ABSOLUTE ZERO Lowest temperature attainable. As a body is cooled, its atoms move more and more sluggishly. At absolute zero, equivalent to –273.15 on the Celsius scale, they cease to move altogether. (Actually, this is not entirely true since even at absolute zero the Heisenberg uncertainty principle produces a residual jitter.)
AFTERGLOW OF CREATION See Cosmic Background Radiation.
ALPHA CENTAURI The nearest star system to the Sun. It consists of three stars and is 4.3 light years distant.
ANDROMEDA The nearest big galaxy to our own Milky Way, about 2.5 million light years distant. Andromeda and the Milky Way are the dominant, big galaxies in a cluster of at least 40 galaxies known as the Local Group.
ANTENNA Any device that converts a free-space electromagnetic wave into a guided electromagnetic wave – for instance, one channelled along a hollow metal ‘wave guide’.
ANTHROPIC PRINCIPLE The idea that the Universe is the way it is because, if it was not, we would not be here to notice it. In other words, the fact of our existence is an important scientific observation.
ANTIMATTER Term for a large accumulation of antiparticles. Antiprotons, anti-neutrons and positrons can in fact come together to make anti-atoms. And there is nothing in principle to rule out the possibility of anti-stars, anti-planets and anti-life. One of the greatest mysteries of physics is why we appear to live in a universe made solely of matter when the laws of physics seem to predict a pretty much 50/50 mix of matter and antimatter.
ANTIPARTICLE Every subatomic particle has an associated antiparticle with opposite properties, such as electrical charge. For instance, the negatively charged electron is twinned with a positively charged antiparticle known as the positron. When a particle and its antiparticle meet, they self-destruct, or ‘annihilate’, in a flash of high- energy light, or gamma rays.
ATOM The building block of all normal matter. An atom consist of a nucleus orbited by a cloud of electrons. The positive charge of the nucleus is exactly balanced by the negative charge of the electrons. An atom is about a ten-millionth of a millimetre across.
ATOMIC NUCLEUS The tight cluster of protons and neutrons (a single proton in the case of hydrogen) at the centre of an atom. The nucleus contains more than 99.9 per cent of the mass of an atom.
AXIS OF EVIL The name given to the anomalous alignment of the biggest temperature splotches seen in the cosmic background radiation by the Wilkinson Microwave Anisotropy Probe. (Technically, the alignment is between the ‘quadrupole’ and ‘octupole’ temperature variations.) Such an alignment is unlikely to happen within the standard inflationary picture of cosmology. No one yet knows whether the anomaly is significant or not.
BIG BANG The titanic explosion in which the Universe is thought to have been born 13.7 billion years ago. ‘Explosion’ is actually a misnomer since the Big Bang happened everywhere at once and there was no pre-existing void into which the Universe erupted. Space and time and energy all came into being in the Big Bang.
BIG BANG THEORY The idea that the Universe began in a superdense, super-hot state 13.7 billion years ago and has been expanding and cooling ever since.
BIG CRUNCH If there is enough matter in the Universe, its gravity will one day halt and reverse the Universe’s expansion so that it shrinks down in a big crunch, a sort of mirror image of the Big Bang.
BLACK BODY A body which absorbs all the heat that falls on it. The heat is shared among the atoms in such a way that the heat radiation it gives out takes no account of what the body is made of but depends solely on its temperature and has a characteristic and easily recognisable form. Also known as thermal radiation. The stars are approximate black bodies.
BLACK HOLE The grossly warped space–time left behind when a massive body’s gravity causes it to shrink down to a point. Nothing, not even light, can escape, hence a black hole’s blackness. The Universe appears to contain at least two distinct types of black hole: stellar- sized black holes, formed when very massive stars can no longer generate internal heat to counterbalance the gravity trying to crush them, and ‘supermassive’ black holes. Most galaxies appear to have a supermassive black hole at their heart. They range from millions of times the mass of the Sun in our Milky Way to billions of solar masses in the powerful quasars.
CARBON MONOXIDE Consisting of one carbon atom bonded to one oxygen atom, CO is the most common molecule in interstellar space after molecular hydrogen, H2.
