Quasinormal modes of charged black holes in Einstein-Maxwell-Weyl gravity

  • We study quasinormal modes (QNMs) of charged black holes in the Einstein-Maxwell-Weyl (EMW) gravity by adopting the test scalar field perturbation. We find that the imaginary part of QNM frequencies is consistently negative for different angular parameters l, indicating that these modes always decay and are therefore stable. We do not observe a linear relationship between the QNM frequency ω and parameter p for these black holes, as their charge Q causes a nonlinear effect. We evaluate the massive scalar field perturbation in charged black holes and find that random long lived modes (i.e., quasiresonances) could exist in this spectrum.
  • As is well known, Einstein’s general relativity theory is non-renormalizable [1]. One of the most popular methods is to append all possible quadratic curvature invariants to the usual Einstein-Hilbert action [2], despite the existence of ghost-like modes in this theory. These terms constitute the so-called Einstein-Weyl (EW) gravity [35], which includes the most general Einstein-Hilbert action with quadratic curvature invariants. Subsequently, the non-Schwarzschild black hole (NSBH) solutions were recovered in four dimensional spacetime [3, 4], where they also satisfy the condition of the Ricci scalar (R=0). Since the Ricci scalar R does not vanish in higher dimensional (n>4) spacetime, Lü et al. found only the perturbed numerical solutions in the EW gravity [6]. Employing the continued fraction method, Kokkotas et al. constructed a numerical black hole solution in an analytical form [7], and its Hawking radiation was discussed in Ref. [8]. Furthermore, some new solutions were derived in anti-de Sitter (AdS) spacetime [9, 10].

    Recently, charged black holes were derived in Refs. [11-13] by applying the four dimensional EW theory coupled with an electromagnetic field. According to the ‘seed’ solution, these solutions can be divided into two groups in the Einstein-Maxwell-Weyl (EMW) theory. Group I solutions correspond to the charged extension of the higher derivative curvature for a Schwarzschild black hole. In Group II, the solutions are constructed from the charged generalization of the NSBH solution. The thermodynamic properties of these charged black holes were investigated in Ref. [13].

    The quasinormal modes (QNMs) of a black hole have been a hot research topic for decades. The concept of QNMs was introduced in 1957 by the work of Regge and Wheeler, who investigated the stability of black holes under small perturbations [14]. The study of QNMs is beneficial to the understanding of the structure of black hole spacetimes and could play an important role in the detection of gravitational waves and some fundamental symmetries for the gauge/gravity duality [15, 16]. To date, QNMs have been calculated either by Einstein’s general relativity theory coupled with Maxwell theory [17-19] or nonlinear electrodynamics [2024], or by modified gravities [2528]. The analysis of QNMs of the non-Schwarzschild black holes was performed in the EW gravity [29, 30], where the linear relation between QNM frequencies and the parameter p=r02α was recovered. Inspired by these results, we evaluate the effect of the charge Q on the QNMs and the stability of charged black holes in the Einstein-Maxwell-Weyl gravity. We will calculate the QNMs by considering test massless and massive scalar field perturbations on the charged black holes, respectively.

    This paper is constructed as follows. We first review charged black hole solutions with an increase of charge Q in the EMW gravity in Section 2. Then, we provide a detailed discussion of QNM frequencies under test scalar field perturbations, including massless and massive scalar fields in Section 3. Finally, we provide concluding remarks in the fourth section.

    The action of the Einstein-Weyl gravity, combined with the electromagnetic field, is given by [12, 13]

    I=116πGd4xg[RαCμνρσCμνρσκFμνFμν],

    (1)

    where Fμν=μAννAμ is the electromagnetic tensor. Here, Cμνρσ is the Weyl tensor and the trace-free part of the Riemann tensor with the form [31, 32]

    Cμνρσ=Rμνρσ(gμ[ρRσ]νgν[ρRσ]μ)+13Rgμ[ρRσ]ν,

    where the part within brackets surrounding the indices refers to the anti-symmetric part. To date, various attempts have been made to formulate the Weyl curvature hypothesis in a rigorous way. The simplest choice of a scalar constructed from the Weyl tensor is CμνρσCμνρσ from Eq. (1).

    The equations of motion are obtained as [12, 13]

    Rμν12gμνR4αBμν2κTμν=0,μFμν=0,

    (2)

    where Bμν is the trace-free Bach tensor, and Tμν is energy-momentum tensor of the Maxwell field

    Bμν=(ρσ+12Rρσ)Cμνρσ,Tμν=FαμFαν14gμνFαβFαβ.

    (3)

    We choose the metric ansatz

    ds2=N(r)e2δ(r)dt2+1N(r)dr2+r2(dθ2+sin2θdφ2)

    (4)

    with a metric function N(r) equal to 12m(r)/r. We constructed numerical charged black hole solutions with α=12 and κ=1 [13], according to two neutral scenarios: the Schwarzschild (SBH) and non-Schwarzschild black hole (NSBH) within the bound of 0.363<r0<1.143 in the EW theory. Fig. 1 presents the relation between the mass M and Hawking temperature T for SBH and NSBH scenarios, where both neutral solutions coalesce at T0.091(r00.876). Taking Group I (Fig. 1(a)) as an example, charged black holes were constructed from the Schwarzschild black hole (Q=0) by increasing the charge Q while maintaining the same horizon radius r0. In particular, new charged black holes on both sides of the coalescent point exhibit different properties: as the charge Q increases, their mass becomes larger (smaller) depending on the temperature decrease (increase) on the left (right) hand side. A similar phenomenon is observed in Group II, as shown in Fig. 1(b).

