arXiv subjects

Ville Vaskonen

Publications and source records attributed to Ville Vaskonen.

At least 37 records · Page 2Linked to original sources

arXiv2024

Gravitational waves from inflation in LISA: reconstruction pipeline and physics interpretation

Various scenarios of cosmic inflation enhance the amplitude of the stochastic gravitational wave background (SGWB) at frequencies detectable by the LISA detector. We develop tools for a template-based analysis of the SGWB and introduce a template databank to describe well-motivated signals from inflation, prototype their template-based searches, and forecast their reconstruction with LISA. Specifically, we classify seven templates based on their signal frequency shape, and we identify representative fundamental physics models leading to them. By running a template-based analysis, we forecast the accuracy with which LISA can reconstruct the template parameters of representative benchmark signals, with and without galactic and extragalactic foregrounds. We identify the parameter regions that can be probed by LISA within each template. Finally, we investigate how our signal reconstructions shed light on fundamental physics models of inflation: we discuss their impact for measurements of \emph{e.g.,} ~the couplings of inflationary axions to gauge fields; the graviton mass during inflation; the fluctuation seeds of primordial black holes; the consequences of excited states during inflation, and the presence of small-scale spectral features.

astro-ph.CO↗

arXiv2024

Statistics of the supermassive black hole gravitational wave background anisotropy

We study the statistical properties of the anisotropy in the gravitational wave (GW) background originating from supermassive black hole (SMBH) binaries. We derive the distribution of the GW anisotropy power spectrum coefficients, $C_{l\geq1}/C_0$, in scenarios including environmental effects and eccentricities of the SMBH binaries. Although the mean of $C_{l\geq1}/C_0$ is the same for all multipoles, we show that their distributions vary, with the low $l$ distributions being the widest. We also find a strong correlation between spectral fluctuations and the anisotropy in the GW signal. We show that the GW anisotropy can break the degeneracy between the scenarios including environmental effects or eccentricity. In particular, we find that existing NANOGrav constraints on GW anisotropy begin to constrain SMBH scenarios with strong environmental effects.

astro-ph.CO↗

arXiv2024

Thermalization effects on the dynamics of growing vacuum bubbles

We study the evolution of growing vacuum bubbles. The bubble walls interact with the surrounding fluid and may, consequently, reach a terminal velocity. If the mean free path of the particles in the fluid is much shorter than the bubble wall thickness, the fluid is locally in thermal equilibrium and the wall's terminal velocity can be determined by entropy conservation. On the other hand, if local thermal equilibrium inside the wall cannot be maintained, the wall velocity can be estimated from the pressure impacted by ballistic particle dynamics at the wall. We find that the latter case leads to slightly slower bubble walls. Expectedly, we find the largest differences in the terminal velocity when the fluid is entirely ballistic. This observation indicates that the non-equilibrium effects inside walls are relevant. To study bubble evolution, we perform hydrodynamic lattice simulations in the case of local thermal equilibrium and $N$-body simulations in the ballistic case to investigate the dynamical effects during expansion. Both simulations show that even if a stationary solution exists in theory it may not be reached depending on the dynamics of the accelerating bubble walls.

hep-ph↗

arXiv2024

Black holes and gravitational waves from slow phase transitions

Slow first-order phase transitions generate large inhomogeneities that can lead to the formation of primordial black holes (PBHs). We show that the gravitational wave (GW) spectrum then consists of a primary component sourced by bubble collisions and a secondary one induced by large perturbations. The latter gives the dominant peak if $β/H_0 < 12$, impacting, in particular, the interpretation of the recent PTA data. The GW signal associated with a particular PBH population is stronger than in typical scenarios because of a negative non-Gaussianity of the perturbations and it has a distinguishable shape with two peaks.

astro-ph.CO↗

arXiv2024

What is the origin of the JWST SMBHs?

