Secrets of the Universe

by Paul Murdin

  Agnes Clerke, The System of the Stars, 1890

  The darkness at night hides surprising secrets. It tells us about our place among the stars, and it shows that the Universe has not existed forever.

  The fact that it is dark at night is such a commonplace that it cannot be said to be a discovery. But within the commonplace fact lie surprisingly deep discoveries about our cosmic situation. In the first place, it tells us that there is only one star in our neighbourhood – the Sun that our planet is orbiting. It is rather rare for a star to be single like this – most stars have companions close by, in double or triple stars, or in groups or clusters. In science fiction, scenes may take place on a planet that has two suns, as in the film Star Wars, where we see them in the background, both in the sky together. Such a planet must have complex day/night patterns. Or a planet might orbit a star in a cluster of stars, in which case the other stars would be scattered all over the sky and shining brightly. It would never be as dark on such a planet as our night, although there may be a bright/less bright cycle.

  These scenes are imagined. But the reality is that we do live in a cluster of stars of sorts – the Galaxy. At night we do see its constituent stars, and they cast a dim light on the surface of the Earth. This is the reason why it is possible to see more on a star-lit night than on a cloudy one, as hunters and soldiers know well.

  How bright is it at night? There is an argument that it ought to be as bright at night as by day. The steps to this argument were discovered progressively in the eighteenth and nineteenth centuries, starting after it became clear that the Universe was not contained within crystal spheres, but extensive, with many stars beyond the edge of the Solar System. It might even be infinite in extent, and therefore, assuming that it is isotropic, contain an infinite number of stars. Edmond Halley was one of the first to consider this possibility in papers published in 1721 under the name ‘Of the infinity of the sphere of the fix’d stars’ and to link the possibility with the question about the light of the night sky. The mathematical argument was fully formulated by the Swiss astronomer Jean-Philippe Loys de Chéseaux in 1744.

  Think of an infinite universe of stars, uniformly distributed around the Earth, which we can visualize as a succession of thin spheres of the same thickness, each sphere getting larger and larger like the shells of an onion. The number of stars in each shell is proportional to its volume, and increases in proportion to the square of the radius of each shell. But the light that reaches the Earth from each star in each shell diminishes according to the inverse square law, so the total light at the Earth from each shell is the same. If there is an infinite number of shells, the light from all of them added together is infinite – there would be no dark sky at night.

  Chéseaux realized that there is a limitation in this argument – the stars in the nearer shells would obscure some of the stars in the shells behind. The most distant stars would not contribute any light at the Earth, just as, in a forest, no matter in which direction we look, we see a tree trunk, whether nearby or further away. We cannot see any of the trunks of the most distant trees. Substitute ‘star surface’ for the words ‘tree trunk’ in this analogy and it becomes clear that based on the argument laid out, no matter where we looked in the night sky our line of sight would end on the surface of a star. The surface brightness of the sky would be the same as the brightness of the surface of an average star, like our Sun. Chéseaux estimated that we could see on average a distance of 3,000 trillion light years, that the number of visible stars would be correspondingly many, many trillions, and that the sky at night would contribute as much light as 90,000 suns. His figures are broadly consistent with modern calculations.

  Chéseaux recoiled from these numbers, and the inevitable conclusion that the sky at night would be as bright as the surface of the Sun, which is even brighter than the sky by day. He proposed that the solution to the problem was either that the starry part of the Universe was finite, terminating well before the sky was completely full of stars in no matter which direction you looked, or that starlight was absorbed in space, diminishing the light from the more distant stars. The enormous difference between his theoretical conclusion and our actual experience demonstrates either that the sphere of fixed stars is not infinite, or that something unknown blocks starlight.

