COSMOLOGY: WHAT ASTRONOMERS DON'T KNOW BUT ARE NO LONGER AFRAID TO ASK
A briefing document prepared for the Royal Society
and the Association of British Science Writers
by Dr Martin Redfern
June 1995
Introduction Humans have known for thousands of years that the Earth
is round; for hundreds of years that the Earth goes round the Sun.
Yet only this century did Harlow Shapley recognise that the Milky
Way was but one typical galaxy among many and Edwin Hubble that the
Universe was expanding. Even then, to speculate about the origin of
that expansion or the evolution of galaxies was considered a
dangerously unscientific pursuit.
Today, bigger, more sensitive telescopes on Earth and in space
detecting radiation at a full spectrum of wavelengths above and
below the visible, are looking out into space and hence back in
time. It has taken light from distant galaxies millions or even
billions of years to reach us, so astronomers see them as they were,
millions or billions of years ago, at earlier phases in the
evolution of the cosmos. Physicists are beginning to recreate the
conditions that existed even earlier in the history of the Universe
in their particle accelerators and theoreticians are building a
conceptual framework to encompass the observations.
As Professor Sir Martin Rees says, the aim of cosmology is to put us
and our planet in cosmic context. But he warns against discussing
well established facts verified by observations in the same tone of
voice as "flaky speculations", since people tend either to believe
both or neither. As the Russian physicist, Lev Landau said
"Cosmologists are often in error but never in doubt!" However,
today, more of cosmology can be backed up by observation and
confidence. There is general consensus that the expansion of the
Universe began in a hot Big Bang and that the structures in the
Universe have evolved over time. Observation and extrapolation make
astronomers fairly confident in their model of the Universe back
through billions of years of its history to within a few seconds of
the Big Bang. Some, particularly the physicists, are as bold as to
extrapolate to within a few microseconds of the Big Bang and they
say that the nature of the Universe today depends on what went on
then. But the conditions during the first tenth of a billionth of a
second must have been so extreme that the physics can be only
speculative.
However, astronomers can now begin to pose questions that, a few
years ago, were the realm only for those flaky speculations -
questions such as how old is the Universe? How dense is the
Universe? How big is the Universe? How did matter first form in it
and how did it develop the structures we see today? They can even
consider what the ultimate fate of the Universe might be.
In his introduction to this cosmology press briefing, Martin Rees
made no apology for the fact that all the contributors are British.
In a recent survey of the most cited papers in astronomy, seven out
of ten are about cosmology and five are by British authors. Four of
those authors were at this briefing!
The formation of galaxies Among the stars in the sky are faint
smudges of light that look different from the sharp points of stars
themselves. Each is a galaxy, a star city. It is only 70 years since
we realised that such galaxies were external to the Milky Way. They
are large and bright, so telescopes can detect them across
unimaginable distances. The distances are so large that the length
of time it takes their light to reach us means that we are seeing
some of these galaxies as they were when the light left them
millions, or even billions, of years ago. Thanks to the
light-collecting power of big, modern telescopes, it is possible to
study galaxies at big 'look back' times and so use them as tracers
of the history of the Universe.
In our own Milky Way, optical light reveals stars and also dark
bands of dust obscuring the centre of the galaxy. Moving to longer
wavelengths, such as infrared and microwaves, enables astronomers to
see through the dust bands and detect interstellar molecules and the
bulging central nucleus of the galaxy. That bulge extends into a
great halo containing very dense clusters of extremely ancient
stars. It seems as if that part of the galaxy is much older than the
dusty spiral arms in which our Sun is situated. Clearly, the galaxy
has not always been the way it is today and is an evolving
structure.
Turn the telescopes and look out into deep space and, thanks to the
sensitive modern electronic CCD detectors, a hundred times more
powerful than a photographic plate, a long exposure begins to reveal
the extent of the visible Universe. Divide the sky up into the 360
degrees of a circle and each square degree contains more than a
million galaxies.
The expansion of the Universe stretches the light coming from
distant galaxies in just the same way as the sound of a receding
siren is stretched to a lower pitch as a police car races away. This
stretching, detected by Edwin Hubble in the 1920s, moves the
wavelength of visible light further towards the red end of the
spectrum so it is called the redshift. Since all of space is
expanding, the further away from us a galaxy is, the faster its
motion and hence the greater its redshift. So the redshift is a
measure of cosmological distance, but only a relative measure unless
we know the expansion rate of the Universe.
