arXiv subjects

Houri Ziaeepour

Publications and source records attributed to Houri Ziaeepour.

At least 19 recordsLinked to original sources

arXiv2026

Energy-momentum and dark energy in $\boldsymbol{SU(\infty)}$-QGR quantum gravity

$SU(\infty)$ Quantum GRavity (QGR) is a recently proposed fundamentally quantum approach to gravity and cosmology. In this model the Hilbert space of the Universe represents $SU(\infty)$ symmetry. Its fragmentation generates approximately isolated subsystems (particles) representing, in addition to $SU(\infty)$, finite-rank local symmetries. The common $SU(\infty)$ is associated to quantum gravity, and at lowest quantum order the effective action for all symmetries is Yang-Mills on a 4D parameter space $Ξ$. Nonetheless, physical processes and measurables must be independent of the geometry of $Ξ$. In previous works we demonstrated that diffeomorphism of $Ξ$ can be neutralized by $SU(\infty)$ gauge transformation. In this work we show that the invariance of action under variation of $Ξ$'s metric leads to a constraint resembling Einstein equation. It consists of energy-momentum tensors for all components of the model, including the spin-1 gravitons. In addition, through calculation of quantum information measures we study the effect of Hilbert Space Fragmentation (HSF) on the emergent classical spacetime and different cosmological era, such as inflation, reheating, and late time accelerating expansion. The results of this preliminary and approximate investigation show that HSF may be classically interpreted as these phenomena. Consequently, inflaton, quintessence, and similar fields associated to these processes may be order parameters that phenomenologically present them.

gr-qc↗

arXiv2025

Quantum state of fields in $SU(\infty)$ Quantum Gravity ($SU(\infty)$-QGR)

Our Universe is ruled by quantum mechanics and should be treated as a quantum system. $SU(\infty)$-QGR is a recently proposed quantum model for the Universe, in which gravity is associated to $SU(\infty)$ symmetry of its Hilbert space. Fragmentation of its infinite dimensional state due to random quantum fluctuations divides the Universe to approximately isolated subsystems. In addition to parameters of their {\it internal} finite rank symmetries, states and dynamics of subsystems are characterized by 4 continuous parameters and the perceived classical spacetime is their effective representation, reflecting quantum states of subsystems and their relative evolution. At lowest order the effective Lagrangian of $SU(\infty)$-QGR~has the form of Yang-Mills gauge theories for both $SU(\infty)$ - gravity - and internal symmetries defined on the aforementioned 4D parameter space. In the present work we study more thoroughly some of the fundamental aspects of $SU(\infty)$-QGR. Specifically, we clarify the impact of the degeneracies of $\mathcal{SU}(\infty)$ algebra on the construction of the model; describe mixed states of subsystems and their purification; calculate measures of their entanglement to the rest of the Universe; and discuss their role in the emergence of local gauge symmetries. We also describe the relationship between what is called {\it internal space} of $SU(\infty)$ Yang-Mills with the 4D parameter space, and analytically demonstrate irrelevance of the geometry of parameter space for physical observables. Along with these topics, we demonstrate the equivalence of two sets of criteria for compositeness of a quantum system, and show uniqueness of the limit of various algebras leading to $\mathcal{SU}(\infty)$.

gr-qc↗

arXiv2025

$SU(\infty)$ Quantum Gravity: Emergence of Gravity in an Infinitely Divisible Quantum Universe

