arXiv subjects

M. M. Sheikh-Jabbari

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

At least 55 records · Page 3Linked to original sources

arXiv2024

Covariant Phase Space Formalism for Fluctuating Boundaries

We reconsider formulating $D$ dimensional gauge theories, with the focus on the case of gravity theories, in spacetimes with boundaries. We extend covariant phase space formalism to the cases in which boundaries are allowed to fluctuate. We analyze the symplectic form, the freedoms (ambiguities), and its conservation for this case. We show that boundary fluctuations render all the surface charges integrable. We study the algebra of charges and its central extensions, charge conservation, and fluxes. We briefly comment on memory effects and questions regarding semiclassical aspects of black holes in the fluctuating boundary setup.

hep-th↗

arXiv2024

Gravitational Stress Tensor and Current at Null Infinity in Three Dimensions

We develop the framework that reveals the intrinsic conserved stress tensor and current associated with the null infinity of a three-dimensional ($3d$) asymptotically flat spacetime. These are, respectively, canonical conjugates of degenerate metric and Ehresmann connection of the boundary Carrollian geometry. Their conservation reproduces the Bondi-mass and angular momentum conservation equations if the asymptotic boundary is endowed with a torsional affine connection that we specify. Our analysis and results shed further light on the $3d$ flat holography; the stress tensor and current give rise to an asymptotically flat fluid/gravity correspondence. The requirement of a well-defined $3d$ action principle yields Schwarzian action at null infinity governing the dynamics induced by reparametrizations over the celestial circle, in accord with the codimension $2$ holography of $3d$ flat spacetimes.

hep-th↗

arXiv2024

On Redshift Evolution & Negative Dark Energy Density in Pantheon+ Supernovae

Within the Friedmann-Lemaître-Robertson-Walker (FLRW) framework, the Hubble constant $H_0$ is an integration constant. Thus, consistency of the model demands observational constancy of $H_0$. We demonstrate redshift evolution of best fit $Λ$CDM parameters $(H_0, Ω_{m})$ in Pantheon+ supernove (SNe). Redshift evolution of best fit cosmological parameters is a prerequisite to finding a statistically significant evolution as well as identifying alternative models that are competitive with $Λ$CDM in a Bayesian model comparison. To assess statistical significance, we employ three different methods: i) Bayesian model comparison, ii) mock simulations and iii) profile distributions. The first shows a marginal preference for the vanilla $Λ$CDM model over an ad hoc model with 3 additional parameters and an unphysical jump in cosmological parameters at $z=1$. From mock simulations, we estimate the statistical significance of redshift evolution of best fit parameters and negative dark energy density ($Ω_m > 1$) to be in the $1-2 σ$ range, depending on the criteria employed. Importantly, in direct comparison to the same analysis with the earlier Pantheon sample we find that statistical significance of redshift evolution of best fit parameters has increased, as expected for a physical effect. Our profile distribution analysis demonstrates a shift in $(H_0, Ω_m)$ in excess of $95\%$ confidence level for SNe with redshifts $z > 1$ and also shows that a degeneracy in MCMC posteriors is not equivalent to a curve of constant $χ^2$. Our findings can be interpreted as a statistical fluctuation or unexplored systematics in Pantheon+ or $Λ$CDM model breakdown. The first two possibilities are disfavoured by similar trends in independent probes.

astro-ph.CO↗

arXiv2024

Putting Flat $Λ$CDM In The (Redshift) Bin

Flat $Λ$CDM cosmology is specified by two constant fitting parameters at the background level in the late Universe, the Hubble constant $H_0$ and matter density (today) $Ω_m$. Mathematically, $H_0$ and $Ω_m$ are either integration constants arising from solving ordinary differential equations or are directly related to integration constants. Seen in this context, if fits of the $Λ$CDM model to cosmological probes at different redshifts lead to different $(H_0, Ω_m)$ parameters, this is a mismatch between mathematics and observation. Here, in mock observational Hubble data (OHD) (geometric probes of expansion history) we demonstrate evolution in distributions of best fit parameters with effective redshift. As a result, considerably different $(H_0, Ω_m)$ best fits from Planck-$Λ$CDM cannot be precluded in high redshift bins. We explore if OHD, Type Ia supernovae and standardisable quasar samples exhibit redshift evolution of best fit $Λ$CDM parameters. In all samples, we confirm a decreasing $H_0$ and increasing $Ω_m$ trend with increasing bin redshift. Through comparison with mocks, we confirm that similar behaviour can arise randomly within the flat $Λ$CDM model with probabilities as low as $p = 0.0021$ ($3.1 \, σ$). We present complementary profile distribution analysis confirming the shifts in cosmological parameters in high redshift bins. In particular, we identify a redshift range where Planck $(H_0, Ω_m)$ values are disfavoured at $99.6 \%$ ($2.9 σ$) confidence level in a combination of OHD and supernovae data.

