The Universe Adventure - Glossary: K to O

Glossary: K to O


Kelvin- The SI base unit of temperature. The coldest possible temperature of anything is 0 Kelvin which corresponds to -273 degrees on the Celsius scale.

Lambda: The Cosmological Constant, Λ- A constant representing the energy density of the smooth vacuum. It was first postulated by Albert Einstein to counteract the attraction of gravity at large distance scales to preserve the hypothesis of a static Universe.

Large Scale Structure- Galaxies, galaxy clusters, and galaxy superclusters

Last Scattering Surface- The earliest time in the history of the Universe in which light could travel freely. In Era 1 in the early Universe, all matter was ionized and interacted with photons, causing the Universe to appear opaque. Between Era 1 and Era 2, the ionized matter began to form into atoms, which no longer interacted strongly with photons. Light could then begin to travel freely. The last scattering surface is when today's CMB photons were last scattered by the ionized matter.

Lepton- Elementary subatomic particles, such as the electron, that do not experience the Strong Force. Leptons can have either a positive or a negative electric charge, depending on whether they are particles or antiparticles.

Light- A general term for electromagnetic radiation, usually referring to wavelengths within the visible part of the electromagnetic spectrum.

Light Cone- A region of spacetime (shaped like two cones placed peak to peak) that can be used to describe the history of an event. For example, if a pulse of light is emitted from a flashlight, then the top (or future cone) represents the future history of the pulse of light, while the bottom (or past cone) represents the directions light could have been received at an event in the past.

Light-year- The distance light travels in a year: approximately 9.5 trillion kilometers (~5,900,000,000,000 miles). A light-year is approximately 63,000 times as long as the distance from the Earth to the Sun.

Local Group- A small cluster of more than 30 galaxies in the neighborhood of our galaxy (the Milky Way), including the Andromeda galaxy and the Magellanic Clouds. The Local Group appears to be part of a larger supercluster of galaxies that lies around 60 million light-years away in the direction of the constellation Virgo.

Luminosity- The total energy released by an object that emits light (for example, a star), per unit of time. Luminosity may also refer to the power (energy per time) of a star. Luminosity is an intrinsic quality of a star: it does not matter where you are (neglecting relativistic effects), a star will always have the same luminosity. For most stars, their luminosity is dependent on their mass.

Luminous Matter- Ordinary, visible matter. It is composed of baryons, as opposed to dark matter.

Main Sequence Star- The average star in the Universe. These stars tend to be somewhat small and are powered by hydrogen fusion. They are so named because they lie along a continuous band, the "main sequence", on the Hertzsprung-Russell diagram.

Mass Defect- The mass that is converted to energy during a nuclear reaction. When two subatomic particles are bound together, their combined mass is slightly more or less than their masses would be if each particle were measured separately. During fusion or fission, the difference in mass is turned into energy, as per Einstein's famous E = mc2

Milky Way- The galaxy we live in. When viewed from the Earth, it looks like a hazy band of white light (hence "milky") consisting of distant stars. The Milky Way is a large spiral galaxy, about 100,000 light years across, with a total mass of about 1012 solar masses, comprising 200-400 billion stars.

Model- A description meant to explain and predict events in the natural world. A cosmological model is a theory which describes the evolution and structure of the Universe. There are three primary models of the Universe: one that expands forever, one that eventually collapses, and one that stays the same size.

Muon- A large fundamental particle. Designated μ (mu), it is the second heaviest lepton (behind the Tau), and it has the same negative electric charge as an electron.

Neutrino- A tiny elementary particle with no electric charge. Denoted by ν (nu), neutrinos travel close to the speed of light and have almost no mass, making them very difficult to detect. They are leptons, and are three known varieties.

Neutron- A subatomic particle found in the nucleus with zero electric charge. Different isotopes are distinguished by the number of neutrons in their nuclei. It is a baryon that consists of two down quarks and one up quark (held together by gluons).

Non-Baryonic Matter- Matter that does not interact with electro-magnetic radiation, visible light included, and is therefore not luminous. This is most probably the nature of dark matter.

Nought- The initial value. Denoted by a subscript, "0", it refers to the first among a range of values. For example, if an object started off traveling 30 meters/second, then v0 or 'velocity nought', = 30.

Nucleon- A proton or neutron. It refers to either baryon that makes up an atomic nucleus.

Nucleosynthesis- The process by which protons and neutrons bind to create new atomic nuclei. This primarily occurred in the recombination epoch where the temperature had cooled below 10 million degrees.

Nucleus- The small, positively charged center of an atom. Although the nucleus is 100,000 times smaller than the atom, nearly 100% percent of the atom's mass is in the nucleus. The nucleus is composed of protons and neutrons.

Observed Brightness- How bright an object looks from earth. Defined as the flux of light emitted by the object as measured on earth, it is inversely proportional the distance to the object squared, as opposed to the how bright the object is up close. For example, the full moon has a greater observed brightness than the star Vega, even though Vega has a greater luminosity.

Omega: Density Parameter, Ω- The ratio of the total density of matter to the critical density required to make the Universe flat. For example, if Ω = 1, the total density is the same as the critical density.

Opaque- Not see-through. When radiation (like light) is absorbed or scattered many times, whether by plasma at the beginning of the Universe or by the top of your desk, we can only see the object and nothing behind it.

Open Universe- A Universe where space is curved like a saddle. An open Universe has less mass than the critical density (Ω < 1), and it expands forever.