Slowly Rotating Black Holes in 4D Einstein Gauss-Bonnet Gravity Michael Gammonand Robert B. Mann

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Slowly Rotating Black Holes
in 4D Einstein Gauss-Bonnet Gravity
Michael Gammonand Robert B. Mann
University of Waterloo
Department of Physics
(Dated: April 2, 2024)
Since the recent derivation of a well-defined D4 limit for regularized 4D Einstein Gauss-Bonnet
(4DEGB) gravity, there has been considerable interest in testing it as an alternative to Einstein’s
general theory of relativity. In this paper we construct slowly rotating black hole solutions for
4DEGB gravity in asymptotically flat, de Sitter, and anti-de Sitter spacetimes. At leading order
in the rotation parameter, exact solutions of the metric functions are derived and studied for all
three of these cases. We compare how physical properties (innermost stable circular orbits, photon
rings, black hole shadow, etc.) of the solutions are modified by varying coupling strengths of the
4DEGB theory relative to standard Einstein gravity results. We find that a vanishing or negative
cosmological constant in 4DEGB gravity enforces a minimum mass on the black hole solutions,
whereas a positive cosmological constant enforces both a minimum and maximum mass with a
horizon root structure directly analogous to the Reissner-Nordstr¨om de Sitter spacetime. Besides
this, many of the physical properties are qualitatively similar to general relativity, with the greatest
deviations typically being found in the low (near-minimal) mass regime.
I. INTRODUCTION
Despite the empirical success and predictive power of
Einstein’s general theory of relativity (GR), its modifi-
cations continue attract attention. Early attempts can
be seen in the writings of Weyl [1] or Eddington [2], and
continue through to present day, motivated by the sin-
gularity problem (ie. geodesic incompleteness), the need
for reconciling quantum physics and gravity, and the need
for phenomenological competitors so as to test GR in the
most stringent manner possible.
A common class of modifications to GR are higher cur-
vature theories (or HCTs), in which it is assumed that
a sum of powers of the curvature tensor is proportional
to stress-energy, extending the assumed linear relation-
ship between spacetime curvature (the Einstein tensor)
and stress-energy in GR. Logically this linear relation-
ship is not required, and it is conceivable that the empir-
ical success of Einstein’s theory could be improved upon
by modifying the left-hand-side of the Einstein equations
with a sum of powers of the curvature tensor.
These HCTs play important roles in many different ar-
eas of physics - they appear in a number of proposals for
quantum gravity [3], and may be necessary to account
for observational evidence of dark matter, early-time in-
flation, or late-time acceleration [4]. Lovelock theories
[5] are the best-known examples of HCTs, and have the
distinct feature that their differential equations are of
2nd order. However, until recently, their equations only
had non-trivial solutions in spacetime dimensions larger
than four (D > 4) [6], so their physical significance had
been unclear. Recently, a new class of HCTs have been
mgammon@uwaterloo.ca
rbmann@uwaterloo.ca
proposed that do allow for higher-order gravity in four
dimensions and satisfy reasonable physical requirements
such as positive energy excitations on constant curva-
ture backgrounds. Such higher curvature theories are
referred to as “generalized quasi-topological gravities”,
or GQTGs [6]. The original HCT was cubic in curvature
[5, 7]; it was soon followed by a class quartic in curvature
[3]. Most recently a procedure was found for construct-
ing physically reasonable HCTs to any desired power of
curvature [6].
Amidst the plethora of higher curvature gravity theo-
ries, the quadratic Lovelock theory, or so-called “Einstein
Gauss-Bonnet” gravity has been of special interest. In
addition to having 2nd order equations of motion, it is
the simplest HCT. For a long time it was thought that
the Gauss-Bonnet (GB) action term
SGB
D=αZdDxgRµνρτ Rµνρτ 4Rµν Rµν +R2
αZdDxgG
(1)
could not contribute to a system’s gravitational dynam-
ics in D4 (since it becomes a total derivative in such
cases), and hence the GB contribution (1) is often re-
ferred to as a topological term of no relevance. Further-
more, a new 3+1-dimensional gravity theory which pos-
sesses diffeomorphism invariance, metricity, and second
order equations of motion would be a violation of the
Lovelock theorem [5] and thus should only be possible
by introducing an additional field into the theory besides
the metric tensor.
