acceleration
Increase in speed or change of direction.
accelerating universe
One where distance increase faster with time, in contradiction to those where the gravitational attraction of ordinary matter slows down the expansion.
angular size
The size of an object as viewed from the earth. For example, the moon subtends (covers) an angle of 1/2 of a degree. If the distance to an object is known, the angular size can be used to determine its diameter.
annihilated/annihilati son
A process in which a particle meets its corresponding antiparticle and both disappear. The energy appears in some other form, perhaps as a different particle and its antiparticle (and their energy), perhaps as many mesons, perhaps as a single neutral boson. The produced particles may be any combination allowed by conservation of energy and momentum and of all the charge types.
antiparticle/antiquark/antielectron/...
For most particle types there is another particle type that has exactly the same mass but the opposite value of all other charges. This is called the antiparticle. For example, the antiparticle of an electron is a particle of positive electric charge called the positron.
antimatter
Matter made up of elementary particles whose masses are identical to their normal-matter counterparts but whose other properties, such as electric charge, are reversed. The positron is the antimatter counterpart of an electron, with a positive charge instead of a negative charge. When an antimatter particle collides with its normal-matter counterpart, both particles are annihilated and energy is released.
apparent luminosities
The brightness of an object as it appears from the earth. The apparent luminosity does not take into account the distance from the earth to the object; the sun appears as the brightest object in the sky but is really dimmer (has less intrinsic luminosity) than many stars.
atoms
The smallest unit of matter that possesses chemical properties. All atoms have the same basic structure: a nucleus containing positively charged protons with an equal number of negatively charged electrons orbiting around it. In addition to protons, most nuclei contain neutral neutrons whose mass is similar to that of protons. Each atom corresponds to a unique chemical element determined by the number of protons in its nucleus.
background radiation
Filling all space, e.g. the radiation from the early, hot universe.
baryon
A massive, strongly interacting elementary particle made up of three quarks. Protons and Neutrons are baryons.
baryonic matter
Ordinary matter, which is made up largely of baryons.
Big Bang
A broadly accepted theory for the origin and evolution of our universe. The theory says that the universe started expanding roughly 14 billion years ago from an extremely dense and incredibly hot initial state.
c, speed of light
The speed that light travels in a vacuum. It is about 3 x 108 m/s (186,000 miles/s). This is the fundamental maximum speed that any particle (including light) or information can travel.
closed universe
A universe where the geometry of space closes upon itself like the surface of a sphere. For a universe containing only ordinary matter this means that it must have more than the critical desnity,  > 1.
clusters of galaxies
Assembly of many galaxies bound together by gravity.
CMB
See cosmic microwave background.
cosmic
Having to do with the universe as a whole.
cosmic microwave background, CMB
Radiation filling the universe, remaining from the early, hot phase. (It is sometimes called the "primal glow.") Today this radiation has cooled to 2.73 K and is strongest in the microwave part of the spectrum.
cosmological constant, 
The energy density of the smooth vacuum. Eisntein introduced it as a component of the universe but later withdrew it.
cosmological principle
This principle states that since the universe looks to us the same in all directions, and we are note in a special position, then the distribution of matter across very large distances is the same everywhere in the universe.
cosmology
The study of the universe as a whole, of the contents, structure, and evolution of the universe from the beginning of time to the future.
coupled to radiation
Matter, such as charged particles, that is strongly affected by radiation.
critical density
The density of matter/energy that would just halt the expansion of the universe. The dividing line between a collapsing and an ever-expanding universe.
curvature of spacetime
According to Einstein's General Theory of Relativity, mass curves space and time.
dark energy
An energy causing the acceleration of the expansion of the universe, detectable through its gravitational effects.
dark matter
Matter not visible to us because it emits no radiation that we can observe, but it is detectable gravitationally. A small fraction of it is baryonic, e.g. dim stars, cool gas, etc.
deceleration
Decreasing the velocity (of expansion).
density fluctuations in matter
Regions of greater and lesser mass density, which can form into structure such as galaxies, clusters, etc.
deuterium
A special form of hydrogen (an isotope called "heavy hydrogen") that has a neutron as well as a proton in its nucleus.
electromagnetic radiation
Sometimes called radiation or light. The entire range of wavelengths of electromagnetic radiation includes radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays, and gamma rays.