    Figure 1

    Figure 1.  (color online) Mass M versus temperature T for Schwarzschild (SBH), non-Schwarzschild (NSBH), and charged black holes for Group I (1) and Group II (b). The arrow denotes the increase in charge Q.

    In this section, we consider the test massive scalar field ψ propagating on charged black holes, which obeys the Klein-Gordon equation

    (μ2)ψ=0,

    (5)

    where μ is the mass of the scalar field, and ψ can be separated into spherical harmonics, temporal, and radial components

    ψ(t,r,θ,ϕ)=lm1rΨl(r)Ylm(θ,ϕ)eiωt,

    (6)

    where Ylm(θ,ϕ) is a usual spherical harmonic, and l is the angular harmonic index.

    Substituting Eqs. (4) and (6) into Eq. (5), we obtain the radial perturbed Schrödinger equation

    (2r+ω2Vl(r))Ψl(r)=0,

    (7)

    where the effective potential is given as

    Vl=l(l+1)Ne2δr2+Ne2δ(NNδ)r+μ2Ne2δ

    (8)

    and the tortoise coordinate r is adopted as

    drdr=1Neδ.

    (9)

    We consider the massless scalar field perturbation (μ=0) in these charged black holes for Groups I and II. First, we choose appropriate boundary conditions. The respective boundary conditions at the black hole horizon and spatial infinity are

    Ψl(r){eiωr(rr0)iω4πTrr0(r)eiωreiωrr2Miωr(r),

    (10)

    To derive these QN modes ω, we employ the shooting method [33-35]. With the initial condition given by Eq. (10) at the event horizon r0, we solve the perturbed Eq. (7) numerically for each ω using the Wolfram Mathematica® built-in function NDSolve for r0rrf, where rfr0. This solution must also satisfy the boundary condition in Eq. (10) at spatial infinity, if ω is the quasinormal frequency.

    In the Tables 14, we present QNM frequencies (real and imaginary parts) for these charged black holes in Groups I and II. The QNM frequencies (for l=0 and 1) of these neutral black holes (SBH and NSBH) are displayed in the first row of Tables 14. In the case of Q0, the action presented in Eq. (1) becomes neutral [3, 4], recovering the Schwarzschild and non-Schwarzschild solutions in four dimensional spacetime. The fundamental QNM frequencies under a massless scalar field perturbation on the SBH background are given in Refs. [36, 37]. QNM frequencies (for l=0 and 1) of the NSBH with a horizon radius r0>0.876 are provided in Refs. [29, 30]. However, for the NSBH with r0<0.876, there is a discrepancy between QNM frequencies and data obtained from the relation expression pω in Ref. [30]. This is probably because this expression, derived in the bound 0.876<r0<1.143, is not valid in the region of 0.363<r0<0.876 for the NSBH in the EW gravity.

    Table 1

    Table 1.  QNM frequencies of black holes within region of r0 < 0.876 in Group I.
    r0=0.5r0=0.75
    Qω(l=0)ω(l=1)ω(l=0)ω(l=1)
    00.4420-0.4196i1.1716-0.3908i0.2947-0.2797i0.7810-0.2605i
    0.020.445730-0.419802i1.17219-0.39126i0.29504-0.284839i0.781750-0.262508i
    0.060.447649-0.423259i1.17636-0.394006i0.300096-0.291943i0.768417-0.266851i
    0.10.451833-0.431002i1.18541-0.399437i0.305035-0.307715i0.795061-0.269681i
    0.140.458975-0.443435i1.19959-0.408586i0.310574-0.309508i0.806376-0.275657i
    0.180.465102-0.456727i1.21463-0.420002i0.315876-0.317400i0.820854-0.281226i
    0.220.467173-0.461698i1.23312-0.434605i0.318256-0.328687i0.825449-0.304661i
    0.260.481790-0.486744i1.25409-0.445270i0.321404-0.342171i0.830678-0.326003i
    0.300.512190-0.519842i1.27810-0.477936i0.324608-0.355701i0.834912-0.334916i
    0.330.522566-0.532159i1.29662-0.490691i0.330838-0.369421i0.838476-0.351239i
    DownLoad: CSV
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    Table 2

