Eur. Phys. J. C (2016) 76 :164 Page 3 of 14 164
In this paper, as we consider the antisymmetric part of the
connection, i.e., S
ρ
μν
=
ρ
[μν]
, as well as the symmetric part
of it the last torsional term in the variation (9) comes out as the
starting point of the torsional contribution to the Eddington
gravity, and here we should also notice the tensorial form of
the variation of the connection δ
ρ
μν
[4].
Then our action becomes
δ I
R
=
1
2
d
4
x
|R|
R
−1
νμ
∇
ρ
δ
ρ
μν
−∇
ν
δ
ρ
μρ
−2S
σ
ρν
δ
ρ
μσ
. (10)
We improve Eq. (10) by the integration by parts for the terms
involving covariant derivatives so that we can take advantage
of the identity [4,18]
d
4
x∇
μ
JV
μ
= 2
d
4
xS
μ
JV
μ
, (11)
where J is any scalar density and it corresponds to
√
|R| in
our formalism.
The identity given above is verified by applying the Gaus-
sian theorem to the relation given by combining the covariant
derivative of a scalar density J [4],
∇
ρ
J = ∂
ρ
J −
σ
σρ
J, (12)
with the same derivative of a contravariant vector V
μ
in
which process we consider a vanishing hypersurface inte-
gral.
Thus, by taking advantage of Eq. (11), the variation under
an arbitrary connection becomes
δ I
R
=
d
4
x
−
1
2
∇
ρ
|R|
R
−1
νμ
+
1
2
∇
σ
|R|
R
−1
σμ
δ
ν
ρ
+
|R|
R
−1
νμ
S
ρ
−
|R|
R
−1
σμ
S
σ
δ
ν
ρ
−
|R|
R
−1
σμ
S
ν
ρσ
δ
ρ
μν
,
(13)
for which the principle of least action, δ I
R
= 0, results in
the most general field equations:
∇
ρ
|R|
R
−1
νμ
−∇
σ
|R|
R
−1
σμ
δ
ν
ρ
−2
|R|
R
−1
νμ
S
ρ
+ 2
|R|
R
−1
σμ
S
σ
δ
ν
ρ
+2
|R|
R
−1
σμ
S
ν
ρσ
= 0. (14)
One may go further than Eq. (14) by carrying out the con-
traction with respect to the indices ρ and ν such that it results
in
∇
σ
|R|
R
−1
σμ
=
4
3
|R|
R
−1
σμ
S
σ
, (15)
which, after plugging it into the most general field equations,
leads to
∇
ρ
|R|
R
−1
νμ
− 2
|R|
R
−1
νμ
S
ρ
+
2
3
|R|
R
−1
σμ
S
σ
δ
ν
ρ
+ 2
|R|
R
−1
σμ
S
ν
ρσ
= 0.
(16)
Here, one more thing to obtain the desired tensorial field
equations is to get rid of the scalar density
√
|R|. With this
aim, multiplying the last equation by R
μν
and implementing
to the first term partial differentiation with the fact that
∇
ρ
|R|=−
1
2
|R|R
μν
∇
ρ
R
−1
νμ
, (17)
we obtain
∇
ρ
|R|=
8
3
|R|S
ρ
. (18)
After checking, one may realize that Eq. (17) is naturally
compatible with Eq. (12) in which we regard the Jacobi for-
mula that the differential of a determinant [19], say Det
[
A
]
(such as Det
R
αβ
in our notation) is equivalent to the trace
of the adjoint of a matrix A multiplied by its differential d A,
that is,
d
(
Det
[
A
]
)
= Det
[
A
]
Tr
A
−1
d A
. (19)
Then after using Eq. (18)inEq.(16), we obtain the field
equations with the inverse Ricci tensor such that
∇
ρ
R
−1
νμ
+
2
3
S
ρ
R
−1
νμ
+
2
3
S
σ
δ
ν
ρ
R
−1
σμ
+2S
ν
ρσ
R
−1
σμ
= 0. (20)
After multiplying with R
μκ
and R
ξν
, the last equation can
also be expressed as
∇
ρ
R
μν
−
2
3
S
ρ
R
μν
−
2
3
S
ν
R
μρ
− 2S
σ
ρν
R
μσ
= 0. (21)
We are now in a position to show significant effects of the
tensorial field equations (21) on our Ricci tensor, where the
situation is now clearly different from Eddington gravity due
to the torsion, i.e. the antisymmetric part of the connection,
contributions. Let us first see the case by multiplying Eq. (21)
by the inverse Ricci tensor
R
−1
νμ
as follows:
R
−1
νμ
∇
ρ
R
μν
=
16
3
S
ρ
, (22)
which can also be seen from Eqs. (17) and (18). Then we
have
R
−1
νμ
∂
ρ
R
μν
=
16
3
S
ρ
+ 2
β
βρ
. (23)
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