杏吧原创

IN the beginning there was light鈥攖he light created by the fireball of
the big bang. However, as the fireball expanded and cooled, the light shifted
out of the visible region of the spectrum, plunging the Universe into a dark
age. For millions of years, the Universe grew and grew鈥攁 hundred times in
size鈥攂ut still the cosmic darkness persisted. What brought it to an end
was the birth of the first stars and galaxies. 鈥淪uddenly light was everywhere,鈥�
says Abraham Loeb of Harvard鈥檚 Center for Astrophysics. 鈥淭he Universe lit up
like a Christmas tree.鈥�

Understanding that dark age鈥攁nd how it ended鈥攈as until now been
little more than an astronomical pipe dream. Though modellers have predicted
what to look for, current telescopes simply cannot see that far back in
time.

Now, however, it looks as if help is finally on its way. Funding for two new
satellites鈥攐ne American and one European鈥攈as just been approved and
they will soon be scanning the heavens for signs of the light from the very
earliest galaxies. Meanwhile, astronomers are reaching out to farther, fainter
sources using nature鈥檚 own telescopes鈥攇iant galaxy clusters that bend the
light from more distant sources and focus it towards the Earth. And the
successor to the Hubble Space Telescope, the Next Generation Space Telescope now
on the drawing board, could soon bring those first stars and galaxies swimming
into view.

Ironically, we can travel back as far as the big bang because its remnants
persist in the cosmic background radiation, and reach us from a time when the
Universe was only 300 000 years old. But even pushing our present telescopes to
the very limits will only let us see the light from galaxies and quasars when
the Universe was about a billion years old鈥攔oughly 14 billion years ago.
We have no direct evidence of what the Universe was like between an age of 300
000 years and a billion years.

Astronomers prefer to express this gap in another way, since the time between
the big bang and any event depends on certain controversial vital statistics of
the Universe鈥攖he famous Hubble constant, for instance. So astronomers use
a phenomenon known as 鈥渞ed shift鈥�. As the Universe expands, the wavelength of
light from distant objects gets stretched out. The more distant the object, the
higher the red shift.

Because light from the most distant objects set off on its journey towards us
billions of years ago, looking at objects with high red shifts is like looking
back in time. Roughly speaking, the red shift is the factor by which the
Universe has expanded in size since the light of a galaxy or star began its
journey to us. The light of the big bang comes from a red shift of 1000 whereas
the light of the most distant observable galaxies from a red shift of just under
5. Astronomers therefore say that the period for which we have no direct
evidence of the Universe extended from a red shift of 1000 to a red shift of
5.

There is plenty of evidence that this youthful Universe was far from empty.
For one thing, the earliest, most distant galaxies we can see already contain
plenty of stars, and they cannot have formed overnight.

Another clue comes from unusually bright galaxies called quasars, which exist
in the far-distant Universe. To reach us, the light from these quasars must pass
through tenuous clouds of hydrogen gas which drift between galaxies. If the
clouds were made up of neutral atoms of hydrogen, which strongly absorb quasar
light, the light from the quasars would be snuffed out before it could reach
Earth. Since we can indeed see the quasars鈥攖he intervening hydrogen gas
has merely taken bites out of their spectra鈥�99.99 per cent of the hydrogen
must be in the form of ionised atoms, stripped of their electrons. Something
must have generated enough light to ionise all of this hydrogen, and since the
Universe is ionised out as far as we can see, it must have happened at red
shifts greater than that.

鈥淥nly intense ultraviolet radiation could have ionised all the Universe鈥檚
hydrogen,鈥� says Loeb. 鈥淎nd that could only have come from a population of hot
stars or quasars at red shifts greater than 5.鈥�

There is also evidence that the early Universe contained heavy elements,
which can only have come from stars. Only light elements came out of the big
bang鈥攈ydrogen, helium and the like. Heavier elements such as carbon have
to be 鈥渃ooked鈥� during nuclear reactions inside stars, and then blown out into
space when the stars explode at the end of their lives.

Spectral bites

The evidence for heavy elements comes from the fine details of the bites
taken out of quasar spectra by islands of neutral hydrogen. Typically, there are
a large number of such islands between a distant quasar and the Earth, all at
different red shifts, and their effect is to produce a large number of bites in
the spectrum. In 1994 David Tytler of the University of California in San Diego
and Len Cowie of the University of Hawaii in Honolulu independently discovered
that some of the gas clouds producing these bites contain not just hydrogen, but
heavier elements too.

Tytler鈥檚 work was done with the giant 10-metre Keck Telescope in Hawaii. In a
bright quasar at a red shift of 3.3 he studied hydrogen absorption features so
weak they must have come from diffuse clouds of gas rather than galaxies. In a
substantial fraction of all such clouds he found the tell-tale signature of
carbon. 鈥淲e do not yet know how widely distributed it is鈥攑erhaps only 10
per cent of gas has carbon in it, or perhaps 90 per cent,鈥� says Tytler.

