arXiv subjects

Joseph Silk

Publications and source records attributed to Joseph Silk.

At least 91 records · Page 5Linked to original sources

arXiv2021

In the Realm of the Hubble tension $-$ a Review of Solutions

The $Λ$CDM model provides a good fit to a large span of cosmological data but harbors areas of phenomenology. With the improvement of the number and the accuracy of observations, discrepancies among key cosmological parameters of the model have emerged. The most statistically significant tension is the $4-6σ$ disagreement between predictions of the Hubble constant $H_0$ by early time probes with $Λ$CDM model, and a number of late time, model-independent determinations of $H_0$ from local measurements of distances and redshifts. The high precision and consistency of the data at both ends present strong challenges to the possible solution space and demand a hypothesis with enough rigor to explain multiple observations--whether these invoke new physics, unexpected large-scale structures or multiple, unrelated errors. We present a thorough review of the problem, including a discussion of recent Hubble constant estimates and a summary of the proposed theoretical solutions. Some of the models presented are formally successful, improving the fit to the data in light of their additional degrees of freedom, restoring agreement within $1-2σ$ between {\it Planck} 2018, using CMB power spectra data, BAO, Pantheon SN data, and R20, the latest SH0ES Team measurement of the Hubble constant ($H_0 = 73.2 \pm 1.3{\rm\,km\,s^{-1}\,Mpc^{-1}}$ at 68\% confidence level). Reduced tension might not simply come from a change in $H_0$ but also from an increase in its uncertainty due to degeneracy with additional physics, pointing to the need for additional probes. While no specific proposal makes a strong case for being highly likely or far better than all others, solutions involving early or dynamical dark energy, neutrino interactions, interacting cosmologies, primordial magnetic fields, and modified gravity provide the best options until a better alternative comes along.[Abridged]

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arXiv2021

Could PBHs and secondary GWs have originated from squeezed initial states?

[Abridged] Recently, the production of primordial black holes (PBHs) and secondary gravitational waves (GWs) due to enhanced scalar power on small scales have garnered considerable attention in the literature. Often, the mechanism considered to arrive at such enhanced power involves a modification of the standard slow roll inflationary dynamics, achieved with the aid of fine-tuned potentials. In this work, we investigate another well known method to generate features in the power spectrum wherein the initial state of the perturbations is assumed to be squeezed states. This approach is technically straightforward to implement since the Bogoliubov coefficients characterizing the squeezed initial states, can be immediately determined from the desired form of the power spectrum. It is known that, for squeezed initial states, the scalar bispectrum is strongly scale dependent and the consistency condition governing the scalar bispectrum in the squeezed limit is violated. In fact, the non-Gaussianity parameter $f_{_{\rm NL}}$ characterizing the scalar bispectrum proves to be inversely proportional to the squeezed mode and this dependence enhances its amplitude at large wave numbers making it highly sensitive to even a small deviation from the standard Bunch-Davies vacuum. These aspects can possibly aid in leading to enhanced formation of PBHs and generation of secondary GWs. However, we find that: (i) the desired form of the squeezed initial states may be challenging to achieve from a dynamical mechanism, and (ii) the backreaction due to the excited states severely limits the extent of deviation from the Bunch-Davies vacuum at large wave numbers. We argue that, unless the issue of backreaction is circumvented, squeezed initial states cannot lead to a substantial increase in power on small scales that is required for enhanced formation of PBHs and generation of secondary GWs.

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arXiv2021

PBHs and secondary GWs from ultra slow roll and punctuated inflation

[Abridged] The primordial scalar power spectrum is well constrained on large scales, primarily by the observations of the anisotropies in the cosmic microwave background (CMB). Over the last few years, it has been recognized that a sharp rise in power on small scales will lead to enhanced formation of primordial black holes (PBHs) and also generate secondary gravitational waves (GWs) of higher and, possibly, detectable amplitudes. It is well understood that scalar power spectra with COBE normalized amplitude on the CMB scales and enhanced amplitudes on smaller scales can be generated due to deviations from slow roll in single, canonical scalar field models of inflation. In fact, an epoch of so-called ultra slow roll inflation can lead to the desired amplification. We find that scenarios that lead to ultra slow roll can be broadly classified into two types, one wherein there is a brief departure from inflation (a scenario referred to as punctuated inflation) and another wherein such a departure does not arise. We consider a set of single field inflationary models involving the canonical scalar field that lead to ultra slow roll and punctuated inflation and examine the formation of PBHs as well as the generation of secondary GWs in these models. Apart from considering specific models, we reconstruct potentials from certain functional choices of the first slow roll parameter leading to ultra slow roll and punctuated inflation and investigate their observational signatures. In addition to the secondary tensor power spectrum, we calculate the secondary tensor bispectrum in the equilateral limit in these scenarios. Moreover, we calculate the inflationary scalar bispectrum that arises in all the cases and discuss the imprints of the scalar non-Gaussianities on the extent of PBHs formed and the amplitude of the secondary GWs.

