Secrets of the Universe

by Paul Murdin

  Eventually the Cambridge radio astronomers came to realize the validity of the criticisms and redesigned their equipment and analysis, producing the 3C catalogue in 1959. Ryle was awarded the Nobel Prize in 1974 for ‘pioneering research in radio astrophysics…for his observations and inventions, in particular of the aperture synthesis technique’. Third parties, like John Bolton, working with a group at Owens Valley, California, joined the debate and pressed the observations towards their ultimate conclusion, which came when Ryle replaced his 3C catalogue with a revision, 4C. This came much more strongly to the same answer, that the Universe was not the same in the past as now: it was not in steady state, but had changed.

  Ryle produced his 4C catalogue in 1965, and invited Hoyle to the press conference at which he would release his results. Hoyle, not informed about what Ryle would say, was sat on the stage and made to listen to Ryle describing what historian of astronomy Simon Mitton has termed ‘the inadequate 1C, the hopeless 2C, the ambiguous 3C and the perfect 4C’. 4C proved that the Steady State theory was wrong – there was indeed an excess of faint radio sources at vast distances and look-back times. Hoyle was thus humiliated in public by the stage-managed presentation of Ryle’s new results, but astronomers accepted the outcome: the Steady State theory, at least in its original form, was dead. Radio astronomers had discovered that the Universe has evolved.

  Cosmic Microwave Background

  The after-glow of the Big Bang

  Philosophically, I liked the steady-state cosmology. So I thought that we should report our results as a simple measurement; the measurement might be true after the cosmology was no longer true!

  Robert Wilson, ‘Discovery of the Microwave Background’,

  in Modern Cosmology in Retrospect, 1990

  The Cosmic Microwave Background is the black-body (thermal electromagnetic) radiation left over from the fireball of the Big Bang. Between 1946 and 1965 three groups of people were tracking it down, none of them aware of the efforts of the others, all of them starting from different theoretical standpoints, and none of them realizing that its existence had first been implied by a curious, unexplained measurement made in 1938.

  In Russia, George Gamow, Ralph Alpher and Robert Herman predicted the existence of the Cosmic Microwave Background (CMB) in 1947 from their theories about the formation of the elements in the Big Bang. In the first seconds of the Big Bang, the heated, expanded material cooled to a temperature of about a billion degrees. Their idea was that during this time elements were created in stages from the simplest element – hydrogen – by the successive addition of neutrons, one at a time, to make heavier and heavier nuclei.

  The theory failed because it could not create elements heavier than lithium. However, their analysis of the conditions of the matter in the early stages of the Big Bang became very important: they realized that the early Universe contained hot radiation. The radiation cooled as the Universe expanded, but became frozen when the Universe became transparent, about 400,000 years after the Big Bang, and the expanding matter could not alter the radiation’s characteristics. From then until now, the radiation propagated unhindered through the Universe, to become the CMB. In 1948 Gamow calculated the temperature of the radiation at 10 K (degrees above absolute zero) – Alpher and Herman got 5 K. These independent calculations were remarkably consistent, considering the uncertainties.

  Meanwhile, in the USA, Robert Dicke at Princeton University was also studying the formation of the elements in the Universe. He thought the Universe was oscillating: expanding and then collapsing periodically, to what he called a Big Crunch. Each cycle destroyed the elements made in the previous period, evaporating them when the temperature reached 10 billion degrees. The radiation formed in this phase then cooled and became frozen, as in Gamow’s theory. Dicke’s colleague, Jim Peebles, calculated the temperature of the radiation. In 1964, the two of them and David Wilkinson set out to detect it by building a so-called ‘Dicke radiometer’, a type of detector that the Princeton researchers had earlier been using to detect the heat of the Moon. In 1946 it had already been used to set an upper limit to the background radiation of 20 K.

