scholarly journals Late-Time Mild Inflation: A Possible Solution of a Dilemma: The Cosmic Age and the Hubble Parameter

1997 ◽  
Vol 06 (01) ◽  
pp. 69-90 ◽  
Author(s):  
Takeshi Fukuyama ◽  
Masae Miyoshi ◽  
Mikiko Hatakeyama ◽  
Masahiro Morikawa ◽  
Akika Nakamichi
Keyword(s):  
Cosmological Constant ◽  
Hubble Parameter ◽  
Gravitational Constant ◽  
High Redshift ◽  
Fine Tuning ◽  
Energy Conditions ◽  
Late Time ◽  
Distant Galaxy ◽  
Cepheid Variables ◽  
Varying Gravitational Constant

We explore the cosmological model in which a late-time mild inflation (LMI) is realized after the star formation epoch. Nonvanishing curvature coupling of a classical boson field yields this mild inflation without a cosmological constant. Accordingly the lifetime of the present Universe is remarkably increased in this LMI model. Thus we show that the present observed high value of the Hubble parameter H0 ≈ 70–80 km/sec/Mpc is compatible with the age of the oldest stars 14 Gyr without introducing the cosmological constant or the dilute Universe model. Moreover in this LMI model, the local Hubble parameter becomes larger than the global one. Thus we show that the present observed local Hubble parameter measured by using the Cepheid variables is compatible with the global Hubble parameter measured by using the Sunyaev–Zeldovich effect. Furthermore we examine several aspects of the LMI model: (a) The energy conditions are violated. We examine the consequences of these violations. (b) There is a natural evolution of the effective gravitational "constant" in high redshift region. This yields a drastic change of the stellar luminosity through the constructive equations of a star. We point out that a distant galaxy becomes much dimmer by this effect. (c) This varying gravitational "constant" affects the cosmic expansion speed and the nucleosynthesis process in the early Universe. We point out that this effect constrains the parameters of the LMI model though fine tuning is always possible.

The possibility of a stable flat dark energy-dominated Swiss-cheese brane-world universe

2020 ◽  
Vol 17 (05) ◽  
pp. 2050075
Author(s):  
Nasr Ahmed ◽  
Kazuharu Bamba ◽  
F. Salama
Keyword(s):  
Dark Energy ◽  
Cosmological Constant ◽  
Cosmological Models ◽  
Brane World ◽  
Fine Tuning ◽  
Energy Conditions ◽  
Late Time ◽  
Swiss Cheese ◽  
Black Strings ◽  
The Stability

In this paper, we study the possibility of obtaining a stable flat dark energy-dominated universe in a good agreement with observations in the framework of Swiss-cheese brane-world cosmology. Two different brane-world cosmologies with black strings have been introduced for any cosmological constant [Formula: see text] using two empirical forms of the scale factor. In both models, we have performed a fine-tuning between the brane tension and the cosmological constant so that the Equation of state (EoS) parameter [Formula: see text] for the current epoch, where the redshift [Formula: see text]. We then used these fine–tuned values to calculate and plot all parameters and energy conditions. The deceleration–acceleration cosmic transition is allowed in both models, and the jerk parameter [Formula: see text] at late-times. Both solutions predict a future dark energy-dominated universe in which [Formula: see text] with no crossing to the phantom divide line. While the pressure in the first solution is always negative, the second solution predicts a better behavior of cosmic pressure where the pressure is negative only in the late-time accelerating era but positive in the early-time decelerating era. Such a positive-to-negative transition in the evolution of pressure helps to explain the cosmic deceleration–acceleration transition. Since black strings have been proved to be unstable by some authors, this instability can actually reflect doubts on the stability of cosmological models with black strings (Swiss-cheese type brane-worlds cosmological models). For this reason, we have carefully investigated the stability through energy conditions and sound speed. Because of the presence of quadratic energy terms in Swiss-cheese type brane-world cosmology, we have tested the new nonlinear energy conditions in addition to the classical energy conditions. We have also found that a negative tension brane is not allowed in both models of the current work as the energy density will no longer be well defined.

scholarly journals A cyclic universe with varying cosmological constant in f(R, T) gravity

2019 ◽  
Vol 97 (10) ◽  
pp. 1075-1082 ◽  
Author(s):  
Nasr Ahmed ◽  
Sultan Z. Alamri
Keyword(s):  
Cosmological Constant ◽  
Early Time ◽  
State Parameter ◽  
Singular Values ◽  
Energy Conditions ◽  
Late Time ◽  
Time Varying ◽  
Causality Condition ◽  
Cyclic Universe ◽  
Cyclic Models

