Secrets of the Universe

by Paul Murdin

  moderately penetrating mind, monsters to which

  those of the oceans bear no sort of comparison.’

  ‘What monsters may they be?’

  ‘Impersonal monsters, namely, Immensities.’

  Thomas Hardy, Two on a Tower, 1882

  At the centre of our Milky Way galaxy, a cluster of a hundred stars is orbiting a mysterious object at incredibly high speeds. Although astronomers cannot see the object, they know its mass is over 4 million times the mass of the Sun. It seems to be a supermassive black hole that is invisible because it is sleeping.

  Karl Jansky’s and Grote Reber’s maps of the radio sky showed that radio waves came from along the line of the Milky Way with a maximum in the direction of Sagittarius. The centre of the Galaxy lies about 25,000 light years in this direction, but in visible light we can only see as far as perhaps 1,000 light years, because the interstellar space contains a smoke of dust particles blown out by stars. There is typically 1 dust particle per 1,000 cubic metres (say one particle in a cathedral-sized volume), but since interstellar space is so large, the individual dust particles add up to an opaque screen. Radio waves and infrared radiation can, however, penetrate through this smoke, and reveal what lies at the centre of the Galaxy beyond.

  At first the discrimination of radio telescopes was not fine enough to show anything other than that there was a strong radio source in Sagittarius. It gathered the name Sagittarius A as the strongest source in that constellation, or Sgr A for short. It soon became clear that Sgr A was complex, and made up of several different things, like nebulae and supernova remnants.

  Radio astronomers discovered that there were two strong radio-emitting sources with distinct radio properties within the region, and so Sgr A was divided into Sagittarius A West and Sagittarius A East. Sgr A East is probably a supernova remnant, but Sgr A West was more of a mystery. It is a complicated spiral shape and it coincides with the highest density of stars in the Galaxy. Radio astronomers interpret it as the centre of our Galaxy, and when in 1959 the International Astronomical Union agreed to set up a coordinate system for the Galaxy, Sgr A West was defined as the central point.

  Because Sgr A West is so complex, it was difficult to study. In February 1974 Bruce Balick and Robert Brown used the Green Bank 35-kilometre radio-linked interferometer of the National Radio Astronomy Observatory to map Sgr A West and discovered a bright point-like radio source at its centre. They concluded: ‘The unusual nature of the sub-arcsecond structure and its positional coincidence with the inner 1-pc [parsec] core of the galactic nucleus strongly suggests that this structure is physically associated with the galactic center (in fact, defines the galactic center).’ Robert Brown invented the name Sagittarius A* (pronounced ‘Sagittarius A-star’ and abbreviated Sgr A*) for the source.

  With the position of Sgr A* clarified and with the development of very large telescopes and detectors optimized for use in the infrared spectrum, it became possible to take infrared pictures of the galactic centre. Infrared pictures show dust, stars and gas lying in the central 30-light-year region. Its very centre is surrounded by the so-called Circumnuclear Disc of dust. The Disc surrounds a cluster of stars, and Sgr A* lies at the centre of the Disc and the cluster.

  The motion of these bright stars has been studied by teams led by the German astronomer Reinhard Genzel and University of California astronomer Andrea Ghez. They have repeatedly imaged the stars over the past decade with telescopes in the European Southern Observatory in Chile and the Keck Observatory in Hawaii and been able to see the progressive displacement of the stars even though they are at a distance of 25,000 light years. Both groups use advanced image-stabilization techniques to overcome the wobble of the star images caused by the Earth’s moving atmosphere, which blurs ordinary images. The instrument produces the main image by reflecting it off a series of movable mirrors. The instrument senses the distortion of the atmosphere and adjusts the mirrors to compensate. The sharpened images make it possible to see easily the small displacements of the stars from year to year.

