Quantum gravity at Hubble scales determines the cosmological constant and the amplitude of primordial perturbations

2017 ◽  
Vol 26 (12) ◽  
pp. 1743002
Author(s):  
T. Padmanabhan ◽  
Hamsa Padmanabhan
Keyword(s):  
Cosmological Constant ◽  
Energy Scale ◽  
Accelerated Expansion ◽  
Late Time ◽  
Quantum Structure ◽  
Scale Invariant ◽  
Expansion Phase ◽  
Perturbation Spectrum ◽  
The Universe

Gravity controls the amount of information that is accessible to any specific observer. We quantify the notion of cosmic information (‘CosmIn’) for the case of an eternal observer in the universe. Demanding the finiteness of CosmIn requires the universe to have a late-time accelerated expansion phase. Combined with some generic features of the quantum structure of spacetime, this leads to the determination of (i) the numerical value of the cosmological constant, as well as (ii) the amplitude of the primordial, scale invariant perturbation spectrum in terms of a single free parameter, which specifies the energy scale at which the universe makes a transition from a pre-geometric phase to the classical phase. This formalism also shows that the quantum gravitational information content of spacetime can be tested by using precision cosmology.

Heuristic determination of the cosmological constant

2021 ◽  
pp. 2150114
Author(s):  
Manuel Urueña Palomo ◽  
Fernando Pérez Lara
Keyword(s):  
Field Theory ◽  
Energy Density ◽  
Cosmological Constant ◽  
Accelerated Expansion ◽  
Fundamental Constants ◽  
Quantum Field ◽  
Brute Force ◽  
Expansion Of The Universe ◽  
The Universe

The vacuum catastrophe results from the disagreement between the theoretical value of the energy density of the vacuum in quantum field theory and the estimated one observed in cosmology. In a similar attempt in which the ultraviolet catastrophe was solved, we search for the value of the cosmological constant by brute-force through computation. We explore combinations of the fundamental constants in physics performing a dimensional analysis, in search of an equation resulting in the measured energy density of the vacuum or cosmological constant that is assumed to cause the accelerated expansion of the universe.

scholarly journals f(R) dual theories of quintessence: expansion-collapse duality

2021 ◽  
Vol 2021 (12) ◽  
pp. 016
Author(s):  
Dipayan Mukherjee ◽  
H.K. Jassal ◽  
Kinjalk Lochan
Keyword(s):  
Dark Energy ◽  
Cosmological Constant ◽  
Einstein Frame ◽  
Time Limit ◽  
Gravity Models ◽  
Accelerated Expansion ◽  
Jordan Frame ◽  
Late Time ◽  
Perturbative Solution ◽  
The Universe

Abstract The accelerated expansion of the universe demands presence of an exotic matter, namely the dark energy. Though the cosmological constant fits this role very well, a scalar field minimally coupled to gravity, or quintessence, can also be considered as a viable alternative for the cosmological constant. We study f(R) gravity models which can lead to an effective description of dark energy implemented by quintessence fields in Einstein gravity, using the Einstein frame-Jordan frame duality. For a family of viable quintessence models, the reconstruction of the f(R) function in the Jordan frame consists of two parts. We first obtain a perturbative solution of f(R) in the Jordan frame, applicable near the present epoch. Second, we obtain an asymptotic solution for f(R), consistent with the late time limit of the Einstein frame if the quintessence field drives the universe. We show that for certain class of viable quintessence models, the Jordan frame universe grows to a maximum finite size, after which it begins to collapse back. Thus, there is a possibility that in the late time limit where the Einstein frame universe continues to expand, the Jordan frame universe collapses. The condition for this expansion-collapse duality is then generalized to time varying equations of state models, taking into account the presence of non-relativistic matter or any other component in the Einstein frame universe. This mapping between an expanding geometry and a collapsing geometry at the field equation level may have interesting potential implications on the growth of perturbations therein at late times.

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.

Cosmological constant

2018 ◽  
Author(s):  
Michael Kachelriess
Keyword(s):  
Dark Energy ◽  
Cosmological Constant ◽  
Accelerated Expansion ◽  
Vacuum Fluctuations ◽  
Present Epoch ◽  
Modify Gravity ◽  
Expansion Of The Universe ◽  
The Universe

The contribution of vacuum fluctuations to the cosmological constant is reconsidered studying the dependence on the used regularisation scheme. Then alternative explanations for the observed accelerated expansion of the universe in the present epoch are introduced which either modify gravity or add a new component of matter, dubbed dark energy. The chapter closes with some comments on attempts to quantise gravity.

INFLATION, QUINTESSENCE PROBLEM AND VARYING SPEED OF LIGHT

2002 ◽  
Vol 17 (05) ◽  
pp. 295-302
Author(s):  
SUBENOY CHAKRABORTY
Keyword(s):  
Cosmological Constant ◽  
Accelerated Expansion ◽  
Speed Of Light ◽  
Expansion Of The Universe ◽  
The Universe

In this paper it is shown that the present accelerated expansion of the Universe can be explained only by considering variation of the speed of light, without taking into account the cosmological constant or quintessence matter.