Looking back even to our nearest neighbour, the great spiral galaxy
in Andromeda, takes us two million years back in history, to before
the emergence of modern man. The far side of the Virgo cluster of
galaxies, of which we are a part, takes us back 150 million years,
to a time before the dinosaurs. Powerful telescopes can still see
galaxies more than 12 billion light years away, at a time when the
Universe was probably less than a billion years old.
There are two main types of big nearby galaxy. Spirals, such as our
own, with a big inner bulge and less structured elliptical galaxies.
The ellipticals are seen as far as the telescope can probe so must
have formed very quickly in the early Universe. They are essentially
passive galaxies. There is little gas to form new stars and most of
the stars are now old, slowly burning up their nuclear fuel until
they die.
Spiral galaxies are more complex. Their inner bulge looks rather
like a miniature elliptical galaxy, full of old stars, but the
spiral arms are still the site of extensive star formation and are
filled with dust and young stars. However, no one is quite sure how
the spiral arms form or over what timescale. Some galaxies appear to
be in the process of bumping into one another, either merging or
passing light great ships in the night and evidence from these and
computer simulations of such interactions suggest that such
interactions could produce the shock waves that form the spiral
arms.
The evolution of galaxies is by no means complete. There are several
dwarf galaxies surrounding our own, including some only recently
discovered. They, together with our companion galaxies, the Small
and Large Magellanic clouds, are slowly moving in an intricate dance
through space, orchestrated by gravity; a dance that seems likely to
conclude with the merging of the smaller galaxies into our own great
spiral Milky Way.
The Hubble Space Telescope, particularly since its in-orbit repair
in December 1993, is the most powerful tool for studying galaxies.
It can resolve details in the cores of relatively nearby galaxies as
well as detecting light from the most distant objects in the
Universe. It has revealed that our neighbouring spiral galaxy,
Andromeda, appears to have a double core, perhaps the remnant of an
earlier merger. And it has shown details of a great jet of high
velocity material coming from the compact core of the galaxy M87 in
Virgo. The energy source for that jet must be so powerful and yet so
compact that astronomers have calculated that it can only be due to
the effects of a massive black hole.
Evidence for the existence of a massive black hole at the centre of
our own galaxy is more controversial. Intervening dust and gas makes
it almost impossible to see into the core except using infrared and
radio waves and their evidence is ambiguous. One bright source
called Sagittarius A*, thought by some to be a black hole, is not at
the precise centre of the galaxy and its brightness could be due to
star formation. If there is a black hole there, it is not
particularly active at the moment.
Moving out through space, telescopes reveal a whole zoo of different
types of galaxies, some of which have very active nuclei indeed. Of
those, the quasars are the brightest and are therefore visible over
the greatest distances. The nuclei of quasars can vary in brightness
over timescales of about a year, so they cannot be larger than about
a light year across. Yet they give out more radiation than an entire
galaxy. Again, matter spiralling into a supermassive black hole is
the only known explanation for their energy source.
Professor Richard Ellis, of the Institute of Astronomy in Cambridge,
has used the Hubble Space Telescope and instruments on the ground to
look for the precursors of galaxies. No one is sure whether huge
structures in the early Universe fragment to form first clusters and
then individual galaxies (a sort of top-down process) or whether
galaxies assemble from smaller components (the bottom-up
explanation). Richard Ellis' observations are beginning to suggest
that galaxies build up, but he needs to measure the relative motions
of what he believes are galactic precursors to see for certain if
they are coming together as part of one gravitational bound system.
Nature may provide the most powerful telescopes of all. Einstein
predicted that the gravitational power of galaxies could act to bend
the path of light passing nearby in much the same way as a lens
does. He thought that such gravitational lenses would be of no
practical use. In 1935, Fritz Zwicky suggested that such phenomena
might be used as natural telescopes and, over the last 20 years,
such natural gravitational lenses have begun to be detected.