    Table 2.  QNM frequencies of black holes within region of r0 > 0.876 in Group I.
    r0=1.3r0=2
    Qω(l=0)ω(l=1)ω(l=0)ω(l=1)
    00.1700-0.1614i0.4506-0.1503i0.1105-0.1049i0.2929-0.0977i
    0.020.169935-0.160131i0.449981-0.149923i0.110434-0.104754i0.292409-0.0975053i
    0.060.169604-0.158960i0.449018-0.149611i0.110072-0.104079i0.290913-0.0960668i
    0.10.169109-0.157354i0.448207-0.148787i0.108803-0.101841i0.287647-0.0937670i
    0.140.168720-0.155810i0.447165-0.147679i0.106439-0.098191i0.283246-0.0906137i
    0.180.168023-0.153998i0.443449-0.145653i0.103804-0.0933381i0.278075-0.0869171i
    0.220.166891-0.151193i0.441312-0.143235i0.101889-0.0875931i0.272255-0.0830565i
    0.260.166014-0.148333i0.437092-0.140916i0.100331-0.0820946i0.266262-0.0790222i
    0.300.165430-0.144852i0.432538-0.137778i0.098978-0.0771625i0.259892-0.0749341i
    0.330.164395-0.143688i0.429952-0.135908i0.098060-0.0736714i0.254996-0.0719146i
    DownLoad: CSV
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    Table 3

    Table 3.  QNM frequencies of black holes within region of r0 < 0.876 in Group II.
    r0=0.5r0=0.7
    Qω(l=0)ω(l=1)ω(l=0)ω(l=1)
    00.1920-0.05801i0.5650-0.1168i0.2484-0.1550i0.5574-0.4347i
    0.020.191974-0.0578165i0.563943-0.115416i0.247626-0.15488i0.556522-0.433647i
    0.060.191900-0.0575649i0.561869-0.114350i0.245508-0.153018i0.551806-0.407181i
    0.10.191801-0.0572173i0.556545-0.112453i0.242262-0.150052i0.550735-0.410594i
    0.120.191707-0.0569421i0.553771-0.111982i0.240109-0.148196i0.548074-0.403119i
    0.1550.191457-0.0563484i0.547261-0.109390i0.235633-0.144682i0.542076-0.386617i
    0.1850.191279-0.0559977i0.543921-0.104531i0.231188-0.141623i0.536208-0.369774i
    0.2150.190982-0.0555578i0.534453-0.100874i0.226304-0.138735i0.530038-0.353073i
    0.250.188420-0.0548792i0.521852-0.094748i0.220221-0.135954i0.518341-0.344284i
    0.290.185355-0.0536175i0.508852-0.087635i0.213025-0.133582i0.508015-0.333045i
    DownLoad: CSV
    Show Table

    Table 4

    Table 4.  QNM frequencies of black holes within region of r0 > 0.876 in Group II.
    r0=1r0=1.1
    Qω(l=0)ω(l=1)ω(l=0)ω(l=1)
    00.2490-0.3900i0.6920-0.2920i0.2967-0.4970i0.7764-0.3774i
    0.020.248326-0.390859i0.697313-0.292519i0.298359-0.497294i0.78196-0.3774330i
    0.0550.249730-0.398086i0.699694-0.294816i0.300096-0.497879i0.786675-0.377930i
    0.090.251139-0.403061i0.706433-0.300164i0.303839-0.507651i0.795747-0.387478i
    0.1250.258110-0.418834i0.713570-0.308565i0.313144-0.495390i0.795217-0.390079i
    0.160.257806-0.426266i0.722897-0.314524i0.315173-0.529731i0.816878-0.410238i
    0.1950.262761-0.434169i0.731343-0.322654i0.322461-0.538773i0.827657-0.422568i
    0.230.266851-0.445229i0.740003-0.332937i0.328884-0.559389i0.842650-0.437051i
    0.280.279679-0.465540i0.749440-0.341363i0.341379-0.571647i0.860521-0.459423i
    DownLoad: CSV
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    For charged black holes, the imaginary parts are always negative, indicating that these modes always decay and are therefore stable. Moreover, for the charged black holes with r0=0.5 and 0.75 in Group I, the absolute values of the imaginary part (|ωi|) and real part (ωr) of QNM frequencies (l=0,1) both increase with increasing of Q (see Table 1 and Fig. 2(a)). In contrast, the corresponding values of |ωi| and ωr of QNM frequencies for charged black holes with r0=1.3 and 2 in Group I both decrease with increasing of Q (see Table 2 and Fig. 2(b)). Similarly, QNM frequencies of these charged black holes in Group II, located each side of the coalescent point T0.091(r00.876), exhibit different trends from the ones given in Tables 3, 4 and Fig. 3. This is because the mass M and temperature T of these charged black holes affect the boundary conditions of the test scalar field Ψl(r) given in Eq. (10). However, based on the free energies (Fig. 7 in Ref. [13]) and QNM frequencies of these charged and neutral black holes [Figs. 2 and 3], the correlation between thermodynamic phase transitions and dynamical stabilities fails.

    Figure 2

    Figure 2.  (color online) Dependence of fundamental QNM frequencies of charged black hole on charge Q with fixed horizon radius in Group I.

    Figure 3

    Figure 3.  (color online) Dependence of fundamental QNM frequencies of charged black hole on charge Q with fixed horizon radius in Group II.

    Notably, the study of Ref. [30] asserted that QNM frequencies ω are linearly dependent on p=r02α for NSBH in the EW gravity. Following a similar path, we assume α=1/2 and a fixed charge Q, and subsequently consider QNMs of the new charged black holes with a different horizon radius r0 (here, we do not show new numerical solutions) for various multipole numbers l, assumingp=r02α=r0. According to QNM frequencies of charged black holes with r0=p=0.5 in Group I and r0=p=0.7 in Group II, these new QNM frequencies are expressed as a function of p in Table 5 and Figs. 4 and 5. In comparison with QNMs of charged black holes with p=r0=0.5, the new charged black holes with different values of p=r0>0.5 exhibit lower real frequencies and a lower damping rate. Moreover, the tendency of the imaginary part of QNM frequencies for l=1 changes, as shown in Fig. 5(b). Finally, we do not observe the strict linear relation between ω and p, as the charge Q causes a nonlinear effect on the frequency for these charged black holes in the EMW gravity.

