arXiv subjects

Niayesh Afshordi

Publications and source records attributed to Niayesh Afshordi.

At least 73 records · Page 4Linked to original sources

arXiv2020

Quantum Black Holes in the Sky

Black Holes are possibly the most enigmatic objects in our Universe. From their detection in gravitational waves upon their mergers, to their snapshot eating at the centres of galaxies, black hole astrophysics has undergone an observational renaissance in the past 4 years. Nevertheless, they remain active playgrounds for strong gravity and quantum effects, where novel aspects of the elusive theory of quantum gravity may be hard at work. In this review article, we provide an overview of the strong motivations for why "Quantum Black Holes" may be radically different from their classical counterparts in Einstein's General Relativity. We then discuss the observational signatures of quantum black holes, focusing on gravitational wave echoes as smoking guns for quantum horizons (or exotic compact objects), which have led to significant recent excitement and activity. We review the theoretical underpinning of gravitational wave echoes and critically examine the seemingly contradictory observational claims regarding their (non-)existence. Finally, we discuss the future theoretical and observational landscape for unraveling the "Quantum Black Holes in the Sky".

gr-qc↗

arXiv2020

Echoes from the Abyss: A highly spinning black hole remnant for the binary neutron star merger GW170817

The first direct observation of a binary neutron star (BNS) merger was a watershed moment in multi-messenger astronomy. However, gravitational waves from GW170817 have only been observed prior to the BNS merger, but electromagnetic observations all follow the merger event. While post-merger gravitational wave signal in general relativity is too faint (given current detector sensitivities), here we present the first tentative detection of post-merger gravitational wave "echoes" from a highly spinning "black hole" remnant. The echoes may be expected in different models of quantum black holes that replace event horizons by exotic Planck-scale structure and tentative evidence for them has been found in binary black hole merger events. The fact that the echo frequency is suppressed by $\log M$ (in Planck units) puts it squarely in the LIGO sensitivity window, allowing us to build an optimal model-agnostic search strategy via cross-correlating the two detectors in frequency/time. We find a tentative detection of echoes at $f_{\rm echo} \simeq 72$ Hz, around 1.0 sec after the BNS merger, consistent with a 2.6-2.7 $M_\odot$ "black hole" remnant with dimensionless spin $0.84-0.87$. Accounting for all the "look-elsewhere" effects, we find a significance of $4.2 σ$, or a false alarm probability of $1.6\times 10^{-5}$, i.e. a similar cross-correlation within the expected frequency/time window after the merger cannot be found more than 4 times in 3 days. If confirmed, this finding will have significant consequences for both physics of quantum black holes and astrophysics of binary neutron star mergers [Note added: This result is independently confirmed by arXiv:1901.04138, who use the electromagnetic observations to infer $t_{\rm coll}=0.98^{+0.31}_{-0.26}$ sec for black hole formation].

gr-qc↗

arXiv2020

Echoes from the Abyss: A Status Update

Gravitational wave echoes provide our most direct and surprising observational window into quantum nature of black holes. Three years ago, the first search for echoes from Planck-scale modifications of general relativity near black hole event horizons led to tentative evidence at false detection probability of 1\% arXiv:1612.00266 . The study introduced a naive phenomenological model and used the public data release by the Advanced LIGO gravitational wave observatory for the first observing run O1 (GW150914, GW151226, and LVT151012, now GW151012). Here, we provide a status update on various observational searches for echoes by independent groups, and argue that they can all be consistent if echoes are most prominent at lower frequencies and/or in binary mergers of more extreme mass ratio. We also point out that the only reported "detection" of echoes (with $>4σ$ confidence) at 1.0 second after the binary neutron star merger GW170817 arXiv:1803.10454 is coincident with the formation time of the black hole inferred from electromagnetic observations.