In most cases, light from a particularly bright distant galaxy, such
as a quasar, is bent around an intervening but almost invisible
galaxy or cluster of galaxies. In some cases, the light follows
several different paths around the gravitational lens, producing
multiple images of the same object. In others, it produces a ring,
or segments of one. They are mostly very imperfect lenses producing
highly distorted images but they are often magnified images and
their light carries important messages about the structure and
processes at work in the most distant galaxies.
Such systems can also tell astronomers something about the mass of
the intervening galaxies and, by simple geometry, they should soon
be giving up enough information to help in measuring the distance
scale of the Universe. Some might magnify the first light from the
very dawn of galaxies, enhancing observations of the formation and
evolution of galaxies.
As the most powerful man-made telescopes peer out into space, the
galaxies get fainter and fainter. As instruments get bigger and more
sensitive, there may come a point where the numbers of galaxies tail
off. That tail-off could represent the edge of the Universe. In
fact, because of the finite speed of light, astronomers are seeing
back to a time rather than a place, and there must be a time before
which the first galaxies had not begun to shine. Exactly how long
ago that was depends on one of the as yet unanswered questions of
cosmology; the rate of expansion of the Universe. But it may be
about 12 or 14 billion years ago. At that point, the Universe was
probably no more than one billion years old and those first galaxies
may have produced the first light after the fireball of the Big Bang
itself, marking the end of the dark age of the Universe.
The structure of the Universe There is one signal that astronomers
can detect which is older even than the galaxies. In 1965, the
American radio astronomers Arno Penzias and Robert Wilson detected a
faint microwave radio signal coming from all directions in the sky.
It was the glow of the Big Bang itself, red-shifted down to just 2.7
degrees above absolute zero. It dates from a mere 300 000 years
after the start of the Bang, when the Universe first became
transparent and radiation could de-couple from matter.
In 1992, faint structure was finally detected in that microwave
background (see below) but, in spite of that, it is remarkably even,
smooth to within one thousandth of one percent. The Universe we see
around us is, by contrast, lumpy. We live on one very beautiful lump
called the Earth. That revolves around a lump called the Sun within
another lump, the Milky Way Galaxy. As astronomers look further and
further out into the Universe and map the positions of galaxies,
they find that the lumpy structure continues. The Milky Way is a
member of a local group of galaxies which in turn is part of the
Virgo cluster. On a larger scale still, there are super-clusters of
thousands of galaxies.
The difficulty is that, as they look into the night sky, quite apart
from all the nearby stars in the way, astronomers see the galaxies
spread out across the sky as if on a two dimensional sheet of paper.
Several painstaking programmes are attempting to map the galaxies in
three dimensions by using their redshifts as a measure of their
distance from us. At first, that meant looking at each galaxy
separately with a big telescope, measuring the spectrum of its light
and finding how far various marker bands in the light had been
shifted. Now, the process is being automated. Optical fibres are
being positioned to coincide with each galaxy in a telescope's field
of view and the light from each fibre analysed simultaneously
enabling up to 400 redshifts to be measure at once.
Even so, it is a long process. There are about two million galaxies
in the deepest catalogue we have (a two-dimensional map of galaxies
up to about two billion light years away). So astronomers are
adopting different strategies to get a glimpse of the deep structure
of the Universe without mapping it all out in detail. The first
surveys, notably one completed in 1986 by Huchra and Geller in the
USA only went a few hundred million light years deep. But they found
more structure than anyone expected. What seemed like a great wall
of galaxies, 200 million light years long, turned out to be part of
what can only be described as a bubble-like structure around great
voids containing comparatively few galaxies.
Attempts to look deeper have sampled the sky along thin pencil beams
in different directions. The first of these revealed what looked
like the posts in a picket fence, concentrations of galaxies at
regular intervals. Of more than 30 subsequent pencil beam surveys,
most show structures on similar scales to the great wall, but they
are probably not straight walls but the sides of bubbles, repeating
in a sort of foam. Another project is attempting to strip-mine the
sky, measuring the redshifts of all galaxies in a long, narrow band.
The redshifts of more than 2 300 galaxies have already been
recorded. The indications are that large-scale structure continues
at every scale, but it is difficult to be certain. It is a bit like
trying to map the Earth by looking at random points from a
satellite. You will probably pick up the major continents and oceans
but you might miss Australia or the Mediterranean.
