Universe: Feature Papers 2023—Cosmology

A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Cosmology".

Deadline for manuscript submissions: closed (31 October 2023) | Viewed by 22892

Special Issue Editor

Special Issue Information

Dear Colleagues,

This Special Issue aims to set itself at the cutting edge of the most recent advances in the intertwined ties of cosmology with other fields at all relevant scales from mutually fertilizing theoretical, phenomenological and experimental perspectives. Topics include, but are not limited to, the following:

  • Cosmological Models
  • Inflationary cosmology
  • Big Bang
  • Bounce Cosmology
  • Cosmic String/Superstring Theory
  • Quantum Cosmology
  • Observational Cosmology
  • LCDM Cosmology
  • Dark Matter and Dark Energy
  • Axion Cosmology
  • Cosmic Acceleration
  • Cosmological constants
  • Cosmological Perturbation Theory
  • Cosmic Microwave Background (CMB)
  • Hubble's Law/Constants
  • Machine Learning and Cosmology
  • The Large-Scale Structure of the Universe

You are welcome to send short proposals for submissions of Feature Papers to our Editorial Office (universe@mdpi.com). They will be evaluated by Editors first, and the selected papers will be thoroughly and rigorously peer reviewed.

Dr. Kazuharu Bamba
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

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Please visit the Instructions for Authors page before submitting a manuscript. Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Published Papers (20 papers)