    Figure 4

    Figure 4.  (color online) Dependence of QNM frequencies of charged black hole on parameter p with Q=0.1 starting from Q=0.1 and p=r0=0.5 in Group I. Solid and dashed lines denote the real and imaginary parts of QNM frequencies, respectively.

    Table 5

    Table 5.  Dependence of QNM frequencies of charged black hole on parameter p with fixed Q=0.1 in Groups I and II.
    Group IGroup II
    pω(l=0)ω(l=1)pω(l=0)ω(l=1)
    0.50.451303-0.428182i1.185030-0.399112i0.70.242462-0.150258i0.551352-0.409716i
    0.540.423578-0.399711i1.099260-0.366381i0.6850.242485-0.150641i0.560200-0.387280i
    0.580.386467-0.369919i1.019980-0.348577i0.6650.242862-0.151787i0.568930-0.372865i
    0.630.356692-0.342205i0.939253-0.320923i0.650.243282-0.153298i0.574191-0.367385i
    0.680.329482-0.312415i0.862011-0.294039i0.640.243481-0.155328i0.583836-0.360624i
    0.720.313943-0.302641i0.823348-0.279923i0.620.244227-0.162025i0.601820-0.359452i
    0.750.300831-0.297926i0.797623-0.271460i0.60.246600-0.169481i0.60298-0.346543i
    DownLoad: CSV
    Show Table

    Figure 5

    Figure 5.  (color online) Dependence of QNM frequencies of charged black hole on parameter p with Q=0.1 starting from Q=0.1 and p=r0=0.7 in Group II.

    We consider how the behavior of QNMs changes depending on the massive scalar field μ0. Under the massive scalar field perturbation, the boundary conditions are different from those in the massless case. From the radial perturbed Schrödinger Eq. (7), the respective ingoing boundary conditions at the black hole event horizon and outgoing boundary conditions at spatial infinity are obtained as

    Ψl(r){eiωr(rr0)iω4πT,rr0ei(μ2ω2)1/2rrme(μ2ω2)1/2rr2M(μ2ω2)1/2r

    (11)

    We take as an example the charged black holes with r0=0.5 and Q=0.1 in Group I. The QNM frequencies as a function of mass μ are shown in Fig. 6 [38]. For the angular parameter l=0, the real part of the QNM frequencies becomes larger, while the imaginary part decreases with increasing mass μ. This indicates that massive modes are scattered more slowly than massless modes. Moreover, these oscillations could become undamped, i.e., ωi=0, under certain conditions, causing the appearance of so-called quasiresonances. This phenomenon emerged for the massive scalar field perturbation in the NSBH background in EW gravity [30] and the charged field perturbation in the RN black hole [39, 40, 41]. Nevertheless, these quasiresonances disappear for l=1 in the EMW gravity.

    Figure 6

    Figure 6.  (color online) Dependence of QNM frequencies of charged black hole on mass \mu with r0=0.5 and Q=0.1 in Group I on the mass .

    Applying the Einstein-Maxwell-Weyl theory, we investigated QNMs and the stability of charged black holes under the test scalar field perturbation. With an increase in the charge Q, QNM frequencies are depicted by larger (smaller) real oscillations and higher (lower) damping rates, than those for the neutral branches at the same side of the coalescent point T0.091(r00.876), in both Groups I and II. Moreover, these phenomena are reflected by the behaviors of the thermodynamic quantities M and T. Furthermore, the linear dependence of the QNM frequency on the parameter p for NSBH in the EW gravity does not manifest for the new charged black holes in the EMW gravity, as the charge Q causes a nonlinear effect on QNM frequencies. Furthermore, we discussed the case of a massive scalar field perturbation, where undamped oscillations occur for sufficiently large masses.

    Recently, (anti-) de Sitter charged black hole solutions in the EMW gravity were presented [11]. Because of the dual conformal field theory, QNM frequencies of AdS black holes have a direct interpretation [42, 43]. Therefore, it would be interesting to consider QNMs and the stability of charged AdS black holes in the EMW gravity.

