SEMPARIS – Séminaires en région parisienne Séminaire du

SEMPARIS – Séminaires en région parisienne
http ://string.lpthe.jussieu.fr/semparis/
Séminaire du GReCO : groupe de
GRavitation et COsmologie à l’IAP
Lundi 21 Mars 2016, 11 :00
IAP, Musco
Domaines : astro-ph
Titre : Causal Nature and Dynamics of Trapping Horizons in Black Hole
Collapse and Cosmology
Orateur : Ilia ( Lab. Univers et Théories (LUTH), Obs. ParisMeudon-Nancay )
Résumé : In calculations of gravitational collapse to form black holes, trapping horizons (foliated by marginally trapped surfaces) make their first appearance either within the collapsing matter or where it joins on to a vacuum
exterior. Those which then move outwards with respect to the matter have
been proposed for use in defining black holes, replacing the global concept
of an “event horizon” which has some serious drawbacks for practical applications. We focus here on studying the properties of trapping horizons within spherical symmetry (which gives some simplifications while retaining the
most essential general features). Their locations are then given by exactly the
same condition (R = 2M ) as for the event horizon in the vacuum Schwarzschild metric, and the same condition also applies for cosmological trapping
horizons. We have investigated the causal nature of these horizons (i.e. whether they are spacelike, timelike or null), making contact with the MisnerSharp formalism, which has often been used for numerical calculations of
spherical collapse. We follow two different approaches, one using a geometrical quantity related to the expansion of null geodesic congruences, and the
other using the horizon velocity measured with respect to the collapsing (or
expanding) matter. Simple expressions are found for each of these in terms
of local fluid parameters, and the connection between them allows a full description of the possible behaviours, depending on the initial density profile
and the equation of state. After revisiting the FLRW universe model and the
pressureless Oppenheimer-Snyder collapse model in the light of this, we have
carried out numerical simulations for stellar collapse with non-zero pressure,
making contact with pioneering calculations from the 1960s where some features of the emergence and subsequent behaviour of trapping horizons could
already be seen.
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