1 (page 1)p. 1The observable Universe

In a galaxy more than a billion light years away, two black holes, one 36, and the other 29 times the mass of the Sun, had become ensnared in each other’s powerful gravitational grip, and were gyrating rapidly around each other. As the pair orbited, the spacetime around them was dynamic and distorted, producing disturbances spreading out at the speed of light across the Universe, as ripples in the fabric of spacetime—gravitational waves. As this drama played out, the black holes spiralled inwards towards their fate—the merger of two black holes into a single, bigger one. A billion years later, in 2015, the final ripples from this merger, GW150914, reached the Earth and jiggled the sensors of a new and exquisitely sensitive type of astronomical instrument, the Laser Gravitational Wave Observatory (LIGO). This was the first ever detection of a gravitational wave.

Modern observational astronomy is exciting and varied, and is capturing dramatic events in the cosmos which may well not involve traditional telescopes. Until just a few decades ago, astronomers had only photons to work with and virtually everything we know about the Universe has come from studying particles of light. Before 1945, this meant observing in the narrow visible region of the electromagnetic spectrum. Since then, great advances in technology have enabled us to probe beyond the visible spectrum, allowing us to explore the Universe at radio, (page 2)p. 2infrared, ultraviolet, X-ray, and gamma ray wavelengths. These new observations, made with increasingly sophisticated technology, have shown us a plethora of exotic phenomena such as young galaxies at the edge of the visible Universe, pulsars, quasars, colliding galaxies, and exploding stars. Pulsars, or pulsating stars, are neutron stars, the relatively tiny stellar cores left behind when massive stars explode as supernovae, spinning rapidly and emitting intense beams of radiation. Quasars, or quasi-stellar radio sources, first observed at radio wavelengths, are extremely distant objects which emit vast amounts of energy and powered by matter falling into supermassive black holes. These exotic objects have been discovered using new types of telescopes that have enabled us to see farther, deeper in time, and to ‘see’ in different ways.

Observational astronomy is not just a story of technological development. To interpret what is observed, astronomers and astrophysicists use theory. Observations and physical theory have together enabled us to piece together how stars produce their energy, the origin of the chemical elements, how black holes form, and how supermassive black holes lurking in the hearts of galaxies can power quasars, spewing out immensely powerful jets of particles and energy thousands of light years into space. Observations have shown us that the Universe is expanding and that the expansion itself is accelerating. Sometimes we discover celestial objects that provoke us into devising ingenious ways of testing theories, and sometimes an observation is so puzzling or surprising that we are driven to revise, expand, or even replace our model of what is happening, how, and why. Either way, theory and observation are inseparably entwined, as will be evident throughout this volume.

The first maps of the stars were made in antiquity by the Babylonians and Egyptians who, 5,000 years ago, were skilled astronomers. The father of observational astronomy is a Greek, Hipparchus, who, in around 129 bc, catalogued 850 stars and (page 3)p. 3invented a way to record their apparent brightness, the magnitude system, still in use today. Magnitudes range from one (the brightest) to six (the faintest visible with the naked eye), a back-to-front scale where a difference of five magnitudes indicates that one star is 100 times brighter than another.

The use of the first telescopes greatly increased our awareness of how big the visible Universe truly is. Galileo Galilei took a 3x power Dutch spyglass, multiplied the magnification 10-fold and, with it, separated the diffuse glow of our galaxy, the Milky Way, into a vast number of points of light from separate stars. As telescopes increased in size they revealed fainter objects, pushing observations ever deeper towards the boundaries of the visible Universe. Today’s biggest telescopes capture 10 million times more photons than the eye, and shepherd them onto electronic retinas which are much more sensitive than those in our eyes.

Light brings us most of the knowledge we discover about the world. Nothing travels faster than light, with a speed, c, of 300,000 kilometres (km) per second. By everyday standards this speed is enormous, so that light appears to move instantaneously. However, the light reaching you from the words you are reading takes a nanosecond, one-billionth of a second, to reach your eyes. And the light from our nearest star, the Sun, takes eight minutes to reach us—we see the Sun not as it is now, but as it was eight minutes ago. The finite speed of light means that we never see the present, we always see the past. Time and space are inextricably intertwined. Astronomers have turned this into a way of measuring big distances in the Universe. The speed of light defines the astronomical distance of a light year, the distance light travels in one year. (One light year is almost 10 trillion km (or 10,000,000,000,000 km).) Moving out deeper in space, the nearest star to the Sun is Proxima Centauri. The light it emits takes four years to reach us; it is four light years away. The faint stars that Galileo saw in the Milky Way are further away still: they emitted the light that we are now seeing 10,000 years ago.