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2. Mustafa, G., Demir, E., Javed, F. et al. Epicyclic oscillations and particle collision with trajectories around quantum corrected black holes[J]. Physics of the Dark Universe, 2024. doi: 10.1016/j.dark.2024.101708
3. Li, S.-Y., Zhang, M., Zou, D.-C. et al. Analytical Approximations to Charged Black Hole Solutions in Einstein–Maxwell–Weyl Gravity[J]. Universe, 2023, 9(8): 371. doi: 10.3390/universe9080371
4. Huang, Y., Liu, D.-J., Zhang, H. Novel black holes in higher derivative gravity[J]. Journal of High Energy Physics, 2023, 2023(2): 57. doi: 10.1007/JHEP02(2023)057
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De-Cheng Zou, Chao Wu, Ming Zhang and Ruihong Yue. Quasinormal modes of charged black holes in Einstein-Maxwell-Weyl gravity[J]. Chinese Physics C. doi: 10.1088/1674-1137/44/5/055102
De-Cheng Zou, Chao Wu, Ming Zhang and Ruihong Yue. Quasinormal modes of charged black holes in Einstein-Maxwell-Weyl gravity[J]. Chinese Physics C.  doi: 10.1088/1674-1137/44/5/055102 shu
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Quasinormal modes of charged black holes in Einstein-Maxwell-Weyl gravity

  • 1. Center for Gravitation and Cosmology and College of Physical Science and Technology, Yangzhou University, Yangzhou 225009, China
  • 2. Faculty of Science, Xi’an Aeronautical University, Xi’an 710077 China

Abstract: We study quasinormal modes (QNMs) of charged black holes in the Einstein-Maxwell-Weyl (EMW) gravity by adopting the test scalar field perturbation. We find that the imaginary part of QNM frequencies is consistently negative for different angular parameters l, indicating that these modes always decay and are therefore stable. We do not observe a linear relationship between the QNM frequency ω and parameter p for these black holes, as their charge Q causes a nonlinear effect. We evaluate the massive scalar field perturbation in charged black holes and find that random long lived modes (i.e., quasiresonances) could exist in this spectrum.

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    1.   Introduction
    • As is well known, Einstein’s general relativity theory is non-renormalizable [1]. One of the most popular methods is to append all possible quadratic curvature invariants to the usual Einstein-Hilbert action [2], despite the existence of ghost-like modes in this theory. These terms constitute the so-called Einstein-Weyl (EW) gravity [35], which includes the most general Einstein-Hilbert action with quadratic curvature invariants. Subsequently, the non-Schwarzschild black hole (NSBH) solutions were recovered in four dimensional spacetime [3, 4], where they also satisfy the condition of the Ricci scalar (R=0). Since the Ricci scalar R does not vanish in higher dimensional (n>4) spacetime, Lü et al. found only the perturbed numerical solutions in the EW gravity [6]. Employing the continued fraction method, Kokkotas et al. constructed a numerical black hole solution in an analytical form [7], and its Hawking radiation was discussed in Ref. [8]. Furthermore, some new solutions were derived in anti-de Sitter (AdS) spacetime [9, 10].

      Recently, charged black holes were derived in Refs. [11-13] by applying the four dimensional EW theory coupled with an electromagnetic field. According to the ‘seed’ solution, these solutions can be divided into two groups in the Einstein-Maxwell-Weyl (EMW) theory. Group I solutions correspond to the charged extension of the higher derivative curvature for a Schwarzschild black hole. In Group II, the solutions are constructed from the charged generalization of the NSBH solution. The thermodynamic properties of these charged black holes were investigated in Ref. [13].

      The quasinormal modes (QNMs) of a black hole have been a hot research topic for decades. The concept of QNMs was introduced in 1957 by the work of Regge and Wheeler, who investigated the stability of black holes under small perturbations [14]. The study of QNMs is beneficial to the understanding of the structure of black hole spacetimes and could play an important role in the detection of gravitational waves and some fundamental symmetries for the gauge/gravity duality [15, 16]. To date, QNMs have been calculated either by Einstein’s general relativity theory coupled with Maxwell theory [17-19] or nonlinear electrodynamics [2024], or by modified gravities [2528]. The analysis of QNMs of the non-Schwarzschild black holes was performed in the EW gravity [29, 30], where the linear relation between QNM frequencies and the parameter p=r02α was recovered. Inspired by these results, we evaluate the effect of the charge Q on the QNMs and the stability of charged black holes in the Einstein-Maxwell-Weyl gravity. We will calculate the QNMs by considering test massless and massive scalar field perturbations on the charged black holes, respectively.

      This paper is constructed as follows. We first review charged black hole solutions with an increase of charge Q in the EMW gravity in Section 2. Then, we provide a detailed discussion of QNM frequencies under test scalar field perturbations, including massless and massive scalar fields in Section 3. Finally, we provide concluding remarks in the fourth section.

    2.   Charged black holes in EMW gravity
    • The action of the Einstein-Weyl gravity, combined with the electromagnetic field, is given by [12, 13]

      I=116πGd4xg[RαCμνρσCμνρσκFμνFμν],

      (1)

      where Fμν=μAννAμ is the electromagnetic tensor. Here, Cμνρσ is the Weyl tensor and the trace-free part of the Riemann tensor with the form [31, 32]

      Cμνρσ=Rμνρσ(gμ[ρRσ]νgν[ρRσ]μ)+13Rgμ[ρRσ]ν,

      where the part within brackets surrounding the indices refers to the anti-symmetric part. To date, various attempts have been made to formulate the Weyl curvature hypothesis in a rigorous way. The simplest choice of a scalar constructed from the Weyl tensor is CμνρσCμνρσ from Eq. (1).

      The equations of motion are obtained as [12, 13]

      Rμν12gμνR4αBμν2κTμν=0,μFμν=0,

      (2)

      where Bμν is the trace-free Bach tensor, and Tμν is energy-momentum tensor of the Maxwell field

      Bμν=(ρσ+12Rρσ)Cμνρσ,Tμν=FαμFαν14gμνFαβFαβ.