Another problem comes in the conversion of redshifts into true
distances. Not only is the precise rate of the expansion of the
Universe unknown, but the expansion may also not be even. Analysis
of relatively nearby galaxies in different directions has shown
average redshifts to be higher in some directions than others. If
the sample of galaxies is genuinely random, that effect could be due
to a streaming motion among the galaxies, as if they were being
pulled by some great gravitational attractor. The existence of a
great attractor is controversial. It now seems that it is unlikely
to be something obvious like a giant cluster of massive galaxies,
but it could be something very big and diffuse, a slightly greater
concentration of mass in one direction than another, but on a very
large scale indeed.
The question that is now preoccupying astronomers such as Professor
George Efstathiou of Oxford University is how all this structure
formed from the relatively smooth Big Bang as recorded by the
microwave background. The complication comes from evidence that the
visible matter of stars and galaxies is only a fraction of the
material in the Universe. It is clear from the way stars rotate
within galaxies that there is at least 10 times as much mass there
than is visible. If such dark matter pervades the spaces between
galaxies as well, it could represent a hundred times the mass which
is visible. Clearly, the gravitational effects of such dark matter
will have influenced the evolving structure of what can be seen.
Cosmologists group the dark matter into two categories: cold dark
matter, which behaves rather like the matter we can see and is
predominantly shaped by gravity into clusters and other structures,
and hot dark matter made of particles that move too quickly to be
dominated by gravity but which are sufficiently numerous to
contribute significant mass to the Universe. So far, any choice
between the two can only be made by comparing computer simulations
and calculations of how structure might develop in universes with
different amounts and forms of dark matter. Simulations beginning
with the sort of faint structures now detected in the microwave
background and using only cold dark matter seem to give rise to
finer structures than those we see in the real universe. So many
astronomers interpret this as meaning that our Universe is not
dominated by cold dark matter. A more likely scenario seems to be a
combination of cold and hot dark matter but the jury is still out
and physicists are still searching for either form.
The weight of the Universe It is possible to calculate the mass of
the Earth from the orbit of the Moon or of an artificial satellite;
the mass of the Sun from the orbits of the planets and so on.
Continuing outwards to galaxies, astronomers were surprised to find
that the orbital velocity of stars round a galaxy does not slow down
much the further from the centre of that galaxy the star is. This
means that there must still be plenty of matter further out in the
galaxy than the visible stars, exerting its gravitational pull. In
fact, calculations suggest that there is 10 times as much invisible
matter than all the stars that can be seen. The same thing is true
for the motions of entire galaxies within clusters. The only
explanation must be that there is matter that cannot be seen, and an
appreciable amount of it. It used to be called missing mass, but in
fact it is not missing, simply invisible, so it is now termed dark
matter.
Some of it is certainly in vast clouds of gas between the galaxies.
It is so hot that it gives out x-rays, but otherwise it is
conventional matter. Ordinary matter (baryonic matter), made of
protons, neutrons and electrons would also be invisible if it were
in the form of planet-like objects too small to shine like stars
with their own light. The same would be true of collapsed objects -
neutron stars or small black holes. The search is on for these
through the effect they would have on light from stars behind them,
and there are already some promising candidates. But if there are
really large amounts of dark matter between the galaxies, it is
probable that it will take unfamiliar particles like massive
neutrinos or WIMPs (weakly interacting massive particles) to explain
it, and physicists are still not sure that such things even exist.
No one knows if the Universe is finite or infinite. The long
look-back times of the most distant quasars and of the microwave
background are not really the edge of the Universe, just the
furthest that light has had time to come from since the formation of
the Universe. To see beyond this horizon, the only thing to do is to
wait for more to come into view, time acting like the mast that a
sailor must climb in order to extend his horizon. To extend the
horizon of the cosmos may take billions of years, and if the
Universe is truly infinite, we would have to wait an infinite time
to know it. So that is a question that can never be answered.
However, astronomers can, in theory at least, answer the question
How dense is the Universe?
There are good reasons for finding the answer to this question. If
the visible stars and gas were all that there was, the Universe
would expand for ever, slowly cooling and darkening as stars ran out
of fuel. But already, from the rotation of galaxies, we know it is
10 times denser than that, and calculations simulating the evolution
of structure in the Universe suggest that it is denser still.
