We consider the simplest anisotropic generalization, as a correction, to the standard ΛCDM model, by replacing the spatially flat Robertson-Walker metric by the Bianchi type-I metric, which brings in ...a new term Ωσ0a−6 (mimicking the stiff fluid) in the average expansion rate H(a) of the Universe. From Hubble and Pantheon data, relevant to the late Universe (z≲2.4), we obtain the constraint Ωσ0≲10−3, in line with the model-independent constraints. When the baryonic acoustic oscillations and cosmic microwave background (CMB) data are included, the constraint improves by 12 orders of magnitude, i.e., Ωσ0≲10−15. We find that this constraint could alter neither the matter-radiation equality redshift nor the peak of the matter perturbations. Demanding that the expansion anisotropy has no significant effect on the standard big bang nucleosynthesis (BBN), we find the constraint Ωσ0≲10−23. We show explicitly that the constraint from BBN renders the expansion anisotropy irrelevant to make a significant change in the CMB quadrupole temperature, whereas the constraint from the cosmological data in our model provides the temperature change up to ~11 mK, though it is much beyond the CMB quadrupole temperature.
We propose a modified theory of gravitation constructed by the addition of the term f(TμνTμν) to the Einstein-Hilbert action, and elaborate a particular case f(TμνTμν)=α(TμνTμν)η, where α and η are ...real constants, dubbed energy-momentum powered gravity (EMPG). We search for viable cosmologies arising from EMPG, especially in the context of the late-time accelerated expansion of the Universe. We investigate the ranges of the EMPG parameters (α,η) on theoretical as well as observational grounds leading to the late-time acceleration of the Universe with pressureless matter only, while keeping the successes of standard general relativity at early times. We find that η=0 corresponds to the ΛCDM model, whereas η≠0 leads to a wCDM-type model. However, the underlying physics of the EMPG model is entirely different in the sense that the energy in the EMPG Universe is sourced by pressureless matter only. Moreover, the energy of the pressureless matter is not conserved, namely, in general it does not dilute as ρ∝a−3 with the expansion of the Universe. Finally, we constrain the parameters of an EMPG-based cosmology with a recent compilation of 28 Hubble parameter measurements, and find that this model describes an evolution of the Universe similar to that in the ΛCDM model. We briefly discuss that EMPG can be unified with Starobinsky gravity to describe the complete history of the Universe including the inflationary era.
We present an explicit detailed theoretical and observational investigation of an anisotropic massive Brans–Dicke (BD) gravity extension of the standard
Λ
CDM model, wherein the extension is ...characterized by two additional degrees of freedom; the BD parameter,
ω
, and the present day density parameter corresponding to the shear scalar,
Ω
σ
2
,
0
. The BD parameter, determining the deviation from general relativity (GR), by alone characterizes both the dynamics of the effective dark energy (DE) and the redshift dependence of the shear scalar. These two affect each other depending on
ω
, namely, the shear scalar contributes to the dynamics of the effective DE, and its anisotropic stress – which does not exist in scalar field models of DE within GR – controls the dynamics of the shear scalar deviating from the usual
∝
(
1
+
z
)
6
form in GR. We mainly confine the current work to non-negative
ω
values as it is the right sign – theoretically and observationally – for investigating the model as a correction to the
Λ
CDM. By considering the current cosmological observations, we find that
ω
≳
250
,
Ω
σ
2
,
0
≲
10
-
23
and the contribution of the anisotropy of the effective DE to this value is insignificant. We conclude that the simplest anisotropic massive BD gravity extension of the standard
Λ
CDM model exhibits no significant deviations from it all the way to the Big Bang Nucleosynthesis. We also point out the interesting features of the model in the case of negative
ω
values; for instance, the constraints on
Ω
σ
2
,
0
could be relaxed considerably, the values of
ω
∼
-
1
(relevant to string theories) predict dramatically different dynamics for the expansion anisotropy.
We propose a new law for the deceleration parameter that varies linearly with time and covers Berman’s law where it is constant. Our law not only allows one to generalize many exact solutions that ...were obtained assuming constant deceleration parameter, but also gives a better fit with data (from SNIa, BAO and CMB), particularly concerning the late time behavior of the universe. According to our law only the spatially closed and flat universes are allowed; in both cases the cosmological fluid we obtain exhibits quintom like behavior and the universe ends with a big-rip. This is a result consistent with recent cosmological observations.
