Eur. Phys. J. C (2018) 78 :771 Page 3 of 13 771
tensor of matter contents defined by
β
a
= e
−1
δ(e
L
m
)
δe
a
β
and
≡ e
−1
∂
μ
(e∂
μ
).
In Ref. [104], the authors investigated cosmological data
analysis by a suitably chosen function f (φ) = A exp(nφ)
and later, in Ref. [105], using a Noether symmetry approach.
In our paper we will fix f (φ) in another simple/adequate
form in the next section.
3 Cosmological background and effective field
equations
The aim of this section is to write the equations of motion
for a cosmological background in the presence of matter
fields in an effective form. Let us suppose that the non-
singular, physical metric of spacetime is given in the form
of the Friedman–Lemaitre–Robertson–Walker (FLRW) met-
ric, given by ds
2
= dt
2
− a(t)
2
(dx
b
dx
b
), where b =
1, 2, 3 is spatial coordinate and a(t ) is scale factor, mea-
suring the expansion of the whole cosmological Universe
as well as its acceleration/deceleration phase. The corre-
sponding suitable, diagonal tetrad basis is given by e
a
μ
=
diag
(
1, a(t), a(t ), a(t)
)
. The set of FLRW equations and
the equations for the scalar fields are written as follows:
3H
2
(1 + ξ − f (φ)) =
1
2
˙
φ
˙
ξ
+κ(ρ
m
+ ρ
+ ρ
r
), (6)
(2
˙
H + 3H
2
)(1 + ξ − f (φ)) =−
1
2
˙
φ
˙
ξ
+2H(
˙
ξ −
˙
f (φ)) − κ(p
+ p
r
), (7)
¨
ξ + 3H
˙
ξ − 6H
2
f
(φ) = 0, (8)
¨
φ + 3H
˙
φ + 6H
2
= 0. (9)
The matter energy-momentum tensor is given in terms of
a diagonal tensors for matter, dark energy, and radiation as
follows:
τ
ν
μ
= e
a
μ
e
ν
b
τ
b
a
= di ag
ρ
m
+ ρ
+ ρ
r
, −p
−p
r
, −p
− p
r
, −p
− p
r
, (10)
where e
a
μ
e
μ
a
= δ
a
b
is the unit matrix. The matter budget of our
model is the dark matter density ρ
m
, the radiation field den-
sity ρ
r
and the scalar field density ρ
φ
. In order to preserve the
acceleration expansion and the existence of late-time de Sit-
ter cosmology we inserted a nonzero cosmological constant
with energy density ρ
. In an attempt to keep simplicity
we assume that all matter contents are given in barotropic
forms, where we define the equation of state (EoS) param-
eter w
a
for each fluid component, namely matter, radiation
and cosmological constant, and as a result for any compo-
nent of matter field we have a linear EoS, i.e., p
a
= w
a
ρ
a
.
Here the Roman index a refers to different matter contents.
Namely we denote it by a ={m,,r}where m is for matter,
is for DE and r is for the radiation field. Note that neither
φ nor ξ are considered as the DE. The reason is that both
fields play the role of auxiliary fields. We cannot guarantee
that the fields φ,ξ will be ghost or not. Actually the appear-
ance of ghost scalar fields in the nonlocal theories for gravity
is an important issue and should be addressed adequately.
For example in the nonlocal extensions of the GR, when the
action is corrected by nonlocal terms
1
R or higher order
terms, one must count the number of degrees of freedom of
the localized form of the Lagrangian. Additionally, one needs
to check the equivalence between local and nonlocal repre-
sentations of the theory both at the action level and equation
of motion levels. It is possible to make a categorization based
on the first form of the auxiliary fields. Based on this classifi-
cation we can find the number of algebraic constraints, which
will limit our ability to write the local or nonlocal represen-
tations of theory. However, in nonlocal extensions of GR, as
long as we have a linear term, we can ascertain the equiva-
lence between frames. However, with higher order terms this
equivalence is broken. That means in a general nonlocal GR
when we have only curvature terms, our theory may suffer
from ghosts. In nonlocal extensions of teleparallel gravity
(TEGR), we can deduce the same as long as we develop the
theory using a nonlocal action made by linear scalar torsion
T , and, hence, no ghost will appear. That is because it was
proved that the Einstein–Hilbert action of GR is dynamically
equivalent to the TEGR at the level of the action as well as
the equations of motion [106]. In our study with the nonlocal
term which we will opt for in next paragraph the model will
be ghost free. Although probably the scalar field φ will not
be a ghost, still we do not have any strong reason to keep it as
the only sector for the acceleration expansion in our model.
For this reason we also keep the cosmological constant
and its energy density ρ
.
The first challenge is to choose one suitable form for f (φ).
Note that ξ =−f
(φ)φ. It is illustrative to expand this
equation and write it in the following equivalent form:
(ξ + f (φ)) = f
(φ)φ.
Here, we suppose that f
(φ) = 0. Thus one suitable class
of models is
f (φ) = Aφ + B. (11)
Note that for A = 0, B = 2 the results reduce to GR as a
limiting case. Note that now (ξ + f (φ)) = 0, and we have
the freedom to take ξ + f (φ) = , where is a harmonic
function over R
4
. A possible option is to consider = 2 Aφ;
consequently the set of Eqs. (6) and (7) are simply written in
the following forms:
3H
2
=
1
2
A
˙
φ
2
+ κ(ρ
m
+ ρ
+ ρ
r
)
1 − 2B
, (12)
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