  The problem of why it is dark at night was revisited seventy years after Chéseaux published his paper by Heinrich Wilhelm Matthias Olbers. Olbers was a doctor in Bremen, Germany, who discovered two of the first asteroids. In 1823 he wrote ‘On the transparency of space’, in which, without acknowledging Chéseaux’s work, he considered Chéseaux’s problem and proposed Chéseaux’s solution. Olbers’ work became well known but Chéseaux’s did not, and as a result the problem formulated carefully by Chéseaux is known as Olbers’ Paradox.

  In fact, one of Chéseaux’s solutions to the paradox raises a further problem, discovered by John Herschel in 1848. If starlight is absorbed by something in space, that something must get hotter, until eventually it gets as hot as an average star’s surface and gives out as much light as it takes in. Putting something in space to block starlight does not, in the long term, help solve Chéseaux’s and Olbers’ problem.

  In modern times Olbers’ Paradox has relevance to cosmology, if you substitute ‘galaxies’ for ‘stars’. In 1960 the Austrian-born British mathematician Hermann Bondi listed the four major assumptions of Olbers’ Paradox as it would be formulated for cosmology: (a) The Universe is uniform throughout space; (b) The Universe is unchanging in time; (c) There are no major systematic motions in space; (d) The laws of physics apply everywhere. With these assumptions about the galaxies in space, Chéseaux’s and Olbers’ calculation holds together. But modern cosmology offers possible solutions, by attacking assumptions (a), (b) and (c): (a) The Universe is not infinite and therefore not uniform beyond a certain distance; we can see out only as far as it has been possible for light to travel since the Universe formed; (b) In fact, we can see galaxies only out to the distance and time at which they were formed – before that time, and beyond that distance, the Universe was different; (c) Moreover, the Universe is expanding, with the galaxies receding, and light from the more distant galaxies is weakened by the recession.

  The last objection is not as important as the other two, which are of course connected. The sky is dark at night because of missing galaxies, not because of missing light from galaxies. The simple fact that the sky is dark at night means the Universe was created a finite time ago, and is limited in dimension.

  FUTURE DISCOVERIES

  Dark Matter

  A known unknown

  There are known knowns: things we know that we know. There are known unknowns: things we know we don’t know. But there are unknown unknowns: things we do not know we don’t know. Each year we discover further unknown unknowns.

  Donald H. Rumsfeld, US Secretary of Defense, 2002

  Astronomers estimate that 80% of the material in the Universe is a substance called ‘dark matter’. It is invisible to all currently available technology, and nearly everything about it is a secret yet to be uncovered, leading some scientists to question whether dark matter exists at all.

  Looking out at the broad view of the Universe, astronomers see mass on a large scale, distributed as stars and gas in galaxies and in clusters of galaxies. There is also a considerable amount of hydrogen and helium in intergalactic space in clouds that do not shine and contain few or no stars – galaxy-sized clouds left over from the Big Bang that have not turned into galaxies. This material makes its presence known by absorbing ultraviolet light from distant quasars – the light from a distant quasar penetrates through the clouds as if they were pieces of meat skewered on a kebab, and each cloud leaves a gap in the spectrum of the quasar’s light.

  There is also a considerable amount of matter in the Universe that astronomers cannot see, either by the light (or other radiation) that it emits or through its absorption of light that encounters it. This mysterious substan
ce is generically called ‘dark matter’.

  The person who discovered dark matter was the notoriously abrasive Swiss-born astronomer Fritz Zwicky of the California Institute of Technology. In the 1930s he set out to provide a complete scheme of every thing in the Universe. Measuring the mass of every known space object was a good place to set the boundaries of such a scheme. In 1933 he estimated the mass of a nearby cluster of galaxies in the constellation Coma by measuring the speeds of motion of its galaxies, each of them pulled in orbit by the mass of the cluster as a whole. They were moving much faster than Zwicky expected, based on his estimate of the mass of all the stars in the cluster as judged by the light they gave off – his estimate was that the Coma Cluster was 400 times the mass of its stars and that 399⁄400ths of the mass was therefore, in his words, ‘missing’.

