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1400 |
Afshordi, Niayesh |
Where Will Einstein Fail? |
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Despite being the most successfully tested theory in physics, there are strong theoretical and observational arguments for why General Relativity should fail. It is not a question of if, but rather a question of where and when! I start by summarizing the pathologies in Einstein's theory of gravity, and then attempt to forecast where (and how) we should first observe its failure. This will include cosmology at early times and/or large scales, as well as astrophysical compact objects. |
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1430 |
Narimani, Ali * |
Modified Gravity in Cosmology |
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The increasing precision of cosmological data provides us with an opportunity to test general relativity (GR) on the largest accessible scales. Parameterizing modified gravity models facilitates the systematic testing of the predictions of GR, and gives a framework for detecting possible deviations from it. Several different parameterizations have already been suggested, some linked to classifications of theories, and others more empirically motivated. Here we describe a particular new approach which casts modifications to gravity through two free functions of time and scale, which are directly linked to the field equations, but also easy to confront with observational data. We compare our approach with other existing methods of parameterizing modied gravity, specifically the parameterized post-Friedmann approach and the older method using the parameter set ${mu,gamma}$. We explain the connection between our parameters and the physics that is most important for generating cosmic microwave background anisotropies. Some qualitative features of this new parameterization, and therefore modifications to the gravitational equations of motion, are illustrated in a toy model, where the two functions are simply assumed to be constant parameters. |
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1445 |
Foucart, Francois |
Numerical simulations of black hole-neutron star mergers |
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Black hole-neutron star binaries are among the sources of gravitational waves that will soon be observed by the Advanced LIGO/VIRGO/KAGRA detector network. Their merger can also be accompanied by the emission of observable electromagnetic signals (short gamma-ray bursts, kilonovae, radio afterglows), which could provide important constraints on the parameters of the binary and of its environment. The existence and characteristics of electromagnetic counterparts is indeed strongly dependent on the characteristics of the binary (mass ratio, black hole spin, neutron star radius), and the density of the surrounding interstellar medium. In this talk, I will present numerical simulations of black hole-neutron star mergers across the most likely range of binary parameters, and show how they help us predict the outcome of these mergers. In particular, I will show that for the most likely black hole masses, electromagnetic counterparts are only possible for rapidly rotating black holes, while for non-spinning black holes, black hole-neutron star mergers are nearly indistinguishable from binary black hole mergers. |
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1500 |
MacDonald, Ilana * |
The Suitability of Hybrid Waveforms for Advanced Gravitational Wave Detectors |
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General relativity predicts that the coalescence of two compact objects, such as black holes, will produce gravitational waves (GWs). Detectors like Advanced LIGO are expected to measure such events within the next few years. In order to be able to characterize the GWs they measure, these detectors require accurate waveform models, which can be constructed by fusing an analytical post-Newtonian (PN) inspiral waveform with a numerical relativity (NR) late-inspiral-merger-ringdown waveform. NR, though the most accurate model, is computationally expensive: the longest simulations to date taking several months to run. PN theory is easily computed but becomes increasingly inaccurate near merger. Because of this, it is important to determine the optimal length of the NR waveform, while maintaining the necessary accuracy for GW detectors. We present a study of the sufficient accuracy of PN and NR waveforms for the most demanding usage case: parameter estimation of strong sources in advanced GW detectors. We perform a comprehensive analysis of errors that enter such “hybrid waveforms” in the case of unequal mass non-spinning binaries. We also explore the possibility of using these hybrid waveforms as a detection template bank for Advanced LIGO. |
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1515 |
Cyr-Racine, Francis-Yan |
How Much do we Really Know about Neutrinos? Limits on Neutrino-Neutrino Scattering in the Early Universe. |
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In the standard model neutrinos are assumed to have streamed across the Universe since they last scattered at the weak decoupling epoch when the temperature of the standard-model plasma was near 1 MeV. The shear stress of free-streaming neutrinos imprints itself gravitationally on the Cosmic Microwave Background (CMB) and makes the CMB a sensitive probe of neutrino scattering. Yet, the presence of nonstandard physics in the neutrino sector may alter this standard chronology and delay neutrino free-streaming until a much later epoch. We use observations of the CMB to constrain the strength of neutrino self-interactions Gν and put limits on new physics in the neutrino sector from the early Universe. Recent measurements of the CMB at large multipoles made by the Planck satellite and high-l experiments are critical for probing this physics. Within the context of conventional ΛCDM parameters, cosmological data are compatible with Gν < 1/(35MeV)^2 and neutrino free-streaming might be delayed until the Universe has cooled to as low as ∼35 eV. Intriguingly, we also find an alternative cosmology compatible with cosmological data in which neutrinos scatter off each other until z ∼ 9000 with a preferred interaction strength in a narrow region around Gν ≃ 1/(9MeV)^2. This distinct self-interacting neutrino cosmology is characterized by somewhat lower values of both the scalar spectral index and the amplitude of primordial fluctuations. While we phrase our discussion here in terms of a specific scenario in which a late onset of neutrino free-streaming could occur, our constraints on the neutrino visibility function are very general. |
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