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arXiv2019

On the origin of the LIGO "mystery" noise and the high energy particle physics desert

One of the most ubiquitous features of quantum theories is the existence of zero-point fluctuations in their ground states. For massive quantum fields, these fluctuations decouple from infrared observables in ordinary field theories. However, there is no "decoupling theorem" in Quantum Gravity, and we recently showed that the vacuum stress fluctuations of massive quantum fields source a red spectrum of metric fluctuations given by $\sim$ mass$^5$/frequency in Planck units. I show that this signal is consistent with the reported unattributed persistent noise, or "mystery" noise, in the Laser Interferometer Gravitational-Wave Observatory (LIGO), for the Standard Model of Particle Physics. If this interpretation is correct, then it implies that: 1) This will be a fundamental irreducible noise for all gravitational wave interferometers, and 2) There is no fundamental weakly-coupled massive particle heavier than those in the Standard Model.

gr-qc↗

arXiv2019

Does History Repeat Itself? Periodic Time Cosmology

It has been suggested that the cosmic history might repeat in cycles, with an infinite series of similar aeons in the past and the future. Here, we instead propose that the cosmic history repeats itself exactly, constructing a universe on a periodic temporal history, which we call Periodic Time Cosmology. In particular, the primordial power spectrum, convolved with the transfer function throughout the cosmic history, would form the next aeon's primordial power spectrum. By matching the big bang to the infinite future using a conformal rescaling (a la Penrose), we uniquely determine the primordial power spectrum, in terms of the transfer function up to two free parameters. While nearly scale invariant with a red tilt on large scales, using Planck and Baryonic Acoustic Oscillation observations, we find the minimal model is disfavoured compared to a power-law power spectrum at $5.1σ$. However, extensions of $Λ$CDM cosmic history change the large scale transfer function and can provide better relative fits to the data. For example, the best fit seven parameter model for our Periodic Time Cosmology, with $w=-1.024$ for dark energy equation of state, is only disfavoured relative to a power-law power spectrum (with the same number of parameters) at $1.8σ$ level. Therefore, consistency between cosmic history and initial conditions provides a viable description of cosmological observations in the context of Periodic Time Cosmology.

astro-ph.CO↗

arXiv2019

$H_0$ tension as a hint for a transition in gravitational theory

We propose a cosmological model, ü$Λ$CDM, based on {\it über-gravity}, which is a canonical ensemble average of many theories of gravity. In this model, we have a sharp transition from (a purely) $Λ$CDM era to a phase in which the Ricci scalar is a constant. This transition occurs when the Ricci scalar reaches a critical scale or alternatively at a transition redshift, $z_\oplus$. We use the observations of baryonic acoustic oscillations (BAO) and Supernovae Ia (SNe), as well as the cosmic microwave background (CMB) data to constrain ü$Λ$CDM. This yields $H_0=70.6_{-1.3}^{+1.1}$ km/s/Mpc, $Ω_m=0.2861 \pm{0.0092} $ and $z_\oplus=0.537_{-0.375}^{+0.277}$, providing a marginally better fit with a Akaike information criterion of 0.8. %After adding in the data from BAO in the Lyman-$α$ forest in quasar spectra, we get $Δχ^2=-2.14$, where the best fit occurs at $H_0=71.2_{-1.5}^{+1.8}$ km/s/Mpc, $Ω_m=0.280\pm 0.013$ and $z_{\oplus}= 0.41^{+0.31}_{-0.29}$. Therefore, ü$Λ$CDM can ease the $H_0$-tension, albeit marginally, with one additional free parameter. We also provide a preliminary study of the linear perturbation theory in ü$Λ$CDM which points to interesting potential {\it smoking guns} in the observations of large scales structure at $z < z_{\oplus}$.

astro-ph.CO↗

arXiv2019

Cooling+Heating Flows in Galaxy clusters: Turbulent heating, spectral modelling, and cooling efficiency

The discrepancy between expected and observed cooling rates of X-ray emitting gas has led to the {\it cooling flow problem} at the cores of clusters of galaxies. A variety of models have been proposed to model the observed X-ray spectra and resolve the cooling flow problem, which involves heating the cold gas through different mechanisms. As a result, realistic models of X-ray spectra of galaxy clusters need to involve both heating {\it and} cooling mechanisms. In this paper, we argue that the heating time-scale is set by the magnetohydrodynamic (MHD) turbulent viscous heating for the Intracluster plasma, parametrised by the Shakura-Sunyaev viscosity parameter, $α$. Using a cooling+heating flow model, we show that a value of $α\simeq 0.05$ (with 10\% scatter) provides improved fits to the X-ray spectra of cooling flow, while at the same time, predicting reasonable cooling efficiency, $ε_{cool} = 0.33^{+0.63}_{-0.15}$. Our inferred values for $α$ based on X-ray spectra are also in line with direct measurements of turbulent pressure in simulations and observations of galaxy clusters. This simple picture unifies astrophysical accretion, as a balance of MHD turbulent heating and cooling, across more than 16 orders of magnitudes in scale, from neutron stars to galaxy clusters.