CEPHEID VARIABLE A very luminous yellow star that swells and shrinks periodically. The pulsation period is related to the intrinsic luminosity of the star. This means whenever a Cepheid is observed, its period reveals its true luminosity. A comparison with its apparent luminosity yields its distance. Cepheids have played a key role in measuring the distance to nearby galaxies such as Andromeda.
COLD LOAD Reference standard with which a celestial source of radio waves can be compared in order to make an ‘absolute’ measurement of its effective temperature. In the case of the cosmic background radiation, a cold load at the temperature of liquid helium is often used, since its temperature of 4.2 degrees above absolute zero is very close to the background temperature of 2.725 degrees.
COPERNICAN PRINCIPLE The idea that there is nothing special about our position in the Universe, either in space or in time. This is a generalised version of Copernicus’s recognition that the Earth is not in a special position at the centre of the Solar System but is just another planet circling the Sun. See also Cosmological Principle.
COSMIC BACKGROUND EXPLORER SATELLITE (COBE) Satellite launched in 1989 to ‘map’ the temperature of the cosmic background radiation – the ‘afterglow’ of the Big Bang fireball – across the sky. COBE found slight variations in the average temperature of the radiation, which were created by matter beginning to clump 380,000 years after the birth of the Universe. The clumps were the ‘seeds’ of giant superclusters of galaxies in today’s Universe.
COSMIC BACKGROUND RADIATION The ‘afterglow’ of the Big Bang fireball. Incredibly, it still permeates all of space 13.7 billion years after the event, a tepid microwave radiation corresponding to a temperature of –270°C.
COSMIC BACKGROUND RADIATION, ANISOTROPY Subtle variations in the temperature of the Big Bang radiation from place to place in the sky. These are related to the clumpiness of matter in the Universe at the epoch of last scattering, 380,000 years after the beginning of the Universe.
COSMIC BACKGROUND RADIATION, DIPOLE ANISOTROPY OF The variation in the temperature of the Big Bang radiation due to the motion of the Sun relative to the radiation. This causes the radiation to be marginally hotter in the direction of motion and marginally cooler in the opposite direction.
COSMIC BACKGROUND RADIATION, POWER SPECTRUM The manner in which the hotness of the hot spots in the cosmic background radiation changes with the size of the hot spots. On a graph, with the size of the hot spots decreasing from left to right, this looks like a series of mountains and valleys. The location and height of each mountain encodes the cosmological numbers that characterise our Universe.
COSMIC MICROWAVE BACKGROUND See Cosmic Background Radiation.
COSMIC RAYS High-speed atomic nuclei, mostly protons, from space. Low-energy ones come from the Sun; high-energy ones probably come from supernovae. The origin of ultra-high-energy cosmic rays, particles millions of times more energetic than anything we can currently produce on Earth, is one of the great unsolved puzzles of astronomy.
COSMIC REIONISATION The splitting apart of each hydrogen atom into its constituent proton and electron, a process which, according to the Wilkinson Microwave Anisotropy Probe (WMAP), began to happen about 400 million years after the Big Bang. The most likely cause was intense ultraviolet light pumped into space by the first stars to have formed, which are suspected to have been very massive and very hot.
COSMIC REPULSION The force which is causing the expansion of the Universe to speed up and which is attributed to the repulsive gravity of the invisible dark energy which fills all of space.
COSMIC RIPPLES See Cosmic Background Radiation, Anisotropy.
COSMOLOGICAL CONSTANT A force of repulsion exerted by empty space. It was originally inserted by Einstein into his equations of the Universe in order to counter gravity and so make the Universe unchanging in time, or static. He later called it his greatest blunder. However, it has been reborn of late as a possible explanation of the fact that something is speeding up the expansion of the Universe.
COSMOLOGICAL PRINCIPLE The idea that the Universe looks the same wherever you are – that is, it is the same in all places (homogeneous) and in all directions (isotropic). This enables Einstein’s equations of gravity as applied to the Universe to be simplified so that they yield the Big Bang solutions.
COSMOLOGICAL PRINCIPLE, PERFECT The idea that the Universe looks the same wherever you are and at all times. This enables Einstein’s equations of gravity as applied to the Universe to be simplified so that they yield the steady-state solutions.