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Research

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20 pages, 384 KiB  
Article
Raychaudhuri Equations, Tidal Forces, and the Weak-Field Limit in Schwarzshild–Finsler–Randers Spacetime
Universe 2024, 10(1), 26; https://doi.org/10.3390/universe10010026 - 09 Jan 2024
Viewed by 685
Abstract
In this article, we study the form of the deviation of geodesics (tidal forces) and the Raychaudhuri equation in a Schwarzschild–Finsler–Randers (SFR) spacetime which has been investigated in previous papers. This model is obtained by considering the structure of a Lorentz tangent bundle [...] Read more.
In this article, we study the form of the deviation of geodesics (tidal forces) and the Raychaudhuri equation in a Schwarzschild–Finsler–Randers (SFR) spacetime which has been investigated in previous papers. This model is obtained by considering the structure of a Lorentz tangent bundle of spacetime and, in particular, the kind of the curvatures in generalized metric spaces where there is more than one curvature tensor, such as Finsler-like spacetimes. In these cases, the concept of the Raychaudhuri equation is extended with extra terms and degrees of freedom from the dependence on internal variables such as the velocity or an anisotropic vector field. Additionally, we investigate some consequences of the weak-field limit on the spacetime under consideration and study the Newtonian limit equations which include a generalization of the Poisson equation. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
18 pages, 2109 KiB  
Article
Cosmological Parameter Estimation with Genetic Algorithms
Universe 2024, 10(1), 11; https://doi.org/10.3390/universe10010011 - 27 Dec 2023
Cited by 1 | Viewed by 955
Abstract
Genetic algorithms are a powerful tool in optimization for single and multimodal functions. This paper provides an overview of their fundamentals with some analytical examples. In addition, we explore how they can be used as a parameter estimation tool in cosmological models to [...] Read more.
Genetic algorithms are a powerful tool in optimization for single and multimodal functions. This paper provides an overview of their fundamentals with some analytical examples. In addition, we explore how they can be used as a parameter estimation tool in cosmological models to maximize the likelihood function, complementing the analysis with the traditional Markov chain Monte Carlo methods. We analyze that genetic algorithms provide fast estimates by focusing on maximizing the likelihood function, although they cannot provide confidence regions with the same statistical meaning as Bayesian approaches. Moreover, we show that implementing sharing and niching techniques ensures an effective exploration of the parameter space, even in the presence of local optima, always helping to find the global optima. This approach is invaluable in the cosmological context, where an exhaustive space exploration of parameters is essential. We use dark energy models to exemplify the use of genetic algorithms in cosmological parameter estimation, including a multimodal problem, and we also show how to use the output of a genetic algorithm to obtain derived cosmological functions. This paper concludes that genetic algorithms are a handy tool within cosmological data analysis, without replacing the traditional Bayesian methods but providing different advantages. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
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11 pages, 943 KiB  
Article
MeV Dark Energy Emission from a De Sitter Universe
Universe 2023, 9(12), 513; https://doi.org/10.3390/universe9120513 - 13 Dec 2023
Viewed by 939
Abstract
The evolution of a de Sitter Universe is the basis for both the accelerated Universe and the late-stationary Universe. So, how do we differentiate between both universes? In this paper, we state that it is not possible to design an experiment using luminous [...] Read more.
The evolution of a de Sitter Universe is the basis for both the accelerated Universe and the late-stationary Universe. So, how do we differentiate between both universes? In this paper, we state that it is not possible to design an experiment using luminous or angular distances to distinguish between the two cases because they are the same during the de Sitter phase. However, this equivalence allows us prediction of the signal of a constant dark energy emission with a signal peak around 29.5 MeV, in which, according to our astrophysical test of survival probability, the radiation must be non-standard photons. Remarkably, experiments by EGRET and COMPTEL have observed an excess of gamma photons in this predicted region, coming from a possible decay process of dark energy emission, which may constitute the smoking gun of a late-stationary Universe with the continuous creation of non-standard radiation, an alternative approach to understanding the current stages of the Universe’s evolution. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
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42 pages, 1151 KiB  
Article
Geodesic Structure of Generalized Vaidya Spacetime through the K-Essence
Universe 2023, 9(12), 510; https://doi.org/10.3390/universe9120510 - 08 Dec 2023
Viewed by 1547
Abstract
This article investigates the radial and non-radial geodesic structures of the generalized K-essence Vaidya spacetime. Within the framework of K-essence geometry, it is important to note that the metric does not possess conformal equivalence to the conventional gravitational metric. This study employs a [...] Read more.
