arXiv subjects

M. M. Sheikh-Jabbari

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

At least 19 recordsLinked to original sources

arXiv2026

Null String Holography, Null Strings Probe Projective Boundary of Their Target Spaces

We prove that the \textit{minimal null string theory} on background manifold ${\cal M}$ is classically equivalent to the ILST theory on the projective boundary of $\mathcal M$, ${\mathcal B}_{_{\text{P}}}={\partial_{_{\text{P}}}\mathcal{M}}$. We conjecture that this equivalence also holds as quantum level, which we dub \textit{null string holography}. In particular, we show classical null string theory on Poincaré patch of $D$ dimensional Anti de Sitter (AdS$_D$) background is described by ILST on $D-1$ dimensional Minkowski background, paralleling the celebrated AdS/CFT. Null strings on cosmological patch of dS$_D$ is governed by ILST on $D-1$ dimensional flat Euclidean space. For null strings on $D$ dimensional Minkowski space, depending on the coordinate patch used, the projective boundary where the ``dual'' ILST theory resides, can by dS$_{D-1}$, $D-1$ dimensional Carrollian space and $D-1$ dimensional hyperboloid, respectively including spacelike asymptotic boundary $ι^0$, asymptotic null boundaries ${\cal I}^\pm$ and timelike infinities $ι^\pm$. We comment on physical implications of the null string holography on black hole backgrounds whose projective boundary includes horizons.

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arXiv2026

What is Classical Null String Theory?

We revisit the classical definition of a null string in a D-dimensional Minkowski target space and distinguish a consistent partially-gauged Carrollian sigma-model from the null string theory. The Isberg-Lindström-Sundborg-Theodoridis (ILST) action gauges worldsheet diffeomorphisms and the common Weyl rescaling, but not the independent relative rescaling of the temporal and spatial representatives of the intrinsic Carrollian structure, Carroll-Weyl scaling. A physical string theory requires every local change of worldsheet representative to be gauged, therefore, the Carroll-Weyl symmetry should be gauged in a null string theory. In the minimal realization, the corresponding gauging is obstructed at finite tension, whereas at zero tension it yields an additional first-class constraint and the associated identification of physical configurations along its gauge orbits. The description of the null worldsheet as a congruence of null geodesics provides a complementary target-space interpretation of gauging of the Carroll-Weyl scaling. Our analysis thus provides the supplementary requirements that turns the ILST theory into the null string theory.

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arXiv2026

A Prediction for DESI Full-Shape: Increasing Tomographic $Ω_m(z)$ Trend

To confirm $Λ$CDM deviations are due to missing physics (not systematics), one should demonstrate that the model fitting parameters exhibit qualitatively similar redshift drift across independent observables. This is the only way one guarantees new physics. Here, we show that a recent Dark Energy Spectroscopic Instrument (DESI) DR2 Full-Shape (FS) modelling Lyman-$α$ constraint at $z_{\rm eff} = 2.33$ combined with earlier DR1 FS modelling constraints with $0.295 \leq z_{\rm eff} \leq 1.491$ leads to a straight line $Ω_m(z) = m z + c$ with slope $m = 0.022 \pm 0.012$, $1.8 σ$ removed from constant $Ω_m$. Akaike Information Criterion and Bayesian evidence confirm that constant $Ω_m$ and increasing $Ω_m(z)$ are statistically indistinguishable. Through the $Om(z)$ diagnostic, we review how increasing and decreasing $Ω_m(z)$ trends map to phantom and quintessence dark energy (DE) regimes, respectively. While FS modelling constraints map to phantom DE, the decreasing and increasing $Ω_m(z)$ trends in DESI BAO and DESI with external data make a phantom crossing inevitable. Since dynamical DE is but one interpretation for $Ω_m(z)$ trends, it is imperative that different datasets converge on their $Ω_m(z)$ trends before one jumps to physical conclusions. We forecast how DESI FS modelling $Ω_m$ constraints will improve up to the final data release and explore the implications for model selection.

