scholarly journals THE EFFECT OF STRUCTURE FORMATION ON THE EXPANSION OF THE UNIVERSE

2008 ◽  
Vol 17 (13n14) ◽  
pp. 2543-2548 ◽  
Author(s):  
SYKSY RÄSÄNEN
Keyword(s):  
Structure Formation ◽  
Cold Dark Matter ◽  
Expansion Rate ◽  
Isotropic Case ◽  
Isotropic Distribution ◽  
Expansion Of The Universe ◽  
Free Parameters ◽  
Matter Transfer ◽  
The Universe ◽  
Dust Universe

Observations of the expansion rate of the universe at late times disagree by a factor of 1.5–2 with the prediction of homogeneous and isotropic models based on ordinary matter and gravity. We discuss how the departure from linearly perturbed homogeneity and isotropy due to structure formation could explain this discrepancy. We evaluate the expansion rate in a dust universe which contains nonlinear structures with a statistically homogeneous and isotropic distribution. The expansion rate is found to increase relative to the exactly homogeneous and isotropic case by a factor of 1.1–1.3 at some tens of billions of years. The time scale follows from the cold dark matter transfer function and the amplitude of primordial perturbations without additional free parameters.

scholarly journals COSMOLOGICAL ACCELERATION FROM STRUCTURE FORMATION

2006 ◽  
Vol 15 (12) ◽  
pp. 2141-2146 ◽  
Author(s):  
SYKSY RÄSÄNEN
Keyword(s):  
Structure Formation ◽  
Gravitational Collapse ◽  
Physical Meaning ◽  
Expansion Rate ◽  
Small Perturbations ◽  
Local Expansion ◽  
The Universe ◽  
Friedmann Robertson Walker ◽  
Dust Universe

We discuss the Buchert equations, which describe the average expansion of an inhomogeneous dust universe. In the limit of small perturbations, they reduce to the Friedmann–Robertson–Walker equations. However, when the universe is very inhomogeneous, the behavior can be qualitatively different from the FRW case. In particular, the average expansion rate can accelerate even though the local expansion rate decelerates everywhere. We clarify the physical meaning of this paradoxical feature with a simple toy model, and demonstrate how acceleration is intimately connected with gravitational collapse. This provides a link to structure formation, which in turn has a preferred time around the era when acceleration has been observed to start.

scholarly journals Taxonomy of Dark Energy Models

Universe ◽  
2021 ◽  
Vol 7 (6) ◽  
pp. 163
Author(s):  
Verónica Motta ◽  
Miguel A. García-Aspeitia ◽  
Alberto Hernández-Almada ◽  
Juan Magaña ◽  
Tomás Verdugo
Keyword(s):  
Dark Energy ◽  
Cold Dark Matter ◽  
Accelerated Expansion ◽  
Energy Component ◽  
The Past ◽  
Energy Models ◽  
The Status ◽  
Expansion Of The Universe ◽  
The Universe ◽  
Unknown Component

The accelerated expansion of the Universe is one of the main discoveries of the past decades, indicating the presence of an unknown component: the dark energy. Evidence of its presence is being gathered by a succession of observational experiments with increasing precision in its measurements. However, the most accepted model for explaining the dynamic of our Universe, the so-called Lambda cold dark matter, faces several problems related to the nature of such energy component. This has led to a growing exploration of alternative models attempting to solve those drawbacks. In this review, we briefly summarize the characteristics of a (non-exhaustive) list of dark energy models as well as some of the most used cosmological samples. Next, we discuss how to constrain each model’s parameters using observational data. Finally, we summarize the status of dark energy modeling.

scholarly journals AN ALTERNATIVE INTERPRETATION FOR THE MODULI FIELDS OF THE COSMOLOGY ASSOCIATED TO TYPE IIB SUPERGRAVITY WITH FLUXES

2008 ◽  
Vol 23 (13) ◽  
pp. 1949-1962 ◽  
Author(s):  
TONATIUH MATOS ◽  
JOSÉ-RUBÉN LUÉVANO ◽  
HUGO GARCÍA-COMPEÁN ◽  
J. ALBERTO VÁZQUEZ
Keyword(s):  
Structure Formation ◽  
Alternative Interpretation ◽  
Linear Regime ◽  
Superstring Theory ◽  
Supergravity Theory ◽  
Cosmological Observations ◽  
Λcdm Model ◽  
Free Parameters ◽  
Quintessence Field ◽  
The Universe

