Retrolensing by a wormhole: π and 3π in the sky? ∗ Naoki Tsukamoto School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China (Dated: February 1, 2017) Abstract 7 Deflection angle of a light ray can be arbitrary large near a light sphere. The time symmetrical 1 0 shapes of light curves of light rays reflected by a light sphere of a lens object does not depend on 2 the details of the lens object. We consider retrolensing light curves of sun lights with deflection n a J angles π and 3π by an Ellis wormhole which is the simplest Morris-Thorne wormhole. If an Ellis 1 3 wormhole with a throat parameter a = 1011 km is at 100 pc away from an observer and if the Ellis ] wormhole, the observer, and the sun are aligned perfectly in this order, the apparent magnitudes c q of a pair of the light rays with deflection angles π and 3π become 11 and 18, respectively. The - r g two pairs of the light rays make a superposed light curve with two separable peaks and they break [ 1 down time symmetry of a retrolensing light curve. The observation of the two separated peaks v 9 of the light curves gives us information on the details of the lens object. If the observer can also 6 1 separate the pair of the images with the deflection angle π into a double image, he or she can say 9 0 whether the retrolensing is caused by an Ellis wormhole or a Schwarzschild black hole. . 1 0 7 1 : v i X r a ∗ [email protected] 1 I. INTRODUCTION Gravitational lensing is one of useful tools to search for dark and compact objects. Grav- itational lensing under a quasi-Newtonian approximation has been discussed widely [1–3] while gravitational lensing without the approximation has been also investigated [4]. In 1959 Darwin pointed out that an infinite number of ghost images appear near a light sphere or photon sphere [5, 6] in the Schwarzschild spacetime [7]. The gravitational lensing of the faint images has been discussed by several authors [8–21]. Gravitational lensing of light rays with a deflection angle which is almost π is called retrolensing. Holz and Wheeler discussed the retrolensing of sun lights reflected by the light sphere of a black hole near the solar system [22]. One can distinguish between the light curves of retrolensing and other light curves since the retrolensing light curves have a characteristic shape and are symmetric in time. If one observe a light curve with the characteristic shape and with solar spectra on the elliptic, he or she can say that it is a retrolensing light curve of the sun. A black hole in the galactic center as a retrolens was also investigated [23–25]. The effects of a electrical charge [23, 26] and rotation [25, 27] of a black hole on a retrolensing light curve and a double image [26] was also considered. Gravitational lensing is also caused by a wormhole [28]. One can survey wormholes with a negative Arnowitt-Deser-Misner (ADM) mass [29–36], a vanishing ADM mass [21, 36–51], and a positive ADM mass [37, 51–56] by gravitational lensing. In 1973 Ellis obtained a static and spherically symmetric wormhole solution of Einstein equations with a phantom scalar field [57]. The traversable wormhole with vanishing ADM masses is called Ellis worm- hole or Ellis-Bronnikov wormhole since it was also obtained by Bronnikov in a scalar-tensor theory in the same year [58]. The wormhole is often also referred the Morris-Thorne worm- hole [59] without mentioning Ellis [57] and Bronnikov’s works [58]. The instability of the Ellis wormhole was revealed by several authors [60], contrary to a conclusion of an earlier work [61]. Some static and spherically symmetric wormholes have the same metric as the one of the Ellis wormhole in a vanishing ADM mass case [62–66]. In 2013 Bronnikov et al. showed that a wormhole with the same metric as the metric of the Ellis wormhole and with electrically charged dust with negative energy density is linearly stable under spherically symmetric and axial perturbations [67]. The quasinormal mode was also investigated [68]. 