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SUMMARY:David Kubiznak (Perimeter Institute)
DTSTART:20200430T170000Z
DTEND:20200430T180000Z
DTSTAMP:20260404T110743Z
UID:PIStrongGravity/1
DESCRIPTION:Title: <a href="https://stable.researchseminars.org/talk/PIStr
 ongGravity/1/">On Gauss-Bonnet gravity in four dimensions</a>\nby David Ku
 biznak (Perimeter Institute) as part of PI Strong Gravity\n\n\nAbstract\nW
 e comment on the recently introduced Gauss-Bonnet gravity in four dimensio
 ns. We argue that it does not make sense to consider this theory to be def
 ined by a set of D->4 solutions of the higher-dimensional Gauss-Bonnet gra
 vity. We show that a well-defined D->4 limit of Gauss-Bonnet Gravity is ob
 tained generalizing a method employed by Mann and Ross to obtain a limit o
 f the Einstein gravity in D=2 dimensions. This is a scalar-tensor theory o
 f the Horndeski type obtained by dimensional reduction methods. By conside
 ring simple spacetimes beyond spherical symmetry (Taub-NUT spaces) we show
  that the naive limit of the higher-dimensional theory to four dimensions 
 is not well defined and contrast the resultant metrics with the actual sol
 utions of the new theory. Theory and solutions in lower dimensions will al
 so be briefly discussed.\n
LOCATION:https://stable.researchseminars.org/talk/PIStrongGravity/1/
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BEGIN:VEVENT
SUMMARY:Maarten van de Meent (Albert Einstein Institute)
DTSTART:20200514T170000Z
DTEND:20200514T180000Z
DTSTAMP:20260404T110743Z
UID:PIStrongGravity/2
DESCRIPTION:Title: <a href="https://stable.researchseminars.org/talk/PIStr
 ongGravity/2/">The small mass-ratio expansion of the relativistic two body
  problem</a>\nby Maarten van de Meent (Albert Einstein Institute) as part 
 of PI Strong Gravity\n\n\nAbstract\nTo predict the gravitational waves emi
 tted by a black hole binary\, one needs to understand the dynamics of the 
 binary in general relativity. No closed form solutions of this problem exi
 st. Instead one must introduce some form of approximation. One such approx
 imation\, can be made if one of the components is much heavier than the ot
 her\, suggesting a perturbative expansion in the mass-ratio. I will review
  this small mass-ratio (SMR) expansion of the dynamics\, and the progress 
 that has been made over the last two decades. In particular\, I will discu
 ss some recent results on the convergence of this series\, suggesting that
  at relatively low orders this SMR expansion can be used to model even equ
 al mass binaries.\n
LOCATION:https://stable.researchseminars.org/talk/PIStrongGravity/2/
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BEGIN:VEVENT
SUMMARY:Hang Yu (Caltech)
DTSTART:20200528T170000Z
DTEND:20200528T180000Z
DTSTAMP:20260404T110743Z
UID:PIStrongGravity/3
DESCRIPTION:Title: <a href="https://stable.researchseminars.org/talk/PIStr
 ongGravity/3/">Nonlinear dynamical tides in white dwarf binaries</a>\nby H
 ang Yu (Caltech) as part of PI Strong Gravity\n\n\nAbstract\nCompact white
  dwarf (WD) binaries are important sources for space-based gravitational-w
 ave (GW) observatories\, and an increasing number of them are being identi
 fied by surveys like ELM and ZTF. We study the effects of nonlinear dynami
 cal tides in such binaries. We focus on the global three-mode parametric i
 nstability and show that it has a much lower threshold energy than the loc
 al wave-breaking condition studied previously. By integrating networks of 
 coupled modes\, we calculate the tidal dissipation rate as a function of o
 rbital period. We construct phenomenological models that match these numer
 ical results and use them to evaluate the spin and luminosity evolution of
  a WD binary. While in linear theory the WD's spin frequency can lock to t
 he orbital frequency\, we find that such a lock cannot be maintained when 
 nonlinear effects are taken into account. Instead\, as the orbit decays\, 
 the spin and orbit go in and out of synchronization.  Each time they go ou
 t of synchronization\, there is a brief but significant dip in the tidal h
 eating rate. While most WDs in compact binaries should have luminosities t
 hat are similar to previous traveling-wave estimates\, a few percent shoul
 d be about ten times dimmer because they reside in heating rate dips. This
  offers a potential explanation for the low luminosity of the CO WD in J06
 51. Lastly\, we consider the impact of tides on the GW signal and show tha
 t LISA and TianGO can constrain the WD's moment of inertia to better than 
 1% for centi-Hz systems.\n
LOCATION:https://stable.researchseminars.org/talk/PIStrongGravity/3/
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