scholarly journals Probing modified gravity in cosmic filaments

2018 ◽  
Vol 619 ◽  
pp. A122 ◽  
Author(s):  
Alex Ho ◽  
Max Gronke ◽  
Bridget Falck ◽  
David F. Mota
Keyword(s):  
Modified Gravity ◽  
Gravity Models ◽  
Accelerated Expansion ◽  
Radial Density ◽  
Post Process ◽  
Global Properties ◽  
Modified Gravity Theories ◽  
Expansion Of The Universe ◽  
Gravity Theories ◽  
The Universe

Multiple modifications of general relativity (GR) have been proposed in the literature in order to understand the nature of the accelerated expansion of the Universe. However, thus far all the predictions of GR have been confirmed with constantly increasing accuracy. In this work, we study the imprints of a particular class of models – “screened” modified gravity theories – on cosmic filaments. We have utilized the N-body code ISIS/RAMSES to simulate the symmetron model and the Hu–Sawicky f(R) model, and we post-process the output with DisPerSE to identify the filaments of the cosmic web. We investigated how the global properties of the filaments – such as their lengths, masses, and thicknesses – as well as their radial density and speed profiles change under different gravity theories. We find that filaments are, on average, shorter and denser in modified gravity models compared to in ΛCDM. We also find that the speed profiles of the filaments are enhanced, consistent with theoretical expectations. Overall, our results suggest that cosmic filaments can be an effective complementary probe of screened modified gravity theories on Mpc scales.

scholarly journals On the temporal evolution of particle production in f(T) gravity

2020 ◽  
pp. 2050328
Author(s):  
Sanjay Mandal ◽  
P. K. Sahoo
Keyword(s):  
Arbitrary Function ◽  
Particle Production ◽  
Particle Number Density ◽  
Gravity Models ◽  
Accelerated Expansion ◽  
Modified Theories Of Gravity ◽  
Torsion Scalar ◽  
Expansion Of The Universe ◽  
Modified Formula ◽  
The Universe

The thermodynamical study of the universe allows particle production in modified [Formula: see text] ([Formula: see text] is the torsion scalar) theory of gravity within a flat FLRW framework for line element. The torsion scalar [Formula: see text] plays the same role as the Ricci scalar [Formula: see text] in the modified theories of gravity. We derived the [Formula: see text] gravity models by taking [Formula: see text] as the sum of [Formula: see text] and an arbitrary function of [Formula: see text] with three different arbitrary function. We observe that the particle production describes the accelerated expansion of the universe without a cosmological constant or any unknown “quintessence” component. Also, we discussed the supplementary pressure, particle number density and particle production rate for three cases.

scholarly journals SOME ASPECTS OF GENERALIZED MODIFIED GRAVITY MODELS

2013 ◽  
Vol 22 (08) ◽  
pp. 1330017 ◽  
Author(s):  
R. MYRZAKULOV ◽  
L. SEBASTIANI ◽  
S. ZERBINI
Keyword(s):  
Modified Gravity ◽  
Correction Term ◽  
High Redshift ◽  
Early Time ◽  
Gravity Models ◽  
High Frequency Oscillation ◽  
Field Equations ◽  
Modified Gravity Theories ◽  
Gravity Theories ◽  
General Aspects

In this paper, we review some general aspects of modified gravity theories, investigating mathematical and physical properties and, more specifically, the feature of viable and realistic models able to reproduce the dark energy (DE) epoch and the early-time inflation. We will discuss the black hole (BH) solutions in generalized theories of gravity: it is of fundamental interest to understand how properties and laws of BHs in General Relativity (GR) can be addressed in the framework of modified theories. In particular, we will discuss the energy issue and the possibility to derive the First Law of thermodynamics from the field equations. Then, in the analysis of cosmological solutions, we will pay particular attention to the occurrence of finite-time future singularities and to the possibility to avoid them in [Formula: see text]-gravity. Furthermore, realistic models of F(R)-gravity will be analyzed in detail. A general feature occurring in matter era will be shown, namely, the high derivatives of Hubble parameter may be influenced by the high frequency oscillation of the DE and some correction term may be required in order to stabilize the theory at high redshift. The inflationary scenario is also carefully analyzed and a unified description of the universe is evolved. In the final part of the work, we will look at the last developments in modified gravity, namely, we will investigate cosmological and BH solutions in a covariant field theory of gravity and we will introduce the extended "teleparallel" F(T)-gravity theories. A nice application to the dark matter (DM) problem will be presented.

