arXiv subjects

Niayesh Afshordi

Publications and source records attributed to Niayesh Afshordi.

At least 19 recordsLinked to original sources

arXiv2026

White paper: 1-10 Hz matter-wave interferometer to test the spin entanglement witness for quantum gravity

In this white paper, we highlight the importance of the ($1-10~{\rm Hz}$) frequency range for laboratory tests of the quantum nature of gravity using the quantum gravity-induced entanglement of masses (QGEM) protocol. QGEM requires matter-wave interferometers with masses ($m\sim10^{-15}-10^{-14}~{\rm kg}$), brought within separations ($d\sim30-50~μ{\rm m}$), while maintaining spatial superpositions of ($1-20~μ{\rm m}$) and coherence for ($τ\sim 0.1 - 1~{\rm s}$). These requirements make low-frequency environmental noise a central experimental challenge and place QGEM in a regime closely related to the low-frequency goals of the Einstein Telescope (ET) and the Cosmic Explorer (CE). In particular, QGEM is sensitive to relative acceleration noise (RAN) and to gravity-gradient noise (GGN) generated by seismic and other environmental mass-density fluctuations. For representative parameters $m=10^{-14}~{\rm kg}$, $Δx=10~μ{\rm m}$, and $τ=1~{\rm s}$, the differential acceleration-noise amplitude spectral density must be suppressed well below the $10^{-15}~{\rm m\,s^{-2}/\sqrt{Hz}}$ level to keep acceleration-induced dephasing below the relevant experimental scale. Achieving this level of low-frequency noise suppression is therefore a key requirement for QGEM and closely parallels the seismic and gravity-gradient noise challenges that ET and CE address.

quant-ph↗

arXiv2026

Is DESI Seeing Dynamical Dark Energy, or a Cosmic Glitch in Gravity?

Combined baryon acoustic oscillation and cosmic microwave background data have begun to mildly favor departures from $Λ\mathrm{CDM}$. This is usually framed as 2-parameter dynamical dark energy. However, a 1-parameter "cosmic glitch in gravity" model, whose cosmological gravitational coupling differs from Newton's constant as in Hořava gravity, fits at least as well. $\textit{Planck}$+ACT+SPT and DESI~DR2 give $G_\mathrm{cosmo}/G_\mathrm{N}=0.9920\pm0.0025$, a $3.3\,σ$ preference for weaker cosmological gravity, robust to adding CMB lensing and supernovae. Fitted to the CMB alone, the glitch predicts DESI distances; $w_0w_a$CDM does not.

astro-ph.CO↗

arXiv2026

A Cuscuton Representation of the Loop Quantum Cosmology Bounce

Loop Quantum Cosmology (LQC) replaces the big bang singularity of the homogeneous universe by a bounce, usually described by the modified Friedmann equation $H^2=ρ(1-ρ/ρ_c)/(3M_p^2)$. We show that this background dynamics follows from a local cuscuton effective theory, whose scalar equation is a constraint rather than a wave equation. One way to establish this is to write the cuscuton in terms of an angular coordinate $θ$, identified with the LQC polymerization angle $2λb$. Its constraint gives $H\propto\sinθ$, while the Einstein constraint gives $ρ=ρ_c\sin^2(θ/2)$, exactly reproducing the LQC bounce. To our knowledge, this is the first closed-form, local, generally covariant realization of the exact standard flat-FLRW LQC background dynamics for minimally coupled matter satisfying null energy condition, without introducing additional local dynamical degrees of freedom. A branchwise Legendre transformation establishes a canonical equivalence between the clock-gauge-fixed homogeneous mimetic system in the case of vanishing mimetic dust energy density and the cuscuton systems. It selects the constant-tension cuscuton coordinate $Θ=\sinθ-θ$, while $θ$ retains its interpretation as the LQC polymerization angle. This equivalence does not establish agreement of the inhomogeneous theories or their perturbations. The angular action also admits an explicit branched $f(K)$ representation, where $K$ is the mean extrinsic curvature of spatial hypersurfaces. The construction is therefore an effective covariant representation of the LQC holonomy correction, not a derivation from full loop quantum gravity.

