Tue03Apr20184:00 pmLewis Hall 101
Colloquium: Presentations by Graduate Students
Mir Emad Aghili, Vishal Baibhav, and Shrobana Ghosh
Department of Physics and Astronomy
University of Mississippi
Presentations by Graduate Students
"Manifoldlikeness of Causal Sets" by Mir Emad Aghili
Abstract: We study the distribution of maximal-chain lengths between two elements of a causal set, and its relationship with the embeddability of the causal set in a region of flat spacetime. We start with causal sets obtained from uniformly distributed points in Minkowski space. After some general considerations we focus on the 2-dimensional case and derive expressions for the expected number nk of maximal chains as a function of their length k, the most probable maximal-chain length k0, and the width Δ of the length distribution, as functions of the number N of causal set elements in the interval between the two points. These results, together with the results of numerical simulations of causal sets embedded in Minkowski space of various dimensionalities, show that for a given N the values of k0 and Δ can be used to estimate the dimensionality of a causal set embeddable in Minkowski space. Other dimension estimators are known for manifoldlike causal sets, but the length distribution also gives us a way to evaluate the embeddability of a causal set. We provide a first test of manifoldlikeness based on k0 and Δ, and end with a few simple examples of nk distributions for non-manifoldlike causal sets.
"Systematic Errors and Energy Estimates in Binary Black Hole Ringdown" by Vishal Baibhav
Abstract: High signal-to-noise ratio gravitational wave observations will enable us to measure the quasinormal frequencies of binary black hole merger remnants. In general relativity, these frequencies depend only on the remnant's mass and spin, so they can be used to test general relativity and the Kerr nature of the remnant. To carry out these tests, systematic errors must be subdominant with respect to statistical errors. I'll talk about how accurately ringdown frequencies can be extracted from state-of-the-art numerical simulations from the Simulating eXtreme Spacetimes (SXS) catalog. I'll also present some results on the relative excitation of different quasinormal modes. To quantify these excitations, one must define a suitable “starting time“, e.g. by maximizing the energy content “parallel” to a quasinormal mode (as suggested by Nollert). We used Nollert's method to quantify the energy radiated in quasinormal modes for aligned-spin binaries, and we produced post-Newtonian inspired fits of the resulting energy estimates.
"Detectability of Gravitational Radiation from Superradiant Instabilities" by Shrobana Ghosh
Abstract: An incident wave, when scattered off a black hole may get amplified, at the expense of the rotational energy of the black hole. This process is known as superradiance and due to this rotating black holes can serve as particle detectors. For a massive field, the mass of the field helps in confining the field. Therefore, even an ultralight bosonic field can form a non-axisymmetric cloud around the black hole due to repeated amplification from the black hole. This leads to emission of gravitational radiation that can be detected by ground-based or space-based gravitational wave detectors, depending on the mass of the boson. Based on astrophysical models we show that adLIGO should see 104 events in a 4 year mission for a scalar field mass of 3×10-12 eV, while LISA will see about 103 events in a 4 year mission for a scalar field mass of 10-17 eV. In the absence of detection at a particular detector, we can rule out the corresponding mass range of the scalar field. We also look at the detectability of such events from the remnants of the merger events already seen by adLIGO at the present and future ground-based detectors.