      (3)

      We choose the metric ansatz

      ds2=N(r)e2δ(r)dt2+1N(r)dr2+r2(dθ2+sin2θdφ2)

      (4)

      with a metric function N(r) equal to 12m(r)/r. We constructed numerical charged black hole solutions with α=12 and κ=1 [13], according to two neutral scenarios: the Schwarzschild (SBH) and non-Schwarzschild black hole (NSBH) within the bound of 0.363<r0<1.143 in the EW theory. Fig. 1 presents the relation between the mass M and Hawking temperature T for SBH and NSBH scenarios, where both neutral solutions coalesce at T0.091(r00.876). Taking Group I (Fig. 1(a)) as an example, charged black holes were constructed from the Schwarzschild black hole (Q=0) by increasing the charge Q while maintaining the same horizon radius r0. In particular, new charged black holes on both sides of the coalescent point exhibit different properties: as the charge Q increases, their mass becomes larger (smaller) depending on the temperature decrease (increase) on the left (right) hand side. A similar phenomenon is observed in Group II, as shown in Fig. 1(b).

      Figure 1.  (color online) Mass M versus temperature T for Schwarzschild (SBH), non-Schwarzschild (NSBH), and charged black holes for Group I (1) and Group II (b). The arrow denotes the increase in charge Q.

    3.   Quasinormal modes for test scalar field
    • In this section, we consider the test massive scalar field ψ propagating on charged black holes, which obeys the Klein-Gordon equation

      (μ2)ψ=0,

      (5)

      where μ is the mass of the scalar field, and ψ can be separated into spherical harmonics, temporal, and radial components

      ψ(t,r,θ,ϕ)=lm1rΨl(r)Ylm(θ,ϕ)eiωt,

      (6)

      where Ylm(θ,ϕ) is a usual spherical harmonic, and l is the angular harmonic index.

      Substituting Eqs. (4) and (6) into Eq. (5), we obtain the radial perturbed Schrödinger equation

      (2r+ω2Vl(r))Ψl(r)=0,

      (7)

      where the effective potential is given as

      Vl=l(l+1)Ne2δr2+Ne2δ(NNδ)r+μ2Ne2δ

      (8)

      and the tortoise coordinate r is adopted as

      drdr=1Neδ.

      (9)
    • 3.1.   Quasinormal modes for massless scalar field

    • We consider the massless scalar field perturbation (μ=0) in these charged black holes for Groups I and II. First, we choose appropriate boundary conditions. The respective boundary conditions at the black hole horizon and spatial infinity are

      Ψl(r){eiωr(rr0)iω4πTrr0(r)eiωreiωrr2Miωr(r),

      (10)

      To derive these QN modes ω, we employ the shooting method [33-35]. With the initial condition given by Eq. (10) at the event horizon r0, we solve the perturbed Eq. (7) numerically for each ω using the Wolfram Mathematica® built-in function NDSolve for r0rrf, where rfr0. This solution must also satisfy the boundary condition in Eq. (10) at spatial infinity, if ω is the quasinormal frequency.

      In the Tables 14, we present QNM frequencies (real and imaginary parts) for these charged black holes in Groups I and II. The QNM frequencies (for l=0 and 1) of these neutral black holes (SBH and NSBH) are displayed in the first row of Tables 14. In the case of Q0, the action presented in Eq. (1) becomes neutral [3, 4], recovering the Schwarzschild and non-Schwarzschild solutions in four dimensional spacetime. The fundamental QNM frequencies under a massless scalar field perturbation on the SBH background are given in Refs. [36, 37]. QNM frequencies (for l=0 and 1) of the NSBH with a horizon radius r0>0.876 are provided in Refs. [29, 30]. However, for the NSBH with r0<0.876, there is a discrepancy between QNM frequencies and data obtained from the relation expression pω in Ref. [30]. This is probably because this expression, derived in the bound 0.876<r0<1.143, is not valid in the region of 0.363<r0<0.876 for the NSBH in the EW gravity.

      r0=0.5r0=0.75
      Qω(l=0)ω(l=1)ω(l=0)ω(l=1)
      00.4420-0.4196i1.1716-0.3908i0.2947-0.2797i0.7810-0.2605i
      0.020.445730-0.419802i1.17219-0.39126i0.29504-0.284839i0.781750-0.262508i
      0.060.447649-0.423259i1.17636-0.394006i0.300096-0.291943i0.768417-0.266851i
      0.10.451833-0.431002i1.18541-0.399437i0.305035-0.307715i0.795061-0.269681i
      0.140.458975-0.443435i1.19959-0.408586i0.310574-0.309508i0.806376-0.275657i
      0.180.465102-0.456727i1.21463-0.420002i0.315876-0.317400i0.820854-0.281226i
      0.220.467173-0.461698i1.23312-0.434605i0.318256-0.328687i0.825449-0.304661i
      0.260.481790-0.486744i1.25409-0.445270i0.321404-0.342171i0.830678-0.326003i
      0.300.512190-0.519842i1.27810-0.477936i0.324608-0.355701i0.834912-0.334916i
      0.330.522566-0.532159i1.29662-0.490691i0.330838-0.369421i0.838476-0.351239i