gr-qc↗

arXiv2026

Current and Future Constraints on the Primordial Power Spectrum

The primordial scalar power spectrum provides a powerful window onto early-universe physics, including a broad class of inflationary scenarios. These models often predict a characteristic scalar spectral index $(n_s)$, nonzero running $(α_s)$, or more general deviations from a pure power-law form. We find that freeing the number of light relic species $(N_{\rm eff})$ and the sum of the neutrino masses $(\sum m_ν)$ broadens the $n_s$ error contours enough that key early-Universe models are no longer excluded. In particular, while both Starobinsky inflation and the Bi-thermal Big Bang model are ruled out at about the 95% confidence level in the six-parameter $Λ$CDM model, we find that in the nine-parameter $Λ{\rm CDM} + α_s + N_{\rm eff} + \sum m_ν$ model, CMB data from Planck, ACT, and SPT (CMB-PAS) combined with DESI DR2 give $n_s = 0.9758 \pm 0.0064$ and $α_s = 0.0080 \pm 0.0064$, consistent with both models. Future CMB facilities will sharply improve constraints on $n_s$, $α_s$, and, more generally, the binned primordial power spectrum $\mathcal{P}(k)$. For example, CMB-PAS constrains $e^{-2τ}\mathcal{P}(k = 0.2\,{\rm Mpc}^{-1})$ to 0.5%, while SO-like and CMB-HD-like surveys would tighten this to 0.1%; CMB-HD would also extend this measurement to smaller, previously unprobed scales, constraining $e^{-2τ}\mathcal{P}(k = 30\,{\rm Mpc}^{-1})$ to within a factor of ten. We release our forecast code and update the public CMB-HD likelihood and Fisher codes to support CLASS in addition to CAMB.

astro-ph.CO↗

arXiv2026

Gauss--Bonnet running and the de Sitter saddle of quadratic gravity inflation

Inflation driven purely by the quantum running of the curvature-squared couplings of quadratic gravity was shown to accommodate the high scalar tilt favored by recent cosmic microwave background observations, starting from a Euclidean four-sphere at a maximum of the running $R^2$ coupling. That maximum exists only in one renormalization scheme, and is not stationary once the Euler trace anomaly is included. We show that the standard one-loop running, with the usually neglected Gauss--Bonnet coefficient retained, recovers the missing de Sitter state: the Euler running balances the scale dependence of the $R^2$ term at a unique coupling ratio, giving a de Sitter solution that is stationary for the gravitational constraint and for the compact Euclidean action alike, and whose scalaron potential is an extremely flat hilltop ($m^2/H^2\simeq-5\times10^{-10}$) joined to an inverse-linear inflationary plateau. The last $N_*\simeq50$--$60$ $e$-folds are scheme independent, with scalar tilt $n_s\simeq1-4/(3N_*)$, while the tensor-to-scalar ratio, $r$, is set by the number of matter fields that enhance the running. The current tensor bound excludes pure gravity and sets a minimum matter content, some $4\times10^6$ conformally coupled scalars or $3\times10^5$ vectors---$2.6$ times fewer than the momentum-induced scheme requires---while one-loop control to the end of inflation sets a maximum about ten times higher. Across that window the model predicts $r\gtrsim0.008$ with $0.973\lesssim n_s\lesssim0.978$, within reach of upcoming CMB polarization surveys.

hep-th↗

arXiv2026

Black hole spectroscopy: from theory to experiment

The "ringdown" radiation emitted by oscillating black holes has great scientific potential. By carefully predicting the frequencies and amplitudes of black hole quasinormal modes and comparing them with gravitational-wave data from compact binary mergers we can advance our understanding of the two-body problem in general relativity, verify the predictions of the theory in the regime of strong and dynamical gravitational fields, and search for physics beyond the Standard Model or new gravitational degrees of freedom. We summarize the state of the art in our understanding of black hole quasinormal modes in general relativity and modified gravity, their excitation, and the modeling of ringdown waveforms. We also review the status of LIGO-Virgo-KAGRA ringdown observations, data analysis techniques, and the bright prospects of the field in the era of LISA and next-generation ground-based gravitational-wave detectors.

gr-qc↗

arXiv2026

Thermal Sunyaev-Zel'dovich cross-correlations with unWISE galaxies: disentangling radio contamination, dust properties, and electron pressure