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arXiv2026

Gravitational Entropy And The Second Law of Thermodynamics for Causal Observers

It is well established that black holes possess entropy and behave as thermodynamic systems. Associating entropy with gravitational fields has not remained limited to black holes, necessitating the notion of the second law of thermodynamics in gravitating systems. There have been many ideas and attempts to prove the second law within gravitating systems starting from first principles. Within the covariant phase space formalism, we define gravitational entropy as the charge associated with the local boosts, detaching the gravitational entropy from horizons or trapped surfaces as well as from the diffeomorphisms as symmetry generators. Using this definition for the Einstein gravity case, we compute variations of the entropy along the path of any causal free-fall observer and establish that the entropy variations are always non-negative if the matter content satisfies the strong energy condition integrated along any segment of the observer's trajectory.

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arXiv2026

Who Writes the Gravitational Second Law of Thermodynamics?

We define gravitational entropy as the manifestly integrable surface charge associated with local transverse Lorentz boosts, entirely bypassing the conventional reliance on spacetime diffeomorphisms. Any notion of entropy must satisfy the second law. To answer the question posed in the title, an analogy with Newtonian classical mechanics is instructive: Newton's second law of motion is written by inertial observers who are defined by the first law of mechanics. We show that the second law of gravitational thermodynamics is written by free-fall observers with path parameterization in which the non-affinity of the geodesics is equal to the expansion of their velocity vector field. We then provide a proof of the local second law by studying variations in entropy as viewed by this class of covariantly-defined causal free-fall observers. We show that the entropy variation is strictly non-decreasing, provided the matter sector satisfies the integrated strong energy condition along the observer path.

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arXiv2026

Null Strings Gauged and Reloaded, I: Null Strings Have Carroll-Weyl Gauge Symmetry

Null strings, strings with Carrollian worldsheets, are traditionally described by the Isberg-Lindström-Sundborg-Theodoridis (ILST) action, which is obtained via a tensionless limit of standard tensile strings. In a recent work, we observed that the ILST action enjoys an overlooked partial-gauge symmetry whose existence calls into question the consistency of standard null-string analyses found in the literature. In this paper, we show that the Carrollian geometry provides us with two Weyl scaling options, in contrast to a single Weyl scaling available for the ordinary tensile string worldsheet. Defining the null string theory by the action that realizes the two Carroll-Weyl scalings as well as the 2D diffeomorphisms as local (gauge) symmetries, we construct the new null string action. We show that the ILST action is obtained after fixing one of the two Carroll-Weyl scalings of the action that we construct, and that the residual part of this symmetry is precisely the overlooked partial-gauge symmetry. We hence clarify the Carroll-geometric origin of the overlooked symmetry and pave the way for a consistent quantization of null strings.

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arXiv2026

An Inconsistency in the Null Strings Literature: The Tale of an Overlooked Symmetry

Null strings, strings with $2d$ null worldsheets, have about half a century of literature and are relevant in many physically interesting cases, especially when strings probe cosmological or black hole horizons. We observe that the null string action possesses a previously overlooked local symmetry, the consideration of which is necessary for the consistency of null string analyses. By correctly accounting for this symmetry, we show that the number of physical propagating degrees of freedom of null strings in $D$ dimensional target space is $D-3$, in contrast to $D-2$ that one finds in the literature. In other words, null strings probe a codimension-1 null surface on a $D$ dimensional target space. The existence of this overlooked symmetry calls for a thorough revision of results in the null string literature. We briefly mention some of its physical consequences.