electron
A negatively charge elementary particle that typically resides outside the nucleus of an atom but is bound to it by electromagnetic forces. An electron's mass is tiny: 1,836 electrons equals the mass of one proton.
elementary particle physics
The study of fundamental particles (such as quarks and leptons) and their interactions (forces).
energy density
The amount of energy per unit volume. The energy density affects, and is affected by, the expansion or contraction of the universe.
era
A time period dominated by a particular physical process.
evolution
Changing with time.
flat universe
A universe where the geometry of space does not curve but is flat like a tabletop. In a flat universe the total energy density equals the critical density.
flatness problem
Under normal gravity, any slight deviation from flatness should quickly get larger - for example a closed universe should recollapse or an open universe should expand so much that no two objects are near each other. The flatness problem asks what has caused the universe to be so flat and so well balanced between these cases.
fundamental interactions
In the Standard Model the fundamental interactions are the strong, electromagnetic, weak, and gravitational interactions (forces). Four interaction types are all that are needed to explain all observed physical phenomena. The theory proposes at least one more fundamental interaction that is responsible for fundamental particle masses.
fundamental particles
A particle with no internal substructure. In the Standard Model the quarks, leptons, photons, gluons, W+ and W- bosons, and the Z bosons are fundamental. All other known objects are made from these.
galaxy
A large grouping of stars (about 100 billion), bound by gravity. Our Sun is part of the Milky Way, a spiral galaxy.
General Theory of Relativity
Einstein's theory of gravitation replaces Newton's Law of Gravitation, which is approximately correct, but breaks down when gravitation is very strong. The key idea of General Relativity, called the Equivalence Principle, is that gravity pulling in one direction is equivalent to an acceleration in the opposite direction, and both are equivalent to the curvature of space.
geometries
Models of the curvature of the universe in which the laws of geometry are like those that would apply on flat, spherical, or saddle-shaped surfaces.
globular clusters
A collection of hundreds of thousands of old stars held together by gravity.
gluon
The carrier particle of the strong interactions. It holds quarks together and the atom's nucleus together.
gravitational attraction
The interaction of particles due to their mass/energy.
H0, Hubble Constant
A measure of the expansion rate and age of the universe. It appears to be between 60 and 80 kilometers per second per megaparsec. (A megaparsec is equal to 3.26 x 106 light-years.)
Hubble's Law
The observation that any two distant celestial objects (e.g., galaxies) move away from each other at a speed v that is proportional to the distance d between them, due to the homogenous expansion of space. v=H0 * d
inflation
A rapid, accelerating expansion occuring in the early universe in some cosmological models, which solves several problems of cosmology.
inhomogeneity
Something not evenly distributed in space; a clump or cluster.
intrinsic luminosity
A measure of the true brightness of an object. It is the amount of energy radiated into space every second by a celestial object, such as a star.
ion
Atomic matter that has unequal number of positive and negative charges, so its net charge is not zero.
isotropic
Same in all directions.
K
The Kelvin temperature scale. 0 Kelvin is -273 Celsius (-452 Fahrenheit) is absolute zero, the lowest possible temperature. K=C+273.
large scale structure
Galaxies, galaxy clusters, and galaxy superclusters.
last scattering surface
The farthest back in the history of the universe that photons could travel freely. Earlier, matter was ionized and interacted with the photons, preventing us from seeing light from longer ago.
light year
The distance that a particle of light (photon) will travel in a year - about 10 trillion kilometers (6 trillion miles). The nearest star to our sun is four light years away, and our galaxy is about 100,000 light years across.
local group
A small cluster of more than 30 galaxies "near" to our galaxy, including the Andromeda galaxy and the Magellanic Clouds.
matter-dominated Universe
When the density of matter is greater than that of any other component.
meson
A kind of matter made from an even number of quark-antiquark constituents. The basic structure of most mesons is one quark and one antiquark.
metastable energy density
A theorized transient energy, often preceding a phase transition.
muon
The second heaviest charged lepton (with electron and tau), with electric charge -1.
neutrino
A lepton with no electric charge. Neutrinos participate only in weak and gravitational interactions and are therefore very difficult to detect. There are three known types of neutrinos, all of which have very little mass.