  Zwicky’s discovery of dark matter in clusters of galaxies was at first not followed up by his colleagues. It was an outlandish idea, made even less palatable because he was so difficult to converse with. However, it was confirmed half a lifetime later on a smaller scale, not within clusters of galaxies but within galaxies individually, by Carnegie Institute astronomer Vera Rubin. When she graduated from Vassar College, New York, in 1948, she applied to study for a further degree at Princeton University but was told that the university did not admit women to its astronomy programme, and so she developed her career instead by studying at Georgetown University, where she was supervised by George Gamow. She went on to work at the Carnegie Institute with Kent Ford, who had developed a sensitive spectrograph that could measure the spectrum of faint galaxies. Rubin and Ford used the new spectrograph to determine how spiral galaxies rotate, and pressed their observations further into the outer, fainter regions of the galaxies than had been possible before.

  The expectation was that stars in the outer regions of a spiral galaxy would move more slowly than stars in the central regions, because most of the mass of stars is concentrated in the central regions, and stars in the outer regions are a long way away. In just the same way, the more distant planets of the Solar System rotate around the Sun more slowly than the inner planets. But Rubin and Ford discovered that, in fact, stars in the outer regions of each spiral galaxy were moving just as fast as those nearer its centre. This means that there is unseen matter in each galaxy, even in the outer regions of galaxies where the visible stars are few. There is at least twice, and up to ten times, more mass in a typical spiral galaxy than is accounted for by stars. By 1975 Rubin had become convinced that ‘What you see in a spiral galaxy is not what you get.’

  Initially she met with the same scepticism that Zwicky had provoked. But the evidence became overwhelming with the construction of radio telescopes like the Westerbork Synthesis Radio Telescope in the Netherlands, which measured the rotation of hydrogen gas in spiral galaxies, which spreads far beyond the starlight, still with the same result.

  In 1937 Zwicky had laid out another way to investigate the mass of galaxies. If by chance a massive galaxy lies along our line of sight to a more distant galaxy, then according to Einstein’s Theory of General Relativity it acts as a ‘gravitational lens’, warping the surrounding space to magnify, distort and displace the image of the background galaxy.

  Zwicky did not live to see a gravitational lens discovered. The first one was discovered in 1979 by Dennis Walsh, Robert Carswell and Ray Weymann. They were using a small telescope at the Kitt Peak National Observatory in Arizona to identify quasars discovered by the University of Manchester’s Jodrell Bank radio telescope, and found a pair of identical quasars right next to each other – in fact, two images of the same quasar produced by a gravitational-lens galaxy. Since then, gravitational-lensed images have been found that have been produced by clusters of galaxies, and have been used to confirm Zwicky’s calculations of the clusters’ mass. The latest estimate is that 5% of the matter in the Universe is made up of stars, 15% of intergalactic gas clouds and 80% of dark matter. Altogether these forms of matter make up 32% of the energy in the Universe, with 68% being dark energy.

  The composition of dark matter is unknown. It might be some sort of unknown massive elementary particle (different models being known under various names, such as axions or ‘WIMPs’ – Weakly Interacting Massive Particles). Laboratory searches for them are under way, and might produce the discovery of the twenty-first century. If dark matter is indeed found in the laboratory, this will be a case like helium, where a fundamental constituent of matter has been discovered in the cosmos before being identified on Earth. However, given that dark matter has not been identified, some astronomers speculate that there is something wrong with our theories of gravity, or that dark matter is ordinary matter in some hard-to-see form. At least we know there is something to explain, somewhere between a known unknown and an unknown unknown.

  Dark Energy

  On the threshold of a profound discovery

  Much later, when I was discussing cosmological problems with Einstein, he remarked that the introduction of the cosmological [constant] was the biggest blunder of his life.

  George Gamow, My World Line, 1970

  Imagine taking a region of space and removing all matter and radiation from it until the area is completely empty, much more so than ordinary interstellar space. The result is a ‘vacuum’. This vacuum has effects that physicists and astronomers call ‘dark energy’. Dark energy is thought to account for nearly ¾ of the energy in the Universe, but, like dark matter, we have not yet discovered a way to see dark energy.