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arXiv2021

Investigating Cosmic Discordance

We show that a combined analysis of CMB anisotropy power spectra obtained by the Planck satellite and luminosity distance data simultaneously excludes a flat universe and a cosmological constant at $99 \%$ CL. These results hold separately when combining Planck with three different datasets: the two determinations of the Hubble constant from Riess et al. 2019 and Freedman et al. 2020, and the Pantheon catalog of high redshift supernovae type-Ia. We conclude that either LCDM needs to be replaced by a different model, or else there are significant but still undetected systematics. Our result calls for new observations and stimulates the investigation of alternative theoretical models and solutions.

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arXiv2021

Testing the general theory of relativity using gravitational wave propagation from dark standard sirens

Alternative theories of gravity predict modifications in the propagation of gravitational waves (GW) through space-time. One of the smoking-gun predictions of such theories is the change in the GW luminosity distance to GW sources as a function of redshift relative to the electromagnetic (EM) luminosity distance expected from EM probes. We propose a multi-messenger test of the theory of general relativity from the propagation of gravitational waves by combining EM and GW observations to resolve these issues from GW sources without EM counterparts (which are also referred to as dark standard sirens). By using the relation between the geometric distances accessible from baryon acoustic oscillation measurements, and luminosity distance measurements from the GW sources, we can measure any deviation from the general theory of relativity via the GW sources of unknown redshift that will be detectable by networks of GW detectors such as LIGO, Virgo, and KAGRA. Using this technique, the fiducial value of the frictional term can be measured to a precision $Ξ_0=0.98^{+0.04}_{-0.23}$ after marginalizing over redshift dependence, cosmological parameters, and GW bias parameters with $\sim 3500$ dark standard sirens of masses $30\,\rm M_\odot$ each distributed up to redshift $z=0.5$. For a fixed redshift dependence, a value of $Ξ_0=0.99^{+0.02}_{-0.02}$ can be measured with a similar number of dark sirens. Application of our methodology to the far more numerous dark standard sirens detectable with next generation GW detectors, such as LISA, Einstein Telescope and Cosmic Explorer, will allow achievement of higher accuracy than possible from use of bright standard sirens.

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arXiv2020

Primordial Kerr Black Holes

Primordial Black Holes (PBHs) are appealing candidates for dark matter in the universe but are severely constrained by theoretical and observational constraints. We will focus on the Hawking evaporation limits extended to Kerr black holes. In particular, we will discuss the possibility to distinguish between black holes of primordial and of stellar origins based on the Thorne limit on their spin. We will also review the isotropic extragalactic gamma ray background constraints and show that the ``window'' in which PBHs can constitute all of the dark matter depends strongly on the PBH spin. Finally, we will consider the possibility that the so-called Planet 9 is a primordial black hole.

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arXiv2020

Fast radio burst repeaters produced via Kozai-Lidov feeding of neutron stars in binary systems

Neutron stars are likely surrounded by gas, debris, and asteroid belts. Kozai-Lidov perturbations, induced by a distant, but gravitationally bound companion, can trigger the infall of such orbiting bodies onto a central compact object. These effects could lead to the emission of fast radio bursts (FRBs), for example by asteroid-induced magnetic wake fields in the wind of the compact object. A few percent of binary neutron star systems in the Universe, such as neutron star-main sequence star, neutron star-white dwarf, double neutron star, and neutron star-black hole systems, can account for the observed non-repeating FRB rates. More remarkably, we find that wide and close companion orbits lead to non-repeating and repeating sources, respectively, and they allow for one to compute a ratio between repeating and non-repeating sources of a few percent, which is in close agreement with the observations. Three major predictions can be made from our scenario, which can be tested in the coming years: 1) most repeaters should stop repeating after a period between 10 years to a few decades, as their asteroid belts become depleted; 2) some non-repeaters could occasionally repeat, if we hit the short period tail of the FRB period distribution; and 3) series of sub-Jansky level short radio bursts could be observed as electromagnetic counterparts of the mergers of binary neutron star systems.