neutron
A baryon with electric charge zero; it has the basic structure of two down quarks and one up quark (held together by gluons). The neutral component of an atomic nucleus is made from neutrons. Different isotopes of the same element are distinguished by having different numbers of neutrons in their nuclei.
nucleon
Proton or neutron, part of atomic nucleus.
nucleus/nuclei
A collection of neutrons and protons that forms the core of an atom (plural: nuclei).
opaque to radiation
A condition when radiation is absorbed or scattered many times. For the first 400,000 years the universe was so hot that the matter was ionized and interacted abundantly with radiation, making the universe opaque.
open universe, 
A universe where the geometry of space is curved like an infinite saddle-shaped surface. For universes with only matter, an open universe has less than critical density, < 1, and expands forever.
particle accelerators
A machine used to accelerate particles to high speeds, and thus, high energy compared to their rest-mass energy.
particle physics
The study of fundamental particles (such as quarks and leptons) and the fundamental interactions (four forces).
phase transition
Change from one state of energy to another.
photon
The force carrier particle of the electromagnetic interactions. A packet of electromagnetic energy, of light. A photon is regarded as a chargeless, massless particle having an indefinitely long lifetime.
Planck time
The period of time immediately after the creation of the universe (10-43 s) during which quantum gravity is the principal phenomenon governing the evolution of the universe.
plasma
A state of matter composed of charged particles, like ions and electrons, and possibly some neutral particles.
positron (  )
The antiparticle of the electron.
primordial nucleosynthesis
Building of the atomic elements in the early universe when nuclei fused with one another. Most of the helium in the universe was created by this process.
quantum fluctuations
Tiny changes in the energy of the vacuum.
quantum mechanics
The laws of physics that apply on very small scales, e.g. to elementary particles and atoms.
quark
A fundamental matter particle that has strong interactions. Quarks have electric charge of either +2/3 (up, charm, top) or -1/3 (down, strange, bottom) in units where the proton charge is 1. The basic building block of hadrons (protons, neutrons and mesons).
radiation
Electromagnetic energy, and particles moving close to the speed of light.
radiation gas
A mixture of photons and fast-moving charged particles.
radiation-dominated universe
When the density of radiation is greater than that of any other component.
redshift
A change in the frequency or wavelength of light. This can arise from motion of the source (Doppler shift), expansion of space, or strong gravitational fields.
relative brightness
How bright one object is compared to another. If they have the same intrinsic brightness then the relative brightness tells us the ratio of distances to the objects.
relics of history
Remnant products or evidence of early physical eras.
spacetime
The theory of relativity says that time cannot be treated absolutely separately from space, only in one observer's relative view. So space and time together describe a four dimensional universe.
spectral line
An emission (bright) or absorption (dark) of light at a specific frequency or wavelength.
standard candle
A celestial object whose intrinsic brightness is known or can be estimated by some physical principle. Therefore its observed brightness tells us its distance.
stochastic
Random.
strong force
The interaction responsible for binding quarks, antiquarks, and gluons to make hadrons. Residual strong interactions provide the nuclear binding force.
supercluster
A vast collection of galaxy clusters that may contain tens of thousands of galaxies spanning over a hundred million light-years of space. Galaxy superclusters are the largest structures in the universe.
supernova
An exploding star. It can briefly be as bright as an entire galaxy of stars.
synthesis of nuclei
Combining protons and neutrons to form nuclei.
Type Ia supernova
Supernova formed from the explosion of an old, compact star (white dwarf). Used as standard candles as they have uniform intrinsic brightness.
Universe
The totality of space and time, along with all the matter and energy in it.
vacuum energy
A theorized energy everywhere in space, arising from quantum mechanics.
variable stars
Stars which brighten and dim, some with precise regularity. For Cepheid variables, their period allows determination of their brightness, and so their distance.
visible mass
Mass that gives off electromagnetic radiation, so that it can be detected. It is ordinary (baryonic) matter.
visible universe
The part of the entire universe from which light would have had time to reach us in the age of the universe.
wavelength
The distance between two wave crests. It is characteristic of the type of wave. Radio waves can have lengths of several feet; the wavelengths of X-rays are roughly the size of atoms.
Lambda,
The cosmological constant, which measures the energy density of the vacuum.
Omega,
The ratio of the average total density of matter to the critical density required to make the universe flat and eventually stop its expansion.
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