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  1. 2. the antiparticle of the proton; a particle having the same mass as the proton but an equal and opposite charge.
  2. 5. the SI base unit of thermodynamic temperature (equivalent in size to the degree Celsius), first introduced as the unit used in the Kelvin scale.
  3. 8. The 'critical density' is the average density of matter required for the Universe to just halt its expansion, but only after an infinite time.
  4. 10. of the same or a similar kind or nature
  5. 11. nonluminous material that is postulated to exist in space and that could take any of several forms including weakly interacting particles ( cold dark matter ) or high-energy randomly moving particles created soon after the Big Bang
  6. 13. is the speed of light; also, informal : an extremely fast rate or speed. How to use light speed in a sentence.
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  1. 1. a particle representing a quantum of light or other electromagnetic radiation. A photon carries energy proportional to the radiation frequency but has zero rest mass.
  2. 2. molecules formed by atoms consisting of antiprotons, antineutrons, and positrons. Stable antimatter does not appear to exist in our universe.
  3. 3. having a physical property which has the same value when measured in different directions
  4. 4. When a body that is emitting radiation has a non-zero radial velocity relative to an observer, the wavelength of the emission will be shortened or lengthened, depending upon whether the body is moving towards or away from an observer.
  5. 6. An alternative term for the radiation era in the Big Bang theory, during which the Universe was hot and dense and was dominated by the effects of radiation.
  6. 7. An atom is the basic building block of chemistry.
  7. 9. the science of the origin and development of the universe. Modern astronomy is dominated by the Big Bang theory, which brings together observational astronomy and particle physics.
  8. 12. In the Big Bang theory, the era that started when the gravitational effect of matter began to dominate the effect of radiation pressure. Although radiation is massless, it has a gravitational effect which increases with the intensity of the radiation.