If it were about a hundred times denser than the material we can
see, it would reach a critical density where the gravitational pull
of matter was just enough to slow and perhaps to halt the expansion
of the Universe. Some theoretical explanations for the Big Bang
predict such a density. At this critical density, it might take
billions of billions of years even to slow down enough for
astronomers or their descendants to be able to tell if it would
grind to a halt altogether. If it exceeded the critical density,
then it could eventually start to collapse again into what has been
termed the Big Crunch. So the density of the Universe is indeed
crucial to its ultimate fate.
Another critical factor is the rate at which the Universe is
expanding. That has been named the Hubble Constant after Edwin
Hubble who first detected that the Universe is expanding. It is
measured in kilometres per second per Megaparsec, where a parsec is
an astronomical unit of length (approximately 3.26 light years).
Hubble estimated that his constant might have a value of about 500,
a very rapid expansion indeed and one that would mean that the Big
Bang must have taken place less than five billion years ago, not
much more than the age of the Earth. Modern measurements have
narrowed it down to somewhere between 40 and 90 but it is still
being hotly debated.
The Hubble Constant would not only reveal the age of the Universe
but also the true scale of the Universe. It would provide a way of
converting the velocities revealed by redshifts into true distances.
For the moment, efforts are concentrating on trying to establish
true distances in order to determine the Hubble Constant more
accurately.
To do this, astronomers are looking for cosmological yardsticks. One
of the best is a sort of star known as a cepheid variable. These
giant stars vary their brightness over a period of a few days which
is directly related to their maximum luminosity. So they act like
standard candles. Find one in another galaxy and measure its period
and you know how bright it is independently of how far away it is.
Thus you can use its apparent brightness to reveal its true distance
- so long as the light is not dimmed by any undetected intervening
dust!
Once a distance scale is established to some of the major clusters
of galaxies, it can then be used to calibrate other potential
yardsticks. Exploding stars or supernovae are bright enough to be
seen in quite distant galaxies and might be used as standard candles
themselves. But they are rare events in our local group of galaxies
and not enough have been seen to be able to calibrate them reliably
for use at greater distances. The dynamics of galaxies themselves
might be yardsticks. Astronomers suspect that the rotation rates of
spiral galaxies are related to their true size but again,
independent calibration is needed.
Radio astronomers are testing another new technique. The eight
dishes of the Ryle radio telescope at Cambridge have been mapping
the sort of radio shadow the hot gas around a distant galaxy cluster
makes against the microwave background. Independent x-ray
measurements of the cloud combine to give its true size enabling the
distance to be calculated by simple geometry. The first results from
that give a low Hubble Constant of about 50, making the Universe
bigger and older than if the constant was larger.
Meanwhile, results were recently published from the Hubble space
telescope and from another group of astronomers using a telescope in
Hawaii that contradict this. They have both measured the period of
cepheid variable stars in galaxies in the Virgo cluster, extending
what they believe to be the most accurate yardstick to much greater
distances than was possible before. The result they come up with
gives the Hubble Constant a value of around 80. This important
result is getting some astronomers very worried because if it is
true then the Universe is no more than about 12 billion years old.
If it is close to its critical density, then the expansion is
slowing down and the Universe is as little as 8 billion years old.
That is unfortunate since stars in groups called globular clusters
have been dated by methods based on well trusted theories of nuclear
processes to be around 15 billion years old. Clearly something is
wrong! Maybe there is something wrong with the measurements. Maybe
there is something wrong with the theory. Perhaps there is something
wrong with the Virgo cluster such that it is streaming away faster
than the general expansion rate. Maybe there is another factor, a
weak repulsive force on large scales which is accelerating the
expansion. Einstein once proposed such a cosmological constant but
afterwards said it was his greatest mistake. Perhaps it was not
after all.
The age of the Universe is one of the questions that astronomers
certainly dare to ask now, but opinion is still divided on the
answer and only more observations will resolve it.
For those who like numbers, Professor Michael Rowan-Robinson of
Imperial College, London, has made some best estimates for the known
Universe:
Density: 0.00000000000000000000000000001 gm/cc (10-29 g/cc)
Size: 100 000 000 000 000 000 000 000 000 ms (1026 m)
Weight: 10 000 000 000 000 000 000 000 000 000 000 000 000
000 000 000
000 000 kgs (1052 kg)
Echoes of the Big Bang As mentioned earlier, the Universe is bathed
in the cooled glow of microwave radiation left from the fireball of
the Big Bang itself. Like the interior of the Sun, the early
Universe was full of ionised matter and was opaque to radiation.
