<div dir="ltr">Dear All,<br><br>next week there will be two gravity-theory webinars:<div><br></div><div><b>1) </b><b>Nico Sanchis-Gual (IST, Lisbon):</b> "<b>Light in the dark: GW190521 as a Proca star merger"</b><b> </b></div><div><div><b>Mon</b> (<b>Nov 30</b><b>th <a class="gmail_plusreply" id="plusReplyChip-0">@</a> noon</b>)</div><div><br>The detections of gravitational waves are opening a new window to the Universe. The nature of black holes and neutron stars may now be unveiled, but gravitational radiation may also lead to exciting discoveries of new exotic compact objects, oblivious to electromagnetic waves. In particular, Advanced LIGO-Virgo recently reported a short gravitational-wave signal (GW190521) interpreted as a quasi-circular merger of black holes, one at least populating the pair-instability supernova gap. We found that GW190521 is also consistent with numerically simulated signals from head-on collisions of two (equal mass and spin) horizonless vector boson stars (aka Proca stars). This provides the first demonstration of close degeneracy between these two theoretical models, for a real gravitational-wave event.</div><div><br></div><div><b>2) Horng Sheng Chia (Princeton, US): "Tidal Deformation and Dissipation of Rotating Black Holes"</b></div><div><b>Wed (Dec, 2nd @ 3pm) </b></div><br>Black holes are never isolated in realistic astrophysical environments; instead, they are often perturbed by complicated external tidal fields. How does a black hole respond to these tidal perturbations? In this talk, I will discuss both the conservative and dissipative responses of the Kerr black hole to a weak and adiabatic gravitational field. The former describes how the black hole would change its shape due to these tidal interactions, and is quantified by the so-called “Love numbers”. On the other hand, the latter describes how energy and angular momentum are exchanged between the black hole and its tidal environment due to the absorptive nature of the event horizon. I will show that the Love numbers of the Kerr black hole vanish identically — in other words, you cannot stretch a black hole. I will also describe how the Kerr black hole's dissipative response implies that energy and angular momentum can either be lost to or extracted from the black hole, with the latter process commonly known as the black hole superradiance. I will end by discussing how these tidal responses leave distinct imprints on the gravitational waves emitted by binary black holes. Time permitting, I will also argue that the ambiguity issue of Love numbers in General Relativity can be resolved through a careful consideration of the boundary condition at asymptotic infinity. <div><br></div><div><br></div><div><div>For both seminars the Zoom link is: <a href="https://uniroma1.zoom.us/j/81283707469?pwd=SzkyMk5jRm0xcEN3WE5MYVh1RVFtQT09" target="_blank">https://uniroma1.zoom.us/j/81283707469?pwd=SzkyMk5jRm0xcEN3WE5MYVh1RVFtQT09</a> </div><div><div>ID meeting: 812 8370 7469<br>Passcode: 915450</div><div><br></div><div>Calendar of the future gravity theory seminars: <a href="https://calendar.google.com/calendar/u/1?cid=Y191MGo3bG9pYzE1bjBxdGFhN3A4MTlzNDg2Y0Bncm91cC5jYWxlbmRhci5nb29nbGUuY29t" target="_blank">https://calendar.google.com/calendar/u/1?cid=Y191MGo3bG9pYzE1bjBxdGFhN3A4MTlzNDg2Y0Bncm91cC5jYWxlbmRhci5nb29nbGUuY29t</a></div></div><div><br></div><div>Cheers,</div><div>Paolo</div></div><div><br></div>-- <br><div dir="ltr" data-smartmail="gmail_signature"><div dir="ltr">Paolo Pani <br>Associate Professor<br>ERC StG Fellow, DarkGRA project<br>Junior Fellow, Scuola Superiore di Studi Avanzati Sapienza<br><br>Physics Department (Marconi Building), 3rd floor, room 334<br>Sapienza University of Rome<br>Piazzale Aldo Moro 5 - 00185 Rome (Italy) <br>+39 06 4991 4472 (int 24472)</div></div></div></div>
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