arXiv subjects

Mairi Sakellariadou

Publications and source records attributed to Mairi Sakellariadou.

At least 73 records · Page 4Linked to original sources

arXiv2021

Eliminating the LIGO bounds on primordial black hole dark matter

Primordial black holes (PBHs) in the mass range $(30$--$100)~M_{\odot}$ are interesting candidates for dark matter, as they sit in a narrow window between microlensing and cosmic microwave background constraints. There are however tight constraints from the binary merger rate observed by the LIGO and Virgo experiments. In deriving these constraints, PBHs were treated as point Schwarzschild masses, while the more careful analysis in an expanding universe we present here, leads to a time-dependent mass. This implies a stricter set of conditions for a black hole binary to form and means that black holes coalesce much more quickly than was previously calculated, namely well before the LIGO/Virgo's observed mergers. The observed binaries are those coalescing within galactic halos, with a merger rate consistent with data. This reopens the possibility for dark matter in the form of LIGO-mass PBHs.

astro-ph.CO↗

arXiv2021

Constraints on extended gravity models through gravitational wave emission

Using recent experimental results of detection of gravitational waves from the binary black hole signals by Advanced LIGO and Advanced Virgo, we investigate the propagation of gravitational waves in the context of fourth order gravity nonminimally coupled to a massive scalar field. Gravitational radiation admits extra massive modes of oscillation and we assume that the amplitude of these modes is comparable to that of the massless mode. We derive the propagation equation and effective mass for each degree of freedom and we infer, from the current observational data, constraints on the free parameters of the gravity models we considered. In particular, for $f(R)=R-R^2/R_0 $, the constraint obtained from the speed of gravitational waves is not compatible with the one set by Solar System tests, which implies that amplitude of the massive modes could not be detectable with current experiments on Earth

gr-qc↗

arXiv2020

Simultaneous estimation of astrophysical and cosmological stochastic gravitational-wave backgrounds with terrestrial detectors

The recent Advanced LIGO and Advanced Virgo joint observing runs have not claimed a stochastic gravitational-wave background detection, but one expects this to change as the sensitivity of the detectors improves. The challenge of claiming a true detection will be immediately succeeded by the difficulty of relating the signal to the sources that contribute to it. In this paper, we consider backgrounds that comprise compact binary coalescences and additional cosmological sources, and we set simultaneous upper limits on these backgrounds. We find that the Advanced LIGO, Advanced Virgo network, operating at design sensitivity, will not allow for separation of the sources we consider. Third generation detectors, sensitive to most individual compact binary mergers, can reduce the astrophysical signal via subtraction of individual sources, and potentially reveal a cosmological background. Our Bayesian analysis shows that, assuming a detector network containing Cosmic Explorer and Einstein Telescope and reasonable levels of individual source subtraction, we can detect cosmological signals $Ω_{\rm{CS}} (25\,\rm{Hz})=4.5 \times 10^{-13}$ for cosmic strings, and $Ω_{\rm BPL}(25\,\rm{Hz})= 2.2 \times 10^{-13}$ for a broken power law model of an early universe phase transition.

gr-qc↗

arXiv2020

Detecting a stochastic gravitational-wave background in the presence of correlated magnetic noise

A detection of the stochastic gravitational-wave background (SGWB) from unresolved compact binary coalescences could be made by Advanced LIGO and Advanced Virgo at their design sensitivities. However, it is possible for magnetic noise that is correlated between spatially separated ground-based detectors to mimic a SGWB signal. In this paper we propose a new method for detecting correlated magnetic noise and separating it from a true SGWB signal. A commonly discussed method for addressing correlated magnetic noise is coherent subtraction in the raw data using Wiener filtering. The method proposed here uses a parameterized model of the magnetometer-to-strain coupling functions, along with measurements from local magnetometers, to estimate the contribution of correlated noise to the traditional SGWB detection statistic. We then use Bayesian model selection to distinguish between models that include correlated magnetic noise and those with a SGWB. Realistic simulations are used to show that this method prevents a false SGWB detection due to correlated magnetic noise. We also demonstrate that it can be used for a detection of a SGWB in the presence of strong correlated magnetic noise, albeit with reduced significance compared to the case with no correlated noise. Finally, we discuss the advantages of using a global three-detector network for both identifying and characterizing correlated magnetic noise.