astro-ph.CO↗

arXiv2024

Heisenberg Soft Hair on Robinson-Trautman Spacetimes

We study 4 dimensional $(4d$) gravitational waves (GWs) with compact wavefronts, generalizing Robinson-Trautman (RT) solutions in Einstein gravity with an arbitrary cosmological constant. We construct the most general solution of the GWs in the presence of a causal, timelike, or null boundary when the usual tensor modes are turned off. Our solution space besides the shape and topology of the wavefront which is a generic compact, smooth, and orientable $2d$ surface $Σ$, is specified by a vector over $Σ$ satisfying the conformal Killing equation and two scalars that are arbitrary functions over the causal boundary, the boundary modes (soft hair). We work out the symplectic form over the solution space using covariant phase space formalism and analyze the boundary symmetries and charges. The algebra of surface charges is a Heisenberg algebra. Only the overall size of the compact wavefront and not the details of its shape appears in the boundary symplectic form and is canonical conjugate to the overall mass of the GW. Hence, the information about the shape of the wavefront can't be probed by the boundary observer. We construct a boundary energy-momentum tensor and a boundary current, whose conservation yields the RT equation for both asymptotically AdS and flat spacetimes. The latter provides a hydrodynamic description for our RT solutions.

hep-th↗

arXiv2023

Carrollian Structure of the Null Boundary Solution Space

We study pure $D$ dimensional Einstein gravity in spacetimes with a generic null boundary. We focus on the symplectic form of the solution phase space which comprises a $2D$ dimensional boundary part and a $2(D(D-3)/2+1)$ dimensional bulk part. The symplectic form is the sum of the bulk and boundary parts, obtained through integration over a codimension 1 surface (null boundary) and a codimension 2 spatial section of it, respectively. Notably, while the total symplectic form is a closed 2-form over the solution phase space, neither the boundary nor the bulk symplectic forms are closed due to the symplectic flux of the bulk modes passing through the boundary. Furthermore, we demonstrate that the $D(D-3)/2+1$ dimensional Lagrangian submanifold of the bulk part of the solution phase space has a Carrollian structure, with the metric on the $D(D-3)/2$ dimensional part being the Wheeler-DeWitt metric, and the Carrollian kernel vector corresponding to the outgoing Robinson-Trautman gravitational wave solution.

hep-th↗

arXiv2023

$S_8$ increases with effective redshift in $Λ$CDM cosmology

Hubble constant $H_0$ and weighted amplitude of matter fluctuations $S_8$ determinations are biased to higher and lower values, respectively, in the late Universe with respect to early Universe values inferred by the Planck collaboration within flat $Λ$CDM cosmology. If these anomalies are physical, i.e. not due to systematics, they naively suggest that $H_0$ decreases and $S_8$ increases with effective redshift. Here, subjecting matter density today $Ω_{m}$ to a prior, corresponding to a combination of Planck CMB and BAO data, we perform a consistency test of the Planck-$Λ$CDM cosmology and show that $S_8$ determinations from $f σ_8(z)$ constraints increase with effective redshift. Due to the redshift evolution, a $\sim 3 σ$ tension in the $S_8$ parameter with Planck at lower redshifts remarkably becomes consistent with Planck within $1 σ$ at high redshifts. This provides corroborating support for an $S_8$ discrepancy that is physical in origin. We further confirm that the flat $Λ$CDM model is preferred over a theoretically ad hoc model with a jump in $S_8$ at a given redshift. In the absence of the CMB+BAO $Ω_m$ prior, we find that $> 3 σ$ tensions with Planck in low redshift data are ameliorated by shifts in the parameters in high redshift data. Results here and elsewhere suggest that the $Λ$CDM cosmological parameters are redshift dependent. Fitting parameters that evolve with redshift is a recognisable hallmark of model breakdown.

astro-ph.CO↗

arXiv2023

Horizon Strings as 3d Black Hole Microstates

We propose that 3d black holes are an ensemble of tensionless null string states. These microstates typically have non-zero winding. We evaluate their partition function in the limit of large excitation numbers and show that their combinatorics reproduces the Bekenstein-Hawking entropy and its semiclassical logarithmic corrections.

hep-th↗

arXiv2023

Towards A Realistic Dipole Cosmology: The Dipole $Λ$CDM Model

Dipole cosmology is the maximally Copernican generalization of the FLRW paradigm that can incorporate bulk flows in the cosmic fluid. In this paper, we first discuss how multiple fluid components with independent flows can be realized in this set up. This is the necessary step to promote ``tilted" Bianchi cosmologies to a viable framework for cosmological model building involving fluid mixtures (as in FLRW). We present a dipole $Λ$CDM model which has radiation and matter with independent flows, with (or without) a positive cosmological constant. A remarkable feature of models containing radiation (including dipole $Λ$CDM) is that the $relative$ flow between radiation and matter can increase at late times, which can contribute to eg., the CMB dipole. This can happen generically in the space of initial conditions. We discuss the significance of this observation for late time cosmic tensions.

