B00B7H7M2E EBOK (41 page)

Figure 8.2 Computer Simulations from Carlos S. Frenk and his colleagues

C
most resembles the universe today. It is based on a mass density only 30 per cent of what we think would be needed to produce omega-equals-one, or critical density, and Frenk also factored in the cosmological constant.

A
and
B
are much less successful models based on greater density.

D
is based on 30 per cent density, without the cosmological constant.

No simulation, by itself, can provide the answers to the questions about expansion rate and age, the cosmological constant and the missing mass. But Frenk, in an interview with the
New York Times
, argued that his team’s simulations do point out strengths and weaknesses in several theoretical models and ‘give us greater confidence in what are, you might say, the best-buy models of the universe’. The project’s results were in line with those of more modest simulations by Jeremiah Ostriker of Princeton and Paul Steinhardt of the University of Pennsylvania, and with models developed by James Peebles of Princeton.

The possibility raised by the simulations that omega does not equal one echoed some recent observational evidence. Studies of the spectra of galaxies in the X-ray range had been raising questions about proportions of ordinary matter and exotic dark matter. Also, the discovery of ever-larger galactic superclusters seemed unexplainable if omega does equal one. On the other hand, simulations by Joel Primack at the University of California, Santa Cruz, and scientists at New Mexico State University seemed to rule out the cosmological constant. ‘No one,’ said Peebles, ‘should start collecting bets on a low-density universe.’

That was how things stood when, at the January 1998 meeting of the American Astronomical Society, the Supernova Cosmology Project – a team that had been studying supernovae to find out whether the expansion of the universe is slowing down – announced that not only does the expansion show no signs of slowing down, it actually appears to be speeding up. Their announcement was another blockbuster.

Saul Perlmutter of the Lawrence Berkeley National Laboratory in California, who heads the project, had always been deeply interested in the most fundamental questions of how the world works. As an undergraduate at Harvard and working towards a PhD at Berkeley, he’d become increasingly convinced that serving on teams involving hundreds of participants – as is
common
in modern world-class particle physics – would give a young physicist little chance to shape the research. How else to ask the fundamental questions? Perlmutter decided to try astrophysics, and that is where he is today, shaping research that may indeed lead to the answers to his questions. But his experience in fundamental physics has inclined him to be more patient and less resistant than some of the astronomy culture can be to projects that take years of single-minded pursuit to complete.

Perlmutter began what promised to be a lengthy and difficult endeavour indeed – and at the outset something of a gamble – using distant supernovae as mile-markers to measure trends in cosmic expansion. When preliminary results satisfied him and others that supernovae could be used effectively for such measurement and that available technology should be up to the task, Perlmutter and his team dug in for a long-range investigation.

The most distant supernovae that Perlmutter’s team had discovered by January 1998 were some seven billion light years away, meaning dial by the time their light reached telescopes on Earth, seven billion years had passed since the stars exploded. By now that light is feeble, red-shifted by the expansion of the universe. The Supernova Cosmology Project involves comparing the light of these distant supernovae with the light of bright nearby supernovae to determine how far the faint supernova light has travelled. The distances combined with red shifts of the supernovae give the rate of expansion of the universe over its history, allowing researchers to determine how much the expansion rate may be speeding up or slowing down.

The remarkable predictability of Type Ia supernovae is what makes this project possible. Although all Type Ia supernovae don’t have the same brightness, it turns out that their absolute luminosity
can
be learned by watching how quickly each supernova fades away. Type Ia supernovae in nearby galaxies are so
predictable
that the time the supernova explosion began can be determined just from a look at its spectrum, and the most distant supernovae also have precisely the right spectrum on the right day of the explosion. ‘The real similarity of the details of these events,’ says Perlmutter, ‘can be seen in the beautiful spectra we get from the Keck telescope in Hawaii, the largest in the world.’ Researchers breathed a sigh of relief when it was clear that Type Ia supernovae that exploded when the universe was half its present age behave essentially the same as supernovae do now, for this eliminated one worry about the reliability of the project’s results – the question whether Type Ia supernovae have been different in different epochs.

Because the most distant supernova explosions appear so faint from Earth, happen at unpredictable times, and last for such a short while, the team performs a tightly choreographed sequence of observations, using telescopes around the world and the Hubble Space Telescope. Some team members survey distant galaxies using the largest telescope in the Andes Mountains of Chile, while others in Berkeley, California, receive that data over the Internet and analyse it to find supernova candidates. Once they find likely supernovae, they rush out to Hawaii to confirm that these
are
supernovae and measure their red shifts. Team members at telescopes outside Tucson, Arizona, and on the Canary Islands are meanwhile standing by to measure the same supernovae as they fade. The Hubble Space Telescope is summoned into action to study the most remote of the supernovae, whose distances make them too difficult to measure accurately from the ground.

By January 1998 Perlmutter’s team had analysed 40 of the roughly 65 supernovae so far discovered by the project. Only a little earlier they had reported that the cosmic expansion rate seemed to have slowed down very little, if at all. Now Perlmutter was ready to report that ‘all the indications from our observations of supernovae spanning a large range of distances are that we live in a universe that will expand
forever
. Apparently there isn’t enough mass in the universe for its gravity to slow the expansion to a halt.’