gr-qc↗

arXiv2020

Primordial black holes from cusp collapse on cosmic strings

Primordial black holes (PBHs) are of fundamental interest in cosmology and astrophysics, and have received much attention as a dark matter candidate and as a potential source of gravitational waves. One possible PBH formation mechanism is the gravitational collapse of cosmic strings. Thus far, the entirety of the literature on PBH production from cosmic strings has focused on the collapse of (quasi)circular cosmic string loops, which make up only a tiny fraction of the cosmic loop population. We demonstrate here a novel PBH formation mechanism: the collapse of a small segment of cosmic string in the neighbourhood of a cusp. Using the hoop conjecture, we show that collapse is inevitable whenever a cusp appears on a macroscopically-large loop, forming a PBH whose rest mass is smaller than the mass of the loop by a factor of the dimensionless string tension squared, $(Gμ)^2$. Since cusps are generic features of cosmic string loops, and do not rely on finely-tuned loop configurations like circular collapse, this implies that cosmic strings produce PBHs in far greater numbers than has previously been recognised. The resulting PBHs are highly spinning and boosted to ultrarelativistic velocities; they populate a unique region of the BH mass-spin parameter space, and are therefore a "smoking gun" observational signature of cosmic strings. We derive new constraints on $Gμ$ from the evaporation of cusp-collapse PBHs, and update existing constraints on $Gμ$ from gravitational-wave searches.

astro-ph.CO↗

arXiv2020

Science Case for the Einstein Telescope

The Einstein Telescope (ET), a proposed European ground-based gravitational-wave detector of third-generation, is an evolution of second-generation detectors such as Advanced LIGO, Advanced Virgo, and KAGRA which could be operating in the mid 2030s. ET will explore the universe with gravitational waves up to cosmological distances. We discuss its main scientific objectives and its potential for discoveries in astrophysics, cosmology and fundamental physics.

astro-ph.CO↗

arXiv2020

Shot noise in the astrophysical gravitational-wave background

We calculate the noise induced in the anisotropies of the astrophysical gravitational-wave background by finite sampling of both the galaxy distribution and the compact binary coalescence event rate. This shot noise leads to a scale-invariant bias term in the angular power spectrum $C_\ell$, for which we derive a simple analytical expression. We find that this bias dominates over the true cosmological power spectrum in any reasonable observing scenario, and that only with very long observing times and removal of a large number of foreground sources can the true power spectrum be recovered.

astro-ph.CO↗

arXiv2020

Estimating the angular power spectrum of the gravitational-wave background in the presence of shot noise

There has been much recent interest in studying anisotropies in the astrophysical gravitational-wave (GW) background, as these could provide us with interesting new information about galaxy clustering and large-scale structure. However, this information is obscured by shot noise, caused by the finite number of GW sources that contribute to the background at any given time. We develop a new method for estimating the angular spectrum of anisotropies, based on the principle of combining statistically-independent data segments. We show that this gives an unbiased estimate of the true, astrophysical spectrum, removing the offset due to shot noise power, and that in the limit of many data segments, it is the most efficient (i.e. lowest-variance) estimator possible.

astro-ph.CO↗

arXiv2020

Constraints on Quasi-dilaton Massive Gravity

The last decade has seen great advancements in the field of modified gravity, motivated by the dark energy problem, or by the search for a fundamental quantum gravity theory. With a phenomenologically-driven approach, we consider dRGT theory and its extension, quasi-dilaton massive gravity (QDMG). When looking for ways to constrain the theory, a promising direction appeared to be astrophysical tests. The scalar gravitational degree of freedom and quasi-dilaton degree of freedom alter the evolution of Bardeen potentials, which in turn affects the galaxy rotation curves. We find an upper bound on graviton mass in QDMG to be $m \leq 10^{-31} {\rm eV}$. This result agrees with bounds from LIGO and numerous Solar System tests. However, the extremely small mass of the graviton remains a detection out of reach, with LISA's sensitivity exploring the parameter space up to $m \leq 10^{-25} {\rm eV}$.