astro-ph.GA↗

arXiv2019

Echoes from Quantum Black Holes

One of the most triumphant predictions of the theory if general relativity was the recent LIGO-Virgo detection of gravitational wave (GW) signals produced in binary black hole (BH) mergers. However, it is suggested that exotic compact objects, proposed in quantum gravity models of BHs, may produce similar classical GW waveforms, followed by delayed repeating "echoes". In a companion paper [1], we have presented different arguments for a universal Boltzmann reflectivity of quantum BH horizons. Here, we investigate the resulting echoes from this prescription. We derive corresponding quasi-normal modes (QNMs) for quantum BHs analytically, and show how their initial conditions can be related to the QNMs of classical BHs. Ergoregion instability is suppressed by the imperfect reflectivity. We then compare the analytic and numerical predictions for echoes in real time, verifying their consistency. In particular, we find that the amplitudes of the first ~20 echoes decay inversely with time, while the subsequent echoes decay exponentially. Finally, we present predictions for the signal-to-noise ratio of echoes for spinning BHs, which should be imminently detectable for massive remnants, subject to the uncertainty in the nonlinear initial conditions of the BH merger.

gr-qc↗

arXiv2019

Bose-Einstein Condensates as Gravitational Wave Detectors

We investigate a Bose-Einstein condensate (BEC) as a gravitational wave detector, and study its sensitivity by optimizing the properties of the condensate and the measurement duration. We show that detecting kilohertz gravitational waves is limited by current experimental techniques in squeezing BEC phonons. Future improvements in technology to squeeze BEC states can make them competitive detectors for gravitational waves of astrophysical and/or cosmological origin.

quant-ph↗

arXiv2019

Probing microstructure of black hole spacetimes with gravitational wave echoes

Quantum nature of black hole horizons has been a subject of recent interest and scrutiny. In particular, a near-horizon quantum violation of the equivalence principle has been proposed as a resolution of the black hole information paradox. Such a violation may lead to a modified dispersion relation at high energies, which could become relevant due to the intense gravitational blueshift experienced by ingoing gravitational waves. We investigate the ringdown for a perturbed black hole with such a modified dispersion relation and find that infalling gravitational waves are partially reflected near the horizon. This results in the appearance of late-time ${\it echoes}$ in the ringdown phase of black hole merger events, with similar properties to those (arguably) seen in the Advanced LIGO observations. Current measurements suggest a Lorentz-violation scale of $10^{13 \pm 2}$ GeV for gravitational waves, with comparable dissipation and dispersion. Therefore, if confirmed, black hole ringdown echoes probe the microstructure of horizons and thus can test Lorentz-violating UV completions, such as in Ho$ř$ava-Lifshitz gravity.

gr-qc↗

arXiv2018

Temperatures of renormalizable quantum field theories in curved spacetime

In this paper we compute the temperature registered by an Unruh-DeWitt detector coupled to a Hadamard renormalizable quantum field in an arbitrary state, moving along an accelerated trajectory in a curved spacetime. For a massless and conformally invariant field, the generalized expression for the temperature is given by the quadratic sum of the 4-acceleration, Raychaudhuri scalar, and renormalized field polarization. We can further find a novel constraint on the renormalized quantum field polarization in relativistic systems that are in global thermal equilibrium.

gr-qc↗

arXiv2018

Searching for dark matter annihilation from individual halos: uncertainties, scatter and signal-to-noise ratios