  These astronomers have discovered that the stars in the cluster are speeding round Sgr A*. Every fifteen years, one of them, the star known as S2, approaches as close as 120 times the Earth–Sun distance of the black hole – the distance of the furthest small planets in the Solar System. The motions of the stars make it possible to estimate the mass in the cluster. It is 4,100,000 times the mass of the Sun. This cannot be the mass of the cluster itself, which consists of something like a hundred stars. The vast majority of it must be accounted for by Sgr A* itself. And the close approach of S2 means that these millions of solar masses are packed into something the size of our Solar System, which contains just one star. The only object that could be so dense is a supermassive black hole.

  If we compare the black hole in our Galaxy with a typical quasar or other Active Galactic Nucleus, it does not rank very high in brightness. It does not rival, say, 3C 273 or M89. One reason for this is that the black hole in our Galaxy is not at the top end of the range of masses for supermassive black holes. Another is that not much matter falls onto it. Collisions between clouds of material in the Circumnuclear Disc causes them to fall in towards Sgr A*, but only a small trickle of material dribbles onto the black hole. Our Galaxy’s black hole is thus dormant, its signs of life faint snores when compared to the terrible rages of 3C 273 or M89.

  Gamma-Ray Bursters

  The biggest bangs since the Big Bang

  From the conception the increase.

  From the increase the swelling.

  From the swelling the thought.

  A Maori song, collected by Richard Taylor, 1850

  Gamma-ray bursters are cosmic explosions of extraordinary energy, which were first discovered during the Cold War by US satellites designed to detect Soviet nuclear-weapons tests. The Partial Test Ban Treaty, signed in October 1963, prohibited tests of nuclear weapons in the atmosphere or in space. To verify whether the other side was keeping to the terms of the treaty, the US Air Force launched a series of satellites, named Vela, to look for signs of illegal tests.

  The satellites watched for the nuclear effects of the explosions – brief bursts of gamma rays. However, from the launch of the first satellites in 1967, the gamma-ray equipment on the satellites discovered a completely unexpected phenomenon – not bursts from man-made terrestrial explosions, but bursts from natural causes. Each burst lasted only about a second and, as they were being detected on an almost weekly basis, were too frequent to be clandestine nuclear explosions from any country.

  As the Vela series of satellites became more sophisticated, they provided more information about the mysterious gamma-ray bursts. Vela 5A and 5B (launched in 1969) and Vela 6A and 6B (1970) were able to characterize the signatures of the bursts, even though they were so brief. In 1972 Ray Klebesadel and Ian Strong of the Los Alamos Scientific Laboratory, New Mexico, studied secret records that revealed the direction from which the gamma rays had originated. They found that on several occasions the same burst had been detected by two satellites. To look for tests on both sides of the Earth, the Vela satellites were operated in pairs, identical satellites on opposite sides of a circular orbit 250,000 kilometres in diameter. The scientists found that the gamma-ray bursts were not quite simultaneous in both satellites; in fact, the light travel time from one spacecraft to another, across the orbital diameter, was about 1 second. The gamma rays, travelling at the speed of light, triggered first one satellite and then the other, thus indicating from which direction they had come.

  By 1973 Klebesadel and Strong were able to prove conclusively that the gamma-ray bursts were of cosmic origin. They had noticed that some bursts had been seen by four satellites – both the Vela 5 and the Vela 6 pairs – and therefore could only have come from random areas of the sky, but astronomers debated their origins. Klebesadel and Strong were given permission to publish this discovery as a scientific paper, but they were not allowed to report all the details of the instrumentation and i
ts capability, as that might have made it possible for other countries to tailor illegal weapons tests that could not be detected. Some astronomers complained that they were not privy to all the relevant scientific background, but they had to put up with it.

  Scientific satellites were deployed on the problem and gamma-ray bursts were discovered almost daily. The phenomenon was given a new name – a gamma-ray burster. Up to 1997 the best clue to the nature of bursters was that gamma-ray bursts come from all directions equally (isotropy) but this evidence was equivocal. Some astronomers thought that bursters might occupy a region surrounding the Solar System at a distance of up to a light year (comets come from this region). Others thought that bursters might occupy a halo, extended around our Galaxy.