$SU(\infty)-QGR$ is a foundationally quantum approach to cosmology and gravity. It assumes that the Hilbert space of the Universe as a whole represents the symmetry group $SU(\infty)$, and demonstrates this symmetry for Hilbert spaces of infinite number of subsystems, which randomly emerge and represent arbitrary finite rank {\it internal} symmetries. The aim of present work is in depth study of the foundation and properties of this model. We show that the global $SU(\infty)$ symmetry manifests itself through the entanglement of each subsystem with the rest of the Universe. We demonstrate that the states of subsystems depend on a dimensionful parameter arising due to the breaking of a global $U(1)$ symmetry. A relative dynamics arises when an arbitrary subsystem is selected as a quantum clock with a time parameter. Thus, states of subsystems are characterized by 4 continuous parameters related to their $SU(\infty)$ symmetry and dynamics, plus discrete parameters characterizing their internal symmetries. We demonstrate the irrelevance of the geometry of this parameter space for observables. On the other hand, we use quantum speed limits to show that the perceived classical spacetime and its Lorentzian geometry emerge as an average path of subsystems in their Hilbert space. In this respect, $SU(\infty)-QGR$ fundamentally deviates from gauge-gravity duality models. Invariance under reparameterization restricts the action of subsystems dynamics to a Yang-Mills quantum field theory defined on the (3+1)-dimensional parameter space for both $SU(\infty)$ - gravity - and internal symmetries. Consequently, $SU(\infty)-QGR$ is renormalizable, but predicts a spin-1 mediator for quantum gravity. Nonetheless, it is proved that when quantum gravity effects are not detectable, the dynamics is perceived as the Einstein-Hilbert action. We briefly discuss $SU(\infty)-QGR$ specific models for dark energy.

gr-qc↗

arXiv2025

$SU(\infty)$ Quantum Gravity and Cosmology

We highlight the structure and properties of an abstract approach to quantum cosmology and gravity, dubbed $SU(\infty)$-QGR. Beginning from the concept of the Universe as an isolated quantum system, the main axiom of is the existence of an infinite number of mutually commuting observables. Consequently, the Hilbert space of the Universe represents $SU(\infty)$ symmetry. This Universe as a whole is static and topological. Nonetheless, quantum fluctuations induce local clustering in its quantum state and divide it into approximately isolated subsystems representing $G \times SU(\infty)$, where $G$ is a generic finite-rank internal symmetry. Due to the global $SU(\infty)$ subsystems are entangled to the rest of the Universe. In addition to parameters characterizing the representation of $G$, their quantum states depend on four continuous parameters: two of them characterize the representation of $SU(\infty)$, a dimensionful parameter arises from the possibility of comparing representations of $SU(\infty)$ by different subsystems; the fourth parameter is a measurable used as time registered by an arbitrary subsystem chosen as a clock. It introduces a relative dynamics for subsystems, formulated by a symmetry-invariant effective Lagrangian defined on the (3+1)D space of the continuous parameters. At lowest quantum order, the Lagrangian is a Yang--Mills field theory for both $SU(\infty)$ and internal symmetries. We identify the common $SU(\infty)$ symmetry and its interaction with gravity. Consequently, $SU(\infty)$-QGR predicts a spin-1 mediator for quantum gravity (QGR). Apparently, this is in contradiction with classical gravity. Nonetheless, we show that an observer who is unable to detect the quantumness of gravity perceives its effect as curvature of the space of average values of the continuous parameters. We demonstrate Lorentzian geometry of this emergent classical spacetime.

gr-qc↗

arXiv2023

$\mathbf {SU(\infty)}$-QGR Quantumania: Everything, Everywhere, All At Once

$SU(\infty)$-QGR is a quantum approach to Universe and gravity. Its main assumption is infinite mutually commuting observables in the Universe, leading to representation of $SU(\infty)$ by its Hilbert spaces and those of its subsystems. The Universe as a whole is static, topological, and characterized by two continuous parameters. Nonetheless, quantum fluctuations induce clustering and finite rank internal symmetries, which approximately divide the Universe to infinite interacting subsystems. Their Hilbert space depends on an additional dimensionful parameter, and selection of a subsystem as clock induces a relative dynamics, with $SU(\infty)$ sector as gravity. The Lagrangian defined on the (3+1)-dimensional parameter space is Yang-Mills for both symmetries. When quantumness of gravity is undetectable, it is perceived as curvature of an effective spacetime.

gr-qc↗

arXiv2023

GRB 221009A: Discovery of an Exceptionally Rare Nearby and Energetic Gamma-Ray Burst

