More than 400 keywords from relativity and related topics, from “absolute zero” to “XMM Newton” – please use the menu on the left to choose a letter.

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In particle physics: collective expression for particles that fundamentally consist of three quarks. The most important examples are protons and neutrons.

In cosmology, the word denotes ordinary matter in contrast with more exotic forms of matter that the greatest part of dark matter is thought to consist of. This usage comes about because cosmologists are mainly interested in what percentage of mass in the universe is represented by ordinary matter. The mass of ordinary matter is mostly contained in atomic nuclei, and these nuclei are built of baryons (protons and neutrons), the total baryonic mass is, to good approximation, the same as the total mass of ordinary matter in any region of our universe.

big bang

The big bang models are the foundation of modern cosmology. Firmly grounded in Einstein’s theory of general relativity, they describe a universe that began in a very hot initial state and has expanded (and cooled down) ever since. They make precise predictions about nucleosynthesis in the early universe, the existence and properties of the cosmic background radiation, and the distribution of distant galaxies in the cosmos, which have been confirmed by astronomical observation.

The word “big bang” has two different meanings. In a strict sense, the big bang is a spacetime singularity, a state of infinite density – the initial state the big bang models predict for our universe. In a more general sense, the term is applied to the earliest cosmic eras, in which the universe was exceedingly hot and dense. Further information about these two meanings and why it is important to distinguish between them can be found in the spotlight text A tale of two big bangs.

The basic features of the big bang models are reviewed in the chapter Cosmology of Elementary Einstein.

Selected aspects of cosmology are described in the category Cosmology of our Spotlights on relativity.

Big Bang Nucleosynthesis

Synonym: primordial nucleosynthesis. The formation of complicated nuclei from constituents such as protons and neutrons in the early universe. According to the big bang models, the early universe was filled with a particle soup of protons and neutrons. At cosmic times between a few seconds and a few minutes, nuclear reactions produced the first light elements, mainly nuclei of deuterium, different varieties of helium and lithium. Heavier nuclei up to those of iron formed and continue to form in the course of fusion processes inside stars; nuclei that are even more massive form in the course of supernova explosions. These explosions also serve to disseminate the complex nuclei formed inside stars (stellar nucleosynthesis) in space.

A brief account of Big Bang Nucleosynthesis can be found in the spotlight text Big Bang Nucleosynthesis, while Equilibrium and change provides more information about the physical processes involved and Elements of the past describes how the predictions of Big Bang Nucleosynthesis can be tested against astronomical observation.

binary star

A system consisting of two stars in orbit around each other. From a relativistic point of view, there are binaries that are of special interest, namely those in which at least one of the partners is a neutron star or a black hole. Potentially, such systems are effective sources of gravitational waves.

Synonyms: binary

binding energy

The energy needed to break up a composite object into its component parts.

More about binding energy, the mass defect it is responsible for and its role in nuclear fission and nuclear fusion can be found in our spotlight topic Is the whole the sum of its parts?

Birkhoff’s theorem

A theorem of general relativity, discovered by J. T. Jebsen (1921) and independently discovered, and named after, George D. Birkhoff (1923): Any spherically symmetric spacetime has the same properties as some region from one of a simple family of spacetimes found by Karl Schwarzschild in 1916. More concretely: The spherically symmetric spacetime around any spherically symmetric matter configuration (approximately, in good approximation, the earth) has the same properties as spacetime around a Schwarzschild black hole of the appropriate mass.

BKL conjecture

Conjecture by the Soviet physicists Vladimir Belinskii, Isaak Khalatnikov and Evgeny Lifshitz that, near a singularity, the contribution of matter to gravity becomes negligible compared with the effects of gravity as a source of further gravity (compare the spotlight text The gravity of gravity), and that near a singularity, the variation of the gravitational field from one location to the next can be neglected – what is much more important is the way gravity changes over time. Further information about this can be found in the spotlight text Of singularities and breadmaking.

black hole

Region in space where a sufficient amount of mass is concentrated so that it forms a gravitational prison – a region into which matter or light can enter from the outside, but from which nothing that has once entered can ever leave.

