scholarly journals On perturbations in Horndeski theories

2018 ◽  
Vol 182 ◽  
pp. 02127
Author(s):  
Victoria Volkova
Keyword(s):  
Correct Result ◽  
Quadratic Action ◽  
Scalar Perturbations

This research has been carried out in collaboration with R. Kolevatov, S. Mironov, V. Rubakov and N. Sukhov. We study the approach suggested by Deffayet et al. for obtaining a derivative part of the quadratic action for scalar perturbations in the cubic Horndeski theory. We analyse the validity of the approach and generalize it for the complete Horndeski theory. We explicitly check that the generalized method gives the correct result.

scholarly journals Properties of perturbations in beyond Horndeski theories

2018 ◽  
Vol 33 (27) ◽  
pp. 1850155 ◽  
Author(s):  
S. Mironov ◽  
V. Volkova
Keyword(s):  
Incorrect Result ◽  
Higher Derivatives ◽  
Quadratic Action ◽  
Derivatives Of ◽  
Scalar Perturbations ◽  
The Way

We study whether the approach of Deffayet et al. (DPSV) can be adopted for obtaining a derivative part of quadratic action for scalar perturbations in beyond Horndeski theories about homogeneous and isotropic backgrounds. We find that even though the method does remove the second and higher derivatives of metric perturbations from the linearized Galileon equation, in the same manner as in the general Horndeski theory, it gives incorrect result for the quadratic action. We analyze the reasons behind this property and suggest the way of modifying the approach, so that it gives valid results.

Real-time subsurface scattering with single pass variance-guided adaptive importance sampling

2020 ◽  
Vol 3 (1) ◽  
pp. 1-21
Author(s):  
Tiantian Xie ◽  
Marc Olano ◽  
Brian Karis ◽  
Krzysztof Narkowicz
Keyword(s):  
Real Time ◽  
Adaptive Sampling ◽  
Monte Carlo Sampling ◽  
Diffusion Profile ◽  
Correct Result ◽  
Subsurface Scattering ◽  
Stochastic Sampling ◽  
Light Gradient ◽  
Temporal Variance ◽  
Importance Function

In real-time applications, it is difficult to simulate realistic subsurface scattering with differing degrees translucency. Burley's reflectance approximation by empirically fitting the diffusion profile as a whole makes it possible to achieve realistic looking subsurface scattering for different translucent materials in screen space. However, achieving a physically correct result requires real-time Monte Carlo sampling of the analytic importance function per pixel per frame, which seems prohibitive to achieve. In this paper, we propose an approximation of the importance function that can be evaluated in real-time. Since subsurface scattering is more pronounced in certain regions (e.g., with light gradient change), we propose an adaptive sampling method based on temporal variance to lower the required number of samples. We propose a one phase adaptive sampling pass that is unbiased, and able to adapt to scene changes due to motion and lighting. To further improve the quality, we explore temporal reuse with a guiding pass prior to the final temporal anti-aliasing (TAA) phase that further improves the quality. Our local guiding pass does not constrain the TAA implementation, and only requires one additional texture to be passed between frames. Our proposed variance-guided algorithm has the potential to make stochastic sampling algorithm effective for real-time rendering.

scholarly journals A cosmic microscope for the preheating era

2021 ◽  
Vol 2021 (1) ◽  
Author(s):  
JiJi Fan ◽  
Zhong-Zhi Xianyu
Keyword(s):  
Scalar Field ◽  
Nonlinear Effect ◽  
Large Scale ◽  
Nonlinear Effects ◽  
Light Fields ◽  
Unique Probe ◽  
Light Scalar ◽  
Spatially Varying ◽  
Near Future ◽  
Scalar Perturbations

Abstract Light fields with spatially varying backgrounds can modulate cosmic preheating, and imprint the nonlinear effects of preheating dynamics at tiny scales on large scale fluctuations. This provides us a unique probe into the preheating era which we dub the “cosmic microscope”. We identify a distinctive effect of preheating on scalar perturbations that turns the Gaussian primordial fluctuations of a light scalar field into square waves, like a diode. The effect manifests itself as local non-Gaussianity. We present a model, “modulated partial preheating”, where this nonlinear effect is consistent with current observations and can be reached by near future cosmic probes.

scholarly journals Accidental Gauge Symmetries of Minkowski Spacetime in Teleparallel Theories

Universe ◽  
2021 ◽  
Vol 7 (5) ◽  
pp. 143
Author(s):  
Jose Beltrán Jiménez ◽  
Tomi S. Koivisto
Keyword(s):  
General Relativity ◽  
Minkowski Spacetime ◽  
Linear Extensions ◽  
Gauge Symmetries ◽  
Linear Spectrum ◽  
Massless Spin ◽  
Non Linear ◽  
Common Framework ◽  
Local Symmetries ◽  
Quadratic Action