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arXiv2026

Redshift Dependence of $H_0$ Dipole in Pantheon+ Supernovae

We examine the dipole structure in the local cosmic expansion rate $H_0$ using the Pantheon+ compilation of Type Ia supernovae. We reconstruct directional maps of $H_0$ across the sky within the low-redshift regime, $z_{\rm min} < z < 0.2$, evaluated in the CMB rest frame. To perform a tomographic analysis, we choose 10 different values for $z_{\rm min}\in [0.015 , 0.045]$ range. We find a dipole amplitude $A_{\rm dip}=1.16\pm0.28\ {\rm km/s/Mpc}$ for the lowest bin, monotonically decreasing to $A_{\rm dip} =0.35\pm0.52\ {\rm km/s/Mpc}$, as we increase $z_{\rm min}$. Since $A_{\rm dip}$ is positive-definite, by construction $A_{\rm dip} > 0$ even in an isotropic universe, thus to assess the statistical significance, we compare to $A_{\rm dip}$ generated from 1000 Monte Carlo simulations based on the $Λ$CDM model and the same redshift ranges. Our results reveal a $2-3σ$ dipole pattern (depending on how the significance level is computed) for $z_{\rm min}\lesssim 0.032$. For redshift thresholds where the signal remains statistically significant ($>2σ$), the inferred dipole direction points close to Shapley supercluster and CMB dipole directions. As $z_{\rm min}$ increases, the dipole amplitude diminishes in line with expectations of a higher redshift isotropic Universe, but the difference in maximal antipodal $H_0$ determinations, $Δ\text{H}_0^{\rm max}$, retain a signal of an anisotropy that disappears in the dipole ansatz. The decreasing statistical significance of all estimators with increasing $z_{\rm min}$ suggests that any $H_0$ dipole is a low-redshift feature.

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arXiv2026

Dynamical Entropy Is a Noether Charge

Black hole thermodynamics for generic dynamical, non-equilibrium regimes remains a fundamental challenge. We establish dynamical entropy as the Noether charge associated with a generic evolving null surface subject to Dirichlet boundary conditions. We specify the symmetry generator associated with the dynamical entropy, which is a null vector on the null surface, upon requiring physically motivated geometric conditions that yield a notion of ``dynamical zeroth law.'' We prove that this Noether charge density satisfies the second law of thermodynamics strictly at each instant in time, bypassing the teleological final conditions traditionally required by event horizons. Thus, we extend and generalize the notion of dynamical entropy introduced in \cite{Hollands:2024vbe}, in some different ways: We do not impose background stationarity; our dynamical entropy and the associated second law are local in time and work for generic dynamical gravitational systems.

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arXiv2026

Null-strings Gauged, Reloaded and Quantized, I: Canonical Quantization in the Light-Cone Gauge

We study the light-cone quantization of null-strings in $D$ dimensional flat target-space. Incorporating the essential new gauge symmetry and the associated constraint structure of the null-string, allows one to solve for one more degree of freedom (DoF) compared to the standard light-cone gauge, reducing the physical phase space to $(D-3)$ propagating DoF. Quantization is formulated directly in the Schrödinger representation, leading to a Hilbert space of wavefunctions on the reduced configuration space. The space of physical states is built on a reduced phase space associated with the corrected gauge structure. We discuss the ground-state wavefunction and a generic class of excited states. As a direct consequence of the overlooked Carroll-Weyl gauge symmetry of the null-strings, we find the remarkable and perhaps unexpected result that null-strings exhibit a discrete spectrum. Our analysis indicate that there is no critical dimension for null-strings, and $D$ can be arbitrary.

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arXiv2026

Geometric Aspects of Covariant Phase Space Formalism: Solution Space Slicings and Surface Charge Integrability