We report the discovery of the unusually bright long-duration gamma-ray burst (GRB), GRB 221009A, as observed by the Neil Gehrels Swift Observatory (Swift), Monitor of All-sky X-ray Image (MAXI), and Neutron Star Interior Composition Explorer Mission (NICER). This energetic GRB was located relatively nearby (z = 0.151), allowing for sustained observations of the afterglow. The large X-ray luminosity and low Galactic latitude (b = 4.3 degrees) make GRB 221009A a powerful probe of dust in the Milky Way. Using echo tomography we map the line-of-sight dust distribution and find evidence for significant column densities at large distances (~> 10kpc). We present analysis of the light curves and spectra at X-ray and UV/optical wavelengths, and find that the X-ray afterglow of GRB 221009A is more than an order of magnitude brighter at T0 + 4.5 ks than any previous GRB observed by Swift. In its rest frame GRB 221009A is at the high end of the afterglow luminosity distribution, but not uniquely so. In a simulation of randomly generated bursts, only 1 in 10^4 long GRBs were as energetic as GRB 221009A; such a large E_gamma,iso implies a narrow jet structure, but the afterglow light curve is inconsistent with simple top-hat jet models. Using the sample of Swift GRBs with redshifts, we estimate that GRBs as energetic and nearby as GRB 221009A occur at a rate of ~<1 per 1000 yr - making this a truly remarkable opportunity unlikely to be repeated in our lifetime.

astro-ph.HE↗

arXiv2021

Making a Quantum Universe: Symmetry and Gravity

So far, none of attempts to quantize gravity has led to a satisfactory model that not only describe gravity in the realm of a quantum world, but also its relation to elementary particles and other fundamental forces. Here, we outline the preliminary results for a model of quantum universe, in which gravity is fundamentally and by construction quantic. The model is based on three well motivated assumptions with compelling observational and theoretical evidence: quantum mechanics is valid at all scales; quantum systems are described by their symmetries; universe has infinite independent degrees of freedom. The last assumption means that the Hilbert space of the Universe has $SU(N\rightarrow \infty) \cong \text{area preserving Diff.} (S_2)$ symmetry, which is parameterized by two angular variables. We show that, in the absence of a background spacetime, this Universe is trivial and static. Nonetheless, quantum fluctuations break the symmetry and divide the Universe to subsystems. When a subsystem is singled out as reference -- observer -- and another as clock, two more continuous parameters arise, which can be interpreted as distance and time. We identify the classical spacetime with parameter space of the Hilbert space of the Universe. Therefore, its quantization is meaningless. In this view, the Einstein equation presents the projection of quantum dynamics in the Hilbert space into its parameter space. Finite dimensional symmetries of elementary particles emerge as a consequence of symmetry breaking when the Universe is divided to subsystems/particles, without having any implication for the infinite dimensional symmetry and its associated interaction - perceived as gravity. This explains why gravity is a universal force.

gr-qc↗

arXiv2021

Comparing Quantum Gravity Models: String Theory, Loop Quantum Gravity, and Entanglement gravity versus $SU(\infty)$-QGR

In a previous work [arXiv:2009.03428] we proposed a new model for Quantum GRavity(QGR) and cosmology, dubbed $SU(\infty)$-QGR. One of the axioms of this model is that Hilbert spaces of the Universe and its subsystems represent $SU(\infty)$ symmetry group. In this framework, the classical spacetime is interpreted as being the parameter space characterizing states of the $SU(\infty)$ representing Hilbert spaces. Using quantum uncertainty relations, it is shown that the parameter space - the spacetime - has a 3+1 dimensional Lorentzian geometry. Here after a review of $SU(\infty)$-QGR, including the demonstration that its classical limit is Einstein gravity, we compare it with several QGR proposals, including: string and M-theories, loop quantum gravity and related models, and QGR proposals inspired by holographic principle and quantum entanglement. The purpose is to find their common and analogous features, even if they apparently seem to have different roles and interpretations. The hope is that such exercise gives a better understanding of gravity as a universal quantum force and clarifies the physical nature of the spacetime. We identify several common features among the studied models: importance of 2D structures; algebraic decomposition to tensor products; special role of $SU(2)$ group in their formulation; necessity of a quantum time as a relational observable. We discuss how these features can be considered as analogous in different models. We also show that they arise in $SU(\infty)$-QGR without fine-tuning, additional assumptions, or restrictions.