This article investigates the radial and non-radial geodesic structures of the generalized K-essence Vaidya spacetime. Within the framework of K-essence geometry, it is important to note that the metric does not possess conformal equivalence to the conventional gravitational metric. This study employs a non-canonical action of the Dirac–Born–Infeld kind. In this work, we categorize the generalized K-essence Vaidya mass function into two distinct forms. Both the forms of the mass functions have been extensively utilized to analyze the radial and non-radial time-like or null geodesics in great detail inside the comoving plane. Indications of the existence of wormholes can be noted during the extreme phases of spacetime, particularly in relation to black holes and white holes, which resemble the Einstein–Rosen bridge. In addition, we have also detected a distinctive indication of the quantum tunneling phenomenon around the singularity (r0). Furthermore, we have found that for certain types of solutions, there exist circular orbits through the event horizon as well as quasicircular orbits. Also, we have noted that there is no central singularity in our spacetime where both r and t tend towards zero. The existence of a central singularity is essential for any generalized Vaidya spacetime. This indicates that spacetime can be geodesically complete, which correlates with the findings of Kerr’s recent work (2023). Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
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14 pages, 705 KiB  
Article
Effective Potential for Quintessential Inflation Driven by Extrinsic Gravity
Universe 2023, 9(12), 497; https://doi.org/10.3390/universe9120497 - 28 Nov 2023
Viewed by 1005
Abstract
We numerically study the evolution of the extrinsic energy density in the context of an inflationary regime at the background level in a five-dimensional model using a Bayesian analysis from a dynamic nested sampler (DYNESTY) code. By means of the Nash–Greene embedding theorem, [...] Read more.
We numerically study the evolution of the extrinsic energy density in the context of an inflationary regime at the background level in a five-dimensional model using a Bayesian analysis from a dynamic nested sampler (DYNESTY) code. By means of the Nash–Greene embedding theorem, we show that the corresponding model provides an effective potential driven by the influence of extrinsic geometry. We obtain a quintessential inflation that defines a model with a potential V(ϕ)=eα1ϕ(1α2ϕ2), where α1 and α2 are dimensionless parameters. Using some known phenomenological parameterizations, such as Chevallier–Polarski–Linder (CPL) and Barboza–Alcaniz (BA) parameterizations, we show that the model reflects a slow-varying inflation preferring a thawing behavior, suggesting an optimistic scenario for further research on the unification of inflation with late cosmic acceleration. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
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7 pages, 250 KiB  
Article
Nothing into Something and Vice Versa: A Cosmological Scenario
Universe 2023, 9(10), 445; https://doi.org/10.3390/universe9100445 - 09 Oct 2023
Viewed by 978
Abstract
In the almost empty universe (with almost no matter in it), stochastic gravitational waves (SGW) of finite amplitude produce a de Sitter regime as a solution, which is invariant with respect to the Wick rotation. Asymptotically, super horizon SGWs do not “feel” difference [...] Read more.
In the almost empty universe (with almost no matter in it), stochastic gravitational waves (SGW) of finite amplitude produce a de Sitter regime as a solution, which is invariant with respect to the Wick rotation. Asymptotically, super horizon SGWs do not “feel” difference between Lorentzian and Euclidean spacetime and belong simultaneously to both of them. The universe is finishing its evolution in Euclidean spacetime, i.e., it disappears into nothing. Quantum fluctuations of the gravitational field (gravitons) produce a de Sitter regime again in Euclidean spacetime where the current universe finished its existence, and due to the invariance of the de Sitter regime with respect to Wick rotation, the next universe starts its life with de Sitter inflation in Lorentzian spacetime. Such a scenario assumes that a permanent process of birth, death and rebirth of an infinite sequence of universes takes place on an infinite time axis. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
16 pages, 775 KiB  
Article
An Effective Sign Switching Dark Energy: Lotka–Volterra Model of Two Interacting Fluids
Universe 2023, 9(10), 437; https://doi.org/10.3390/universe9100437 - 30 Sep 2023
Cited by 9 | Viewed by 1137
Abstract
One of the recent attempts to address the Hubble and S8 tensions is to consider that the Universe started out not as a de Sitter-like spacetime, but rather anti-de Sitter-like. That is, the Universe underwent an “AdS-to-dS” transition at some point. We [...] Read more.
One of the recent attempts to address the Hubble and S8 tensions is to consider that the Universe started out not as a de Sitter-like spacetime, but rather anti-de Sitter-like. That is, the Universe underwent an “AdS-to-dS” transition at some point. We study the possibility that there are two dark energy fluids, one of which gave rise to the anti-de Sitter-like early Universe. The interaction is modeled by the Lotka–Volterra equations commonly used in population biology. We consider “competition” models that are further classified as “unfair competition” and “fair competition”. The former involves a quintessence in competition with a phantom, and the second involves two phantom fluids. Surprisingly, even in the latter scenario it is possible for the overall dark energy to cross the phantom divide. The latter model also allows a constant w “AdS-to-dS” transition, thus evading the theorem that such a dark energy must possess a singular equation of state. We also consider a “conversion” model in which a phantom fluid still manages to achieve “AdS-to-dS” transition even if it is being converted into a negative energy density quintessence. In these models, the energy density of the late time effective dark energy is related to the coefficient of the quadratic self-interaction term of the fluids, which is analogous to the resource capacity in population biology. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