astro-ph.CO↗

arXiv2023

Shallow Water Memory: Stokes and Darwin Drifts

It has been shown in \cite{Tong:2022gpg} that shallow water in the Euler description admits a dual gauge theory formulation. We show in the Lagrange description this gauge symmetry is a manifestation of the 2 dimensional area-preserving diffeomorphisms. We find surface charges associated with the gauge symmetry and their algebra, and study their physics in the shallow water system. In particular, we provide a reinterpretation of the Kelvin circulation theorem in terms of conserved charges. In the linear shallow water case, the charges form a u(1) current algebra with level proportional to the Coriolis parameter over the height of the fluid. We also study memory effect for the gauge theory description of the linearized shallow water and show Euler, Stokes and Darwin drifts can be understood as a memory effect and/or change of the surface charges in the gauge theory description.

hep-th↗

arXiv2023

Dipole Cosmology: The Copernican Paradigm Beyond FLRW

We introduce the $dipole$ $cosmological$ $principle$, the idea that the Universe is a maximally Copernican cosmology, compatible with a cosmic flow. It serves as the most symmetric paradigm that generalizes the FLRW ansatz, in light of the increasingly numerous (but still tentative) hints that have emerged in the last two decades for a non-kinematic component in the CMB dipole. Einstein equations in our "dipole cosmology" are still ordinary differential equations -- but instead of the two Friedmann equations, now we have four. The two new functions can be viewed as an anisotropic scale factor that breaks the isotropy group from $SO(3)$ to $U(1)$, and a "tilt" that captures the cosmic flow velocity. The result is an axially isotropic, tilted Bianchi V/VII$_h$ cosmology. We assess the possibility of model building within the dipole cosmology paradigm, and discuss the dynamics of expansion rate, anisotropic shear and tilt, in various examples. A key observation is that the cosmic flow (tilt) can grow even while the anisotropy (shear) dies down. Remarkably, this can happen even in an era of late time acceleration.

astro-ph.CO↗

arXiv2023

Hydro & Thermo Dynamics at Causal Boundaries, Examples in 3d Gravity

We study 3-dimensional gravity on a spacetime bounded by a generic 2-dimensional causal surface. We review the solution phase space specified by 4 generic functions over the causal boundary, construct the symplectic form over the solution space and the 4 boundary charges and their algebra. The boundary charges label boundary degrees of freedom. Three of these charges extend and generalize the Brown-York charges to the generic causal boundary, are canonical conjugates of boundary metric components and naturally give rise to a fluid description at the causal boundary. Moreover, we show that the boundary charges besides the causal boundary hydrodynamic description, also admit a thermodynamic description with a natural (geometric) causal boundary temperature and angular velocity. When the causal boundary is the asymptotic boundary of the 3d AdS or flat space, the hydrodynamic description respectively recovers an extension of the known conformal or conformal-Carrollian asymptotic hydrodynamics. When the causal boundary is a generic null surface, we recover the null surface thermodynamics of [1] which is an extension of the usual black hole thermodynamics description.

hep-th↗

arXiv2023

A Tilt Instability in the Cosmological Principle

We show that the Friedmann-Lemaître-Robertson-Walker (FLRW) framework has an instability towards the growth of fluid flow anisotropies, even if the Universe is accelerating. This flow (tilt) instability in the matter sector is invisible to Cosmic No-Hair Theorem-like arguments, which typically only flag shear anisotropies in the metric. We illustrate our claims in the setting of ``dipole cosmology'', the maximally Copernican generalization of FLRW that can accommodate a flow. Simple models are sufficient to show that the cosmic flow need not track the shear, even in the presence of a positive cosmological constant. We also emphasize that the growth of the tilt hair is fairly generic if the total equation of state $w(t) \rightarrow -1$ at late times (as it does in standard cosmology), irrespective of the precise model of dark energy. The generality of our theoretical result puts various recent observational claims about late time anisotropies and cosmic dipoles in a new light.

astro-ph.CO↗

arXiv2023

High Redshift $Λ$CDM Cosmology: To Bin or not to Bin?