gr-qc↗

arXiv2019

Properties of jet and surrounding material of GW/GRB~170817A

We use published data in radio, optical and X-ray bands to analyze and model afterglows of GW/GRB 170817A. Our analysis is based on a phenomenological gamma-ray burst generator model which we previously used to study the prompt gamma-ray emission of this important transient. We find a multi-component model and a few of its variants that are consistent with broad band $\sim 1$ year observations of afterglows, once the contribution of kilonova in optical/IR band is taken into account. Considering beaming and off-axis view, we interpret the components of the model as approximately presenting the profile of a relativistic structured jet with a rapidly declining Lorentz factor from our line of sight, where it had a Lorentz factor of $\mathcal {O}(100)$, to outer boundaries, where it became a mildly relativistic cocoon with a relative velocity to light of $\sim 0.4-0.97$. Properties of the jet core obtained here are consistent with conclusions from analysis of the prompt gamma-ray emission. In particular, our results show that after prompt internal shocks the remnant of the jet retains in some extent its internal collimation and coherence. Slow rise of the afterglows can be associated to low density of circum-burst material and low column density of the jet. The long distance of external shocks from the merger, which could have been in part responsible for extensive thinning of the jet through expansion and energy dissipation before occurrence of external shocks is responsible for the peak of emission being at $\gtrsim 110$~days after the merger. We discuss implications of these observations for properties of circum-burst material around binary neutron stars. This analysis confirms our previous results showing that an outflow with a Lorentz factor of $\sim 2-5$ cannot explain observed afterglows without an additional X-ray source or significant absorption of optical/IR photons.

astro-ph.HE↗

arXiv2019

Binary Neutron Star (BNS) merger: What we learned from relativistic ejecta of GW/GRB~170817A

Gravitational waves from coalescence of a Binary Neutron Star (BNS) and its accompagning short Gamma-Ray Burst GW/GRB~170817A confirmed the presumed origin of these puzzeling transients and opened up the way for relating properties of short GRBs to those of their progenitor stars and their surroundings. Here we review an extensive analysis of the prompt gamma-ray and late afterglows of this event. We show that a fraction of polar ejecta from the merger had been accelerated to ultra-relativistic speeds. This structured jet had an initial Lorentz factor of about $260$ in our direction - $\mathcal{O}(10^\circ)$ from the jet's axis - and was a few orders of magnitude less dense than in typical short GRBs. At the time of arrival to circum-burst material the ultra-relativistic jet had a close to Gaussian profile and a Lorentz factor $\gtrsim 130$ in its core. It had retained in some extent its internal collimation and coherence, but had extended laterally to create mildly relativistic lobes - a {\it cocoon}. External shocks on the far from center inhomogeneous circum-burst material and low density of colliding shells generated slow rising afterglows. The circum-burst material was somehow correlated with the merger and it is possible that it contained recently ejected material from glitching, which had resumed due to the deformation of neutron stars crust by tidal forces in the latest stages of inspiral but well before their merger. By comparing these findings with the results of relativistic MHD simulations and observed gravitational waves we conclude that progenitor neutron stars were old, had close masses and highly reduced magnetic fields. In addition, they probably had oppositely directed spins due to the encounter and gravitational interaction with other stars.

