Eur. Phys. J. C (2016) 76:497
DOI 10.1140/epjc/s10052-016-4355-7
Letter
Stability in higher-derivative matter fields theories
Petr V. Tretyakov
1,2,a
1
Bogoliubov Laboratory of Theoretical Physics, Joint Institute for Nuclear Research, Joliot-Curie 6, Dubna 141980, Russia
2
Department of General Relativity and Gravitation, Institute of Physics, Kazan Federal University, Kremlevskaya street 18, Kazan 420008, Russia
Received: 19 August 2016 / Accepted: 1 September 2016 / Published online: 12 September 2016
© The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract We discuss possible instabilities in higher-
derivative matter field theories. These theories have two free
parameters β
1
and β
4
. By using a dynamical system approach
we explicitly demonstrate that for the stability of Minkowski
space in an expanding universe we need the condition β
4
< 0.
By using the quantum field theory approach we also find an
additional restriction for the parameters, β
1
> −
1
3
β
4
, which
is needed to avoid a tachyon-like instability.
1 Introduction
The unsolved problems of General Relativity such as dark
energy [1,2] and dark matter [3] force us to investigate dif-
ferent alternatives. One of the most popular alternatives is
well-known f (R)-gravity [4,5] in the different forms [6–8].
There are also a number of another possibilities to modify
gravity, such as Palatini f (R)-gravity [9], teleparallel grav-
ity [10], Horndeski theory [11], theories with non-minimal
kinetic coupling [12], and so on [13].
A new type of modified gravity was recently proposed
[14]. The higher-derivative matter fields are implied in such
a kind of theory, which can be interpreted as non-dynamical
auxiliary fields,
G
μν
+ g
μν
= T
μν
+ S
μν
(g, T). (1)
The most general form of the term S
μν
if we take into
account terms up to fourth order in derivatives is
S
μν
= α
1
g
μν
T + α
2
g
μν
T
2
+ α
3
TT
μν
+ α
4
g
μν
T
αβ
T
αβ
+α
5
T
α
μ
T
αν
+ β
1
∇
μ
∇
ν
T + β
2
g
μν
T + β
3
T
μν
+2β
4
∇
α
∇
(μ
T
ν)α
, (2)
a
e-mail: tpv@theor.jinr.ru
where since we impose the divergence free condition S
μν
;μ
=
0
1
there are a number of relations between the coefficients.
So finally we have only two independent parameters, β
1
and
β
4
, and Eq. (1) takes the form [14]
G
μν
= T
μν
− g
μν
− β
1
g
μν
T +
1
2
β
4
g
μν
T
αβ
T
αβ
+
1
4
(1 − 2β
1
)(β
1
− β
4
)g
μν
T
2
+[β
4
(1 − 2β
1
) − β
1
]TT
μν
− 2β
4
T
α
μ
T
αν
+β
1
∇
μ
∇
ν
T − β
1
g
μν
T − β
4
T
μν
+2β
4
∇
α
∇
(μ
T
ν)α
. (3)
Some of the terms from Eq. (3) also appear in f (R, T )-
theory [15]. In this sense there may a further generaliza-
tions of such a kind of theories, for instance, in the light of
f (R, T, R
μν
T
μν
)-theory [16] or maybe f (R, T, R
μν
T
μν
,
T
μν
T
μν
)-theory. Note that for a special choice of parame-
ters some of the well-known theories are contained in the
representation (3) as a limit. For instance, the case β
1
= 0,
β
4
=−κ/2 corresponds to EiBI gravity in the small coupling
limit, or β
4
= 0 corresponds to generic Palatini f (R) gravity
[14]. In this sense the theory of (1) also may be interpreted
as some kind of phenomenological theory of modified grav-
ity. Cosmology in such a kind of theories was investigated in
[17,18]. There was obtained an effective dark energy sector
and one found accelerating, de Sitter, and non-accelerating
solutions. Also it was demonstrated that in such a kind of
theories the effective dark energy e.o.s. may be quintessence-
like, cosmological constant-like or even phantom-like. Nev-
ertheless we need to note that the problem of multiple de
Sitter solutions (which was successfully solved in f (R)-
gravity [19]) is unsolved yet, and it seems that it cannot
be solved without some additional terms in the equation.
Generalizations for brane theories were studied in [20–22].
Some interesting remarks about this theory may be found also
in [23].
1
Note that this implies T
μν
;μ
= 0aswell.
123