In 2020, Glavan and Lin [8] claimed to have bypassed
the Lovelock theorem via the following rescaling of the
Gauss-Bonnet coupling constant:
(D4)αα, (2)
arXiv:2210.01909v3 [gr-qc] 30 Mar 2024
2
and then taking the D4 limit of exact solutions
to Einstein-Gauss-Bonnet gravity. This interesting ap-
proach allowed for non-vanishing contributions from the
GB action term in D= 4. In doing so, a number of 4-
dimensional metrics can be obtained (for spherical black
holes [8–12], cosmological solutions [8, 13, 14], star-like
solutions [15, 16], radiating solutions [17], collapsing so-
lutions [18], etc.) carrying imprints of higher curvature
corrections inherited from their D > 4 counterparts.
Unfortunately the existence of limiting solutions does
not actually imply the existence of a 4D theory, and a
number of objections in this vein quickly appeared [19–
21]. However, the conclusion that there ultimately was no
four-dimensional Gauss-Bonnet theory of gravity proved
to be premature when it was shown that a D4 limit
of the action (2) could be taken [22, 23], generalizing
a previous procedure for taking the D2 limit of GR
[24]. It is also possible to employ a Kaluza-Klein-like pro-
cedure [25], compactifying D-dimensional Gauss-Bonnet
gravity on a (D4)-dimensional maximally symmetric
space and then re-scaling the coupling constant according
to equation (2). The resultant 4D scalar-tensor theory is
a special case of Horndeski theory [26], which surprisingly
has spherical black hole solutions whose metric functions
match those from the na¨ıve D4 limiting solutions de-
rived by Glavan & Lin [8]. Such solutions can be obtained
without ever referencing a higher dimensional spacetime
[22].
This theory, referred to as 4D Einstein Gauss-Bonnet
gravity, provides an interesting phenomenological com-
petitor to GR [27]. Conformally transforming the metric
gµν e2ϕgµν in (1) and subtracting it from the original
GB action (1) yields the 4DEGB action after trivial field
redefinitions: [22]
SG
4=αZd4xghϕG+ 4Gµν µϕνϕ4(ϕ)2ϕ
+ 2(ϕ)4i(3)
using (2), where ϕis an additional scalar field. No further
assumptions about particular solutions to higher dimen-
sional theories or background spacetimes are required.
Adding to this the Einstein-Hilbert action with a cos-
mological term,
S=Zd4xg[R2Λ] + SG
4(4)
the equations of motion follow from the standard varia-
tional principle, with that for the scalar being given by
Eϕ=−G + 8Gµν νµϕ+ 8Rµν µϕνϕ8(ϕ)2
+ 8(ϕ)2ϕ+ 16aϕνϕνµϕ+ 8νµϕνµϕ
= 0
(5)
and the variation with respect to the metric yields
Eµν = Λgµν +Gµν +α"ϕHµν 2R[(µϕ) (νϕ) + νµϕ]
+ 8Rσ
(µν)σϕ+ 8Rσ
(µν)ϕ(σϕ)2Gµν (ϕ)2+ 2ϕ
4 [(µϕ) (νϕ) + νµϕ]ϕ+ 8 (µϕν)σϕσϕ
gµν (ϕ)24 (µϕ) (νϕ)(ϕ)2+ 4 (σνϕ) (σµϕ)
4gµν Rσρhσρϕ+ (σϕ) (ρϕ)i2gµν (σρϕ) (σρϕ)
4gµν (σϕ) (ρϕ) (σρϕ)+4Rµνσρ [(σϕ) (ρϕ) + ρσϕ]
+ 2gµν (ϕ)2#= 0
(6)
where His the Gauss-Bonnet tensor:
Hµν = 2hRRµν 2Rµανβ Rαβ +RµαβσRαβσ
ν2RµαRα
ν
1
4gµν RαβρσRαβρσ 4Rαβ Rαβ +R2i.
(7)
These field equations satisfy the following relationship
0 = gµν Eµν +α
2Eϕ= Rα
2G(8)
which can act as a useful consistency check to see whether
prior solutions generated via the Glavin/Lin method are
even possible solutions to the theory. For example, using
(8) it is easy to verify that the rotating metrics generated
from a Newman-Janis algorithm [11, 28] are not solutions
to the field equations of the 4DEGB theory.