The carbon could only have come from huge numbers of stars that had already
gone through their lives. 鈥淗ere we have the unmistakable fingerprint of
pre-galactic generation of stars,鈥� says Loeb.

Given this tantalising evidence, it鈥檚 clear that there must have been stars
or quasars lighting up the Universe at some point between the big bang and as
far back as we can see at present. But when and how did they form?

To date, the only way to try to answer this question has been to use
computers to simulate how the Universe evolved from its initial state, which was
smooth and featureless, to its state today, which is clumpy. Loeb and his
colleagues have done just that, recreating the birth of quasars and ordinary
galaxies. It was a process entirely orchestrated by gravity, says Loeb. Some
regions of the cooling fireball of the big bang were slightly denser than
others. The higher gravity in the denser regions attracted matter more quickly
than the rest, which boosted their gravity, which pulled in more matter, and so
on.

As always, there is a complication. Most of the Universe鈥檚 mass is invisible
and no one really knows what it is. However, assuming that most of this 鈥渄ark
matter鈥� clumps like ordinary matter, it is possible to make progress and
estimate, for instance, how many clumps of gas of a given mass collapse at a
given red shift. 鈥淚n the beginning, the Universe was very boring,鈥� says Loeb.
鈥淏ut between a red shift of 30 and 10 we find a rapid growth of structure with
lights coming on everywhere.鈥�

Quasars are thought to be galaxies whose light is generated when an extremely
hot disc of matter swirls into a central, supermassive black hole. Different
quasars shine with different brightnesses, and astronomers know how many quasars
they can see at each level of brightness. Loeb and his colleagues find that if
they assume that the quasars shine for about a million years鈥攕omewhat
shorter than generally accepted鈥攖he models produce just the right number
of quasars at each brightness between red shifts of 2 and 5.

Encouraged by the way this simple model matches the quasars we can see, Loeb
and his student Zoltan Haiman last year extrapolated the quasar luminosity
function to much higher red shifts. They found that the number of very bright
quasars鈥攐nes about as bright as 100 Milky Ways鈥攄rops off suddenly at
red shifts greater than 4. This makes sense鈥攂right quasars are formed when
a large clump of gas collapses. The bigger the clumps of gas, the longer it
takes to collapse, and before a red shift of 4, clumps big enough to spawn very
bright quasars had simply not had enough time.

However, Loeb鈥檚 most dramatic result is that at very high red shifts there
should exist a gigantic population of faint quasars鈥攐nes from the
brightness of the Milky Way down to a tenth the brightness of the Milky Way. In
fact, his team鈥檚 calculations indicate that faint quasars existed in such
extraordinary numbers that their combined light had already totally ionised the
Universe by a red shift of 10.

If such an early population of quasars existed, we should be able to see
their effects. For one thing, the free electrons stripped from hydrogen atoms by
the intense quasar radiation are excellent at 鈥渟cattering鈥�, or redirecting,
photons of light. The most numerous photons in the Universe are the ones left
over from the big bang. The way their intensity varies across the sky today
mirrors the clumpiness of the matter from which they emerged 300 000 years after
the moment of creation. However, Loeb predicts that free electrons at high red
shifts will influence part of this variation in a way that he can predict
accurately. NASA鈥檚 Cosmic Background Explorer (COBE) satellite studied the big
bang photons back in 1992, but its resolution was too low to pick up this
effect.

The good news, though, is that it should soon be within reach. Both the
American 鈥淢icrowave Anisotropy Probe鈥� (MAP), due for launch in 4 years鈥� time,
and the European Planck satellite (formerly COBRAS/SAMBA), to be launched 5
years later
(鈥淕enesis to Exodus鈥�, New 杏吧原创, 19 October 1996, p 30)
will be able to study the big bang photons at a much higher resolution than COBE
managed.

Stardust

Big bang radiation may harbour another signal from an early population of
quasars. Dust ejected by the stars in the host galaxies of quasars absorbs
optical and ultraviolet light from the quasars. It then re-radiates it in the
far-infrared, close to the peak of the spectrum of the big bang radiation. This
鈥渞eprocessed鈥� light should enhance the 2.726 kelvin temperature of the radiation
at short wavelengths. The effect is small enough for COBE to miss it but a more
sensitive instrument on DIMES, a satellite currently seeking approval by NASA,
could well strike lucky.

Loeb鈥檚 simulations concentrated on quasars, and it seems that there are
plenty of ways to test if quasars were responsible for lighting up the Universe.
But what about stars? Could they have formed very early in the Universe? This is
a difficult question, admits Loeb. Star formation in general is shrouded in
mystery. Generally speaking, a star forms when a cloud of gas collapses under
gravity. However, if the cloud is too hot, pressure will combat the effect of
gravity and prevent the cloud from collapsing. So to form a star, the gas cloud
must have a way of cooling down.