  To common sense, the vacuum of space is nothing. To a scientist, the vacuum is not nothing; it is a physical state and it has an energy. In the absence of gravity, there is no way of measuring the energy of a state on an absolute scale; the best we can do is to compare energy differences. The vacuum energy itself would be arbitrary. According to the theory of General Relativity, however, any form of energy has a gravitational effect, so the vacuum energy might be a crucial ingredient in the evolution of the Universe. Colloquially, the vacuum energy is known as ‘dark energy.’

  Dark energy is as important to the dynamical history of the Universe as dark matter. The Universe is expanding, and it is expanding nearly freely. The galaxies and dark matter in the Universe mutually pull one another, a process that should slow the expansion down. This has been checked by the Hubble Space Telescope. In looking out into the distant Universe, the HST is looking back in time, because light travels at a finite speed and carries pictures of the Universe from far away and in the past to here and now. So the furthest galaxies, representing the earlier Universe, should be moving more quickly than nearby ones. In 1998–99 astronomers from the Supernova Cosmology Project and the High-Z Supernova Search Team used supernovae observed with the HST to check the distances of distant galaxies and the largest ground-based optical telescopes to check how fast they were moving. The two teams discovered the reverse of what was expected. The expansion of the Universe is speeding up, not slowing down. There is some progressive input of energy into the Universe. It goes under the name of ‘dark energy’, and its nature is a mystery.

  Curiously, dark energy has similar effects to a concept hypothesized a century ago by Albert Einstein. Einstein formulated his Theory of General Relativity before the discovery that the Universe is expanding. He used it to develop a theory of a static Universe. The force of gravity is what attracts all galaxies together. If gravity was the only force there was, it would be impossible for the Universe to be static – Einstein needed something to stop the galaxies falling together. Quite arbitrarily, he added a term to his equations called the ‘Cosmological Constant’, an opposing force cancelling out the effects of gravity, symbolized by the Greek letter Λ (lambda). When the expansion of the Universe was discovered, Einstein retracted the concept as unnecessary and regarded introducing it as a blunder. As history has turned out, Einstein was ahead of his time by inventing Λ to provide a theoretical solution to a problem that did not then exist. Quite what the theoretical solution really means is
still not clear, but some of its consequences are. A cosmological constant has the tendency to cause galaxies to accelerate away from us. In a Universe with both matter and vacuum energy, there is a competition between the tendency of Λ to cause acceleration and the tendency of matter to cause deceleration. This has a big effect on the ‘formation of structure’, the name that astronomers give to the way that the earliest irregularities in the material of the Big Bang grew. The denser bits drew in surrounding matter and grew to intergalactic-size clouds, which nucleated into clusters of galaxies, in which condensed stars and planets and, indeed, people. The balance between gravitation and Λ controlled the way our Universe turned out.

  In a major calculation called the Millennium Simulation (plate XIX), astronomers of the University of Durham and the Max Planck Institute for Astrophysics in Garching, Germany, showed how the formation of a cluster of galaxies starts weakly, because the fluctuations in Big Bang material are not very pronounced. Gravity, principally from dark matter, draws in surrounding material, but then the release of dark energy tends to stabilize the infall, which peters out as the galaxies orbit one another. The Millennium Simulation uses theories of cosmology, hydrodynamics and General Relativity. The output is a sample of the distribution of galaxies, and the actual distribution of galaxies in the Universe can be compared with the simulation. The astronomers can fine-tune the output of the simulation by altering the amounts of presumed dark matter and dark energy so that the calculated and real distributions match. The evidence suggests that the Universe has something like 32% of its density in matter and 68% of its density in dark energy. This indicates the scale of the problem – it is fundamental to physics. As with the problem of dark matter, the problem of dark energy may indicate that there is something wrong with our theories of gravity.