# UMD ASTR 498 - Frontiers: Observational Signatures of Strong Gravity (7 pages)

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**View the full content.**## Frontiers: Observational Signatures of Strong Gravity

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## Frontiers: Observational Signatures of Strong Gravity

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Lecture Notes

- Pages:
- 7
- School:
- University of Maryland, College Park
- Course:
- Astr 498 - Special Problems In Astronomy

**Unformatted text preview: **

Frontiers Observational Signatures of Strong Gravity As we said a few lectures ago general relativity is in a unique position among theories of fundamental interactions because of the relative weakness of gravity One can for example probe EM or strong weak interactions using particle accelerators and by this can test the predictions of these theories in relatively extreme environments But experimental laboratory tests of GR predictions are limited to weak gravity These include things like the gravitational redshift of light light deflection by the Sun delays of radio waves and GR precession of planets However GR corrections are typically of order M r compared to the Newtonian predictions This is very small in things to which we have access for example M r 2 10 6 for the Sun and M r 10 9 for the Earth Even for signals from binary pulsars it is their separation of 1011 cm that matters so again M r 1 Therefore many of the predictions of GR in strong gravity are untested experimentally Since these predictions are used to model all black holes and neutron stars the actual behavior of gravity in these regimes is very important Here s an example Suppose that black holes are pseudo Newtonian in the sense that they have horizons but no ISCO Therefore gas will spiral in nearly circular orbits right down to the horizon then get sucked in This means that they will release 50 of their mass energy as they spiral Ask class how would we use this plus the Eddington luminosity to estimate how long it would take a black hole to grow in mass Since LE is the maximum luminosity of accretion the maximum accretion rate is M E LE c2 which is 3 1017 g s 1 M M or 2 2 108 yr for an e folding time If black holes are originally formed with roughly stellar masses 10 100 M then they need more than 10 e foldings to reach supermassive status This would take 2 3 billion years so we wouldn t expect any AGN at z 4 5 even if the black holes all accrete at Eddington This would pose problems In contrast

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