JUNE 1992, THE NATIONAL SCIENTIFIC BALLOON FACILITY, PALESTINE, TEXAS, US:
As the sun goes down over Palestine, a giant white balloon is straining at its
rigging. The helium fill-line has been retracted and a flat-bed truck, stacked
with now-empty orange cylinders, is backing away in a cloud of exhaust smoke.
The only thing stopping the balloon floating off into the clear blue sky is
Tiny Tim, a monstrous mobile crane, which is holding the balloon鈥檚 2-tonne
payload on its jib. A man on the tarmac lifts his arms and the crane driver
answers with a thumbs-up. Tiny Tim releases its charge.
The payload of the Medium Scale Anisotropy Measurement (MSAM) is now rising
into the air taking with it the hopes of the team of scientists who built it.
They are both exhilarated and racked with nerves as they watch the balloon
dwindle in size. 鈥淥ne time, our balloon actually burst,鈥� says Ed Cheng of
NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland. 鈥淭here was
absolutely nothing we could do about it.鈥�
MSAM has been designed to measure tiny 鈥渞ipples鈥� in the cosmic microwave
background radiation, the tepid afterglow of the big bang explosion in which
the Universe was born. Incredibly, the radiation left over from the primordial
fireball still pervades all of space 15 billion years after the event. It
rains down on us from every direction in the sky. If the experiment succeeds,
it will afford Cheng and his colleagues a unique glimpse of the Universe as it
was just 300 000 years after the big bang.
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The balloon does not burst. Cheng and his fellow team members breathe
again. In the control room everyone is glued to computer monitors. Soon, MSAM
is at an altitude of 38 kilometres, where the temperature is 鈭�40 掳C
and the air pressure barely half a per cent of that at sea level. The balloon
has grown to fill a volume of nearly a million cubic metres and is big enough
to fill a football stadium. Lowered on a cable so that it dangles 100 metres
below the balloon, MSAM is about to open its eyes to the radiation from the
beginning of time. 鈥淢onths of hard work are riding on the next few hours,鈥�
says Cheng.
The equipment has to be hoisted to the edge of space, because water vapour
in the atmosphere also radiates microwaves at the wavelength where the relic
radiation is at its brightest. A satellite, like NASA鈥檚 Cosmic Background
Explorer (COBE), can do the job even better. But balloon experiments take
considerably less planning and are much cheaper.
The cosmic background radiation which the MSAM telescope is designed to
observe comes to us directly from the epoch when galaxies like our own Milky
Way first started to congeal out of the primordial stuff of the big bang.
Before this period, about 300 000 years after the big bang, the Universe
consisted of a hot, plasma fireball of free-flying electrons and atomic nuclei
caught in a maelstrom of highly energetic radiation in the form of photons. In
this early Universe, the force of gravity, which would have pulled matter
together, was overwhelmed by the violence of the interactions between photons
and free electrons and between free electrons and nuclei.
Everything changed dramatically, however, when the expanding Universe
cooled to about 3000 掳C. As the violence of the interactions lessened,
atomic nuclei were at last able to begin to capture the free electrons and
keep them in orbit. Photons 鈥� which do not interact easily with electrons held
in orbit 鈥� were suddenly free to fly across the now 鈥渢ransparent鈥� Universe,
instead of zigzagging their way through space, constantly scattered by free
electrons.
At this point, at the end of the 鈥渆poch of last scattering鈥�, matter and the
photons of radiation went their separate ways. Matter, now 鈥渄ecoupled鈥� from
the disruptive bombardment of radiation, began to clump together under the
influence of gravity, eventually forming galaxies. And the photons continued
their way across the Universe. It is these photons, greatly cooled by the
expansion of the Universe, that we can detect as the cosmic microwave
background radiation.
MSAM鈥檚 sensitive detectors are designed to search for tiny variations or
鈥渞ipples鈥� in the temperature of the radiation from this period. These ripples
鈥� if they can be found 鈥� are potentially far more interesting than the vast
ripples picked up by the COBE satellite in 1992. Despite the storm of global
publicity that greeted NASA鈥檚 announcement at the time, the ripples picked up
by COBE are probably the result of tiny differences in the density of the
Universe during its first split-second of existence. If so, they tell us only
about the physics of exotic particles in an impossibly remote epoch.
Galaxy ripples
On the other hand the ripples that MSAM is looking for reflect later
conditions when the galaxies began to form. These ripples are much smaller 鈥�
on a scale of half a degree, or the apparent diameter of the Moon as seen from
Earth, rather than the 7掳 or more found by COBE. 鈥淭he small-scale ripples
are the Rosetta stone for many theories of galaxy formation,鈥� says Cheng.
