arXiv subjects

M. M. Sheikh-Jabbari

Publications and source records attributed to M. M. Sheikh-Jabbari.

At least 37 records · Page 2Linked to original sources

arXiv2026

Freelance Fluid/Gravity Correspondence, 3d Analysis

Freelance holography program is an extension of gauge/gravity correspondence, where the gravity theory is defined on a portion of AdS with an arbitrary timelike boundary, with any desired boundary conditions. It is also known that gauge/gravity correspondence admits a fluid/gravity correspondence limit, where the gauge theory side is well described by a fluid. In this work, combining the two, we work through ``freelance fluid/gravity''. In particular, we study in detail the 2d fluid (3d Einstein gravity) case, where one has a good analytical control over the bulk equations due to their integrability and absence of viscosity in the 2d fluid. We study consistency and validity requirements for the freelance fluid/gravity and how the fluid changes along the renormalization group (RG) flow. We prove the $v_g$-theorem, stating that the group velocity of fluid waves $v_g$ is a decreasing function as we move toward the infrared region along the RG flow, regardless of the adopted boundary conditions. We also study examples of holographic fluid with various asymptotic boundary conditions.

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arXiv2025

AdS$_3$ Freelance Holography, A Detailed Analysis

Freelance holography program is an extension of the gauge/gravity correspondence in which the boundary theory can reside on any timelike codimension-one surface in AdS space, and the boundary conditions on the bulk fields can be chosen arbitrarily. Freelance holography provides the framework for a systematic study of various boundary conditions and associated bulk geometries. In this work, we analyze in detail the AdS$_3$ freelance holography. One can explicitly solve for the bulk AdS$_3$ Einstein gravity equations of motion. For generic boundary conditions, the solutions are described by two arbitrary functions of one variable. We study holographic renormalization group (RG) flows, the interpolation between different boundary conditions at different boundaries and associated surface charges and their algebras.

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arXiv2025

On the Analysis Dependence of DESI Dynamical Dark Energy

We continue scientific scrutiny of the DESI dynamical dark energy (DE) claim by explicitly demonstrating that the result depends on the analysis pipeline. Concretely, we define a likelihood that converts the $w_0 w_a$CDM model back into the (flat) $Λ$CDM model, which we fit to DESI constraints on the $Λ$CDM model from DR1 Full-Shape (FS) modelling and BAO. We further incorporate CMB constraints. Throughout, we find that $w_0$ and $w_a$ are within $1 σ$ of the $Λ$CDM model. Our work makes it explicit that, in contrast to DR1 and DR2 BAO, there is no dynamical DE signal in FS modelling, even when combined with BAO and CMB. Moreover, one confirms late-time accelerated expansion today $(q_0 < 0)$ at $ \gtrsim 3.4 σ$ in FS modelling + BAO. On the contrary, DR1 and DR2 BAO fail to confirm $q_0 < 0$ under similar assumptions. Our analysis highlights the fact that trustable scientific results should be independent of the analysis pipeline.

astro-ph.CO↗

arXiv2025

Gravity Is Induced By Renormalization Group Flow

We revisit the holographic renormalization group (RG) setting in which a 4-dimensional ($4d$) quantum field theory at a finite cutoff corresponds to/is described by the Einstein gravity on a part of AdS$_{5}$ space, cutoff at a finite radius. This holographic setting has interesting and important implications for the $4d$ field theory: Deformation of the field theory by a certain combination involving the square of its energy-momentum tensor can be alternatively viewed as formulating the field theory on a background with a dynamical metric. Explicitly, starting with a non-gravitating $4d$ field theory in the UV, flowing to the IR, quantum effects that we compute using the classical $5d$ Einstein gravity theory, induce an effective $4d$ Einstein gravity theory. In other words, we show that gravity is not a fundamental force and is an effective description of quantum effects in the IR limit.