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14 pages, 6874 KiB  
Article
Revise the Phase-Space Analysis of the Dynamical Spacetime Unified Dark Energy Cosmology
Universe 2023, 9(9), 406; https://doi.org/10.3390/universe9090406 - 05 Sep 2023
Viewed by 719
Abstract
We analyze the phase-space of an alternate scalar field cosmology that aims to combine the concepts of dark energy and the dark sector. The investigation focuses on stationary points within this phase-space, considering different functional forms of the two potential functions. Our findings [...] Read more.
We analyze the phase-space of an alternate scalar field cosmology that aims to combine the concepts of dark energy and the dark sector. The investigation focuses on stationary points within this phase-space, considering different functional forms of the two potential functions. Our findings indicate that a de Sitter universe is achievable solely when at the asymptotic limit the potential function is constant. For constant potential function, the de Sitter universe is recovered in the finite regime; however, for the exponential potential, the de Sitter universe exists at the infinity regime. The cosmological viability of the present theory is discussed. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
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16 pages, 342 KiB  
Article
Anisotropic Cosmology in the Local Limit of Nonlocal Gravity
Universe 2023, 9(9), 377; https://doi.org/10.3390/universe9090377 - 23 Aug 2023
Cited by 1 | Viewed by 574
Abstract
Within the framework of the local limit of nonlocal gravity (NLG), we investigate a class of Bianchi type I spatially homogeneous but anisotropic cosmological models. The modified field equations are presented in this case, and some special solutions are discussed in detail. This [...] Read more.
Within the framework of the local limit of nonlocal gravity (NLG), we investigate a class of Bianchi type I spatially homogeneous but anisotropic cosmological models. The modified field equations are presented in this case, and some special solutions are discussed in detail. This modified gravity theory contains a susceptibility function S(x) such that general relativity (GR) is recovered for S = 0. In the modified anisotropic cosmological models, we explore the contribution of S(t) and its temporal derivative to the local anisotropic cosmic acceleration. The implications of our results for observational cosmology are briefly discussed. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
14 pages, 417 KiB  
Article
Quantum Big Bounce of the Isotropic Universe Using Relational Time
Universe 2023, 9(8), 373; https://doi.org/10.3390/universe9080373 - 16 Aug 2023
Cited by 2 | Viewed by 627
Abstract
We analyze the canonical quantum dynamics of the isotropic Universe with a metric approach by adopting a self-interacting scalar field as relational time. When the potential term is absent, we are able to associate the expanding and collapsing dynamics of the Universe with [...] Read more.
We analyze the canonical quantum dynamics of the isotropic Universe with a metric approach by adopting a self-interacting scalar field as relational time. When the potential term is absent, we are able to associate the expanding and collapsing dynamics of the Universe with the positive- and negative-frequency modes that emerge in the Wheeler–DeWitt equation. On the other side, when the potential term is present, a non-zero transition amplitude from positive- to negative-frequency states arises, as in standard relativistic scattering theory below the particle creation threshold. In particular, we are able to compute the transition probability for an expanding Universe that emerges from a collapsing regime both in the standard quantization procedure and in the polymer formulation. The probability distribution results similar in the two cases, and its maximum takes place when the mean values of the momentum essentially coincide in the in-going and out-going wave packets, as it would take place in a semiclassical Big Bounce dynamics. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
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6 pages, 244 KiB  
Communication
A Simple, Exact Formulation of Number Counts in the Geodesic-Light-Cone Gauge
Universe 2023, 9(7), 327; https://doi.org/10.3390/universe9070327 - 10 Jul 2023
Viewed by 577
Abstract
In this article, we compare different formulations of the number count prescription using the convenient formalism of the Geodesic-Light-Cone gauge. We then find a simple, exact, and very general expression of such a prescription which is suitable for generalised applications. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
41 pages, 520 KiB  
Article
Cosmological Fluctuations in Delta Gravity
Universe 2023, 9(7), 315; https://doi.org/10.3390/universe9070315 - 30 Jun 2023
Cited by 2 | Viewed by 570
Abstract
About 70% of the Universe is Dark Energy, but the physics community still does not know what it is. Delta gravity (DG) is an alternative theory of gravitation that could solve this cosmological problem. Previously, we studied the Universe’s accelerated expansion, where DG [...] Read more.
About 70% of the Universe is Dark Energy, but the physics community still does not know what it is. Delta gravity (DG) is an alternative theory of gravitation that could solve this cosmological problem. Previously, we studied the Universe’s accelerated expansion, where DG was able to explain the SNe-Ia data successfully. In this work, we computed the cosmological fluctuations in DG that give rise to the CMB through a hydrodynamic approximation. We calculated the gauge transformations for the metric and the perfect fluid to present the equations of the evolution of cosmological fluctuations. This provided the necessary equations to solve the scalar TT power spectrum in a semi-analytical way. These equations are useful for comparing the DG theory with astronomical observations and thus being able to constrain the DG cosmology. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