(page 4)p. 4But the light that we start with is that from our star, the Sun. The Sun is a small, rather commonplace star. Its most obvious observational feature is its brightness, quantified by the solar constant, the life-maintaining flux of energy arriving at the Earth, measured as 1.37 kilowatt (kW) per square metre (m). Integrating this flux over the incident surface of the Earth, and allowing for the radiation reflected back into space, the amount of energy absorbed by the Earth is enormous and greatly exceeds all human-made power sources. The total power emitted by the Sun (the standard astronomical unit of luminosity) is 3.86 × 10²⁶ watts, its mass (the unit of 1 solar mass) is 1.99 × 10³⁰ kilogrammes (kg), and it is a nearly perfect sphere with a diameter of 1.39 million km.

The Sun is the most studied of all stars. It is the only star close enough to reveal surface features such as sunspots, granular surface texture, solar flares, prominences, and a hot outer atmosphere, the corona. Other stars present such small angular sizes to us that making images of the surfaces of even the nearest ones requires specialized high-resolution techniques. The light emitted by stars comes from the photosphere and is radiated as a broad thermal continuous spectrum. The surface temperatures of stars range from 3,000 kelvin (K) to 30,000K; for the Sun, it is 5,800K. (The absolute temperature scale used here is measured in kelvin starting from absolute zero, 0K, or −273 degrees Celsius (°C).) An obvious feature of the stars is that they present different colours. A star’s colour relates to its temperature: hot stars radiate strongly at short wavelengths (peaking at the blue end of the spectrum), and cooler stars radiate at the red end. The Sun’s spectrum peaks near the middle, in the green part. The Sun is composed of 73 per cent hydrogen and 25 per cent helium, with the remainder comprising carbon, nitrogen, and oxygen, and traces of heavier elements. (It is usual for astronomers to refer to chemical elements heavier than helium as ‘metals’.) The abundances of the elements in the Sun and the gas giant planets are similar to those in the Universe as a whole.

(page 5)p. 5Extensive solar observations have led to the development of what is generally known as the standard stellar model, a mathematical model that not only describes the main observational features of the Sun, but can also be applied to understanding other stars. The model includes the mechanism describing the release of energy in the Sun’s hot core by thermonuclear fusion reactions, the transport of the energy to the surface by radiation and convection, and the evolution of the star. In the core of the Sun, the temperature is 15 million K, and the weight of the overlying layers press down on it squeezing the particles to densities of over 10 times that of lead. There, hydrogen nuclei (protons) collide at very high speeds. By overcoming the natural electrostatic repulsion that exists between positive nuclear charges, conditions exist that enable four hydrogen nuclei (protons) to fuse together to make helium. This process releases the nuclear binding energy that powers the Sun and all stars.

One of the most powerful tools available to observational astronomers is spectroscopy—the study of the emission and absorption of electromagnetic radiation of different wavelengths and its interaction with matter. Isaac Newton famously used a simple triangular glass prism to split sunlight into its constituent colours and demonstrated that what we have come to call ‘white light’ is really a mixture of light of all the colours of the rainbow. The true power of spectroscopy only began to be realized when 19th-century astronomers turned spectroscopes towards the stars. (A spectroscope is a prism combined with a small telescope designed to study a spectrum in detail.) In the Sun, hundreds of mysterious narrow dark lines (known as Fraunhofer lines) were seen superimposed on the underlying broad thermal spectrum. It was as if numerous narrow ‘slices’ had been removed from the spectrum. These dark lines are caused by the absorption of light at specific wavelengths, relating to the quantum energy transitions of atoms in the Sun’s cooler atmosphere. The different patterns of atomic lines in a spectrum are like fingerprints revealing the presence of different chemical elements; the patterns identify a (page 6)p. 6given type of atom as unambiguously as a supermarket barcode identifies its product. Spectroscopic observations therefore provided the first profoundly important evidence that the matter in the stars is the same type as matter found on Earth. Spectroscopic observations are now being made over the entire electromagnetic spectrum, and they play a central role in providing us with information about the physical state and motion of matter in distant celestial objects.