The aim of this work is to provide a basis to interpret the dilaton as the dark matter of the universe, in the context of a particular cosmological model derived from type IIB supergravity theory with fluxes. In this theory, the dilaton is usually interpreted as a quintessence field. But, with this alternative interpretation we find that (in this supergravity model) the model gives a similar evolution and structure formation of the universe compared with the ΛCDM model in the linear regime of fluctuations of the structure formation. Some free parameters of the theory are fixed using the present cosmological observations. In the nonlinear regime there are some differences between the type IIB supergravity theory with the traditional CDM paradigm. The supergravity theory predicts the formation of galaxies earlier than the CDM and there is no density cusp in the center of galaxies. These differences can distinguish both models and might give a distinctive feature to the phenomenology of the cosmology coming from superstring theory with fluxes.

scholarly journals SOLVING THE INVERSE PROBLEM WITH INHOMOGENEOUS UNIVERSES

2011 ◽  
Vol 01 ◽  
pp. 234-239
Author(s):  
CHUL-MOON YOO ◽  
TOMOHIRO KAI ◽  
KEN-ICHI NAKAO
Keyword(s):  
Inverse Problem ◽  
Dark Matter ◽  
Cold Dark Matter ◽  
Big Bang ◽  
Universe Model ◽  
Cosmological Redshift ◽  
Λcdm Model ◽  
The Big Bang ◽  
The Universe ◽  
Dust Universe

We construct the Lemaître-Tolman-Bondi (LTB) dust universe whose distance-redshift relation is equivalent to that in the concordance Λ cold dark matter (ΛCDM) cosmological model. In our model, the density distribution and velocity field are not homogeneous, whereas the big-bang time is uniform, which implies that the universe is homogeneous at its beginning. We also study the temporal variation of the cosmological redshift and show that, by the observation of this quantity, we can distinguish our LTB universe model from the concordance ΛCDM model, even if their redshift-distance relations are equivalent to each other.

scholarly journals Acceleration in Friedmann cosmology with torsion

2019 ◽  
Vol 79 (11) ◽  
Author(s):  
S. H. Pereira ◽  
R. de C. Lima ◽  
J. F. Jesus ◽  
R. F. L. Holanda
Keyword(s):  
Energy Density ◽  
Critical Density ◽  
Spin Vector ◽  
Torsion Function ◽  
Different Types ◽  
Expansion Of The Universe ◽  
Cosmological Data ◽  
Free Parameters ◽  
The Universe ◽  
Matter Energy

AbstractA Friedmann like cosmological model in Einstein–Cartan framework is studied when the torsion function is assumed to be proportional to a single $$\phi (t)$$ϕ(t) function coming just from the spin vector contribution of ordinary matter. By analysing four different types of torsion function written in terms of one, two and three free parameters, we found that a model with $$\phi (t)=- \alpha H(t) \big ({\rho _{m}(t)}/{\rho _{0c}}\big )^n$$ϕ(t)=-αH(t)(ρm(t)/ρ0c)n is totally compatible with recent cosmological data, where $$\alpha $$α and n are free parameters to be constrained from observations, $$\rho _m$$ρm is the matter energy density and $$\rho _{0c}$$ρ0c the critical density. The recent accelerated phase of expansion of the universe is correctly reproduced by the contribution coming from torsion function, with a deceleration parameter indicating a transition redshift of about 0.65.

scholarly journals IR quantum gravity solves naturally cosmic acceleration and its coincidence problem

2019 ◽  
Vol 28 (14) ◽  
pp. 1944013
Author(s):  
Fotios K. Anagnostopoulos ◽  
Georgios Kofinas ◽  
Vasilios Zarikas
Keyword(s):  
Quantum Gravity ◽  
Cosmic Acceleration ◽  
Fine Tuning ◽  
Accelerated Expansion ◽  
The Novel ◽  
Positive Cosmological Constant ◽  
Coincidence Problem ◽  
Expansion Of The Universe ◽  
Free Parameters ◽  
The Universe

