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# Glossary

axions
  Hypothetical particles which may be a component of dark matter.

baryon
  A term used to describe strongly interacting particles that have half odd-integer spin i.e. spin 1/2, 3/2 etc. Each baryon is a combination of three quarks. The lowest mass baryons are the proton and the neutron. A type of hadron.

baryonic matter
  Matter composed of baryons.

big bang
  The name given to the current standard cosmological model (see cosmology), in which the Universe began in a very hot, dense state and has been expanding and cooling ever since. The big bang model successfully explains the observed recession of distant galaxies (see Hubble law), the properties of the cosmic microwave background radiation, and the abundances of the light elements in the Universe. As a result of the cosmological principle, the expansion of the Universe can be described in terms of the evolution of a single quantity, the scale factor, which describes the changing physical distance between typical points in the Universe. At the present time the scale factor is increasing with time, giving rise to the observed expansion. The behaviour of the scale factor depends on the amount of matter (and energy) in the Universe, and the ultimate fate of the Universe is determined by whether the gravitational effects of matter are strong enough to overcome the expansion.

black-body spectrum
  The spectrum emitted by a black body.

Chandrasekhar limit
  The theoretical upper limit to the mass of a white dwarf, about , also called the Chandrasekhar mass.

CMB
  See cosmic microwave background radiation.

cosmic microwave background
  Low-energy black-body radiation seen with almost identical properties in all directions. Its black-body spectrum corresponds to a temperature around 2.7 K. In the big bang cosmological model, the background radiation is a relic of the early stages of the Universe, when the temperatures and densities were much higher, the whole Universe was optically thick, and matter and photons were in thermal equilibrium. Study of the microwave background radiation therefore gives important information about the structure of the young Universe.

cosmological constant
  A non-zero value of in the Einstein field equations. Its value is given by .

cosmological redshift
  Redshift arising from the expansion of the Universe. It is related to the scale factor by where tem and tob are the times at which the radiation was emitted and observed respectively.

cosmology
  The branch of science that is concerned with the study of the Universe as a whole, including its structure and history.

critical density
  With reference to cosmological models, the quantity defined by .

curvature
  See curvature parameter.

curvature parameter
  The quantity k that has a value of 0 for a spatially flat geometry, or can take values <0 or >0 for spatial geometries that have positive or negative curvature.

dark energy
  A proposed form of energy that affects the Universe on the largest scales. Its primary effect is to drive the accelerating expansion of the Universe.

dark matter
  Matter that does not produce radiation, and so can only be detected (at present) by its gravitational effects on other matter. Evidence from the rotation curve of spiral galaxies, the velocity dispersion of clusters of galaxies, gravitational lensing and observations of the cosmic microwave background radiation suggest that there is more dark matter than luminous matter in the Universe by a large factor, and that most of it is non-baryonic (that is, not made primarily of protons and neutrons as normal matter is). The nature of the non-baryonic dark matter is one of the major puzzles of modern astrophysics.

density
  Also known as mass density. The ratio of mass to volume for a homogeneous system. It is possible to define the density at a given point in any system by taking a small volume element around that point and evaluating the ratio of mass to volume for that volume element. Contrast with number density.

density parameter
  One of the fractional densities defined by: matter density, ; radiation density, ; dark energy density, where is the critical density. The effective energy density associated with curvature is .

equation of state parameter
  The parameter in the equation of state relating the pressure and density of a perfect fluid: . For non-interacting matter (referred to as ‘dust’) , for radiation , and for dark energy .

flat
  A space that is not curved, i.e. the curvature parameter .

flatness problem
  The recognition of the fact that the initial density of the Universe is apparently very finely tuned, such that the density is currently extremely close to the critical value required for a flat universe.

Friedmann equation
  The equation relating the scale factor a and its derivatives to the density parameters and the curvature parameter k: , where is the density (of matter, radiation and dark energy).

general relativity
  See theory of general relativity.

horizon problem
  The recognition of the fact that objects that are further apart than a certain distance could not have been in causal contact in the past. This poses a problem in understanding how parts of the cosmic microwave background radiation that are more than a few degrees apart ever managed to look so similar.

hot big bang
  See big bang.

Hubble diagram
  A plot of apparent magnitude against redshift.

Hubble law
  See Hubble-Lemaître law.

Hubble constant
  The value of the Hubble parameter at the current time.

Hubble parameter
  In terms of the scale factor a(t), the Hubble parameter at any given time can be written . The value of the Hubble parameter at the current time is called the Hubble constant.

Hubble-Lemaître law
  The linear relationship, discovered independently by Edwin Hubble and Georges Lemaître, between the distance of a galaxy and its cosmological redshift, expressed as an apparent recession speed. The law states v = H0D where v is the apparent recession speed in km s-1 and D is the distance in megaparsecs. H0 is the Hubble constant.

inflation
  A hypothetical epoch in the very early development of the Universe, when the Universe is supposed to have undergone a brief period of very rapid expansion.

lookback time
  The time elapsed between the emission of a photon by a distant astronomical source and its detection by us. For objects at cosmological distances, the lookback time can be a significant fraction of the age of the Universe.

matter-energy density
  The equivalent mass per unit volume of a source of matter and/or energy, since matter and energy are related by , where is the speed of light.

monopole problem
  Grand unified theories predict about one magnetic monopole per horizon size at the time the Universe was at the critical GUT temperature. Therefore the present-day Universe should have many magnetic monopoles and they would dominate the energy density of the Universe. The fact that we see none is known as the monopole problem.

negatively curved
  The situation when the curvature parameter k has a value < 0.

non-baryonic matter
  Matter not composed of baryons. See dark matter.