Despite much exploration of the theory [29], there has
been relatively little work investigating rotating black
hole solutions. Attempts have been made using a na¨ıve
rescaling of the 4DEGB coupling constant [30], or by
implementing the Newman-Janis approach [11, 28, 31],
neither of which produce valid solutions to this theory in
general (the latter case not satisfying the positive energy
condition). A notable exception is a recently obtained
class of asymptotically flat slowly rotating black hole so-
lutions [16] that were obtained for the 4DEGB theory.
However, a detailed study of the geodesics of particles
surrounding such black holes (particularly in de Sitter
and anti-de Sitter spacetimes) was not done.
In this paper we address this issue, obtaining slowly
rotating black hole solutions to the field equations of the
4DEGB theory in asymptotically flat/(A)dS space. We
then analyze their physical properties (innermost stable
circular orbits, photon rings, black hole shadow, etc.)
to see how they differ from standard results in Einstein
gravity. We find for Λ 0 that 4DEGB gravity enforces
a minimum mass on the black hole solutions, whereas
for Λ >0 both a minimum and maximum mass occur,
whose horizon structure is directly analogous to that of
Reissner-Nordstr¨om de Sitter spacetime. Besides this,
the results are similar in form to general relativity. The
incredible empirical success of GR necessitates this simi-
3
larity of solutions for a correct theory, but makes differen-
tiation via measurement difficult. We find that the great-
est deviations from GR are typically in the low (near-
minimal) mass regime, motivating a search for the small-
est observable astrophysical black holes.
II. SOLUTIONS
A. Metric Functions
To construct slowly rotating solutions for the new
4DEGB theory, we begin with the following metric
ansatz:
ds2=f(r)dt2+dr2
h(r)+ 2ar2p(r) sin2θdtdϕ
+r22+ sin2θdϕ2
(9)
where ais a small parameter governing the rate of rota-
tion. Of particular interest are Schwarzschild-like solu-
tions where h(r) = f(r). With this, and the substitution
x= cos θ, our line element can be written
ds2=f(r)dt2+dr2
f(r)+ 2ar2p(r)(1 x2)dtdϕ
+r2hdx2
1x2+ (1 x2)2i.
(10)
Inserting this into the equations of motion eqs. (5)
and (6) and considering the combination E0
0− E1
1, we
derive the following equation for the scalar field
(ϕ2+ϕ′′)(1 (rϕ1)2f) = 0 (11)
which admits either the solution ϕ= ln( rr0
l) (with r0, l
integration constants), or the solutions
ϕ±=Zf±1
fr dr, (12)
where the latter solution with a minus sign reproduces
previous results [8] in the spherically symmetric case, and
falls off as 1/r when Λ = 0. We shall choose this solution
henceforth.
Using (11) we can solve for the metric function f(r)
from the geometric expression (8). It can easily be shown
that
r2(1 f(r)) Λr4
3+αf(r)22αf(r)αC2r+C1= 0.
(13)
As we wish to recover the Schwarzschild-AdS solution
when α= 0, we set C1=αand C2=2M
αyielding
f±= 1 + r2
2α1±r1 + 8αM
r3+4
3αΛ(14)
where the f(or Einstein) branch is the one yielding the
Schwarzschild AdS solution in the limit α0. From
here it is straightforward to show that ϕfalls off as 1/r
when Λ = 0, whereas all other solutions for ϕdiverge
logarithmically at large r.
Remarkably the solution (14) is still valid to leading
order in a. The only remaining independent equation
from (6) to this order is given by E03:
rp′′ 24αM +r3(4αΛ + 3)+4p15αM +r3(4αΛ + 3)= 0
(15)
and admits the exact solution
p=C2C1q1 + 8αM
r3+4
3αΛ
12αM .(16)
We require that the metric match the slowly rotating
Kerr-(A)dS metric function pin the large rlimit. An
expansion of the function f(r) in this limit indicates
that
Λeff =3
2α"r1 + 4αΛ
31#=
1 + q1 + 4αΛ
3
(17)
is the effective cosmological constant, yielding
C1= 2M12αΛ+9 C2=3+4αΛ2αΛeff
6α(18)
This leaves us with the final expression
p(r) =
3
q1 + 4αΛ
3
1 + q1 + 4αΛ
3
(19)
+1
2α"1r1 + 4αΛ
3s1 + 4α
3Λ + 6M
r3#
which, in the Λ = 0 limit, matches1the result [16] for
asymptotically flat spacetime. Additionally we note that
we can rewrite the metric functions in terms of Λeff as
f±= 1 + r2
2α
1±s2
3αΛeff + 12
+8αM
r3
(20)
p(r) = Λeff
32
3αΛeff + 1(21)
+1
2α
12
3αΛeff + 1s2
3αΛeff + 12
+8αM
r3
.