COSMOLOGY The ultimate science. The science whose subject matter is the origin, evolution and fate of the entire Universe.
COSMOS Another word for Universe.
CYANOGEN A molecule consisting of a carbon atom joined to a nitrogen atom (CN). Interstellar cyanogen molecules spin like tiny dumb-bells, and they spin faster than expected because of the buffeting they receive from the photons of the cosmic background radiation.
DARK ENERGY Mysterious ‘stuff ’ with repulsive gravity. Discovered unexpectedly in 1998, it is invisible, fills all of space and appears to be pushing apart the galaxies and so be speeding up the expansion of the Universe. It accounts for 73 per cent of the mass-energy of the Universe, compared with 4 per cent for ordinary – atomic – matter. Nobody has much of a clue what it is.
DARK MATTER Matter which gives out no discernible light and whose existence is inferred from the gravitational pull it exerts on visible matter such as stars and galaxies. The Universe’s dark matter outweighs its normal matter by a factor of about six. It may consist of hitherto unknown subatomic particles.
DARK MATTER, COLD Invisible dark matter made of subatomic particles moving much slower than the speed of light. It can therefore be tamed by gravity and tends to form clumps.
DARK MATTER, HOT Invisible dark matter made of subatomic particles moving at speeds close to that of light. It cannot be tamed by gravity and tends to be smeared out uniformly throughout the Universe.
DECELERATION PARAMETER The number in the Big Bang models which encapsulates the braking effect of gravity on the expansion of the Universe.
DENSITY The mass of an object divided by its volume. Air has a low density, and iron has a high density.
EINSTEIN’S THEORY OF GRAVITY See Relativity, General Theory of.
ELECTRIC CHARGE A property of microscopic particles which comes in two types – positive and negative. Electrons, for instance, carry a negative charge and protons a positive charge. Particles with the same charge repel each other, while particles with opposite charge attract.
ELECTRIC CURRENT A river of charged particles, usually electrons, which can flow through a conductor.
ELECTROMAGNETIC WAVE A wave that consists of an electric field which periodically grows and dies alternating with a magnetic field which periodically dies and grows. An electromagnetic wave is generated by a vibrating electric charge and travels through space at the speed of light.
ELECTRON Negatively charged subatomic particle typically found orbiting the nucleus of an atom. As far as anyone can tell, it is a truly elementary particle, incapable of being subdivided.
ELEMENT A substance which cannot be reduced any further by chemical means. All atoms of a given element possess the same number of protons in their nucleus. For instance, all atoms of hydrogen have one proton, all atoms of chlorine 17, and so on.
ELEMENT, HEAVY Any element heavier than helium and lithium forged in the internal furnaces of stars since the Big Bang.
ELEMENT, LIGHT Any element such as hydrogen, helium and lithium, forged in the fireball of the Big Bang between about one and ten minutes after the beginning of the Universe.
ENERGY A quantity which is almost impossible to define. Energy can never be created or destroyed, only converted from one form to another. Among the many familiar forms are heat energy, energy of motion, electrical energy, sound energy, and so on.
ENERGY, CONSERVATION OF The principle that energy can never be created or destroyed, only converted from one form to another.
EPOCH OF LAST SCATTERING The period about 380,000 years after the beginning of the Universe when the fireball of the Big Bang had cooled sufficiently for electrons and nuclei to combine to form the first atoms. Since free electrons are very good at redirecting, or ‘scattering’, photons, before this time light could not travel in a straight line and the Universe was opaque. Once the electrons were mopped up by atoms, it was possible for photons to travel unhindered in straight lines and the Universe became transparent. Today, we pick up photons from this epoch, greatly cooled by the expansion of the Universe in the past 13.7 billion years, as the cosmic background radiation.
EVENT HORIZON The one-way ‘membrane’ that surrounds a black hole. Anything that falls through – whether matter or light – can never get out again.
EXPANDING UNIVERSE See Universe, Expanding.
FIREBALL RADIATION See Cosmic Background Radiation.
FREQUENCY How fast a wave oscillates up and down. Frequency is measured in hertz (Hz), where 1 Hz is one oscillation per second.
FREQUENCY BAND A range of frequencies.
