Physics Glossary 

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Dark matter: Matter that is detected only by its gravitational pull on visible matter. At least 90%, and possibly 99% of the matter in the universe is dark. The composition is unknown; it might consist of very low mass stars or supermassive black holes, but big-bang nucleosynthesis calculations limit the amount of such baryonic matter to a small fraction of the critical mass density.

If the mass density is critical, as predicted by the simplest versions of inflation, then the bulk of the dark matter must be a gas of weakly interacting non-baryonic particles, sometimes called WIMPS (Weakly Interacting Massive Particles). Various extensions of the standard model of particle physics suggest specific candidates for the WIMPs.

Decay of the false vacuum: Since the universe is not inflating today, the inflationary theory depends on the fact that the false vacuum which drives inflation is metastable, so it will eventually decay. The theory requires that this decay happen after more than 100 doubling times of the exponential expansion, but it still happens when the universe is much less than one second old. 

If the inflation fields are in a local valley of the energy density diagram, then the decay happens via the formation of bubbles, like the boiling of water, and the randomness of the bubble formation destroys the homogeneity of the universe and causes the theory to fail. This failure is called the graceful exit problem of the original inflationary theory. If however, the inflation field is at the top of a local plateau, as assumed in the new inflationary universe theory, then inflation continues as the fields roll gently towards a minimum energy value, and a single bubble becomes large enough to encompass the entire observed universe.

Density perturbations: Non-uniformities in the density of matter in the universe. Such non-uniformities are gravitationally unstable, which means that they are amplified by gravity. Any region with a mass density slightly higher than average produces a stronger-than-average gravitational field, which pulls in more matter and further increases the mass density. Thus, very mild non-uniformities in the early universe can serve as seeds for the formation of galaxies.

Deuterium: An isotope of hydrogen in which each nucleus contains one proton and one neutron, instead of only one proton as in normal hydrogen. Water containing deuterium instead of ordinary hydrogen, called heavy water, is sometimes used as a moderator in nuclear reactors. See also tritium.

Down: A flavour of quark. See flavour.

Doppler shift: The shift in the received frequency and wavelength of a sound wave or electromagnetic wave that occurs when either the source or the observer are in motion. Approach causes a shift toward shorter wavelengths and higher frequencies, called a blue-shift. Recession has the opposite effect, called a redshift.

Dyne: The force necessary to cause a mass of one gram to accelerate at one centimetre per second per second.

Electro-magnetic wave: A pattern of electric and magnetic fields that moves through space. Depending on the wavelength, an electromagnetic wave can be a radio wave, a microwave, an infrared wave, a wave of visible light, an ultraviolet wave, a beam of X rays, or a beam of gamma rays. See photon.

Electro-magnetism: The phenomena associated with electrical and magnetic forces. Electrical and magnetic forces are intimately related, since a changing electric field produces a magnetic field, and vice versa. Electromagnetic waves are an example of electromagnetism.

Electron-volt: The energy released when a single electron passes through an ideal one-volt battery.

Electro-weak interactions: The unified description of the weak interactions and electro-magnetism, developed between 1967 and 1970 by Sheldon Glashow, Steven Weinberg, and Abdus Salam.

Erg: Twice the amount of energy necessary to accelerate a one gram mass to a speed of one centimetre per second.

Exponential expansion: An expansion described by a fixed doubling time. The size doubles after one doubling time, quadruples after two doubling times, octuples after three doubling times, etc.

Extended inflationary universe theory: A version of the inflationary universe theory proposed in 1989 by Paul Steinhardt and Daile La. Its key new feature was the suggestion that a new field interacts directly with the gravitational field, causing the strength of gravity to change with time. This causes the expansion of the universe to slow clown, allowing the bubbles forming at the end of inflation to catch up with the expansion and smoothly fill the universe.

False vacuum: The false vacuum is a peculiar form of matter which is predicted to exist by many (probably most) current theories of elementary particles. Particle physicists describe the false vacuum, like all forms of matter, in terms of fields. Originally the phrase "false vacuum" was used to describe a region in which one or more fields have a set of values that do not minimize the energy density, but which are in a local valley of the energy density diagram. Classically such a state would be absolutely stable, since there would be no energy available for the fields to jump over the hills that surround the valley. By the rules of quantum theory, however, the fields can tunnel through the hill of the energy density diagram and settle eventually in the true vacuum, the state of lowest energy density. The false vacuum is nonetheless metastable, and in fact it can endure for a period which is long by early universe standards. The false vacuum has a negative pressure which creates a repulsive gravitational field, capable of driving the universe into a period of exponential expansion called inflation. Similar effects occur if the fields are at the top of a broad plateau, instead of in a valley, so in this book I have stretched the phrase "false vacuum" to include this case as well.

False vacuum bubble: A bubble which has false vacuum on the inside, and true vacuum on the outside. In principle, the creation of such a bubble offers the possibility of creating a new universe in a hypothetical laboratory.

Field: A quantity that can be defined at each point in space, such as an electric, magnetic, or gravitational field. When the rules of quantum theory are applied to the electromagnetic field, one concludes that the energy is concentrated into indivisible lumps, called photons, which behave essentially as particles. In present-day particle theory the photon is used as the prototype for all particles, so a field is introduced for the electron, for each type of quark, etc. Ali particles are viewed as the quantized lumps of energy in a field.

