2015 saw the first observation of gravitational waves from a binary black hole merger event by the ligo/Virgo Collaborations. Since then, the ligo-Virgo-Kagra collaboration has finished its third observing run, and has detected about 50 gravitational-wave events. These include binary neutron stars — of which one, GW170817, was also observed in electromagnetic radiation — neutron-star black-hole binaries, and finally numerous black-hole binaries. This has enabled many tests of the gravitational interaction and of cosmology, such as tests of General Relativity, the measurement of the Hubble factor today, and constraints on different early universe phenomena.
The science case of the lisa, which will fly in ∼ 2034, is very rich, spanning from astrophysics, to fundamental physics, to cosmology. We work on several aspects of the science that can be done with lisa.
In particular, lisa has a great potential to provide cosmological tests. It can detect a stochastic gravitational wave background (SGWB) from the primordial Universe (which can be sourced e.g. by inflation, phase transitions, topological defects, primordial black holes), which would furnish unique information about the status of the primordial Universe and help discriminating among model.
The gravitational-wave signal from compact binaries measured by lisa (e.g. massive black hole binaries) can also be used to probe the redshift-distance relationship and determine cosmological parameters, either in combination with the coincident detection of an electro-magnetic counterpart, or through statistical identification of the binary host galaxy.
Gravitational waves
The research of our group mainly focuses on probing cosmology with gravitational waves, both with the present data from the ligo/Virgo collaboration, and providing forecasts for future gravitational-wave observatories, in particular lisa (see also Sedda:2019uroSesana:2019vho).
C. Caprini wrote an important and timely review on the stochastic back-ground of gravitational waves (SGWB) of cosmological origin Caprini:2018mtu, which is already a reference for this area of physics.
She also worked extensively on the science case of lisa, for instance on the ability of lisa to detect a SGWB from a first order electroweak phase transition in the early universe Caprini:2019egz.
The electroweak energy scale corresponds indeed to the mHz frequency range, detectable by lisa, and the measurement of a SGWB signal generated at electroweak phase transition epoch in the early universe would bring invaluable information on the fundamental theory describing the universe at this scale, opening up interesting connections with beyond the Standard Model physics, baryogenesis, and dark matter candidates. Exploiting this amazing discovery potential for lisa resides however on the possibility to disentangle the SGWB from other astrophysical foregrounds and from the detector noise Caprini:2019pxz.
Lisa can also be used to probe late-universe cosmology, and in particular its background acceleration. Together with A. Mangiagli, we work on forecasting the ability of lisa to constrain cosmological parameters via coincident observation of the gravitational-wave emission from massive black hole binaries coalescence and of the EM emission associated to the merger and/or to the host galaxy. This entails performing accurate parameter estimation of the binary waveform (to get the luminosity distance and sky-position of the binary) and constructing models for the electromagnetic counterpart at several frequencies (optical, radio, X-ray) as well as building counterparts detection strategies adapted to the different telescopes (lsst, ELT, ska, athena).
D.A. Steer has worked extensively on the signatures of cosmic strings (a type of topological defect) in gravitational wave, both at lisa and ligo/Virgo frequencies. In particular, together with PhD student Pierre Auclair, and using latest data from ligo/Virgo, the most up-to-date constraints on cosmic strings are found in LIGOScientific:2021nrg. Constraints at lisa frequencies were studied in Auclair:2019wcv.
D.A. Steer has also contributed a number of theoretical developments on the subject of cosmic strings. Of particular note is the joint constraint on cosmic strings from both gravitational wave and γ-ray observations Auclair:2019jip.
D.A. Steer has also been involved in the work of the ligo/Virgo collabora tion centered on measuring cosmological parameters with gravitational wave. In particular, together with members of the gravitation group, she contributed to the first measurement of the Hubble constant H0 using GW170817 LIGOScientific:2017adf. Finally, amongst our main results are constraints on modified theories of gravity using gravitational-wave observations Mastrogiovanni:2020gua.
Modified gravity
We have studied modified gravity in the context of dhost theories. These theories can be considered for dark energy, as discussed earlier, but they can also describe, with a different set of parameters, modified gravity in astrophysical bodies.
We have investigated some dhost models in neutron stars in Sakstein:2016oel, showing in particular that one can obtain a higher maximal mass than in general relativity, with the same equation of state for nuclear matter. We have also analysed linear perturbations of stealth Kerr black holes Charmousis:2019fre.
More recently we have studied the linear perturbations of black holes in dhost theories, by introducing a new analytical approach to obtain the asymptotic behaviour of metric and scalar perturbations at spatial infinity and near the horizon. This enables us to identify the boundary conditions that define quasi-normal modes and, in principle, to compute numerically their (complex) frequencies Langlois:2021aji,Langlois:2021xzq.
We have shown that so-called mimetic theories, introduced by Chamseddine and Mukhanov, can be seen as a particular subset of dhost theories Langlois:2018jdg. In a similar vein, dhost theories can be useful to effectively recover some results of loop quantum cosmology Langlois:2017hdf.
Beyond dhost theories, we have investigated theories at the “boundary” of dhost theories: these theories, which we named U-dhost theories, are degenerate
only in the unitary gauge (where the scalar field is uniform) and contain a second scalar degree of freedom, which however does not propagate and can be described as instantaneous DeFelice:2018ewo.
Ref. Crisostomi:2017ugk explores metric theories described by Lagrangians that contain up to second-order derivatives of the metric, as well as scalar-tensor
theories involving couplings between the Riemann tensor and first or second-order derivatives of a scalar field. We used the criterium of degeneracy to select viable theories.
More recently Langlois:2020xbc, we have presented quadratic dhost theories in a novel and remarkably simple formulation, using disformal transformations to rewrite the Lagrangian as the sum of an Einstein-Hilbert term and a few simple geometrical quantities characterising the uniform scalar field hypersurfaces.
Gauge Gravity
The Teleparallel Equivalent to General Relativity (TEGR) theory is a theory of gravity, leading to the same observable predictions as General relativity (GR). Contrary to GR in which the gravity is encoded in the curvature only, that of the Levi-Civita connection, the TEGR encodes gravity in torsion only, through the Weitzenb¨ock connection, the curvature being equal to zero. The TEGR is often described as the gauge theory for the translations group.
With M. Fontanini, M. Le Delliou an Z.-C. Lin (University of Lanzhou, China) we pointed out that the gauge potential in gauge-translation TEGR does not correspond to a connection as required in a gauge theory. We proposed a formulation of TEGR using a Cartan connection - different from the Ehresmann one used in gauge theory of particle physics. This allowed us to describe TEGR, including the coupling to matter, in a coherent mathematical framework and to show how the usual gauge theory formalism has to be modified if one insist to interpret it as a gauge theory for the translation group (see Huguet:2021roy).
P. Auclair (PhD 2018-2021), C. Caprini (DR CNRS), E. Huguet (MCF, U-Paris), D. Langlois (DR CNRS) , A. Mangiagli (Postdoc, 2021-2023), K. Noui ((visiteur associé), H. Roussille, D. Steer (Professeure)