  By contrast with these possible ‘local’ origins, some daring astronomers began to consider that the bursters might be ‘cosmological’ – distributed among the most distant galaxies, at distances of hundreds of millions of light years. This was daring because, if bursters were really this distant, their energy was enormous.

  In 1997 the question of the distance and energy of bursters was answered through a key observation by an Italian–Dutch satellite called BeppoSAX. At 5 a.m. on 28 February, BeppoSAX detected a gamma-ray burst known as GRB 970228 – gamma-ray bursts are numbered by date. The satellite’s operations team in Rome rescheduled its observing programme to deploy more accurate X-ray sensitive instruments for follow-up observations only eight hours later. They saw a new X-ray source, which quickly faded. Both the positional coincidence and the variability indicated that the gamma-ray burst and the X-ray emission originated from the same object.

  Less than one day after the first detection of the gamma-ray burst, Dutch astronomer Jan van Paradijs used the William Herschel Telescope (WHT) on the island of La Palma, Spain, to discover a faint optical source at the same position in the sky as the X-ray source. The WHT’s optical images showed that the optical burst was surrounded by a fuzzy patch. The Keck 10-metre telescope on Mauna Kea, Hawaii, then the largest in the world, and the Hubble Space Telescope revealed that the fuzzy patch was a galaxy at an enormous distance from Earth.

  To be able to be detected at such distances, gamma-ray bursters let loose as much energy as a supernova in a few seconds or less. In fact, at least some gamma-ray bursters have subsequently proved to have spectral properties that are just the same as imploding supernovae. For some reason the implosion is ‘naked’ and the gamma rays get out, rather than being absorbed in stellar debris surrounding the explosion.

  But not all gamma-ray bursters are the same. About a third of the bursts are much shorter-lived than the majority, typically less than a second in duration. The favoured theory is that they are the result of the merger of two neutron stars, spiralling together in a double-star system. One such event was seen in 2017 as an accompaniment to a burst of gravitational waves. The signature of the gravitational waves was of the merger of two neutron stars, each between 1 and 2 times the mass of the Sun. A short gamma-ray burst followed 1.7 seconds afterwards, and then an optical source that lasted for a month and an X-ray source that lasted over a year. It was clearly a complicated event, which offers the promise of understanding these short-period bursters. It is rare for bursters to happen in a given galaxy, but, since the gamma-ray bursters are powerful enough to bring the entire Universe of galaxies within view and there are so many galaxies in the Universe, astronomers see one happening practically every day.

  The Evolving Universe

  The past, the present and the future

  Of the forces which are imperceptible forces, none is greater than that of change. All things are ever in the state of change. Therefore the I of the past is no longer the I of today.

  Chuang Tzu, fourth-century BCE Taoist philosopher

  Is the size of the Universe fixed and unchanging, or is it continually expanding? Has the Universe always existed, or did it have a discrete, explosive beginning in an event called the Big Bang? Throughout the twentieth century, rival camps of physicists and astronomers fought over these questions about the Universe’s past, present and future.

  In the seventeenth century Isaac Newton applied his theory of gravity to the Universe, on the assumptions that the mass in it was uniform and static. Each mass particle – he thought of them as stars, but we would think of them as galaxies – attracted all the others, and Newton realized that the Universe would therefore logically collapse in on itself. Manifestly it has not done so.

  Newton never resolved this difficulty. When in 1915 Albert Einstein discovered a new formulation of the theory of gravity, which came to be called the General Relativity, he also tried to apply it to the entire Universe, with the same result as Newton – a static Universe of galaxies was fundamentally unstable. His solution was to invent the so-called Cosmological Constant, which acted like a repulsive force that held the Universe up.

  That very year, Willem de Sitter discovered in the mathematics of General Relativity something that Einstein had overlooked: the Universe need not be static, but could be expanding. Around 1927 the Belgian astronomer Georges Lemaître, visualizing the start of the Universe as an exploding atom, proposed theories of the expanding Universe that were valid without need for Einstein’s Cosmological Constant. In 1929 the US astronomer Edwin Hubble used the 100-inch Mount Wilson telescope to map the motions of the galaxies, and discovered Hubble’s Law, in which distant galaxies are moving away from us at a rate that is proportional to their distance. The interpretation is that the Universe is expanding. In his 1927 paper Lemaître had implicitly predicted a linear velocity–distance relation of this kind.

