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SUMMARY:David Kubiznak (Perimeter Institute)
DTSTART;VALUE=DATE-TIME:20200430T170000Z
DTEND;VALUE=DATE-TIME:20200430T180000Z
DTSTAMP;VALUE=DATE-TIME:20230925T222957Z
UID:PIStrongGravity/1
DESCRIPTION:Title: On Gauss-Bonnet gravity in four dimensions\nby David Kubiznak
(Perimeter Institute) as part of PI Strong Gravity\n\n\nAbstract\nWe comme
nt on the recently introduced Gauss-Bonnet gravity in four dimensions. We
argue that it does not make sense to consider this theory to be defined by
a set of D->4 solutions of the higher-dimensional Gauss-Bonnet gravity. W
e show that a well-defined D->4 limit of Gauss-Bonnet Gravity is obtained
generalizing a method employed by Mann and Ross to obtain a limit of the E
instein gravity in D=2 dimensions. This is a scalar-tensor theory of the H
orndeski type obtained by dimensional reduction methods. By considering si
mple spacetimes beyond spherical symmetry (Taub-NUT spaces) we show that t
he naive limit of the higher-dimensional theory to four dimensions is not
well defined and contrast the resultant metrics with the actual solutions
of the new theory. Theory and solutions in lower dimensions will also be b
riefly discussed.\n
LOCATION:https://researchseminars.org/talk/PIStrongGravity/1/
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SUMMARY:Maarten van de Meent (Albert Einstein Institute)
DTSTART;VALUE=DATE-TIME:20200514T170000Z
DTEND;VALUE=DATE-TIME:20200514T180000Z
DTSTAMP;VALUE=DATE-TIME:20230925T222957Z
UID:PIStrongGravity/2
DESCRIPTION:Title: The small mass-ratio expansion of the relativistic two body proble
m\nby Maarten van de Meent (Albert Einstein Institute) as part of PI S
trong Gravity\n\n\nAbstract\nTo predict the gravitational waves emitted by
a black hole binary\, one needs to understand the dynamics of the binary
in general relativity. No closed form solutions of this problem exist. Ins
tead one must introduce some form of approximation. One such approximation
\, can be made if one of the components is much heavier than the other\, s
uggesting a perturbative expansion in the mass-ratio. I will review this s
mall mass-ratio (SMR) expansion of the dynamics\, and the progress that ha
s been made over the last two decades. In particular\, I will discuss some
recent results on the convergence of this series\, suggesting that at rel
atively low orders this SMR expansion can be used to model even equal mass
binaries.\n
LOCATION:https://researchseminars.org/talk/PIStrongGravity/2/
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SUMMARY:Hang Yu (Caltech)
DTSTART;VALUE=DATE-TIME:20200528T170000Z
DTEND;VALUE=DATE-TIME:20200528T180000Z
DTSTAMP;VALUE=DATE-TIME:20230925T222957Z
UID:PIStrongGravity/3
DESCRIPTION:Title: Nonlinear dynamical tides in white dwarf binaries\nby Hang Yu
(Caltech) as part of PI Strong Gravity\n\n\nAbstract\nCompact white dwarf
(WD) binaries are important sources for space-based gravitational-wave (GW
) observatories\, and an increasing number of them are being identified by
surveys like ELM and ZTF. We study the effects of nonlinear dynamical tid
es in such binaries. We focus on the global three-mode parametric instabil
ity and show that it has a much lower threshold energy than the local wave
-breaking condition studied previously. By integrating networks of coupled
modes\, we calculate the tidal dissipation rate as a function of orbital
period. We construct phenomenological models that match these numerical re
sults and use them to evaluate the spin and luminosity evolution of a WD b
inary. While in linear theory the WD's spin frequency can lock to the orbi
tal frequency\, we find that such a lock cannot be maintained when nonline
ar effects are taken into account. Instead\, as the orbit decays\, the spi
n and orbit go in and out of synchronization. Each time they go out of sy
nchronization\, there is a brief but significant dip in the tidal heating
rate. While most WDs in compact binaries should have luminosities that are
similar to previous traveling-wave estimates\, a few percent should be ab
out ten times dimmer because they reside in heating rate dips. This offers
a potential explanation for the low luminosity of the CO WD in J0651. Las
tly\, we consider the impact of tides on the GW signal and show that LISA
and TianGO can constrain the WD's moment of inertia to better than 1% for
centi-Hz systems.\n
LOCATION:https://researchseminars.org/talk/PIStrongGravity/3/
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