Article
Astronomy & Astrophysics
Damiano Anselmi
Summary: In this study, inflationary perturbation spectra and the quantity r + 8n(T) were computed to the next-to-next-to-leading log order in quantum gravity with solely virtual particles. The spectra are functions of inflationary running coupling and satisfy cosmic renormalization group flow equations. Tensor fluctuations receive contributions from the spin-2 fakeon chi(mu nu), while the scalar spectrum's dependence on chi(mu nu) starts from alpha(2) corrections. The theoretical predictions have errors ranging from alpha(4) to alpha(3), and nontrivial issues regarding the fakeon projection at higher orders are discussed.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Ichiro Oda
Summary: In this paper, we perform the manifestly covariant quantization of a scale invariant gravity with a scalar field in the de Donder gauge condition. By deriving various equal-time (anti)commutation relations, it is shown that the general action with the de Donder gauge possesses a Poincare-like IOSp(8 vertical bar 8) supersymmetry. The paper also derives the equal-time commutation relations for the gravitational sector involving the metric tensor and scalar fields, and explains how scale symmetry is spontaneously broken in quantum gravity, showing that the dilaton is a massless Nambu-Goldstone particle.
Article
Astronomy & Astrophysics
Shinji Mukohyama, Ryo Namba
Summary: The study focuses on the completeness and caustic problem of shift-symmetric k-essence models, demonstrating through numerical methods that introducing a heavy field can resolve the caustic problem. Further research is conducted on the cosmological application of the model, showing that EFT reduction is successful when taking the limit of infinite curvature.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Gaoping Long, Xiangdong Zhang
Summary: In this paper, we explore the gauge reduction of the simplicity constraint in both classical and quantum theory of loop quantum gravity (LQG) in all dimensions. We find that the simplicity reduced holonomy fails to capture the degrees of freedom of intrinsic curvature, preventing the construction of a correct scalar constraint operator. To address this issue, we introduce a new type of holonomy corresponding to the simplicity reduced connection, which properly captures the degrees of freedom of both intrinsic and extrinsic curvature. Based on this new holonomy, we propose three new strategies to construct scalar constraint operators, which could serve as valuable candidates for studying the dynamics of all dimensional LQG in the future.
Article
Astronomy & Astrophysics
Luca Marchetti, Daniele Oriti
Summary: This research derives effective dynamics for scalar cosmological perturbations from quantum gravity in the framework of group field theory condensate cosmology. It establishes a connection between fundamental quantum gravity models and cosmological observations.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2022)
Article
Astronomy & Astrophysics
Damiano Anselmi
Summary: This study investigates primordial cosmology with two scalar fields participating in inflation, revealing the existence of an asymptotically de Sitter cosmic RG flow in two couplings. The importance of quantum gravity theory's predictiveness and the role of RG invariance are highlighted.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
G. Fanizza, G. Marozzi, M. Medeiros, G. Schiaffino
Summary: Inspired by the fully non-linear Geodesic Light-Cone (GLC) gauge, a new cosmological perturbation theory is built and the connection with standard cosmological perturbation theory is explored through gauge transformations related to a new set of perturbations. In addition, various definitions for gauge invariant combinations are utilized, with emphasis on the relationships with linearized GLC gauge perturbations. The GLC gauge is shown to offer advantages in terms of providing simple expressions for light-like observables and matching the proper time of a free-falling observer.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Yonadav Barry Ginat
Summary: This paper discusses the consideration of small-scale inhomogeneities in modeling the universe on a cosmic scale, introducing a multiple-scales method based on singular perturbation theory and deriving an effective Einstein equation. Conditions are established to ensure that deviations from the large-scale space-time metric do not grow unboundedly, and it is shown that these conditions are satisfied in the case of non-relativistic matter.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Steffen Gielen, Lisa Mickel
Summary: We investigate the behavior of scalar perturbations in cosmological scenarios with modified Friedmann equations. We use a separate universe approximation to study wavelengths larger than the cosmological horizon and examine the conservation of curvature perturbation (zeta) and comoving curvature perturbation (R). While these quantities are conserved on super-horizon scales in standard cosmological models, we find that this is not always the case in scenarios like quantum gravity bouncing cosmologies. Our results provide insights for understanding the dynamics of scalar perturbations in modified cosmological scenarios.
Article
Astronomy & Astrophysics
Katsuki Aoki, Shinji Mukohyama, Ryo Namba
Summary: In a Lorentz-breaking background, a class of multi-field scalar-field theories imposes consistency conditions on its effective theory of a single field, demonstrating an example of order-unity violation of a naively applied positivity bound when a large hierarchy exists between the masses of the lightest field and the others.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Alvaro de la Cruz Dombriz, Francisco Jose Maldonado Torralba, David F. Mota
Summary: The stable pseudo-scalar degree of freedom in the quadratic Poincare Gauge theory of gravity is identified as a suitable candidate for dark matter. The study determines the parameter space in the theory that can explain all predicted cold dark matter phenomena and constrains these parameters with astrophysical observations.
