TOWARDS PROBING THE STRONG FIELD GRAVITY USING BINARY BLACK-HOLE RINGDOWNS

In this thesis, we study black hole ringdowns as a probe to understand the strong-field clssical gravity. When two black holes merge, they form a single distorted black hole that then radiates gravitational waves and settles into a final stable Kerr state. The signal produced during this process is called the ringdown and can be modelled as perturbations on the space-time of the final Kerr black hole. Ringdowns contain information about the strong field dynamics close to the black holes and thus, can be used to test our understanding of gravity. There are three specific questions that we explore in this thesis: First, at what point after the merger of two black holes can one use a perturbative description to model the space-time? This tells where in the gravitational wave signal of a binary black hole merger can one start a ringdown-based test. Second, how likely are we to realistically find a signal that allows us to perform a ringdown-based test given the current and future gravitational wave observatories. And finally we discuss, how should the data analysis be carried out in order to extract information from the ringdown of an observed GW signal

The cosmic variance of testing general relativity with gravitational-wave catalogs

Combining multiple gravitational-wave observations allows for stringent tests of general relativity, targeting effects that would otherwise be undetectable using single-event analyses. We show that the finite size of the observed catalog induces a significant source of variance. If not appropriately accounted for, general relativity can be excluded with arbitrarily large credibility even if it is the underlying theory of gravity. This effect is generic and entirely analogous to the so-called "cosmic variance" of cosmology: in essence, we only have one catalog that contains all the events. We show that the cosmic variance holds for arbitrarily large catalogs and cannot be suppressed by selecting "golden" observations with large signal-to-noise ratios. We present a mitigation strategy based on bootstrapping (i.e. resampling with repetition) that allows assigning uncertainties to one's credibility on the targeted test. We demonstrate our findings using both toy models and real gravitational-wave data. In particular, we quantify the impact of the cosmic variance on the ringdown properties of black holes using the latest LIGO/Virgo catalog.Comment: 6 pages, 2 figure

Charging up India’s electric vehicles

India’s ambitious electric mobility targets are highly dependent on the availability of robust charging infrastructure and readiness of the power system to integrate the additional flexible EV load. Indian policymakers at state and national level are proactively pursuing actions for developing EV charging infrastructure (EV service providers) on supply-side and EV users’ (demand side). Further enhancements for supply-side can come from the role of distribution companies (DISCOMs), tariff design, incentives, permitting processes and data privacy, and on the demand side from payment methods, minimum facilities, charging station user registration and consumer complaints. EV charging business in India is at its early stage, and it has a large scope for business model innovation. As EV penetration increases and market grows, innovations can be expected in the areas of service provision, partnership and pricing. EV load can increase peak demand and distribution grid congestion. Solutions are emerging to avoid more investment in generation and network capacity such as time-varying tariff and flexibility measures taken by the DISCOM to deal with these issues. V2X is still in an early stage but would become relevant as the market matures. The accuracy in predicting the availability of V2X resource and minimising market entry barriers for V2X service provision can be improved through aggregation, allowing smaller minimum bid volumes and contract periods, asymmetric products and shorter lead times. The search for the most appropriate solutions would benefit from regulatory sandboxes both at the national and state level

Analytical model for gravitational-wave echoes from spinning remnants

Gravitational-wave echoes in the postmerger signal of a binary coalescence are predicted in various scenarios, including near-horizon quantum structures, exotic states of matter in ultracompact stars, and certain deviations from general relativity. The amplitude and frequency of each echo is modulated by the photon-sphere barrier of the remnant, which acts as a spin- and frequency-dependent high-pass filter, decreasing the frequency content of each subsequent echo. Furthermore, a major fraction of the energy of the echo signal is contained in low-frequency resonances corresponding to the quasinormal modes of the remnant. Motivated by these features, in this work we provide an analytical gravitational-wave template in the low-frequency approximation describing the postmerger ringdown and the echo signal of a spinning ultracompact object. Besides the standard ringdown parameters, the template is parametrized in terms of only two physical quantities: the reflectivity coefficient and the compactness of the remnant. We discuss novel effects related to the spin and to the complex reflectivity, such as a more involved modulation of subsequent echoes, the mixing of two polarizations, and the ergoregion instability in the case of perfectly reflecting spinning remnants. Finally, we compute the errors in the estimation of the template parameters with current and future gravitational-wave detectors using a Fisher matrix framework. Our analysis suggests that models with almost perfect reflectivity can be excluded/detected with current instruments, whereas probing values of the reflectivity smaller than 80% at the 3σ confidence level requires future detectors (Einstein Telescope, Cosmic Explorer, LISA). The template developed in this work can easily be implemented to perform a matched-filter based search for echoes and to constrain models of exotic compact objects

