4,820 research outputs found
Surface singularities in Eddington-inspired Born-Infeld gravity
Eddington-inspired Born-Infeld gravity was recently proposed as an
alternative to general relativity that offers a resolution of spacetime
singularities. The theory differs from Einstein's gravity only inside matter
due to nondynamical degrees of freedom, and it is compatible with all current
observations. We show that the theory is reminiscent of Palatini f(R) gravity
and that it shares the same pathologies, such as curvature singularities at the
surface of polytropic stars and unacceptable Newtonian limit. This casts
serious doubts on its viability.Comment: 5 pages. v2: minor corrections to match published versio
Gravitational fields with sources, regular black holes, quasiblack holes, and analogue black holes
We discuss recent developments in gravitational fields with sources, regular
black holes, quasiblack holes, and analogue black holes, related to the talks
presented at the corresponding Parallel Session AT3 of the 13th Marcel
Grossmann Meeting.Comment: Report of the Parallel Session AT3 at the Marcel Grossmann Meeting
13, Stockholm 2012, Proceedings of the Conference. 9 page
Quasinormal Modes Beyond Kerr
The quasinormal modes (QNMs) of a black hole spacetime are the free, decaying
oscillations of the spacetime, and are well understood in the case of Kerr
black holes. We discuss a method for computing the QNMs of spacetimes which are
slightly deformed from Kerr. We mention two example applications: the
parametric, turbulent instability of scalar fields on a background which
includes a gravitational QNM, and the shifts to the QNM frequencies of Kerr
when the black hole is weakly charged. This method may be of use in studies of
black holes which are deformed by external fields or are solutions to
alternative theories of gravity.Comment: Proceedings of the Sant Cugat Forum on Astrophysics (2014). Session
on 'Gravitational Wave Astrophysics.' 7 page
Gravity with Auxiliary Fields
Modifications of General Relativity usually include extra dynamical degrees
of freedom, which to date remain undetected. Here we explore the possibility of
modifying Einstein's theory by adding solely nondynamical fields. With the
minimal requirement that the theory satisfies the weak equivalence principle
and admits a covariant Lagrangian formulation, we show that the field equations
generically have to include higher-order derivatives of the matter fields. This
has profound consequences for the viability of these theories. We develop a
parametrization based on a derivative expansion and show that - to next to
leading order - all theories are described by just two parameters. Our approach
can be used to put stringent, theory-independent constraints on such theories,
as we demonstrates using the Newtonian limit as an example.Comment: 5 pages, no figures; v2: clarifications and minor improvements,
matches published versio
Black holes with surrounding matter in scalar-tensor theories
We uncover two mechanisms that can render Kerr black holes unstable in
scalar-tensor gravity, both associated to the presence of matter in the
vicinity of the black hole and the fact that this introduces an effective mass
for the scalar. Our results highlight the importance of understanding the
structure of spacetime in realistic, astrophysical black holes in scalar-tensor
theories.Comment: 5 pages. Published version. arXiv admin note: substantial text
overlap with arXiv:1305.693
Testing strong gravity with gravitational waves and Love numbers
The LIGO observation of GW150914 has inaugurated the gravitational-wave astronomy era and the possibility of testing gravity in extreme regimes. While distorted black holes are the most convincing sources of gravitational waves, similar signals might be produced also by other compact objects. In particular, we discuss what the gravitational-wave ringdown could tell us about the nature of the emitting object, and how measurements of the tidal Love numbers could help us in understanding the internal structure of compact dark objects
Matter around Kerr black holes in scalar-tensor theories: scalarization and superradiant instability
In electrovacuum stationary, asymptotically flat black holes in scalar-tensor
theories of gravity are described by the Kerr-Newman family of solutions, just
as in general relativity. We show that there exist two mechanisms which can
render Kerr black holes unstable when matter is present in the vicinity of the
black hole, as this induces an effective mass for the scalar. The first
mechanism is a tachyonic instability that appears when the effective mass
squared is negative, triggering the development of scalar hair --- a black hole
version of "spontaneous scalarization". The second instability is associated
with superradiance and is present when the effective mass squared is positive
and when the black hole spin exceeds a certain threshold. The second mechanism
is also responsible for a resonant effect in the superradiant scattering of
scalar waves, with amplification factors as large as 10^5 or more.Comment: 11 pages, 3 figures. v2: published versio
Searches for direct pair production of third generation squarks with the ATLAS detector
Naturalness arguments for weak-scale supersymmetry favour supersymmetric partners of the third generation quarks with masses not too far from those of their Standard Model counterparts. Top or bottom squarks with masses less than a few hundred GeV can also give rise to direct pair production rates at the LHC that can be observed in the data sample recorded by the ATLAS detector. The talk presents recent ATLAS results from searches for direct stop and sbottom pair production based on 20:7f
To the bottom of the stop: calibration of bottom-quark jets identification algorithms and search for scalar top-quarks and dark matter with the Run I ATLAS data
In the first part of this thesis, the results of a calibration of bottom quark jets identification algorithms are reported. The analysis is performed with 5 fb of proton-proton collisions at 7 TeV centre-of-mass energy recorded by the ATLAS detector at the LHC. A b-jet enriched sample from fully reconstructed was used to validate the Monte Carlo simulation of b-jets properties and identification algorithms to the experimental data. In the second part of the thesis, the results of a search for top squark pair production in final states with one isolated lepton, jets, and missing transverse momentum are reported. The search is based on 20 fb of pp collisions at 8 TeV centre-of-mass energy recorded by the ATLAS detector. No significant excess over the Standard Model prediction is observed. The results were interpreted in terms of exclusion limits on a wide range of scenarios with different decay modes of the top squark and mass assumptions for the top squark and the other particles considered in the decay chain. Finally, the search was interpreted in terms of dark matter plus top pairs associate production. Limits were derived on dark matter nucleon scattering cross section for two contact operators (D1 and C1) in the context of dark matter effective field theory
Stochastic gravitational-wave background at 3G detectors as a smoking gun for microscopic dark matter relics
Microscopic horizonless relics could form in the early universe either
directly through gravitational collapse or as stable remnants of the Hawking
evaporation of primordial black holes. In both cases they completely or
partially evade cosmological constraints arising from Hawking evaporation and
in certain mass ranges can explain the entirety of the dark matter. We
systematically explore the stochastic gravitational-wave background associated
with the formation of microscopic dark-matter relics in various scenarios,
adopting an agnostic approach and discussing the limitations introduced by
existing constraints, possible ways to circumvent the latter, and expected
astrophysical foregrounds. Interestingly, this signal is at most marginally
detectable with current interferometers but could be detectable by
third-generations instruments such as the Einstein Telescope, strengthening
their potential as discovery machines.Comment: 11 pages, 4 figure
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