gr-qc↗

arXiv2020

Probing non-Gaussian Stochastic Gravitational Wave Backgrounds with LISA

The stochastic gravitational wave background (SGWB) contains a wealth of information on astrophysical and cosmological processes. A major challenge of upcoming years will be to extract the information contained in this background and to disentangle the contributions of different sources. In this paper we provide the formalism to extract, from the correlation of three signals in the Laser Interferometer Space Antenna (LISA), information about the tensor three-point function, which characterizes the non-Gaussian properties of the SGWB. This observable can be crucial to discriminate whether a SGWB has a primordial or astrophysical origin. Compared to the two-point function, the SGWB three-point function has a richer dependence on the gravitational wave momenta and chiralities. It can be used therefore as a powerful discriminator between different models. For the first time we provide the response functions of LISA to a general SGWB three-point function. As examples, we study in full detail the cases of an equilateral and squeezed SGWB bispectra, and provide the explicit form of the response functions, ready to be convoluted with any theoretical prediction of the bispectrum to obtain the observable signal. We further derive the optimal estimator to compute the signal-to-noise ratio. Our formalism covers general shapes of non-Gaussianity, and can be extended straightaway to other detector geometries. Finally, we provide a short overview of models of the early universe that can give rise to a non-Gaussian SGWB.

astro-ph.CO↗

arXiv2019

Shortcomings of Shapiro delay-based tests of the equivalence principle on cosmological scales

The "Shapiro delay" experienced by an astronomical messenger traveling through a gravitational field has been used to place constraints on possible deviations from the equivalence principle. The standard Shapiro delay used to obtain these constraints is not itself an observable in general relativity, but is rather obtained by comparing with a fiducial Euclidean distance. There is not a mapping between the constraints obtained in this manner and alternative theories that exhibit equivalence principle violations. However, even assuming that the comparison with the fiducial Euclidean distance is carried out in a way that is useful for some class of alternative theories, we show that the standard calculation of these constraints cannot be applied on cosmological scales, as is often done. Specifically, we find that the Shapiro delay computed in the standard way (taking the Newtonian potential to vanish at infinity) diverges as one includes many remote sources. We use an infinite homogeneous lattice model to illustrate this divergence, and also show how the divergence can be cured by using Fermi coordinates associated with an observer. With this correction, one finds that the Shapiro delay is no longer monotonic with the number of sources. Thus, one cannot compute a conservative lower bound on the Shapiro delay using a subset of the sources of the gravitational field without further assumptions and/or observational input. As an illustration, we compute the Shapiro delay by applying the Fermi coordinate expression to two catalogs of galaxy clusters, illustrating the dependence of the result on the completeness of the catalogue and the mass estimates.

gr-qc↗

arXiv2019

Gravitational-wave luminosity distance in quantum gravity

Dimensional flow, the scale dependence of the dimensionality of spacetime, is a feature shared by many theories of quantum gravity (QG). We present the first study of the consequences of QG dimensional flow for the luminosity distance scaling of gravitational waves in the frequency ranges of LIGO and LISA. We find generic modifications with respect to the standard general-relativistic scaling, largely independent of specific QG proposals. We constrain these effects using two examples of multimessenger standard sirens, the binary neutron-star merger GW170817 and a simulated supermassive black-hole merger event detectable with LISA. We apply these constraints to various QG candidates, finding that the quantum geometries of group field theory, spin foams and loop quantum gravity can give rise to observable signals in the gravitational-wave spin-2 sector. Our results complement and improve GW propagation-speed bounds on modified dispersion relations. Under more model-dependent assumptions, we also show that bounds on quantum geometry can be strengthened by solar-system tests.

gr-qc↗

arXiv2019

Quantum gravity and gravitational-wave astronomy

We investigate possible signatures of quantum gravity which could be tested with current and future gravitational-wave (GW) observations. In particular, we analyze how quantum gravity can influence the GW luminosity distance, the time dependence of the effective Planck mass and the instrumental strain noise of interferometers. Using both model-dependent and model-independent formulae, we show that these quantities can encode a non-perturbative effect typical of all quantum-gravity theories, namely the way the dimension of spacetime changes with the probed scale. Effects associated with such dimensional flow might be tested with GW observations and constrained significantly in theories with a microscopically discrete spacetime geometry, more strongly than from propagation-speed constraints. Making use of public LIGO data as well as of a simulated higher-redshift LISA source, we impose the first, respectively, actual and mock constraints on quantum-gravity parameters affecting the GW luminosity distance and discuss specific theoretical examples. If also the Newtonian potential is modified but light geodesics are not, then solar-system bounds may be stronger than GW ones. Yet, for some theories GW astronomy can give unique information not available from solar-system tests.