The Sun sits about halfway out from the centre of our galaxy, the Milky Way, a giant disc-like spiral galaxy, and home to 200 billion stars. Surrounding the nucleus is a central stellar bulge, embedded in a flat disc of stars and gas. If we could move out the galactic disc and look back at it, it would resemble the spiral galaxy Messier 74 (M74), shown in Figure 1. (The M74 galaxy was listed by the 18th-century French astronomer Charles Messier, a comet seeker. At that time comet hunting was a major activity and M74 was put on a watch-list of around 100 non-cometary but fuzzy objects to be avoided.) In between the stars, spiral galaxies contain a tenuous interstellar medium of gas and dust grains in the disc. From our place in the galaxy we see the Milky Way as a diffuse band of light, with dark blotches of dust clouds obscuring the light (page 7)p. 7from countless distant stars. The Sun orbits around the centre of the galaxy at a speed of 230km per second, and makes one complete revolution, a galactic ‘year’, in 226 million Earth years.

Stars are the building blocks of galaxies, and galaxies gather into larger structures. The image of M74 gives the impression that the stars in a galaxy are densely concentrated, especially in the central bulge. But the average spacing of stars in spirals is about five light years (30 million solar diameters). This means that the chance of stars physically colliding with each other is negligible. Hence the stars in a galaxy are, in effect, collisionless. The other main class of galaxies are ellipticals, which are shaped like larger versions of the central bulges of spirals but contain little gas or dust. When we look out into space, beyond the Milky Way, we see groups and other assemblies of galaxies in the local Universe. When astronomers talk about the ‘local Universe’ they mean everything in our immediate vicinity for which the effects of cosmic evolution can be ignored. In practical terms this is a spherical volume roughly a billion light years across. Since the average separation of galaxies is around 10 million light years, the local Universe contains some 60 million galaxies. But when we look out further than this, we discover plenty of evidence that the Universe has undergone dramatic evolution in its history, evidence that points to its origin in the Big Bang.

The age of the Universe, namely the time that has elapsed since the Big Bang, is 13.8 billion years. It is reasonable to ask how big the observable Universe is. The oldest photons that we can observe are those of the Cosmic Microwave Background (CMB) radiation, the relic of an early hot and dense phase, emitted at a time before the stars and galaxies had formed. When it was emitted, the CMB radiation would have appeared to our eyes as visible light, but, owing to the expansion of the Universe, it has now been shifted in wavelength (or redshifted) by such a large factor that we now observe it in the microwave waveband. The CMB radiation has been travelling through space for nearly (page 8)p. 813.8 billion years. We might expect that if it has been travelling for that length of time, and is only just now entering our telescopes, the visible Universe should be a sphere, centred on the Earth, with a radius of 13.8 billion light years. Correct? No, this is completely wrong.

Where this line of reasoning goes wrong is in the assumption that the Universe is static, namely that the distances between objects are constant. The Universe is not static. Observations have shown us that it was once smaller and hotter, and is expanding. When we measure the distance between two points on Earth, we take it for granted that we can make measurements simultaneously. But that is not the case: we are only just now seeing the CMB radiation that was emitted shortly after the Big Bang, nearly 13.8 billion years later. In the time it has taken for the radiation to reach us, space has expanded, and the place where the original light was emitted is now further away from us. The simplest calculation of the radius of the sphere around us from which the CMB was emitted suggests 41 billion light years. But even that is too small. In 1998 astronomers discovered an extra component of the Universe, dark energy, a repulsive anti-gravity energy field, filling all of space and pushing the galaxies apart. When dark energy is taken into consideration, the radius of the sphere from which the CMB light was emitted increases to 46 billion light years. So, in short: when we observe the CMB radiation with our telescopes, we are seeing it as it was when it was emitted a (relatively) short distance away; but now, because of the expansion of space, the location from which it was emitted has moved 1,000 times further away from us than it used to be. When we see this radiation, we see it as it was then, but not as it is now.

We therefore find ourselves sitting in the middle of a 46-billion-light-year-radius sphere containing all the observable matter in the Universe. There may, of course, be more matter outside the sphere, but its light has not had time to reach us yet, and so it is unobservable. The sphere of the observable Universe contains (page 9)p. 9several hundred billion galaxies, equivalent to a matter content of some 10⁸⁰ hydrogen atoms. Although it is difficult to imagine such a vast number, the Universe is so big that the average density of matter, spread out through the whole volume, amounts to only a few hydrogen atoms per cubic metre. By comparison, the Earth’s density is almost 10³⁰ times larger than that. Planets are therefore unusually dense regions of the Universe. But there are objects even denser than planets. The density of a neutron star is over 10¹³ times greater than that in a planet. These numbers begin to make sense when it is appreciated that the Universe is enormous, and consists of mostly empty space. But even then, space is not really truly empty: it contains dark energy, dark matter, the CMB radiation, subatomic neutrino particles, and highly energetic cosmic ray particles. The fabric of spacetime is also crossed by the expanding ripples of gravitational waves, emitted by the violent mergers of black holes and neutron stars.