Glossary: P to T

Parallax- A method of finding astronomical distances. Using a distant background of stars as a reference plane, one observes an object moving across the sky as the earth moves around the sun. The closer the object, the bigger its parallax motion.

Parsec- A unit of length used in astronomy. Equal to 3.1 x 10¹³ km or 3.26 light years, its name means it is the distance to a star with a PARallax of one SECond.

Particle Accelerators- Machines used to propel particles to high speeds. They use electric fields to contain and accelerate charged particles.

Particle Physics- The study of fundamental particles (such as quarks and leptons) and their fundamental interactions (four forces).

Period- The time between two peaks of a wave. Usually designated as T, this is the time it takes a wave to make one complete oscillation.

Phase Transition- Change from one state of energy to another.

Photon- An elementary particle that makes up electromagnetic radiation (including light). Even though a photon is a particle, it is still a wave and has a wavelength, frequency, and so on like any other wave. Unlike many other particles, a photon has no mass.

Planck Time / Length- The shortest possible units of time and length. A Planck time is about 5.4x10^-44 seconds, and a Planck length is about 1.6x10^-35 meters. At any smaller time or length, our current understanding of physics would break down.

Plasma- A state of matter composed of free charged particles. Plasma, such as fire, stars, or lightning, consists of high energy ions and electrons. A gas becomes a plasma, or ionized gas, when electrons have enough energy to escape from their atoms.

Positron- The antiparticle of the electron. It carries a charge exactly opposite that of an electron.

Primordial- Earliest; original; initial.

Primordial Nucleosynthesis- Building of atomic elements in the early Universe when temperatures were high enough to fuse nuclei together. Most of the helium in the Universe was created by this process.

Probability Density- The likelihood of an object being found in a particular location. In quantum mechanics, it is impossible to know an object's exact location; a high probability density tells us where the particle is most likely to be found.

Proton- A subatomic particle found in the nucleus with positive electric charge. The number of protons in an atom determines its element. It is a baryon that consists of two up quarks and one down quark (held together by gluons).

Quantum Fluctuations- Tiny, temporary changes in the energy of a vacuum. Because of inherent uncertainties at subatomic scales, energy is not guaranteed to be conserved.

Quantum Mechanics- A theory of physics that applies to systems on extremely small scales (i.e., the size of an atom). Quantum mechanics describes the "fuzziness" of the Universe. It can only predict the probability that an event will happen.

Quark- A fundamental particle that interacts through the strong force. Quarks have an electric charge of either +2/3 (up, charm, top) or -1/3 (down, strange, bottom) the charge of a proton. They are the basic building blocks of hadrons (protons, neutrons, and mesons).

Quasar (QUASi-stellAR radio source)- A quasar is the nucleus of a young galaxy powered by a super massive black hole. Every so often, a black hole devours some form of mass and becomes extremely bright. The brightest known quasar is more than one hundred times as bright as our entire galaxy, even though it is only several light weeks across (as opposed to the Milky Way -- 100,000 light-years across).

Quintessence- In ancient and medieval philosophy quintessence was thought to be the fifth classical element after air, earth, fire, and water. Quintessence is the element that the cosmos and all celestial beings are made of.

Radiation- Energy that is carried in waves. It includes electromagnetic energy and subatomic particles moving close to the speed of light.

Radiation Pressure- The pressure caused by light or other radiation. When any kind of electromagnetic radiation hits a surface, it transfers momentum to the object, potentially propelling it forward.

Recede- To move away from the observer (us, in most cases).

Recombination- An epoch in which the universe is 300,000 years old and temperatures have cooled sufficient amounts for nucleosynthesis to occur. The density of the Universe rapidly decreases.

Redshift- An increase in the wavelength of light. This can arise from motion of the source or receiver (Doppler shift), expansion of space, or strong gravitational fields.

Relative Brightness- How bright one object is compared to another. Even if two objects have the same intrinsic brightness, their relative brightness tells us which is closer and by how much.

Relic- Anything left over from a previous time period. In cosmology, they include evidence of early physical eras.

Scatter- When an object absorbs light from one direction and then emits light in all directions. For example, when light from our sun comes into contact with the clouds in our atmosphere, it is absorbed by the clouds and then re-emitted in all directions. On a clear day, sunlight comes from a small point (the sun), but on cloudy days it seems to come from all directions.

Scalar- A constant number. 1, -16, .3534, 7346 are all scalars.

Simultaneous- Occurring at the same time

Spacetime- The combination of space and time into a four dimensional Universe. The first three are spatial: up/down, north/south, east/west; the fourth is temporal: past/future.

Spectrum- A range of possible measurements. In physics, a spectrum usually refers to all the different wavelengths or colors (if in the visible range) of light.

Spectral Line- A line that appears in an otherwise uniform spectrum at a specific frequency or wavelength. They are due either to emission of light, which makes a bright line, or absorption of light, making a dark line.

Standard Candle- A celestial object whose intrinsic brightness is known or can be well estimated by some physical principle. Because we can see its observed brightness as well, we can determine its distance.

Static- Unchanging over a length of time

Steady State Theory- An cosmological model that says the Universe has always been and will always be the same as it is now. It claims that the Universe is homogenous and isotropic not just in space, but throughout time. The discovery and analysis of the Cosmic Microwave Background Radiation, however, has provided conclusive evidence against the Steady State Theory.

Stochastic- Random

Strong Force- An extremely strong but extremely short range attractive force. It is the interaction responsible for binding quarks and antiquarks together to make hadrons, such as protons and neutrons. The leftover strong interactions make up forces that keep the nucleus together.

Supercluster- A group of galaxy clusters. A supercluster may contain tens of thousands of galaxies spanning over a hundred million light-years of space. Galaxy superclusters are the largest confirmed structures in the Universe.

Supernova- An enormous and extremely bright explosion of a star at the end of its lifetime. Dying stars that grow too large may collapse in on themselves, or white dwarfs that get too heavy may trigger a thermonuclear explosion. The shock waves and expelled matter from supernovae are responsible for the birth of new stars.

Tangential Velocity- The velocity of an object in its current direction of motion. Even if an object is moving in a complicated path, its tangential velocity is its speed in a straight line at any given moment.

Type Ia Supernova- A supernova formed from the explosion of an old, compact star (white dwarf). Because of their uniform brightness, they make good standard candles.

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The Universe Adventure

Glossary: P to T