Article
Astronomy & Astrophysics
Marcin Kisielowski
Summary: In this paper, a method is proposed to restrict the quantum gravity model to the spatially flat sector, reducing the number of degrees of freedom and allowing for numerical and analytical calculations. The resulting model reveals the periodic evolution of the Universe, with quantum gravity effects resolving the Big Bang singularity and causing a Big Bounce followed by a contraction phase.
CLASSICAL AND QUANTUM GRAVITY
(2023)
Article
Physics, Particles & Fields
Kevin Falls
Summary: The geometric formulation of Wilson's exact renormalisation group is based on a gauge invariant ultraviolet regularisation scheme without the need for a background field. By introducing Pauli-Villars determinants in the path integral measure and requiring quasi-locality under a change in the cutoff scale, an exact renormalisation group flow equation for the Wilsonian effective action is derived without fixing the gauge. This approach allows for the calculation of key quantities such as the one-loop beta function in Yang-Mills and the one-loop divergencies of General Relativity.
EUROPEAN PHYSICAL JOURNAL C
(2021)
Article
Astronomy & Astrophysics
Mingzhe Li, Dehao Zhao
Summary: This paper explores the cosmological applications and features of cosmological perturbations in parity violating models based on symmetric teleparallel gravity. It reveals that in addition to the known parity violation in tensor perturbations, vector perturbations in this model exhibit unique characteristics, with one mode behaving as a ghost at high momentum scales, leading to vacuum instability in the quantum theory of cosmological perturbations.
Article
Astronomy & Astrophysics
Zhe Chang, Xukun Zhang, Jing-Zhi Zhou
Summary: This study systematically investigates the second order and third order vector modes in a radiation-dominated era. The results show that a monochromatic primordial power spectrum can generate third order vector modes, and the third order vector modes sourced by the second order scalar perturbations dominate the two point function and corresponding power spectra.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2022)
Article
Pediatrics
Ranjith Kumar Manokaran, Harshavardhan Mahalingam, Shubha Shankaranarayanan, Devaram Sowmya, Padmasani Venkat Ramanan
Summary: AESD is a clinicoradiological syndrome in children caused by viral or bacterial infections. This case report presents an infant with dengue infection and AESD, who recovered completely following treatment with human immunoglobulin therapy. The study expands the reported spectrum of neurological manifestations of dengue infection.
JOURNAL OF TROPICAL PEDIATRICS
(2021)
Article
Astronomy & Astrophysics
Joseph P. Johnson, Archana Sangwan, S. Shankaranarayanan
Summary: In this study, we investigate an interacting field theory model for the interaction between dark energy and dark matter. By comparing with cosmological data, we find that this interacting model is consistent with observations and obtain quantitative tools to distinguish between interacting and non-interacting dark energy scenarios.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2022)
Article
Physics, Multidisciplinary
Manu Srivastava, S. Shankaranarayanan
Summary: This study investigates the effects of quantum fluctuations of scalar field stress-tensors in black-hole space-times using semi-classical analysis. A critical length-scale is obtained at which gravitational interactions become significant.
Review
Astronomy & Astrophysics
S. Shankaranarayanan, Joseph P. Johnson
Summary: This review discusses three aspects of modified gravity (MG) theories: why we need to consider MG theories, how to modify general relativity (GR), and the observational consequences. The review aims to serve as a reference for theorists, observers, and those interested in bridging the gap between theory and observations, by exploring the differences between GR and modified gravity theories.
GENERAL RELATIVITY AND GRAVITATION
(2022)
Article
Astronomy & Astrophysics
Ashu Kushwaha, Abhishek Naskar, Debottam Nandi, S. Shankaranarayanan
Summary: At astrophysical and cosmological scales, there is a detectable amount of magnetic field. This paper proposes an Effective Field Theory (EFT) approach to describe the origin and generation mechanism of magnetic field in the early Universe, based on expansion about the Hubble parameter and its derivatives. By analyzing a specific model, it is confirmed that broken conformal invariance and causal propagation are two necessary conditions for the generation of primordial magnetic fields.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2023)
Article
Astronomy & Astrophysics
Saurya Das, S. Shankaranarayanan, Vasil Todorinov
Summary: A recent study establishes the correspondence between the Generalized Uncertainty Principle (GUP) and Modified theories of gravity, particularly Stelle gravity. The study investigates the implications of this correspondence for inflation and cosmological observables by evaluating the power spectrum of perturbations using two different methods. By utilizing PLANCK observations, the GUP parameter gamma(0) is determined and its value is used to study the effects of quantum gravity on the power spectrum of primordial gravitational waves and their detectability in future detectors.