First order: A phase transition is called first order if it occurs in a manner similar to the way water boils. Bubbles of the new phase (steam) form in the midst of the old phase (water), so that temporarily the two distinct phases (steam and water) coexist. In a second order phase transition, by contrast, one phase evolves into the other as the temperature changes, so the two phases never coexist.

Flat universe: A homogeneous, isotropic universe is called flat if it is just on the borderline between being spatially closed and spatially open, so the geometry is precisely Euclidean. If Einstein's cosmological constant is zero then a flat universe will go on expanding forever, but the velocity of recession between any two objects would approach zero at large times.

Flatness problem: A problem of the traditional big bang theory (without inflation) related to the precision required for the initial value of omega, the ratio of the actual mass density to the critical mass density. If the description is started at one second after the big bang, for example, omega must have been equal to one to an accuracy of fifteen decimal places, or else the resulting universe would not resemble our own. Yet the traditional big bang theory offers no explanation for this special value, which must be incorporated as an arbitrary postulate about the initial conditions. See also horizon problem.

Flavour: The known quarks exist in six different types, or Flavours: up, clown, charmed, strange, top, and bottom. The up and clown quarks belong to the first generation, the charmed and strange quarks belong to the second, and the top and bottom quarks belong to the third. The up, charmed, and top quarks each have an electrical charge 2/3 that of a proton, while the clown, strange, and bottom quarks have a charge -1/3 that of a proton.

Force carriers: Particles that act as the transmitters of forces. The best known example is the photon, which transmits electromagnetic forces. The gluons are the transmitters of the strong interactions, and the w+' w-, and zº particles are the transmitters of the weak interactions.

Fractal: A geometric figure in which a pattern is repeated ad infinitum on smaller and smaller scales. A classic example is Von Koch's snowflake, for which the construction begins with an equilateral triangle. Trisect each side, and replace the middle section by two sides of a smaller equilateral triangle, bulging outward. The snowflake is obtained by repeating this process for each side of the resulting figure, then for each side of the subsequent figure, and continuing forever.

Free parameter: A number which is needed to define a theory well enough so that predictions can be made, but which must be determined by experiment or observation

Frequency: The number of peaks (often called crests) of a propagating wave that cross a given point in a unit of time. For example, if 1000 peaks cross a given point in one second, one says that the frequency is 1000 cycles per second, or 1000 hertz.

Gamma ray: An electromagnetic wave with a wavelength in the range of 10-l 3 to 10-10 meter, corresponding to photons with energy in the range of 104 to 107 electron volts.

Gauge theories: See Yang-Mills theories. (Technically speaking, electromagnetism is also an example of a gauge theory.

Generation: The fundamental particles of the standard model of particle physics can be grouped into quarks, leptons, force carriers, and the Higgs particle. The quarks and leptons have been found to exist in three generations, each of which is a copy of the first, except that the masses increase with each generation. (The first generation includes all the building blocks of ordinary matter-electrons and the up and clown quarks that make up protons and neutrons. The particles of the other generations were discovered by studying high energy collisions of first generation particles.)

Gluons: The force -carrying particles associated with the strong interactions, the forces which bind quarks inside of protons and neutrons. For more details, see Yang-Milis theories

Graceful exit problem: A problem of the original formulation of the inflationary theory, in which the formation of bubbles at the end of inflation destroys the homogeneity of the universe. See decay of the false vacuum, and percolation.

Grand unified theories: A speculative class of theories of particle interactions, first developed in 1974, which attempt to describe electromagnetism, the weak interactions, and the strong interactions in a fully unified theory. Of the known forces, only gravitation is omitted.

Gravitation: The mutual attraction between any two masses, as was first described accurately by Newton. Gravity appears strong because it has infinite range and it is always attractive (except for a false vacuum), but on a subatomic leve] gravity is the weakest of the known interactions; the gravitational force between a proton and an electron is 2 x 1039 times weaker than the electrical attraction.

Gravitational energy: Same as gravitational potential energy.

Gravitational field: Instead of describing gravity as an action-at-a-distance force, modern physicists describe it in terms of a gravitational field. At each point of space, the field is defined as the force that would be experienced by a standard mass, if the mass were positioned at that point. While Newton's law of gravity can be expressed equally well in terms of an action-at-a-distance or a field, Einstein's theory of general relativity, which is now the accepted description of gravity, can be formulated only in terms of fields.

Gravitational field lines: A method of depicting a gravitational field by drawing lines. The direction of the field is indicated by the direction of the lines, and the strength of the field is indicated by how closely the lines are spaced.

Gravitational potential energy: When we lift a weight from the floor to a table top, we clearly put energy into it. The energy is not lost, however, because we can retrieve it by allowing the weight to fall back to the floor. While the weight is on the table, we say that the energy is stored as gravitational potential energy. The energy is stored in the gravitational field

Gravitationally unstable: See density perturbations.

Great Wall: A sheet of galaxies which stretches more than 500 million light-years across the sky.