FUNDAMENTAL FORCE One of the four basic forces which are believed to underlie all phenomena. The four forces are the gravitational force, electromagnetic force, strong force and weak force. The strong suspicion among physicists is that these forces are actually merely facets of a single superforce. In fact, experiments have already shown the electromagnetic and weak forces to be different sides of the same coin.
FUNDAMENTAL PARTICLE One of the basic building blocks of all matter. Currently, physicists believe there are six different quarks and six different leptons, making a total of 12 truly fundamental particles. The hope is that the quarks will turn out to be merely different faces of the leptons.
GALAXY One of the building blocks of the Universe. Galaxies are great islands of stars. Our own island, the Milky Way, is spiral in shape and contains about 200 billion stars.
GALAXY CLUSTER A group of galaxies bound together under their mutual gravity. Such clusters may contain anywhere from a few tens of galaxies, such as our own Local Group, to hundreds or even thousands of galaxies.
GALAXY SUPERCLUSTER A cluster of galaxy clusters bound together under their mutual gravity.
GAMMA RAY The highest energy form of light, generally produced when an atomic nucleus rearranges itself.
GAS A collection of atoms that fly about through space like a swarm of tiny bees.
GENERAL THEORY OF RELATIVITY Einstein’s theory of gravity which shows gravity to be nothing more than the warpage of space–time. The theory incorporates several ideas that were not incorporated in Newton’s theory of gravity. One was that nothing, not even gravity, can travel faster than light. Another was that all forms of energy have mass and so are sources of gravity. Among other things, the theory predicted black holes, the expanding Universe and that gravity would bend the path of light.
GRAVITATIONAL FORCE The weakest of the four fundamental forces of nature. Gravity is approximately described by Newton’s universal law of gravity but more accurately by Einstein’s theory of gravity – the general theory of relativity. General relativity breaks down at the singularity in the heart of a black hole and the singularity at the birth of the Universe. Physicists are currently looking for a better description of gravity. The theory, already dubbed quantum gravity, will explain gravity in terms of the exchange of particles called gravitons.
GRAVITY See Gravitational Force.
HEISENBERG UNCERTAINTY PRINCIPLE A principle of quantum theory stating that there are pairs of quantities, such as a particle’s location and speed, that cannot simultaneously be known with absolute precision. The uncertainty principle puts a limit on how well the product of such a pair of quantities can be known. In practice this means that if the speed of a particle is known precisely, it is impossible to have any idea where the particle is. Conversely, if the location is known with certainty, the particle’s speed is unknown. By limiting what we can know, the Heisenberg uncertainty principle imposes a ‘fuzziness’ on nature. If we look too closely, everything blurs like a newspaper picture dissolving into dots.
HELIUM Second-lightest element in nature and the only one to have been discovered on the Sun before it was discovered on the Earth. Helium is the second most common element in the Universe after hydrogen, accounting for about 10 per cent of all atoms. Most helium was forged in the Big Bang.
HELIUM-3 A light form, or isotope, of helium, containing only one neutron and two protons instead of the two neutrons and two protons of the common form, helium-4.
HELIUM, LIQUID The liquid with the lowest boiling point. Below 4.2 degrees above absolute zero, helium condenses into a liquid. Below 2.17 degrees, it becomes a ‘superfluid’ with the ability to run uphill and squeeze through impossibly small holes.
HORIZON, LIGHT See Light Horizon, Cosmic.
HORIZON PROBLEM The problem that far-flung parts of the Universe which could never have been in contact with each other, even in the Big Bang, have almost identical properties, such as density and temperature. Technically, they were always beyond each other’s horizon. The theory of inflation provides a way for such regions to have been in contact in the Big Bang, and so can potentially solve the horizon problem.
HUBBLE CONSTANT The number in the Big Bang models which encapsulates the current expansion rate of the Universe.
HUBBLE’S LAW The fact that the recession velocity of galaxies is proportional to their distance, so a galaxy that is twice as far away as another is fleeing twice as fast, three times the distance three times as fast, and so on.
HYDROGEN The lightest element in nature. A hydrogen atom consists of a single proton orbited by a single electron. Close to 90 per cent of all atoms in the Universe are hydrogen atoms.