      Table 1.  QNM frequencies of black holes within region of r0 < 0.876 in Group I.

      r0=1.3r0=2
      Qω(l=0)ω(l=1)ω(l=0)ω(l=1)
      00.1700-0.1614i0.4506-0.1503i0.1105-0.1049i0.2929-0.0977i
      0.020.169935-0.160131i0.449981-0.149923i0.110434-0.104754i0.292409-0.0975053i
      0.060.169604-0.158960i0.449018-0.149611i0.110072-0.104079i0.290913-0.0960668i
      0.10.169109-0.157354i0.448207-0.148787i0.108803-0.101841i0.287647-0.0937670i
      0.140.168720-0.155810i0.447165-0.147679i0.106439-0.098191i0.283246-0.0906137i
      0.180.168023-0.153998i0.443449-0.145653i0.103804-0.0933381i0.278075-0.0869171i
      0.220.166891-0.151193i0.441312-0.143235i0.101889-0.0875931i0.272255-0.0830565i
      0.260.166014-0.148333i0.437092-0.140916i0.100331-0.0820946i0.266262-0.0790222i
      0.300.165430-0.144852i0.432538-0.137778i0.098978-0.0771625i0.259892-0.0749341i
      0.330.164395-0.143688i0.429952-0.135908i0.098060-0.0736714i0.254996-0.0719146i

      Table 2.  QNM frequencies of black holes within region of r0 > 0.876 in Group I.

      r0=0.5r0=0.7
      Qω(l=0)ω(l=1)ω(l=0)ω(l=1)
      00.1920-0.05801i0.5650-0.1168i0.2484-0.1550i0.5574-0.4347i
      0.020.191974-0.0578165i0.563943-0.115416i0.247626-0.15488i0.556522-0.433647i
      0.060.191900-0.0575649i0.561869-0.114350i0.245508-0.153018i0.551806-0.407181i
      0.10.191801-0.0572173i0.556545-0.112453i0.242262-0.150052i0.550735-0.410594i
      0.120.191707-0.0569421i0.553771-0.111982i0.240109-0.148196i0.548074-0.403119i
      0.1550.191457-0.0563484i0.547261-0.109390i0.235633-0.144682i0.542076-0.386617i
      0.1850.191279-0.0559977i0.543921-0.104531i0.231188-0.141623i0.536208-0.369774i
      0.2150.190982-0.0555578i0.534453-0.100874i0.226304-0.138735i0.530038-0.353073i
      0.250.188420-0.0548792i0.521852-0.094748i0.220221-0.135954i0.518341-0.344284i
      0.290.185355-0.0536175i0.508852-0.087635i0.213025-0.133582i0.508015-0.333045i

      Table 3.  QNM frequencies of black holes within region of r0 < 0.876 in Group II.

      r0=1r0=1.1
      Qω(l=0)ω(l=1)ω(l=0)ω(l=1)
      00.2490-0.3900i0.6920-0.2920i0.2967-0.4970i0.7764-0.3774i
      0.020.248326-0.390859i0.697313-0.292519i0.298359-0.497294i0.78196-0.3774330i
      0.0550.249730-0.398086i0.699694-0.294816i0.300096-0.497879i0.786675-0.377930i
      0.090.251139-0.403061i0.706433-0.300164i0.303839-0.507651i0.795747-0.387478i
      0.1250.258110-0.418834i0.713570-0.308565i0.313144-0.495390i0.795217-0.390079i
      0.160.257806-0.426266i0.722897-0.314524i0.315173-0.529731i0.816878-0.410238i
      0.1950.262761-0.434169i0.731343-0.322654i0.322461-0.538773i0.827657-0.422568i
      0.230.266851-0.445229i0.740003-0.332937i0.328884-0.559389i0.842650-0.437051i
      0.280.279679-0.465540i0.749440-0.341363i0.341379-0.571647i0.860521-0.459423i

      Table 4.  QNM frequencies of black holes within region of r0 > 0.876 in Group II.

      For charged black holes, the imaginary parts are always negative, indicating that these modes always decay and are therefore stable. Moreover, for the charged black holes with r0=0.5 and 0.75 in Group I, the absolute values of the imaginary part (|ωi|) and real part (ωr) of QNM frequencies (l=0,1) both increase with increasing of Q (see Table 1 and Fig. 2(a)). In contrast, the corresponding values of |ωi| and ωr of QNM frequencies for charged black holes with r0=1.3 and 2 in Group I both decrease with increasing of Q (see Table 2 and Fig. 2(b)). Similarly, QNM frequencies of these charged black holes in Group II, located each side of the coalescent point T0.091(r00.876), exhibit different trends from the ones given in Tables 3, 4 and Fig. 3. This is because the mass M and temperature T of these charged black holes affect the boundary conditions of the test scalar field Ψl(r) given in Eq. (10). However, based on the free energies (Fig. 7 in Ref. [13]) and QNM frequencies of these charged and neutral black holes [Figs. 2 and 3], the correlation between thermodynamic phase transitions and dynamical stabilities fails.

      Figure 2.  (color online) Dependence of fundamental QNM frequencies of charged black hole on charge Q with fixed horizon radius in Group I.