The Covariant Phase Space Formalism (CPSF) provides a robust framework for deriving symplectic structures and surface charges in diffeomorphism-invariant theories. By construction, the CPSF operates on two distinct manifolds: the spacetime and the Solution Phase Space (SPS). In this paper, we advance the formalism by establishing a strictly parallel geometric formulation for both manifolds. Within this framework, we systematically analyze diffeomorphisms and frame changes on both spaces. While spacetime diffeomorphisms have been extensively studied in the literature, transformations on the SPS have been largely overlooked; we rigorously define and investigate these as changes of slicing on SPS. We demonstrate that the standard Wald-Zoupas criterion for the integrability of surface charge variations is inherently slicing-dependent. To resolve this issue, we develop the Frobenius theorem on the SPS and use it to extends the Wald-Zoupas condition into an inherently slicing-independent criterion for integrability. The Frobenius theorem on the SPS also yields a rigorous and natural definition of fundamental geometric quantities on the solution space, specifically the SPS connection, torsion, and curvature. Furthermore, this geometric machinery naturally distinguishes between fundamentally different surface fluxes: "fake" fluxes are identified mathematically as pure gauge artifacts of the SPS connection, while "genuine" fluxes manifest as non-vanishing SPS torsion, which directly relates to the physical gravitational News tensor. Finally, we present a geometric formulation of the Liouville theorem on the SPS, offering a unified classification scheme for theories with and without propagating bulk degrees of freedom.

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arXiv2026

Null Strings Gauged and Reloaded, II: Consistent Classical Treatment of the Null Strings

We observed that the null strings, tensionless strings with Carrollian worldsheets, exhibit an extra gauge symmetry, \textit{Carroll-Weyl} gauge symmetry, which cannot be obtained from ultra-relativistic Carrollian limit of tensile strings. Due to the existence of this symmetry, the BMS$_3$ algebra of constraints, which is obtained as the Carrollian limit of two Virasoro algebras of the standard tensile strings, should be replaced with an BMS$_3$ algebra extended by a weight one operator. To establish further the existence and necessity of the Carroll-Weyl gauge symmetry, we carefully work through Hamiltonian analyses of constrained/gauged systems. We also discuss the extended BMS$_3$ algebra of constraints.

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arXiv2026

Crosschecking Cosmic Distances from DESI BAO and DES SNe

We perform a consistency check of DESI DR2 BAO constraints ($D_M/r_d, D_H/r_d)$ by reconstructing the same quantities from DES supernovae (SNe) in bins with the same effective redshift $z_{\textrm{eff}} \in \{ 0.510, 0.706, 0.934 \}$ and a Planck $r_d$ prior. Through mock analysis we show that $D_M(z_{\rm eff})$ and $D_{H}(z_{\rm eff})$ can be locally reconstructed model agnostically from $Λ$CDM and extended models, but only if one employs frequentist methods; purely Bayesian reconstructions from Markov Chain Monte Carlo (MCMC) exhibit bias. We find that the ratio of the three $D_M/r_d$ values at different $z_{\textrm{eff}}$ are consistent with a horizontal, thus confirming that the distance duality relation holds up to calibration. However, the $D_H/r_d$ ratio shows a decreasing trend driven by the $z_{\textrm{eff}} = 0.934$ bin, the significance of which varies from $2.5 σ$ with Bayesian methods down to $1.4 σ$ with frequentist methods. We show that replacing DES with DES-Dovekie SNe reduces the significance to $1.7 σ$ and $1.2 σ$ in Bayesian and frequentist approaches, respectively. We conclude that distances reconstructed from SNe show good agreement with DESI BAO distances across the redshifts studied. We also note that $D_M(z_{\rm eff} = 0.510)/r_d$ reconstructed from SNe favours DESI BAO over transversal BAO against a backdrop of a $3.7 σ$ disagreement.

