Nico Yunes - Graduate Student
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Physical Address:322 Whitmore Laboratory |
Mailing Address:The Pennsylvania State University |
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Research Interests
Constructing Astrophysically Realistic Initial Data for Numerical Relativistic Simulations
Gravitational wave detection requires precise knowledge of the waveforms produced by strongly gravitating objects, like supermassive black hole binaries. Such precise knowledge of these waveforms can only be achieved via nonlinear numerical simulations, which require astrophysically realistic initial data. Analytical understanding of approximate solutions to the Einstein equations is essential in order to construct such initial data. We are currently investigating the formulation of the initial value problem and the construction of approximate global solutions, in order to obtain a better analytical understanding of astrophysically realistic initial data.
Constructing initial data requires knowledge of global solutions to the Einstein equations. However, there does not exist a closed-form exact global solution to some astrophysically scenarios, such as the 2 body problem of binary black holes. We have concentrated on this physical scenario and constructed approximate global solutions, via asymptotically matching approximate solutions that are valid in different regimes (see figure). In particular, we used a post-Newtonian near zone expansion (1PN), which is valid reltively far away from both black holes, and an inner zone expansion, obtained via black hole perturbation theory, which is valid close to either black hole. From this approximate global solution, we can calculate the lapse, shift and the extrinsic curvature. However, all these initial data is intrinsically piecewise, so we further constructed transition functions to convert these data into an infinitely differntiable set. Not only is this set infinitely differentiable, but since it has been consructed using valid approximatin methods in different regimes, it is also astrophysically realistic.
Testing Alternative Theories of Gravity
Even though General Relativity is the standard model for astrophysics, there exists several alternative competing theories. Tests that place bounds on the validity of these theories, are, hence, essential for progress in science. We have developed tests that could be performed through gravitational wave astronomy. Particular emphasis has been put n scalar tensor theories of the Brans-Dicke type and hypothetical massive graviton theories.
Alternative theories of gravity depend on external parameters (like the mass or angular momentum of the physical system) and on intrinsic parameters, some of which describe how similar the theory is to General Relativity in the strong field regime. Gravitational waves allow us to test alternative theories of gravity by placing upper and lower bounds on these intrinsic parameter. The systems under consideration are mostly inspiralling compact binaries before the time of coalescence. Post-newtonian approximations are used to model the waveforms and, via matched filtering and Information theory, it is possible to look for bounds. Depending on the detector used (LIGO, LISA, GEO, VIRGO, TAMA etc), different physical systems can be studied and yield different bounds. In this manner, we can model how bounds on alternative theories will change as a function of the system observed and the detector used to place them. In particular, we have analyzed how different versions of LISA (different laser power, arm length, etc), could produce different bounds.
N. Yunes and C.M. Will, "Testing Alternative Theories of Gravity using LISA", Class. Quantum Gravity 21, 4367-4383 (2004)
