Browsing by Author "Sreenath, V."

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Alleviating the Tension in the Cosmic Microwave Background Using Planck-Scale Physics
(American Physical Society subs@aip.org;revtex@aps.org;prx@aps.org;prxtex@aps.org;help@aps.org;prb@aps.org, 2020) Ashtekar, A.; Gupt, B.; Jeong, D.; Sreenath, V.
Certain anomalies in the CMB bring out a tension between the six-parameter flat ?CDM model and the CMB data. We revisit the PLANCK analysis with loop quantum cosmology (LQC) predictions and show that LQC alleviates both the large-scale power anomaly and the tension in the lensing amplitude. These differences arise because, in LQC, the primordial power spectrum is scale dependent for small k, with a specific power suppression. We conclude with a prediction of larger optical depth and power suppression in the B-mode polarization power spectrum on large scales. © 2020 American Physical Society.
Anomalies in the CMB from a cosmic bounce
(Springer, 2021) Agullo, I.; Kranas, D.; Sreenath, V.
We explore a model of the early universe in which the inflationary epoch is preceded by a cosmic bounce, and argue that this scenario provides a common origin to several of the anomalous features that have been observed at large angular scales in the cosmic microwave background (CMB). More concretely, we show that a power suppression, a dipolar asymmetry, and a preference for odd-parity correlations, with amplitude and scale dependence in consonance with observations, are expected from this scenario. The model also alleviates the tension in the lensing amplitude. These signals originate from the indirect effect that non-Gaussian correlations between CMB modes and super-horizon wavelengths induce in the power spectrum. We follow a phenomenological approach, restricted to a family of bouncing models, and complement our analysis by pointing out to well established theories where our ideas are materialized. Â© 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature.
Anomalies in the Cosmic Microwave Background and Their Non-Gaussian Origin in Loop Quantum Cosmology
(Frontiers Media S.A., 2021) Agullo, I.; Kranas, D.; Sreenath, V.
Anomalies in the cosmic microwave background (CMB) refer to features that have been observed, mostly at large angular scales, and which show some tension with the statistical predictions of the standard ?CDM model. In this work, we focus our attention on power suppression, dipolar modulation, a preference for odd parity, and the tension in the lensing parameter AL. Though the statistical significance of each individual anomaly is inconclusive, collectively they are significant, and could indicate new physics beyond the ?CDM model. In this article, we present a brief, but pedagogical introduction to CMB anomalies and propose a common origin in the context of loop quantum cosmology. © Copyright © 2021 Agullo, Kranas and Sreenath.
Cosmic Tango Between the Very Small and the Very Large: Addressing CMB Anomalies Through Loop Quantum Cosmology
(Frontiers Media S.A., 2021) Ashtekar, A.; Gupt, B.; Sreenath, V.
While the standard, six-parameter, spatially flat (Formula presented.) model has been highly successful, certain anomalies in the cosmic microwave background bring out a tension between this model and observations. The statistical significance of any one anomaly is small. However, taken together, the presence of two or more of them imply that according to standard inflationary theories we live in quite an exceptional Universe. We revisit the analysis of the PLANCK collaboration using loop quantum cosmology, where an unforeseen interplay between the ultraviolet and the infrared makes the primordial power spectrum scale dependent at very small k. Consequently, we are led to a somewhat different (Formula presented.) Universe in which anomalies associated with large scale power suppression and the lensing amplitude are both alleviated. The analysis also leads to new predictions for future observations. This article is addressed both to cosmology and loop quantum gravity communities, and we have attempted to make it self-contained. © Copyright © 2021 Ashtekar, Gupt and Sreenath.
Estimation of imprints of the bounce in loop quantum cosmology on the bispectra of cosmic microwave background
(Institute of Physics, 2023) Roshna, K.; Sreenath, V.