Article
Physics, Multidisciplinary
Indranil Das, Joseph P. Johnson, S. Shankaranarayanan
Summary: In this study, two classes of Ricci-inverse models are considered and numerically solved. The results show that these two classes of models cannot explain the late-time evolution of the Universe. By using the reduced action approach, it is found that the no-go theorem for Ricci-inverse gravity models cannot be bypassed. Finally, the implications for early-Universe cosmology are discussed.
EUROPEAN PHYSICAL JOURNAL PLUS
(2022)
Article
Astronomy & Astrophysics
Swastik Bhattacharya, S. Shankaranarayanan
Summary: This work describes the interaction between non-stationary black hole horizons and external fields using effective field theory methods, providing insights beyond classical black hole physics. The authors construct a low-energy effective field theory description for the horizon-fluid of a 4-dimensional, asymptotically flat, Einstein black hole, finding that a deformed Conformal Field Theory can naturally satisfy the requirements of near-horizon symmetries and length scales due to external perturbations. The study also determines the mass and viscosity of the horizon-fluid, which affects the scrambling time of black holes.
Article
Multidisciplinary Sciences
Avijit Chowdhury, Semin Xavier, S. Shankaranarayanan
Summary: Motivations for modified gravity have grown in the last two decades, both at theoretical and observational levels. While f(R) and Chern-Simons gravity have received more attention as simpler generalizations, they lack other modes present in modified theories of gravity. In contrast, quadratic gravity (Stelle gravity) is a more general second-order modification that includes a massive spin-2 mode. Through the study of gravitational wave energy-flux and backreaction on the spacetime of black holes, it has been demonstrated that the massive spin-2 mode carries more energy than the spin-0 mode. These effects are particularly pronounced for intermediate-mass black holes, making them important targets for LISA.
SCIENTIFIC REPORTS
(2023)
Article
Astronomy & Astrophysics
Sandra Byju, Kinjalk Lochan, S. Shankaranarayanan
Summary: We investigate generalized thermalization in an isolated free fermionic chain that undergoes a sudden quench from an out of equilibrium initial state. We focus on the evolution of local observables after the quench, such as occupation number, hopping between nearest neighbors, information sharing, and out-of-time-order correlations. We find that the system relaxes to a generalized Gibbs ensemble, despite the belief that noninteracting or free fermionic models do not relax to such an ensemble. Our analysis reveals that the internal interactions within the system become less important once the quench is sufficiently strong.
Article
Astronomy & Astrophysics
S. Mahesh Chandran, S. Shankaranarayanan
Summary: In time-independent quantum systems, the entanglement entropy possesses a scaling symmetry that the energy of the system does not have. We extend this symmetry to time-dependent systems including coupled harmonic oscillators and quantum scalar fields. These time-dependent systems exhibit a dynamical scaling symmetry that preserves the evolution of various measures of quantum correlations. It is shown that instabilities in these systems can be quantified using scrambling time and Lyapunov exponents, and that the delayed decay of the Loschmidt echo is determined by inverted modes. We also discuss the implications of zero modes and inverted modes in time-dependent massive scalar fields in different spacetimes, such as cosmological and black hole spacetimes.
Article
Education, Scientific Disciplines
Karthik Rajeev, S. Shankaranarayanan
Summary: Formulating a satisfactory quantum theory of gravity is a challenging task in theoretical physics, and Thanu Padmanabhan's pioneering works in black hole physics, quantum cosmology, and quantum gravity are considered insightful and unique. This article briefly reviews his contributions in these fields.
RESONANCE-JOURNAL OF SCIENCE EDUCATION
(2023)
Article
Astronomy & Astrophysics
Susmita Jana, S. Shankaranarayanan
Summary: The gravitational memory effect and its electromagnetic analog are potential probes in the strong gravity regime. Researchers evaluated the electromagnetic (EM) memory effect for comoving observers in arbitrary curved spacetimes, providing a transparent and easily applicable expression for the EM memory in general curved spacetimes. The master equation derived in this study allows for a physical understanding of the contribution to the EM memory and demonstrates the advantages of this approach in obtaining EM memory for specific spacetime geometries.
Article
Statistics & Probability
S. Farzana, Gayathri Harikumar, S. Shankaranarayanan, N. Vikram
Summary: Banks are no longer solely focused on satisfying consumers, but are now paying attention to providing a delightful banking experience. Research shows that e-services have an impact on customer satisfaction in the banking industry, with responsiveness and reliability being the most important factors.
JOURNAL OF STATISTICS AND MANAGEMENT SYSTEMS
(2022)
Article
Astronomy & Astrophysics
Semin Xavier, Alan Sunny, S. Shankaranarayanan
Summary: In this study, an exact time-dependent solution modeling evaporating black holes in the cosmological background was obtained, considering mass-loss, mass distribution, and cosmological background as the three aspects of primordial black holes (PBHs). The research also found that the decay rate of PBHs is positively correlated with their mass. This is significant for understanding the theoretical constraints on PBHs as dark matter.