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More than 350 keywords from relativity and related topics, from "absolute zero" to "X-rays" - please use the menu on the left to choose a letter.

 dark energy

Comparing astronomical observations with the predictions of the big bang models (which link the properties of matter and the speed of the universe's expansion), it turns out that more than 70 percent of the density of the universe is supplied by what is called dark energy, a type of energy that is associated with empty space itself. Ordinary matter or energy are conserved when the universe expands: If I have 10 hydrogen atoms in a certain region of space, and if that region now expands to twice its initial volume, it will still contain no more than the initial 10 hydrogen atoms, now spread over the larger volume. On the other hand, the amount of dark energy in that region of space doubles in the process, just as the volume, the "amount of space" is twice as large than it was in the beginning.

There's another crucial difference between ordinary energy and dark energy. The gravitational influence of ordinary masses and ordinary energy is attractive - it is aimed at pulling all the contents of the universe closer together. Dark energy, on the other hand, acts to accelerate the universe's expansion. In that way, it is equivalent to a certain type of what is called a cosmological constant.

As yet, nobody knows how (and if) dark energy fits somewhere into our current picture of the fundamental constitutents of the universe, for instance: into the standard model of particle physics or some extension of that model. This makes dark energy one of the greatest mysteries of modern physics.

 dark matter

Astronomical observations of galaxies and galaxy clusters as well as comparison of observations with the predictions of the big bang models show that only about 15 percent of matter in the universe announces its presence by giving off light or other kinds of electromagnetic radiation. The other 85 percent of the mass are supplied by dark matter, and cosmologists have convincing evidence that most of that dark matter is not in the form of the usual atomic constitutents protons and neutrons. The exact properties of these unusual matter particles are not yet known, other than that they do not interact with ordinary matter and radiation other than by gravitational influence.

 deflection of light, relativistic

One of the basic predictions of general relativity is that light is influenced by gravity. For instance, light passing a massive body is slightly deflected. This is the basis for what is called gravitational lensing.

General information about this topic can be found in the spotlight text The gravitational deflection of light, while its connection with one of the fundamental principles of general relativity is examined in The equivalence principle and the deflection of light.

 degeneracy pressure

For a gas made up of electrons, quantum effects become important. Roughly, it is strictly forbidden for two electrons to be present at the same location (this is called the Pauli exclusion principle), and if anyone attempts to concentrate electrons in a small volume of space, they will start to flit back and forth madly (cf. Heisenberg's uncertainty principle). Just like with regular gases, this flitting back and forth leads to pressure, in this case to what is called degeneracy pressure.

For instance, it is this kind of electron degeneracy pressure that stabilizes a white dwarf, preventing further collapse.

Degeneracy pressure is not only possible for electrons, but for a whole class of quantum particles, namely for all fermions (for example for neutrons or protons.


In a stricter sense synonymous with "mass density": The average density of matter in a region of space is the total mass of all matter contained in that region, divided by the region's volume.

More generally, density can refer to other physical quanti ties as well. The energy density, for instance, is the total sum of energy localized in a region divided by that region's volume.


Variant of the chemical element hydrogen where the atomic nucleus consists not of a single proton, but of a proton and a neutron.

In the context of general relativity, it is of interest as one of the species of light atomic nuclei that formed in the early universe during Big Bang Nucleosynthesis.

 Deutsches Elektronensynchrotron (DESY)

Literally "German electron synchrotron" (a synchrotron being a type of particle accelerator). German research centre for particle physics, founded in 1959 and located in Hamburg in Northern Germany. Site of the particle accelerator HERA, among others.

DESY website


If we put a drop of ink into clear water, then even without stirring, the ink will slowly spread throughout the water. Behind this is the motion of the ink molecules associated with the temperature of the system. The motion of each molecule is purely random, but eventually the sum of many random steps will carry a sizeable number of the molecules far away from the location where we have put the ink drop in. Processes like this where random motions lead to a spreading-out of an ensemble of molecules or other entities are called diffusion.


The number of independent directions within a set of points, alternatively: the number of coordinates needed to give each point a unique name. This is rather abstract - time for some examples:

A line is one-dimensional. There's only one direction to go on the line (the opposite direction isn't counted extra): Back-forth. A single number is sufficient to define a point of the line. For instance, on a motorway, given the statement "the accident happened 4 kilometres from the beginning of the I95 (or M1, or whatever)" is sufficient information for the rescue workers to know exactly where to go.

Surfaces are two-dimensional, as there are two independent directions: back-forth and left-right, say. On the earth's surface, the two coordinate numbers geographical longitude and latitude uniquely define each location.

The space that surrounds us is three-dimensional. There are three independent directions, say back-forth, left-right and up-down. In order to define a location in space, one needs to specify three numbers - for instance, two to specify where a house is located on the earth's surface (latitude/longitude, see above) and one floor number (or, more precisely, the height above the earth's surface).

Adding time to the three space coordinates (a must for defining an appointment - where and when?), the result is four-dimensional space-time. In order to define an event in space-time, one needs to give four numbers: three of them determine where in space the event happens, the fourth gives the time where it happens.

According to some of the models that have been studied as candidates for a theory of guantum gravity, our world should have even more space dimensions than the usual three. Some information about these extra dimensions can be found in the spotlight topics  "Extra dimensions, and how to hide them", "Hunting for extra dimensions" and Extra dimensions and simplicity.

 Dirac equation

Equation regulating the behaviour of relativistic quantum particles that have a spin of 1/2, for instance electrons. It was first formulated by Paul Dirac in 1928 and led directly to his successful prediction of the existence of the first species of anti-particle, the positron.

 Doppler effect

Effect named after the Austrian scientist Christian Doppler concerning the emission of waves by moving sources. Consider a wave-source (for instance, a device that sends out sound-waves or light-waves). Also consider two observers A and B, with observer A moving relative to the source, while observer B is at rest relative to it. When a source that moves relative to an observer emits a wave, the frequency measured by this observer is different from what a measuring instrument would record that is at rest relative to the source: If source and observer approach each other, the observer measures a higher frequency, if they move away from each other, a lower frequency.

In everyday life, the Doppler effect is readily apparent when we listen to sound waves from moving sources. If a police car or fire truck with blaring horns first races towards us, then passes us and races away, the characteristic horn sounds change dramatically in pitch the moment the car passes us. This is because, at first, the car is moving towards us, and there is a Doppler shift towards higher pitch compared with a listener in the car. From the moment the car passes us, it becomes a source that moves away from us, with all sounds being shifted to lower pitch.

In the context of relativity, the most important Doppler effect is that for light waves. In this context, a shift towards higher frequencies is called blueshift, one to lower frequencies redshift.

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