**Absolute zero:** The lowest possible temperature, at which substances contain no heat energy.

**Acceleration:** The rate at which the speed of an object is changing.

**Anthropic principle:** We see the universe the way it is because if it were different we would not be here to observe it.

**Antiparticle:** Each type of matter particle has a corresponding antiparticle. When a particle collides with its antiparticle, they annihilate, leaving only energy.

**Atom:** The basic unit of ordinary matter, made up of a tiny nucleus (consisting of protons and neutrons) surrounded by orbiting electrons.

**Big bang:** The singularity at the beginning of the universe.

**Big crunch:** The singularity at the end of the universe.

**Black hole:** A region of space-time from which nothing, not even light, can escape, because gravity is so strong.

**Black hole area theorem:** Ignoring the effects of quantum mechanics, the surface area of a black hole can only grow with time.

**Casimir effect:** The attractive pressure between two flat, parallel metal plates placed very near to each other in a vacuum. The pressure is due to a reduction in the usual number of virtual particles in the space between the plates.

**Chandrasekhar limit:** The maximum possible mass of a stable cold star, above which it must collapse into a black hole.

**Conservation of energy:** The law of science that states that energy (or its equivalent in mass) can neither be created nor destroyed.

**Coordinates:** Numbers that specify the position of a point in space and time.

**Cosmological constant:** A mathematical device used by Einstein to give space-time an inbuilt tendency to expand.

**Cosmology:** The study of the universe as a whole.

**Dark energy:** An unknown form of energy which is accelerating the expansion of the universe and is believed to permeate all of space.

**Dark matter:** Matter in galaxies, clusters, and possibly between clusters, that can not be observed directly but can be detected by its gravitational effect. As much as 90 percent of the mass of the universe may be in the form of dark matter.

**Duality:** A correspondence between apparently different theories that lead to the same physical results.

**Edwin Hubble:** American astronomer (1889–1953) who played a pivotal role in establishing that distant galaxies recede from Earth, and therefore that space is expanding.

**Einstein-Rosen bridge:** A thin tube of space-time linking two black holes. *Also see* **Wormhole**.

**Electric charge:** A property of a particle by which it may repel (or attract) other particles that have a charge of similar (or opposite) sign.

**Electromagnetic force:** The force that arises between particles with electric charge; the second strongest of the four fundamental forces.

**Electron:** A particle with negative electric charge that orbits the nucleus of an atom.

**Electroweak unification energy:** The energy (around 100 GeV) above which the distinction between the electromagnetic force and the weak force disappears.

**Elementary particle:** A particle that, it is believed, cannot be subdivided.

**Eternal inflation:** A possible variant of inflation, in which much of space continues expanding rapidly while relatively small individual portions drop their expansion rate to become universes such as our own. If correct, this implies our entire cosmos is embedded within a much larger inflating multiverse.

**Event:** A point in space-time, specified by its time and place.

**Event horizon:** The boundary of a black hole.

**Exclusion principle:** The idea that two identical spin-½ particles cannot have (within the limits set by the uncertainty principle) both the same position and the same velocity.

**Field:** Something that exists throughout space and time, as opposed to a particle that exists at only one point at a time.

**Frequency:** For a wave, the number of complete cycles per second.

**Friedmann models:** Descriptions of the evolution and fate of the universe, based on Einstein’s general relativity. For sufficiently high densities of matter, these models predict that the universe will eventually collapse in on itself in a “big crunch.”

**Gamma rays:** Electromagnetic rays of very short wavelength, produced in radioactive decay or by collisions of elementary particles.

**General relativity:** Einstein’s theory based on the idea that the laws of science should be the same for all observers, no matter how they are moving. It explains the force of gravity in terms of the curvature of a four-dimensional space-time.

**Geodesic:** The shortest (or longest) path between two points.

**Grand unification energy:** The energy above which, it is believed, the electromagnetic force, weak force, and strong force become indistinguishable from each other.

**Grand unified theory (GUT):** A theory which unifies the electromagnetic, strong, and weak forces.

**Gravitational waves:** Ripples in the curvature of space, predicted in 1916 by Albert Einstein and first directly detected in 2016 by the LIGO consortium.

**Imaginary time:** Time measured using imaginary numbers.

**Inflation:** The theory that space expanded rapidly in the early universe, giving rise to an almost but not perfectly uniform universe today, which agrees with observations.

**Information paradox:** The information content of objects falling into a black hole at first sight seems to be lost, but this contradicts the laws of quantum mechanics. It is now believed that the information is returned in an altered form in the Hawking radiation escaping the black hole.

**Large Hadron Collider:** The world’s largest and most powerful particle collider. Built by the European Organization for Nuclear Research (CERN), it is situated deep in the earth near Geneva.

**Light cone:** A surface in space-time that marks out the possible directions for light rays passing through a given event.