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arXiv2025

The CosmoVerse White Paper: Addressing observational tensions in cosmology with systematics and fundamental physics

The standard model of cosmology has provided a good phenomenological description of a wide range of observations both at astrophysical and cosmological scales for several decades. This concordance model is constructed by a universal cosmological constant and supported by a matter sector described by the standard model of particle physics and a cold dark matter contribution, as well as very early-time inflationary physics, and underpinned by gravitation through general relativity. There have always been open questions about the soundness of the foundations of the standard model. However, recent years have shown that there may also be questions from the observational sector with the emergence of differences between certain cosmological probes. In this White Paper, we identify the key objectives that need to be addressed over the coming decade together with the core science projects that aim to meet these challenges. These discordances primarily rest on the divergence in the measurement of core cosmological parameters with varying levels of statistical confidence. These possible statistical tensions may be partially accounted for by systematics in various measurements or cosmological probes but there is also a growing indication of potential new physics beyond the standard model. After reviewing the principal probes used in the measurement of cosmological parameters, as well as potential systematics, we discuss the most promising array of potential new physics that may be observable in upcoming surveys. We also discuss the growing set of novel data analysis approaches that go beyond traditional methods to test physical models. [Abridged]

astro-ph.CO↗

arXiv2025

Freelance Holography, Part I: Setting Boundary Conditions Free in Gauge/Gravity Correspondence

We explore AdS/CFT duality in the large $N$ limit, where the duality reduces to gauge/gravity correspondence, from the viewpoint of covariant phase space formalism (CPSF). In particular, we elucidate the role of the $W, Y$, and $Z$ freedoms (also known as ambiguities) in the CPSF and their meaning in the gauge/gravity correspondence. We show that $W$-freedom is associated with the choice of boundary conditions and slicing of solution space in the gravity side, which has been related to deformations by multi-trace operators in the gauge theory side. The gauge/gravity correspondence implies the equivalence of on-shell symplectic potentials on both sides, thereby the $Y$-freedom of the gravity side specifies the on-shell symplectic form of the gauge theory side. The $Z$-freedom, which determines the corner Lagrangian on the gravity side, establishes the boundary conditions and choice of slicing in the boundary theory and its solution space. We utilize these results to systematically formulate freelance holography in which boundary conditions of the fields on the gravity side are chosen freely and are not limited to Dirichlet boundary conditions, and discuss some examples with different boundary conditions.

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arXiv2025

Does DESI 2024 Confirm $Λ$CDM?

We demonstrate that a $\sim 2 σ$ discrepancy with the Planck-$Λ$CDM cosmology in DESI Luminous Red Galaxy (LRG) data in the redshift range $0.4 < z < 0.6$ with effective redshift $z_{\textrm{eff}} = 0.51$ translates into an unexpectedly large $Ω_m$ value, $Ω_m = 0.67^{+0.18}_{-0.17}$. We independently confirm that this anomaly drives the preference for $w_0 > -1$ in DESI data \textit{alone} confronted to the $w_0 w_a$CDM model. Given that LRG data at $z_{\textrm{eff}} = 0.51$ is at odds with Type Ia supernovae in overlapping redshifts, we expect that this anomaly will decrease in statistical significance with future DESI data releases leaving an increasing $Ω_m$ trend with effective redshift at higher redshifts. We estimate the current significance of the latter in DESI data at $\sim 1.8 σ$ and comment on how it dovetails with independent observations. It is imperative to understand what makes DESI LRG data at $z_{\textrm{eff}} = 0.51$ an outlier when it comes to $Ω_m$ determinations.

astro-ph.CO↗

arXiv2025

Strings, Virasoro Sandwiches and Worldsheet Horizons

We revisit the canonical quantization of free bosonic closed string theory and observe that the physicality of states requires vanishing of the worldsheet Virasoro algebra generators sandwiched between any two physical states. This requirement yields four classes of physical states, depending on discrete worldsheet symmetries: parity and time reversal. The usual string states which are highest weight states of the Virasoro algebra, preserve both, while the other new three classes break one or both. We apply our formulation to an accelerated worldsheet with horizons, initiating the worldsheet formulation of a thermal string theory and strings probing horizon of black holes.

hep-th↗

arXiv2025

Do high redshift QSOs and GRBs corroborate JWST?

