Antimatter – The known elementary particles have antiparticle partners with opposite charges; e.g. the proton has an antiproton partner with a negative charge. When a proton encounters an antiproton or an electron encounters an antielectron, both particles are annihilated in a burst of energetic radiation. Antimatter is quite rare in the universe.
Arrow of time – We experience that time only moves forward, not backward. The arrow of time from past to future is related to the Second Law of Thermodynamics and the requirement for ever-increasing entropy.
Axions – Non-baryonic dark matter candidates that have small masses and are force carriers. They change to and from photons in the presence of strong magnetic fields.
Baryons – Elementary particles which are composed of 3 quarks. The proton and neutron are the best known of the baryons and the building blocks for atomic nuclei. They are the prime contributors to “ordinary matter”.
Black Hole – An object whose mass is confined within such a small distance that even light cannot escape. For a non-rotating black hole, if the radius is less than 2*G*M/c^2 where G is the gravitational constant, M the mass of the object and c the speed of light, then a black hole will result. Primordial black holes are one candidate for dark matter, with interest in the intermediate black holes of around 30 solar masses.
Brane – A membrane in two or more dimensions; these are objects from string theory. Open strings must terminate on a membrane. The vibrational modes of branes, like those of strings, give rise to fundamental particles of different types.
Cosmic microwave background (CMB) – Isotropic blackbody relic radiation from the Big Bang emitted at a time of 380,000 years after the origin; it is now redshifted by 1100 times into the millimeter waveband and has characteristic temperature T = 2.73 K (Kelvins, degrees above absolute zero). The radiation was originally emitted at T ~ 3000 K.
Dark matter – Matter which is detected by measuring its gravitational effects and through large-scale cosmological measurements, but which does not radiate detectably in either radio, infrared, visible, UV, X-ray or gamma ray wavelengths.
Dark energy – Non-matter contribution to the overall mass-energy of space throughout the universe. It may be due to the quantum mechanical vacuum energy or due to a time varying field “quintessence”.
Dark gravity – There are two senses in which the term dark gravity is used: (1) Gravity as a much weaker force than the other fundamental forces. (2) Formulations of gravity that are different from 4-dimensional general relativity. Dark gravity refers to theories which postulate additional terms in the general relativity equations, or which invoke additional dimensions in which gravity, unlike other forces, operates.
General relativity – The highly successful theory of gravity, due to Einstein, that states that the presence of mass and/or energy causes space to curve, and the curvature of space determines the way in which masses move.
Hubble flow – The Hubble flow is the measure of the rate at which galaxies recede from one another. The Hubble constant relates the speed of recession as being proportional to the distance between galaxies. The Hubble constant (70 kilometers/sec/Megaparsec) is constant in space, but changes with time. It is the same everywhere at a given time in the case of a homogeneous and isotropic expanding universe.
Luminous matter – Matter that is detectable through optical, infrared or emission of photons at other wavelengths. It is measured to amount to about 4% to 5% of the total mass-energy of the universe. Also known as ordinary matter.
Redshift – The shift of light emitted from distant galaxies towards the red (lower frequency) end of the spectrum due to the galaxies’ movement away from us as the universe expands (see Hubble flow).
z – Redshift measure of the epoch and scale factor of the universe. z =0 is now. Larger z is earlier in the history of the universe. z ~ 1100 is the era of the cosmic microwave background (at this time the universe became transparent to radiation), z ~ 11 is the reionization era due to the earliest star formation, z ~ 6 corresponds to the earliest observable quasars (energetic galaxies), and z = 0.4 the epoch when dark matter and dark energy were equal in strength, after which acceleration began to dominate.