  While the Princeton detector was being built, Arno Penzias and Robert Woodrow Wilson were trying to identify systematically all the sources of noise in a very sensitive receiver-antenna combination at Holmdel, New Jersey, being used by Bell Labs for early communication satellite experiments (bouncing radio waves off the Echo satellites to reflect radio signals from one continent to another). They used a giant antenna in the shape of a huge horn that could be rotated up and down around a horizontal axis, and around the horizon on a circular ground track. Radio waves that entered the horn were reflected along the axis and through a hole where they could be detected by equipment in a cabin. They were able to eliminate or measure all the instrumental effects, including the effect of droppings from two pigeons that had decided to nest in the horn, but they always saw excess noise, for which they could not account.

  Penzias and Wilson decided to investigate whether the radiation had an astronomical origin. It seemed to come equally from all parts of the sky. Meanwhile, at Johns Hopkins University, Peebles gave a lecture mentioning Dicke’s idea on the cosmic fireball, and his remark found its way to Penzias. In 1965 the Holmdel and Princeton groups got together and decided that what Penzias and Wilson had detected was the Cosmic Microwave Background radiation. They measured its temperature at 3 K, near enough to the early estimates by Gamow, Alpher and Herman. The discovery was published and became a sensation, because it was evidence against the Steady State theory of the Universe. Penzias and Wilson were awarded the Nobel Prize in 1978 ‘for their discovery of cosmic microwave background radiation’.

  The 3 K radiation had in fact already been discovered by Canadian astronomer Andrew McKellar in 1941, but he had not fully realized the implication of what he saw. He found from measurements of spectral lines in interstellar molecules called cyanogen (CN) made by S. W. Adams in 1938 that the molecules of interstellar space were warmed to about 2.3 K. The CN molecules are interstellar thermometers and the effect was clear, but the source of the warmth was only successfully identified by N. J. Woolf and George Field after the discovery of the CMB had been announced. The latest measurement of the temperature of the CMB is 2.7255 ± 0.0006 K, the most accurately measured temperature of a black-body ever.

  Several measurements confirmed that the CMB is basically isothermal and isotropic. Isothermal means that the radiation had almost the same temperature everywhere, and therefore must have had a single origin, such as an explosion. If it originated from, say, something happening in galaxies, its properties would alter somehow with the age, or type, or distribution of galaxies. Isotropic means that the CMB looks the same in every direction. This suggests that the CMB is a property of the Universe as a whole, because there is no reason to think that, by and large, any part of the Universe is different from any other. However, isotropy and isothermality are bound to break down at some level, the closer you look at the CMB and the closer you peer into the details of its origin: the CMB may have originated in one explosion, but the explosion must have had some structure.

  After some previous unsuccessful searches for irregularities in the brightness of the CMB, NASA’s Cosmic Background Explorer (COBE) spacecraft was launched in 1989, carrying an instrument to search for anisotropies. The spacecraft repeatedly scanned the sky, measuring the intensity and the temperature of the CMB everywhere, looking for differences from place to place. The COBE team, led by Berkeley physicist George Smoot and NASA astrophysicist John Mather, discovered that that the CMB had ‘shady patches’ at the level of 40 parts in a million (much more uniform than the most perfect white paper), indicating minute variations in temperature (plate XVI). In 1992 Stephen Hawking described this as the ‘discovery of the century, if not of all time’.

  These anisotropies formed when gravity acted on the minuscule fluctuations in density caused by quantum mechanics during the first moments of the Bi
g Bang. They developed into the major structures in the Universe. Eventually these structures made possible galaxies, stars, planets, and you and me, and the characteristics of the structures make it possible to calculate very precisely some of the properties of the Universe, like its age, size and density.

  Smoot and Mather shared the Nobel Prize for the discovery in 2006. The patches have been mapped in greater detail by the Wilkinson Microwave Anisotropy Probe (WMAP), launched in June 2001. Definitive measurements, which achieved the ultimate limit of accuracy at which the fluctuations can be measured (because of complicating interference from other astronomical radiations) were achieved by ESA’s Planck satellite, launched in 2009.

  Gravitational Waves

  Whispers of black holes, neutron stars and the Big Bang

  The weakness of the gravitational interaction makes it unlikely that gravitational radiation will ever be observed.

