The Experts below are selected from a list of 168 Experts worldwide ranked by ideXlab platform
Yifu Cai - One of the best experts on this subject based on the ideXlab platform.
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Primordial black holes and gravitational waves from resonant amplification during inflation
2020Co-Authors: Zhou Zihan, Yifu Cai, Jiang Jie, Sasaki Misao, Pi ShiAbstract:We present a new realization of the resonant production of primordial black holes as well as gravitational waves in a two-stage inflation model consisting of a scalar field \phi with an axion-monodromy-like periodic structure in the potential that governs the first stage and another field \chi with a hilltop-like potential that dominates the second stage. The parametric resonance seeded by the periodic structure at the first stage amplifies the perturbations of both fields inside the Hubble Radius. While the evolution of the background trajectory experiences a turn as the oscillatory barrier height increases, the amplified perturbations of \chi remain as they are and contribute to the final curvature perturbation. It turns out that the primordial power spectrum displays a significant resonant peak on small scales, which can lead to an abundant production of primordial black holes. Furthermore, gravitational waves are also generated from the resonantly enhanced field perturbations during inflation, the amplitude of which may be constrained by future gravitational wave interferometers.Comment: 14 pages, 8 figure
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towards a nonsingular bouncing cosmology
Journal of Cosmology and Astroparticle Physics, 2012Co-Authors: Yifu Cai, Damien A Easson, Robert BrandenbergerAbstract:We present a nonsingular bouncing cosmology using single scalar field matter with non-trivial potential and non-standard kinetic term. The potential sources a dynamical attractor solution with Ekpyrotic contraction which washes out small amplitude anisotropies. At high energy densities the field evolves into a ghost condensate, leading to a nonsingular bounce. Following the bounce there is a smooth transition to standard expanding radiation and matter dominated phases. Using linear cosmological perturbation theory we track each Fourier mode of the curvature fluctuation throughout the entire cosmic evolution. Using standard matching conditions for nonsingular bouncing cosmologies we verify that the spectral index does not change during the bounce. We show there is a controlled period of exponential growth of the fluctuation amplitude for the perturbations (but not for gravitational waves) around the bounce point which does not invalidate the perturbative treatment. This growth induces a natural suppression mechanism for the tensor to scalar ratio of fluctuations. Moreover, we study the generation of the primordial power spectrum of curvature fluctuations for various types of initial conditions. For the pure vacuum initial condition, on scales which exit the Hubble Radius in the phase of Ekpyrotic contraction, the spectrum is deeply blue. For thermal particle initial condition, one possibility for generating a scale-invariant spectrum makes use of a special value of the background equation of state during the contracting Ekpyrotic phase. If the Ekpyrotic phase is preceded by a period of matter-dominated contraction, the primordial power spectrum is nearly scale-invariant on large scales (scales which exit the Hubble Radius in the matter-dominated phase) but acquires a large blue tilt on small scales. Thus, our model provides a realization of the "matter bounce" scenario which is free of the anisotropy problem.
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towards a nonsingular bouncing cosmology
arXiv: High Energy Physics - Theory, 2012Co-Authors: Yifu Cai, Damien A Easson, Robert BrandenbergerAbstract:We present a nonsingular bouncing cosmology using single scalar field matter with non-trivial potential and non-standard kinetic term. The potential sources a dynamical attractor solution with Ekpyrotic contraction which washes out small amplitude anisotropies. At high energy densities the field evolves into a ghost condensate, leading to a nonsingular bounce. Following the bounce there is a smooth transition to standard expanding radiation and matter dominated phases. Using linear cosmological perturbation theory we track each Fourier mode of the curvature fluctuation throughout the entire cosmic evolution. Using standard matching conditions for nonsingular bouncing cosmologies we verify that the spectral index does not change during the bounce. We show there is a controlled period of exponential growth of the fluctuation amplitude for the perturbations (but not for gravitational waves) around the bounce point which does not invalidate the perturbative treatment. This growth induces a natural suppression mechanism for the tensor to scalar ratio of fluctuations. Moreover, we study the generation of the primordial power spectrum of curvature fluctuations for various types of initial conditions. For the pure vacuum initial condition, on scales which exit the Hubble Radius in the phase of Ekpyrotic contraction, the spectrum is deeply blue. For thermal particle initial condition, one possibility for generating a scale-invariant spectrum makes use of a special value of the background equation of state during the contracting Ekpyrotic phase. If the Ekpyrotic phase is preceded by a period of matter-dominated contraction, the primordial power spectrum is nearly scale-invariant on large scales (scales which exit the Hubble Radius in the matter-dominated phase) but acquires a large blue tilt on small scales.