**Light-second (light-year):** The distance traveled by light in one second (year).

**Magnetic field:** The field responsible for magnetic forces, now incorporated along with the electric field, into the electromagnetic field.

**Mass:** The quantity of matter in a body; its inertia, or resistance to acceleration.

**Microwave background radiation:** The radiation from the glowing of the hot early universe, now so greatly red-shifted that it appears not as light but as microwaves (radio waves with a wavelength of a few centimeters). Also see COBE.

**Naked singularity:** A space-time singularity not surrounded by a black hole.

**Neutrino:** An extremely light (possibly massless) particle that is affected only by the weak force and gravity.

**Neutron:** An uncharged particle, very similar to the proton, which accounts for roughly half the particles in an atomic nucleus.

**Neutron star:** A cold star, supported by the exclusion principle repulsion between neutrons.

**No boundary condition/proposal:** The idea that the universe is finite but has no boundary (in imaginary time).

**Nuclear fusion:** The process by which two nuclei collide and coalesce to form a single, heavier nucleus.

**Nucleus:** The central part of an atom, consisting only of protons and neutrons, held together by the strong force.

**Particle accelerator:** A machine that, using electromagnets, can accelerate moving charged particles, giving them more energy.

**Phase:** For a wave, the position in its cycle at a specified time: a measure of whether it is at a crest, a trough, or somewhere in between.

**Photon:** A quantum of light.

**Planck’s quantum principle:** The idea that light (or any other classical waves) can be emitted or absorbed only in discrete quanta, whose energy is proportional to their wavelength.

**Planck satellite:** Operated from 2009–13 and named after the German physicist Max Planck, this produced our most precise all-sky map of the cosmic microwave background.

**Positron:** The (positively charged) antiparticle of the electron.

**Primordial black hole:** A black hole created in the very early universe.

**Proportional:** “X is proportional to Y” means that when Y is multiplied by any number, so is X. “X is inversely proportional to Y” means that when Y is multiplied by any number, X is divided by that number.

**Proton:** A positively charged particle, very similar to the neutron, that accounts for roughly half the particles in the nucleus of most atoms.

**Pulsar:** A rotating neutron star that emits regular pulses of radio waves.

**Quantum:** The indivisible unit in which waves may be emitted or absorbed.

**Quantum chromodynamics (QCD):** The theory that describes the interactions of quarks and gluons.

**Quantum mechanics:** The theory developed from Planck’s quantum principle and Heisenberg’s uncertainty principle.

**Quark:** A (charged) elementary particle that feels the strong force. Protons and neutrons are each composed of three quarks.

**Radar:** A system using pulsed radio waves to detect the position of objects by measuring the time it takes a single pulse to reach the object and be reflected back.

**Radioactivity:** The spontaneous breakdown of one type of atomic nucleus into another.

**Red shift:** The reddening of light from a star that is moving away from us, due to the Doppler effect.

**Singularity:** A point in space-time at which the space-time curvature becomes infinite.

**Singularity theorem:** A theorem that shows that a singularity must exist under certain circumstances—in particular, that the universe must have started with a singularity.

**Space-time:** The four-dimensional space whose points are events.

**Spatial dimension:** Any of the three dimensions that are spacelike—that is, any except the time dimension.

**Special relativity:** Einstein’s theory based on the idea that the laws of science should be the same for all observers, no matter how they are moving, in the absence of gravitational phenomena.

**Spectrum:** The component frequencies that make up a wave. The visible part of the sun’s spectrum can be seen in a rainbow.

**Spin:** An internal property of elementary particles, related to, but not identical to, the everyday concept of spin.

**Stationary state:** One that is not changing with time: a sphere spinning at a constant rate is stationary because it looks identical at any given instant.

**String theory:** A theory of physics in which particles are described as waves on strings. Strings have length but no other dimension.

**Strong force:** The strongest of the four fundamental forces, with the shortest range of all. It holds the quarks together within protons and neutrons, and holds the protons and neutrons together to form atoms.

**Uncertainty principle:** The principle, formulated by Heisenberg, that one can never be exactly sure of both the position and the velocity of a particle; the more accurately one knows the one, the less accurately one can know the other.

**Virtual particle:** In quantum mechanics, a particle that can never be directly detected, but whose existence does have measurable effects.

**Wave/particle duality:** The concept in quantum mechanics that there is no distinction between waves and particles; particles may sometimes behave like waves, and waves like particles.

**Wavelength:** For a wave, the distance between two adjacent troughs or two adjacent crests.

**Weak force:** The second weakest of the four fundamental forces, with a very short range. It affects all matter particles, but not force-carrying particles.

**Weight:** The force exerted on a body by a gravitational field. It is proportional to, but not the same as, its mass.

**White dwarf:** A stable cold star, supported by the exclusion principle repulsion between electrons.

**Wormhole:** A thin tube of space-time connecting distant regions of the universe. Wormholes might also link to parallel or baby universes and could provide the possibility of time travel.