Conference

The binary black hole merger events recently discovered by the LIGO and Virgo Collaboration offer us excellent testbeds for exploring extreme (strong and dynamical-field) gravity that was previously inaccessible. In this talk, I will first explain the current status of probing fundamental pillars of General Relativity using the inspiral part of the gravitational waveform. I will next describe how well one can constrain one type of quantum black holes, collapsed polymers, with the GW150914 ringdown. I will conclude with a list of important open problems.

The noise dominated nature of the gravitational wave detectors requires an assessment of the noise background in the search for astrophysical signals. Starting with a frequentist approach, the original analysis used about 16 seconds of data after the merger signal to find how frequently random noise mimics the expected signal. We present the results of extending the background estimation to 4096 seconds of public LIGO data and discuss the concerns arising from subtleties in the analysis for the long and self-similar echo templates.

Exotic compact objects (e.g. boson stars, dark matter stars, gravastars), and certain quantum modifications to black holes (e.g. firewalls) are speculated to give out ``echoes'' or bursts of radiation appearing at regular time intervals due to a perturbation by any infalling matter or field. In particular, these echoes are also expected to appear soon after their formation. The presence (or absence) of gravitational-wave echoes following observations of coalescences of compact binaries by detectors like Advanced LIGO and Virgo, might be able to directly probe (or constrain) the nature of the remnant compact object. For a large class of these objects, the echoes are expected to appear at time scales that are amenable to a search. However, there can be a substantial variation in the detailed waveform models, and this might make a template-based search inffective. We propose and demonstrate a model-independent search method that relies only on the constancy of the time difference between subsequent echoes.

The recent detections of merging black holes allow for observational tests of the nature of these objects, such as searching for the GW echo signals proposed in some models. Tentative evidence for these was presented, found in an analysis based upon methods for GW data analysis as demonstrated on the Ligo Open Science Center. We present the results of characterising these method's behaviour when applied to the specific form of the echo signals, and address problems and improvements based on our findings.

In classical General Relativity (GR), an observer falling into an astrophysical black hole is not expected to experience anything dramatic as she crosses the event horizon. However, tentative resolutions to problems in quantum gravity, such as the cosmological constant problem, or the black hole information paradox, invoke significant departures from classicality in the vicinity of the horizon. It was recently pointed out that such near-horizon structures can lead to late-time echoes in the black hole merger gravitational wave signals that are otherwise indistinguishable from GR. We search for observational signatures of these echoes in the gravitational wave data released by advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), following the three black hole merger events GW150914, GW151226, and LVT151012. In particular, we look for repeating damped echoes with time-delays of 8MlogM (+spin corrections, in Planck units), corresponding to Planck-scale departures from GR near their respective horizons. Accounting for the "look elsewhere" effect due to uncertainty in the echo template, we find tentative evidence for Planck-scale structure near black hole horizons at false detection probability of 1% (corresponding to 2.5σ significance level). We also report the results of same search for echoes in the new black hole merger event GW170104. Future observations from interferometric detectors at higher sensitivity, along with more physical echo templates, will be able to confirm (or rule out) this finding, providing possible empirical evidence for alternatives to classical black holes, such as in firewall or fuzzball paradigms.