Robert H Brandenberger - One of the best experts on this subject based on the ideXlab platform.
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creating spatial flatness by combining string gas cosmology and power law inflation
Physical Review D, 2020Co-Authors: Vahid Kamali, Robert H BrandenbergerAbstract:We show that it is possible to combine an early phase of String Gas Cosmology which can explain the origin of the observed structures on cosmological scales with a short later period of power law inflation which creates spatial flatness. The resulting model is consistent with the ``swampland criteria'' and the constraints coming from the {\it Trans-Planckian Censorship Conjecture}. Such a construction is not possible using only canonical slow-roll inflation, but it can emerge in the warm inflation scenario or in cold inflation with an exponential potential. The resulting cosmology is non-singular. We discuss the spectrum of cosmological perturbations resulting in our scenario. On large scales (scales which remain larger than the Hubble Radius after the initial string gas phase) the spectrum is determined by the thermal string gas fluctuations set up in the primordial phase, and it is almost scale-invariant with a slight red tilt. On small scales, the perturbations produced during the inflationary phase dominate. On these scales, the spectrum is once again nearly scale-invariant. There is an intermediate range (scales which enter the Hubble Radius during the period of inflation) where the string gas fluctuations are damped but continue to dominate over those produced during the period of inflation. On these scales the spectrum has a sharp red spectral index of $n_s - 1 \sim -2$.
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termination of the phase of quintessence by gravitational back reaction
Physical Review D, 2001Co-Authors: Wenbin Lin, Xinmin Zhang, Robert H BrandenbergerAbstract:We study the effects of gravitational back reaction in models of quintessence. The effective energy-momentum tensor with which cosmological fluctuations back react on the background metric will in some cases lead to a termination of the phase of acceleration. The fluctuations we make use of are the perturbations in our present Universe. Their amplitude is normalized by recent measurements of anisotropies in the cosmic microwave background; their slope is taken to be either scale invariant or characterized by a slightly blue tilt. In the latter case, we find that the back-reaction effect of fluctuations whose present wavelength is smaller than the Hubble Radius but which are stretched beyond the Hubble Radius by the accelerated expansion during the era of quintessence domination can become large. Since the back-reaction effects of these modes oppose acceleration, back reaction will lead to a truncation of the period of quintessence domination. This result impacts on the recent discussions of the potential incompatibility between string theory and quintessence.
Kenichi Saikawa - One of the best experts on this subject based on the ideXlab platform.
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on the estimation of gravitational wave spectrum from cosmic domain walls
Journal of Cosmology and Astroparticle Physics, 2014Co-Authors: Takashi Hiramatsu, Masahiro Kawasaki, Kenichi SaikawaAbstract:We revisit the production of gravitational waves from unstable domain walls analyzing their spectrum by the use of field theoretic lattice simulations with grid size 10243, which is larger than the previous study. We have recognized that there exists an error in the code used in the previous study, and the correction of the error leads to the suppression of the spectrum of gravitational waves at high frequencies. The peak of the spectrum is located at the scale corresponding to the Hubble Radius at the time of the decay of domain walls, and its amplitude is consistent with the naive estimation based on the quadrupole formula. Using the numerical results, the magnitude and the peak frequency of gravitational waves at the present time are estimated. It is shown that for some choices of parameters the signal of gravitational waves is strong enough to be probed in the future gravitational wave experiments.