Cross-correlations between the thermal Sunyaev-Zel'dovich (tSZ) effect and galaxy surveys provide a sensitive probe of hot gas in low-mass halos, but on the angular scales of greatest astrophysical interest they are also highly vulnerable to residual foreground contamination. We analyze the cross-correlation of two low-redshift unWISE galaxy samples with Planck PR4 and ACT DR6 microwave temperature sky maps directly in harmonic space, fitting simultaneously for correlated tSZ, cosmic infrared background (CIB), and radio emission. Using the nine Planck bands to perform model selection, we find that a three-component model consisting of tSZ, radio emission, and a CIB amplitude term is strongly preferred over a model that omits radio contamination, with a significance of $9.5σ$ for the unWISE Low-z sample and $11σ$ for the unWISE Mid-z sample respectively. For the unWISE Low-z sample ($\bar{z}=0.14$) the preferred Planck fit gives an effective CIB emissivity index $β_0=1.79\pm0.29$, an effective CIB dust temperature $T_0=22.0\pm5.1 K$, and a radio spectral index $β_r=-2.18\pm0.24$; for the unWISE Mid-z sample ($\bar{z}=0.23$) we find $β_0=1.41\pm0.13$, $T_0=28.0\pm4.2 K$, and $β_r=-2.62\pm0.26$. We apply this radio-inclusive recipe to the ACT+Planck 90, 150, and 220 GHz maps to obtain the best small-scale measurements. This removes the apparent negative galaxy-tSZ cross-correlation seen in the fiducial ACT DR6 NILC reconstruction and in no-radio fits. The cleaned tSZ cross unWISE spectra remain positive to $\ell \simeq 6000$ and are broadly consistent with the Planck-only reconstruction on overlapping scales. We show that these radio-cleaned spectra are well described by a conventional halo model calibrated to the galaxy population and a two-parameter generalized NFW electron pressure profile.

astro-ph.CO↗

arXiv2026

Self-regularized entropy: What does black hole entropy predict for tests of Kerr no-hair theorem?

We compute the canonical, or brick-wall, entropy of a massless scalar field in a quantum black hole model whose strong field exterior is described phenomenologically by the static $q$-metric, also known as the Zipoy-Voorhees metric. This geometry is an exact vacuum deformation of Schwarzschild with a small quadrupolar distortion parameter, $q$. Using WKB counting of trapped near horizon cavity modes, we show that this deformation changes the near horizon density of states so that the usual Schwarzschild brick-wall ultraviolet divergence is self-regularized, eliminating the need for an ad hoc proper distance cutoff within the perturbative regime studied here. Treating the Hawking temperature and Bekenstein-Hawking entropy of a Schwarzschild black hole of the same mass as external thermodynamic benchmarks, we obtain an analytic entropy-motivated deformation scale, $|q|\sim 0.2$, across the stellar-to-supermassive black hole mass range. Through a stationary extension, this scale maps phenomenologically onto percent-to-tens-of-percent violations of the Kerr multipole relations, providing observational targets for ngEHT imaging, LISA extreme mass ratio inspirals, and third generation ground based gravitational wave tests.

gr-qc↗

arXiv2026

Big Mysteries Survey: Physicists' Views on Cosmology, Black Holes, Quantum Mechanics, and Quantum Gravity

We present results from the Big Mysteries Survey, a large-scale survey conducted through the American Physical Society's Physics Magazine on foundational and controversial topics in contemporary physics. The survey provides a snapshot of physicists' views on issues in cosmology, black-hole physics, quantum mechanics, quantum gravity, and anthropic coincidences. A central finding is that several positions often described publicly as field-wide ``consensus'' views are, in practice, supported by much narrower majorities or by pluralities rather than majorities.

physics.soc-ph↗

arXiv2026

Waveform Modelling for the Laser Interferometer Space Antenna

LISA, the Laser Interferometer Space Antenna, will usher in a new era in gravitational-wave astronomy. As the first anticipated space-based gravitational-wave detector, it will expand our view to the millihertz gravitational-wave sky, where a spectacular variety of interesting new sources abound: from millions of ultra-compact binaries in our Galaxy, to mergers of massive black holes at cosmological distances; from the beginnings of inspirals that will venture into the ground-based detectors' view to the death spiral of compact objects into massive black holes, and many sources in between. Central to realising LISA's discovery potential are waveform models, the theoretical and phenomenological predictions of the pattern of gravitational waves that these sources emit. This white paper is presented on behalf of the Waveform Working Group for the LISA Consortium. It provides a review of the current state of waveform models for LISA sources, and describes the significant challenges that must yet be overcome.

