| annihilation | The mutual destruction of a matter–antimatter pair of particles. The charges on the two particles cancel and the mass of the particles is entirely converted to energy.
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| Big Bang | The explosive event at the beginning of the universe. The expansion produced the Big Bang that continues today.
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| cosmic background radiation (CBR) | Radiation observed to haveal most perfectly uniform brightness in all directions in the sky. The CBR is highly redshifted radiation produced about a million years after the universe began to expand.
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| cosmological constant | A self-repelling property of space first proposed by Einstein.
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| cosmology | The study of the universe as a whole.
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| critical density | The value that the average density of the universe must equal or exceed if the universe is closed. If the density of the universe is less than the critical density, the universe will continue to expand forever.
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| dark energy | A form of energy that may be causing the expansion of the universe to accelerate.
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| decoupling epoch | The time about a million years after the expansion of the universe began when the universe became transparent and light could, for the first time, travel great distances before being absorbed or scattered. The cosmic background radiation was produced at the decoupling epoch.
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| deuteron | A nucleus of deuterium, an isotope of hydrogen. A deuteron contains one proton and one neutron.
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| Hubble time | An estimate of the age of the universe obtained by taking the inverse of Hubble’s constant. The estimate is only valid if there has been no acceleration or deceleration of the expansion of the universe.
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| inflation | A brief period of extremely rapid and enormous expansion that may have occurred very early in the history of the universe.
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| isotropic | Looking the same in all directions.
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| pair production | A process in which gamma rays are transformed into a particle and its antiparticle (such as an electron and a positron).
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| radiation era | The period of time, before about 1 million years after the expansion of the universe began, when radiation rather than matter was the dominant constituent of the universe.
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| recombination epoch | The time, about 1 million years after the expansion of the universe began, when most of the ions and electrons in the universe combined to form atoms.
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| scalar field | A form of energy that has been proposed as the cause of inflation in the early universe.
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