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study of gravitational radiation from cosmic domain walls
Journal of Cosmology and Astroparticle Physics, 2011Co-Authors: Masahiro Kawasaki, Kenichi SaikawaAbstract:In this paper, following the previous study, we evaluate the spectrum of gravitational wave background generated by domain walls which are produced if some discrete symmetry is spontaneously broken in the early universe. We apply two methods to calculate the gravitational wave spectrum: One is to calculate the gravitational wave spectrum directly from numerical simulations, and another is to calculate it indirectly by estimating the unequal time anisotropic stress power spectrum of the scalar field. Both analysises indicate that the slope of the spectrum changes at two characteristic frequencies corresponding to the Hubble Radius at the decay of domain walls and the width of domain walls, and that the spectrum between these two characteristic frequencies becomes flat or slightly red tilted. The second method enables us to evaluate the GW spectrum for the frequencies which cannot be resolved in the finite box lattice simulations, but relies on the assumptions for the unequal time correlations of the source.
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study of gravitational radiation from cosmic domain walls
arXiv: Cosmology and Nongalactic Astrophysics, 2011Co-Authors: Masahiro Kawasaki, Kenichi SaikawaAbstract:In this paper, following the previous study, we evaluate the spectrum of gravitational wave background generated by domain walls which are produced if some discrete symmetry is spontaneously broken in the early universe. We apply two different methods to calculate the gravitational wave spectrum: One is to calculate the gravitational wave spectrum directly from numerical simulations, and another is to calculate it indirectly by estimating the unequal time anisotropic stress power spectrum of the scalar field. Both analysises indicate that the slope of the spectrum changes at two characteristic frequencies corresponding to the Hubble Radius at the decay of domain walls and the width of domain walls, and that the spectrum between these two characteristic frequencies becomes flat or slightly red tilted. The second method enables us to evaluate the GW spectrum semi-analytically for the frequencies which can not be resolved in the finite box lattice simulations, but relies on the assumptions for the unequal time correlations of the source.
L Sriramkumar - One of the best experts on this subject based on the ideXlab platform.
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the scalar bi spectrum in the starobinsky model the equilateral case
Journal of Cosmology and Astroparticle Physics, 2012Co-Authors: Jerome Martin, L SriramkumarAbstract:While a featureless, nearly scale invariant, primordial scalar power spectrum fits the most recent Cosmic Microwave Background (CMB) data rather well, certain features in the spectrum are known to lead to a better fit to the data (although, the statistical significance of such results remains an open issue). In the inflationary scenario, one or more periods of deviations from slow roll are necessary in order to generate features in the scalar perturbation spectrum. Over the last couple of years, it has been recognized that such deviations from slow roll inflation can also result in reasonably large non-Gaussianities. The Starobinsky model involves the canonical scalar field and consists of a linear inflaton potential with a sudden change in the slope. The change in the slope causes a brief period of departure from slow roll which, in turn, results in a sharp rise in power, along with a burst of oscillations in the scalar spectrum for modes that leave the Hubble Radius just before and during the period of fast roll. The hallmark of the Starobinsky model is that it allows the scalar power spectrum to be evaluated analytically in terms of the three parameters that describe the model, viz. the twomore » slopes that describe the potential on either side of the discontinuity and the Hubble scale at the time when the field crosses the discontinuity. In this work, we evaluate the bi-spectrum of the scalar perturbations in the Starobinsky model in the equilateral limit. Remarkably, we find that, just as the power spectrum, all the different contributions to the the bi-spectrum too can be evaluated completely analytically and expressed in terms of the three paramaters that describe the model. We show that the quantity f{sub NL}, which characterizes the extent of non-Gaussianity, can be expressed purely in terms of the ratio of the two slopes on either side of the discontinuity in the potential. Further, we find that, for certain values of the parameters, f{sub NL} in the Starobinsky model can be as large as the mean value that has been arrived at from the analysis of the recent CMB data. We also demonstrate that the usual hierarchy of contributions to the bi-spectrum can be altered for certain values of the parameters. Altogether, we find that the Starobinsky model represents a unique scenario wherein, even when the slow roll conditions are violated, the background, the perturbations as well as the corresponding two and three point correlation functions can be evaluated completely analytically. As a consequence, the Starobinsky model can also be used to calibrate numerical codes aimed at computing the non-Gaussianities.« less