astro-ph.HE↗

arXiv2018

Late afterglows of GW/GRB 170817A

The gamma-ray burst that followed the first detection of gravitational waves from the merger of a Binary Neutron Stars and its low energy counterparts were in many respects unusual and challenge our understanding of mechanisms involved in their production. In a previous work we used a phenomenological formulation of relativistic shocks and synchrotron emission to analyse the prompt gamma-ray emission of GW/GRB 170817A. Here we use the same model to analyse late afterglows of this event. The main goal is to see whether synchrotron emission alone can explain late afterglows. We find that collision between a mildly relativistic outflow from the merger with a Lorentz factor of $\sim 1.2-3$ and the ISM/circumburst material can explain observations, if the synchrotron self-absorption of radio emission and extinction of optical/IR photons are taken into account. Alternatively, an additional source of X-ray is necessary to explain the data. These conclusions are independent of the model used here and can be deduced directly from data. We also show that at the time of its encounter with circumburst material the outflow could have been still mildly magnetized. The origin for optical extinction could be a dust rich old faint star cluster surrounding the BNS, which had also helped its formation and merger. Such environment evades present observational constraints and is consistent with our conclusions about properties and evolution of the progenitor neutron stars obtained from the prompt gamma-ray. If the synchrotron emission was produced by collision between density shells, the extinction might have occurred inside the outflow itself rather than externally. A plausible source of additional X-ray is decay of medium and heavy isotopes produced by the kilonova, including r-processes, and recombination of cooled electrons. Contribution of these processes should be quantified in future works.

astro-ph.HE↗

arXiv2018

Non-equilibrium evolution of quantum fields during inflation and late accelerating expansion

To understand mechanisms leading to inflation and late acceleration of the Universe it is important to see how one or a set of quantum fields may evolve such that the classical energy-momentum tensor behave similar to a cosmological constant. In this work we consider a toy model including 3 scalar fields with very different masses to study the formation of a light axion-like condensate, presumed to be responsible for inflation and/or late accelerating expansion of the Universe. Despite its simplicity, this model reflects hierarchy of masses and couplings of the Standard Model and its candidate extensions. The investigation is performed in the framework of non-equilibrium quantum field theory in a consistently evolved FLRW geometry. We discuss in details how the initial conditions for such a model must be defined in a fully quantum setup and show that in a multi-component model interactions reduce the number of independent initial degrees of freedom. Numerical simulation of this model shows that it can be fully consistent with present cosmological observations. For the chosen range of parameters we find that quantum interactions rather than effective potential of a condensate is the dominant contributor in the energy density of the Universe and triggers both inflation and late accelerating expansion. Nonetheless, despite its small contribution in the energy density, the light scalar field - in both condensate and quasi free particle forms - has a crucial role in controlling the trend of heavier fields. Furthermore, up to precision of our simulations we do not find any IR singularity during inflation. These findings highlight uncertainties in attempts to extract information about physics of the early Universe by naively comparing predictions of local effective classical models with cosmological observations, neglecting inherently non-local nature of quantum processes.

gr-qc↗

arXiv2018

Prompt gamma-ray emission of GRB 170817A associated to GW 170817: A consistent picture

The short GRB 170817A associated to the first detection of gravitation waves from a Binary Neutron Star (BNS) merger was in many ways unusual. Possible explanations are emission from a cocoon or cocoon break out, off-axis view of a structured or uniform jet, and on-axis ultra-relativistic jet with reduced density and Lorentz factor. Here we use a phenomenological model of shock evolution and synchrotron/self-Compton emission to simulate the prompt emission of GRB 170817A and to test above proposals. We find that synchrotron emission from a mildly relativistic cocoon with a Lorentz factor of 2-3, as considered in the literature, generates a too soft, too long, and too bright prompt emission. Off-axis view of an structured jet with a Lorentz factor of about 10 can reproduce observations, but needs a very efficient transfer of kinetic energy to electrons in internal shocks, which is disfavored by particle in cell simulations. We also comment on cocoon breakout as a mechanism for generation of the prompt gamma-ray. A relativistic jet with a Lorentz factor of about 100 and a density lower than typical short GRBs seems to be the most plausible model and we conclude that GRB 170817A was intrinsically faint. Based on this result and findings of relativistic magnetohydrodynamics simulations of BNS merger in the literature we discuss physical and astronomical conditions, which may lead to such faint short GRBs. We identify small mass difference of progenitor neutron stars, their old age and reduced magnetic field, and anti-alignment of spin-orbit angular momentum induced by environmental gravitational disturbances during the lifetime of the BNS as causes for the faintness of GRB 170817A. We predict that BNS mergers at lower redshifts generate on average fainter GRBs.