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arXiv2026

GR from RG, $2d$ Example: JT-Gravity Induced from Renormalization Group Flow

We demonstrate how the two-dimensional gravity emerges within ``GR from RG'' program initiated in \cite{Adami:2025pqr, Sheikh-Jabbari:2026uol}. To achieve this, we consider a generic 2d CFT with a 3d holographic description, which we assume to be well-described by pure Einstein-AdS$_3$ gravity in the bulk. We study the holographic RG flow for the 2d CFT action and show that the renormalization group (RG) corrected action at an arbitrary energy scale contains a 2d scalar-tensor gravity theory. In the simplest case, the flow induces Jackiw-Teitelboim (JT) gravity, where the bulk radial lapse function seeds the dynamical dilaton field of the JT gravity. We show that the standard T$\bar{\text{T}}$ deformation of the 2d CFT is recovered as a special case in the Fefferman-Graham limit where the lapse is fixed. We further establish the robustness of the RG induced gravity picture by verifying its consistency under holographic renormalization and by generalizing the result to a one-parameter family of boundary conditions. Our results provide a first-principles derivation of the JT gravity at a finite cutoff as an intrinsic manifestation of the holographic RG flow in a non-Fefferman-Graham gauge

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arXiv2026

GR from RG: Gravity Is Induced From Renormalization Group Flow In The Infrared

In this essay and utilizing the holographic Renormalization Group (RG) flow, we demonstrate how the effective action of a non-gravitating quantum field theory in the ultraviolet (UV) develops an Einstein-Hilbert term in the infrared (IR). That is, gravity is induced by the RG flow. An inherent outcome of holography that plays a crucial role in our analysis is the \textit{RG flow of boundary conditions}: the rigid Dirichlet conditions on the background metric in the UV become an admixture of Dirichlet and Neumann as we flow to the IR, thereby ``unfreezing'' the metric and transforming it from a non-dynamical background into a dynamical field. This mechanism, which is a conceptually new addition to the standard Wilsonian RG flow, also provides the mechanism to evade the Weinberg-Witten no-go theorem. Within the GR from RG picture outlined here, the search for a quantum theory of gravity by treating the metric as a fundamental field may be a hunt for a phantom--akin to seeking the atomic structure of water by quantizing the equations of hydrodynamics.

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arXiv2026

How much has DESI dark energy evolved since DR1?

DESI has reported a dynamical dark energy (DE) signal based on the $w_0 w_a$CDM model that is in conflict with Hubble tension. Recalling that the combination of DESI DR1 BAO and DR1 full-shape (FS) modeling are consistent with $Λ$CDM, in this letter we comment on the status of fluctuations in DR1 BAO documented in \cite{DESI:2024mwx, Colgain:2024xqj} in the DR2 update. In particular, we note that neither DR1 BAO nor DR2 BAO nor DR2 BAO+CMB confronted to the $w_0 w_a$CDM model with relaxed model parameter priors confirm late-time accelerated expansion today. Translating DESI BAO constraints into flat $Λ$CDM constraints, we observe that the LRG1 constraint remains the most prominent outlier, a distinction now held jointly with ELG1, LRG2 switches from smaller to larger $Ω_m$ values relative to Planck-$Λ$CDM, and ELG data drive the relatively low $Ω_m$ in the full DR2 BAO. We observe that one cannot restore $w_0 = -1$ within one $1 σ$ by removing either LRG1 or ELG1 or LRG2, but LRG2 in DR2, in contrast to LRG1 in DR1, now has the greatest bearing on $w_0 > -1$. We conclude that BAO has yet to stabilise, but the general trend is towards greater consistency with DESI DR1 FS modeling results, where there may be no dynamical DE signal in DESI data alone.

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arXiv2026

Revisiting Quantization of Gauge Field Theories: Sandwich Quantization Scheme

Quantization of field theories with gauge symmetry is an extensively discussed and well-established topic. In this short note, we revisit this old problem. While we confirm all details of the existing literature, we highlight a potentially important point which may provide a better understanding of and insights on the quantization of gauged systems. The gauge degrees of freedom have vanishing momenta, and hence their equations of motion appear as constraints on the system. We argue that to ensure consistency of quantization one can impose these constraints as ``sandwich conditions'': The physical Hilbert space of the theory consists of all states for which the constraints sandwiched between any two physical states vanish. We solve the sandwich constraints and show they have solutions not discussed in the gauge field theory literature. We briefly discuss the physical meaning of these solutions and the implications of the "sandwich quantization scheme".

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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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