INFLATION, ETERNAL A generic property of inflation. Although the inflationary, or false, vacuum continually decays into bubbles of normal vacuum – creating Big Bang universes – the false vacuum grows in volume at a faster rate than it is lost. Consequently, inflation, once begun, is unstoppable.
INFLATION, THEORY OF The idea that in the first split second of creation the Universe underwent a fantastically fast expansion. In a sense inflation preceded the conventional Big Bang explosion. If the Big Bang is likened to the explosion of a grenade, inflation was like the explosion of an H-bomb. Inflation can solve some problems with the Big Bang theory, such as the horizon problem.
INFRARED Type of invisible light which is given out by warm bodies.
INTERSTELLAR MEDIUM The tenuous gas and dust floating between the stars. In the vicinity of the Sun this gas comprises about one hydrogen atom in every three cubic centimetres, making it a vacuum far better than anything achievable on the Earth.
INTERSTELLAR SPACE The space between the stars.
ION An atom or molecule which has generally been stripped of one or more of its orbiting electrons and so has a net positive electrical charge.
ISOTOPE One possible form of an element. Isotopes are distinguishable by their differing masses. For instance, chlorine comes in two stable isotopes, with a mass of 35 and 37. The mass difference is due to a differing number of neutrons in their nuclei. For instance, chlorine-35 contains 18 neutrons, and chlorine-37 contains 20 neutrons. (Both contain the same number of protons – 17 – since this determines the identity of an element.)
LAGRANGE-2 POINT One of the five locations in the Sun–Earth system where the gravitational and centrifugal forces on a body balance so that, in principle, it can stay becalmed for ever. L2 is 1.5 million kilometres beyond the Earth on the extension of the line joining the Sun to the Earth.
LIGHT A wave of alternating electricity and magnetism, also known as an electromagnetic wave.
LIGHT, SPEED OF The cosmic speed limit – 300,000 kilometres per second.
LIGHT HORIZON, COSMIC The Universe has a horizon much like the horizon that surrounds a ship at sea. The reason for the Universe’s horizon is that light has a finite speed and the Universe has been in existence for only a finite time. This means that we see only objects whose light has had time to reach us since the Big Bang. The observable Universe is therefore like a bubble centred on the Earth, with the horizon being the surface of the bubble. Every day the Universe gets older (by one day), so every day the horizon expands outwards and new things become visible, just like ships coming over the horizon at sea.
LIGHT YEAR Convenient unit for expressing the distances in the Universe. It is simply the distance light travels in one year, which turns out to be 9.46 trillion kilometres.
LUMINOSITY The total amount of light pumped into space each second by a celestial body such as a star.
MAGNETIC FIELD The field of force which surrounds a magnet or magnetic material.
MAP See WMAP.
MASS A measure of the amount of matter in a body. Mass is the most concentrated form of energy. A single gram contains the same amount of energy as 100 tonnes of dynamite.
MASS-ENERGY The energy a body possesses by virtue of its mass. This is given by the most famous equation in all of physics – E = mc2, where E is energy, m is mass and c is the speed of light.
MATTER The most concentrated form of energy.
MATTER, ATOMIC A minority constituent of the Universe. Although it composes you and me and the stars and the planets, it accounts for a mere 4 per cent of the mass-energy of the Universe, the rest being dark matter and dark energy.
MATTER-DOMINATED ERA The era in which the energy density of matter exceeds the energy density of light. This inevitably happens since the expansion of the Universe dilutes the energy density of light faster than it does matter. We live in the matter-dominated era.
MICROWAVE A type of electromagnetic wave with a wavelength in the range of centimetres to tens of centimetres.
MICROWAVE HORN Funnel-shaped antenna for collecting and concentrating microwaves from the sky.
MILKY WAY Our Galaxy.
MOLECULE Collection of atoms glued together by electromagnetic forces. One atom, carbon, can link with itself and other atoms to make a huge number of molecules. For this reason, chemists divide molecules into ‘organic’ – those based on carbon – and ‘inorganic’ – the rest.
MOLECULE, INTERSTELLAR One of the more than 100 kinds of molecule found floating in space. They include ethyl alcohol and the simple amino acid glycine. Each emits characteristic light which can be picked up by telescopes.