This isn鈥檛 as easy as it sounds. In today鈥檚 Universe this is accomplished by
a huge array of molecules which collide and radiate away the heat. However, the
atoms necessary for making all but the simplest molecule鈥攎olecular
hydrogen鈥攈ave to be made inside stars. It is a chicken-and-egg
situation.

One possibility is that molecular hydrogen acted as the necessary coolant for
star formation in the early Universe. Unfortunately, it is fragile and easily
shattered by ultraviolet light. Even worse, it forms most readily on dust
grains, but dust too has to be made in stars. This leaves only atomic hydrogen
as a possible coolant. The trouble is that atomic hydrogen is not very efficient
at losing heat if the temperature is below about 10 000 kelvin, and you need
temperatures at least 100 times lower to make stars. 鈥淭he truth is that we don鈥檛
understand star formation at a fundamental level,鈥� says Loeb.

Nature鈥檚 telescopes

Yet we known that carbon existed in the early Universe, so the stars must
have found some way to form. But how can we look for them? Present telescopes
are simply not powerful enough to go beyond a red shift of around 5.
Improvements in the Keck Telescope in the next year will help it to see galaxies
that are much fainter, says Tytler. But stars will be much harder.

One clever way around this is to use nature鈥檚 own telescopes. These are
鈥済ravitational lenses鈥�, clusters of galaxies whose gravity bends and magnifies
the faint light coming from distant objects. This technique has already yielded
the most distant galaxy鈥攁 lensed red arc with a red shift of 4.92. But it
could also help astronomers to see distant stars, which can briefly outshine
galaxies when they explode as supernovas at the end of their lifetime. Some
theorists, such as Jordi Miralda-Escude of the University of Pennsylvania and
Martin Rees of the University of Cambridge have predicted how such an explosion
should look at high red shift. 鈥淪ince neutral hydrogen was more common in the
early Universe, supernovae should show a sharp absorption feature known as the
Gunn-Peterson trough,鈥� says Miralda-Escude. 鈥淚 believe lensing could reveal
supernovas up to a red shift of about 6.鈥�

If supernovas are a good way to see stars, gamma-ray bursters could be even
better. These massive bursts of energy are widely believed to originate in
mergers between super-dense neutron stars. 鈥淭his is a very exciting
possibility,鈥� says Miralda-Escude. 鈥淕amma-ray bursters are between 100 and 1000
times brighter than a supernova at its peak and could get us beyond a red shift
of 5 before we get more powerful telescopes.鈥�

However, some astronomers are worried that they may not be seeing objects at
high red shifts because their star-forming regions are shrouded in dust, which
strongly absorbs ultraviolet and visible light. Unfortunately, large
ground-based telescopes are hot objects which glow at the very same mid-infrared
wavelengths where astronomers would expect to see the first stars if they were
embedded in dust. 鈥淚f this is the case, we will have to wait for a large cold
telescope in space,鈥� says Tytler.

Fortunately, just such a telescope is being planned. The Next Generation
Space Telescope (NGST) is the eagerly awaited successor of the Hubble Space
Telescope. 鈥淚t鈥檚 as close to being approved as it could be,鈥� says Study
杏吧原创 John Mather of NASA鈥檚 Goddard Space Flight Center in Greenbelt,
Maryland.

Mather hopes that the $500-million NGST will be approved by Congress
in 2002 for a launch in 2007. It will have a primary mirror between 4 and 8
metres across, compared with the 2.5 metre primary of Hubble, and will be
sensitive to infrared light between 1 and 5 micrometres in wavelength. 鈥淲e鈥檙e
pretty sure that if there鈥檚 anything to see at high red shift, we鈥檒l see it,鈥�
says Mather. 鈥淚鈥檇 expect the earliest things will be around a red shift 10 to
15.鈥�

The NGST is widely seen as the most exciting project in astronomy. 鈥淲e鈥檙e
getting extraordinary support from scientists who hope to use it,鈥� says Mather.
鈥淲hen Dan Goldin, the head of NASA, told the American Astronomical Society about
it, he got a standing ovation.鈥�

The NGST will have a field of view of about 3 per cent of the area of the
Moon, and according to Loeb it should see hundreds of quasars and star clusters
at a red shift of 10 in each field.

Mather firmly believes the NGST will be even more influential than the Hubble
Space Telescope. 鈥淭he cosmic dark age is one of the great mysteries of
astronomy,鈥� says Mather. 鈥淲e have no direct evidence of how galaxies were
formed, how the first stars formed without the help of the prior generations of
stars, how galaxies evolved, whether they were formed from aggregations of
smaller units or from subdivisions of large ones. Everything happened in the
cosmic dark age. It goes right to the heart of the question of how we got here.鈥�
If Loeb and his colleagues are right, we could soon have the answer.