2 The trajectory of a light ray in the Ellis wormhole spacetime was investigated by Ellis in Ref. [57]. The deflection angle of the light ray was calculated first by Chetouani and Clement [47] and then by several authors [21, 37–40, 48–50]. Various gravitational lensing effects [37, 39–46, 51, 69–74], a particle collision [75], a shadow [76–78], visualization [79], several observables like a rotation curve [80] in the Ellis wormhole spacetime were also investigated. Takahashi and Asada gave the upper bound of the number density N upper ≤ 10−4h3Mpc−3 of the Ellis wormhole with a throat parameter 10 a 104pc [36] with strong ≤ ≤ lensing of quasars in the data of Sloan Digital Sky Survey Quasar Lens Search [81] and Yoo et al. gave N 10−9AU−3 for a 1cm [41] with femto lensing of gamma-ray bursts [82] upper ≤ ∼ in the data of Fermi Gamma-Ray Burst Monitor [83]. Recently, Tsukamoto andHarada madea conjecture thatthe shape oflight curves formed by light rays which are reflected by a light sphere does not depend on the details of a static spherically symmetric and asymptotically flat spacetime and a lens configuration [51]. If the conjecture is true, one cannot distinguish between black holes and wormholes with the shape of their retrolensing light curves. Can we distinguish between black holes and wormholes by retrolensing? In this paper, we consider the details of retrolensing by an Ellis wormhole and a black hole near the solar system to answer the question. This paper is organized as follows. In Sec. II we briefly review a deflection angle in a strong deflection angle in an Ellis wormhole spacetime. In Sec. III we review retrolensing in a general static spherically symmetric and asymptotically flat spacetime. In Sec. IV, we investigate the effect of the light rays with deflection angle 3π on retrolensing by the Ellis wormhole and we discuss our results in Sec. V. In this paper we use the units in which the light speed and Newton’s constant are unity. II. DEFLECTION ANGLE IN A STRONG DEFLECTION LIMIT In this section, we review briefly the deflection angle α of a light ray in a strong deflec- tion angle limit in an Ellis wormhole spacetime and in the Schwarzschild spacetime in the following form, b ¯ α = a¯log 1 +b+O((b b )log(b b )), (2.1) c c − (cid:18)b − (cid:19) − − c 3 ¯ where a¯ and b are parameters and b is the impact parameter of the light ray and b is the c critical impact parameter. 1 A. Ellis wormhole The line element is given by [57, 58] 2 2 2 2 2 2 2 2 ds = dt +dr +(r +a )(dθ +sin θdφ ), (2.2) − where a is a positive constant. The wormhole throat exists at r = 0. The coordinates are defined in a range < t < , < r < ,0 θ π, and 0 φ < 2π but we −∞ ∞ −∞ ∞ ≤ ≤ ≤ concentrate on a region r 0. We assume θ = π/2 without loss of generality because of ≥ spherical symmetry. From ds2 = 0, the trajectory of a light ray is given by 2 1 dr 1 1 = , (2.3) (r2 +a2)2 (cid:18)dφ(cid:19) b2 − r2 +a2 where b L/E is the impact parameter of the light ray and the conserved energy E t˙> 0 ≡ ≡ and angular momentum L (r2+a2)φ˙, where the dot denotes a differentiation with respect ≡ to an affine parameter, are constant along the trajectory. A light ray does not pass through the throat if b > a while it passes through the throat if b < a. We only consider b > a. | | | | | | The Ellis wormhole spacetime has a light sphere at r = 0 which is coincident with the throat. In the strong deflection limit b b a, where b is the critical impact parameter, light c c → ≡ rays wind around the wormhole throat at r = 0. From Eq. (2.3), the deflection angle of a light ray is given by [47] ∞ bdr α(b) = 2 π Z (r2 +a2)(r2 +a2 b2) − ro − ap = 2K π, (2.4) b − (cid:16) (cid:17) where r √b2 a2 is the closet distant of the light ray and K(k) is the complete elliptic o ≡ − integral of the first kind given by 1 dx K(k) = , (2.5) Z0 (1 x2)(1 k2x2) − − p 1 Bozzaestimatedtheorderofthe errortermisO(b bc)inRef.