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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Universe | 6/10 | https://en.wikipedia.org/wiki/Universe | reference | science, encyclopedia | 2026-05-05T13:33:50.833322+00:00 | kb-cron |
A lepton is an elementary, half-integer spin particle that does not undergo strong interactions but is subject to the Pauli exclusion principle; no two leptons of the same species can be in exactly the same state at the same time. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). Electrons are stable and the most common charged lepton in the universe, whereas muons and taus are unstable particles that quickly decay after being produced in high energy collisions, such as those involving cosmic rays or carried out in particle accelerators. Charged leptons can combine with other particles to form various composite particles such as atoms and positronium. The electron governs nearly all of chemistry, as it is found in atoms and is directly tied to all chemical properties. Neutrinos rarely interact with anything, and are consequently rarely observed. Neutrinos stream throughout the universe but rarely interact with normal matter. The lepton epoch was the period in the evolution of the early universe in which the leptons dominated the mass of the universe. It started roughly 1 second after the Big Bang, after the majority of hadrons and anti-hadrons annihilated each other at the end of the hadron epoch. During the lepton epoch, the temperature of the universe was still high enough to create lepton–anti-lepton pairs, so leptons and anti-leptons were in thermal equilibrium. Approximately 10 seconds after the Big Bang, the temperature of the universe had fallen to the point where lepton–anti-lepton pairs were no longer created. Most leptons and anti-leptons were then eliminated in annihilation reactions, leaving a small residue of leptons. The mass of the universe was then dominated by photons as it entered the following photon epoch.
==== Photons ====
A photon is the quantum of light and all other forms of electromagnetic radiation. It is the carrier for the electromagnetic force. The effects of this force are easily observable at the microscopic and at the macroscopic level because the photon has zero rest mass; this allows long distance interactions. The photon epoch started after most leptons and anti-leptons were annihilated at the end of the lepton epoch, about 10 seconds after the Big Bang. Atomic nuclei were created in the process of nucleosynthesis which occurred during the first few minutes of the photon epoch. For the remainder of the photon epoch the universe contained a hot dense plasma of nuclei, electrons and photons. About 380,000 years after the Big Bang, the temperature of the universe fell to the point where nuclei could combine with electrons to create neutral atoms. As a result, photons no longer interacted frequently with matter and the universe became transparent. The highly redshifted photons from this period form the cosmic microwave background. Tiny variations in the temperature of the CMB correspond to variations in the density of the universe that were the early "seeds" from which all subsequent structure formation took place.
== Habitability == The frequency of life in the universe has been a frequent point of investigation in astronomy and astrobiology, being the issue of the Drake equation and the different views on it, from identifying the Fermi paradox, the situation of not having found any signs of extraterrestrial life, to arguments for a biophysical cosmology, a view of life being inherent to the physical cosmology of the universe.
== Cosmological models ==
=== Model of the universe based on general relativity ===
General relativity is the geometric theory of gravitation formulated by Albert Einstein in 1915 and remains the standard description of gravity in modern physics. It extends special relativity and Newton's law of universal gravitation by describing gravity as a manifestation of the curvature of space and time (spacetime). In this framework, the curvature of spacetime is determined by the energy and momentum of matter and radiation. This relationship is expressed through the Einstein field equations, which link the distribution of matter and energy to the geometry of spacetime. The resulting geometry governs the motion of matter, so that solutions of these equations describe how the universe evolves over time. Under the cosmological principle, which assumes that the universe is homogeneous and isotropic on large scales, the field equations admit a class of solutions described by the metric tensor known as the Friedmann–Lemaître–Robertson–Walker metric. In this description, the universe is characterized by two quantities: a scale factor, which describes how its overall size changes with time, and a curvature index, which specifies its spatial geometry. The curvature can be flat, positively curved, or negatively curved. The evolution of the scale factor depends on both the spatial curvature and the cosmological constant, which represents the energy density of empty space and may be associated with dark energy. The relation governing this evolution is known as the Friedmann equation, introduced by Alexander Friedmann. The curvature determines the global geometry of space. A positively curved universe has a finite volume and can be visualized as a three-dimensional sphere. A flat or negatively curved universe is spatially infinite. Although this may seem counterintuitive, models with flat or negative curvature allow an infinite universe to emerge from an initial state in which the scale factor vanishes, consistent with the cosmological principle. Analogies include an infinite plane (flat) or other geometries such as a torus. The ultimate fate of the universe depends on both the curvature and the cosmological constant. A sufficiently dense universe with positive curvature would eventually recollapse in a Big Crunch, possibly followed by a Big Bounce. In contrast, a flat or negatively curved universe would expand indefinitely, approaching a Big Freeze and eventual heat death of the universe. Observations indicate that the expansion of the universe is accelerating, raising the possibility of a Big Rip. Current data suggest that the universe is close to flat, with a density near the critical value separating recollapse from eternal expansion.
=== Multiverse hypotheses ===