It is straightforward to show that the metric (9) is
asymptotically of constant curvature, with
Rµν Λeff gµν (22)
1up to an errant factor of the rotation parameter included in the
metric function in [16]
4
as r→ ∞.
We pause to comment on the range of validity of the
metric (9) (with f(r) and p(r) given by respectively by
(14) and (19)), which is within the class of generalized
Lense-Thirring metrics recently discussed in [32]. Unlike
Einstein gravity, the solution (9) is characterized by two
metric functions, as was shown for this class [32]. It gen-
eralizes the asymptotically flat case previously obtained
[16] and reduces to the slowly rotating Kerr-(A)dS met-
ric if α= 0. We first note that if M= 0 the metric is
that of a spacetime of constant curvature Λeff in rotating
coordinates, and that if a= 0 the metric is an exact so-
lution to the field equations (5), (6), with event horizons
given by the real solutions of
Λr43r2+ 6Mr 3α= 0 (23)
where f(r) = 0. If Λ >0 there are, in decreasing order
of magnitude, three solutions rc,r+, and rto (23); if
Λ<0 only the latter two are present. In both cases r+is
the outer horizon of the black hole. Note that the values
of these solutions to (23) are independent of a, as is the
case for the Kerr solution to leading order in a, though
the latter has only a single horizon in this approximation.
Using analytic continuation via a Kruskal-type extension
to continue to values of r < r+, our solution will be
valid for all values of r > rprovided a << r. In
analyzing the structure of the metric, we shall make this
assumption2. However phenomenologically we need only
consider physics in regions where r > r+, in which case
it is sufficient to require a << r+, ensuring that the g
component of the metric remains small in this region.
For α= 0 = Λ this criterion becomes a < 2m.
B. Analytic Properties
The slowly rotating 4DEGB metric (10), with pgiven
by (19) and f=hgiven by (14), is singular near r= 0,
which can be seen by computing the Ricci scalar to lowest
order in r. In doing so we find that
R(r)r015
4r2M
αr3(24)
regardless of the value of Λ. We consider here only solu-
tions where this singularity is behind the horizon. Com-
puted explicitly to leading order in the rotation parame-
ter, the Kretschmann scalar is
K=f′′(r)2+4f(r)2
r2+4(f(r)1)2
r4,(25)
2It is possible to consider values of the scalar field for r < r+
if the solution (12) is extended to the time-dependent solution
ϕ=qt +Rff+q2r2
f r dr where qis a constant of integration.
Remarkably this does not affect the solution for the metric [16].
and the only other condition leading to a curvature sin-
gularity is
4αΛ + 24αM
r3+ 3 = 0 (26)
which always occurs inside the horizon and is never true
since the quantity 4αΛ+30 in the allowed region of
(α, Λ) parameter space (see section III). Therefore the
4DEGB metric is regular everywhere but the black hole
singularity.
The metric (9) has two Killing vectors ξ(t)=/∂t and
ξ(ϕ)=/∂ϕ. It can be shown easily that parameter
M=MK≡ Q(ξ(t)) is the Komar mass, where
Q(ξ) = 1
8πZΣ
⋆K =c
8πZΣ
+32
eff (27)
is the Komar charge [33] associated with the Killing form
ξ=ξµdxµ(with the Hodge dual); the normalization
constant c=1
2q1 + 4
3αΛ, and Σ is the 2-surface of
constant tat spatial infinity. Likewise, it is straightfor-
ward to show that J= 2Mac is the angular momentum
associated with the Komar charge Q(ξ(ϕ)).
For black hole solutions, the scalar field (12) is ill-
defined inside the horizon where f(r)<0. Such solutions
can be made regular across the horizon [16]. As we are
primarily interested in the phenomenological aspects of
the slowly rotating solutions, we shall focus on exterior
solutions in the sequel.