We present a new semi-analytical model for the evolution of galaxies and supermassive black holes (SMBHs) that is based on the extended Press-Schechter formalism and phenomenological modelling of star formation. The model yields BH mass-stellar mass relations that reproduce both the JWST and pre-JWST observations. If the efficiency for BH mergers is high the JWST data prefer light seeds while the pre-JWST data prefers heavy seeds. The fit improves for a smaller merger efficiency, $O(0.1)$, for which both data prefer heavy seeds, while also accommodating the PTA GW background data.

astro-ph.CO↗

arXiv2024

Gravitational waves from first-order phase transitions in LISA: reconstruction pipeline and physics interpretation

We develop a tool for the analysis of stochastic gravitational wave backgrounds from cosmological first-order phase transitions with LISA: we initiate a template databank for these signals, prototype their searches, and forecast their reconstruction. The templates encompass the gravitational wave signals sourced by bubble collisions, sound waves and turbulence. Accounting for Galactic and extra-Galactic foregrounds, we forecast the region of the parameter space that LISA will reconstruct with better than $\sim 10\,\%$ accuracy, if certain experimental and theoretical uncertainties are solved by the time LISA flies. We illustrate the accuracy with which LISA can reconstruct the parameters on a few benchmark signals, both in terms of the template parameters and the phase transition ones. To show the impact of the forecasts on physics beyond the Standard Model, we map the reconstructed benchmark measurements into the parameter spaces of the singlet extension of the Standard Model and of the classically conformal invariant $U(1)_{B-L}$ model.

astro-ph.CO↗

arXiv2024

Formation of primordial black hole binaries and their merger rates

We review the theory behind the formation of primordial black hole binaries and their merger rates. We consider the binary formation in the early and late Universe, emphasising the former as it gives the dominant contribution of the present primordial black hole merger rate. The binaries formed in the early Universe are highly eccentric so their interactions with other primordial black holes can significantly increase their coalescence times and thereby suppress the merger rate. We discuss in detail how the suppression of the merger rate arising from such interactions can be estimated and how such interactions lead to the formation of another, much harder, binary population that contributes to the present merger rate if more than 10% of dark matter consists of primordial black holes with a relatively narrow mass distribution. When the primordial abundance is below 1%, encounters between primordial black holes are unlikely and their effect on the merger rate becomes negligible.

astro-ph.CO↗

arXiv2024

Irreducible cosmological backgrounds of a real scalar with a broken symmetry

We explore the irreducible cosmological implications of a singlet real scalar field. Our focus is on theories with an approximate and spontaneously broken $\mathbb{Z}_2$ symmetry where quasi-stable domain walls can form at early times. This seemingly simple framework bears a wealth of phenomenological implications that can be tackled by means of different cosmological and astrophysical probes. We elucidate the connection between domain wall dynamics and the production of dark matter and gravitational waves. In particular, we identify three main benchmark scenarios. The gravitational wave signal observed by pulsar timing arrays can be generated by the domain walls if the mass of the singlet is $m_s \sim\,$PeV. For lower masses, but with $m_s \gtrsim 10\,$GeV, scalars produced in the annihilation of the domain walls can be dark matter with a distinctive feature in their power spectrum. Finally, the thermal bath provides an unavoidable source of unstable scalars via the freeze-in mechanism whose subsequent decays can be tested by their imprints on cosmological and terrestrial observables.

hep-ph↗

arXiv2024

In Search of the Biggest Bangs since the Big Bang

Many galaxies contain supermassive black holes (SMBHs), whose formation and history raise many puzzles. Pulsar timing arrays have recently discovered a low-frequency cosmological "hum" of gravitational waves that may be emitted by SMBH binary systems, and the JWST and other telescopes have discovered an unexpectedly large population of high-redshift SMBHs. We argue that these two discoveries may be linked, and that they may enhance the prospects for measuring gravitational waves emitted during the mergers of massive black holes, thereby opening the way towards resolving many puzzles about SMBHs as well as providing new opportunities to probe general relativity.

gr-qc↗

arXiv2024

Consistency of JWST Black Hole Observations with NANOGrav Gravitational Wave Measurements