After the first 300 000 years, when it was still only about 1/1 400
of its present size, it cooled sufficiently for the radiation to
de-couple from matter and shine out into the expanding space it
still occupies. Like the inside of a furnace, the microwave
background is purely thermal radiation. It has the spectrum of a
perfectly uniform black body radiating simply because of its heat.
Originally, the temperature must have been about 4 000 degrees above
absolute zero. Now it has cooled to 2.7 degrees.
One of the first things that the Cosmic Background Explorer
satellite (COBE) did after its launch was to confirm that the
radiation is precisely that of a black body, though more recent
observations at lower frequencies have found some interesting
deviations. COBE then went on to search for any variations in
brightness. There were none down to about 1 part in 100 000 but at
that level COBE did detect fluctuations - ripples in the cosmic
background. Initially, the only evidence was statistical and the
image of what looked like ripples presented triumphantly at a press
conference in 1992 showed little more than random noise from the
receiver. But more data have confirmed that there really are
ripples.
COBE mapped the microwave background across the whole sky but to a
very coarse resolution of about 10 degrees. Experiments on the
ground are looking on finer scales. British astronomers working on a
mountain in Tenerife have been mapping strips of sky for nearly 10
years to a resolution of 5 degrees and 3.5 times the sensitivity of
COBE. They have now announced that they too have detected structures
and have begun to map individual ripples in the microwave background
and measure their sizes. Experiments at Cambridge (Cosmic Anisotropy
Telescope or CAT) and in several other projects on the ground, in
space and in balloons are also making fine resolution measurements
at different frequencies. It is at last possible to look at the
astronomy of the cosmic background.
So what caused the ripples and how do they relate to the structures
we see in the Universe today? The direct cause of the ripples is
radiation de-coupling from the matter in the Big Bang fireball
slightly earlier in some parts than others. This in turn was
probably due to density variations in the fireball. But the Universe
has expanded so much since then that even something like the Great
Wall, 200 million light years long, would be less than 1 degree
across at the distance of the microwave background. The ripples
mapped so far in the background are far bigger and that has led
cosmologists to ask how they got there. The Universe was so new at
this stage that, even travelling at the speed of light, signals
could not have crossed distances on the scale of the ripples so
there is no conventional explanation for how coherent structures
that big were made. A solution may come from what is called
inflation theory; a period of extremely rapid expansion very early
in the life of the Universe that has been described mathematically
by some theorists (notably Alan Guth of MIT).
If inflation really took place, then random fluctuations caused on
subatomic scales by quantum processes before inflation would have
expanded to the size of the ripples and the largest features in the
Universe today. The only other explanation is that the ripples were
caused by defects or phase transitions in the early Universe, rather
like the boundaries between crystals in a crystallising solid.
The first second Studies of the visible Universe and of the cosmic
background all give observational clues about the Big Bang itself
and help to explain why the Universe turned out the way it did. But
to probe beyond the microwave background requires a marriage of the
physics of the very small and the cosmology of the very large. As
astronomers can peer out and study the frontiers of the visible
universe, so physicists can recreate the conditions of the Big Bang
in their particle accelerators. So what seem elegant if sometimes
incredible theories can be put to the test.
As scientists rewind the tape, as it were, back through the first
second of the Universe, they are beginning to work out some of the
conditions that must have existed in this incredibly dense, hot
phase. The numbers that they use are often so great for the
temperatures and so small for the times that they speak in powers of
10 where, for example, 106 is a million and 10-6 is a millionth. As
the temperature increases, so the available energy gets higher;
sufficient, in fact, to create fundamental particles freely and
breakdown the bonds and the differences between them.
For example, 300 000 years after the Big Bang, the temperature
cooled to about 4 000oK, allowing electrons to recombine with atomic
nuclei and enabling light to shine out. But that was a very late
event in the history that physicists are concerned with. Most of the
important things happened within the first second.