gr-qc↗

arXiv2026

Exploring Born-Infeld f(T) teleparallel gravity through accretion disk dynamics

Teleparallel Born-Infeld gravity (TBI) is a modified theory of gravity that aims to maintain second order field equations, leading to alternative scenarios for strong gravity and cosmological settings. In this study, we examine the impact of TBI gravity on the physical characteristics of thin (Novikov-Thorne) accretion disks, focusing on quantities such as flux, pressure, temperature, etc. We also examine the spectral luminosity, comparing it to disks around the Schwarzschild black holes. By comparing the theoretical predictions to observational data in the low frequency regime, we demonstrate the model's ability to match real astrophysical systems and distinguish subtle differences between TBI gravity and general relativity, with improved sensitivity. Furthermore, the results suggest that observations of X-ray spectra from the inner disk regions can provide valuable insights into the properties of TBI gravity, potentially offering constraints on this modified gravity theory through future astrophysical observations.

gr-qc↗

arXiv2026

Ultraviolet Completion of the Big Bang in Quadratic Gravity

We present a quantum quadratic gravity inflationary scenario that can accommodate the new cosmological constraints, which have disfavored Starobinsky inflation. The theory is asymptotically free in the ultraviolet, but 1-loop running is found to dynamically lead to slow-roll inflation toward the infrared. When a large number of matter fields contribute to the beta functions, the spectral index and the tensor-to-scalar ratio can be phenomenologically viable. We find that as inflation ends, the theory approaches its strong coupling regime and general relativity must emerge, as an effective field theory, as the universe must reheat and enter its standard radiation era. In order to avoid strong coupling, a minimum tensor-to-scalar ratio of 0.01 is predicted for this theory. Our framework offers a laboratory for connecting a concrete ultraviolet completion (quantum quadratic gravity) with inflationary dynamics, reheating, and precise cosmological observations.

hep-th↗

arXiv2025

Smart Holes: Analogue black holes with the right temperature and entropy

In analogue gravity studies, the goal is to replicate black hole phenomena, such as Hawking radiation, within controlled laboratory settings. In the realm of condensed matter systems, this may happen in 2D tilted Dirac cone materials based on honeycomb lattice. In particular, we compute the entropy of this system, and find it has the same form as black hole Bekenstein-Hawking entropy, if an analogue horizon forms. Hence, these systems can be potential analogues of quantum black holes. We show that this entropy is primarily concentrated in the region where the tilt parameter is close to one, which corresponds to the location of the analogue black hole horizon. Additionally, when nonlinear effects are taken into account, the entropy is peaked in a small pocket of the Fermi sea that forms behind the analogue event horizon, which we call the \textit{Fermi puddle}. We further refer to this new type of analogue black hole as a {\it smart hole}, since, in contrast to dumb holes, it can simulate both the correct temperature {\it and} entropy of general relativistic black holes. These results provide an opportunity to illuminate various quantum facets of black hole physics in a laboratory setting.

hep-th↗

arXiv2025

Where are all the dark galaxies? Predicting galaxy/halo locations from their bright neighbors

Astronomical objects in our universe that are too faint to be directly detectable exist and are important - an obvious example being dark matter. The same can also apply to very faint baryonic objects, such as low luminosity dwarf galaxies and gravitationally compact objects (e.g., rogue planets, white dwarfs, neutron stars, black holes, dark sirens). While they are very difficult to observe directly, they have locations that are highly important when studying astrophysical phenomena. Here, we use a machine learning algorithm known as symbolic regression to model the probability of a dark object's existence as a function of their separation distances to their closest two ``bright" (directly observable) neighbors, and the distances of these bright objects to each other. An advantage of this algorithm is that it is interpretable by humans and can be used to make reproducible predictions. Galaxies with masses above $10^9 M_{\odot}$ and halos above $10^{12} M_{\odot} $ are the objects that we separate into ``bright" and ``dark" to be used in our analysis. We find that it is possible to predict the density of dark objects using an analytic expression that depends on their distances to their closest bright neighbors in Illustris-TNG galaxy formation simulations, which is significantly better than the (linear) scale-dependent biasing prediction for $k \sim 1.0~ h$Mpc$^{-1}$ (and potentially beyond, if allowed by the resolution). This could potentially open the avenue for finding dark objects based on their vicinity to directly observable bright sources and make future surveys more targeted and efficient.