MULTIVERSE Hypothetical enlargement of the cosmos in which our Universe turns out to be one among an enormous number of separate and distinct universes. Most universes are dead and uninteresting. Only in a tiny subset do the laws of physics promote the emergence of stars and planets and life.
NASA The National Aeronautics and Space Agency, the US space agency.
NEBULA A cloud of tenuous gas in space. If young, hot stars are embedded in the gas, they will cause it to glow brightly. If there are no such stars, it may still reveal itself as a black splotch which blots out the light of more distant stars.
NEUTRINO Neutral subatomic particle with a very small mass that travels very close to the speed of light. Neutrinos hardly ever interact with matter. However, when created in huge numbers they can blow a star apart, as in a supernova.
NEUTRON One of the two main building blocks of the atomic nucleus at the centre of atoms. Neutrons have essentially the same mass as protons but carry no electrical charge. They are unstable outside of a nucleus and disintegrate in about ten minutes.
NEUTRON STAR A star that has shrunk under its own gravity to such an extent that most of its material has been compressed into neutrons. Typically, such a star is only 20 to 30 kilometres across. A sugar cube of neutron star stuff would weigh as much as the entire human race.
NEWTON’S UNIVERSAL LAW OF GRAVITY The idea that all bodies pull on each other across space with a force which depends on the product of their individual masses and the inverse square of their distance apart. In other words, if the distance between the bodies is doubled, the force becomes four times weaker; if it is tripled, nine times weaker; and so on. Newton’s theory of gravity is perfectly good for everyday applications but turns out to be an approximation. Einstein provided an improvement with the general theory of relativity.
NOVA Close binary star system in which one star is a super-dense white dwarf. Matter sucked from the other star spirals down to the white dwarf and, when enough has accumulated, this can trigger an orgy of heat-generating nuclear reactions and an explosion.
NUCLEAR FUSION The welding together of two light nuclei to make a heavier nucleus, a process which results in the liberation of nuclear binding energy. The most important fusion process for human beings is the gluing together of hydrogen nuclei to make helium in the core of the Sun, since its byproduct is sunlight.
NUCLEAR REACTION Any process which converts one type of atomic nucleus into another type of atomic nucleus.
NUCLEAR STATISTICAL EQUILIBRIUM Furious state in which the nuclear reactions to make a particular nucleus are as fast as the nuclear reactions to un-make it. Despite the chaos, the abundance of each nucleus remains constant, dependent only on the temperature and the properties of the nucleus.
NUCLEOSYNTHESIS The gradual build-up of heavy elements from light elements, either in the Big Bang – Big Bang nucleosynthesis – or inside stars – stellar nucleosynthesis.
NUCLEOSYNTHESIS, BIG BANG The build-up of light elements between about one and ten minutes after the beginning of the Universe. This is responsible for making most of the Universe’s helium.
NUCLEOSYNTHESIS, STELLAR The build-up of heavy elements such as carbon and iron inside the furnaces of stars.
NUCLEUS, RADIOACTIVE A nucleus which is unstable and seething with surplus energy. It sheds this, or decays, by emitting a radioactive particle.
OLBERS’ PARADOX The paradox, publicised by the nineteenth- century German astronomer Heinrich Olbers, that the sky at night is dark whereas, if the Universe is infinite in extent, it should be as bright as the surface of a typical star. In fact, this was first pointed out by the German astronomer Johannes Kepler in 1610.
PHOTON Particle of light.
PLASMA An electrically charged gas of ions and electrons.
POSITRON Antiparticle of the electron.
PROTON One of the two main building blocks of the nucleus. Protons carry a positive electrical charge, equal and opposite to that of electrons.
PULSAR A rapidly rotating neutron star which sweeps an intense beam of radio waves around the sky much like a lighthouse.
QUANTUM The smallest chunk into which something can be divided. Photons, for instance, are quanta of the electromagnetic field.
QUANTUM COSMOLOGY Quantum theory applied to the whole Universe. Since the Universe was once smaller than an atom, such a theory is necessary to try and understand the birth of the Universe in the Big Bang.