Primordial non-Gaussianity has set strong constraints on models of the early universe. Studies have shown that Loop Quantum Cosmology (LQC), which is an attempt to extend inflationary scenario to planck scales, leads to a strongly scale dependent and oscillatory non-Gaussianity. In particular, the non-Gaussianity function f NL (k 1, k 2, k 3) generated in LQC, though similar to that generated during slow roll inflation at small scales, is highly scale dependent and oscillatory at long wavelengths. In this work, we investigate the imprints of such a primordial bispectrum in the bispectrum of Cosmic Microwave Background (CMB). Inspired by earlier works, we propose an analytical template for the primordial bispectrum in LQC. We write the template as a sum of strongly scale dependent and oscillatory part, which captures the contribution due to the bounce, and a part which captures the scale invariant behaviour similar to that of slow roll. We then compute the reduced bispectra of temperature and electric polarisation and their three-point cross-correlations corresponding to these two parts. We show that the contribution from the bounce to the reduced bispectrum is negligible compared to that from the scale-independent part. Thus, we conclude that the CMB bispectra generated in LQC will be similar to that generated in slow roll inflation. We conclude with a discussion of our results and its implications to LQC. © 2023 IOP Publishing Ltd and Sissa Medialab.
Hamiltonian theory of classical and quantum gauge invariant perturbations in Bianchi I spacetimes
(American Physical Society revtex@aps.org, 2020) Agullo, I.; Olmedo, J.; Sreenath, V.
We derive a Hamiltonian formulation of the theory of gauge invariant, linear perturbations in anisotropic Bianchi I spacetimes, and describe how to quantize this system. The matter content is assumed to be a minimally coupled scalar field with potential V(?). We show that a Bianchi I spacetime generically induces both anisotropies and quantum entanglement on cosmological perturbations, and provide the tools to compute the details of these features. We then apply this formalism to a scenario in which the inflationary era is preceded by an anisotropic Bianchi I phase, and discuss the potential imprints in observable quantities. The formalism developed here paves the road to a simultaneous canonical quantization of both the homogeneous degrees of freedom and the perturbations, a task that we develop in a companion paper. © 2020 American Physical Society.
Large scale anomalies in the CMB and non-Gaussianity in bouncing cosmologies
(IOP Publishing Ltd, 2021) Agullo, I.; Kranas, D.; Sreenath, V.
We propose that several of the anomalies that have been observed at large angular scales in the CMB have a common origin in a cosmic bounce that took place before the inflationary era. The bounce introduces a new physical scale in the problem, which breaks the almost scale invariance of inflation. As a result, the state of scalar perturbations at the onset of inflation is no longer the Bunch–Davies vacuum, but it rather contains excitations and non-Gaussianity, which are larger for infrared modes. We argue that the combined effect of these excitations and the correlations between CMB modes and longer wavelength perturbations, can account for the observed power suppression, for the dipolar asymmetry, and it can also produce a preference for odd-parity correlations. The model can also alleviate the tension in the lensing amplitude AL. We adopt a phenomenological viewpoint by considering a family of bounces characterized by a couple of parameters. We identify the minimum set of ingredients needed for our ideas to hold, and point out examples of theories in the literature where these conditions are met. © 2021 IOP Publishing Ltd Printed in the UK
Observational consequences of Bianchi i spacetimes in loop quantum cosmology
(American Physical Society subs@aip.org;revtex@aps.org;prx@aps.org;prxtex@aps.org;help@aps.org;prb@aps.org, 2020) Agullo, I.; Olmedo, J.; Sreenath, V.
Anisotropies generically dominate the earliest stages of expansion of a homogeneous universe. They are particularly relevant in bouncing models, since shears grow in the contracting phase of the cosmos, making the isotropic situation unstable. This paper extends the study of cosmological perturbations in loop quantum cosmology (LQC) to anisotropic Bianchi I models that contain a bounce followed by a phase of slow-roll inflation. We show that, although the shear tensor dilutes and the universe isotropizes soon after the bounce, cosmic perturbations retain memory of this short anisotropic phase. We develop the formalism needed to describe perturbations in anisotropic, effective LQC, and apply it to make predictions for the cosmic microwave background (CMB), while respecting current observational constraints. We show that the anisotropic bounce induces: (i) anisotropic features in all angular correlation functions in the CMB, and in particular a quadrupolar modulation that can account for a similar feature observed in the temperature map by the Planck satellite, and (ii) quantum entanglement between scalar and tensor modes, that manifests itself in temperature-polarization (T-B and E-B) correlations in the CMB. © 2020 American Physical Society.