We present a detailed investigation of the Rastall gravity extension of the standard
Λ
CDM model. We review the model for two simultaneous modifications of different nature in the Friedmann equation ...due to the Rastall gravity: the new contributions of the material (actual) sources (considered as effective source) and the altered evolution of the material sources. We discuss the role/behavior of these modifications with regard to some low redshift tensions, including the so-called
H
0
tension, prevailing within the standard
Λ
CDM. We constrain the model at the level of linear perturbations, and obtain the first constraints through a robust and accurate analysis using the latest full Planck cosmic microwave background (CMB) data, with and without including baryon acoustic oscillations (BAO) data. We find that the Rastall parameter
ϵ
(null for general relativity) is consistent with zero at 68% CL (with a tendency towards positive values,
-
0.0001
<
ϵ
<
0.0007
(CMB+BAO) at 68% CL), which in turn implies no significant statistical evidence for deviation from general relativity, and also a precision of
O
(
10
-
4
)
for the coefficient
-
1
/
2
of the term
g
μ
ν
R
in the Einstein field equations of general relativity (guaranteeing the local energy-momentum conservation). We explore the consequences led by the Rastall gravity on the cosmological parameters in the light of the observational analyses. It turns out that the effective source, with a present-day density parameter
Ω
X
0
=
-
0.0010
±
0.0013
(CMB+BAO, 68% CL), dynamically screens the usual vacuum energy at high redshifts, but this mechanism barely works due to the opposition by the altered evolution of cold dark matter. Consequently, two simultaneous modifications of different nature in the Friedmann equation by the Rastall gravity act against each other, and do not help to considerably relax the low redshift tensions, including the so-called
H
0
tension. Our results may offer a guide for the research community that studies the Rastall gravity in various aspects of gravitation and cosmology.
ABSTRACT
In this paper, we study the gravitational-wave (GW) radiation and radiative behaviour of relativistic compact binary systems in the scale-independent energy–momentum squared gravity (EMSG). ...The field equations of this theory are solved approximately. The gravitational potential of a gravitational source is then obtained by considering two matter Lagrangian densities that both describe a perfect fluid in general relativity (GR). We derive the GW signals emitted from a compact binary system. The results are different from those obtained in GR. It is shown that the relevant non-GR corrections modify the wave amplitude and leave the GW polarizations unchanged. Interestingly, this modification depends on the choice of the matter Lagrangian density. This means that for different Lagrangian densities, this theory presents different predictions for the GW radiation. In this case, the system loses energy to modified GWs. This leads to a change in the secular variation of the Keplerian parameters of the binary system. In this work, we investigate the non-GR effects on the radiative parameter, that is, the first time derivative of the orbital period. Next, applying these results together with GW observations from the relativistic binary systems, we constrain/test the scale-independent EMSG theory in the strong-field regime. After assuming that GR is the valid gravity theory, as a priori expectation, we find that the free parameter of the theory is of the order 10−5 from the direct GW observation, the GW events GW190425 and GW170817, as well as the indirect GW observation, the double pulsar PSR J0737−3039A/B experiment.
In this work, we first discuss the possibility that dark energy models with negative energy density values in the past can alleviate the H0 tension, as well as the discrepancy with the baryon ...acoustic oscillation (BAO) Lymanα data, both which prevail within the Λ CDM model. We then investigate whether two minimal extensions of the Λ CDM model, together or separately, can successfully realize such a scenario: (i) the spatial curvature, which, in the case of spatially closed universe, mimics a negative density source and (ii) simple-graduated dark energy (gDE), which promotes the null inertial mass density of the usual vacuum energy to an arbitrary constant-if negative, the corresponding energy density decreases with redshift similar to the phantom models, but unlike them crosses below zero at a certain redshift. We find that, when the Planck data are not included in the observational analysis, the models with simple-gDE predict interesting and some significant deviations from the Λ CDM model. In particular, a spatially closed universe along with a simple-gDE of positive inertial mass density, which work in contrast to each other, results in minor improvement to the H0 tension. The joint dataset, including the Planck data, presents no evidence for a deviation from spatial flatness but almost the same evidence for a cosmological constant and the simple-gDE with an inertial mass density of order O ( 10−12 ) eV4. The latter case predicts almost no deviation from the Λ CDM model up until today-so that it results in no improvement regarding the BAO Lyα data-except that it slightly aggravates the H0 tension. We also study via dynamical analysis the history of the Universe in the models, as the simple-gDE results in futures different than the de Sitter future of the Λ CDM model.
Screening Λ in a new modified gravity model Akarsu, Özgür; Barrow, John D.; Board, Charles V. R. ...
European physical journal. C, Particles and fields,
10/2019, Volume:
79, Issue:
10
Journal Article
Peer reviewed
Open access
We study a new model of Energy-Momentum Squared Gravity (EMSG), called Energy-Momentum Log Gravity (EMLG), constructed by the addition of the term
f
(
T
μ
ν
T
μ
ν
)
=
α
ln
(
λ
T
μ
ν
T
μ
ν
)
, ...envisaged as a correction, to the Einstein–Hilbert action with cosmological constant
Λ
. The choice of this modification is made as a specific way of including new terms in the right-hand side of the Einstein field equations, resulting in constant effective inertial mass density and, importantly, leading to an explicit exact solution of the matter energy density in terms of redshift. We look for viable cosmologies, in particular, an extension of the standard
Λ
CDM model. EMLG provides an effective dynamical dark energy passing below zero at large redshifts, accommodating a mechanism for screening
Λ
in this region, in line with suggestions for alleviating some of the tensions that arise between observational data sets within the standard
Λ
CDM model. We present a detailed theoretical investigation of the model and then constrain the free parameter
α
′
, a normalisation of
α
, using the latest observational data. The data does not rule out the
Λ
CDM limit of our model (
α
′
=
0
), but prefers slightly negative values of the EMLG model parameter (
α
′
=
-
0.032
±
0.043
), which leads to the screening of
Λ
. We also discuss how EMLG relaxes the persistent tension that appears in the measurements of
H
0
within the standard
Λ
CDM model.
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