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The study of the nature, origin, and evolution of the universe.
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| The conflict between theory and evidence regarding the darkness of the night sky. |
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| The part of the universe that you can see from your location in space and in time. |
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| The high-density, high-temperature state from which the expanding universe of galaxies began. |
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| The age of the universe, equivalent to 1 divided by the Hubble constant. The Hubble time is the age of the universe if it has expanded since the big bang at a constant rate. |
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| cosmic microwave background radiation (CMB) |
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| Radiation from the hot matter of the universe soon after the big bang. The large redshift makes it appear to come from a blackbody with a temperature of 2.7 K. |
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| Matter composed of antiparticles, which upon colliding with a matching particle of normal matter annihilate and convert the mass of both particles into energy. The antiproton is the antiparticle of the proton, and the positron is the antiparticle of the electron. |
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| The stage, within 400,000 years of the big bang, when the gas became transparent to radiation. |
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| The period of time after the glow of the big bang faded into the infrared and before the birth of the first stars, during which the universe expanded in darkness. |
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| The stage in the early history of the universe when ultraviolet photons from the first stars ionized the gas filling space. |
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| The observation that, in its general properties, the universe looks the same in every direction. |
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| The observation that, on the large scale, matter is uniformly spread throughout the universe |
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| The assumption that any observer in any galaxy sees the same general features of the universe. |
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| A model of the universe in which space-time is curved in such a way that the universe is infinite. |
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| A model of the universe in which space-time is curved to meet itself and the universe is finite. |
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| A model of the universe in which space-time is not curved. |
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| The average density of the universe needed to make its curvature flat. |
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| Proposed dark matter made up of particles other than protons and neutrons (baryons). |
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| Dark matter that is made of slow-moving particles. |
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| In cosmology, the peculiar circumstances that the early universe must have contained almost exactly the right amount of matter to make space-time flat. |
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| In cosmology, the circumstances that the primordial background radiation seems much more isotropic than can be explained by the standard big bang theory. |
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| A version of the big bang theory, derived from grand unified theories of particle physics, that includes a rapid expansion when the universe was very young. |
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| A constant in Einstein's equations of space and time that represents a force of repulsion. |
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| A possible form of dark energy that can change in strength as the universe ages. |
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| The fate of the universe if dark energy increases with time and galaxies, stars, and even atoms are eventually ripped apart by the accelerating expansion of the universe. |
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| A cluster of galaxy clusters. |
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| The distribution of clusters and superclusters of galaxies in filaments and walls enclosing voids. |
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