaved ends. But they are a bugbear of many
theories, which predict that they would have been produced in such huge numbers
by the primordial symmetry-breaking transitions that they should now dominate
the Universe, even though none has ever been seen. Cosmic strings are another
kind of topological defect. They are long thin regions made of stacks of the
vortices described above.
Atomic conformists
In 1985 Wojciech Zurek, a physicist at the Los Alamos National Laboratory in
New Mexico, realised that the mathematical theory that described the formation
of cosmic strings in the early Universe applied equally well to the formation of
topological defects in superfluid helium as it is cooled through the transition
from the normal liquid state to its superfluid state. In the normal liquid
state, each atom does its own thing: they all move about at random. But the
peculiar thing about a superfluid is that every atom must do the same
thing—so the motions become coherent. It’s like a crowd in which everyone
must try to look in the same direction as their neighbours.
The topological defects in a superfluid occur when different parts of the
fluid choose, as the liquid cools through the transition, to move in different
directions. Where these regions meet, little whirlpools—rotating
vortices—are created with a tiny thread of normal (non-superfluid, liquid)
helium at their centre. Lines of such vortices are like cosmic strings in a
beaker.
Or so the theory said. But creating and detecting such vortices
experimentally is extremely difficult. Zurek suggested looking for them in the
superfluid form of the isotope helium-4, which exists below 2.17 degrees kelvin.
To freeze-in topological defects, the temperature of the helium must drop
quickly through the transition. But this is very difficult at only a couple of
degrees above absolute zero. Zurek teamed up with Los Alamos physicists Jim
Hoffer and S.S. Shiah in the late 1980s to carry out the experiments, but these
proved too difficult and the results were inconclusive.
Then in 1991, researchers recreated the big bang in a system that was easier
to handle. Bernard Yurke from AT&T Bell Laboratories in New Jersey joined
cosmologist Neil Turok at Princeton University and his co-workers to use a soap
bubble-like film of a liquid crystal—a liquid in which the molecules are
relatively ordered. Most liquids are totally disordered: you see the same jumble
of molecules no matter where you look. But for liquid crystals not all
orientations are the same. On average, all of the molecules tend to point one
way, like logs in a log jam. Liquid crystals are mostly rod-like molecules, and
it is easier for them to pack together in a liquid if they all point in the same
direction, just as it is easier to stay standing than to lie down in a dense
crowd of people. They are still genuine liquids, because the molecules are still
free to move though they stay aligned.
Tangled topology
But the direction of alignment can get into tangles—topological
defects. The handy thing about using liquid crystals as a model for cosmic
strings is that the symmetry-breaking phase transition happens at around room
temperature and the defects show up as dark lines under a microscope. You can
even see them with the naked eye. Yurke and colleagues used an organic compound
that changed from a true liquid (the symmetric state) to a liquid crystal (the
broken-symmetry state) at around 35 °C. They cooled the substance rapidly
through this temperature and saw many topological defects, including linear,
string-like defects and isolated, monopole-like ones.
These experiments seemed to confirm that Kibble’s mechanism for the formation
of cosmic strings was viable. But how about the numbers involved? If you scaled
up the experiments to match the conditions (such as cooling rate) of the early
Universe, would enough cosmic strings be produced to make the galaxy-seeding
idea plausible?
In 1994, Mark Bowick and his colleagues at Syracuse University in New York
looked into this question using similar experiments on liquid crystals. They
found that the number of strings formed relative to the number of separate
regions that grew in the broken-symmetry phase was about the same as predicted
for the early Universe by Kibble’s mechanism. But it was hard to pin down the
numbers completely using this approach, and the estimates were very rough.
Yet at the same time that these experiments were being carried out,
researchers at Lancaster and Exeter universities, led by Peter McClintock,
finally found a way to carry out Zurek’s proposed experiment in superfluid
helium-4. They devised a way of rapidly expanding liquid helium, carrying it
through the transition from normal fluid (at high density) to superfluid (at low
density).
They looked for the presence of defects by sending sound waves through the
superfluid. The idea was that the more vortices were generated, the more the
sound waves would be scattered. Sure enough, they discovered that huge numbers
of string-like vortices were being generated, implying that there could have
been plenty of cosmic strings around after the big bang to seed galaxy
formation.
Tricky tracking
But their technique for detecting vortices was still rather rough and ready
and they could not track individual vortices. All they could do was look at the
sound waves that came out of the beaker and infer what was happening inside.
That is why Kibble has now teamed up with researchers from around the world to
experiment at the Low Temperature Laboratory of Helsinki University of
Technology in Finland with the other helium isotope, helium-3. Because helium-3
becomes superfluid at just a few thousandths of a degree above absolute zero, it
is much trickier to handle.
But the advantage of helium-3 is that, unlike helium-4, it can be studied
using nuclear magnetic resonance spectroscopy, which is sensitive enough to
track individual vortices. What’s more, the researchers use an ingenious method
to carry the helium rapidly through the superfluid transition temperature. They
fire neutrons into a chamber of rotating superfluid helium-3, inducing a nuclear
reaction in which some helium nuclei absorb a neutron and then spit out a
proton, along with a considerable amount of nuclear energy.
This release of energy warms a small bubble of helium above the transition
temperature, and it quickly cools again to the temperature of the surrounding
superfluid. In the process, a tangle of vortex loops is generated in the bubble.
If they are big enough, the rotation of the vessel as a whole allows some of
these to expand and survive indefinitely. This provided the opportunity to put
the cosmic strings idea through a stringent numerical hoop. If Kibble’s original
idea about cosmic strings was right, Zurek calculated, there should be a clear
relationship between the smallest vortices that can expand and be detected, and
the chamber’s rotation speed.
Meanwhile, an almost identical experiment was performed by researchers at the
Centre for Very Low Temperature Research in Grenoble, France, and Lancaster
University, who decided instead to look at how far apart, on average, the
vortices were. In both experiments, the numbers compared well with those
predicted by the Kibble/Zurek theory, adding still more weight to the idea that
comparable topological defects were created in the early moments of the big
bang.
This is still a long way from a proof that galaxies were strung across the
sky by cosmic strings. But experiments like these are the best proof we have
that cosmic strings exist. They also prove that we can learn something about the
birth of the cosmos without straining our eyes staring at the night sky or
straining our research budgets funding megabuck science.