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| ACDM malli. Avaruuden kiihtyvä laajeneminen kuva wikimedia |
Wikipedia kertoo, että
The Friedmann–Lemaître–Robertson–Walker (FLRW) metric is an exact solution of Einstein's field equations of general relativity; it describes a homogeneous, isotropic expanding or contracting universe that may be simply connected or multiply connected. (If multiply connected, then each event in spacetime will be represented by more than one tuple of coordinates.)
The general form of the metric follows from the geometric properties of homogeneity and isotropy; Einstein's field equations are only needed to derive the scale factor of the universe as a function of time.
Depending on geographical or historical preferences, a subset of the four scientists — Alexander Friedmann, Georges Lemaître, Howard Percy Robertson and Arthur Geoffrey Walker — may be named (e.g., Friedmann–Robertson–Walker (FRW) or Robertson–Walker (RW) or Friedmann–Lemaître (FL)).
This model is sometimes called the Standard Model of modern cosmology. It was developed independently by the named authors in the 1920s and 1930s.
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The FLRW metric starts with the assumption of homogeneity and isotropy of space. It also assumes that the spatial component of the metric can be time-dependent. The generic metric which meets these conditions is
whereranges over a 3-dimensional space of uniform curvature, that is, elliptical space, Euclidean space, or hyperbolic space. It is normally written as a function of three spatial coordinates, but there are several conventions for doing so, detailed below.
does not depend on t — all of the time dependence is in the function a(t), known as the "scale factor".
FLWR merkitys oletusarvona
These equations are the basis of the standard big bang cosmological model including the current ΛCDM model.
Because the FLRW model assumes homogeneity, some popular accounts mistakenly assert that the big bang model cannot account for the observed lumpiness of the universe. In a strictly FLRW model, there are no clusters of galaxies, stars or people, since these are objects much denser than a typical part of the universe.
Nonetheless, the FLRW model is used as a first approximation for the evolution of the real, lumpy universe because it is simple to calculate, and models which calculate the lumpiness in the universe are added onto the FLRW models as extensions.
Most cosmologists agree that the observable universe is well approximated by an almost FLRW model, i.e., a model which follows the FLRW metric apart from primordial density fluctuations. As of 2003, the theoretical implications of the various extensions to the FLRW model appear to be well understood, and the goal is to make these consistent with observations from COBE and WMAP.
Havaintojen antamia todisteita
By combining the observation data from some experiments such as WMAP and Planck with theoretical results of Ehlers–Geren–Sachs theorem and its generalization, astrophysicists now agree that the universe is nearly an isotropic and homogeneous FLRW spacetime.
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Eräitä käsitteitä ja termejä FLWR tekstissä
Einsteins's field equations of general relativity
The Einstein field equations (EFE) or Einstein's equations are a set of 10 equations in Albert Einstein's general theory of relativity which describe the fundamental interaction of gravitation as a result of spacetime being curved by matter and energy.Homogenous - Corresponding in structure because of a common origin.
First published by Einstein in 1915 as a tensor equation, the EFE equate local spacetime curvature (expressed by the Einstein tensor) with the local energy and momentum within that spacetime (expressed by the stress–energy tensor).
wikipedia
Isotropic - Identical in all directions; invariant with respect to direction-
Ehlers–Geren–Sachs theorem
The Ehlers–Geren–Sachs theorem, published in 1968 by Jürgen Ehlers, P. Geren and Rainer Sachs, shows that if, in a given universe, all freely falling observers measure the cosmic background radiation to have exactly the same properties in all directions (that is, they measure the background radiation to be isotropic), then that universe is an isotropic and homogeneous FLRW spacetime.
wikipedia
ACDM model
The ΛCDM or Lambda-CDM model is a parametrization of the Big Bang cosmological model in which the universe contains a cosmological constant, denoted by Lambda (Greek Λ), and cold dark matter (abbreviated CDM). It is frequently referred to as the standard model of Big Bang cosmology, since it is the simplest model that provides a reasonably good account of the following properties of the cosmos:
- the existence and structure of the cosmic microwave background
- the large-scale structure in the distribution of galaxies
- the abundances of hydrogen (including deuterium), helium, and lithium
- the accelerating expansion of the universe observed in the light from distant galaxies and supernovae
The model assumes that general relativity is the correct theory of gravity on cosmological scales. It emerged in the late 1990s as a concordance cosmology, after a period of time when disparate observed properties of the universe appeared mutually inconsistent, and there was no consensus on the makeup of the energy density of the universe.
The ΛCDM model can be extended by adding cosmological inflation, quintessence, and other elements that are current areas of speculation and research in cosmology.
Some alternative models challenge the assumptions of the ΛCDM model. Examples of these are: modified Newtonian dynamics, modified gravity, and theories of large-scale variations in the matter density of the universe.
wikipedia
COBE
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| Kuva NASA |
The COBE satellite was developed by NASA's Goddard Space Flight Center to measure the diffuse infrared and microwave radiation from the early universe to the limits set by our astrophysical environment. It was launched November 18, 1989 and carried three instruments,
- a Diffuse Infrared Background Experiment (DIRBE) to search for the cosmic infrared background radiation,
- a Differential Microwave Radiometer (DMR) to map the cosmic radiation sensitively, and
- a Far Infrared Absolute Spectrophotometer (FIRAS) to compare the spectrum of the cosmic microwave background radiation with a precise blackbody.
Each COBE instrument yielded a major cosmological discovery
Planck
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| Planck satelliitti taustana universumin mikroaalto kartta kuva ESA |
Planck will help provide answers to some of the most important questions in modern science: how did the Universe begin, how did it evolve to the state we observe today, and how will it continue to evolve in the future? Planck's objective is to analyse, with the highest accuracy ever achieved, the remnants of the radiation that filled the Universe immediately after the Big Bang - this we observe today as the Cosmic Microwave Background.
European Space Agency ESA
WMAP
The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA Explorer mission that launched June 2001 to make fundamental measurements of cosmology -- the study of the properties of our universe as a whole. WMAP has been stunningly successful, producing our new Standard Model of Cosmology. WMAP's data stream has ended. Full analysis of the data is now complete. Publications have been submitted as of 12/20/2012.
WMAP page NASA
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