Phase space analysis of the Umami Chaplygin model

2021 ◽  
pp. 2150052
Author(s):  
Qihong Huang ◽  
Ruanjing Zhang ◽  
Jun Chen ◽  
He Huang ◽  
Feiquan Tu
Keyword(s):  
Phase Space ◽  
Cosmic Acceleration ◽  
Accelerated Expansion ◽  
Late Time ◽  
Phase Space Analysis ◽  
Space Analysis ◽  
History Of ◽  
Evolutionary Trajectories ◽  
The Universe ◽  
Universe Evolution

In this paper, we analyze the universe evolution and phase space behavior of the Umami Chaplygin model, where the Umami Chaplygin fluid replaces both a dark energy and a dark and baryonic matter. We find the Umami Chaplygin model can be stable against perturbations under some conditions and can be used to explain the late-time cosmic acceleration. The results of phase space analysis show that there exists a late-time accelerated expansion attractor with [Formula: see text], which indicates the Umami Chaplygin fluid can behave as a cosmological constant. Moreover, the Umami Chaplygin model can describe the expansion history of the universe. The evolutionary trajectories of the statefinder diagnostic pairs and the finite time future singularities are also discussed.

Five-dimensional Brans–Dicke compactified universe dominated by a varying speed of light

2020 ◽  
Vol 35 (30) ◽  
pp. 2050252
Author(s):  
Rami Ahmad El-Nabulsi
Keyword(s):  
Killing Vector ◽  
Scalar Fields ◽  
Analytic Solutions ◽  
Accelerated Expansion ◽  
Late Time ◽  
Speed Of Light ◽  
Orthogonal Space ◽  
Spacetime Curvature ◽  
The Universe ◽  
Data Analytic

We extend the model of a 5D Brans–Dicke gravity theory reduced to 4D through the presence of a hypersurface-orthogonal space-like killing vector field in the underlying 5D spacetime by including a varying speed of light. The resulting model is characterized by the presence of two scalar fields. We focus on late-time power law solutions which emerge in general when scalar fields couple to spacetime curvature and do not contradict the SNIa astrophysical data. Analytic solutions in 4-dimensions are derived and late-time accelerated expansion was found. The universe is dominated by dark energy, free from phantom field and is characterized by a decaying energy matter density, decaying scalar fields, and a decreasing celerity of light. The model is confronted with astrophysical observations and is found to fit these data.

scholarly journals Dynamical Stability of Bulk Viscous Isotropic and Homogeneous Universe

Universe ◽  
2019 ◽  
Vol 5 (8) ◽  
pp. 185
Author(s):  
Muhammad Sharif ◽  
Qanitah Ama-Tul-Mughani
Keyword(s):  
Accelerated Expansion ◽  
Stationary Points ◽  
Late Time ◽  
Energy Models ◽  
Bulk Viscous ◽  
Expansion Of The Universe ◽  
Homogeneous Universe ◽  
The Stability ◽  
The Universe ◽  
Autonomous Set

In this paper, we study the phase space portrait of homogeneous and isotropic universe by taking different coupling functions between dark energy models and bulk viscous dark matter. The dimensionless quantities are introduced to establish an autonomous set of equations. To analyze the stability of the cosmos, we evaluate critical points and respective eigenvalues for different dynamical quantities. For bulk viscous matter and radiation in tachyon coupled field, these points show stable evolution when γ ≫ δ but accelerated expansion of the universe for δ > 1 9 . The stability of the universe increases for some stationary points which may correspond to the late-time expansion for the coupled phantom field.

Analysis of observational parameters and stability in extended teleparallel gravity

2020 ◽  
Vol 17 (11) ◽  
pp. 2050158
Author(s):  
A. Y. Shaikh ◽  
B. Mishra
Keyword(s):  
Thermodynamic Temperature ◽  
Teleparallel Gravity ◽  
Entropy Density ◽  
Accelerated Expansion ◽  
Late Time ◽  
General Relativistic ◽  
Basic Equations ◽  
The Stability ◽  
The Universe ◽  
Early Phases

In this paper, we have investigated the stability of General Relativistic Hydrodynamics (GRHD) model in a Friedmann–Robertson–Walker space-time with the volumetric power law in teleparallel gravity. The basic equations are derived along with its thermodynamical aspects. Thermodynamic temperature and entropy density of the model are also obtained. The state finder diagnostic pair and jerk parameter are analyzed to characterize different phases of the universe and the well-known astrophysical phenomena such as look-back time, the luminosity distance with redshift are derived. The model shows an accelerated expansion with inflationary era in the early and the very late time of the cosmic evolution. The GRHD model is stable at the early phases of the universe and is unstable at late times.

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