astro-ph.CO↗

arXiv2025

Copenhagen Survey on Black Holes and Fundamental Physics

The purpose of this survey is to take a snapshot of the attitudes of physicists working on some of the most pressing questions in modern physics, which may be useful to sociologists and historians of science. For this study, a total of 85 completed surveys were returned out of 151 registered participants of the ``Black holes Inside and out'' conference, held in Copenhagen in 2024. The survey asked questions about some of the most contentious issues in fundamental physics, including the nature of black holes and dark energy. A number of surprising results were found. For example, some of the leading frameworks, such as the cosmological constant, cosmic inflation, or string theory - while most popular - gain less than the majority of votes from the participants. The only statement that gains majority approval (by 68\% of participants) was that the Big Bang meant ``the universe evolved from a hot dense state'', not ``an absolute beginning time''. These results provide reasons for caution in describing ideas as consensus in the scientific community when a more nuanced view may be justified.

gr-qc↗

arXiv2025

Doppler bias: impact of peculiar velocities on color selection and the large scale structure of galaxy surveys

Lightcone selection effects on cosmic observables must be precisely accounted for in the next generation of surveys, including the Dark Energy Spectroscopic Instrument (DESI) survey. This will allow us to correctly model the data and extract subtle shifts from general-relativistic effects. We examine the effects of peculiar velocities on color selection in spectroscopic galaxy surveys, with a focus on their implications for the galaxy clustering dipole $P_1(k)$. Using DESI Emission Line Galaxy (ELG) targets, we show that peculiar velocities can shift spectral emission features into or out of filter bands, modifying galaxy colors and thereby changing galaxy selection. This phenomenon mimics the effect of evolution bias, and we refer to it as the Doppler bias, $b_D$. The Doppler bias is of comparable size to the evolution bias at $0.8 < z < 1$, where it is largest. This enhances the ELG-LRG (Luminous Red Galaxy) cross-correlation dipole by 25-50%. This could be detectable at the $\sim$6$σ$ level for the full DESI survey. Additionally, we found that our $b_D$ estimate is impacted by the incompleteness of the parent ELG sample. Therefore, this work highlights the essential need for careful consideration of spectral-dependent biases caused by peculiar velocities during the selection phase of galaxy surveys, to enable unbiased analyses.

astro-ph.CO↗

arXiv2025

Conformal Cores of Quantum Black Holes in Quadratic Gravity

We explore the possibility that quadratic gravity, as a renormalizable theory, describes the interior of quantum black holes. We find new exact power-law solutions to pure quadratic gravity under spherical symmetry, which are complex valued. The resulting solutions, dubbed powerballs, are horizonless compact objects that become Schwarzschild-like a small distance (of the order of the Planck length) outside the would-be Schwarzschild horizon. We present a description of the global eternal geometry, whose right and left exteriors are Lorentzian and Euclidean Schwarzschild-like regions, respectively, while the complex interior is a form of spiraling spacetime. We compute the total on-shell action integral as a saddle point to a gravitational path integral and discuss the Lorentzian and Euclidean interpretations thereof.

hep-th↗

arXiv2025

Formulating the complete initial boundary value problem in numerical relativity to model black hole echoes

In an attempt to simulate black hole echoes (generated by potential quantum-gravitational structure) in numerical relativity, we recently described how to implement a reflecting boundary outside of the horizon of a black hole in spherical symmetry. Here, we generalize this approach to spacetimes with no symmetries and implement it numerically using the generalized harmonic formulation. We cast the evolution equations and the numerical implementation into a Summation By Parts (SBP) scheme, which seats our method closer to a class of provably numerically stable systems. We implement an embedded boundary numerical framework that allows for arbitrarily shaped domains on a rectangular grid and even boundaries that evolve and move across the grid. As a demonstration of this framework, we study the evolution of gravitational wave scattering off a boundary either inside, or just outside, the horizon of a black hole. This marks a big leap toward the goal of a generic framework to obtain gravitational waveforms for behaviors motivated by quantum gravity near the horizons of merging black holes.

gr-qc↗