gr-qc↗

arXiv2019

Probing the gravitational wave background from cosmic strings with LISA

Cosmic string networks offer one of the best prospects for detection of cosmological gravitational waves (GWs). The combined incoherent GW emission of a large number of string loops leads to a stochastic GW background (SGWB), which encodes the properties of the string network. In this paper we analyze the ability of the Laser Interferometer Space Antenna (LISA) to measure this background, considering leading models of the string networks. We find that LISA will be able to probe cosmic strings with tensions $Gμ\gtrsim \mathcal{O}(10^{-17})$, improving by about $6$ orders of magnitude current pulsar timing arrays (PTA) constraints, and potentially $3$ orders of magnitude with respect to expected constraints from next generation PTA observatories. We include in our analysis possible modifications of the SGWB spectrum due to different hypotheses regarding cosmic history and the underlying physics of the string network. These include possible modifications in the SGWB spectrum due to changes in the number of relativistic degrees of freedom in the early Universe, the presence of a non-standard equation of state before the onset of radiation domination, or changes to the network dynamics due to a string inter-commutation probability less than unity. In the event of a detection, LISA's frequency band is well-positioned to probe such cosmic events. Our results constitute a thorough exploration of the cosmic string science that will be accessible to LISA.

astro-ph.CO↗

arXiv2019

Fermionic spectral action and the origin of nonzero neutrino masses

We propose that the fermionic part of the action in the framework of the noncommutative description of the Standard Model is spectral, in an analogous way to the bosonic part of the action that is customary considered as being spectral. We then discuss the terms that appear in the asymptotic expansion of the fermionic spectral action.

hep-th↗

arXiv2019

Cosmic string loop production functions

Numerical simulations of Nambu-Goto cosmic strings in an expanding universe show that the loop distribution relaxes to an universal configuration, the so-called scaling regime, which is of power law shape on large scales. Precise estimations of the power law exponent are, however, still matter of debate while numerical simulations do not incorporate all the radiation and backreaction effects expected to affect the network dynamics at small scales. By using a Boltzmann approach, we show that the steepness of the loop production function with respect to loops size is associated with drastic changes in the cosmological loop distribution. For a scale factor varying as a(t)~t^nu, we find that sub-critical loop production functions, having a Polchinski-Rocha exponent chi < (3nu-1)/2, yield scaling loop distributions which are mostly insensitive to infra-red (IR) and ultra-violet (UV) assumptions about the cosmic string network. For those, cosmological predictions are expected to be relatively robust, in accordance with previous results. On the contrary, critical and super-critical loop production functions, having chi >= (3nu-1)/2, are shown to be IR-physics dependent and this generically prevents the loop distribution to relax towards scaling. In the latter situation, we discuss the additional regularisations needed for convergence and show that, although a scaling regime can still be reached, the shape of the cosmological loop distribution is modified compared to the naive expectation. Finally, we discuss the implications of our findings.

astro-ph.CO↗

arXiv2019

Anisotropies in the astrophysical gravitational-wave background: The impact of black hole distributions

We use population inference to explore the impact that uncertainties in the distribution of binary black holes (BBH) have on the astrophysical gravitational-wave background (AGWB). Our results show that the AGWB monopole is sensitive to the nature of the BBH population (particularly the local merger rate), while the anisotropic $C_\ell$ spectrum is only modified to within a few percent, at a level which is insignificant compared to other sources of uncertainty (such as cosmic variance). This is very promising news for future observational studies of the AGWB, as it shows that (i) the monopole can be used as a new probe of the population of compact objects throughout cosmic history, complementary to direct observations by LIGO and Virgo and (ii) we are able to make surprisingly robust predictions for the $C_\ell$ spectrum, even with only very approximate knowledge of the black hole population. As a result, the AGWB anisotropies have enormous potential as a new probe of the large-scale structure of the Universe, and of late-Universe cosmology in general.

astro-ph.CO↗

arXiv2019

Cosmology and the Early Universe

This Astro-2020 White Paper deals with what we might learn from future gravitational wave observations about the early universe phase transitions and their energy scales, primordial black holes, Hubble parameter, dark matter and dark energy, modified theories of gravity and extra dimensions.

astro-ph.HE↗