The James Webb Space Telescope (JWST) is reporting massive high redshift galaxies that appear challenging from the $Λ$CDM perspective. Interpreted as a cosmological problem, this necessitates the Planck collaboration underestimating either matter density $Ω_m$ or physical matter density $Ω_m h^2$ at higher redshifts. Through standard frequentist profile likelihoods, we identify corroborating quasar (QSO) and gamma-ray burst (GRB) data sets where $Ω_m$ increases with effective redshift $z_{\textrm{eff}}$ and remains anomalously large at higher redshifts. We relax the traditional priors by allowing for $Ω_m > 1$, consistent with negative dark energy density, to locate profile likelihood peaks where possible. While the variation of $Ω_m$ with $z_{\textrm{eff}}$ is at odds with the $Λ$CDM model, demarcating frequentist confidence intervals through differences in $χ^2$ in profile likelihoods, the prevailing technique in the literature, points to $3.9 σ$ and $7.9 σ$ tensions between GRBs and QSOs, respectively, and Planck-$Λ$CDM. We explain the approximations inherent in the existing profile likelihood literature, and highlight fresh methodology that generalises the prescription. We show that alternative methods, including Bayesian approaches, lead to similar tensions. Finally, in the large sample limit, we show that Feldman-Cousins prescription for frequentist confidence intervals in the presence of a boundary (prior) leads to confidence intervals that are bounded above by Wilks' theorem.

astro-ph.CO↗

arXiv2025

Towards Quantizing Null p-branes: Light-Cone Gauge Analysis and Physical Hilbert Space

We study null $p$-branes, $p$-branes with a Carrollian $p+1$-dimensional worldvolume embedded in a generic $D$-dimensional flat Minkowski target space. This theory has a generalized BMS$_{p+1}$ gauge symmetry. By fixing the light-cone gauge, the BMS symmetry is partly fixed, leaving $p$ ``momentum constraints'' alongside $p$-dimensional area-preserving diffeomorphisms. We quantize the theory in the light-cone gauge via canonical quantization and construct the physical Hilbert space by imposing the remaining constraints using sandwich conditions: the constraints must vanish when sandwiched between any two physical states. We show that solutions to the sandwich conditions are classified into $p+1$ distinct classes, which we completely specify. In addition, we discuss special and interesting case of membranes in four dimensions and examine the physical implications of the quantized null $p$-brane and its associated physical Hilbert space.

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arXiv2025

Freelance Holography, Part II: Moving Boundary in Gauge/Gravity Correspondence

We continue developing the freelance holography program, formulating gauge/gravity correspondence where the gravity side is formulated on a space bounded by a generic timelike codimension-one surface inside AdS and arbitrary boundary conditions are imposed on the gravity fields on the surface. Our analysis is performed within the Covariant Phase Space Formalism (CPSF). We discuss how a given boundary condition on the bulk fields on a generic boundary evolves as we move the boundary to another boundary inside AdS and work out how this evolution is encoded in deformations of the holographic boundary theory. Our analyses here extend the extensively studied T$\bar{\text{T}}$-deformation by relaxing the boundary conditions at asymptotic AdS or at the cutoff surface to be any arbitrary one (besides Dirichlet). We discuss some of the implications of our general freelance holography setting.