Predictions for the Cosmic Microwave Background from an Anisotropic Quantum Bounce
(American Physical Society revtex@aps.org, 2020) Agullo, I.; Olmedo, J.; Sreenath, V.
We introduce an extension of the standard inflationary paradigm on which the big bang singularity is replaced by an anisotropic bounce. Unlike in the big bang model, cosmological perturbations find an adiabatic regime in the past. We show that this scenario accounts for the observed quadrupolar modulation in the temperature anisotropies of the cosmic microwave background, and we make predictions for the polarization angular correlation functions E-E, B-B, and E-B, together with temperature-polarization correlations T-B and T-E, that can be used to test our ideas. We base our calculations on the bounce predicted by loop quantum cosmology, but our techniques and conclusions apply to other bouncing models as well. © 2020 American Physical Society.
Quantum-to-classical transition and imprints of continuous spontaneous localization in classical bouncing universes
(World Scientific, 2021) Jaffino Stargen, D.J.; Sreenath, V.; Sriramkumar, L.
The perturbations in the early universe are generated as a result of the interplay between quantum field theory and gravitation. Since these primordial perturbations lead to the anisotropies in the cosmic microwave background and eventually to the inhomogeneities in the Large Scale Structure (LSS), they provide a unique opportunity to probe issues which are fundamental to our understanding of quantum physics and gravitation. One such fundamental issue that remains to be satisfactorily addressed is the transition of the primordial perturbations from their quantum origins to the LSS which can be characterized completely in terms of classical quantities. Classical bouncing universes provide an alternative to the more conventional inflationary paradigm as they can help overcome the horizon problem in a fashion very similar to inflation. While the problem of the quantum-to-classical transition of the primordial perturbations has been investigated extensively in the context of inflation, we find that there has been a rather limited effort toward studying the issue in classical bouncing universes. In this work, we analyze certain aspects of this problem with the example of tensor perturbations produced in classical matter and near-matter bouncing universes. We investigate the issue mainly from two perspectives. First, we approach the problem by examining the extent of squeezing of a quantum state associated with the tensor perturbations with the help of the Wigner function. Second, we analyze the issue from the perspective of the quantum measurement problem. In particular, we study the effects of wave function collapse, using a phenomenological model known as continuous spontaneous localization, on the tensor power spectra. We conclude with a discussion of results. © 2021 World Scientific Publishing Company.
xAct implementation of the theory of cosmological perturbation in bianchi I spacetimes
(2020) Agullo, I.; Olmedo, J.; Sreenath, V.
This paper presents a computational algorithm to derive the theory of linear gauge invariant perturbations on anisotropic cosmological spacetimes of the Bianchi I type. Our code is based on the tensor algebra packages xTensor and xPert, within the computational infrastructure of xAct written in Mathematica. The algorithm is based on a Hamiltonian, or phase space formulation, and it provides an efficient and transparent way of isolating the gauge invariant degrees of freedom in the perturbation fields and to obtain the Hamiltonian generating their dynamics. The restriction to Friedmann-Lemaitre-Robertson-Walker spacetimes is straightforward. � 2019 by the authors.
xAct implementation of the theory of cosmological perturbation in bianchi I spacetimes
(MDPI AG indexing@mdpi.com Postfach Basel CH-4005, 2020) Agullo, I.; Olmedo, J.; Sreenath, V.
This paper presents a computational algorithm to derive the theory of linear gauge invariant perturbations on anisotropic cosmological spacetimes of the Bianchi I type. Our code is based on the tensor algebra packages xTensor and xPert, within the computational infrastructure of xAct written in Mathematica. The algorithm is based on a Hamiltonian, or phase space formulation, and it provides an efficient and transparent way of isolating the gauge invariant degrees of freedom in the perturbation fields and to obtain the Hamiltonian generating their dynamics. The restriction to Friedmann-Lemaitre-Robertson-Walker spacetimes is straightforward. © 2019 by the authors.