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Observations suggest that the expansion of the universe will continue forever. If so, the universe will cool as it expands, eventually becoming too cold to sustain life. For this reason, this future scenario is popularly called the ''Big Freeze''.〔(WMAP – Fate of the Universe ), ''WMAP's Universe'', NASA. Accessed on line July 17, 2008.〕 If dark energy—represented by the cosmological constant, a ''constant'' energy density filling space homogeneously, or scalar fields, such as quintessence or moduli, ''dynamic'' quantities whose energy density can vary in time and space—accelerates the expansion of the universe, then the space between clusters of galaxies will grow at an increasing rate. Redshift will stretch ancient, incoming photons (even gamma rays) to undetectably long wavelengths and low energies.〔 Stars are expected to form normally for 1012 to 1014 (1–100 trillion) years, but eventually the supply of gas needed for star formation will be exhausted. And as existing stars run out of fuel and cease to shine, the universe will slowly and inexorably grow darker, one star at a time.〔A dying universe: the long-term fate and evolution of astrophysical objects, Fred C. Adams and Gregory Laughlin, ''Reviews of Modern Physics'' 69, #2 (April 1997), pp. 337–372. . .〕〔Adams & Laughlin (1997), §IIE.〕 According to theories that predict proton decay, the stellar remnants left behind will disappear, leaving behind only black holes, which themselves eventually disappear as they emit Hawking radiation.〔Adams & Laughlin (1997), §IV.〕 Ultimately, if the universe reaches a state in which the temperature approaches a uniform value, no further work will be possible, resulting in a final heat death of the universe.〔Adams & Laughlin (1997), §VID〕 ==Cosmology== Infinite expansion does not determine the spatial curvature of the universe. It can be open (with negative spatial curvature), flat, or closed (positive spatial curvature), although if it is closed, sufficient dark energy must be present to counteract the gravitational attraction of matter and other forces tending to contract the universe. Open and flat universes will expand forever even in the absence of dark energy.〔Chapter 7, ''Calibrating the Cosmos'', Frank Levin, New York: Springer, 2006, ISBN 0-387-30778-8.〕 Observations of the cosmic background radiation by the Wilkinson Microwave Anisotropy Probe and the Planck mission suggest that the universe is spatially flat and has a significant amount of dark energy.〔(Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Data Processing, Sky Maps, and Basic Results ), G. Hinshaw et al., ''The Astrophysical Journal Supplement Series'' (2008), submitted, , .〕〔(Planck 2015 results. XIII. Cosmological parameters )〕 In this case, the universe should continue to expand at an accelerating rate. The acceleration of the universe's expansion has also been confirmed by observations of distant supernovae.〔 If, as in the concordance model of physical cosmology (Lambda-cold dark matter or ΛCDM), the dark energy is in the form of a cosmological constant, the expansion will eventually become exponential, with the size of the universe doubling at a constant rate. If the theory of inflation is true, the universe went through an episode dominated by a different form of dark energy in the first moments of the Big Bang; but inflation ended, indicating an equation of state much more complicated than those assumed so far for present-day dark energy. It is possible that the dark energy equation of state could change again resulting in an event that would have consequences which are extremely difficult to parametrize or predict. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Future of an expanding universe」の詳細全文を読む スポンサード リンク
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