hep-th↗

arXiv2025

Implications of DES 5YR SNe Dataset for $Λ$CDM

Dark Energy Survey five-year supernovae data (DES 5YR SNe) in conjunction with Planck CMB and Dark Energy Spectroscopic Instrument (DESI) BAO data has detected a strong dynamical dark energy (DE) deviation from the $Λ$CDM model.Here we shift the focus of DES data to the pressureless matter sector in the $Λ$CDM model by studying the matter density parameter $Ω_m$. Employing primarily frequentist profile likelihoods, supported by complementary Bayesian methods, we demonstrate that $Ω_m$ increases with effective redshift in the DES data up to a point that there is a $2.5 σ$ discrepancy with Planck. We relax the traditional $Ω_m \leq 1$ prior to demonstrate negative DE densities $Ω_m > 1$ at the highest effective redshift probed. Nevertheless, the largest discrepancy with Planck occurs for profile likelihoods and posteriors peaked at $Ω_m < 1$ in the traditional $Λ$CDM regime. Our findings corroborate earlier observations in Pantheon and Pantheon+ datasets with an independent SNe dataset with a higher effective redshift. In an appendix, we confirm that curvature $Ω_k$ decreases with effective redshift disfavouring a flat Universe in higher redshift DES SNe at $> 3 σ$. Our choice of $Ω_k$ prior leads to an underestimation of the tension with a flat Universe.

astro-ph.CO↗

arXiv2025

DESI and SNe: Dynamical Dark Energy, $Ω_m$ Tension or Systematics?

Dark Energy Spectroscopic Instrument (DESI) observations, when combined with Cosmic Microwave Background (CMB) and Type Ia supernovae (SNe), have led to statistically significant dynamical dark energy (DDE) claims. In this letter we reconstruct the $Λ$CDM parameter $Ω_m$ from the $w_0 w_a$CDM cosmologies advocated by the DESI collaboration. We identify i) a mild increasing $Ω_m$ trend at high redshift and ii) a sharp departure from $Λ$CDM at low redshift. The latter, which is statistically significant, is driven by SNe that are $1.9 σ- 2.5 σ$ discrepant with DESI full-shape galaxy clustering in overlapping redshift ranges. We identify a low redshift subsample of the Dark Energy Survey (DES) SNe sample that is discrepant with DESI at $3.4 σ$ despite both observables probing the same effective redshift. SNe and BAO/full-shape modeling should not disagree on $Ω_m$ at the same effective redshift. This ``$Ω_m$ tension'' most likely points to unexplored systematics. In general, any \textit{bona fide} DDE signal should be confirmed independently across observables, so the DDE claims are premature.

astro-ph.CO↗

arXiv2025

Big Bang in Dipole Cosmology

We continue the study of dipole cosmology framework put forward in \cite{Krishnan:2022qbv}, a beyond FLRW setting that has a preferred direction in the metric which may be associated with a cosmological tilt, a cosmic dipole. In this setup the shear and the tilt can be positive or negative given the dipole direction. We thoroughly analyze evolution of the universe in this setting, particularly focusing on the behaviour near the Big Bang (BB). We first analyze a single fluid model with a generic constant equation of state $w$. While details of the behavior near the BB depends on $w$ and the other initial conditions, we find that when the shear is negative we have a shear dominated BB singularity, whereas for a positive shear we have a much milder singularity, the whimper singularity \cite{Ellis:1974ug}, at which the tilt blows up while curvature invariants remain finite. We then consider dipole $Λ$CDM model which besides the shear has two tilt parameters, one for radiation and one for the pressureless matter. For positive (negative) shear we again find whimper (curvature) singularity near the BB. Moreover, when the tilt parameters have opposite signs, the shear can change sign from negative to positive in the course of evolution of the Universe. We show that the relative tilt of the radiation and the matter generically remains sizable at late times.

astro-ph.CO↗

arXiv2025

A comparison of Bayesian and frequentist confidence intervals in the presence of a late Universe degeneracy

Hubble tension is a problem in one-dimensional (1D) posteriors, since local $H_0$ determinations are only sensitive to a single parameter. Projected 1D posteriors for $Λ$CDM cosmological parameters become more non-Gaussian with increasing effective redshift when the model is fitted to redshift-binned data in the late Universe. We explain mathematically why this non-Gaussianity arises and show using observational Hubble data (OHD) that Markov Chain Monte Carlo (MCMC) marginalisation leads to 1D posteriors that fail to track the $χ^2$ minimum at $68\%$ confidence level in high redshift bins. To gain a second perspective, we resort to profile likelihoods as a complementary technique. Doing so, we observe that $z \gtrsim 1$ cosmic chronometer (CC) data currently prefers a non-evolving (constant) Hubble parameter over a Planck-$Λ$CDM cosmology at $\sim 2 σ$. Within the Hubble tension debate, it is imperative that subsamples of data sets with differing redshifts yield similar $H_0$ values. In addition, we confirm that MCMC degeneracies observed in 2D posteriors are not due to curves of constant $χ^2$. Finally, on the assumption that the Planck-$Λ$CDM cosmological model is correct, using profile likelihoods we confirm a $>2 σ$ discrepancy with Planck-$Λ$CDM in a combination of CC and baryon acoustic oscillations (BAO) data beyond $ z \sim 1.5$. This confirms a discrepancy reported earlier with fresh methodology.