Individual extragalactic dark matter halos, such as those associated with nearby galaxies and galaxy clusters, are promising targets for searches for gamma-rays from dark matter annihilation. We review the predictions for the annihilation flux from individual halos, focusing on the effect of current uncertainties in the concentration-mass relation and the contribution from halo substructure, and also estimating the intrinsic halo-to-halo scatter expected. After careful consideration of recent simulation results, we conclude that the concentrations of the smallest halos, while well determined at high redshift, are still uncertain by a factor of 4-6 when extrapolated to low redshift. This in turn produces up to two orders of magnitude uncertainty in the predicted annihilation flux for any halo mass above this scale. Substructure evolution, the small-scale cutoff to the power spectrum, cosmology, and baryonic effects all introduce smaller, though cumulative, uncertainties. We then consider intrinsic variations from halo to halo. These arise from variations in concentration and substructure, leading to a scatter of $\sim$ 2.5 in the predicted annihilation luminosity. Finally, we consider the problem of detecting gamma-rays from annihilation, given the expected contributions from other sources. We estimate the signal-to-noise ratio for gamma-ray detection as a function of halo mass, assuming that cosmic rays from star formation are the main noise source in the detection. This calculation suggests that group-scale halos, individually or in stacks, may be a particularly interesting target for the next generation of annihilation searches.

astro-ph.CO↗

arXiv2018

Forward modelling of quasar light curves and the cosmological matter power spectrum on milliparsec scales

We devise an optimal method to measure the temporal power spectrum of the lensing and intrinsic fluctuations of multiply-imaged strongly lensed quasar light curves, along with the associated time delays. The method is based on a Monte-Carlo Markov Chain (MCMC) sampling of a putative gaussian likelihood, and accurately recovers the input properties of simulated light curves, as well as the "Time Delay Challenge". We apply this method to constrain the dimensionless cosmological (non-linear) matter power spectrum on milliparsec scales (comparable to the size of the solar system), to $Δ_{\rm NL}^2< 4 \times 10^7$ at $k_{\rm NL} \sim 10^3 {\rm pc}^{-1}$. Using a semi-analytic nonlinear clustering model which is calibrated to simulations, the corresponding constraint on the primordial (linear) scalar power spectrum is ${\cal P}_{\cal R} < 3 \times 10^{-9}$ at $k_{\rm L}\sim$ 3 pc$^{-1}$. This is the strongest constraint on primordial power spectrum at these scales, and is within an order of magnitude from the standard $Λ$CDM prediction. We also report measurements of temporal spectra for intrinsic variabilities of quasar light curves, which can be used to constrain the size of the emitting region in accretion disks. Future cadenced optical imaging surveys, such as LSST, should increase the number of observed strongly lensed quasars by 3 orders of magnitude and significantly improve these measurements, even though improvements in modelling quasar accretion and stellar microlensing are necessary.

astro-ph.CO↗

arXiv2018

Black Hole Echology: The Observer's Manual

While recent detections of gravitational waves from the mergers of binary black holes match well with the predictions of General Relativity (GR), they cannot directly confirm the existence of event horizons. Exotic compact objects (ECOs) are motivated by quantum models of black holes, and can have exotic structure (or a "wall") just outside the (would-be) horizon. ECOs produce similar ringdown waveforms to the GR black holes, but they are followed by delayed "echoes". By solving linearized Einstein equations we can model these echoes and provide analytic templates that can be used to compare to observations. For concreteness, we consider GW150914 event, detected by the LIGO/Virgo collaboration, and study the model dependence of its echo properties. We find that echoes are reasonably approximated by complex gaussians, with amplitudes that decay as a power law in time, while their width in time (frequency) grows (shrinks) over subsequent echoes. We also show that trapped modes between a perfectly reflecting wall and angular momentum barrier in Kerr metric can exhibit superradiant instability over long times, as expected.