QUANTUM FLUCTUATION The appearance of energy out of the vacuum as permitted by the Heisenberg uncertainty principle. Usually, the energy is in the form of virtual particles.
QUANTUM THEORY Essentially, the theory of the microscopic world of atoms and their constituents. Those who favour the Many Worlds interpretation believe it also describes the large-scale world.
QUANTUM VACUUM See Vacuum, Quantum.
QUASAR A galaxy which derives most of its energy from matter heated to millions of degrees as it swirls into a central giant black hole. Quasars can generate as much light as a hundred normal galaxies from a volume smaller than the Solar System, making them the most powerful objects in the Universe.
RADIATION-DOMINATED ERA The era in the early Universe when the energy density of radiation – light – exceeded that of matter. Coincidentally, the era ended just before the epoch of last scattering.
RADIO WAVE A type of electromagnetic wave with a long wavelength, longer than about a centimetre.
RADIOACTIVE DECAY The disintegration of unstable, heavy atoms into lighter, more stable atoms. The process is accompanied by the emission of either alpha particles, beta particles or gamma rays.
RADIOACTIVITY Property of atoms which undergo radioactive decay.
RED DWARF Star less massive than the Sun which glows like a dying ember. About 70 per cent of the stars in the solar neighbourhood are red dwarfs, exploding the myth that the Sun is a typical star. In fact, it is more massive, and therefore more luminous, than the average star.
RED SHIFT The loss of energy of light caused by the expansion of the Universe. The effect can be visualised by drawing a wiggly light wave on a balloon and inflating it. The wave becomes stretched out. Since red light has a longer wavelength than blue light, astronomers talk of the cosmological red shift. (A red shift can also be caused by the Doppler effect when a body emitting light is flying away from us. And it can be caused when light loses energy climbing out of the strong gravity of a compact star such a white dwarf. This is known as a gravitational red shift.)
RELATIVITY, GENERAL THEORY OF Einstein’s generalisation of his special theory of relativity. General relativity relates what one person sees when they look at another person accelerating relative to them. Because acceleration and gravity are indistinguishable – the principle of equivalence – general relativity is also a theory of gravity.
RELATIVITY, SPECIAL THEORY OF Einstein’s theory which relates what one person sees when they look at another person moving at constant speed relative to them. It reveals, among other things, that the moving person appears to shrink in the direction of their motion, while their time slows down, effects which become ever more marked as they approach the speed of light.
SOLAR SYSTEM The Sun and its family of planets, moons, comets and other assorted rubble.
SPACE–TIME In the general theory of relativity, space and time are seen to be essentially the same thing. They are therefore treated as a single entity – space–time. It is the warpage of space–time that turns out to be gravity.
SPECTRAL LINE Atoms and molecules absorb and give out light at characteristic wavelengths. If they swallow more light than they emit, the result is a dark line in the spectrum of a celestial object. Conversely, if they emit more than they swallow, the result is a bright line.
SPECTRUM The separation of light into its constituent ‘rainbow’ colours.
STANDARD CANDLE A class of celestial objects believed to have a standard intrinsic luminosity. If astronomers see one that is fainter than another, they can deduce that it is farther away. Cepheid variable stars are believed to be standard candles, as are Type Ia supernovae.
STAR A giant ball of gas which replenishes the heat it loses to space by means of nuclear energy generated in its core.
STEADY-STATE THEORY The idea that although the Universe is expanding, its galaxies receding from each other, new matter fountains into existence out of the vacuum, congealing into galaxies that fill the gaps so that the Universe looks the same at all times. It has no beginning and no end. The discovery of the Big Bang radiation killed off the theory, at least in its simplest version.
STRING THEORY See Superstring Theory.
STRONG NUCLEAR FORCE The powerful short-range force which holds protons and neutrons together in an atomic nucleus.
SUBATOMIC PARTICLE A particle smaller than an atom, such as an electron or a neutron.
SUN The nearest star.
SUPERCLUSTER See Galaxy Supercluster.
SUPERFORCE Hypothetical force from which each of the four fundamental forces of nature ‘froze out’ as the Universe cooled in the aftermath of the Big Bang.