astro-ph.HE↗

arXiv2018

Quantum Mechanics in symmetry language

We consider symmetry as a foundational concept in quantum mechanics and rewrite quantum mechanics and measurement axioms in this description. We argue that issues related to measurements and physical reality of states can be better understood in this view. In particular, the abstract concept of symmetry provides a basis-independent definition for observables. Moreover, we show that the apparent projection/collapse of the state as the final step of measurement or decoherence is the result of breaking of symmetries. This phenomenon is comparable with a phase transition by spontaneous symmetry breaking, and makes the process of decoherence and classicality a natural fate of complex systems consisting of many interacting subsystems. Additionally, we demonstrate that the property of state space as a vector space representing symmetries is more fundamental than being an abstract Hilbert space, and its $L2$ integrability can be obtained from the imposed condition of being a representation of a symmetry group and general properties of probability distributions.

quant-ph↗

arXiv2015

Quantum coherent states in cosmology

Coherent states consist of superposition of infinite number of particles and do not have a classical analogue. We study their evolution in a FLRW cosmology and show that only when full quantum corrections are considered, they may survive the expansion of the Universe and form a global condensate. This state of matter can be the origin of accelerating expansion of the Universe, generally called dark energy, and inflation in the early universe. Additionally, such a quantum pool may be the ultimate environment for decoherence at shorter distances. If dark energy is a quantum coherent state, its dominant contribution to the total energy of the Universe at present provides a low entropy state which may be necessary as an initial condition for a new Big Bang in the framework of bouncing cosmology models.

hep-ph↗

arXiv2015

Symmetry as a foundational concept in Quantum Mechanics

Symmetries are widely used in modeling quantum systems but they do not contribute in postulates of quantum mechanics. Here we argue that logical, mathematical, and observational evidence require that symmetry should be considered as a fundamental concept in the construction of physical systems. Based on this idea, we propose a series of postulates for describing quantum systems, and establish their relation and correspondence with axioms of standard quantum mechanics. Through some examples we show that this reformulation helps better understand some of ambiguities of standard description. Nonetheless its application is not limited to explaining confusing concept and it may be a necessary step toward a consistent model of quantum cosmology and gravity.

quant-ph↗

arXiv2014

Classical, quantum, and phenomenological aspects of dark energy models

The origin of accelerating expansion of the Universe is one the biggest conundrum of fundamental physics. In this paper we review vacuum energy issues as the origin of accelerating expansion - generally called dark energy - and give an overview of alternatives, which a large number of them can be classified as interacting scalar field models. We review properties of these models both as classical field and as quantum condensates in the framework of non-equilibrium quantum field theory. Finally, we review phenomenology of models with the goal of discriminating between them.

gr-qc↗

arXiv2012

Issues with vacuum energy as the origin of dark energy

In this letter we address some of the issues raised in the literature about the conflict between a large vacuum energy density, apriori predicted by quantum field theory, and the observed dark energy which must be the energy of vacuum or include it. We present a number of arguments against this claim and in favour of a null vacuum energy. They are based on the following arguments: A new definition for the vacuum in quantum field theory as a frame-independent coherent state; Results from a detailed study of condensation of scalar fields in FLRW background performed in a previous work; And our present knowledge about the Standard Model of particle physics. One of the predictions of these arguments is the confinement of nonzero expectation value of Higgs field to scales roughly comparable with the width of electroweak gauge bosons or shorter. If the observation of Higgs by the LHC is confirmed, accumulation of relevant events and their energy dependence in near future should allow to measure the spatial extend of the Higgs condensate.

hep-th↗