gr-qc↗

arXiv2018

Cosmological Tests of Everpresent $Λ$

Everpresent $Λ$ is a cosmological scenario in which the observed cosmological "constant" $Λ$ fluctuates between positive and negative values with a vanishing mean, and with a magnitude comparable to the critical density at any epoch. In accord with a longstanding heuristic prediction of causal set theory, it postulates that $Λ$ is a stochastic function of cosmic time that will vary from one realization of the scenario to another. Herein, we consider two models of "dark energy" that exhibit these features. Via Monte Carlo Markov chains, we explore the space of cosmological parameters and the set of stochastic realizations of these models, finding that Everpresent $Λ$ can fit the current cosmological observations as well as the $Λ$CDM model does. Furthermore, it removes observational tensions with $Λ$CDM, for low redshift measurements of Hubble constant, and the Baryonic Acoustic Oscillations (BAO) in Lyman-$α$ forest at $z\sim 2-3$. However, we also find that Everpresent $Λ$ does not significantly help with the growth of ultramassive black holes at high redshift, and the Lithium problem in Big Bang Nuclesynthesis. Future measurements of "dark energy" at high redshifts will further test the viability of Everpresent $Λ$ as an alternative to the $Λ$CDM cosmology.

gr-qc↗

arXiv2018

Instantaneous Temperatures à la Hadamard: Towards a generalized Stefan-Boltzmann law for curved spacetime

In the celebrated Unruh effect, we learn that a uniformly accelerating detector in a Minkowski vacuum spacetime registers a constant temperature. Building on prior work, we present a technique based on derivative couplings of the two-point Wightman function and the Hadamard renormalization procedure to define an instantaneous temperature for a massive scalar field, non-minimally coupled to gravity. We find the temperature contains local contributions from the acceleration of the detector, the curvature of spacetime, and the renormalized stress-energy tensor of the field. Our result, which can be considered as a generalized Stefan-Boltzmann law for curved spacetimes, agrees with the familiar expressions found in 4D Rindler, thermal Minkowski, and de Sitter.

gr-qc↗

arXiv2018

Comment on: "Low significance of evidence for black hole echoes in gravitational wave data"

In a recent publication (1612.00266), we demonstrated that the events in the first observing run of the Advanced LIGO gravitational wave observatory (aLIGO O1) showed tentative evidence for repeating "echoes from the abyss" caused by Planck-scale structure near black hole horizons. By considering a phenomenological echo model, we showed that the pure noise hypothesis is disfavored with a p-value of 1%, i.e. higher amplitude for echoes than those in aLIGO O1 events are only recovered in 1% of random noise realizations. A recent preprint by Westerweck, et al. (1712.09966) provides a careful re-evaluation of our analysis which claims "a reduced statistical significance ... entirely consistent with noise". It is a mystery to us why the authors make such a statement, while they also find a p-value of 2 $\pm$ 1% (given the Poisson error in their estimate) for the same model and dataset. This is p-erfectly consistent with our results, which would be commonly considered as disfavoring the null hypothesis, or "moderate to significant" evidence for "echoes". Westerweck, et al. also point to diversity of the observed echo properties as evidence for statistical fluke, but such a diversity is neither unique nor surprising for complex physical phenomena in nature.

gr-qc↗

arXiv2017

Echoes from the Abyss: Tentative evidence for Planck-scale structure at black hole horizons

In classical General Relativity (GR), an observer falling into an astrophysical black hole is not expected to experience anything dramatic as she crosses the event horizon. However, tentative resolutions to problems in quantum gravity, such as the cosmological constant problem, or the black hole information paradox, invoke significant departures from classicality in the vicinity of the horizon. It was recently pointed out that such near-horizon structures can lead to late-time echoes in the black hole merger gravitational wave signals that are otherwise indistinguishable from GR. We search for observational signatures of these echoes in the gravitational wave data released by advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), following the three black hole merger events GW150914, GW151226, and LVT151012. In particular, we look for repeating damped echoes with time-delays of $8 M \log M$ (+spin corrections, in Planck units), corresponding to Planck-scale departures from GR near their respective horizons. Accounting for the "look elsewhere" effect due to uncertainty in the echo template, we find tentative evidence for Planck-scale structure near black hole horizons at false detection probability of $1\%$ (corresponding to $2.5σ$ significance level). Future observations from interferometric detectors at higher sensitivity, along with more physical echo templates, will be able to confirm (or rule out) this finding, providing possible empirical evidence for alternatives to classical black holes, such as in ${\it firewall}$ or ${\it fuzzball}$ paradigms.

gr-qc↗