arXiv subjects

Shahin Rouhani

Publications and source records attributed to Shahin Rouhani.

At least 19 recordsLinked to original sources

arXiv2026

Cosmological Tensions in a Gauge-Invariant Modified Gravity

In this work, we investigate a gauge-invariant formulation of modified gravity (GIMOG) wherein the gravitational interaction emerges dynamically from a scalar field following a first-order phase transition. This framework offers a unified cosmological history: it naturally generates a pre-inflationary phase, smoothly recovers the standard radiation and matter-dominated eras, and accounts for late-time cosmic acceleration without the need for a cosmological constant or dark energy. We evaluate the phenomenological viability of the model by confronting it with observational data across distinct cosmological epochs. At late times, the model is constrained using Pantheon+ Type Ia supernova data. In the early Universe, we impose bounds from Big Bang Nucleosynthesis (BBN), specifically utilizing the primordial $^{4}\mathrm{He}$ abundance. Our analysis reveals a distinct phenomenological tension: while late-time observations favor a stronger effective gravitational coupling, BBN constraints tightly restrict early-universe deviations from general relativity. We demonstrate that reconciling these constraints requires a smooth time variation of the effective gravitational constant, $G$, establishing a clear theoretical target for future precision cosmological tests.

astro-ph.CO↗

arXiv2026

Constraining the Lifespan of Intelligent Technological Civilization in the Galaxy

In this work, we explore constraints on the emergence and longevity of technologically intelligent civilizations in our Galaxy, considering the Fermi paradox. We argue that under optimistic assumptions about the probability of life and intelligence emerging on Earth-like planets, the absence of contact with extraterrestrial civilizations imposes limits on their lifespan. Our analysis suggests that if intelligent life is common, technological civilizations must be relatively short-lived, with lifetimes constrained to $\lesssim 5\times10^3$ years under our most optimistic scenario. Considering electromagnetic communication, we note that our current light cone encompasses the entire Galactic history over the past $\sim 10^5$ years, making the lack of detected signals particularly puzzling for long-lived civilizations. We emphasize that these results should be interpreted as upper bounds derived from the Fermi paradox, not as predictions of actual lifespans.

astro-ph.GA↗

arXiv2023

MOG as symmetry breaking in Scalar-Vector-Tensor gravity

The Modified Gravity Model (MOG) has been proposed as a solution to the dark matter problem, but it does not meet the gauge invariant condition. The aim of this work is to propose a gauge-invariant theory, which suggests that symmetry can break at a low temperature in the Universe, leading to the MOG theory. This theory has the potential to alter the dynamics of the early and late Universe and naturally produce cosmological inflation.

gr-qc↗

arXiv2020

Collective Movement with Signaling

We consider a population of mobile agents able to make noisy observation of the environment and communicate their observation by production and comprehension of signals. Individuals try to align their movement direction with their neighbors. Besides, they try to collectively find and travel towards an environmental direction. We show that, when the fraction of informed individuals is small, by increasing the noise in communication, similarly to the Viscek model, the model shows a discontinuous order-disorder transition with strong finite size effects. In contrast, for large fraction of informed individuals, it is possible to go from the ordered phase to the disordered phase without passing any phase transition. The ordered phase is composed of two phases separated by a discontinuous transition. Informed collective motion, in which the population collectively infers the correct environmental direction, occurs for high fraction of informed individuals. When the fraction of informed individuals is low, misinformed collective motion, where the population fails to find the environmental direction becomes stable as well. Besides, we show that an amount of noise in the production of signals is more detrimental for the inference capability of the population, and increases the density fluctuations and the probability of group fragmentation, compared to the same amount of noise in the comprehension.

cond-mat.stat-mech↗

arXiv2019

The expectation value of the number of loops and the left-passage probability in the double-dimer model

We study various statistical properties of the double-dimer model, a generalization of the dimer model, on rectangular domains of the square lattice. We take advantage of the Grassmannian representation of the dimer model, first to calculate the probability distribution of the number of nontrivial loops around a cylinder, which is consistent with the previously known result, and then to calculate the expectation value of the number of loops surrounding two faces and the left-passage probability, both in the discrete and the continuum cases. We also briefly explain the calculation of some related observables. As a by-product, we obtain the partition function of the dimer model in the presence of two and four monomers, and a single monomer on the boundary.

cond-mat.stat-mech↗

arXiv2019

Connection probabilities in the double-dimer model -- the case of two connectivity patterns