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10 pages, 290 KiB  
Article
Electromagnetic Waves in Cosmological Spacetime
Universe 2023, 9(6), 292; https://doi.org/10.3390/universe9060292 - 16 Jun 2023
Cited by 1 | Viewed by 774
Abstract
We consider the propagation of electromagnetic waves in the Friedmann–Lemaître–Robertson–Walker metric. The exact solutions for plane and spherical wave are written down. The corresponding redshift, amplitude change, and dispersion are discussed. We also speculate about the connection of the electromagnetic wave equation to [...] Read more.
We consider the propagation of electromagnetic waves in the Friedmann–Lemaître–Robertson–Walker metric. The exact solutions for plane and spherical wave are written down. The corresponding redshift, amplitude change, and dispersion are discussed. We also speculate about the connection of the electromagnetic wave equation to the Proca equation and its significance for the early Universe. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
14 pages, 783 KiB  
Article
Fitting Type Ia Supernova Data to a Cosmological Model Based on Einstein–Newcomb–De Sitter Space
Universe 2023, 9(5), 204; https://doi.org/10.3390/universe9050204 - 25 Apr 2023
Viewed by 1297
Abstract
Einstein–Newcomb–de Sitter (ENdS) space is de Sitter’s modification of spherical space used by Einstein in his first cosmological model paper published in 1917. The modification by de Sitter incorporated the topological identification of antipodal points in space previously proposed by Newcomb in 1877. [...] Read more.
Einstein–Newcomb–de Sitter (ENdS) space is de Sitter’s modification of spherical space used by Einstein in his first cosmological model paper published in 1917. The modification by de Sitter incorporated the topological identification of antipodal points in space previously proposed by Newcomb in 1877. De Sitter showed that space topologically modified in this way (called elliptical or projective space) satisfies Einstein’s field equations. De Sitter also found that in a space with constant positive curvature, spectral lines of remote galaxies would be red-shifted (called the de Sitter effect). However, de Sitter’s formulae relating distances to red shifts do not satisfy observational data. The likely reason for this mismatch is that de Sitter mainly focused on space curvature and ignored the identification of antipodal points. Herein, we demonstrate that it is this particular feature that allows an almost perfect fit of the ENdS-based cosmological model to observational data. We use 1701 sources from the type Ia supernovae data sample called Pantheon+, which was previously used to fit the ΛCDM model. ΛCDM and ENdS diverge in their predictions for red shifts exceeding z2.3. Since there are no available type Ia supernovae (SNe) data for higher red shifts, both models can be validated by using an additional sample of 193 gamma-ray bursts (GRBs) spanning red shifts up to z8. This validation shows that the minimum χ2 for the SNe+GRBs sample is about 2.7% smaller for the ENdS space model than for the ΛCDM model. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
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9 pages, 1615 KiB  
Article
Do White Holes Exist?
Universe 2023, 9(4), 194; https://doi.org/10.3390/universe9040194 - 19 Apr 2023
Viewed by 1037
Abstract
In a paper published in 1939, Albert Einstein argued that Black Holes (BHs) did not exist “in the real world”. However, recent astronomical observations indicate otherwise. Does this mean that we should also expect White Holes (WHs) to exist in the real world? [...] Read more.
In a paper published in 1939, Albert Einstein argued that Black Holes (BHs) did not exist “in the real world”. However, recent astronomical observations indicate otherwise. Does this mean that we should also expect White Holes (WHs) to exist in the real world? In classical General Relativity (GR), a WH refers to the time reversed version of a collapsing BH solution that allows the crossing of the BH event horizon inside out. Such solution has been disputed as not possible because escaping an event horizon violates causality. Despite such objections, the Big Bang model is often understood as a WH (the reverse of a BH collapse). Does this mean that the Big Bang breaks causality? Recent measurements of cosmic acceleration indicate that our Big Bang solution is not really a WH, but a BH. Events decelerate when the expansion accelerates and this prevents the crossing of the event horizon from inside out. We present a general explanation of why this happens; the explanation resolves the above causality puzzle and indicates that such apparent WH solutions have a regular Schwarzschild BH exterior. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
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19 pages, 3246 KiB  
Article
Quantum Fractionary Cosmology: K-Essence Theory
Universe 2023, 9(4), 185; https://doi.org/10.3390/universe9040185 - 13 Apr 2023
Cited by 6 | Viewed by 769
Abstract
Using a particular form of the quantum K-essence scalar field, we show that in the quantum formalism, a fractional differential equation in the scalar field variable, for some epochs in the Friedmann–Lemaı^tre–Robertson–Walker (FLRW) model (radiation and inflation-like epochs, for example), [...] Read more.