Planck time
  A fundamental timescale, given by

positively curved
  The situation when the curvature parameter k has a value > 0.

quintessence
  The name given to a postulated fifth fundamental force (in addition to the established four fundamental forces of nature: electromagnetic interaction, gravitational interaction, strong nuclear interaction and weak nuclear interaction). It is one form of dark energy with a time-varying equation of state parameter.

recombination
  The process in which a free electron combines with an ion, releasing energy in the form of a photon; the reverse of ionisation.

redshift
  A shift of a spectral line to redder (longer) wavelengths. There are three important types of redshift: (1) Doppler shift - due to the motion of the emitting object away from the observer. (2) Gravitational redshift - due to strong gravity at the surface of the emitting object. (3) Cosmological redshift - due to the expansion of the Universe (see the Hubble constant, big bang). Numerically, the redshift z is defined by where is the original emitted wavelength (the wavelength that the emission line would have in the laboratory) and is the difference between the observed and emitted wavelengths. If the redshift is a small Doppler shift, then z = v/c, where v is the speed of recession. For a cosmological redshift, the same formula can be used together with Hubble’s law to infer distances, but only if z < 1; otherwise more complex results, depending on the geometry of the Universe, must be applied.

rest wavelength
  The wavelength of a spectral line in a frame of reference in which the material emitting the line is itself at rest.

scale factor
  A numerical quantity used to describe the expansion of the Universe in big bang cosmology; the scale factor gives the relationship between the true distance between two objects and their separation in co-moving coordinates (which do not change with time). If we adopt an Earth-centred co-moving coordinate system, in which r is the radial distance (we are at r = 0) then, in the simple case of a spatially flat Universe, the distance d to an object is given by d = a(t)r. Because the scale factor describes the expansion of the Universe, the ratio of the scale factors when a photon was emitted and when it is observed give us the redshift: (1 + z) = a(observed)/a(emitted). The usefulness of the scale factor is that (1) the equations describing the expansion of the Universe can easily be written in terms of a and its time derivatives and (2) observable cosmological quantities such as the Hubble constant can be described in the same way. If the scale factor increases with time, the Universe is expanding; if the scale factor decreases with time, the Universe is contracting.

theory of general relativity
  The theory published by Albert Einstein in 1915 that generalises the ideas of his earlier special theory of relativity by extending them to non-inertial frames of reference. An important principle of the theory asserts that an accelerating frame of reference is locally equivalent to one that is located in a gravitational field. Consequently, the general theory of relativity is also a theory of gravitation, and as such supersedes Newton’s theory of gravity. (The predictions of Newton’s theory approximate those of general relativity in situations where the gravitational fields are weak.) According to general relativity, gravity manifests itself in the geometric structure (curvature) of spacetime. Mass and other sources of gravity determine that curvature, and moving bodies respond to that curvature, giving rise to the appearance of a gravitational force.

time of last scattering
  The epoch, about 380,000 years after the big bang, at which electrons combined with protons (see recombination) to form neutral atoms. After this time, radiation ceased to scatter from matter in the universe. The cosmic microwave background radiation is a relic of this epoch.

type Ia supernovae
  Type Ia supernovae are thought to occur when accretion onto the surface of a white dwarf in a binary system takes its mass over the Chandrasekhar limit. When this happens, the star can no longer be supported by degeneracy pressure, and so it starts to collapse, igniting runaway thermonuclear reactions between the heavy nuclei in the star. The resulting explosion destroys the star and gives rise to the observed supernova. Type Ia supernovae show no hydrogen lines in their spectra, consistent with an origin in a massive white dwarf; absorption features from heavy elements such as silicon are common. Because all Type Ia supernovae have a mass around the Chandrasekhar mass, they have a very similar peak absolute magnitude and so can be used as standard candles.

weakly interacting massive particles
  A possible class of dark matter particles, which includes the neutralino. Often abbreviated to WIMPs.

white dwarf
  A stellar-mass compact object, with a mass below the Chandrasekhar mass (1.4 solar masses) supported against gravitational collapse by the degeneracy pressure of electrons. White dwarfs are the final products of the evolution of low-mass stars, after thermonuclear reactions have ceased and the outer regions of the star have been lost in stellar winds or as a planetary nebula. If left isolated they will gradually cool and contract until they become invisible, but since the luminosity is low, the cooling time is long. White dwarfs in binary systems may meet a different fate: when their companion star moves off the main sequence, mass transfer may take the white dwarf over the Chandrasekhar mass. In this case, the white dwarf can end its life as a Type Ia supernova.

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