A new kind of evolution for cyclic models in which the Hubble parameter oscillates and remains positive has been explored in a specific f(R, T) gravity reconstruction. A singularity-free cyclic universe with negative varying cosmological constant has been obtained, which supports the role suggested for negative Λ in stopping the eternal acceleration. The cosmological solutions have been obtained for the case of a flat universe, supported by observations. The cosmic pressure grows without singular values; it is positive during the early-time decelerated expansion and negative during the late-time accelerating epoch. The time-varying equation of state parameter ω(t) shows quintom behavior and is restricted to the range –2.25 ≤ ω(t) ≲ 1/3. The validity of the classical linear energy conditions and the sound speed causality condition has been studied. The non-conventional mechanism of negative cosmological constant that are expected to address the late-time acceleration has been discussed.

The “still point” cosmology

2005 ◽  
Vol 201 ◽  
pp. 514-515
Author(s):  
Ivan I. Shevchenko
Keyword(s):  
Scalar Field ◽  
Cosmological Constant ◽  
Hubble Parameter ◽  
Gravitational Constant ◽  
Critical Density ◽  
Matter Density ◽  
Future Evolution ◽  
The Future ◽  
The Universe

Recent results on supernovae as standard candles (Riess et al. 1998; Perlmutter et al. 1999) and on CMB anisotropy (Lineweaver 1998) indicate that ΩM ≍ 0.3-0.4, Ωv ≍ 0.6-0.7, ΩM + Ωv ≍ 1. By definition, ΩM = ρM/ρcr, ΩV = ρv/ρcr, where ρM is the matter density, ρv is the vacuum density; the critical density ρcr = 3H2/8πG; H is the Hubble parameter, G is the gravitational constant. In the standard Friedmann-Lemaître cosmologies, these results seriously constrain the non-dimensional cosmological constant (as defined below): Δ ≫ 1, meaning that the Universe expands forever. If a scalar field is present, the future evolution may be different.

scholarly journals Dynamical symmetry breaking and negative cosmological constant

2019 ◽  
Vol 34 (24) ◽  
pp. 1950136 ◽  
Author(s):  
Wei Lu
Keyword(s):  
Symmetry Breaking ◽  
Cosmological Constant ◽  
Hubble Parameter ◽  
Friedmann Equation ◽  
Dynamical Symmetry ◽  
Boson Mass ◽  
Minor Role ◽  
Late Time ◽  
Dynamical Symmetry Breaking ◽  
Negative Cosmological Constant

In the context of Clifford functional integral formalism, we revisit the Nambu–Jona-Lasinio-type dynamical symmetry breaking model and examine the properties of the dynamically generated composite bosons. Given that the model with 4-fermion interactions is nonrenormalizable in the traditional sense, the aim is to gain insight into the divergent integrals without resorting to explicit regularization. We impose a restriction on the linearly divergent primitive integrals, thus resolving the long-standing issue of momentum routing ambiguity associated with fermion–antifermion condensations. The removal of the ambiguity paves the way for the possible calculation of the true ratio of Higgs boson mass to top quark mass in the top condensation model. In this paper, we also investigate the negative vacuum energy resulted from dynamical symmetry breaking and its cosmological implications. In the framework of modified Einstein–Cartan gravity, it is demonstrated that the late-time acceleration is driven by a novel way of embedding the Hubble parameter into the Friedmann equation via an interpolation function, whereas the dynamically generated negative cosmological constant only plays a minor role for the current epoch. Two cosmic scenarios are proposed, with one of which suggesting that the universe may have been evolving from an everlasting coasting state towards the accelerating era characterized by the deceleration parameter approaching −0.5 at low redshift. One inevitable outcome of the modified Friedmannian cosmology is that the directly measured local Hubble parameter should in general be larger than the Hubble parameter calibrated from the conventional Friedmann equation. This Hubble tension becomes more pronounced when the Hubble parameter is comparable or less than a characteristic Hubble scale.

scholarly journals THEORETICAL MODELS OF DARK ENERGY

2012 ◽  
Vol 21 (12) ◽  
pp. 1230002 ◽  
Author(s):  
JAEWON YOO ◽  
YUKI WATANABE
Keyword(s):  
Dark Energy ◽  
Cosmological Constant ◽  
Modified Gravity ◽  
Fine Tuning ◽  
Gravity Models ◽  
Accelerated Expansion ◽  
Late Time ◽  
Spatial Homogeneity ◽  
The Universe ◽  
Modified Gravity Models