scholarly journals HALO MODELS IN MODIFIED GRAVITY THEORIES WITH SELF-ACCELERATED EXPANSION

2011 ◽  
Vol 20 (12) ◽  
pp. 2383-2397 ◽  
Author(s):  
TATSUYA NARIKAWA ◽  
RAMPEI KIMURA ◽  
TATSUNOSUKE YANO ◽  
KAZUHIRO YAMAMOTO
Keyword(s):  
Gravity Model ◽  
Modified Gravity ◽  
Outer Region ◽  
Gravity Models ◽  
Accelerated Expansion ◽  
Newtonian Gravity ◽  
Sphere Model ◽  
Modified Gravity Theories ◽  
The Common ◽  
The Difference

We investigate the structure of halos in the sDGP (self-accelerating branch of the Dvali–Gavadadze–Porrati braneworld gravity) model and the Galileon modified gravity model on the basis of the static and spherically symmetric solutions of the collisionless Boltzmann equation, which reduce to the singular isothermal sphere model and the King model in the limit of Newtonian gravity. The common feature of these halos is that the density of a halo in the outer region is larger (smaller) in the sDGP (Galileon) model, respectively, in comparison with Newtonian gravity. This comes from the suppression (enhancement) of the effective gravity at large distance in the sDGP (Galileon) model, respectively. However, the difference between these modified gravity models and Newtonian gravity only appears outside the halo due to the Vainshtein mechanism, which makes it difficult to distinguish between them. We also discuss the case in which the halo density profile is fixed independently of the gravity model for comparison between our results and previous work.

scholarly journals Cosmic voids in modified gravity scenarios

2019 ◽  
Vol 632 ◽  
pp. A52 ◽  
Author(s):  
Eder L. D. Perico ◽  
Rodrigo Voivodic ◽  
Marcos Lima ◽  
David F. Mota
Keyword(s):  
General Relativity ◽  
Modified Gravity ◽  
Gravitational Theory ◽  
Gravity Models ◽  
Consistency Check ◽  
Dark Sector ◽  
Modified Gravity Theories ◽  
Acceleration Of The Universe ◽  
Best Fit ◽  
The Universe

Modified gravity (MG) theories aim to reproduce the observed acceleration of the Universe by reducing the dark sector while simultaneously recovering General Relativity (GR) within dense environments. Void studies appear to be a suitable scenario to search for imprints of alternative gravity models on cosmological scales. Voids cover an interesting range of density scales where screening mechanisms fade out, which reaches from a density contrast δ ≈ −1 close to their centers to δ ≈ 0 close to their boundaries. We present an analysis of the level of distinction between GR and two modified gravity theories, the Hu–Sawicki f(R) and the symmetron theory. This study relies on the abundance, linear bias, and density profile of voids detected in N-body cosmological simulations. We define voids as connected regions made up of the union of spheres with a mean density given by ρ̅v = 0.2 ρ̅m, but disconnected from any other voids. We find that the height of void walls is considerably affected by the gravitational theory, such that it increases for stronger gravity modifications. Finally, we show that at the level of dark matter N-body simulations, our constraints allow us to distinguish between GR and MG models with |fR0| > 10−6 and zSSB >  1. Differences of best-fit values for MG parameters that are derived independently from multiple void probes may indicate an incorrect MG model. This serves as an important consistency check.

scholarly journals RECONSTRUCTION OF MODIFIED GRAVITY WITH GHOST DARK ENERGY MODELS

2012 ◽  
Vol 27 (18) ◽  
pp. 1250100 ◽  
Author(s):  
A. KHODAM-MOHAMMADI ◽  
M. MALEKJANI ◽  
M. MONSHIZADEH
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Modified Gravity ◽  
Deceleration Parameter ◽  
Accelerated Expansion ◽  
Ghost Dark Energy ◽  
Energy Models ◽  
Expansion Of The Universe ◽  
The Universe ◽  
Generalized Ghost Dark Energy

In this work, we reconstruct the f(R) modified gravity for different ghost and generalized-ghost dark energy (DE) models in FRW flat universe, which describes the accelerated expansion of the universe. The equation of state and deceleration parameter of reconstructed f(R) gravity have been calculated. The equation of state and deceleration parameter of reconstructed f(R)-ghost/generalized-ghost DE, have been calculated. We show that the corresponding f(R) gravity of ghost/generalized-ghost DE model can behave like phantom or quintessence. Also the transition between deceleration to acceleration regime is indicated by deceleration parameter diagram for reconstructed f(R) generalized-ghost DE model.

