Glossary of Technical Terms

Angular resolution
The smallest angular size that the instrument can resolve.

Anisotropy
The difference in the property of a system with changes in direction. In this case, anisotropy refers to the difference in the temperature of the cosmic microwave background radiation with direction.

Antenna temperature
Antenna temperature is a way of expressing the brightness of a radiation source - it is proportional to the power per unit area emitted by the source. In most cases where it is used it corresponds to the thermodynamic or physical temperature of the source being observed. It thus relates the power emitted by the source to an interesting physical property of that source.

When we observe the brightness of the CMB, we often express the measurements in units of antenna temperature for the same reason. However, because the CMB is so cold, 2.75 K (degrees above absolute zero), the correspondence between antenna temperature and physical temperature breaks down above ~70 GHz in frequency, as shown for example in this plot.

Cosmological Constant
Einstein first proposed the cosmological constant (not to be confused with the Hubble Constant), usually symbolized by the greek letter "lambda" (Λ), as a mathematical fix to the theory of general relativity. In its simplest form, general relativity predicted that the universe must either expand or contract. Einstein thought the universe was static, so he added this new term to stop the expansion. When Hubble's study of nearby galaxies showed that the universe was in fact expanding, Einstein regretted modifying his elegant theory and viewed the cosmological constant term as his "greatest mistake".

Many cosmologists advocate reviving the cosmological constant term on theoretical grounds, as a way to explain the rate of expansion of the universe. Modern field theory now associates this term with the energy density of the vacuum. For this energy density to be comparable to other forms of matter in the universe, it would require new physics theories. So the addition of a cosmological constant term has profound implications for particle physics and our understanding of the fundamental forces of nature.

Energy Spectrum
The amount of energy emitted at a specific wavelength of light. For example, the Sun emits light at many wavelengths, which can be seen by passing sunlight through a prism. The energy spectrum of sunlight is strongest at the wavelength of yellow light, thus the Sun appears yellow to our eyes.

Feed
A component that "feeds" the signal from the optical system to the amplification electronics. Also called a "feed horn" or "horn".

HEMT
High Electron Mobility Transistor. This is a particular kind of transistor that is especially appropriate for use in microwave amplifiers.

Horizon
The limiting distance from which we can have received information since the Big Bang, 13.7 billion years ago, due to the finite speed of light. Since the universe has been expanding throughout its history, the "proper" distance to our horizon today is close to 45 billion light-years. This bounds our observable universe.

Ionized
The state in which an atom is missing one or more of its electrons, and is therefore positively charged. An ionized gas is one in which some or all of the atoms are ionized, rather than electrically neutral. The ionized electrons behave as free particles in this gas.

Isotope
Atoms which have the same number of protons in the nucleus (this defines an element), but a different number of neutrons. This changes the atomic mass, producing a "variety" of the element.

Kelvin
One Kelvin degree is equivalent to one Celsius degree. The difference between the two temperature scales: All motion within an atom ceases at zero Kelvin (K) -- this point is called absolute zero. Water freezes at zero degrees Celsius, which is approximately 273.16K.

Lagrange Points
Positions in space where the gravitational pull of the two orbiting masses precisely equals the centripetal force required to rotate a third smaller mass with them in a constant pattern.

Last Scattering Surface
Also known as the Surface of Last Scattering. This is the point at which the ionized plasma that filled the early universe cooled to less than 2967 kelvin. At this temperature electrons and protons were then able to combine to make neutral hydrogen, allowing photons to travel through space without scattering. While this happened everywhere at the same time, our vantage point on Earth, 13.7 billion years later, results in the view of a surrounding bubble of faint microwave light to reach us from that long ago time.

MIDEX
NASA's Mid-Class Explorer.

Sensitivity
The level of random noise in the temperature measurement.

Structure
The patterns of the distribution of matter in the universe, as probed by the cosmic microwave background radiation or observations of galaxies.

Systematic Errors
Measurement errors that are not random.

Theory
A scientifically testable general principle or body of principles offered to explain observed phenomena. In scientific usage, a theory is distinct from a hypothesis (or conjecture) that is proposed to explain previously observed phenomena. For a hypothesis to rise to the level of theory, it must predict the existence of new phenomena that are subsequently observed. A theory can be overturned if new phenomena are observed that directly contradict the theory.