Glossary: A to E

Aberration- Discontinuity or distortion in an image produced by a lens. In astronomy, this is the apparent change in direction of a source of light caused by an observer's motion.

Absorption Spectrum- The range of electromagnetic radiation transmitted through a substance with dark lines indicating the absorption of specific wavelengths. The dark lines indicate the absorption of photons at frequencies specific to that particular element or molecule.

Acceleration- The rate of change of velocity per unit of time. Acceleration implies that either an object's speed (rate of change of position) or direction of motion is changing.

Accelerating Universe- A cosmological term referring to the discovery that Hubble Expansion is speeding up over time rather than slowing under the influence of gravity

Angular Size- A measure of size based on the angle that the object subtends, or covers. For example, the moon subtends an angle of 1/2 of a degree. If the distance to an object is known, the angular size (the angle of 1/2 a degree) can be used to determine its diameter.

Angular Velocity- How quickly something travels around a circle. This velocity describes what angle an object travels across in a given amount time. It is usually measured in radians per second.

Anisotropy- Physical property values that vary when measured from different directions. In cosmology, anisotropies refer to the difference in temperature of the CMB with direction.

Annihilated/Annihilation- A process in which a particle meets its corresponding antiparticle and both disappear. This apparent loss in energy appears in some other form, perhaps as a different particle and its antiparticle (and their energy), perhaps as many mesons, perhaps as a single neutral boson. The produced particles may be any combination allowed by conservation of energy and momentum and of all the charge types.

Anomaly- A deviation from the ordinary; something peculiar or unusual.

Antiparticle- A particle type that has exactly the same mass but the opposite value of all other charges from its corresponding particle. For example, the antiparticle of an electron is a particle of positive electric charge called the positron.

Antimatter- Matter made up of elementary particles whose masses are identical to their normal-matter counterparts but whose other properties, such as electric charge, are reversed. The positron is the antimatter counterpart of an electron, with a positive charge instead of a negative charge. When an antimatter particle collides with its normal-matter counterpart, both particles are annihilated and energy is released.

Apparent Brightness- The brightness of an object as it appears without taking into account the distance from the observer to the object. For example, the sun appears to be the brightest object in the sky but is really dimmer (has less intrinsic luminosity) than many other stars.

Atoms- The smallest unit of matter that possesses chemical properties. All atoms have the same basic structure: a nucleus containing positively charged protons with an equal number of negatively charged electrons orbiting around it. In addition to protons, most nuclei contain neutral neutrons whose mass is similar to that of protons. Each atom corresponds to a unique chemical element determined by the number of protons in its nucleus.

Background Radiation- Radiation filling all space. For example, the Cosmic Microwave Background Radiation from the early, hot Universe is visible throughout the Universe today.

Baryon- Subatomic particles comprised of three quarks having a mass equal to or greater than a proton. Protons (comprised of two up quarks and one down quark) and neutrons (made up of two down quarks and one up quark) are examples of baryons.

Baryonic Matter- Matter comprised of protons and neutrons.

Big Bang- The violent cosmic explosion of an incredibly small amount of matter at high temperature and density approximately 20 billion years ago.

Black Body- A body that absorbs all light. At normal temperatures, it would appear black (hence, "black body"), but as it heats up, it emits a distinct spectrum of thermal radiation based only on its temperature. For example, the metal coil of an electric stove is black when cool, but it turns red and then yellow as it heats up.

Black Body Spectrum- A plot of intensity of radiation versus wavelength for an object that absorbs all electromagnetic radiation (a black body).

bulk flow- The relative motion of galaxies in a cluster due to inter-gravitational forces. These are local deviations form the cosmological principle that states that matter is homogeneously distributed throughout the universe.

c (speed of light)- The speed that light travels in a vacuum. It is about 3 x 10⁸ m/s (186,000 miles/s). This is the maximum speed that any particle (including light) or information can travel.