鈥淭hey contain a wealth of information about the origin of structures in
today鈥檚 Universe.鈥�
The ripples reflect differences in the energy of photons. Slightly hotter
areas were created where photons picked up energy during collisions with free
electrons which had not been mopped up by atomic nuclei. These leftover
electrons were moving rapidly, under the influence of gravity, as matter
rushed together to form clusters of galaxies. When they interacted with
photons, they transferred energy to the photons, boosting their energy so that
they appear hotter. From the amplitude and scale of the ripples, cosmologists
may be able to answer several questions about conditions in this era of the
early Universe.
One thing in particular they would like to know is how many thousands of
years it took atomic nuclei to mop up the Universe鈥檚 free electrons.
Cosmologists can estimate this time interval by measuring the smallest angular
scale at which ripples first appear in the microwave background. To understand
why, we need to think in terms of time as well as space. The photons in the
cosmic background appear to be coming from a spherical shell surrounding the
Earth. But not all photons broke free of matter at the same time. Those that
escaped near the beginning of the epoch of last scattering have travelled
farther, and for longer, than those coming from near its end. The shell can
therefore be thought of as having a radius in time (see Figure).
Within this 鈥渟urface of last scattering鈥� will be regions that are slightly
denser than average and regions that are slightly less dense than average. The
dense regions will be slightly hotter than average and the rarefied region
slightly colder. They will be of all sizes.FIG-mg19483801.GIF
Now if we observe the sky on a small enough angular scale, we will pick up
radiation from many such regions along the line of sight. On average, as many
will be cold as hot, so they will cancel each other out and we will pick up no
signal whatsoever. A signal should first appear when the sky is observed on an
angular scale which is comparable with the width of the surface of last
scattering. The best bet is that this will be between a tenth and a half of a
degree.
The ripples could also tell us whether the Universe will continue to expand
forever or will one day stop expanding and collapse down to a 鈥渂ig crunch鈥�.
Precisely what happens depends on how much mass there is in the Universe,
something scientists don鈥檛 know for certain. However, the more massive the
Universe, the more its expansion has been braked by the gravity of that mass.
The greater the check on the expansion of the Universe, the larger the
features on the surface of last scattering will appear to us. Thus the angular
scale on which the ripples have their greatest amplitude is sensitive to the
mass of the Universe.
As the sun comes up over Palestine, Cheng and his colleagues order MSAM to
close its shutter. The balloon has drifted for six hours and observed 20
patches of sky at four wavelengths between 0.4 and 2 millimetres. Now the
number crunching begins.
AUGUST 1994, GODDARD SPACE FLIGHT CENTER, MARYLAND: It took a while but
Cheng and his colleagues finally concluded that their balloon experiment had
indeed picked up ripples on a scale of half a degree in the background
radiation, varying in temperature between 5 and 19 parts in a million
(Astrophysical Journal, vol 422, p L37). And Cheng鈥檚 group was not alone.
Indeed, so great is the interest in detecting small-scale ripples that about
ten experiments worldwide are busy searching, and picking up, ripples.
Lyman Page of Princeton University, another member of the MSAM team, is
involved in a ground-based experiment in Saskatoon called Big Plate. Two
groups of experimenters have been busy at the South Pole, where temperatures
are so low that that water vapour is frozen out of the air. Mark Dragovan and
his colleagues at Princeton University claimed to have detected small-scale
ripples with their Python experiment. So too did a team led by Phil Lubin of
the University of California at Santa Barbara.
Lubin, together with Paul Richards and Andrew Lange of the University of
California at Berkeley was also involved in a balloon experiment called MAX,
which was flown by Joshua Gunderson at the University of California at Santa
Barbara and his colleagues. And these are just the experiments in the US. In
Europe, an Anglo-Spanish group has been probing the background radiation for
many years from the summit of a volcano in Tenerife.
Many of the research groups are claiming to have detected small-scale
rippples that look encouraging, but no one has yet dared to break out the
champagne. 鈥淓veryone is seeing something,鈥� says Page. 鈥淭he trouble is no one
is quite sure that what they are seeing is really in the big bang radiation or
has another explanation altogether.鈥�
Interpreting the data is just as hard as obtaining them. All the readings
can be scuppered by spurious radiation from the surrounding environment or
even bremsstrahlung radiation from electrons moving in gas in our Galaxy or
beyond it. Local sources of error can be fairly easily dealt with but the only
way to rule out bremsstrahlung, and confirm that the observed ripples are in
the big bang radiation itself, is to make parallel observations at longer
wavelengths, where the spectra of bremsstrahlung are different from the cosmic
background radiation and therefore easy to distinguish. There is plenty of
confusion: MAX recorded a 鈥渨hopping signal鈥� in one part of the sky. Yet the
experiment saw only a very small signal in another patch of sky.