Using a particular form of the quantum K-essence scalar field, we show that in the quantum formalism, a fractional differential equation in the scalar field variable, for some epochs in the Friedmann–Lemaı^tre–Robertson–Walker (FLRW) model (radiation and inflation-like epochs, for example), appears naturally. In the classical analysis, the kinetic energy of scalar fields can falsify the standard matter in the sense that we obtain the time behavior for the scale factor in all scenarios of our Universe by using the Hamiltonian formalism, where the results are analogous to those obtained by an algebraic procedure in the Einstein field equations with standard matter. In the case of the quantum Wheeler–DeWitt (WDW) equation for the scalar field ϕ, a fractional differential equation of order β=2α2α1 is obtained. This fractional equation belongs to different intervals, depending on the value of the barotropic parameter; that is to say, when ωX[0,1], the order belongs to the interval 1β2, and when ωX[1,0), the order belongs to the interval 0<β1. The corresponding quantum solutions are also given. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
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18 pages, 685 KiB  
Article
Effective f(R) Actions for Modified Loop Quantum Cosmologies via Order Reduction
Universe 2023, 9(4), 181; https://doi.org/10.3390/universe9040181 - 11 Apr 2023
Cited by 1 | Viewed by 950
Abstract
General Relativity is an extremely successful theory, at least for weak gravitational fields; however, it breaks down at very high energies, such as in correspondence to the initial singularity. Quantum Gravity is expected to provide more physical insights in relation to this open [...] Read more.
General Relativity is an extremely successful theory, at least for weak gravitational fields; however, it breaks down at very high energies, such as in correspondence to the initial singularity. Quantum Gravity is expected to provide more physical insights in relation to this open question. Indeed, one alternative scenario to the Big Bang, that manages to completely avoid the singularity, is offered by Loop Quantum Cosmology (LQC), which predicts that the Universe undergoes a collapse to an expansion through a bounce. In this work, we use metric f(R) gravity to reproduce the modified Friedmann equations which have been obtained in the context of modified loop quantum cosmologies. To achieve this, we apply an order reduction method to the f(R) field equations, and obtain covariant effective actions that lead to a bounce, for specific models of modified LQC, considering a massless scalar field. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
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13 pages, 693 KiB  
Article
Bayesian Implications for the Primordial Black Holes from NANOGrav’s Pulsar-Timing Data Using the Scalar-Induced Gravitational Waves
Universe 2023, 9(4), 157; https://doi.org/10.3390/universe9040157 - 24 Mar 2023
Cited by 17 | Viewed by 1196
Abstract
Assuming that the common-spectrum process in the NANOGrav 12.5-year dataset has an origin of scalar-induced gravitational waves, we study the enhancement of primordial curvature perturbations and the mass function of primordial black holes, by performing the Bayesian parameter inference for the first time. [...] Read more.
Assuming that the common-spectrum process in the NANOGrav 12.5-year dataset has an origin of scalar-induced gravitational waves, we study the enhancement of primordial curvature perturbations and the mass function of primordial black holes, by performing the Bayesian parameter inference for the first time. We obtain lower limits on the spectral amplitude, i.e., A102 at 95% confidence level, when assuming the power spectrum of primordial curvature perturbations to follow a log-normal distribution function with width σ. In the case of σ0, we find that the primordial black holes with 2×104102 solar mass are allowed to compose at least a fraction 106 of dark matter. Such a mass range is shifted to more massive regimes for larger values of σ, e.g., to a regime of 4×1030.2 solar mass in the case of σ=1. We expect the planned gravitational-wave experiments to have their best sensitivity to A in the range of 104 to 107, depending on the experimental setups. With this level of sensitivity, we can search for primordial black holes throughout the entire parameter space, especially in the mass range of 1016 to 1011 solar masses, where they could account for all dark matter. In addition, the importance of multi-band detector networks is emphasized to accomplish our theoretical expectation. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
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17 pages, 329 KiB  
Article
Cosmological Constant from Boundary Condition and Its Implications beyond the Standard Model
Universe 2023, 9(2), 103; https://doi.org/10.3390/universe9020103 - 17 Feb 2023
Viewed by 1396
Abstract
Standard cosmology has long been plagued by a number of persistent problems. The origin of the apparent acceleration of the cosmic expansion remains enigmatic. The cosmological constant has been reintroduced as a free parameter with a value in energy density units that “happens” [...] Read more.
Standard cosmology has long been plagued by a number of persistent problems. The origin of the apparent acceleration of the cosmic expansion remains enigmatic. The cosmological constant has been reintroduced as a free parameter with a value in energy density units that “happens” to be of the same order as the present matter energy density. There is an internal inconsistency with regards to the Hubble constant, the so-called H0 tension. The derived value of H0 depends on the type of data that is used. With supernovae as standard candles, one gets a H0 that is 4–5 σ larger than the value that one gets from CMB (Cosmic Microwave Background) data for the early universe. Here we show that these problems are related and can be solved if the cosmological constant represents a covariant integration constant that arises from a spatial boundary condition, instead of being a new type of hypothetical physical field, “dark energy”, as assumed by standard cosmology. The boundary condition only applies to the bounded 3D subspace that represents the observable universe, the hypersurface of the past light cone. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)