In this paper, we provide a general framework for the construction of the Einstein frame within non-linear extensions of the teleparallel equivalents of General Relativity. These include the metric teleparallel and the symmetric teleparallel, but also the general teleparallel theories. We write the actions in a form where we separate the Einstein–Hilbert term, the conformal mode due to the non-linear nature of the theories (which is analogous to the extra degree of freedom in f(R) theories), and the sector that manifestly shows the dynamics arising from the breaking of local symmetries. This frame is then used to study the theories around the Minkowski background, and we show how all the non-linear extensions share the same quadratic action around Minkowski. As a matter of fact, we find that the gauge symmetries that are lost by going to the non-linear generalisations of the teleparallel General Relativity equivalents arise as accidental symmetries in the linear theory around Minkowski. Remarkably, we also find that the conformal mode can be absorbed into a Weyl rescaling of the metric at this order and, consequently, it disappears from the linear spectrum so only the usual massless spin 2 perturbation propagates. These findings unify in a common framework the known fact that no additional modes propagate on Minkowski backgrounds, and we can trace it back to the existence of accidental gauge symmetries of such a background.

Linear groups generated by a pair of quadratic action subgroups

1985 ◽  
Vol 44 (1) ◽  
pp. 15-19
Author(s):  
B. Baumann ◽  
C. Ho
Keyword(s):  
Linear Groups ◽  
Quadratic Action

Scalar perturbations in string-dilaton cosmology

1994 ◽  
Vol 109 (7) ◽  
pp. 783-794 ◽  
Author(s):  
S. Capozziello ◽  
R. Ritis
Keyword(s):  
Scalar Perturbations

On Primordial Black Holes and secondary gravitational waves generated from inflation with solo/multi-bumpy potential

2021 ◽  
Author(s):  
Rui feng Zheng ◽  
Jia ming Shi ◽  
Taotao Qiu
Keyword(s):  
Black Holes ◽  
Black Hole ◽  
Gravitational Waves ◽  
Power Spectra ◽  
Small Scale ◽  
Primordial Black Hole ◽  
Primordial Black Holes ◽  
Slow Roll ◽  
Spherical Collapse ◽  
Scalar Perturbations

Abstract It is well known that primordial black hole (PBH) can be generated in inflation process of the early universe, especially when the inflaton field has some non-trivial features that could break the slow-roll condition. In this paper, we investigate a toy model of inflation with bumpy potential, which has one or several bumps. We found that potential with multi-bump can give rise to power spectra with multi peaks in small-scale region, which can in turn predict the generation of primordial black holes in various mass ranges. We also consider the two possibilities of PBH formation by spherical collapse and elliptical collapse. And discusses the scalar-induced gravitational waves (SIGWs) generated by the second-order scalar perturbations.

scholarly journals Building models of inflation in no-scale supergravity

2020 ◽  
pp. 2030011
Author(s):  
John Ellis ◽  
Marcos A. G. García ◽  
Natsumi Nagata ◽  
Dimitri V. Nanopoulos ◽  
Keith A. Olive ◽  
...  
Keyword(s):  
Cold Dark Matter ◽  
Planck Scale ◽  
Neutrino Masses ◽  
De Sitter ◽  
Building Models ◽  
Scale Models ◽  
Cosmological Inflation ◽  
Text Model ◽  
Hilbert Action ◽  
Scalar Perturbations

After reviewing the motivations for cosmological inflation formulated in the formalism of supersymmetry, we argue that the appropriate framework is that of no-scale supergravity. We then show how to construct within this framework inflationary models whose predictions for the tilt in the spectrum of scalar perturbations, [Formula: see text], and the ratio, [Formula: see text], of tensor and scalar perturbations coincide with those of the [Formula: see text] model of inflation proposed by Starobinsky. A more detailed study of no-scale supergravity reveals a structure that is closely related to that of [Formula: see text] modifications of the minimal Einstein–Hilbert action for general relativity, opening avenues for constructing no-scale de Sitter and anti-de Sitter models by combining pairs of Minkowski models, as well as generalizations of the original no-scale Starobinsky models of inflation. We then discuss the phenomenology of no-scale models of inflation, including inflaton decay and reheating, and then the construction of explicit scenarios based on SU(5), SO(10) and string-motivated flipped SU(5)×U(1) GUT models. The latter provides a possible model of almost everything below the Planck scale, including neutrino masses and oscillations, the cosmological baryon asymmetry and cold dark matter, as well as [Formula: see text] and [Formula: see text].

scholarly journals SPECTRUM OF GRAVITATIONAL WAVES IN KREIN SPACE QUANTIZATION

2011 ◽  
Vol 26 (36) ◽  
pp. 2697-2702 ◽  
Author(s):  
M. MOHSENZADEH ◽  
A. SOJASI ◽  
E. YUSOFI
Keyword(s):  
Gravitational Waves ◽  
Krein Space ◽  
Slow Roll ◽  
Primordial Power Spectrum ◽  
Alternative Approach ◽  
Special Cases ◽  
Kreĭn Space ◽  
Field Quantization ◽  
Krein Space Quantization ◽  
Scalar Perturbations

The main goal of this paper is to derive the primordial power spectrum for the scalar perturbations generated as a result of quantum fluctuations during an inflationary period by an alternative approach of field quantization.1–3 Formulas are derived for the gravitational waves, special cases of which include power law inflation and inflation in the slow roll approximation, in Krein space quantization.

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