Basic information on this key phenomenon of Einstein’s general theory of relativity can be found in the chapter Black holes & Co. of Elementary Einstein.

Selected aspects of the physics of black holes and neutron stars are described in the category Black holes & Co. of our Spotlights on relativity.

In Einstein’s theory, black holes are truly black, due to the fact that no radiation or light can ever escape them. Once quantum theory is taken into account, this assumption no longer holds – on the contrary, it seems as if black holes should emit so-called Hawking radiation. However, for astrophysical black holes (that typically have more or even much more than one solar mass), that radiation would be undetectable, even if we could transport today’s most sensitive sensors into the immediate vicinity of the black hole.

Synonyms: black holes

black hole perturbation theory

Black hole perturbation theory is an approach to calculating theoretical gravitational waveforms, so-called templates. It assumes a binary system consisting of a black hole and a compact small object which is treated as point particle perturbing spactetime. The system thus qualifies as an extreme mass-ratio inspiral (EMRI).

black hole uniqueness theorems

Theorems proved in the context of general relativity that answer the question: How many different kinds of black holes are there? If that question is restricted to stationary black holes (namely black holes that have settled down and do not change over time), then the answer is: Surprisingly few. Once you know a stationary black hole’s mass, angular momentum (roughly speaking, how fast it rotates) and electric charge, its properties are determined completely.

More information can be found in the spotlight text How many different kinds of black hole are there?


Idealized body that is capable of absorbing and emitting all forms of electromagnetic radiation, regardless of their wavelength. The thermal radiation emitted by such a body is governed by a set of especially simple laws, like Planck’s radiation law, the Stefan-Boltzmann law and Wien’s law.


The frequency of a simple light wave is directly related to its colour (cf. spectrum). For the highest possible frequencies of visible light, the colour is blue-violet. If the frequency of a light wave is shifted towards higher frequencies (for instance by the doppler shift), that corresponds to a colour shift towards the blue-violet end of the spectrum, and is hence called a blueshift.

In the light of that, the term ‘blueshift’ has acquired a more general meaning. It is used to refer to any shift towards higher frequencies, including types of electromagnetic radiation where the frequencies do not correspond to any visible colour, and more generally to other types of waves (for example, gravitational waves).

See also redshift.


See Brookhaven National Laboratory (BNL)


Collective expression for quantum particles with an integer spin, mainly spin 0, spin 1, spin 2.

Among the elementary particles, bosons are carrier particles in charge of transmitting the influences of forces. Photons, for instance, the carrier particles of the electromagnetic force, are bosons. In contrast, the elementary particles that make up matter, such as electrons or quarks, are so-called fermions.


In string theory: An object that is the analogue of a two-dimensional membrane embedded in three-dimensional space – an entity with a certain number of dimensions (one-brane, two-brane, three-brane…) embedded in the higher-dimensional space of string theory. A one-brane or 1-brane has one spatial dimension, a two-brane has two, and so on.

brane world

The notion that our world with its three dimensions of space is a three-brane embedded in a higher-dimensional space, akin to a two-dimensional surface embedded in ordinary three-dimensional space.

More information about this can be found in the spotlight text The embedded universe.

Brookhaven National Laboratory (BNL)

National laboratory in the United States, located on Long Island, New York. The BNL operates the Relativististic Heavy Ion Collider (RHIC), a particle accelerator that enables researchers to recreate the state of matter fractions of a second after the big bang. Also, BNL operates accelerators used to produce synchrotron radiation.

> Brookhaven National Laboratory Website

> National Synchrotron Light Source II at the Brookhaven National Laboratory

brown dwarf

A failed star: A gas ball in space that has between one and ten percent solar mass – not enough for the temperature and pressure in its core to reach the values required for the nuclear fusion to start that would transform the gas ball into a shining star.