We construct observational Hubble $H(z)$ and angular diameter distance $D_{A}(z)$ mock data with baseline Planck $Λ$CDM input values, before fitting the $Λ$CDM model to study evolution of probability density functions (PDFs) of best fit cosmological parameters $(H_0, Ω_m, Ω_k)$ across redshift bins. We find that PDF peaks only agree with the input parameters in low redshift ($z \lesssim 1$) bins for $H(z)$ and $D_{A}(z)$ constraints, and in all redshift bins when $H(z)$ and $D_{A}(z)$ constraints are combined. When input parameters are not recovered, we observe that PDFs exhibit non-Gaussian tails towards larger $Ω_m$ values and shifts to (less pronounced) peaks at smaller $Ω_m$ values. This flattening of the PDF is expected as $H(z)$ and $D_{A}(z)$ observations only constrain combinations of cosmological parameters at higher redshifts, so uniform PDFs are expected. Our analysis leaves us with a choice to bin high redshift data in the knowledge that we may be unlikely to recover Planck values, or conduct full sample analysis that biases $Λ$CDM inferences to the lower redshift Universe.

astro-ph.CO↗

arXiv2023

Is the Observable Universe Consistent with the Cosmological Principle?

The Cosmological Principle (CP) -- the notion that the Universe is spatially isotropic and homogeneous on large scales -- underlies a century of progress in cosmology. It is conventionally formulated through the Friedmann-Lemaître-Robertson-Walker (FLRW) cosmologies as the spacetime metric, and culminates in the successful and highly predictive $Λ$-Cold-Dark-Matter ($Λ$CDM) model. Yet, tensions have emerged within the $Λ$CDM model, most notably a statistically significant discrepancy in the value of the Hubble constant, $H_0$. Since the notion of cosmic expansion determined by a single parameter is intimately tied to the CP, implications of the $H_0$ tension may extend beyond $Λ$CDM to the CP itself. This review surveys current observational hints for deviations from the expectations of the CP, highlighting synergies and disagreements that warrant further study. Setting aside the debate about individual large structures, potential deviations from the CP include variations of cosmological parameters on the sky, discrepancies in the cosmic dipoles, and mysterious alignments in quasar polarizations and galaxy spins. While it is possible that a host of observational systematics are impacting results, it is equally plausible that precision cosmology may have outgrown the FLRW paradigm, an extremely pragmatic but non-fundamental symmetry assumption.

astro-ph.CO↗

arXiv2022

Gauge Invariant Derivation of Zeroth and First Laws of Black Hole Thermodynamics

In gauge invariant theories, like Einstein-Maxwell theory, physical observables should be gauge invariant. In particular, mass, entropy, angular momentum, electric charge and their respective chemical potentials, temperature, horizon angular velocity and electric potential which appear in the laws of black hole thermodynamics should be gauge invariant. In the usual construction of the laws of black hole thermodynamics, gauge invariance of the intensive quantities is subtle; here we remedy this and provide a gauge invariant derivation and the proof of the zeroth and first laws of black hole thermodynamics.

hep-th↗

arXiv2022

On Symplectic Form for Null Boundary Phase Space

To formulate gravity in spacetimes bounded by a null boundary, an arbitrary hypothetical null surface, boundary degrees of freedom (d.o.f) should be added to account for the d.o.f and dynamics in the spacetime regions excised behind the null boundary. In the D dimensional example, boundary d.o.f are labelled by D charges defined at D-2 dimensional spacelike slices at the null boundary. While boundary modes can have their own boundary dynamics, their interaction with the bulk modes is governed by flux-balance equations which may be interpreted as a diffusion equation describing "dissolution" of bulk gravitons into the boundary. From boundary viewpoint, boundary d.o.f obey local thermodynamical equations at the boundary. Our description suggests a new "semiclassical" quantization of the system in which boundary d.o.f are quantized while bulk is classical. This semiclassical treatment may be relevant to questions in black hole physics.

gr-qc↗

arXiv2022

Revealing Intrinsic Flat $Λ$CDM Biases with Standardizable Candles

Emerging high redshift cosmological probes, in particular quasars (QSOs), show a preference for larger matter densities, $Ω_{m} \approx 1$, within the flat $Λ$CDM framework. Here, using the Risaliti-Lusso relation for standardizable QSOs, we demonstrate that the QSOs recover the \textit{same} Planck-$Λ$CDM Universe as Type Ia supernovae (SN), $Ω_m \approx 0.3$ at lower redshifts $ 0 < z \lesssim 0.7$, before transitioning to an Einstein-de Sitter Universe ($Ω_m =1$) at higher redshifts $z \gtrsim 1$. We illustrate the same trend, namely increasing $Ω_{m}$ and decreasing $H_0$ with redshift, in SN but poor statistics prevent a definitive statement. We explain physically why the trend is expected in the flat $Λ$CDM cosmology, illustrate the intrinsic bias and non-Gaussian tails with mock Pantheon data, and identify a similar trend in BAO below $z=1$. Our results highlight an intrinsic bias in the flat $Λ$CDM Universe, whereby $Ω_m$ increases, $H_0$ decreases and $S_8$ increases with effective redshift, thus providing a new perspective on $Λ$CDM tensions; even in a Planck-$Λ$CDM Universe the current tensions might have been expected.

astro-ph.CO↗