JWST observations have opened a new chapter in studies of supermassive black holes (SMBHs), stimulating discussion of two puzzles: the abundance of SMBHs in the early Universe and the fraction of dual AGNs. In this paper we argue that the answers to these puzzles may be linked to an interpretation of the data on the nHz gravitational wave (GWs) discovered by NANOGrav and other Pulsar Timing Arrays (PTAs) in terms of SMBH binaries losing energy by interactions with their environments as well as by GW emission. According to this interpretation, the SMBHs in low-$z$ AGNs are the tip of the iceberg of the local SMBH population, which are mainly in inactive galaxies. This interpretation would favour the observability of GW signals from BH binaries in LISA and deciHz GW detectors.

astro-ph.CO↗

arXiv2024

Probing supermassive black hole seed scenarios with gravitational wave measurements

The process whereby the supermassive black holes populating the centers of galaxies have been assembled remains to be established, with the relative importance of seeds provided by collapsed Population-III stars, black holes formed in nuclear star clusters via repeated mergers, or direct collapses of protogalactic disks yet to be determined. In this paper we study the prospects for casting light on this issue by future measurements of gravitational waves emitted during the inspirals and mergers of pairs of intermediate-mass black holes, discussing in particular the roles of prospective measurements by LISA and the proposed atom interferometers AION and AEDGE. We find that, the expected number of detectable IMBH binaries is $O(100)$ for LISA and AEDGE and $O(10)$ for AION in low-mass seeds scenarios and goes down to $O(10)$ for LISA and below one for AEDGE and AION in high-mass seed scenarios. This allows all of these observatories to probe the parameters of the seed model, in particular if at least a fraction of the SMBHs arise from a low-mass seed population. We also show that the measurement accuracy of the binary parameters is, in general, best for AEDGE that sees very precisely the merger of the binary.

astro-ph.CO↗

arXiv2024

Gravitational Waves from SMBH Binaries in Light of the NANOGrav 15-Year Data

The NANOGrav Collaboration has recently announced evidence for nHz gravitational waves (GWs), in the form of a Hellings-Downs angular correlation in the common-spectrum process that had been observed previously by them and other Pulsar Timing Arrays (PTAs). We analyze the possibility that these GWs originate from binary supermassive black holes (SMBHs) with total masses $\gtrsim 10^9\, M_{\odot}$. The spectral index of the GW signal differs at 95 % CL from that predicted for binary evolution by GW emission alone, and we find $> 3 σ$ evidence that environmental effects such as dynamical friction with gas, stars, and dark matter may be affecting the binary evolution. We estimate the required magnitude and spectrum of such environmental effects and comment on their possible implications for measurements of GWs at higher frequencies.

astro-ph.CO↗

arXiv2023

The recent gravitational wave observation by pulsar timing arrays and primordial black holes: the importance of non-gaussianities

We study whether the signal seen by pulsar timing arrays (PTAs) may originate from gravitational waves (GWs) induced by large primordial perturbations. Such perturbations may be accompanied by a sizeable primordial black hole (PBH) abundance. We improve existing analyses and show that PBH overproduction disfavors Gaussian scenarios for scalar-induced GWs at 2σ and single-field inflationary scenarios, accounting for non-Gaussianity, at 3σ as the explanation of the most constraining NANOGrav 15-year data. This tension can be relaxed in models where non-Gaussianites suppress the PBH abundance. On the flip side, the PTA data does not constrain the abundance of PBHs.

astro-ph.CO↗

arXiv2023

What is the source of the PTA GW signal?

The most conservative interpretation of the nHz stochastic gravitational wave background (SGWB) discovered by NANOGrav and other Pulsar Timing Array (PTA) Collaborations is astrophysical, namely that it originates from supermassive black hole (SMBH) binaries. However, alternative cosmological models have been proposed, including cosmic strings, phase transitions, domain walls, primordial fluctuations and "audible" axions. We perform a multi-model analysis (MMA) to compare how well these different hypotheses fit the NANOGrav data, both in isolation and in combination with SMBH binaries, and address the questions: Which interpretations fit the data best, and which are disfavoured? We also discuss experimental signatures that can help discriminate between different sources of the PTA GW signal, including fluctuations in the signal strength between frequency bins, individual sources and how the PTA signal extends to higher frequencies.

