Editing General relativity (section)

=== Black holes and other compact objects ===
{{Main|Black hole}}
[[File:Star collapse to black hole.png|thumb|left|Simulation based on the equations of general relativity: a star collapsing to form a black hole while emitting gravitational waves]]
Whenever the ratio of an object's mass to its radius becomes sufficiently large, general relativity predicts the formation of a black hole, a region of space from which nothing, not even light, can escape. In the currently accepted models of [[stellar evolution]], neutron stars of around 1.4 [[solar mass]]es, and stellar black holes with a few to a few dozen solar masses, are thought to be the final state for the evolution of massive stars.<ref>{{Harvnb|Miller|2002|loc=lectures 19 and 21}}</ref> Usually a galaxy has one [[supermassive black hole]] with a few million to a few [[1000000000 (number)|billion]] solar masses in its center,<ref>{{Harvnb|Celotti|Miller|Sciama|1999|loc=sec. 3}}</ref> and its presence is thought to have played an important role in the formation of the galaxy and larger cosmic structures.<ref>{{Harvnb|Springel|White|Jenkins|Frenk|2005}} and the accompanying summary {{Harvnb|Gnedin|2005}}</ref>

Astronomically, the most important property of compact objects is that they provide a supremely efficient mechanism for converting gravitational energy into electromagnetic radiation.<ref>{{Harvnb|Blandford|1987|loc=sec. 8.2.4}}</ref> [[Accretion (astrophysics)|Accretion]], the falling of dust or gaseous matter onto stellar or supermassive black holes, is thought to be responsible for some spectacularly luminous astronomical objects, notably diverse kinds of active galactic nuclei on galactic scales and stellar-size objects such as microquasars.<ref>For the basic mechanism, see {{Harvnb|Carroll|Ostlie|1996|loc=sec. 17.2}}; for more about the different types of astronomical objects associated with this, cf. {{Harvnb|Robson|1996}}</ref> In particular, accretion can lead to [[relativistic jet]]s, focused beams of highly energetic particles that are being flung into space at almost light speed.<ref>For a review, see {{Harvnb|Begelman|Blandford|Rees|1984}}. To a distant observer, some of these jets even appear to move [[superluminal motion|faster than light]]; this, however, can be explained as an optical illusion that does not violate the tenets of relativity, see {{Harvnb|Rees|1966}}</ref>
General relativity plays a central role in modelling all these phenomena,<ref>For stellar end states, cf. {{Harvnb|Oppenheimer|Snyder|1939}} or, for more recent numerical work, {{Harvnb|Font|2003|loc=sec. 4.1}}; for supernovae, there are still major problems to be solved, cf. {{Harvnb|Buras|Rampp|Janka|Kifonidis|2003}}; for simulating accretion and the formation of jets, cf. {{Harvnb|Font|2003|loc=sec. 4.2}}. Also, relativistic lensing effects are thought to play a role for the signals received from [[X-ray pulsar]]s, cf. {{Harvnb|Kraus|1998}}</ref> and observations provide strong evidence for the existence of black holes with the properties predicted by the theory.<ref>The evidence includes limits on compactness from the observation of accretion-driven phenomena ("[[Eddington luminosity]]"), see {{Harvnb|Celotti|Miller|Sciama|1999}}, observations of stellar dynamics in the center of our own [[Milky Way]] galaxy, cf. {{Harvnb|Schödel|Ott|Genzel|Eckart|2003}}, and indications that at least some of the compact objects in question appear to have no solid surface, which can be deduced from the examination of [[X-ray burst]]s for which the central compact object is either a neutron star or a black hole; cf. {{Harvnb|Remillard|Lin|Cooper|Narayan|2006}} for an overview, {{Harvnb|Narayan|2006|loc=sec. 5}}. Observations of the "shadow" of the Milky Way galaxy's central black hole horizon are eagerly sought for, cf. {{Harvnb|Falcke|Melia|Agol|2000}}</ref>

Black holes are also sought-after targets in the search for gravitational waves (cf. [[#Gravitational waves|Gravitational waves]], above). Merging [[binary black hole|black hole binaries]] should lead to some of the strongest gravitational wave signals reaching detectors here on Earth, and the phase directly before the merger ("chirp") could be used as a "[[standard candle]]" to deduce the distance to the merger events–and hence serve as a probe of cosmic expansion at large distances.<ref>{{Harvnb|Dalal|Holz|Hughes|Jain|2006}}</ref> The gravitational waves produced as a stellar black hole plunges into a supermassive one should provide direct information about the supermassive black hole's geometry.<ref>{{Harvnb|Barack|Cutler|2004}}</ref>