SUPERNOVA A cataclysmic explosion of a star. A supernova may, for a short time, outshine an entire galaxy of 100 billion ordinary stars. It is thought to leave behind a highly compressed neutron star.
SUPERNOVA, TYPE IA The explosion of a white dwarf star triggered by the dumping of matter on it from a companion star. Since all such supernovae arise from essentially the same type of star, they are considered to be of equivalent intrinsic brightness. This makes them useful as cosmological distance indicators since we can be sure that a Type Ia that is fainter than another is also farther away.
SUPERSTRING THEORY Theory which postulates that the fundamental ingredients of the Universe are tiny strings of matter. The strings vibrate in a space–time of ten dimensions. The great pay-off of this idea is that it may be able to unite, or ‘unify’, quantum theory and the general theory of relativity.
TEMPERATURE The degree of hotness of a body. Related to the energy of motion of the particles that compose it.
THERMAL RADIATION See Black Body Radiation.
THERMODYNAMICS, SECOND LAW OF The decree that entropy can never decrease. This is equivalent to saying that heat can never flow from a cold body to a hot body.
ULTRAVIOLET Type of invisible light which is given out by very hot bodies. Responsible for sunburn.
UNCERTAINTY PRINCIPLE See Heisenberg Uncertainty Principle.
UNIFICATION The idea that at extremely high energy the four fundamental forces of nature were one, united in a single theoretical framework.
UNIVERSE All there is. This is a flexible term which was once used for what we now call the Solar System. Later, it was used for what we call the Milky Way. Now it is used for the sum total of all the galaxies, of which there appear to be about 100 billion within the observable Universe.
UNIVERSE, AGE The current best estimate, obtained by the Wilkinson Microwave Anisotropy Probe, is 13.7 billion years.
UNIVERSE, BOUNCING See Universe, Oscillating.
UNIVERSE, COLLIDING The possibility, within the inflationary scenario, that another Big Bang-containing bubble formed in the inflationary vacuum beside our own, and that it collided with us. Such a collision would leave an imprint on the cosmic background radiation. There is disputed evidence that such an imprint is present.
UNIVERSE, EXPANDING The fleeing of the galaxies from each other in the aftermath of the Big Bang.
UNIVERSE, OBSERVABLE All we can see out to the Universe’s horizon. The Universe has a horizon because it was born only 13.7 billion years ago. This means that we can see only the stars and galaxies whose light has taken less than 13.7 billion years to reach us. All other objects are currently beyond the horizon of the observable Universe.
UNIVERSE, OSCILLATING The idea that our Big Bang was triggered after a previous contracting phase in which the Universe shrunk down to a big crunch and bounced into a new expansion phase. Big bangs and big crunches therefore alternate throughout eternity. The idea has several fatal flaws and so is no longer considered a viable possibility.
UNIVERSE, STATIC A universe in which the galaxies are suspended essentially motionless in space so that it appears the same at all times.
URANIUM The heaviest naturally occurring element.
VACUUM, INFLATIONARY See Vacuum, False.
VACUUM, FALSE An unusual state of the vacuum with sufficient negative pressure that it generates repulsive gravity. Such a state is believed to have existed at the beginning of the Universe and to have driven the super-fast expansion of ‘inflation’.
VACUUM, QUANTUM The quantum picture of empty space. Far from empty, it seethes with ultra-short-lived microscopic particles which are permitted by the Heisenberg uncertainty principle to blink into existence and blink out again.
WATER VAPOUR Molecule which absorbs strongly in the far infrared and so makes the atmosphere pretty much opaque to the cosmic background radiation. On the other hand, it is the principal ‘greenhouse’ gas, responsible for keeping the Earth from freezing solid.
WAVELENGTH The distance for a wave to go through a complete oscillation cycle.
WEAK NUCLEAR FORCE The second force experienced by protons and neutrons in an atomic nucleus, the other being the strong nuclear force. The weak nuclear force can convert a neutron into a proton and so is involved in beta decay.
WHITE DWARF A star which has run out of fuel and which gravity has compressed until it is about the size of the Earth. A white dwarf is supported against further shrinkage by electron degeneracy pressure. A sugar cube of white dwarf material weighs about as much as a family car.
X-RAYS A high-energy form of light.