Supernova Science Center
Adam Burrows (University of Arizona)
Warner A. Miller (LANL)
Christian D. Ott (Universities of Arizona and Heidelberg)
IntroductionEinstein's General Theory of Relativity explains the emission of gravitational waves from time changing gravitational fields (i. e. mass distributions) much like electromagnetic theory does for time changing electromagnetic fields (i. e. charge distributions). As there is (as far as we know today) just one gravitational "charge", there is no dipolarity in gravitation and hence no classical Hertz dipole radiation in a multipole expansion of the time changing gravitational field and only multipole orders starting from the quadrupole order contribute to the radiation field. (MTW)Gravitational wave generation takes places in highly localized areas of space, where (highly aspherical) rapid motions of large quantities of matter are present and hence basic Newtonian gravitation is not anymore valid. However, once the waves are fully formed, i.e. in the local- and far-wave zones, gravitational wave propagation can be described using a linear approximation to General Relativity, the so called Linearized Theory, which gives a wave equation identical to the wave equation encountered in electromagnetic theory. Hence, one can apply on the propagation of gravitational waves the same wave propagation laws/principles as one does for electromagnetic waves - with one very important exception: Gravitational waves are so weakly absorbed by matter that absorption is astrophysically negligible (and only of importance near the Planck era of the big-bang) (Thorne1987) Gravitational waves are weak - no matter how strong a potential source of gravitational waves might be, once its waves are fully formed (i. e. when looking at the radiative local and far zones), they have dimensionless strain amplitudes h very small compared to unity. (illustrated by "Why are Gravitational Waves so weak?", Christian D. Ott, 2001 (PDF) and Thorne1987) Gravitational Waves from Core-Collapse Supernovae
As gravitational waves are so extremely weakly absorbed by matter, they can provide us with insight into regions of space from which no electromagnetic radiation ever reaches us. One of these areas might be the collapsing core of a massive star whose gravitational collapse eventually leads to a supernova explosion (for more information on core collapse supernovae go to: Supernova Science Center at U of A). In the regimes encountered during such events and especially in the inner regions of the collapsing core, matter is almost completely opaque to electromagnetic radiation and only the extremely weakly interacting neutrinos may escape the dying giant star before the actual (electromagnetically visible) explosion is manifest. By looking at the gravitational wave signal from core-collapse supernovae one hopes to learn more about what is occurring during the initial collapse of the iron core to nuclear densities (or subnuclear densities, depending on potentially present stabilizing centrifugal forces due to large angular momentum), the subsequent rebound of the core that stalls as material from the outer core falls in, and the eventual explosion. Of interest are also expected contributions of the escaping neutrinos to the gravitational radiation field. However, since the estimated amplitudes for gravitational waves from most astrophysical sources are below or right at the limit of detectability of gravitational wave detectors such as LIGO, advanced LIGO, GEO 600, LISA and VIRGO, one has a chance to detect a signal only if one can filter it out of the dominant noise, and then only if one knows what to look for. Hence, our goal is to predict the gravitational wave signature from stellar core collapse as precisely as possible, using the numerical data provided by the core-collapse simulations of the Supernova Science Center collaboration and a new, fully three-dimensional algorithm for the extraction of the gravitational wave signatures from simulation data. An overview on what is known so far about the gravitational wave signature of core collapse and the contributing physical processes has been presented by Christian Ott in his recent talk. |
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