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Title
Abstract
Introduction
Issues Related to Cosmography
Experimental Procedures
The Cosmic Microwaves Background Measurement
Cosmic Microwaves Background Observations
Conclusions and Perspectives
References

Cosmographic constraints and cosmic fluids

Profile image of Mariafelicia De LaurentisMariafelicia De Laurentis

Galaxies 2013, 1(3), 216-260; doi:10.3390/galaxies1030216

https://doi.org/10.3390/GALAXIES1030216
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Abstract

The problem of reproducing dark energy effects is reviewed here with particular interest devoted to cosmography. We summarize some of the most relevant cosmological models, based on the assumption that the corresponding barotropic equations of state evolve as the universe expands, giving rise to the accelerated expansion. We describe in detail the ΛCDM (Λ-Cold Dark Matter) and ωCDM models, considering also some specific examples, e.g., Chevallier–Polarsky–Linder, the Chaplygin gas and the Dvali–Gabadadze–Porrati cosmological model. Finally, we consider the cosmological consequences of f(R) and f(T) gravities and their impact on the framework of cosmography. Keeping these considerations in mind, we point out the model-independent procedure related to cosmography, showing how to match the series of cosmological observables to the free parameters of each model. We critically discuss the role played by cosmography, as a selection criterion to check whether a particular model passes or does not present cosmological constraints. In so doing, we find out cosmological bounds by fitting the luminosity distance expansion of the redshift, z, adopting the recent Union 2.1 dataset of supernovae, combined with the baryonic acoustic oscillation and the cosmic microwave background measurements. We perform cosmographic analyses, imposing different priors on the Hubble rate present value. In addition, we compare our results with recent PLANCK limits, showing that the ΛCDM and ωCDM models seem to be the favorite with respect to other dark energy models. However, we show that cosmographic constraints on f(R) and f(T) cannot discriminate between extensions of General Relativity and dark energy models, leading to a disadvantageous degeneracy problem.

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Edivaldo Santos

Journal of Cosmology and Astroparticle Physics, 2008

Modified f (R) gravity in the Palatini approach has been presently applied to Cosmology as a realistic alternative to dark energy. In this concern, a number of authors have searched for observational constraints on several f (R) gravity functional forms using mainly data of type Ia supenovae (SNe Ia), Cosmic Microwave Background (CMB) radiation and Large Scale Structure (LSS). In this paper, by considering a homogeneous and isotropic flat universe, we use determinations of the Hubble function H(z), which are based on differential age method, to place bounds on the free parameters of the f (R) = R − β/R n functional form. We also combine the H(z) data with constraints from Baryon Acoustic Oscillations (BAO) and CMB measurements, obtaining ranges of values for n and β in agreement with other independent analyses. We find that, for some intervals of n and β, models based on f (R) = R − β/R n gravity in the Palatini approach, unlike the metric formalism, can produce the sequence of radiation-dominated, matter-dominated, and accelerating periods without need of dark energy.

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Dark Energy Cosmology: The Equivalent Description via Different Theoretical Models and Cosmography Tests -Kazuharu Bamba - A Comparative Analysis of Theoretical Frameworks, Observational Constraints, and the Role of Scalar Fields, Modified Gravity Theories, & Future Singularities -RARE Text!
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The New Alexandria Library of Texas