We apply the Grassmannian representation of the dimer model, an equivalent approach to Kasteleyn's solution to the close-packed dimer problem, to calculate the connection probabilities for the double-dimer model with wired/free/wired/free boundary conditions, on a rectangular subdomain of the square lattice with four marked boundary points at the corners. Using some series identities related to Schwarz-Christoffel transformations, we show that the continuum of the result is consistent with the corresponding one in the upper half-plane (previously obtained by Kenyon-Wilson), which is in turn identical to the connection probabilities for 4SLE$_4$ emanating from the boundary, or equivalently, to a conditioned version of CLE$_4$ with wired/free/wired/free boundary conditions in the context of conformal loop ensembles.

math-ph↗

arXiv2018

Model of contact friction based on extreme value statistics

We propose a model based on extreme value statistics (EVS) and combine it with different models for single asperity contact, including adhesive and elasto-plastic contacts, to derive a relation between the applied load and the friction force on a rough interface. We find that when the summit distribution is Gumbel, and the contact model is Hertzian we have the closest conformity with Amontons law. The range over which Gumbel distribution mimics Amontons law is wider than the Greenwood-Williamson Model. However exact conformity with Amonton's law does not seem for any of the well-known EVS distributions. On the other hand plastic deformations in contact area reduce the relative change of pressure slightly with Gumbel distribution. Elastic-plastic contact mixes with Gumbel distribution for summits. it shows the best conformity with Amonton`s law. Other extreme value statistics are also studied, and results presented. We combine Gumbel distribution with GW-Mc Cool model which is an improved case of GW model, it takes into account a bandwidth for wavelengths of α. Comparison of this model with original GW-Mc Cool model and other simplified versions of BGT reveals that Gumbel distribution has a better conformity with Amonton`s law for all values of α. When adhesive contact model is used, the main observation is that for zero or even negative applied load, there is some friction. Asperities with height even less than the separation of two surfaces are in contact. For a small value of adhesion parameter, a better conformity with Amontons law is observed. Relative pressure increases for stronger adhesion which means that adhesion controlled friction dominated by load controlled friction. We also observe that adhesion increases on a surface with a lower value of roughness.

cond-mat.stat-mech↗

arXiv2018

Evolutionary value of collective sensing

We propose a mathematical model for collective sensing in a population growing in a stochastically varying environment. In the population, individuals use an information channel for sensing the environment, and two channels for signal production and comprehension to communicate among themselves. We show that existence of such system has a positive effect on population growth, hence can have a positive evolutionary effect. We show that the gain in growth due to the collective sensing is related to information theoretic entities, which can be considered as the information content of this system from the environment. We further show that heterogeneity in communication resulted from network or spatial structure increases growth. We compute the growth rate of a population residing on a lattice and show that growth rate near the maximum noise level in observation or communication, increases exponentially as noise decreases. This exponential effect makes the emergence of collective observation an easy outcome in an evolutionary process. Furthermore, we are able to quantify interesting effects such as accelerated growth, and simplification of decision making due to information amplification by communication. Finally, we show that an amount of noise in representation formation has more disadvantageous effect compared to the same noise in signal production.

q-bio.PE↗

arXiv2017

Aspects of Ultra-Relativistic Field Theories via Flat-space Holography

Recently it was proposed that asymptotically flat spacetimes have a holographic dual which is an ultra-relativistic conformal field theory. In this paper, we obtain the conformal anomaly for such a theory via the flat-space holography technique. Furthermore, using flat-space holography we obtain a C-function for this theory which is monotonically decreasing from the UV to the IR by employing the null energy condition in the bulk.

hep-th↗

arXiv2016

Fractional Galilean Symmetries

We generalize the differential representation of the operators of the Galilean algebras to include fractional derivatives. As a result a whole new class of scale invariant Galilean algebras are obtained. The first member of this class has dynamical index $z=2$ similar to the Schrödinger algebra. The second member of the class has dynamical index $z=3/2$, which happens to be the dynamical index Kardar-Parisi-Zhang equation.

hep-th↗

arXiv2013

Logarithmic Exotic Conformal Galilean Algebras

Logarithmic representations of the conformal Galilean algebra (CGA) and the Exotic Conformal Galilean algebra ({\sc ecga}) are constructed. This can be achieved by non-decomposable representations of the scaling dimensions or the rapidity indices, specific to conformal galilean algebras. Logarithmic representations of the non-exotic CGA lead to the expected constraints on scaling dimensions and rapidities and also on the logarithmic contributions in the co-variant two-point functions. On the other hand, the {\sc ecga} admits several distinct situations which are distinguished by different sets of constraints and distinct scaling forms of the two-point functions. Two distinct realisations for the spatial rotations are identified as well. The first example of a reducible, but non-decomposable representation, without logarithmic terms in the two-point function is given.