Mounting observational data confirm that about 73% of the energy density consists of dark energy which is responsible for the current accelerated expansion of the Universe. We present observational evidences and dark energy projects. We then review various theoretical ideas that have been proposed to explain the origin of dark energy; they contain the cosmological constant, modified matter models, modified gravity models and the inhomogeneous model. The cosmological constant suffers from two major problems: one regarding fine-tuning and the other regarding coincidence. To solve them there arose modified matter models such as quintessence, k-essence, coupled dark energy and unified dark energy. We compare those models by presenting attractive aspects, new rising problems and possible solutions. Furthermore, we review modified gravity models that lead to late-time accelerated expansion without invoking a new form of dark energy; they contain f(R) gravity and the Dvali–Gabadadze–Porrati (DGP) model. We also discuss observational constraints on those models and on future modified gravity theories. Finally we review the inhomogeneous Lemaître–Tolman–Bondi (LTB) model that drops an assumption of the spatial homogeneity of the Universe. We also present basics of cosmology and scalar field theory, which are useful especially for students and novices to understand dark energy models.

scholarly journals Constraining the Swiss-Cheese IR-Fixed Point Cosmology with Cosmic Expansion

Universe ◽  
2021 ◽  
Vol 7 (8) ◽  
pp. 263
Author(s):  
Ayan Mitra ◽  
Vasilios Zarikas ◽  
Alfio Bonanno ◽  
Michael Good ◽  
Ertan Güdekli
Keyword(s):  
Fixed Point ◽  
Cosmological Constant ◽  
Cosmic Acceleration ◽  
Fine Tuning ◽  
Clusters Of Galaxies ◽  
Swiss Cheese ◽  
Coincidence Problem ◽  
Cosmological Expansion ◽  
Cosmic Evolution ◽  
Previous Assumption

A recent work proposed that the recent cosmic passage to a cosmic acceleration era is the result of the existence of small anti-gravity sources in each galaxy and clusters of galaxies. In particular, a Swiss-cheese cosmology model, which relativistically integrates the contribution of all these anti-gravity sources on a galactic scale has been constructed assuming the presence of an infrared fixed point for a scale dependent cosmological constant. The derived cosmological expansion provides an explanation for both the fine tuning and the coincidence problem. The present work relaxes the previous assumption on the running of the cosmological constant and allows for a generic scaling around the infrared fixed point. Our analysis reveals that, in order to produce a cosmic evolution consistent with the best ΛCDM model, the IR-running of the cosmological constant is consistent with the presence of an IR-fixed point.

scholarly journals DARK ENERGY, WITH SIGNATURES

2010 ◽  
Vol 19 (14) ◽  
pp. 2325-2330
Author(s):  
SOURISH DUTTA ◽  
ROBERT J. SCHERRER ◽  
STEPHEN D. H. HSU
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Energy Scale ◽  
Fine Tuning ◽  
Time Dependent ◽  
Late Time ◽  
Particle Content ◽  
Energy Field ◽  
Energy Models ◽  
New Energy

We propose a class of simple dark energy models which predict a late-time dark radiation component and a distinctive time-dependent equation of state w(z) for redshift z < 3. The dark energy field can be coupled strongly enough to standard model particles to be detected in colliders, and the model requires only modest additional particle content and little or no fine-tuning other than a new energy scale of order milli-electron volts.

scholarly journals Almost-Pseudo-Ricci Symmetric FRW Universe with a Dynamic Cosmological Term and Equation of State

Universe ◽  
2021 ◽  
Vol 7 (7) ◽  
pp. 205
Author(s):  
Sanjay Mandal ◽  
Avik De ◽  
Tee-How Loo ◽  
Pradyumn Kumar Sahoo
Keyword(s):  
Equation Of State ◽  
Cosmological Parameters ◽  
Cosmological Term ◽  
Ricci Scalar ◽  
Energy Conditions ◽  
Late Time ◽  
Frw Universe ◽  
Accelerating Expansion ◽  
Expansion Of The Universe ◽  
The Universe

The objective of the present paper is to investigate an almost-pseudo-Ricci symmetric FRW spacetime with a constant Ricci scalar in a dynamic cosmological term Λ(t) and equation of state (EoS) ω(t) scenario. Several cosmological parameters are calculated in this setting and thoroughly studied, which shows that the model satisfies the late-time accelerating expansion of the universe. We also examine all of the energy conditions to check our model’s self-stability.

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