Landscape of stellar-mass black-hole spectroscopy with third-generation gravitational-wave detectors

Gravitational-wave black-hole spectroscopy provides a unique opportunity to test the strong-field regime of gravity and the nature of the final object formed in the aftermath of a merger. Here we investigate the prospects for black-hole spectroscopy with third-generation gravitational-wave detectors, in particular the Einstein Telescope in different configurations, possibly in combination with Cosmic Explorer. Using a state-of-the-art population model for stellar-origin binary black holes informed by LIGO-Virgo-KAGRA data, we compute the average number of expected events for precision black-hole spectroscopy using a Fisher-matrix analysis. We find that Einstein Telescope will measure two independent quasinormal modes within ${\cal O}(1)\%$ (resp. ${\cal O}(10)\%$) relative uncertainty for at least ${\cal O}(1)$ (resp. ${\cal O}(500)$) events per year, with similar performances in the case of a single triangular configuration or two L-shaped detectors with same arm length. A 15-km arm-length configuration would improve rates by roughly a factor of two relative to a 10-km arm-length configuration. When operating in synergy with Cosmic Explorer the rates will improve significantly, reaching few-percent accuracy for ${\cal O}(100)$ events per year.Comment: 13 pages, 9 figure

Spectroscopy of binary black hole ringdown using overtones and angular modes

The black hole uniqueness and the no-hair theorems imply that the quasinormal spectrum of any astrophysical black hole is determined solely by its mass and spin. The countably infinite number of quasinormal modes of a Kerr black hole are thus related to each other and any deviations from these relations provide a strong hint for physics beyond the general theory of relativity. To test the no-hair theorem using ringdown signals, it is necessary to detect at least two quasinormal modes. In particular, one can detect the fundamental mode along with a subdominant overtone or with another angular mode, depending on the mass ratio and the spins of the progenitor binary. Also in the light of the recent discovery of GW190412, studying how the mass ratio affects the prospect of black hole spectroscopy using overtones or angular modes is pertinent, and this is the major focus of our study. First, we provide ready-to-use fits for the amplitudes and phases of both the angular modes and overtones as a function of mass ratio $q\in[0,10]$. Using these fits we estimate the minimum signal-to-noise ratio for detectability, resolvability, and measurability of subdominant modes/tones. We find that performing black-hole spectroscopy with angular modes is preferable when the binary mass ratio is larger than $q\approx 1.2$ (provided that the source is not located at a particularly disfavoured inclination angle). For nonspinning, equal-mass binary black holes, the overtones seem to be the only viable option to perform a spectroscopy test of the no-hair theorem. However this would require a large ringdown signal-to-noise ratio ($\approx 100$ for a $5\%$ accuracy test with two overtones) and the inclusion of more than one overtone to reduce modelling errors, making black-hole spectroscopy with overtones impractical in the near future.Comment: 18 pages, 14 figures. Version consistent with PR

Novel Ringdown Amplitude-Phase Consistency Test

The ringdown signal emitted during a binary black hole coalescence can be modeled as a linear superposition of the characteristic damped modes of the remnant black hole that get excited during the merger phase. While checking the consistency of the measured frequencies and damping times against the Kerr BH spectrum predicted by general relativity (GR) is a cornerstone of strong-field tests of gravity, the consistency of measured excitation amplitudes and phases have been largely left unexplored. For a nonprecessing, quasicircular binary black hole merger, we find that GR predicts a narrow region in the space of mode amplitude ratio and phase difference, independently of the spin of the binary components. Using this unexpected result, we develop a new null test of strong-field gravity which demands that the measured amplitudes and phases of different ringdown modes should lie within this narrow region predicted by GR. We call this the amplitude-phase consistency test and introduce a procedure for performing it using information from the ringdown signal. Lastly, we apply this test to the GW190521 event, using the multimodal ringdown parameters inferred by Capano et al. [arXiv:2105.05238]. While ringdown measurements errors for this event are large, we show that GW190521 is consistent with the amplitude-phase consistency test. Our test is particularly well suited for accommodating multiple loud ringdown detections as those expected in the near future, and can be used complementarily to standard black-hole spectroscopy as a proxy for modified gravity, compact objects other than black holes, binary precession and eccentricity