scholarly journals COSMOLOGICAL DYNAMICS OF f(R) GRAVITY SCALAR DEGREE OF FREEDOM IN EINSTEIN FRAME

2013 ◽  
Vol 22 (14) ◽  
pp. 1350083 ◽  
Author(s):  
UMANANDA DEV GOSWAMI ◽  
KABITA DEKA
Keyword(s):  
Scalar Field ◽  
Power Law ◽  
Einstein Frame ◽  
Degree Of Freedom ◽  
Gravity Models ◽  
Accelerated Expansion ◽  
Power Law Model ◽  
Starobinsky Model ◽  
Expansion Of The Universe ◽  
The Universe

f(R) gravity models belong to an important class of modified gravity models where the late time cosmic accelerated expansion is considered as a manifestation of the large scale modification of the force of gravity. f(R) gravity models can be expressed in terms of a scalar degree of freedom by redefinition of model's variable. The conformal transformation of the action from Jordan frame to Einstein frame makes the scalar degree of freedom more explicit and can be studied conveniently. We have investigated the features of the scalar degree of freedoms and the consequent cosmological implications of the power-law (ξRn) and the Starobinsky (disappearing cosmological constant) f(R) gravity models numerically in the Einstein frame. Both the models show interesting behavior of their scalar degree of freedom and could produce the accelerated expansion of the universe in the Einstein frame with the negative equation of state of the scalar field. However, the scalar field potential for the power-law model is the well-behaved function of the field, whereas the potential becomes flat for higher value of field in the case of the Starobinsky model. Moreover, the equation of state of the scalar field for the power-law model is always negative and less than -1/3, which corresponds to the behavior of the dark energy, that produces the accelerated expansion of the universe. This is not always the case for the Starobinsky model. At late times, the Starobinsky model behaves as cosmological constant Λ as behaves by power-law model for the values of n → 2 at all times.

scholarly journals Thermodynamic properties of modified gravity theories

2016 ◽  
Vol 13 (06) ◽  
pp. 1630007 ◽  
Author(s):  
Kazuharu Bamba
Keyword(s):  
Thermodynamic Properties ◽  
Modified Gravity ◽  
Equations Of Motion ◽  
Apparent Horizon ◽  
Second Law Of Thermodynamics ◽  
Second Law ◽  
Modified Gravity Theories ◽  
Gravity Theories ◽  
Clausius Relation ◽  
The Universe

We review thermodynamic properties of modified gravity theories, such as [Formula: see text] gravity and [Formula: see text] gravity, where [Formula: see text] is the scalar curvature and [Formula: see text] is the torsion scalar in teleparallelism. In particular, we explore the equivalence between the equations of motion for modified gravity theories and the Clausius relation in thermodynamics. In addition, thermodynamics of the cosmological apparent horizon is investigated in [Formula: see text] gravity. We show both equilibrium and nonequilibrium descriptions of thermodynamics. It is demonstrated that the second law of thermodynamics in the universe can be met, when the temperature of the outside of the apparent horizon is equivalent to that of the inside of it.

Fast approximate methods for modified gravity cosmological simulations

2020 ◽  
Vol 493 (2) ◽  
pp. 2085-2100
Author(s):  
Michal Vraštil ◽  
Salman Habib
Keyword(s):  
Modified Gravity ◽  
Acoustic Oscillation ◽  
Accelerated Expansion ◽  
Minimal Extension ◽  
Linear Regime ◽  
Approximate Methods ◽  
Perturbative Methods ◽  
Non Linear ◽  
Expansion Of The Universe ◽  
The Universe

ABSTRACT The accelerated expansion of the Universe poses a major theoretical puzzle. Although the assumption of a non-zero cosmological constant provides a minimal extension of general relativity that is consistent with observational data, many theories of modified gravity have been suggested as possible alternatives. Predictions of structure formation for these models in the fully non-linear regime are very expensive and it is difficult, if not impossible, to explore such a huge space of models and parameters using high-resolution N-body simulations. Even in the mildly non-linear regime, perturbative methods can become extremely complex. We explore whether simplified dynamical approximations, applicable for a certain set of cosmological probes, can be used to investigate models of modified gravity with acceptable accuracy in the latter instance. For the case of chameleon gravity, we find that these methods can indeed be used to explore the region around the baryon acoustic oscillation scale, $k\sim 0.1~h\, \text{Mpc}^{-1}$ but not much further.

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