The structure of the field equations is such that all
quantities can be rescaled into a unitless form, relative
to some length scale. Writing
Λ = ±3
L2(28)
where the positive and negative branches correspond to
de Sitter and anti-de Sitter space respectively, and Lis
the Hubble length, we shall perform the rescalings:
α¯αL2M¯
ML r ¯rL a ¯aL (29)
in units of L. The 4DEGB metric functions then become
¯
f(¯r) = 1 + ¯r2
2¯α 1r1±4¯α+8¯α¯
M
¯r3!(30)
and r2p(r)¯r2¯p(¯r), where
¯p(¯r) = ±21±4¯α
1 + 1±4¯α+1
2¯α 11±4¯αr1±4¯α+8¯α¯
M
¯r3!.
(31)
If instead we consider an asymptotically flat spacetime,
we can directly set Λ = 0 in eqs. (14) and (19) and can
rescale all quantities in units of some fiducial mass. In
what follows we shall take this to be a solar mass.
5
III. PROPERTIES OF THE SOLUTION
In this section we study physical properties of the so-
lutions derived above. We discuss the location and an-
gular velocity of the black hole horizons, the equatorial
geodesics – including the innermost stable circular orbit,
photon rings (and associated Lyapunov exponents) – as
well as the black hole shadow. In each case we compare
the properties of the 4DEGB solution for multiple val-
ues of the coupling constant to the analogous GR result,
and discuss how the rotational corrections affect these
properties.
A. Location and Angular Velocity of the Black
Hole Horizons
The angular velocity of the black hole horizon is de-
fined as
h=g
gϕϕ |r=rh=ap(rh) (32)
where rhis the radius at which grr diverges (ie. f(rh) =
0).
To determine the locations of the horizons, it is conve-
nient to rewrite the metric function f(r) as
f(r) = r2
α
F(r)
f+(r)(33)
where
F(r)=12M
r+α
r21
3Λr2.
The denominator f+does not vanish, and the horizons
are given by the roots of the numerator, which obey the
equation
Λr4
h3r2
h+ 6Mrh3α= 0 (34)
which has exact solutions for all values of Λ.
1. Λ=0
In asymptotically flat space, (34) admits the following
simple solutions:
rh=M±pM2α(35)
with r+(the outer horizon) recovering the Schwarzschild
value as α0. Assuming α > 0, this equation sets a
minimum value for black hole mass in the theory, namely
Mmin =α(36)
when Λ = 0. For smaller masses, the metric function
f(r) does not vanish anywhere and thus no horizon exists.
These solutions have naked singularities.
Since we have an exact solution for p(r) from (19) and
a simple analytic form for rh, the angular velocity of the
horizon
h=a
1r1 + 8αM
(M+M2α)3
2α(37)
is straightforward to compute. In figure 1 we plot
χ(in
units of inverse seconds) as a function of M(in units of
solar mass) for a variety of values of αalongside the GR
solution for comparison (where χ=a/M). The main
new feature introduced by the 4DEGB theory is the ex-
istence of a maximal angular velocity, indicated by the
termination points of the blue curves at any given α, due
to the presence of a minimum mass. As αincreases, this
maximal value decreases.
For a fixed αwe observe that the 4DEGB theory pre-
dicts a significantly larger angular velocity at a given
(small) mass than does GR, but quickly converges to the
GR result when the mass is large.
0.0 0.5 1.0 1.5 2.0
M(M
)0
2×108
4×108
6×108
8×108
Ω/χ(s-1)
FIG. 1: Angular velocity of the black hole horizon as a
function of mass when Λ = 0 for α/M 2
= 0.01, 0.1, 0.2,
0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 (in blue from left to right)
plotted against the Einstein (α= 0) solution Ω = χ/4M
(in red).
2. AdS (Λ<0)
Solutions to (34) yield rather cumbersome expressions
for the two different positive values of rh. For this reason,
we solve numerically for the horizon as a function of black
hole mass, illustrating the results in figure 2a. As in
the asymptotically flat case, a minimum mass black hole
exists at which the inner and outer horizons merge, given
by
Mmin =q1 + 12αΛp(1 4αΛ)3
3(38)
which yields (36) in the limit Λ 0.
There are also upper and lower bounds on the Gauss-
Bonnet coupling constant αfor any fixed Λ. These are
摘要:

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