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the scalar bi spectrum in the starobinsky model the equilateral case
arXiv: Cosmology and Nongalactic Astrophysics, 2011Co-Authors: Jerome Martin, L SriramkumarAbstract:(Shortened abstract) While a featureless, nearly scale invariant, primordial scalar power spectrum fits the most recent CMB data rather well, certain features in the spectrum are known to lead to a better fit to the data (although, the statistical significance of such results remains an open issue). In the inflationary scenario, one or more periods of deviations from slow roll are necessary in order to generate features in the scalar perturbation spectrum. Over the last couple of years, it has been recognized that such deviations from slow roll inflation can also result in reasonably large non-Gaussianities. The Starobinsky model involves the canonical scalar field and consists of a linear inflaton potential with a sudden change in the slope. The change in the slope causes a brief period of departure from slow roll which, in turn, results in a sharp rise in power, along with a burst of oscillations in the scalar spectrum for modes that leave the Hubble Radius just before and during the period of fast roll. The hallmark of the Starobinsky model is that it allows the scalar power spectrum to be evaluated analytically in terms of the three parameters that describe the model. We evaluate the bi-spectrum of the scalar perturbations in the Starobinsky model in the equilateral limit. Remarkably, we find that, all the different contributions to the the bi-spectrum too can be evaluated completely analytically. We show that the quantity $\fnl$, which characterizes the extent of non-Gaussianity, can be expressed purely in terms of the ratio of the two slopes on either side of the discontinuity in the potential. Further, we find that, for certain values of the parameters, $\fnl$ in the Starobinsky model can be as large as the mean value that has been arrived at from the analysis of the recent CMB data.
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punctuated inflation and the low cmb multipoles
Journal of Cosmology and Astroparticle Physics, 2009Co-Authors: Rajeev Kumar Jain, Pravabati Chingangbam, Jinnouk Gong, L Sriramkumar, T SouradeepAbstract:We investigate inflationary scenarios driven by a class of potentials which are similar in form to those that arise in certain minimal supersymmetric extensions of the standard model. We find that these potentials allow a brief period of departure from inflation sandwiched between two stages of slow roll inflation. We show that such a background behavior leads to a step like feature in the scalar power spectrum. We set the scales such that the drop in the power spectrum occurs at a length scale that corresponds to the Hubble Radius today — a feature that seems necessary to explain the lower power observed in the quadrupole moment of the Cosmic Microwave Background (CMB) anisotropies. We perform a Markov Chain Monte Carlo analysis to determine the values of the model parameters that provide the best fit to the recent WMAP 5-year data for the CMB angular power spectrum. We find that an inflationary spectrum with a suppression of power at large scales that we obtain leads to a much better fit (with just one extra parameter, χeff2 improves by 6.62) of the observed data when compared to the best fit reference ΛCDM model with a featureless, power law, primordial spectrum.
Tirthabir Biswas - One of the best experts on this subject based on the ideXlab platform.
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swiss cheese inhomogeneous cosmology and the dark energy problem
Journal of Cosmology and Astroparticle Physics, 2008Co-Authors: Tirthabir Biswas, Alessio NotariAbstract:We study an exact Swiss-cheese model of the universe, where inhomogeneous LTB patches are embedded in a flat FLRW background, in order to see how observations of distant sources are affected. We focus mainly on the redshift, both perturbatively and non-perturbatively: the net effect given by one patch is suppressed by (L/RH)3 (where L is the size of one patch and RH is the Hubble Radius). We disentangle this effect from the Doppler term (which is much larger and has been used recently (Biswas et al 2007 J. Cosmol. Astropart. Phys. JCAP12(2007)017 [astro-ph/0606703]) to try to fit the SN curve without dark energy) by making contact with cosmological perturbation theory. Then, the correction to the angular distance is discussed analytically and estimated to be larger, , perturbatively and non-perturbatively (although it should go to zero after angular averaging).
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swiss cheese inhomogeneous cosmology the dark energy problem
arXiv: Astrophysics, 2007Co-Authors: Tirthabir Biswas, Alessio NotariAbstract:We study an exact swiss-cheese model of the Universe, where inhomogeneous LTB patches are embedded in a flat FLRW background, in order to see how observations of distant sources are affected. We find negligible integrated effect, suppressed by (L/R_{H})^3 (where L is the size of one patch, and R_{H} is the Hubble Radius), both perturbatively and non-perturbatively. We disentangle this effect from the Doppler term (which is much larger and has been used recently \cite{BMN} to try to fit the SN curve without dark energy) by making contact with cosmological perturbation theory.