Cosmos Cosmos is the Universe regarded as an ordered system.[1] The philosopher Pythagoras is regarded as the first person to apply the term cosmos (Greek κόσμος) to the order of the Universe.[2] Cosmology[edit] Cosmology is the study of the cosmos in several of the above meanings, depending on context. All cosmologies have in common an attempt to understand the implicit order within the whole of being. In this way, most religions and philosophical systems have a cosmology. In physical cosmology, the term cosmos is often used in a technical way, referring to a particular spacetime continuum within the (postulated) multiverse. Theology[edit] In theology, the term can be used to denote the created Universe, not including the creator. See also[edit] References[edit] External links[edit]
Holographic principle In a larger sense, the theory suggests that the entire universe can be seen as a two-dimensional information structure "painted" on the cosmological horizon[clarification needed], such that the three dimensions we observe are an effective description only at macroscopic scales and at low energies. Cosmological holography has not been made mathematically precise, partly because the particle horizon has a finite area and grows with time.[4][5] The holographic principle was inspired by black hole thermodynamics, which conjectures that the maximal entropy in any region scales with the radius squared, and not cubed as might be expected. Black hole entropy[edit] An object with entropy is microscopically random, like a hot gas. But Jacob Bekenstein noted that this leads to a violation of the second law of thermodynamics. Bekenstein assumed that black holes are maximum entropy objects—that they have more entropy than anything else in the same volume. Black hole information paradox[edit] General
Deceleration parameter The deceleration parameter in cosmology is a dimensionless measure of the cosmic acceleration of the expansion of space in a Friedmann–Lemaître–Robertson–Walker universe. It is defined by: where is positive (recent measurements suggest it is), and in this case the deceleration parameter will be negative.[1] The minus sign and name "deceleration parameter" are historical; at the time of definition was thought to be positive, now it is believed to be negative. The Friedmann acceleration equation can be written as is the energy density of the universe, is its pressure, and This can be rewritten as by using the first Friedmann equation, where is the Hubble parameter and or depending on whether the universe is hyperspherical, flat or hyperbolic respectively. The derivative of the Hubble parameter can be written in terms of the deceleration parameter: Except in the speculative case of phantom energy (which violates all the energy conditions), all postulated forms of matter yield a deceleration parameter .
Conservation of energy Law of physics and chemistry In physics and chemistry, the law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be conserved over time.[1] This law, first proposed and tested by Émilie du Châtelet,[2][3] means that energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another. For instance, chemical energy is converted to kinetic energy when a stick of dynamite explodes. Classically, conservation of energy was distinct from conservation of mass. , the equation representing mass–energy equivalence, and science now takes the view that mass-energy as a whole is conserved. Given the stationary-action principle, conservation of energy can be rigorously proven by Noether's theorem as a consequence of continuous time translation symmetry; that is, from the fact that the laws of physics do not change over time. History[edit] was conserved so long as the masses did not interact.
Cosmic string Cosmic strings are hypothetical 1-dimensional (spatially) topological defects which may have formed during a symmetry breaking phase transition in the early universe when the topology of the vacuum manifold associated to this symmetry breaking was not simply connected. It is expected that at least one string per Hubble volume is formed. Their existence was first contemplated by the theoretical physicist Tom Kibble in the 1970s. The formation of cosmic strings is somewhat analogous to the imperfections that form between crystal grains in solidifying liquids, or the cracks that form when water freezes into ice. Theories containing cosmic strings[edit] The prototypical example of a quantum field theory with cosmic strings is the Abelian Higgs model. Dimensions[edit] Cosmic strings, if they exist, would be extremely thin with diameters of the same order of magnitude as that of a proton, i.e. ~ 1 fm, or smaller. Gravitation[edit] Negative Mass Cosmic String[edit] Observational evidence[edit]
Entropic gravity Entropic gravity is a hypothesis in modern physics that describes gravity as an entropic force; not a fundamental interaction mediated by a quantum field theory and a gauge particle (like photons for the electromagnetic force, and gluons for the strong nuclear force), but a probabilistic consequence of physical systems' tendency to increase their entropy. The proposal has been intensely contested in the physics community but it has also sparked a new line of research into thermodynamic properties of gravity. Origin[edit] The probabilistic description of gravity has a history that goes back at least to research on black hole thermodynamics by Bekenstein and Hawking in the mid-1970s. These studies suggest a deep connection between gravity and thermodynamics, which describes the behavior of heat. Erik Verlinde's theory[edit] Criticism and experimental tests[edit] Even so, entropic gravity in its current form has been severely challenged on formal grounds. See also[edit] References[edit]
Scale factor (cosmology) The scale factor, cosmic scale factor or sometimes the Robertson-Walker scale factor[1] parameter of the Friedmann equations is a function of time which represents the relative expansion of the universe. It relates the proper distance (which can change over time, unlike the comoving distance which is constant) between a pair of objects, e.g. two galaxy clusters, moving with the Hubble flow in an expanding or contracting FLRW universe at any arbitrary time to their distance at some reference time . The formula for this is: where is the proper distance at epoch is the distance at the reference time and is the scale factor.[2] Thus, by definition, The scale factor is dimensionless, with counted from the birth of the universe and set to the present age of the universe: [3] giving the current value of as or The Hubble parameter is defined: where the dot represents a time derivative. one can see that , and also that , so combining these gives which is just Hubble's law. is increasing with time.
Energy current Explanation[edit] "Energy current" is a somewhat informal term that is used, on occasion, to describe the process of energy transfer in situations where the transfer can usefully be viewed in terms of a flow. It is particularly used when the transfer of energy is more significant to the discussion than the process by which the energy is transferred. The units of energy current are those of power (W). Energy current in electromagnetism[edit] A specific use of the concept of energy current was promulgated by Oliver Heaviside in the last quarter of the 19th century. Heaviside's approach had some adherents at the time—enough, certainly, to quarrel with the "traditionalists" in print. After the discovery of the electron in 1897, the Drude model, which describes electrical conduction in metals, was developed very quickly. Poynting-flow diagrams are part of E&M engineering, transmission line theory, and antenna design, but rare in electronics texts.[4] References[edit]