When the Universe was about one second old, the temperature was
about 1010 degrees. That is sufficient to give particles a kinetic
energy of about 1 MeV (Mega electron Volt), about the energy
equivalent to the mass of the electron. So particle accelerators
must reach energies higher than 1 MeV to recreate conditions within
the first second. At about 10-5 seconds the energy is reached where
the structure of protons and neutrons breaks down and their
component quarks form a sort of plasma. Experiments are underway at
the moment at the European physics centre, CERN, to create and study
this quark plasma.
At about 10-10 seconds we reach the energy needed to create
particles called the W and Z. In our present Universe they are very
massive and cannot travel more than a fraction of the size of an
atomic nucleus. At this early, energetic time, it was hot enough for
W and Z particles to behave like massless photons do today and the
differences between the electromagnetic force carried by photons and
the weak nuclear force carried by the W and Z particles breaks down
and the two forces become identical. The theory that unites these
two forces is called the Standard Model. To go further and attempt
to include the strong nuclear force and even gravity, physicists
have developed what they call Grand Unified Theory. One of the
achievements of the LEP accelerator at CERN has been to measure the
properties of the W and Z sufficiently precisely to predict, or
rather confirm suspicions, that there are no more than three sorts
of neutrino in the particle family, corresponding to three leptons
(electron, muon and tau) and three pairs of quarks (up and down,
strange and charm, bottom and top).
Preliminary evidence for the sixth and final quark, the top, has
been reported from the Tevatron accelerator at Fermilab near
Chicago. It appears to have a mass (about 150 GeV) that fits the
predictions of the Standard Model very well.
The next big frontier for a particle accelerator will be to recreate
the conditions of about 10-12 seconds. Up until this time that,
according to theory, particles called Higgs Bosons were common and
it is these that may account for the different masses of particles.
Dr John Ellis, Head of the Theory Division at CERN, likens the Higgs
mechanism to superconductivity. Like superconductivity, it
disappears above a certain temperature but unlike superconductivity,
that temperature is very high indeed, about 1016 degrees, the
temperature when the Universe was a millionth of a millionth (10-12)
of a second old. The Higgs may not be found until such temperatures
can be reached, perhaps in the Large Hadron Collider planned at
CERN.
Though it has not yet been found, there are good theoretical reasons
for believing that the Higgs mechanism exists. It may well have what
is called a super-symmetric partner. If that is the case, a partner
of the Higgs may be one of the best candidates for cold dark matter.
To go back still further is to go deeper and deeper into the realms
of theoretical speculation and further from anything that is within
our technology to test.
Somewhere between 10-35 and 10-10 of a second lies the answer to the
question Why is there matter in the Universe? Matter and anti-matter
are normally created from raw energy in equal quantities. But the
Universe we inhabit is made almost entirely of matter, with
virtually no anti-matter. It would only have taken a very slight
excess of matter in the early Universe to leave a surplus after all
the anti-matter had been annihilated by matter. That excess could
come from a slight asymmetry between the interactions of matter and
anti-matter, something suggested by laboratory experiments and by
the theory of the Standard Model. If that is the reason, then the
density of matter in the early Universe might be demonstrated by
laboratory measurements.
Physics can also tell us a lot about the nature of dark matter. The
relative abundances of elements, notably the helium and lithium
created alongside hydrogen in the Big Bang allows theoretical
predictions of the total density of normal or baryonic matter in the
Universe. The abundances observed in clouds of gas that have not
been enriched by nucleosynthesis in stars suggest that there is only
about 1% of the critical density required to halt the expansion of
the Universe in normal matter. So the search is on for other
particles that interact only weakly with normal matter but still
have mass. Of these, only the neutrino has been proved to exist and
the electron neutrino, at least, has little or no mass. But
experiments to measure neutrinos coming from the Sun find fewer than
expected and the only explanation (other than something going
seriously wrong with the Sun) is that the neutrinos change type on
their journey; something they can only do if they have a little
mass. Preliminary, unconfirmed results from Los Alamos in the US
also provide evidence for such so-called neutrino oscillations.
Neutrinos are so abundant in the Universe that even a little mass
would mean that they contribute significantly to hot dark matter.
The particles of cold dark matter - axions, photinos, gravitinos,
etc are at present only blank spaces on the family tree of particles
and nothing is known of their mass or even their existence.
To go back to the incredibly early hot, dense Universe before it was
10-35 of a second old is to go back into the phase of inflation.
