Physics Letters B 797 (2019) 134848
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Cosmological solutions of a nonlocal square root gravity
I. Dimitrijevic
a
, B. Dragovich
b,c,∗
, A.S. Koshelev
d
, Z. Rakic
a
, J. Stankovic
e
a
Faculty of Mathematics, University of Belgrade, Studentski trg 16, Belgrade, Serbia
b
Institute of Physics, University of Belgrade, Belgrade, Serbia
c
Mathematical Institute, Serbian Academy of Sciences and Arts, Belgrade, Serbia
d
Departamento de Física and Centro de Matemática e Aplicações, Universidade da Beira Interior, 6200 Covilhã, Portugal
e
Teacher Education Faculty, University of Belgrade, Kraljice Natalije 43, Belgrade, Serbia
a r t i c l e i n f o a b s t r a c t
Article history:
Received
30 June 2019
Received
in revised form 6 August 2019
Accepted
7 August 2019
Available
online 9 August 2019
Editor:
M. Trodden
In this paper we consider modification of general relativity extending R −2 by nonlocal term of the
form
√
R −2 F (2)
√
R −2, where F (2) is an analytic function of the d’Alembert operator 2. We
have found some exact cosmological solutions of the corresponding equations of motion without matter
and with = 0. One of these solutions is a(t) = A t
2
3
e
14
t
2
, which imitates properties similar to an
interplay of the dark matter and the dark energy. For this solution we calculated some cosmological
parameters which are in a good agreement with observations. This nonlocal gravity model has not the
Minkowski space solution. We also found several conditions which function F(2) has to satisfy.
© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
Despite of numerous significant phenomenological confirma-
tions
and many nice theoretical properties, General relativity (GR)
[1]is not final theory of gravity. Problems mainly come from quan-
tum
gravity, cosmology and astrophysics. For example, if GR (as
Einstein theory of gravity) is applicable to the universe as a whole
and the universe is homogeneous and isotropic, then it follows that
the universe contains about 68% of dark energy (DE), 27% of dark
matter (DM) and only about 5% of visible matter. However, valid-
ity
of GR at the cosmological scale is not verified, as well as DE
and DM are not yet observed in laboratory experiments. Also, from
GR follows cosmological singularity. These and other problems give
rise to extensions of GR. Note that there is no firm theoretical
principle which could tell us in which direction to look for a so-
lution.
Therefore, there are many attempts to modify GR, e.g. see
review articles [2–6]. One of actual approaches to modification of
GR in domain of cosmology is nonlocal modified gravity, see e.g.
[7–25].
All
nonlocal gravity models contain the gravitational d’Alembert
operator 2, which mainly appears in two ways: 1) in the form of
an analytic function F (2) =
+∞
n=0
f
n
2
n
and 2) in the form 2
−1
.
Models with inverse d’Alembert operator are considered as possi-
*
Corresponding author.
E-mail
address: dragovich@ipb.ac.rs (B. Dragovich).
ble explanation of the late cosmic acceleration without use of a
dark matter. Usually the form of such models is
S =
1
16π G
√
−g
(
R + L
nl
)
d
4
x, (1)
where, for examples: L
nl
= R f (2
−1
R) (see [4,8,22] and references
therein), and L
nl
=−
1
6
m
2
R2
−2
R (see [10] and references therein),
and R is the scalar curvature.
Usage of F (2) =
+∞
n=0
f
n
2
n
is influenced by existence of ana-
lytic
expressions with 2 in string field theory and p-adic string
theory (see [26] and references therein). Some modified gravity
models with analytic nonlocality, which have been so far consid-
ered,
are concrete examples of the action (without matter)
S =
1
16π G
√
−g(R − 2 + P (R) F (2) Q (R))d
4
x, (2)
where is the cosmological constant, P (R) and Q (R) are some
differentiable functions of R. For a better insight into nonlocal
effects, preliminary consideration of these models is usually with-
out
matter. The most attention has been paid to the case when
P(R) = Q (R) = R, e.g. see [11,12,16,18,27–34].
Nonlocal gravity model which we investigate in this paper has
P(R) = Q (R) =
√
R −2 and action is given explicitly below in
(3).
https://doi.org/10.1016/j.physletb.2019.134848
0370-2693/
© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.