This very fascinating obscure paper we investigate the nature of dark energy within the framework of cosmology, focusing on different theoretical models and their equivalence in explaining the accelerated expansion of the universe. We explore the ΛCDM model, the standard model of cosmology, as well as alternative models such as scalar field theories, including quintessence, phantom energy, and tachyon fields, alongside modified gravity theories like F(R) and f(T) gravity. The paper provides a detailed comparison of these models through cosmographic tests, using observational data from Type Ia supernovae, Baryon Acoustic Oscillations, and Cosmic Microwave Background radiation to constrain their validity. In addition, we delve into the concept of future singularities, including type I, II, III, and IV singularities, as they relate to the fate of the universe. We also examine the role of holographic dark energy and the generalized Chaplygin gas model as potential candidates for describing the universe's dark energy content. Through the analysis of these models, we aim to provide a comprehensive understanding of the dynamics of dark energy and its implications for the evolution and ultimate fate of the cosmos. By bridging theoretical frameworks with observational constraints, this study contributes to advancing our knowledge of the mysterious force driving the universe's accelerated expansion. Here is a brief summary for each chapter in the paper "Dark Energy Cosmology: The Equivalent Description via Different Theoretical Models and Cosmography Tests": I. Introduction (6) This section introduces the fundamental problem of dark energy in cosmology, the accelerated expansion of the universe, and provides an overview of various theoretical models and cosmographic tests. It sets the stage for exploring how different models attempt to explain dark energy and its implications for the universe's evolution. II. The Λ Cold Dark Matter (ΛCDM) Model (10) This chapter outlines the standard ΛCDM model, which includes the cosmological constant (Λ) as a form of dark energy. It discusses observational tests that validate the model, such as Type Ia Supernovae (SNe Ia), Baryon Acoustic Oscillations (BAO), and Cosmic Microwave Background (CMB) radiation, which support the ΛCDM framework. III. Cosmology of Dark Fluid Universe (17) This chapter explores alternative models of dark energy, including the concept of a "dark fluid" that combines dark energy and dark matter into a unified framework. It covers equations of state (EoS), finite-time future singularities, and the behavior of various models, such as the Generalized Chaplygin Gas (GCG) and coupled dark energy with dark matter, as well as phantom scenarios, including the Little Rip and Pseudo-Rip cosmologies. IV. Scalar Field Theory as Dark Energy of the Universe (46) Scalar field theories are examined as possible explanations for dark energy. This chapter discusses the equivalence between fluid descriptions and scalar field theories, presenting various cosmological models such as the ΛCDM, Quintessence, Phantom, and unified inflationary models. It also explores scalar fields crossing the phantom divide and their implications for cosmic acceleration. V. Tachyon Scalar Field Theory (56) This section focuses on tachyon scalar fields as an alternative form of dark energy. It explores how tachyon fields can be incorporated into cosmological models to explain the accelerated expansion of the universe and examines their relationship to other scalar field models. VI. Multiple Scalar Field Theories (59) This chapter delves into models with multiple scalar fields, both standard and new types of multi-scalar field theories. It provides an overview of two-scalar field theories and explores the complexities introduced by models with n (≥ 2) scalar fields, which could offer more flexible explanations for dark energy. VII. Holographic Dark Energy (70) The concept of holographic dark energy is introduced in this chapter, which links dark energy to the holographic principle. It includes models of holographic dark energy, generalized forms, and the role of the Hubble entropy in cosmological evolution. VIII. Accelerating Cosmology in F(R) Gravity (78) This section examines F(R) gravity, a modified theory of gravity that includes a general function of the Ricci scalar R. It discusses the reconstruction methods of F(R) gravity, its relation to scalar field theories, and different cosmological scenarios such as the ΛCDM, quintessence, and phantom models. IX. f(T) Gravity (94) This chapter focuses on f(T) gravity, an extension of general relativity based on torsion instead of curvature. It discusses the basic formalism, the reconstruction of f(T) models, and the implications of finite-time future singularities within this framework. It also explores thermodynamic aspects of f(T) gravity. X. Testing Dark Energy and Alternative Gravity by Cosmography: Generalities (104) This chapter presents cosmographic tests as a method for testing dark energy models and alternative gravity theories. It introduces general principles and techniques used to constrain cosmological models through observational data and the expansion history of the universe. XI. An Example: Testing F(R) Gravity by Cosmography (112) A practical example of using cosmographic tests to evaluate F(R) gravity models is provided here. The chapter discusses how different F(R) models, including the CPL model, the ΛCDM case, and constant EoS models, are tested using observational data. XII. Theoretical Constraints on the Model Parameters (125) This chapter provides a detailed analysis of the theoretical constraints on model parameters, including the double power law Lagrangian and the Hu-Sawicki model. These constraints help refine the accuracy of dark energy models and assess their viability in explaining cosmic acceleration. XIII. Constraints Coming from Observational Data (130) This section focuses on the observational data used to test and constrain dark energy models. It reviews the latest measurements from supernovae, CMB, BAO, and other cosmological probes to establish the validity of various dark energy theories. XIV. Conclusion (137) The conclusion summarizes the main findings of the paper, highlighting the strengths and weaknesses of the different theoretical models of dark energy and the role of cosmographic tests in improving our understanding of the universe's accelerated expansion. It suggests potential directions for future research. Acknowledgments (139) This section acknowledges the contributions and support received from various researchers and funding bodies involved in the study. References (140) A comprehensive list of references is provided, detailing the sources and works cited throughout the paper. This summary gives a concise overview of the chapters and their key topics. Let me know if you'd like further details! Tags dark energy, cosmology, ΛCDM model, dark matter, accelerated expansion, cosmic acceleration, cosmographic tests, observational data, supernovae, Type Ia Supernovae, Baryon Acoustic Oscillations, Cosmic Microwave Background, scalar field theory, quintessence, phantom energy, coupled dark energy, Generalized Chaplygin Gas, GCG model, future singularities, finite-time singularities, inhomogeneous universe, dark fluid universe, dark energy equation of state, cosmological constant, universe evolution, cosmic fate, cosmological parameters, observational constraints, cosmological surveys, inflation, unified inflationary models, tachyon field, multiple scalar fields, F(R) gravity, f(T) gravity, modified gravity theories, torsion gravity, dark fluid, cosmological models, phantom phase, Little Rip, Pseudo-Rip cosmology, cosmic structure, future singularities, scalar field reconstruction, cosmology of dark energy, dark energy models, alternative gravity theories, scalar-tensor theories, cosmological reconstruction, holographic dark energy, holographic principle, dark energy reconstruction, modified theories of gravity, curvature, Ricci scalar, torsion, f(T) models, cosmic history, type I singularities, type II singularities, type III singularities, type IV singularities, cosmological observations, parameter fitting, dark energy density, universe expansion rate, cosmological probes, late-time acceleration, accelerated universe, cosmic history tests, energy conditions, model-independent tests, supernova constraints, large-scale structure, cosmic observations, dynamical dark energy, cosmological evolution, cosmic destiny, observational cosmology, dark matter interaction, dark energy theories, scalar-tensor theory equivalence, general relativity, gravitational theories, cosmological inflation, dark energy densities, gravitational lensing, cosmic surveys, dark energy equation of state (EoS), phantom divide, dark energy models, observational constraints, cosmology tests, equation of state, cosmological constant value, redshift surveys, dark energy components, cosmological dynamics, future cosmological probes, cosmic parameters, dark energy constraints, modified gravitational models, cosmic acceleration models, cosmic microwave background anisotropies, perturbation theory, universe parameter constraints, cosmological theory comparisons, structure formation, cosmological tests, exotic matter, cosmic thermodynamics, future cosmic fate, future singularity models, universe's fate, quantum field theories, dark energy interpretation, cosmological data, scalar fields, field theory models, cosmic theory predictions, dark energy density fluctuations, cosmic acceleration measurements, energy content of the universe, unified cosmological models, cosmological limits, gravitational wave signals, testing dark energy, dark energy inflation, quantum cosmology, cosmology of the future, gravity theories, quantum perturbations, large-scale structure measurements, cosmic structure formation, cosmic fluid equations, future testing of cosmology, ...