Closed Universe- A model of the Universe in which there is sufficient gravitational force to stop the expansion initiated by the Big Bang and the geometry of space closes upon itself like the surface of a sphere. For a Universe containing only ordinary matter this means that it must have more than the critical density, Ω > 1.

Cluster- A group of galaxies held together by the force of gravity.

Constant of Proportionality- A number that relates the units of two quantities in a mathematical relationship. For example, if you knew that force was proportional to distance, but you wanted a concrete equation, you could say force = (constant) x distance. The (constant) is the constant of proportionality -- the ratio between force and distance. You could find it by plugging observed data into the equation and solving for it.

Cosmic- Of or relating to the Universe as a whole.

Cosmic Microwave Background (CMB)- Ancient radiation leftover from a time roughly 380,000 years after the Big Bang when the hot, dense plasma that permeated the Universe cooled with the expansion of space. At a nearly uniform temperature of 2.7 Kelvin, the CMB fills the entire Universe.

Cosmological Constant (Λ)- The energy density of the smooth vacuum. Einstein initially introduced the term into equations as a way to counteract the attraction caused by gravity at large scales, while preserving the hypothesis of the static Universe. He would later regret this move calling it his "greatest blunder."

Cosmological Principle- When observing the Universe on a large enough scale, it is homogeneous and isotropic i.e. the Universe is large enough that it looks the same from every point within it.

Cosmology- The scientific study of the origin, evolution, and fate of the Universe.

Coupled to Radiation- Matter that is strongly influenced by the presence of radiation.

Critical Density- The calculated mass density of the Universe that stops the expansion of space. Critical density is the dividing line between models of the Universe that expand forever ("open models"), and those that recollapse ("closed models").

Curvature of Spacetime- The distortion of space and time by the presence of matter as dictated by Einstein's General Theory of Relativity.

Dark Energy- The force or energy postulated by cosmologists to explain the acceleration of the expansion of the Universe. Presently, Dark Energy is detectable only through its gravitational effects.

Dark Matter- Matter that emits no observable radiation but is detectable through its gravitational effects.

Deceleration- Decreasing velocity

Decoupling- Stop interacting. When light and matter interact, matter gets in the way of light and scatters it, which is what makes things opaque. When matter became neutral during the cooling of the Universe, it stopped interfering with light, allowing light to travel uninterrupted.

Density Fluctuations in Matter- Regions of greater and lesser mass density, which can form into large-scale cosmological structures such as galaxies, clusters, etc.

Deuterium- A special form of hydrogen (an isotope called "heavy hydrogen") that has a neutron as well as a proton in its nucleus.

Doppler Effect- The apparent change in frequency of a wave due to the relative motion of the source and observer. For example, when the source and observe move apart, the wavelength is stretched, lowering its frequency.

Dynamics- The branch of mechanics in physics concerned with the effect of forces on the motion of bodies during a particular length of time.

Electromagnetic Force- The fundamental force that binds electrons to atoms and governs the physics of light. Any charge feels an electric force from another charge, while a moving charge feels a magnetic force from another other moving charge.

Electromagnetic Radiation- A kind of radiation including radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays, and gamma rays. Electromagnetic radiation is created when electric fields and magnetic fields vary simultaneously.

Electron- A negatively charged elementary particle that typically resides outside the nucleus of an atom, but is bound to it by electromagnetic force.

Elementary Particle Physics- The study of the properties, relationships, and interactions of fundamental, subatomic particles (such as quarks and leptons).

Energy Density- The amount of energy per unit volume. The energy density affects, and is affected by, the expansion or contraction of the Universe.

Equilibrium- To be in a state of perfect balance. As long as the system (whatever is in balance) is untouched by external forces, it remains in balance. All systems naturally fall toward a state of equilibrium. For example, a spinning top is in a kind of equilibrium until it falls, where it enters a different state of equilibrium.

Era- A time period in the evolution of the Universe dominated by a particular physical process.

Evolution- The gradual development of something over a period of time.

Extrapolate- To extend an estimate or conclusion to an unknown situation by assuming existing trends will continue.

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The Universe Adventure

Glossary: A to E