Confusing signals
鈥淔or ten years, we鈥檝e been trying to see something 鈥� anything,鈥� says
Stephan Meyer, another member of the MSAM team. 鈥淣ow that our experiments are
sensitive enough to see things, we鈥檙e in a quite different boat. We鈥檝e now got
to do experiments to try and eliminate confusing signals.鈥�
One worrying possibility is that everyone is being deceived by a new class
of objects emitting signals in the range that the astronomers are trying to
pick up. 鈥淚f there is a new class of objects whose peak emission is in the
millimetre or sub-millimetre region, then it could limit the cosmology we will
be able to do,鈥� says Meyer. Everyone had hoped that a clean signal from the
cosmic background would provide a window into the early Universe. 鈥淥ne worry
is that we鈥檝e stumbled on a new class of sources that could make it hard,鈥� he
says.
鈥淭he current problem with all the measurements is that they need
confirmation,鈥� says Page. Things will be clearer when the results from each
group are either confirmed by another experiment by the same group or by the
observations of another group. 鈥淪o far, that hasn鈥檛 happened,鈥� says Page.
In June of this year, MSAM rose into the blue skies above Palestine for the
second time. Its instruments observed a portion of the sky which had a 50 per
cent overlap with the area observed in the first flight. 鈥淎t the moment, we鈥檙e
still analysing our data,鈥� says Page. 鈥淏y the end of the year, we should know
whether or not we鈥檝e confirmed our earlier result.鈥� Many of the other
experiments have been repeated as well and everyone is waiting with baited
breath. 鈥淭he results are expected within the next six months,鈥� says Page.
鈥淓verything鈥檚 going to look a lot clearer this time next year.鈥�
Hunting for ripples
SO EXCITED are astronomers by the prospect of measuring small-scale ripples
in the big bang radiation that several new groups of experimenters are
planning to join the fray. One of the most exciting projects being planned is
an array of 10 microwave horns, designed specifically to map the microwave
background. In this age of scientific superlatives, when every instrument is
called a super-this or a very-large-that, it鈥檚 refreshing to hear of an
exception to the rule.
The Very Small Array has been proposed by a team of astronomers from the
University of Cambridge, Jodrell Bank at the University of Manchester and the
Astrophysical Institute of the Canaries. The cost will be 拢3 million. If
funded, and a decision is expected next month, it will be built on a mountain
top in Tenerife, one of the places in the world with the least atmospheric
water vapour.
At present, astronomers hunt for small-scale ripples with arrays of radio
telescopes designed for other purposes, or with balloon-borne experiments
which afford at most a 10-hour glimpse of the big bang radiation. But the Very
Small Array, by being a dedicated array capable of continuous observations of
the sky, will overcome both these problems.
Last April, Anthony Lasenby of the University of Cambridge and his
colleagues submitted their proposal to the Science and Engineering Research
Couoncil. They envisage the 10 microwave horns arranged on a 鈥渢ilt table鈥� and
operating at four frequencies. A prototype of the Very Small Array, called the
Cosmic Anisotropy Telescope, or CAT, began operating at Cambridge in August.
The instrument, which consists of only three microwave horns which map a
4掳-by-4掳 portion of the sky, has proved its worth.
Meanwhile, a similar instrument is being planned by Anthony Readhead and
his colleagues at the Owens Valley Radio Observatory near Bishop in northern
California. But there are more ambitious plans in the air.
After the 15-year slog to make COBE a reality, some of the scientists are
proving themselves gluttons for punishment. Dave Wilkinson of Princeton
University, John Mather and Charles Bennett of the Goddard Space Flight
Center, Maryland, and others are already planning a new space mission,
tentatively dubbed Microwave Anisotropy Probe or MAP. They want to put a
satellite at the Lagrange-2 point. This is a point beyond the Moon on the
Earth鈥檚 orbit around the Sun where the gravity of the Earth, Moon and Sun
balance.
The advantage of such a location is that it is far away from the Earth, the
major source of confusing microwaves. The disadvantage is that a satellite at
L2 is in unstable equilibrium. It could be kept in position only by constant
nudges from onboard rockets.
The plan is to build a cheap, lightweight satellite to probe ripples in the
big bang radiation on a scale of half a degree. It would be less ambitious
than COBE, which also carried a sophisticated instument for spectral
measurements. 鈥淲e鈥檙e lookking at a timescale of at least 5 years 鈥� if we get
the funding from NASA,鈥� says Bennett.
The American satellite is not the only one being planned. In Europe, an
international team including Lasenby, Rod Davis of Jodrell Bank, George
Efstathiou of the University of Oxford and George Smoot of the University of
California at Berkeley has submitted a proposal to the European Space Agency.
The satellite, called COBRAS for cosmic background radiation satellite, would
observe the three COBE frequencies plus one other, and map a 20掳-by-
30掳 portion of the sky with a resolution of half a degree.
鈥淐OBRAS would complement the Very Small Array,鈥� says Lasenby. Currently,
the COBRAS proposal is undergoing 鈥渁 year of assessment鈥�. 鈥淥ptimistically, we
might see the satellite in space in 2002,鈥� says Lasenby.