astro-ph.CO↗

arXiv2025

Charges in General Relativity and Black Hole Thermodynamics

We shed a new light on the longstanding problem of covariant charges in diffeomorphism invariant theories like General Relativity (GR) by noting the other important feature of the theory, the background independence. To this end, we develop covariant phase space formalism in which we allow for the boundaries of spacetime to have arbitrary fluctuations. Within this formalism we show non-covariance of charges appear in inevitable integration constants which also break background independence in the expression of charges. We then apply the same formalism to black hole thermodynamics. We generalize the seminal Iyer-Wald derivation the first law of bl1ack hole thermodynamics by relaxing the need for the assumptions at a bifurcation surface and asymptotic infinity, as well as addressing questions regarding the integrability of charges. We also present a first principles derivation of the Smarr relation within our framework.

hep-th↗

arXiv2024

Further support for $S_8$ increasing with effective redshift

In Adil et al. 2023, we reported an increasing trend in $S_8$ with effective redshift $z_{\textrm{eff}}$ based on $f σ_8(z)$ constraints over the redshift range $0 \lesssim z \lesssim 2$, and predicted that this trend would be observable in independent datasets. Recently, the studies by Artis et al. and the ACT+DESI collaboration appeared, presenting data that aligns with the expected trends. In this letter, we quantify the statistical significance of the increasing $S_8$ trends in recent studies by fitting a linear model to estimate the slope $Δ\,S_8/Δ\, z_{\textrm{eff}}$, and comparing the results to mock simulations. We find probabilities of $p = 0.0163$ and $p = 0.01893$, corresponding to approximately $2.1σ$ for each dataset. Using Fisher's method to combine the independent probabilities, we obtained $p=0.0027$ ($2.8 σ$). When we incorporate our earlier findings, the combined statistical significance reaches between $3σ$ and $3.7σ$. Finally, we discuss how ``scatter" in $σ_8$/$S_8$ constraints from recent DESI full-shape galaxy clustering fits this picture at low statistical significance. This letter continues a series of studies initiated in 2020 that explore redshift-dependent $Λ$CDM parameters as an indication of a breakdown in the standard cosmological model.

astro-ph.CO↗

arXiv2024

$Λ$CDM Tensions: Localising Missing Physics through Consistency Checks

$Λ$CDM tensions are by definition model dependent; one sees anomalies through the prism of $Λ$CDM. Thus, progress towards tension resolution necessitates checking the consistency of the $Λ$CDM model to localise missing physics either in redshift or scale. Since the Universe is dynamical and redshift is a proxy for time, it is imperative to first perform consistency checks involving redshift, then consistency checks involving scale, as the next steps to settle the ``systematics versus new physics" debate and foster informed model building. We present a review of the hierarchy of assumptions underlying the $Λ$CDM cosmological model and comment on whether relaxing them can address the tensions. We focus on the lowest lying fruit of identifying missing physics through the identification of redshift dependent $Λ$CDM model fitting parameters. We highlight recent progress made on ${S_8:= σ_8 \sqrt{Ω_{\rm m}/0.3}}$ tension and elucidate how similar progress can be made on $H_0$ tension. Our discussions indicate that $H_0$ tension, equivalently a redshift dependent $H_0$, and a redshift dependent $S_8$ imply a problem with background $Λ$CDM cosmology.

astro-ph.CO↗