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Curved spacetime
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=== Pressure and stress === Quantities that are directly related to energy and momentum should be sources of gravity as well, namely internal [[pressure]] and [[Stress (physics)|stress]]. Taken together, {{nowrap|1=mass-energy}}, momentum, pressure and stress all serve as sources of gravity: Collectively, they are what tells spacetime how to curve. General relativity predicts that pressure acts as a gravitational source with exactly the same strength as mass–energy density. The inclusion of pressure as a source of gravity leads to dramatic differences between the predictions of general relativity versus those of Newtonian gravitation. For example, the pressure term sets a maximum limit to the mass of a [[neutron star]]. The more massive a neutron star, the more pressure is required to support its weight against gravity. The increased pressure, however, adds to the gravity acting on the star's mass. Above a certain mass determined by the [[Tolman–Oppenheimer–Volkoff limit]], the process becomes runaway and the neutron star collapses to a [[black hole]].<ref name=Schutz />{{rp|243,280}} The stress terms become highly significant when performing calculations such as hydrodynamic simulations of core-collapse supernovae.<ref>{{Cite journal|last1=Kuroda |first1=Takami |last2=Kotake |first2=Kei |last3=Takiwaki |first3=Tomoya |title=Fully General Relativistic Simulations of Core-Collapse Supernovae with An Approximate Neutrino Transport |journal=The Astrophysical Journal |volume=755 |issue=1 |pages=11 |arxiv=1202.2487 |year=2012 |doi=10.1088/0004-637X/755/1/11 |bibcode=2012ApJ...755...11K |s2cid=119179339 }}</ref> These predictions for the roles of pressure, momentum and stress as sources of spacetime curvature are elegant and play an important role in theory. In regards to pressure, the early universe was radiation dominated,<ref>{{cite web |last=Wollack |first=Edward J. |title=Cosmology: The Study of the Universe |work=Universe 101: Big Bang Theory |publisher=[[NASA]] |date=10 December 2010 |url=http://map.gsfc.nasa.gov/universe/ |access-date=15 April 2017 |archive-url=https://web.archive.org/web/20110514230003/http://map.gsfc.nasa.gov/universe/ |archive-date=14 May 2011 |url-status=dead}}</ref> and it is highly unlikely that any of the relevant cosmological data (e.g. [[nucleosynthesis]] abundances, etc.) could be reproduced if pressure did not contribute to gravity, or if it did not have the same strength as a source of gravity as mass–energy. Likewise, the mathematical consistency of the Einstein field equations would be broken if the stress terms did not contribute as a source of gravity.
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