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A constrained cosmological model in f(R,Lm) gravity
HARSHNA BALHARA

New Astronomy

In this article, we study the expanding nature of universe in the contest of f (R, Lm) gravity theory, here R represents the Ricci scalar and Lm is the matter Lagrangian density. With a specific form of f (R, Lm), we obtain the field equations for flat FLRW metric. We parametrize the deceleration parameter in terms of the Hubble parameter and from here we find four free parameters, which are constraints and estimated by using H(z), P antheon, and their joint data sets. Further, we investigate the evolution of the deceleration parameter which depicts a transition from the deceleration to acceleration phases of the universe. The evolution behaviour of energy density, pressure, and EoS parameters shows that the present model is an accelerated quintessence dark energy model. To compare our model with the ΛCDM model we use some of the diagnostic techniques. Thus, we find that our model in f (R, Lm) gravity supports the recent standard observational studies and delineates the late-time cosmic acceleration.

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Constraints on Dark Energy from the Observed Expansion of Our Cosmic Horizon
Fulvio Melia

International Journal of Modern Physics D, 2009

Within the context of standard cosmology, an accelerating universe requires the presence of a third "dark" component of energy, beyond matter and radiation. The available data, however, are still deemed insufficient to distinguish between an evolving dark energy component and the simplest model of a time-independent cosmological constant. In this paper, we examine the cosmological expansion in terms of observer-dependent coordinates, in addition to the more conventional comoving coordinates. This procedure explicitly reveals the role played by the radius Rh of our cosmic horizon in the interrogation of the data. (In Rindler's notation, Rh coincides with the "event horizon" in the case of de Sitter, but changes in time for other cosmologies that also contain matter and/or radiation.) With this approach, we show that the interpretation of dark energy as a cosmological constant is clearly disfavored by the observations. Within the framework of standard Friedmann-Robertson-Walker cosmology, we derive an equation describing the evolution of Rh, and solve it using the WMAP and Type Ia supernova data. In particular, we consider the meaning of the observed equality (or near-equality) Rh(t0) ≅ ct0, where t0 is the age of the universe. This empirical result is far from trivial, for a cosmological constant would drive Rh(t) toward ct (t is the cosmic time) only once — and that would have to occur right now. Though we are not here espousing any particular alternative model of dark energy, for comparison we also consider scenarios in which dark energy is given by scaling solutions, which simultaneously eliminate several conundrums in the standard model, including the "coincidence" and "flatness" problems, and account very well for the fact that Rh(t0) ≈ ct0.