The novel idea is that the undergoing accelerated expansion of the universe happens due to infrared quantum gravity modifications at intermediate astrophysical scales of galaxies or galaxy clusters, within the framework of Asymptotically Safe gravity. The reason is that structures of matter are associated with a scale-dependent positive cosmological constant of quantum origin. In this context, no extra unproven energy scales or fine-tuning are used. Furthermore, this model was confronted with the most recent observational data from a variety of probes, and with the aid of Bayesian analysis, the most probable values of the free parameters were extracted. Finally, the model proved to be statistically equivalent with [Formula: see text]CDM, and thus being able to resolve naturally the concept of dark energy and its associated cosmic coincidence problem.

scholarly journals A Cosmological Scenario from the Starobinsky Model within the f(R,T) Formalism

2019 ◽  
Vol 2019 ◽  
pp. 1-8 ◽  
Author(s):  
P. H. R. S. Moraes ◽  
P. K. Sahoo ◽  
G. Ribeiro ◽  
R. A. C. Correa
Keyword(s):  
Functional Form ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Energy Conditions ◽  
Starobinsky Model ◽  
Cosmological Scenario ◽  
Expansion Of The Universe ◽  
Theory Of Gravitation ◽  
Free Parameters ◽  
The Universe

In this paper we derive a novel cosmological model from the f(R,T) theory of gravitation, for which R is the Ricci scalar and T is the trace of the energy-momentum tensor. We consider the functional form f(R,T)=f(R)+f(T), with f(R) being the Starobinsky Model, named R+αR2, and f(T)=2γT, with α and γ being constants. We show that a hybrid expansion law form for the scale factor is a solution for the derived Friedmann-like equations. In this way, the model is able to predict both the decelerated and the accelerated regimes of expansion of the universe, with the transition redshift between these stages being in accordance with recent observations. We also apply the energy conditions to our material content solutions. Such an application makes us able to obtain the range of acceptability for the free parameters of the model, named α and γ.

Ω0 Concordance

2005 ◽  
Vol 201 ◽  
pp. 271-281
Author(s):  
Masataka. Fukugita
Keyword(s):  
Dark Matter ◽  
Cosmological Constant ◽  
Structure Formation ◽  
Cold Dark Matter ◽  
Mass Density ◽  
Hubble Constant ◽  
Density Parameter ◽  
Microwave Background ◽  
The Hierarchical Structure ◽  
The Universe

The determinations of the mass density parameter Ω0 are examined with a particular emphasis given to the new cosmic microwave background (CMB) experiments. It is shown that the Ω0 and the Hubble constant H0 from CMB are quite consistent with those from other observations with the aid of the hierarchical structure formation models based on cold dark matter dominance with the cosmological constant that makes the universe flat. The concordance value of Ω0 is 0.25-0.45.

scholarly journals Universe opacity and Type Ia supernova dimming

2019 ◽  
Vol 489 (1) ◽  
pp. L63-L68 ◽  
Author(s):  
Václav Vavryčuk
Keyword(s):  
Cold Dark Matter ◽  
Hubble Constant ◽  
Light Extinction ◽  
Universe Model ◽  
Type Ia Supernova ◽  
Accelerating Expansion ◽  
Λcdm Model ◽  
Type Ia ◽  
Expansion Of The Universe ◽  
The Universe

ABSTRACT In this paper, I revoke a debate about an origin of Type Ia supernova (SN Ia) dimming. I argue that except for a commonly accepted accelerating expansion of the Universe, a conceivable alternative for explaining this observation is universe opacity caused by light extinction by intergalactic dust, even though it is commonly assumed that this effect is negligible. Using data of the Union2.1 SN Ia compilation, I find that the standard Λ cold dark matter (ΛCDM) model and the opaque universe model fit the SN Ia measurements at redshifts z < 1.4 comparably well. The optimum solution for the opaque universe model is characterized by the B-band intergalactic opacity $\lambda _{B} = 0.10 \pm 0.03 \, \mathrm{Gpc}^{-1}$ and the Hubble constant $H_0 = 68.0 \pm 2.5 \, \mathrm{km\, s^{-1}\, Mpc^{-1}}$. The intergalactic opacity is higher than that obtained from independent observations but still within acceptable limits. This result emphasizes that the issue of the accelerating expansion of the Universe as the origin of the SN Ia dimming is not yet definitely resolved. Obviously, the opaque universe model as an alternative to the ΛCDM model is attractive, because it avoids puzzles and controversies associated with dark energy and the accelerating expansion.

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