      Figure 3.  (color online) Dependence of fundamental QNM frequencies of charged black hole on charge Q with fixed horizon radius in Group II.

      Notably, the study of Ref. [30] asserted that QNM frequencies ω are linearly dependent on p=r02α for NSBH in the EW gravity. Following a similar path, we assume α=1/2 and a fixed charge Q, and subsequently consider QNMs of the new charged black holes with a different horizon radius r0 (here, we do not show new numerical solutions) for various multipole numbers l, assumingp=r02α=r0. According to QNM frequencies of charged black holes with r0=p=0.5 in Group I and r0=p=0.7 in Group II, these new QNM frequencies are expressed as a function of p in Table 5 and Figs. 4 and 5. In comparison with QNMs of charged black holes with p=r0=0.5, the new charged black holes with different values of p=r0>0.5 exhibit lower real frequencies and a lower damping rate. Moreover, the tendency of the imaginary part of QNM frequencies for l=1 changes, as shown in Fig. 5(b). Finally, we do not observe the strict linear relation between ω and p, as the charge Q causes a nonlinear effect on the frequency for these charged black holes in the EMW gravity.

      Figure 4.  (color online) Dependence of QNM frequencies of charged black hole on parameter p with Q=0.1 starting from Q=0.1 and p=r0=0.5 in Group I. Solid and dashed lines denote the real and imaginary parts of QNM frequencies, respectively.

      Group IGroup II
      pω(l=0)ω(l=1)pω(l=0)ω(l=1)
      0.50.451303-0.428182i1.185030-0.399112i0.70.242462-0.150258i0.551352-0.409716i
      0.540.423578-0.399711i1.099260-0.366381i0.6850.242485-0.150641i0.560200-0.387280i
      0.580.386467-0.369919i1.019980-0.348577i0.6650.242862-0.151787i0.568930-0.372865i
      0.630.356692-0.342205i0.939253-0.320923i0.650.243282-0.153298i0.574191-0.367385i
      0.680.329482-0.312415i0.862011-0.294039i0.640.243481-0.155328i0.583836-0.360624i
      0.720.313943-0.302641i0.823348-0.279923i0.620.244227-0.162025i0.601820-0.359452i
      0.750.300831-0.297926i0.797623-0.271460i0.60.246600-0.169481i0.60298-0.346543i

      Table 5.  Dependence of QNM frequencies of charged black hole on parameter p with fixed Q=0.1 in Groups I and II.

      Figure 5.  (color online) Dependence of QNM frequencies of charged black hole on parameter p with Q=0.1 starting from Q=0.1 and p=r0=0.7 in Group II.

    • 3.2.   Quasinormal modes for massive scalar field

    • We consider how the behavior of QNMs changes depending on the massive scalar field μ0. Under the massive scalar field perturbation, the boundary conditions are different from those in the massless case. From the radial perturbed Schrödinger Eq. (7), the respective ingoing boundary conditions at the black hole event horizon and outgoing boundary conditions at spatial infinity are obtained as

      Ψl(r){eiωr(rr0)iω4πT,rr0ei(μ2ω2)1/2rrme(μ2ω2)1/2rr2M(μ2ω2)1/2r

      (11)

      We take as an example the charged black holes with r0=0.5 and Q=0.1 in Group I. The QNM frequencies as a function of mass μ are shown in Fig. 6 [38]. For the angular parameter l=0, the real part of the QNM frequencies becomes larger, while the imaginary part decreases with increasing mass μ. This indicates that massive modes are scattered more slowly than massless modes. Moreover, these oscillations could become undamped, i.e., ωi=0, under certain conditions, causing the appearance of so-called quasiresonances. This phenomenon emerged for the massive scalar field perturbation in the NSBH background in EW gravity [30] and the charged field perturbation in the RN black hole [39, 40, 41]. Nevertheless, these quasiresonances disappear for l=1 in the EMW gravity.

      Figure 6.  (color online) Dependence of QNM frequencies of charged black hole on mass \mu with r0=0.5 and Q=0.1 in Group I on the mass .

    4.   Conclusions and discussions
    • Applying the Einstein-Maxwell-Weyl theory, we investigated QNMs and the stability of charged black holes under the test scalar field perturbation. With an increase in the charge Q, QNM frequencies are depicted by larger (smaller) real oscillations and higher (lower) damping rates, than those for the neutral branches at the same side of the coalescent point T0.091(r00.876), in both Groups I and II. Moreover, these phenomena are reflected by the behaviors of the thermodynamic quantities M and T. Furthermore, the linear dependence of the QNM frequency on the parameter p for NSBH in the EW gravity does not manifest for the new charged black holes in the EMW gravity, as the charge Q causes a nonlinear effect on QNM frequencies. Furthermore, we discussed the case of a massive scalar field perturbation, where undamped oscillations occur for sufficiently large masses.

      Recently, (anti-) de Sitter charged black hole solutions in the EMW gravity were presented [11]. Because of the dual conformal field theory, QNM frequencies of AdS black holes have a direct interpretation [42, 43]. Therefore, it would be interesting to consider QNMs and the stability of charged AdS black holes in the EMW gravity.

Reference (43)

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