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Universe, Volume 5, Issue 11 (November 2019)

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Cover Story (view full-size image) Gravitational wave astronomy allows us to study objects and events invisible in electromagnetic [...] Read more.
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Open AccessArticle
Observational Constraints on Dynamical Dark Energy with Pivoting Redshift
Universe 2019, 5(11), 219; https://doi.org/10.3390/universe5110219 - 16 Nov 2019
Abstract
We investigate the generalized Chevallier–Polarski–Linder (CPL) parametrization, which contains the pivoting redshift zp as an extra free parameter, in order to examine whether the evolution of the dark energy equation of state can be better described by a different parametrization. We use [...] Read more.
We investigate the generalized Chevallier–Polarski–Linder (CPL) parametrization, which contains the pivoting redshift z p as an extra free parameter, in order to examine whether the evolution of the dark energy equation of state can be better described by a different parametrization. We use various data combinations from cosmic microwave background (CMB), baryon acoustic oscillations (BAO), redshift space distortion (RSD), weak lensing (WL), joint light curve analysis (JLA), and cosmic chronometers (CC), and we include a Gaussian prior on the Hubble constant value, in order to extract the observational constraints on various quantities. For the case of free z p we find that for all data combinations it always remains unconstrained, and there is a degeneracy with the value of the dark energy equation of state w 0 p at z p . For the case where z p is fixed to specific values, and for the full data combination, we find that with increasing z p the mean value of w 0 p slowly moves into the phantom regime, however the cosmological constant is always allowed within 1 σ confidence-level. In fact, the significant effect is that with increasing z p , the correlations between w 0 p and w a (the free parameter of the dark energy equation of state quantifying its evolution with redshift), change from negative to positive, with the case z p = 0.35 corresponding to no correlation. The fact that the two parameters describing the dark energy equation of state are uncorrelated for z p = 0.35 justifies why a non-zero pivoting redshift needs to be taken into account. Full article
(This article belongs to the Special Issue Probing the Dark Universe with Theory and Observations)
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Open AccessReview
The Effects of Finite Distance on the Gravitational Deflection Angle of Light
Universe 2019, 5(11), 218; https://doi.org/10.3390/universe5110218 - 01 Nov 2019
Abstract
In order to clarify the effects of the finite distance from a lens object to a light source and a receiver, the gravitational deflection of light has been recently reexamined by using the Gauss–Bonnet (GB) theorem in differential geometry (Ishihara et al. 2016). [...] Read more.
In order to clarify the effects of the finite distance from a lens object to a light source and a receiver, the gravitational deflection of light has been recently reexamined by using the Gauss–Bonnet (GB) theorem in differential geometry (Ishihara et al. 2016). The purpose of the present paper is to give a short review of a series of works initiated by the above paper. First, we provide the definition of the gravitational deflection angle of light for the finite-distance source and receiver in a static, spherically symmetric and asymptotically flat spacetime. We discuss the geometrical invariance of the definition by using the GB theorem. The present definition is used to discuss finite-distance effects on the light deflection in Schwarzschild spacetime for both the cases of weak deflection and strong deflection. Next, we extend the definition to stationary and axisymmetric spacetimes. We compute finite-distance effects on the deflection angle of light for Kerr black holes and rotating Teo wormholes. Our results are consistent with the previous works if we take the infinite-distance limit. We briefly mention also the finite-distance effects on the light deflection by Sagittarius A*. Full article
(This article belongs to the Special Issue Gravitational Lensing and Optical Geometry: A Centennial Perspective)
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Open AccessReview
Continuous Gravitational Waves from Neutron Stars: Current Status and Prospects
Universe 2019, 5(11), 217; https://doi.org/10.3390/universe5110217 - 31 Oct 2019
Abstract
Gravitational waves astronomy allows us to study objects and events invisible in electromagnetic waves. It is crucial to validate the theories and models of the most mysterious and extreme matter in the Universe: the neutron stars. In addition to inspirals and mergers of [...] Read more.
Gravitational waves astronomy allows us to study objects and events invisible in electromagnetic waves. It is crucial to validate the theories and models of the most mysterious and extreme matter in the Universe: the neutron stars. In addition to inspirals and mergers of neutrons stars, there are currently a few proposed mechanisms that can trigger radiation of long-lasting gravitational radiation from neutron stars, such as e.g., elastically and/or magnetically driven deformations: mountains on the stellar surface supported by the elastic strain or magnetic field, free precession, or unstable oscillation modes (e.g., the r-modes). The astrophysical motivation for continuous gravitational waves searches, current LIGO and Virgo strategies of data analysis and prospects are reviewed in this work. Full article
(This article belongs to the Special Issue Neutron Star Astrophysics)
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Open AccessArticle
Gravitational Fluctuations as an Alternative to Inflation II. CMB Angular Power Spectrum
Universe 2019, 5(11), 216; https://doi.org/10.3390/universe5110216 - 31 Oct 2019
Abstract
Power spectra always play an important role in the theory of inflation. In particular, the ability to reproduce the galaxy matter power spectrum P(k) and the CMB temperature angular power spectrum Cl’s to high accuracy is often considered [...] Read more.
Power spectra always play an important role in the theory of inflation. In particular, the ability to reproduce the galaxy matter power spectrum P ( k ) and the CMB temperature angular power spectrum C l ’s to high accuracy is often considered a triumph of inflation. In our previous work, we presented an alternative explanation for the matter power spectrum based on nonperturbative quantum field-theoretical methods applied to Einstein’s gravity, instead of inflation models based on scalar fields. In this work, we review the basic concepts and provide further in-depth investigations. We first update the analysis with more recent data sets and error analysis, and then extend our predictions to the CMB angular spectrum coefficients C l , which we did not consider previously. Then we investigate further the potential freedoms and uncertainties associated with the fundamental parameters that are part of this picture, and show how recent cosmological data provides significant constraints on these quantities. Overall, we find good general consistency between theory and data, even potentially favoring the gravitationally-motivated picture at the largest scales. We summarize our results by outlining how this picture can be tested in the near future with increasingly accurate astrophysical measurements. Full article
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Open AccessArticle
On the Constant-Roll Inflation with Large and Small ηH
Universe 2019, 5(11), 215; https://doi.org/10.3390/universe5110215 - 25 Oct 2019
Cited by 5
Abstract
We study the apparent duality between large and small ηH for the constant-roll inflation with the second slow-roll parameter ηH being a constant. In the previous studies, only the constant-roll inflationary models with small ηH are found to be consistent [...] Read more.
We study the apparent duality between large and small η H for the constant-roll inflation with the second slow-roll parameter η H being a constant. In the previous studies, only the constant-roll inflationary models with small η H are found to be consistent with the observations. The apparent duality suggests that the constant-roll inflationary models with large η H may be also consistent with the observations. We find that the duality between the constant-roll inflation with large and small η H does not exist, because both the background and scalar perturbation evolutions are very different. By fitting the constant-roll inflationary models to the observations, we get 0.016 η H 0.0078 at the 95% C.L if we take N = 60 for the models with increasing ϵ H , in which inflation ends when ϵ H = 1 . For the models with decreasing ϵ H , we obtain 3.0135 η H 3.021 at the 68% C.L. and 3.0115 η H 3.024 at the 95% C.L. Full article
(This article belongs to the Special Issue Inflation, Black Holes and Gravitational Waves)
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