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Cited by

Dark Energy: The Shadowy Reflection of Dark Matter?
Kostas Kleidis

Entropy, 2016

In this article, we review a series of recent theoretical results regarding a conventional approach to the dark energy (DE) concept. This approach is distinguished among others for its simplicity and its physical relevance. By compromising General Relativity (GR) and Thermodynamics at cosmological scale, we end up with a model without DE. Instead, the Universe we are proposing is filled with a perfect fluid of self-interacting dark matter (DM), the volume elements of which perform hydrodynamic flows. To the best of our knowledge, it is the first time in a cosmological framework that the energy of the cosmic fluid internal motions is also taken into account as a source of the universal gravitational field. As we demonstrate, this form of energy may compensate for the DE needed to compromise spatial flatness, while, depending on the particular type of thermodynamic processes occurring in the interior of the DM fluid (isothermal or polytropic), the Universe depicts itself as either decelerating or accelerating (respectively). In both cases, there is no disagreement between observations and the theoretical prediction of the distant supernovae (SNe) Type Ia distribution. In fact, the cosmological model with matter content in the form of a thermodynamically-involved DM fluid not only interprets the observational data associated with the recent history of Universe expansion, but also confronts successfully with every major cosmological issue (such as the age and the coincidence problems). In this way, depending on the type of thermodynamic processes in it, such a model may serve either for a conventional DE cosmology or for a viable alternative one.

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Anisotropic bulk viscous string cosmological models of the Universe under a time-dependent deceleration parameter
Anirudh Pradhan

Pramana

We investigate a new class of LRS Bianchi type-II cosmological models by revisiting in the paper of Mishra et al (2013) by considering a new deceleration parameter (DP) depending on the time in string cosmology for the modified gravity theory suggested by Sáez & Ballester (1986). We have considered the energy-momentum tensor proposed by Leteliar (1983) for bulk viscous and perfect fluid under some assumptions. To make our models consistent with recent astronomical observations, we have used scale factor (Sharma et al 2018; Garg et al 2019) a(t) = exp [ 1 β √ 2βt + k], where β and k are positive constants and it provides a time-varying DP. By using the recent constraints (H 0 = 73.8, and q 0 = −0.54) from SN Ia data in combination with BAO and CMB observations (Giostri et al, arXiv:1203.3213v2[astro-ph.CO]), we affirm β = 0.0062 and k = 0.000016. For these constraints, we have substantiated a new class of cosmological transit models for which the expansion takes place from early decelerated phase to the current accelerated phase. Also, we have studied some physical, kinematic and geometric behavior of the models, and have found them consistent with observations and well established theoretical results. We have also compared our present results with those of Mishra et al (2013) and observed that the results in this paper are much better, stable under perturbation and in good agreement with cosmological reflections.

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Estimating the Parameters of Extended Gravity Theories with the Schwarzschild Precession of S2 Star
Alexander Zakharov

Universe

After giving a short overview of previous results on constraining of Extended Gravity by stellar orbits, we discuss the Schwarzschild orbital precession of S2 star assuming the congruence with predictions of General Relativity (GR). At the moment, the S2 star trajectory is remarkably fitted with the first post-Newtonian approximation of GR. In particular, both Keck and VLT (GRAVITY) teams declared that the gravitational redshift near its pericenter passage for the S2 star orbit corresponds to theoretical estimates found with the first post-Newtonian (pN) approximation. In 2020, the GRAVITY Collaboration detected the orbital precession of the S2 star around the supermassive black hole (SMBH) at the Galactic Center and showed that it is close to the GR prediction. Based on this observational fact, we evaluated parameters of the Extended Gravity theories with the Schwarzschild precession of the S2 star. Using the mentioned method, we estimate the orbital precession angles for some Exte...

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Raychaudhuri equation in scalar–tensor theory
Ananda Dasgupta

International Journal of Geometric Methods in Modern Physics, 2021

Implications of the Raychaudhuri equation in focusing of geodesic congruences are studied in the framework of scalar–tensor theory of gravity. Specifically, we investigate the Brans–Dicke theory and Bekenstein’s scalar field theory. In both of these theories, we deal with a static spherically symmetric distribution and a spatially homogeneous and isotropic cosmological model as specific examples. We find that it is possible to violate the convergence condition under reasonable physical assumptions. This leads to the possibility of avoiding a singularity.

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On the Electromagnetic Vacuum Origins of Dark Energy, Dark Matter, and Astrophysical Jets
Stuart Marongwe

Advances in High Energy Physics, 2021

We use a semiclassical version of the Nexus paradigm of quantum gravity in which the quantum vacuum at large scales is dominated by the second quantized electromagnetic field to demonstrate that a virtual photon field can affect the geometric evolution of Einstein manifolds or Ricci solitons. This phenomenon offers a cogent explanation of the origins of astrophysical jets, the cosmological constant, and a means of detecting galactic dark matter.

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