Eur. Phys. J. C (2018) 78:584 Page 3 of 15 584
α
4
=
β
4
+
1 − α
12
. (4)
U
2
=[K]
2
−[K
2
], (5)
U
3
=[K]
3
− 3[K][K
2
]+2[K
3
], (6)
U
4
=[K]
4
− 6[K]
2
[K
2
]+8[K][K
3
]+3[K
2
]
2
− 6[K
4
].
(7)
α and β are free parameters. K
μ
ν
= δ
μ
ν
−
g
μσ
f
ab
∂
σ
φ
a
∂
ν
φ
b
.
[K]=K
μ
μ
and [K
n
]=(K
n
)
μ
μ
. We will work in unitary
gauge for which the four Stückelberg fields take the form
φ
a
= x
μ
δ
a
μ
. The fiducial metric f
ab
is chosen to be,
f
ab
= diag(0, 0,α
2
g
h
2
0
, h
2
0
), (8)
for the coordinates (t, r, z,ϕ)where α
g
and h
0
are arbitrary
constants.
2.1 Field equations
1. Einstein’s equations
R
μν
−
1
2
Rg
μν
=−m
2
g
X
μν
+ T
μν
(9)
where the full expression for X
μν
can be found in [11,17].
The energy-momentum tensor is given by,
T
μν
=∇
μ
∇
ν
−
1
2
g
μν
g
σρ
∇
σ
∇
ρ
+ m
2
s
2
(10)
2. Scalar-field equation
∇
a
∇
a
− m
2
s
= 0. (11)
2.2 Static solutions
The Einstein field equation (9) admits cylindrically symmet-
ric solution or “dRGT black string” in the absence of scalar
field, i.e., T
μν
= 0. The dRGT black string solution is defined
as [17],
ds
2
=−f (r)dt
2
+ f
−1
dr
2
+r
2
α
2
g
dz
2
+r
2
dϕ
2
, (12)
where,
f (r) = α
2
m
r
2
−
4M
α
g
r
+ γ r + , (13)
α
2
m
= m
2
g
(1 + α + β) ≡−
3
, (14)
γ =−
α
2
m
h
0
(1 + 2α + 3β)
1 + α + β
, (15)
=
α
2
m
h
2
0
(α + 3β)
1 + α + β
. (16)
M is mass per unit length in z direction of black string. One
can clearly see that the graviton mass m
g
naturally generates
the effect of cosmological constant. With these definitions,
<0 case associates with α
2
m
> 0 while >0 is obtained
by letting α
2
m
< 0. This is the unique character of black
string in dRGT massive gravity. Since there is no de-Sitter
(dS) analogue of black string in standard four dimensional
general relativity [15]. The presence of linear term γ and
constant term makes black string with positive possible.
As will be shown later, these two branches of black string
solutions have different horizon and asymptotic structures.
Therefore, to determine their QNMs, we need to consider
each branch of solution separately. Note that, in the limit,
γ = 0, = 0,α
g
= α
m
, this metric (12) reduces to four
dimensional black string solution in general relativity found
by Lemos [15].
We shall now investigate the root structure of dRGT black
string with negative α
2
m
. Generally speaking, the metric func-
tion (13) has three zeros. With a proper parameter choice, it is
possible that all three roots will be real number. More specif-
ically, for α
2
m
< 0 there are two positive real roots and one
negative real root. These two positive roots will be treated as
black string’s event horizon r
h
and cosmological horizon r
c
,
where r
h
< r
c
. As an example, the roots structure of met-
ric (13) is displayed in Fig. 1. In this plot, the black string
mass M, cosmological constant α
2
m
, α
g
and are fixed to be
M = 1,α
2
m
=−0.01,α
g
= 1, = 0 respectively. Each
curve represents different values of γ . These black strings
shown in this figure have two positive roots. The inner root
is black string’s event horizon and the outer root is cosmic
horizon. As γ increases, the cosmic horizon increases, but the
event horizon slightly decreases. In our previous work [19],
we showed that the charged dRGT black hole has different
properties ranging from naked singularity to a black hole and
extremal black hole. However, this is not the case for neutral
black string with positive . Without the charge term, the
metric (13) cannot develop a naked singularity or extremal
scenario (for charged black string [17], naked singularity can
exist just like in the black hole case). Note that throughout
this work, will be fixed to zero [19]. However, this setting
prevents us from having black string solution with negative
and vanishing γ .
An example of black string solution with α
2
m
> 0 ( < 0)
can be found in Fig. 2. In this plot, we set the black string
mass, cosmological constant and other constant to be M =
1,α
2
m
= 4,α
g
= 1, = 0 and consider the effect of γ on the
spacetime. As can be seen from the plot, the horizon structure
dramatically changes from the positive case. There is only
one positive real root, i.e., black string’s event horizon, for
vanishing [26]. Unlike the positive case, the black string
exists with all possible value of γ (negative, zero and posi-
tive). We can see that the black string becomes large when γ
is more negative. It is clear from the metric (13) that, the black
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