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arXiv2020

Inferring the lensing rate of LIGO-Virgo sources from the stochastic gravitational wave background

Strong lensing of gravitational waves is more likely for distant sources but predicted event rates are highly uncertain with many astrophysical origins proposed. Here we open a new avenue to estimate the event rate of strongly lensed systems by exploring the amplitude of the stochastic gravitational wave background (SGWB). This method can provide a direct upper bound on the high redshift binary coalescing rates, which can be translated into an upper bound on the expected rate of strongly lensed systems. We show that from the ongoing analysis of the Laser Interferometer Gravitational-wave Observatory (LIGO)-Virgo and in the future from the LIGO-Virgo design sensitivity stringent bounds on the lensing event rate can be imposed using the SGWB signal. Combining measurements of loud gravitational wave events with an unresolved stochastic background detection will improve estimates of the numbers of lensed events at high redshift. The proposed method is going to play a crucial in understanding the population of lensed and unlensed systems from gravitational wave observations.

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arXiv2020

Boosting small-scale structure via primordial black holes and implications for sub-GeV dark matter annihilation

We explore the possibility that the annihilation of dark matter (DM) is boosted due to enhanced substructure in the presence of primordial black holes (PBHs) which constitute a sub-component of DM. The PBHs can generate entropy fluctuations at the small scales which trigger early structure formation, and a large fraction of the whole DM can reside in these collapsed objects that formed at high redshift ($z\gtrsim 100$). Such early forming minihalos consequently possess higher densities than those in the conventional scenarios (without PBHs) and would be more resilient to tidal disruptions. Our scenarios of the annihilation boost due to DM substructures are of particular interest for light ($< 1$ GeV) DM which has been less explored compared to heavier DM in the presence of the PBHs.

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arXiv2020

The limits of cosmology: role of the Moon

The lunar surface allows a unique way forward in cosmology, to go beyond current limits. The far side provides an unexcelled radio-quiet environment for probing the dark ages via 21 cm interferometry to seek elusive clues on the nature of the infinitesimal fluctuations that seeded galaxy formation. Far-infrared telescopes in cold and dark lunar polar craters will probe back to the first months of the Big Bang and study associated spectral distortions in the CMB. Optical and IR megatelescopes will image the first star clusters in the universe and seek biosignatures in the atmospheres of unprecedented numbers of nearby habitable zone exoplanets. The goals are compelling and a stable lunar platform will enable construction of telescopes that can access trillions of modes in the sky, providing the key to exploration of our cosmic origins.

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arXiv2020

Cosmology Intertwined III: $f σ_8$ and $S_8$

The standard $Λ$ Cold Dark Matter cosmological model provides a wonderful fit to current cosmological data, but a few tensions and anomalies became statistically significant with the latest data analyses. While these anomalies could be due to the presence of systematic errors in the experiments, they could also indicate the need for new physics beyond the standard model. In this Letter of Interest we focus on the tension of the Planck data with weak lensing measurements and redshift surveys, about the value of the matter energy density $Ω_m$, and the amplitude or rate of the growth of structure ($σ_8,fσ_8$). We list a few interesting models for solving this tension, and we discuss the importance of trying to fit with a single model a full array of data and not just one parameter at a time.

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arXiv2020

Cosmology Intertwined II: The Hubble Constant Tension

The current cosmological probes have provided a fantastic confirmation of the standard $Λ$ Cold Dark Matter cosmological model, that has been constrained with unprecedented accuracy. However, with the increase of the experimental sensitivity a few statistically significant tensions between different independent cosmological datasets emerged. While these tensions can be in portion the result of systematic errors, the persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the need for new physics. In this Letter of Interest we will focus on the $4.4σ$ tension between the Planck estimate of the Hubble constant $H_0$ and the SH0ES collaboration measurements. After showing the $H_0$ evaluations made from different teams using different methods and geometric calibrations, we will list a few interesting new physics models that could solve this tension and discuss how the next decade experiments will be crucial.

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arXiv2020

Cosmology Intertwined I: Perspectives for the Next Decade

The standard $Λ$ Cold Dark Matter cosmological model provides an amazing description of a wide range of astrophysical and astronomical data. However, there are a few big open questions, that make the standard model look like a first-order approximation to a more realistic scenario that still needs to be fully understood. In this Letter of Interest we will list a few important goals that need to be addressed in the next decade, also taking into account the current discordances present between the different cosmological probes, as the Hubble constant $H_0$ value, the $σ_8 - S_8$ tension, and the anomalies present in the Planck results. Finally, we will give an overview of upgraded experiments and next-generation space-missions and facilities on Earth, that will be of crucial importance to address all these questions.

