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In 1990, Michael Turner of the University of Chicago and Fermi National Laboratory proposed a recipe to add up to critical density: 5 per cent ordinary matter; 25 per cent cold dark matter (including both invisible and ‘exotic’ types); 70 per cent the cosmological constant or something like it. According to Turner, the energy of the cosmological constant could compensate for some of the missing mass and serve as an additional brake on cosmic expansion, balancing things out in such a way that the universe would neither eventually collapse nor expand into an ever darker, thinner, colder infinity, but instead perch for all eternity on that highly desirable knife edge between the two. The lower density of matter that such a cosmological constant value would allow might be an added boon to theorists, making it easier to explain how matter congealed into such enormous structures as the Great Wall of galaxy clusters.
After Freedman’s team’s discoveries in late 1994, physicists began to consider much more seriously these suggestions that the cosmological constant might not be zero. Adjust the dial, and the cosmological constant’s energy, over time, could change the rate at which the universe expanded. If expansion was slower when the universe was young, that would have given more time for stars and large structures to develop. Later, the energy of the cosmological constant could have influenced the expansion to speed up. The current measurements of the rate of expansion, by Freedman and others, would be only measurements of the present rate of expansion, and unreliable indicators of the age of the universe.
However, though a non-zero cosmological constant was looking more and more tempting in terms of explanatory power (in other words, useful to explain what was going on), there was still the major hitch that no one had yet been able to find any direct observational evidence that the value was anything other than zero. The first hint that this might change came in 1996, not, itself, from direct observation.
The age-old method of testing alternative ideas and making up for insufficient evidence by using mathematical simulations came into its heyday with the advent of supercomputers. In the 1980s and 1990s, as never before, it was possible to feed in some observed and assumed conditions in the early universe and find out what this might lead to after billions of years. In 1996, an international team headed by Carlos Frenk of Durham University in England, using Cray supercomputers at Munich and Edinburgh, ran simulations to find out whether temperature fluctuations observed in the early universe could have led from a Big Bang fireball, where everything was almost uniform, to today’s universe of galaxies, clusters and voids.
The starting point for the simulations was the universe as it is thought to have existed 300,000 years after the Big Bang. That was when the cosmic microwave background radiation originated (the radiation that Penzias and Wilson detected in 1964). In 1992 George Smoot and his colleagues had been able to discern wrinkles, tiny energy fluctuations predicted by Big Bang theory, in this otherwise smooth cosmic fabric. Frenk’s team simulated the growth of those initial wrinkles. The results supported the possibility that the cosmological constant should indeed be summoned from limbo.
Frenk and Simon White at Munich (assisted by Adrian Jenkins, Frazer Pearce and Joerg Colberg) ran four different simulations. One of the ways they differed from one another was in the estimates of the mass density of the universe. A second difference was in whether or not Frenk and his colleagues allowed the cosmological constant to be other than zero. Of the four (see Figure 8.2), the one capable of producing the universe as we know it today was the model in which the mass density was only 30 per cent of what experts think would be needed to produce omega-equals-one, or critical density, and in which Frenk also factored in a non-zero cosmological constant.
In this simulation, the expansion rate changed over time and was slower in the early universe than it is today. The computers demonstrated how the wrinkles might have attracted surrounding matter. Lumps of matter, according to the simulation, collapsed onto themselves and grew larger by merging with other lumps, eventually forming a complex filamentary network of large, twisting ridges surrounding vast empty regions. Gas and dark matter flowed along these filaments. Where the filaments intersected, galaxies and galaxy clusters formed. In the simulation, the last few billion years don’t show much gross alteration, for the universe is expanding fast and the mass density is too low for the large structures to change very much.
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
When the universe was created, we were not consulted.
Andrei Linde
WHETHER THERE IS an edge to the universe, and what, if anything, might be beyond, are old questions. German astronomer Heinrich Wilhelm Olbers, who lived from 1758 to 1840, pointed out what is now known as Olbers’s paradox: if space has no edge and is infinite and contains an infinite number of stars, the night sky should be as bright as the Sun. It isn’t. He wasn’t the first to worry about that and certainly not the last. Suppose, instead, space is infinite but the number of the stars is not, and the stars are limited to some sort of system ‘inside’ infinite space. That creates another problem: their system will collapse because of their mutual gravitational attraction. Try to solve that by saying that the star system rotates and its centrifugal force keeps it from collapsing, and someone will surely think to ask: In relation to what is it rotating, since it’s the only thing in an infinite universe?