Inflation is the most popular way of explaining at present how the
Universe got from something smaller than an atom to something about
the size of a grapefruit in the incredibly short space of time
between 10-41 and 10-35 of a second. It is only a theory but it fits
in very neatly with what is known and it also offers an explanation
for the structures seen in the cosmic microwave background.
Structure on the scale of the microwave ripples could have formed as
spontaneous quantum fluctuations before inflation and have been
enlarged into the macroscopic universe through inflation. It is a
theory that remains distant from any imaginable experiment but it
makes a mathematically elegant start to the worlds we now know.
Milestones towards answering questions of the Universe (proposed by
Michael Rowan-Robinson)
* Secure local, interstellar distance scale (3 000 light years)
- Hipparcos satellite; data now complete.
* Accurate age determination for the galaxy - soon.
* Accurate Hubble Constant - once contradictory values are
understood, especially those from cepheid variable stars in Virgo.
* Good estimate of the density of the Universe - various
redshift surveys - 5-10 years.
* Improved estimate of primordial density fluctuations - IRAS
galaxy survey, COBE data and ground based microwave background
experiments - 2 years.
* Theoretical ideas about dark matter.
* Determination of neutrino masses, if any - 2 years.
* Detection of normal dark matter, eg brown dwarves - MACHO
experiment - 5 years.
* Detection of cold dark matter particles eg in Boulby mine,
Yorkshire - 5 years.
* Detection of Higgs Boson - LHC at CERN - 10 years.
Contacts
Professor Sir Martin Rees F.R.S.
Institute of Astronomy
University of Cambridge
Madingley Road
Cambridge tel: 01223 337520
CB3 0HA fax: 01223 337523
Professor Richard Ellis
Institute of Astronomy
University of Cambridge
Madingley Road
Cambridge tel: 01223 330879
CB3 0HA fax: 01223 337523
Professor George Efstathiou F.R.S.
Department of Physics
University of Oxford
Keble Road
Oxford tel: 01865 273300
OX1 3RH fax: 01865 273390
Professor Michael Rowan Robinson
Imperial College of Science, Technology and Medicine
Blackett Laboratory
Prince Consort Road
London tel: 0171 594 7530
SW7 2BZ fax: 0171 594 7777
Dr Anthony Lasenby
Cavendish Laboratory
University of Cambridge
Madingley Road
Cambridge tel: 01223 337293
CB3 0HE fax: 01223 363263
Dr John Ellis F.R.S.
Head of Theoretical Studies Division
CERN
CH-1211
Geneva 23 tel: 00 41 22 767 0000
SWITZERLAND fax: 00 41 22 767 0000
The Press Office
CERN
CH-1211
Geneva 23 tel: 00 41 22 767 4101
SWITZERLAND fax: 00 41 22 785 0247
The Press Office (contact: Andrew Bennett)
PPARC
Polaris House
North Star Avenue
Swindon tel: 01793 442000
SN2 1SZ fax: 01793 442003
The Press Office (contact: Peter Andrews)
Royal Greenwich Observatory
Madingley Road
Cambridge tel: 01223 374000
CB3 0HA fax: 01223 374700
The Press Office (contact: Mark McAuly)
Royal Observatory Edinburgh
Blackford Hill
Edinburgh tel: 0131 668 8100
EH9 3HJ fax: 0131 668 8264
The Press Office (contact: Jacqueline Mitton)
Royal Astronomical Society
Burlington House
Piccadilly
London tel: 01223 564914
W1V 0NL fax: 01223 572892
Miss Anna Link
Science Promotion Section
The Royal Society
6 Carlton House Terrace
London
SW1Y 5AG
Telephone: 0171 839 5561 ext 315 20th June 1995
INFORMATION NOTE
Cosmology: What astronomers don't know
but are no longer afraid to ask
On 15th July 1993 a press briefing on cosmology, organized jointly
by the Royal Society and the Association of British Science Writers,
was held at the Royal Society. The enclosed document was prepared
afterwards to summarize key issues raised by the speakers and also
to provide a list of relevant contacts for future reference.
This document does not necessarily constitute the views of the Royal
Society, and views expressed in it should not be attributed to the
Society. The document is free of copyright and may be used without
reference to source.