[15]andthenTsukamotopointedoutthat − it should be read as O((b bc)log(b bc)) [40]. − − 4 where 0 < k < 1. In the strong deflection limit b b = a, the deflection angle becomes c → b α(b) = log 1 +3log2 π +O((b b )log(b b )). (2.6) c c − (cid:18)b − (cid:19) − − − c ¯ Thus, a¯ = 1 and b = 3log2 π in Eq. (2.1). Here we have used − 1 3 limK(k) = log(1 k)+ log2+O((1 k)log(1 k)), (2.7) k→1 −2 − 2 − − which is obtained from Eq. (10) in section 13. 8 in Ref. [84]. See Ref. [40] for the details of the deflection angle in the strong deflection limit in the Ellis wormhole spacetime. B. Schwarzschild black hole The line element in the Schwarzschild spacetime is given by 2M dr2 2 2 2 2 2 2 ds = 1 dt + +r (dθ +sin θdφ ), (2.8) −(cid:18) − r (cid:19) 1 2M − r where M is the ADM mass. The critical impact parameter of a right ray is given by b 3√3M [7, 15, 17]. The deflection angle α of the light ray in the strong deflection angle c ≡ limit b b is expressed as [7, 15, 17, 21] c → b α(b) = log 1 +log 216(7 4√3) π − (cid:18)b − (cid:19) − − c h i +O((b b )log(b b )). (2.9) c c − − Thus, we obtain a¯ = 1 and ¯b = log 216(7 4√3) π. − − (cid:2) (cid:3) III. RETROLENSING Inthissection, we review retrolensing [22–24, 26] ina general staticspherically symmetric and asymptotically flat spacetime. A. Lens equation We consider that a light ray emitted by the sun S is deflected by a lens L or a light sphere with an deflection angle α and then it reaches an observer O with an image angle θ. Figure 1 shows the retrolensing configuration. We concentrate on the case where the impact 5 I L O _ D OL D LS D OS _ S FIG. 1. Retrolensing Configuration. A light ray emitted by the sun S is deflected by a lens L or a light sphere with an deflection angle α. An observer O sees an image I with an image angle θ. α¯ is the effective deflection angle of the light ray which rotates N times around the light sphere defined as α¯ α 2πN. θ¯ is an angle at S between the light ray and line LS and β is a source ≡ − angle defined by ∠OLS. D , D , and D are the distances between the observer and the lens, OL LS OS between the lens and the source, and between the observer and the source, respectively. parameter b is positive. We define the effective deflection angle α¯ of the light ray as α¯ α 2πN (3.1) ≡ − where N is a nonnegative integer which denotes the winding number of the light ray around the light sphere. We use a lens equation considered by Ohanian [10, 19], ¯ β = π α¯(θ)+θ +θ, (3.2) − where β is a source angle ∠OLS and θ¯ is an angle between the light ray and the line LS at S. We assume that the lens L, the observer O, and the sun S are almost aligned in this order and that the size of the light sphere is apparently small for the observer, i.e., b D , (3.3) c OL ≪ where D is the distance between the lens and the observer. Under the assumptions, we OL obtain α π +2πN, (3.4) ∼ α¯ π, (3.5) ∼ β 0, (3.6) ∼ 6 D = D +D , (3.7) LS OL OS where D and D are the distances between the lens and source and between the observer LS OS and the source, respectively, b b c = θ 1, (3.8) D ∼ D ≪ OL OL and b ¯ c θ 1. (3.9) ∼ D ≪ LS ¯ Under the assumptions, we can justify to neglect the terms θ and θ in the Ohanian lens equation (3.2). B. Image angle Inserting the deflection angle in the strong deflection limit (2.1), the definition of the effective deflection angle (3.1), and Eq. (3.8), into the Ohanian lens equation (3.2) neglected ¯ the terms θ and θ, we obtain positive solutions θ = θ (β) for every N, as obtained in +N Ref. [24], where b c θ (β) (1+e (β)), (3.10) +N +N ≡ D OL where e (β) is defined as +N ¯ b (1+2N)π +β e (β) exp − +N ≡ (cid:20) a¯ (cid:21) ¯ b (1+2N)π exp − . (3.11) ∼ (cid:20) a¯ (cid:21) When N = 0, we obtain ¯ b b π +β c θ (β) = 1+exp − +0 D (cid:20) (cid:18) a¯ (cid:19)(cid:21) OL ¯ b b π c 1+exp − . (3.12) ∼ D (cid:20) (cid:18) a¯ (cid:19)(cid:21) OL From spherical symmetry, the negative solution θ = θ−N(β) of the Ohanian lens equation denoting the image angle of a light ray with a negative impact parameter is given by θ−N(β) = θ+N( β) θ+N(β) (3.13) − − ∼ − for each N. The image separation ∆θ between the positive and negative images for every N nonnegative integer N is obtained as ∆θN θ+N(β) θ−N(β) 2θ+N(β). (3.14) ≡ − ∼ 7 C. Magnification We assume that the sun is regarded as a uniform-luminous disk [85–87] with a radius R , s where R = 7 105km is the radius of the sun. The magnification µ (β) of the image with s +N × θ (β) is obtained as [24, 26] +N 2D2 θ dθ µ (β) = OS +N +NI(β), (3.15) +N − πD R2 dβ LS s where I(β) is given by, for βD R , LS s ≤ I(β) = π(R βD ) s LS − Rs+βDLS R2 +β2D2 +R2 + arccos − s LS dR Z 2βD R Rs−βDLS LS (3.16) and, for R βD , s LS ≤ Rs+βDLS R2 +β2D2 +R2 I(β) = arccos − s LS dR. (3.17) Z−Rs+βDLS 2βDLSR From Eqs. (3.10) and (3.15), the magnification is obtained as 2D2 b2 e (β)(1+e (β)) µ (β) = OS c +N +N I(β). (3.18) +N −πD D2 R2 a¯ LS OL s The magnification µ−N(β) of the image with image angle θ−N(β) is given by µ−N(β) µ+N(β). (3.19) ∼ − The total magnification µ (β) of a pair of images for each N is given as totN µtotN(β) µ+N(β) + µ−N(β) ≡ | | | | 4D2 b2 e (β)(1+e (β)) = OS c +N +N I(β) . πD D2 R2 a¯ | | LS OL s (3.20) In a perfect-aligned case, β = 0, the total magnification becomes 4D2 b2 e (0)(1+e (0)) µ (0) = OS c +N +N (3.21) totN D D2 R a¯ LS OL s for every N. Here we have used I(0) = πR . s 8 D. Source plane The sun is on a source plane defined as a plane that is orthogonal to the optical axis, i.e., an axis β = 0. We denote the closest separation between the center of the sun and the intersection of the source plane and the optical axis by β . Figure 2 shows the source plane. m β=0 Sun FIG. 2. Retrolensing. A source plane is defined as a plane on which the sun is and which is orthogonal to the optical axis, an axis β = 0. The closest separation β is defined by the smallest m source angle during retrolensing. We assume that the sun moves with the orbital velocity of the sun v = 30km/s on the source plane. E. Light curves The retrolensing light curves do not diverge because of the finite size of the sun. Figure 3 shows the retrolensing light curves by an Ellis wormhole with a = 106km at D = 1pc OL away. One can estimate β D /R from the shape of the peak of the light curves since the m LS s light curves has a characteristic shape depending on β D /R . The characteristic time m LS s scale of the peak is obtained as 2R /v = 12 hours. In general, if a lensing object is static s 9 and has spatial spherical symmetry, the whole shape and time scale of the retrolensing light curves of the sun do not depend on the parameters of the lens such as the ADM mass, the electrical charge, the size of the wormhole throat, and D [26, 51] while the peak shape LS strongly depends on β D /R . m LS s 21 0 21.5 0.5Rs Rs 22 1.5Rs 22.5 m 23 23.5 24 24.5 2 1.5 1 0.5 0 0.5 1 1.5 2 − − − − t[days] FIG. 3. Retrolensing light curves by an Ellis wormhole with a = 106km at D = 1pc. The (red) OL solid, (green) dashed, (blue) dotted, and (purple) dash-dotted curves denote the light curves with β D = 0, 0.5R , R , and 1.5R , respectively. m is the apparent magnitude. m LS s s s Figure 4 shows the retrolensing light curves by an Ellis wormhole and a Schwarzschild black hole with b = 106km and β = 0 at D = 1pc. We notice that the shapes of the c m OL retrolensing light curves are similar while the apparent magnitudes m are different. 20 Wormhole 20.5 Blackhole 21 21.5 22 m 22.5 23 23.5 24 24.5 2 1.5 1 0.5 0 0.5 1 1.5 2 − − − − t[days] FIG. 4. Retrolensing light curves by an Ellis wormhole and a Schwarzschild black hole with b = 106km and β = 0 at D = 1pc. The (red) solid and (green) dashed curves denote c m OL retrolensing light curves by the wormhole and the black hole, respectively. 10