Review

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53 pages, 5295 KiB  
Review
The Unsettled Number: Hubble’s Tension
Universe 2023, 9(12), 501; https://doi.org/10.3390/universe9120501 - 29 Nov 2023
Cited by 1 | Viewed by 3182
Abstract
One of main sources of uncertainty in modern cosmology is the present rate of the universe’s expansion, H0, called the Hubble constant. Once again, different observational techniques bring about different results, causing new “Hubble tension”. In the present work, we review [...] Read more.
One of main sources of uncertainty in modern cosmology is the present rate of the universe’s expansion, H0, called the Hubble constant. Once again, different observational techniques bring about different results, causing new “Hubble tension”. In the present work, we review the historical roots of the Hubble constant from the beginning of the twentieth century, when modern cosmology originated, to the present. We develop the arguments that gave rise to the importance of measuring the expansion of the Universe and its discovery, and we describe the different pioneering works attempting to measure it. There has been a long dispute on this matter, even in the present epoch, which is marked by high-tech instrumentation and, therefore, in smaller uncertainties in the relevant parameters. It is, again, currently necessary to conduct a careful and critical revision of the different methods before one invokes new physics to solve the so-called Hubble tension. Full article
(This article belongs to the Special Issue Universe: Feature Papers 2023—Cosmology)
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