  F. A. E. Pirani, Gravitational radiation: an introduction to current research, 1962

  When the mass distribution of an object changes, the gravity around it changes, sending ripples through space at the speed of light. These ripples are called ‘gravitational waves’. At one time, even when Pirani was writing as recently as the 1960s, it was thought that the effects were too subtle to be detected, but with new technology astronomers have detected gravitational waves from merging black holes, and are now trying to develop space technology to detect gravitational waves generated split seconds after the Big Bang.

  Gravitational waves are a feature of Einstein’s Theory of General Relativity. They are emitted from revolving binary stars, rotating non-spherical stars and collapsing stars (if they do not collapse straight down, but splatter). Indeed, they are emitted from any event in which the distribution of mass changes. A car driving by gives out gravitational waves, but of course its mass is small, so the gravitational waves are very weak.

  Even from something the mass of a star, gravitational waves are weak. It was only in 2015 that gravitational waves were first detected, generated in a far-distant galaxy by the merger of two black holes. This event briefly radiated more energy than the entire luminous Universe; however, except for two of the most exquisitely sensitive sets of equipment, no one noticed this event, because the gravitational waves pass right through the Earth, leaving behind almost no energy – they whisper quietly and few people can hear them.

  Although gravitational waves are themselves a recent discovery, their effect was discovered even as long ago as the 1970s in the case of the binary pulsars PSR B1913+16 and PSR B1534+12.

  The first binary pulsar, B1913+16, was discovered by Russell Hulse and his PhD supervisor Joseph Taylor in 1973 with the Arecibo Observatory’s radio telescope in Puerto Rico (both men were awarded the Nobel Prize 1993 ‘for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation’). The binary pulsar is a pulsar in a small, high-eccentricity, short-period (in this case eight-hour) orbit around a second neutron star. Its pulses can be tracked precisely by radio telescopes, and arrive early or late according to whether the pulsar is near or far as it revolves in its orbit. It was clear right from the initial discovery that the pulsar would be a precise test of gravity – so precise that radio astronomers could observe the effects of General Relativity. In fact, after only two years, while Hulse was writing up his thesis in 1975, the first effects of General Relativity had been seen in the pulsar timings. By 1980 it was possible for Taylor to see the effect of the gravitational waves that the pulsar emits. The loss of energy from the system, as the gravitational waves radiate away, causes its orbit to shrink by 1.5 centimetres every orbit, so it has shrunk by 500 metres since it was discovered.

  Pulsar B1534+12 was discovered in 1990 by Aleksander Wolszczan. It has not been observed for as long as the Hulse–Taylor pulsar, but its pulses are both stronger and narrower, so its orbit is clearer. A ‘double pulsar’, PSR J0737-3039, was discovered in 2003, in which both neutron stars are pulsars. The extra precision that this enables makes this by far the best pulsar system with which to test General Relativity. In all these cases, it has been possible to measure not gravitational waves themselves, but their effects, and the calculations of General Relativity fit amazingly well, to an accuracy much more precise than 0.1%.

  The first successful attempts to detect gravitational waves directly were made in 2015 by detectors in Hanford, Washington, and Livingston, Louisiana, respectively, which together make up the Laser Interferometer Gravitational-Wave Observatory (LIGO), working in conjunction with a similar detector near Pisa, Italy, called VIRGO, with further detectors being developed in Hanover (GEO 600), Germany, and in Tokyo (TAMA 300), Japan. The principle of the instruments is to measure with a laser the distance between two freely hanging pendulum mirrors. The passage of a gravitational wave causes the mirrors to bob like corks on the sea, changing their separation.