hep-th↗

arXiv2013

Logarithmic correlators or responses in non-relativistic analogues of conformal invariance

Recent developments on emergence of logarithmic terms in correlators or response functions of models which exhibit dynamical symmetries analogous to conformal invariance in not necessarily relativistic systems are reviewed. The main examples of these are logarithmic Schrödinger-invariance and logarithmic conformal Galilean invariance. Some applications of these ideas to statistical physics are described.

hep-th↗

arXiv2011

Irregular Motion of a Falling Spherical Object Through Non-Newtonian Fluid

The falling of an object through a non-Newtonian fluid is an interesting problem, depending on the details of the rheology of the fluid. In this paper we report on the settling of spherical objects through two non-Newtonian fluids: Laponite and hair Gel. A falling object's behavior in passing through a thixotropic colloidal suspension of synthetic clay, Laponite, has been reported to have many behavioral regimes. Here we report observation of a new regime where irregular motion is observed. We argue that this irregular motion may be interpreted as onset of chaos. Observation of this regime depends on the size of the falling sphere, relaxation time of fluid and concentration of particles in the suspension. Similar experiments in Gel, a yield stress polymeric fluid, do not reveal such behavior.

cond-mat.soft↗

arXiv2010

Logarithmic Correlators in Non-relativistic Conformal Field Theory

We show how logarithmic terms may arise in the correlators of fields which belong to the representation of the Schrodinger-Virasoro algebra (SV) or the affine Galilean Conformal Algebra (GCA). We show that in GCA, only scaling operator can have a Jordanian form and rapidity can not. We observe that in both algebras logarithmic dependence appears along the time direction alone.

hep-th↗

arXiv2010

Generating maximally entangled distant pair in invariant stratification spin networks

In this paper we study the generation of Bell states between distant vertices in a permanently coupled quantum spin network, interacting via invariant stratification graphs. To begin with we establish a class of upper bounds over achievable entanglement between the reference site and various vertices. We observe that the maximum of these upper bounds is 1 e-bit. We conclude that the reference site can generate a Bell state with a vertex if the corresponding upper bound of the vertex is 1 e-bit. Thus for generation of a Bell state this upper bound must be saturated. Taking this into account, we obtain the characteristic constraint of the proper graphs. We introduce a special class of antipodal invariant stratification graphs, which is called reflective, whereas the antipode vertex obeys the characteristic constraint. We also show that the antipodal association scheme graphs are reflective so Bell states can be generated between the antipodal vertices. Moreover we observe that in such graphs the proper Hamiltonian that enables creation of Bell state is the Heisenberg interaction between vertex pairs.

quant-ph↗

arXiv2009

Affine Extension of Galilean Conformal Algebra in 2+1 Dimensions

We show that a class of nonrelativistic algebras including non centrally-extended Schrodinger algebra and Galilean Conformal Algebra (GCA) has an affine extension in 2+1 hitherto unknown. This extension arises out of the conformal symmetries of the two dimensional complex plain. We suggest that this affine form may be the symmetry that explains the relaxation of some classical phenomena towards their critical point. This affine algebra admits a central extension and maybe realized in the bulk. The bulk realization suggests that this algebra may be derived by looking at the asymptotic symmetry of an AdS theory. This suggests that AdS/CFT duality may take on a special form in four dimensions.

hep-th↗

arXiv2009

Asymptotic symmetry of geometries with Schrodinger isometry

We show that the asymptotic symmetry algebra of geometries with Schrodinger isometry in any dimension is an infinite dimensional algebra containing one copy of Virasoro algebra. It is compatible with the fact that the corresponding geometries are dual to non-relativistic CFTs whose symmetry algebra is the Schrodinger algebra which admits an extension to an infinite dimensional symmetry algebra containing a Virasoro subalgebra.

hep-th↗

arXiv2002

Transitions in Oscillatory Dynamics of Two Connected Neurons with Excitatory Synapses

It is shown that long term behavior of two connected Integrate- and- Fire neurons with excitatory synapses is determined by some fixed-points. In the case of equal synaptic weights four different dynamic phases are found. Between these phases there is a specific phase with a global attractor fixed-point, which is of interest from different viewpoints. Simulations support our analytic work. When synaptic weights are equal we observe no synchronization but with different weight we do observe an almost synchronous state. Simulations show that when there is only one non-trivial fixed-point the period of oscillations is stable against small changes in synaptic weights

cond-mat.dis-nn↗