In March 1998 a second research group reported similar findings. This team was headed by Brian Schmidt of the Mount Stromlo and Siding Spring Observatory in Australia and included Adam Reiss, a young astronomer at the University of California at Berkeley, and Kirshner from Harvard-Smithsonian. They reported that they had found indication that the expansion rate is approximately 15 per cent greater now than it was when the universe was half its current age.

No sooner were the words out of Perlmutter’s and Schmidt’s mouths than speculation began in earnest about what this news might mean for inflation theory and for the cosmological constant. Inflation theory predicts a flat universe. The new findings were indicating an open universe. Or were they? One extremely intriguing implication of the discovery was that these teams of astrophysicists might actually be looking at the first strong observational evidence that there is a repulsive force operating in the universe, that the universe is indeed getting an anti-gravity boost from somewhere. The evidence, said Perlmutter, strongly suggested a cosmological constant.

No one was jumping to the conclusion that there are no other possible explanations. Michael Turner, who had proposed the recipe for critical density, reflected the caution of the scientific community when he said, ‘If it’s true, this is a remarkable discovery. It means that most of the universe is influenced by an abundance of some weird form of energy whose force is repulsive.’ Schmidt said his own reaction was ‘somewhere between amazement and horror. Amazement, because I just did not expect this result, and horror in knowing that it will likely be disbelieved by a majority of astronomers who, like myself, are extremely sceptical of the unexpected.’ Reiss commented, ‘We are trying not to rush to judgement on the cosmological constant. There could be some other sneaky little effect we have overlooked, something that makes the
supernovae
dimmer and appear to be farther away than they really are, or some variation in the behaviour of more distant supernovae that are deceiving us.’

In spite of such reservations, it seems Schmidt had overestimated his colleagues’ scepticism, for by May a straw vote at a workshop at Fermi National Laboratory indicated that most scientists present agreed the two teams had made strong cases for an accelerating expansion rate and the existence of something resembling a cosmological constant.

It was all beginning to fit: the slowing down caused by the mass density of the universe appeared to be overwhelmed by the speeding up caused by the cosmological constant. Study of the relationship was telling researchers how much larger the energy density due to cosmological constant energy must be than the energy density due to mass density. Inflation theory predicts that omega equals one, and calculations showed that the cosmological constant energy could provide .75 of the total and the mass density .25. This proportion was close to the numbers coming from computer simulations and in Michael Turner’s recipe. The discovery also held out hope for solving the age of the universe glitch, allowing the universe to have expanded more slowly at an earlier age.

But is the secret ingredient really that old ghost, the cosmological constant? Some have been calling it ‘X-matter’ and ‘quintessence’ (named after an element suggested by Aristotle) – speculative concepts in which textures in the early universe created conditions for a cosmic background energy. By the time of the workshop at FermiLab, cosmologists were referring to the ‘missing energy’ of the universe in the same way they had long spoken of the ‘missing matter’. Some were calling it ‘funny energy’. The mystery of what it is is still unsolved in the second decade of the 21st century.

The Supernova Cosmology Project and Brian Schmidt’s group hoped to observe supernovas even further back in time, to about 10 billion light years’ distance. There were
also
proposals for studies involving new X-ray astronomy spacecraft and for surveys of the cosmic microwave background radiation from the ground and from space. Clues to the density of the universe and the value of the cosmological constant are encoded in that radiation as the minuscule temperature variations that Smoot and his colleagues discovered.

Is it still necessary to ask about the deceleration parameter? There seems to be no deceleration! However, a discovery that the expansion rate is speeding up doesn’t mean that the deceleration parameter must relinquish its place in the equation for omega. It can be either a positive or a negative number . . . and it can change over time.

With these new discoveries of the late 1990s, inflationary Big Bang theorists found themselves torn between glee and discouragement. One of the theory’s greatest assets, its ability to solve the flatness problem, had become a potential embarrassment, for researchers were continuing to find insufficient matter to maintain a flat universe. The evidence that the expansion rate was speeding up could be taken as another nail in the coffin of a flat universe. If the universe was ‘open’, what good was a theory that predicted a flat universe? The theory had gone to a great deal of effort and brilliantly predicted a situation that might simply not exist.

However, speculation is rampant about the cosmological constant value, and whether ‘quintessence’ or ‘funny energy’ might in fact make up the deficit left by insufficient mass density, producing precisely the omega-equals-one flat universe that inflation theory predicts. Furthermore, since inflation theorists had already suggested that the cosmological constant might be the agent behind the inflation period in the early universe, observational evidence for its existence would be all to the good.

Another possibility for redeeming the theory is to reinterpret it to predict an open universe rather than a flat one, but most theorists balk at such a move. There is that danger –
reminiscent
of Ptolemaic theory – of adding complications to a theory until it flounders not under its inability to explain and predict but because it can explain and predict too many contradictory findings.

This chapter has only barely sketched the problem of the elusive omega, giving a taste of the complications and the high hopes of modern researchers, and providing some background to explain announcements that will come in the next months and years. No one knows, at the moment, whether the arguments will continue for a long time with more and more disparate voices and conflicting data or whether there might actually be more definitive answers in the near future.

CHAPTER 9

Lost Horizons