arXiv:1802.06778v2 [gr-qc] 21 Feb 2018

Letters in High Energy Physics

LHEP xx, xxx, 2018

Quasi-Newtonian Cosmological Models in Scalar-Tensor Theories of Gravity Heba Sami and Amare Abebe Center for Space Research, North-West University, South Africa Department of Physics, North-West University, South Africa

arXiv:1802.06778v2 [gr-qc] 21 Feb 2018

Abstract In this contribution, classes of shear-free cosmological dust models with irrotational fluid flows will be investigated in the context of scalar-tensor theories of gravity. In particular, the integrability conditions describing a consistent evolution of the linearised field equations of quasi-Newtonian universes are presented. We also derive the covariant density and velocity propagation equations of such models and analyse the corresponding solutions to these perturbation equations. Keywords: f ( R) gravity, scalar field, quasi-Newtonian cosmologies, perturbations

Einstein-Hilbert action [5, 6] as Z  p  1 S f ( R) = d4 x − g f ( R ) + 2L m , 2

1. INTRODUCTION

(1.1)

where L m is the matter Lagrangian and g is the determinant of the metric tensor gµν . Another modified theory of gravity is the scalar-tensor theory of gravitation. This is a broad class of gravitational models that tries to explain the gravitational interaction through both a scalar field and a tensor field. A subclass of this theory, known as the action the Brans-Dicke (BD) theory, has an action of the form   Z p 1 ω S BD = d4 x − g φR − ∇µ φ∇µ φ + 2L m , (1.2) 2 φ

Although general relativity theory (GR) is a generalization of Newtonian Gravity in the presence of strong gravitational fields, it has no properly defined Newtonian limit on cosmological scales. Newtonian cosmologies are an extension of the Newtonian theory of gravity and are usually referred to as quasi-Newtonian, rather than strictly Newtonian formulations [1, 2, 3]. The importance of investigating the Newtonian limit for general relativity on cosmological contexts is that, there is a viewpoint that cosmological studies can be done using Newtonian physics, with the relativistic theory only needed for examination of some observational relations [1]. General relativistic quasi-Newtonian cosmologies have been studied in the context of large-scale structure formation and non-linear gravitational collapse in the late-time universe. This despite the general covariant inconsistency of these cosmological models except in some special cases such as the spatially homogeneous and isotropic, spherically symmetric, expanding (FLRW) spacetimes. Higher-order or modified gravitational theories of gravity such as f ( R) theories of gravity have been shown to exhibit more shared properties with Newtonian gravitation than does general relativity. In [1], a covariant approach to cold matter universes in quasi-Newtonian cosmologies has been developed and it has been applied and extended in [2] in order to derive and solve the equations governing density and velocity perturbations. This approach revealed the existence of integrability conditions in GR. In this work, we derive the evolution of the velocity and density perturbations in the comoving (Lagrangian) and quasi-Newtonian frames. We investigate the existence of integrability conditions of a class of irrotational and shear-free perfect fluid cosmological models in the context of scalar-tensor gravity. Such work has been done in the context of f ( R) gravity [4], where some models of f ( R) gravity have been shown to exhibit Newtonian behaviour in the shear-free regime.

where φ is the scalar field and ω is a coupling constant considered to be independent of the scalar field φ. An interesting aspect of f ( R) theories of gravity is their proven equivalence with the BD theory of gravity [6, 7] with ω = 0. If we define the f ( R) extra degree of freedom 1 as φ ≡ f′ −1 ,

(1.3)

then the actions 1.1 and 1.2 become dynamically equivalent. In a FLRW background universe, the resulting non-trivial field equations lead to the following Raychaudhuri and Friedmann equations that govern the expansion history of the Universe [8]: h 1 ˙ + 1 Θ2 = − Θ µ m + 3pm + f − R(φ + 1) + Θ φ˙ 3 2( φ + 1) ′  φ˙ 2  ′′ φ˙ φ˙ i ¨ −3 ′ , + 3 φ +3φ φ φ′ 2 Θ2 =

i 3 h R ( φ + 1) − f µm + + Θφ˙ , ( φ + 1) 2

(1.4)

(1.5)

where Θ ≡ 3H = 3 aa˙ , H being the Hubble parameter, a(t) is the scale factor, µ m and pm are the energy density and isotropic pressure of standard matter, respectively. The linearised thermodynamic quantities for the scalar field are the energy density µ φ , the pressure pφ , the energy flux

f ( R) and scalar-tensor models of gravitation The so-called f ( R) theories of gravity are among the simplest modification of Einstein’s GR. These theories come about by a straightforward generalisation of the Lagrangian in the

1 ′

f , f ′′ , etc. are the first, second, etc. derivatives of f w.r.t. the Ricci scalar R.

