Next Issue
Volume 4, August
Previous Issue
Volume 4, June

Universe, Volume 4, Issue 7 (July 2018) – 8 articles

Cover Story (view full-size image): A graphic representation of the next generation of space-based gravitational-wave detectors such as LISA and DECIGO, comprising three spacecrafts exchanging laser beams across 1000 km. These experiments will shed light on the physics of cosmological inflation, a process which gives rise to the observable Universe, but whose mathematical details are still poorly constrained. The link between inflation and gravitational waves is explored in a paper by S. Chongchitnan in this issue. View this paper
  • Issues are regarded as officially published after their release is announced to the table of contents alert mailing list.
  • You may sign up for e-mail alerts to receive table of contents of newly released issues.
  • PDF is the official format for papers published in both, html and pdf forms. To view the papers in pdf format, click on the "PDF Full-text" link, and use the free Adobe Readerexternal link to open them.
Order results
Result details
Select all
Export citation of selected articles as:
Article
Cosmological Constant from Condensation of Defect Excitations
Universe 2018, 4(7), 81; https://doi.org/10.3390/universe4070081 - 19 Jul 2018
Cited by 7 | Viewed by 1393
Abstract
A key challenge for many quantum gravity approaches is to construct states that describe smooth geometries on large scales. Here we define a family of (2+1)-dimensional quantum gravity states which arise from curvature excitations concentrated at point like [...] Read more.
A key challenge for many quantum gravity approaches is to construct states that describe smooth geometries on large scales. Here we define a family of (2+1)-dimensional quantum gravity states which arise from curvature excitations concentrated at point like defects and describe homogeneously curved geometries on large scales. These states represent therefore vacua for three-dimensional gravity with different values of the cosmological constant. They can be described by an anomaly-free first class constraint algebra quantized on one and the same Hilbert space for different values of the cosmological constant. A similar construction is possible in four dimensions, in this case the curvature is concentrated along string-like defects and the states are vacua of the Crane-Yetter model. We will sketch applications for quantum cosmology and condensed matter. Full article
Show Figures

Figure 1

Article
A Scale at 10 MeV, Gravitational Topological Vacuum, and Large Extra Dimensions
Universe 2018, 4(7), 80; https://doi.org/10.3390/universe4070080 - 18 Jul 2018
Cited by 2 | Viewed by 1378
Abstract
We discuss a possible scale of gravitational origin at around 10 MeV, or 1012 cm, which arises in the MacDowell–Mansouri formalism of gravity due to the topological Gauss–Bonnet term in the action, as pointed out by Bjorken several years ago. A [...] Read more.
We discuss a possible scale of gravitational origin at around 10 MeV, or 1012 cm, which arises in the MacDowell–Mansouri formalism of gravity due to the topological Gauss–Bonnet term in the action, as pointed out by Bjorken several years ago. A length scale of the same size emerges also in the Kodama solution in gravity, which is known to be closely related to the MacDowell–Mansouri formulation. We particularly draw attention to the intriguing incident that the existence of six compact extra dimensions originated from TeV-scale quantum gravity as well points to a length scale of 1012 cm, as the compactification scale. The presence of six such extra dimensions is also in remarkable consistency with the MacDowell–Mansouri formalism; it provides a possible explanation for the factor of ∼10120 multiplying the Gauss–Bonnet term in the action. We also comment on the relevant implications of such a scale regarding the thermal history of the universe motivated by the fact that it is considerably close to 1–2 MeV below which the weak interactions freeze out, leading to Big Bang Nucleosynthesis. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
Show Figures

Figure 1

Conference Report
relxill_nk: A Relativistic Reflection Model for Testing Einstein’s Gravity
Universe 2018, 4(7), 79; https://doi.org/10.3390/universe4070079 - 11 Jul 2018
Cited by 17 | Viewed by 1404
Abstract
Einstein’s theory of general relativity was proposed over 100 years ago and has successfully passed a large number of observational tests in the weak field regime. However, the strong field regime is largely unexplored, and there are many modified and alternative theories that [...] Read more.
Einstein’s theory of general relativity was proposed over 100 years ago and has successfully passed a large number of observational tests in the weak field regime. However, the strong field regime is largely unexplored, and there are many modified and alternative theories that have the same predictions as Einstein’s gravity for weak fields and present deviations when gravity becomes strong. relxill_nk is the first relativistic reflection model for probing the spacetime metric in the vicinity of astrophysical black holes and testing Einstein’s gravity in the strong field regime. Here, we present our current constraints on possible deviations from Einstein’s gravity obtained from the black holes in 1H0707–495, Ark 564, GX 339–4, and GS 1354–645. Full article
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
Show Figures