astro-ph.CO↗

arXiv2023

Primordial black holes from strong first-order phase transitions

We study the formation of primordial black holes (PBHs) in strongly supercooled first-order phase transitions. The mechanism is based on the presence of remnants dominated by the false vacuum that scale slower with the expansion of the Universe than their surroundings where this energy was already converted into radiation. We compute the PBH formation from these remnants including the contribution from the false vacuum and the bubble walls, by estimating the collapse using the hoop conjecture and by considering both regions collapsing immediately when entering the horizon and sub-horizon regions that collapse as their compactness grows. We show that for exponential bubble nucleation rate, $Γ\propto e^{βt}$, the primordial black hole formation implies $β/H \gtrsim 3.8$, where $H$ denotes the Hubble rate, if the potential energy of the false vacuum is $ΔV \lesssim (10^{12} {\rm GeV})^4$, as otherwise a too large abundance of long-lived PBHs forms. The observed dark matter abundance can be formed in asteroid mass PBHs if $β/H \simeq 3.8$ and $10^5 {\rm GeV} \lesssim ΔV^{1/4} \lesssim 10^8 {\rm GeV}$. Finally, we consider also the effect of the second order correction to the exponential nucleation rate showing that the PBH abundance is mainly determined by the average radius of the true vacuum bubbles.

astro-ph.CO↗

arXiv2023

Prospects for Future Binary Black Hole GW Studies in Light of PTA Measurements

NANOGrav and other Pulsar Timing Arrays (PTAs) have discovered a common-spectrum process in the nHz range that may be due to gravitational waves (GWs): if so, they are likely to have been generated by black hole (BH) binaries with total masses $> 10^9 M_{\odot}$. Using the Extended Press-Schechter formalism to model the galactic halo mass function and a simple relation between the halo and BH masses suggests that these binaries have redshifts $z = {O}(1)$ and mass ratios $\gtrsim 10$, and that the GW signal at frequencies above ${O}(10)$~nHz may be dominated by relatively few binaries that could be distinguished experimentally and would yield observable circular polarization. Extrapolating the model to higher frequencies indicates that future GW detectors such as LISA and AEDGE could extend the PTA observations to lower BH masses $\in (10^6, 10^9 ) M_{\odot}$ and $\in (10^3, 10^9) M_{\odot}$.

astro-ph.CO↗

arXiv2023

Microlensing of gravitational waves by dark matter structures

Gravitational lensing of gravitational waves provides a potential new probe of dark matter structures. In this work, we consider the microlensing effect on gravitational wave signals from black hole binaries induced by low-mass dark matter halos that do not retain enough baryonic matter to hold stars. We clarify systematically when this microlensing effect is relevant and study in detail its detectability by future gravitational wave observatories. We consider lensing by cold dark matter halos and by solitonic cores that reside in fuzzy dark matter halos. Our results show that although the effect can be detectable at relatively large impact parameters, the probability of detecting such lensed events is low. In particular, we find that the expected number of events lensed by cold dark matter halos is $O(0.01)$ per year for BBO and the expected number of events lensed by solitonic cores inside fuzzy dark matter halos is $O(0.01)$ per year for ET. In the case that a significant fraction of dark matter consists of $=O(100 M_\odot)$ objects that are relatively compact, $R < O(0.1\,{\rm pc})$, we show that the expected number of lensed events per year ET can be very large, $0(1000)$.

astro-ph.CO↗

arXiv2023

Gravitational wave microlensing by dressed primordial black holes

We study gravitational wave microlensing by primordial black holes (PBHs), accounting for the effect of a particle dark matter minihalo surrounding them. Such minihaloes are expected when PBHs make up only a fraction of all dark matter. We find that the LIGO-Virgo detections imply a $1σ$ bound on the abundance of PBHs heavier than $50 M_{\odot}$. The next generation observatories can potentially probe PBHs as light as $0.01 M_\odot$ and down to $2\times10^{-4}$ fraction of all dark matter. We also show that these detectors can distinguish between dressed and naked PBHs, providing a novel way to study the distribution of particle dark matter around black holes and potentially shed light on the origins of black holes.

astro-ph.CO↗