1

Letters in High Energy Physics φ

LHEP xx, xxx, 2018 φ

q a and the anisotropic pressure π ab , respectively given by  i 1 h1 ˜ 2φ , R(φ + 1) − f − Θ φ˙ + ∇ ( φ + 1) 2 ′  φ˙ φ˙ 1 h1 f − R(φ + 1) + φ¨ − ′ pφ = ( φ + 1) 2 φ ′′ 2 i 2 φ φ˙ ˜ 2 φ) , + ′ 2 + (Θφ˙ − ∇ 3 φ µφ =

φ

′ 1 h φ˙ 1 i˜ Θ ∇a φ , ′ − ( φ + 1) φ 3 ′  ˙ i h φ ˜ ha∇ ˜ bi R − σab φ′ = ∇ . ( φ + 1) φ

qa = − φ

π ab

and the linearised constraint equations are given by ˜ h a Abi − 1 π ab = 0 , C0ab ≡ E ab − ∇ 2

(1.22)

˜ b σ ab − η abc ∇ ˜ b ωc − 2 ∇ ˜ a Θ + qa = 0 , C1a ≡ ∇ 3

(1.23)

˜ a ωa = 0 , C2 ≡ ∇

(1.24)

(1.8)

˜ c σd + ∇ ˜ h a ω bi − H ab = 0 , C3ab ≡ ηcd( ∇ b)

(1.25)

(1.9)

˜ π ab − 1 ∇ ˜ a η + 1 Θq a = 0 , ˜ b E ab + 1 ∇ C5a ≡ ∇ 2 b 3 3

(1.26)

˜ b H ab + (µ + p)ω a + 1 η abc ∇ ˜ b qa = 0 . Cba ≡ ∇ 2

(1.27)

(1.6) (1.7)

The total (effective) energy density, isotropic pressure, anisotropic pressure and heat flux of standard matter and scalar field combination are given by µm + µφ , ( φ + 1) m π ab φ ≡ + π ab , ( φ + 1)

µ≡ π ab

2. QUASI-NEWTONIAN SPACETIMES

pm + pφ , ( φ + 1) qm φ a qa ≡ + qa . ( φ + 1) p≡

If a comoving 4-velocity u˜ a is chosen such that, in the linearised form u˜ a = u a + v a ,

Covariant equations

(1.10)

pi m ab = σab = Eab = 0 = Hab ,

(1.12)

(1.11)

where Θ, A a , ω a , and σab are the expansion, acceleration, vorticity and the shear terms. Eab and Hab are the “gravito-electric” and “gravito-magnetic” components of the Weyl tensor Cabcd a as defined from the Riemann tensor Rbcd R [ a b] g g , 3 [c d]

(1.13)

1 ≡ ηae gh Cghbd u e u d . 2

(1.14)

C ab cd = R ab cd − 2g[ a [c Rb] d] + g h

Eab ≡ Cagbh u u ,

Hab

pm = 0 ,

(2.1)

qm a = µm va ,

m π ab = 0 , ωa = 0 ,

σab = 0 .

(2.2)

The gravito-magnetic constraint Eq. 1.25 and the shear-free and irrotational condition 2.2 show that the gravito-magnetic component of the Weyl tensor automatically vanishes: H ab = 0 .

(2.3)

The vanishing of this quantity implies no gravitational radiation in quasi-Newtonian cosmologies, and Eq. 1.27 together with Eq. 2.2 show that q m a is irrotational and thus so is v a :

The covariant linearised evolution equations in the general case are given by [2, 3] ˙ = − 1 Θ2 − 1 (µ + 3p) + ∇ ˜ a Aa , Θ 3 2 ˜ a qm µ˙ m = − µ m Θ − ∇ a , 4 m q˙ m a = − Θq a − µ m A a , 3 2 1 ˜ b Ac , ω˙ h ai = − Θω a − η abc ∇ 3 2 1 2 ˜ A , σ˙ ab = − Θσab − Eab + π ab + ∇ h a bi 3 2 ˜ h a qbi ˜ c H i b − ΘE ab − 1 π˙ ab − 1 ∇ E˙ h abi = η cdh a ∇ d 2 2 1 − Θπ ab , 6 h ab i ˜ c E i b + 1 η cdh a ∇ ˜ c π ib , ˙ H = − ΘH ab − η cdh a ∇ d d 2

v a v a