  In 2015 LIGO saw the first detected event of gravitational waves from two merging black holes. By the end of 2018, ten such phenomena had been observed, together with an eleventh event that was generated by two merging neutron stars. In 2017 the Nobel Prize in Physics was awarded to Rainer Weiss, Kip Thorne and Barry Barish for their role in making the first detections. The first event, GW150914, lasted only two tenths of a second, as the two black holes circled each other in their final orbits, quickly spiralling inwards so that their orbital speed increased in a ‘chirp’ from 35 revolutions per second to 250 per second. They touched and merged into one black hole, oscillating afterwards for a few hundredths of a second. The two black holes were 35 and 30 times the mass of the Sun, with the outcome being a black hole of a mass 62 times the Sun. The missing 3 solar masses had been converted to the energy of the gravitational waves, through the E = mc2 equation. The merger took place in a galaxy at a distance of about 1.5 billion light years from Earth, so practically the entire Universe lies within sight of the detectors for this kind of event.

  Over the succeeding years, LIGO astronomers refined their data analysis capabilities and identified in their archived data a black-hole merger that preceded the first one identified, a gargantuan collision seen on 29 July 2017. Two black holes, one 50 and the second 34 times the mass of our Sun, merged to make a single black hole over 80 times the mass of our Sun. Its galaxy is 9 billion light years away. The 50-solar-mass black hole is the largest of the twenty black holes found so far through gravitational wave observations of black-hole mergers.

  The signature of the gravitational waves from the mergers was precisely what had been expected, which made analysis of the observation straightforward – the way to do it had all been worked out before. What was unexpected was that the black holes were so big. Black holes in a binary star system come from supernova explosions of the progenitor stars. Cutting a long story short, astronomers believed until then that only smaller black holes could be the result of a supernova. There must be something that astronomers do not understand – but what? The answer is a discovery yet to be made.

  The neutron-star merger event, GW170817, took place in 2017 (plate XXI). The two neutron stars spiralled into each other over a period of nearly two minutes, speeding up from 24 revolutions per second to about 300. The merger seemingly produced a neutron star of up to about 3 solar masses. Considering that neutron stars can survive only if their mass is less than 2 solar masses, it seems that this one was hypermassive and may then have collapsed to a black hole. Black hole mergers produce no light, radio or X-ray energy, but a neutron-star merger splatters about material that picks up energy from the event and radiates in ways that optical, radio and X-ray telescopes can see. The event was seen by seventy observatories world-wide and in space as a brief optical flash lasting a few days, and a burst of X-rays.

  The first space-borne gravitational-wave detector, called the Laser Interferometer Space Antenna (LISA), is being developed by the European Space Agency. It is planned as three spacecraft, spaced at 2.5-million-kilometre intervals in an equilateral-
triangle formation, following the Earth around in its orbit. It will contain devices to counter the effects of the solar wind as it buffets the spacecraft. The effects of gravitational waves as they pass through the Solar System will be detected by seeing how the spacecraft oscillate. The spacecraft will communicate by lasers that measure how far apart they are. A test vehicle, the LISA Pathfinder, was successfully used to verify some of the engineering techniques in space in 2015. LISA will be the largest man-made construction ever. LISA will be more sensitive than current gravitational-wave detectors partly because it will not be subject to the earth-tremors that shake the ground-based instruments, but also because of its size – the earth-based detectors are only 300 metres to 4 kilometres long.

  Gravitational waves have been detected only from binary stars, but it is predicted that they could also come from any spinning neutron star that has a bump on one side. Usually neutron stars have strong gravity and are spherical, but some neutron stars in some binary stars sit under an infalling stream of gas that heats the surface of the neutron star and may cause a hill. Gravitational waves can also come in bursts from supernovae that collapse to a neutron star or to a black hole. Finally, gravitational waves were made in the early history of the Universe and carry a picture of the conditions in the Big Bang as it was 1-million-million-million-millionth of a second old, just as the Cosmic Microwave Background carries a picture of the Universe when it was 400,000 years old. LISA should detect this Gravitational Wave Background.

  Darkness at Night

  The missing galaxies

  From innumerable stars a limitless sum total of radiations should be derived, by which darkness would be abolished from our skies and the ‘intense inane [void]’, glowing with the mingled beams of suns individually indistinguishable, would bewilder our feeble senses with its momentous splendour.