  Lemaître identified the explosion of the original atom as the moment of the creation of the Universe – the Big Bang. There were some astronomers in the 1950s who had philosophical or religious objections to this, among them Cambridge mathematicians Hermann Bondi and Thomas Gold, who wished to construct a theory in which there was no question of how the Universe originated. They were joined by the physicist Fred Hoyle. After the failure of George Gamow’s theory of the origin of the elements in the hot Big Bang, Hoyle had been successful in formulating a theory of the origin of the chemical elements in stars. If there was no Big Bang then clearly there must be other places where they formed, and Hoyle thought he could say what those places were.

  The outcome of these discussions was the Steady State theory, which holds that the Universe has always been the same, rather than evolving. Since Hubble’s Law showed that it was expanding, getting ever bigger, then something must be filling the space thus created at just the right rate. Fred Hoyle invented the idea of the Continuous Creation of hydrogen, spontaneously in the gaps that developed between the galaxies. In his arguments for the Steady State/Continuous Creation theory, Hoyle invented the derisory term ‘Big Bang’ to describe the origin of the Universe in the rival theory that it was expanding expontentially – to his surprise, the term was taken up without rancour.

  The main difference between the two contrasting theories was that in the one case the Universe had not changed, and in the other it had. In the Big Bang theory, the fundamental difference between the past and the present is that in the past the Universe was denser, with galaxies closer together. Looking back into the past is the equivalent, for astronomers, of looking into the distance: light from a galaxy and its neighbours travels at the speed of light so it carries an image of the galaxy and its neighbours as they were when the light left. The distances to which optical telescopes could see were not so great that this could be investigated, but radio waves travel at the same speed as light, so radio astronomy does just as well. In the 1950s radio telescopes were discovering thousands of radio galaxies at large distances, and the question could be addressed.

  Several groups of radio astronomers began to address this issue at about the same time, and gathered themselves in two camps, one led by Martin Ryle at the University of Cambridge and the other a loose alliance of research groups in Australia at the CSIRO Division of R
adiophysics and the University of Sydney led by John Pawsey, Bernard Mills and Bruce Slee.

  The cosmological question boiled down to arcane arguments about what was called ‘log N-log S’, which was a relationship that expressed the number, N, of radio galaxies of a given brightness, S. The fainter galaxies are in general further away. Radio waves have travelled a long time to get from the further galaxies and we view them as they were in the past. If there are more of the fainter further galaxies than you would have expected, as indicated by the log N-log S distribution, this must mean that the Universe was denser in the past than it is now, and the Universe must have evolved.

  The log N-log S debates got bitter. After the first experimental surveys, Ryle’s group published in 1955 a catalogue of nearly 2,000 radio sources, called 2C. It clearly showed a large overabundance of faint radio sources. In Australia, Bernard Mills had just started making a survey with a radio telescope built on lines that were different from the Cambridge ones. Fred Hoyle wrote a worried letter to Mills asking if his results confirmed Ryle’s. Even early on they did not confirm Ryle’s results. By 1955 the Australian radio-source survey was showing a slight excess of faint sources, but nothing like the excess found by Ryle. The Australians privately expressed reservations about the 2C catalogue, and suggested that most faint sources in the 2C catalogue were spurious instrumental effects.

  Ryle publicly ignored the criticisms, and relations between the two groups soured. In 1957 Mills and Slee published a catalogue that amounted to a devastating criticism of 2C: ‘…there is a striking disagreement between the two catalogues…discrepancies, in the main, reflect errors in the Cambridge catalogue and accordingly deductions of cosmological interest derived from its analysis are without foundation…’. Hoyle was relieved and continued to develop his Steady State theory.