Figure 1

Erratum
Erratum: Frame- and Metric-Like Higher-Spin Fermions
Universe 2018, 4(7), 78; https://doi.org/10.3390/universe4070078 - 09 Jul 2018
Viewed by 898
Abstract
The Universe Editorial Office would like to report errors in the published paper [1].[...] Full article
Article
Primordial Gravitational Waves and Reheating in a New Class of Plateau-Like Inflationary Potentials
Universe 2018, 4(7), 77; https://doi.org/10.3390/universe4070077 - 02 Jul 2018
Cited by 1 | Viewed by 1527
Abstract
We study a new class of inflation model parametrized by the Hubble radius, such that aHexp(αφ)n. These potentials are plateau-like, and reduce to the power-law potentials in the simplest case n=2 [...] Read more.
We study a new class of inflation model parametrized by the Hubble radius, such that aHexp(αφ)n. These potentials are plateau-like, and reduce to the power-law potentials in the simplest case n=2. We investigate the range of model parameters that is consistent with current observational constraints on the scalar spectral index and the tensor-to-scalar ratio. The amplitude of primordial gravitational waves in these models is shown to be accessible by future laser interferometers such as DECIGO. We also demonstrate how these observables are affected by the temperature and equation of state during reheating. We find that a large subset of this model can support instantaneous reheating, as well as very low reheating temperatures of order a few MeV, giving rise to interesting consequences for dark-matter production. Full article
(This article belongs to the Special Issue Gravitational Waves: Prospects after the First Direct Detections)
Show Figures

Figure 1

Article
Universal Thermodynamics in the Context of Dynamical Black Hole
Universe 2018, 4(7), 76; https://doi.org/10.3390/universe4070076 - 01 Jul 2018
Viewed by 1184
Abstract
The present work is a brief review of the development of dynamical black holes from the geometric point view. Furthermore, in this context, universal thermodynamics in the FLRW model has been analyzed using the notion of the Kodama vector. Finally, some general conclusions [...] Read more.
The present work is a brief review of the development of dynamical black holes from the geometric point view. Furthermore, in this context, universal thermodynamics in the FLRW model has been analyzed using the notion of the Kodama vector. Finally, some general conclusions have been drawn. Full article
(This article belongs to the Special Issue Black Hole Thermodynamics)
Article
From Relativistic to Quantum Universe: Observation of a Spatially-Discontinuous Particle Dynamics beyond Relativity
Universe 2018, 4(7), 75; https://doi.org/10.3390/universe4070075 - 25 Jun 2018
Cited by 2 | Viewed by 1170
Abstract
We perform an experimental test where we directly observe light-induced electron transitions with a macroscopic spatial discontinuity. The effect is related to the fundamental indivisibility of macroscopic orbit-like quantum states reminiscent of so-called extended states in the integer quantum Hall system. The test [...] Read more.
We perform an experimental test where we directly observe light-induced electron transitions with a macroscopic spatial discontinuity. The effect is related to the fundamental indivisibility of macroscopic orbit-like quantum states reminiscent of so-called extended states in the integer quantum Hall system. The test has become realizable due to the discovering of a quantum phase with spontaneous pervasive quantum ordering reminiscent of that of a single atom. The observed transitions may be regarded as a peculiar quantum dynamics beyond relativity, which implies that the current relativistic model of universe should be replaced by a deeper quantum model. It is the Bohm’s model of undivided universe which now should involve a deeper-than-classical concept of absolute simultaneity and a deeper-than-relativistic concept of space and time. Ultimately, our test thus establishes a new hierarchy of fundamental physical theories where the de Broglie-Bohm realistic quantum theory is the deepest theory which does not contradict either classical physics or relativity but rather is beyond both. This is because the fact that quantum theory is dealing with a deeper reality where physical objects are not self-sufficient entities and therefore they can transit discontinuously within an overall quantum system determined by their wavefunctions. Full article
Show Figures

Figure 1

Article
Variations of the Energy of Free Particles in the pp-Wave Spacetimes
Universe 2018, 4(7), 74; https://doi.org/10.3390/universe4070074 - 21 Jun 2018
Cited by 7 | Viewed by 1059
Abstract
We consider the action of exact plane gravitational waves, or pp-waves, on free particles. The analysis is carried out by investigating the variations of the geodesic trajectories of the particles, before and after the passage of the wave. The initial velocities of the [...] Read more.
We consider the action of exact plane gravitational waves, or pp-waves, on free particles. The analysis is carried out by investigating the variations of the geodesic trajectories of the particles, before and after the passage of the wave. The initial velocities of the particles are non-vanishing. We evaluate numerically the kinetic energy per unit mass of the free particles and obtain interesting, quasi-periodic behavior of the variations of the kinetic energy with respect to the width λ of the Gaussian that represents the wave. The variation of the energy of the free particle is expected to be exactly minus the variation of the energy of the gravitational field, and therefore provides an estimation of the local variation of the gravitational energy. The investigation is carried out in the context of short bursts of gravitational waves, and of waves described by normalized Gaussians, that yield impulsive waves in a certain limit. Full article
Show Figures

Figure 1

Previous Issue
Next Issue
Back to TopTop