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arXiv2020

Cosmology Intertwined IV: The Age of the Universe and its Curvature

A precise measurement of the curvature of the Universe is of primeval importance for cosmology since it could not only confirm the paradigm of primordial inflation but also help in discriminating between different early Universe scenarios. The recent observations, while broadly consistent with a spatially flat standard $Λ$ Cold Dark Matter ($Λ$CDM) model, are showing tensions that still allow (and, in some cases, even suggest) a few percent deviations from a flat universe. In particular, the Planck Cosmic Microwave Background power spectra, assuming the nominal likelihood, prefer a closed universe at more than 99\% confidence level. While new physics could be in action, this anomaly may be the result of an unresolved systematic error or just a statistical fluctuation. However, since a positive curvature allows a larger age of the Universe, an accurate determination of the age of the oldest objects provides a smoking gun in confirming or falsifying the current flat $Λ$CDM model.

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arXiv2020

Detectability of Gravitational Waves from the Coalescence of Massive Primordial Black Holes with Initial Clustering

We show that the effect of initial non-Gaussian clustering can significantly enhance the event rate for primordial black hole (PBH) coalescence. The impact of such clustering is studied in a specific scenario of multi-stream inflation. Initial clustering enables the possibility of detecting massive PBH coalescence by space-based gravitational wave interferometers such as LISA and DECIGO/BBO. The parameter regime for the ground-based detectors to detect PBH coalescence is also extended.

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arXiv2020

Ejection of supermassive black holes and implications for merger rates in fuzzy dark matter haloes

Fuzzy dark matter (FDM) consisting of ultra-light axions has been invoked to alleviate galactic-scale problems in the cold dark matter scenario. FDM fluctuations, created via the superposition of waves, can impact the motion of a central supermassive black hole (SMBH) immersed in an FDM halo. The SMBH will undergo a random walk, induced by FDM fluctuations, that can result in its ejection from the central region. This effect is strongest in dwarf galaxies, accounting for wandering SMBHs and the low detection rate of AGN in dwarf spheroidal galaxies. In addition, a lower bound on the allowed axion masses is inferred both for Sagittarius $A^*$ and heavier SMBH; to avoid ejection from the galactic centres, axion masses of the order of $10^{-22}{\rm eV}$ or lighter are excluded. Stronger limits are inferred for merging galaxies. We find that the event rate of SMBH mergers in FDM haloes and the associated SMBH growth rates can be reduced by at least an order of magnitude.

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arXiv2020

Repeated mergers and ejection of black holes within nuclear star clusters

Current stellar evolution models predict a dearth of black holes (BHs) with masses $\gtrsim 50$ M$_\odot$ and $\lesssim 5$ M$_\odot$, and intermediate-mass black holes (IMBHs; $\sim10^2- 10^5$ M$_\odot$) have not yet been detected beyond any reasonable doubt. A natural way to form massive BHs is through repeated mergers, detectable via gravitational wave emission with current LIGO/Virgo or future LISA and ET observations. Nuclear star clusters (NSCs) have masses and densities high enough to retain most of the merger products, which acquire a recoil kick at the moment of merger. We explore the possibility that IMBHs may be born as a result of repeated mergers in NSCs, and show how their formation pathways depend on the NSC mass and density, and BH spin distribution. We find that BHs in the pair-instability mass gap can be formed and observed by LIGO/Virgo, and show that the typical mass of the ejected massive BHs is $400$--$500$ M$_\odot$, with velocities of up to a few thousand km s$^{-1}$. Eventually some of these IMBHs can become the seeds of supermassive BHs, observed today in the centers of galaxies. In dwarf galaxies, they could potentially solve the abundance, core-cusp, too-big-to-fail, ultra-faint, and baryon-fraction issues via plausible feedback scenarios.

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arXiv2020

Stellar Signatures of Inhomogeneous Big Bang Nucleosynthesis

We evaluate abundance anomalies generated in patches of the universe where the baryon-to-photon ratio was locally enhanced by possibly many orders of magnitude in the range $η= 10^{-10} - 10^{-1}$. Our study is motivated by the possible survival of rare dense regions in the early universe, the most extreme of which, above a critical threshold, collapsed to form primordial black holes. If this occurred, one may expect there to also be a significant population of early-forming stars that formed in similar but subthreshold patches. We derive a range of element abundance signatures by performing BBN simulations at high values of the baryon-to-photon ratio that may be detectable in any surviving first generation stars of around a solar mass. Our predictions apply to metal-poor galactic halo stars, to old globular star clusters and to dwarf galaxies, and we compare with observations in each of these cases.

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