Comment on “Impact of a Global Quadratic Potential on Galactic Rotation Curves” Kamal K. Nandi1,2,a and Arunava Bhadra3,b
arXiv:1208.5330v1 [gr-qc] 27 Aug 2012
1
Ya.B. Zel’dovich International Center for Astrophysics, Ufa 450000, Russia 2 Department of Mathematics, University of North Bengal, Siliguri 734013, India 3 High Energy & Cosmic Ray Research Centre, University of North Bengal, Siliguri 734013, India a
[email protected] , b aru
[email protected]
Conformal gravity theory can explain observed flat rotation curves of galaxies without invoking hypothetical dark matter. Within this theory, we obtain a generic formula for the sizes of galaxies exploiting the stability criterion of circular orbits. It is found that different galaxies have different finite sizes uniquely caused by the assumed quadratic potential of cosmological origin. Observations on where circular orbits might actually terminate could thus be very instructive in relation to the galactic sizes predicted here. PACS numbers: 04.50.Kd, 95.30.Sf Keywords: Flat rotation curve, Conformal gravity, stability of orbits
In a recent Letter, Mannheim and O’Brien [1] have pre2 sented conformal gravity rotational velocity vtotal fits to the observed data of several galaxy samples, which seem good enough to indicate that conformal gravity could be an interesting alternative to dark matter hypothesis. An important prediction of the theory is the testable upper 2 limit on the size of the galaxies projected from vtotal →0 global (hence effectively the global limit Rproj ≤ γ0 /κ ≈ 100 Kpc). The purpose of this Comment is to correct that this upper limit should be fixed by the criterion of stability of orbits. If the canonical stable limit is observationally surpassed, conformal theory would be falsified even global if the last observed orbit remains within Rproj . Note that emission occurs from stable circular material orbits with information propagating along null geodesics (see, for instance [2]). The stability criterion can severely constrain the extent of the H1 gas and we observe that conformal gravity endows each galaxy with a maximal global max stable limit R = Rstable that falls within the limit Rproj . The two limits often differ significantly, by as much as 20max 30%. For instance, with U GC2885, we find Rstable = 191 global Kpc but Rproj = 253 Kpc and this difference (∼ 62 Kpc) could be well distinguished. Since stability is an max essential physical condition, we think that only Rstable should be regarded as the testable upper limit on the size of a galaxy. With their metric ansatz, the geodesic for a single test particle yieldsthe tangential velocity for circular orbits v 2 = Rc2 /2 B ′ (primes denote derivatives 2 with respect to R). With approximate vtotal , it integrates to 2N ∗ β ∗ + (N ∗ γ ∗ + γ0 ) R − κR2 + R 15R04 N ∗ γ ∗ − 24R02 N ∗ β ∗ 3R02 N ∗ γ ∗ . (1) + 2R 8R3 The radial geodesic is given by B(R) = 1 −
dR dt
2
= B 2 (R) − a
B 3 (R) − bB 3 (R), R2
(2)
where a and b are constants fixed by the usual conditions for circular orbits. The condition for stability is that the second derivative of the right hand side (“effective potential”) of Eq.(2) with respect to R must be negative, which leads to the generic requirement that f (R) ≡ 2B ′2 (R) − B(R)B ′′ (R) − 3B(R)B ′ (R)/R < 0. (3) We illustrate our comments here only for U GC2885 (Figs.1a,b). Rest of the samples yield similar patterns. max The very fact that there exists a finite limit Rstable , caused entirely by the quadratic potential Vκ (r) = −κc2 r2 /2, clearly distinguishes conformal theory from some dark matter models because in the latter there is no such limit, see e.g., [3,4]. Note that we don’t know precisely what would happen beyond this special radius max Rstable , but gas in non-circular motions at larger radii is not certainly excluded. Interestingly, the predicted max does not even much exceed the current Rlast for Rstable max many samples, e.g., U GC0128 has Rstable = 65.6 Kpc, while Rlast = 54.8 Kpc, so we might not have to wait too long. The main thing to watch is whether or not any max updated Rlast shoots past Rstable , which fortunately has not happened yet. Updated observations on Rlast would thus provide a nice test of conformal gravity prediction max of Rstable and hence of the global quadratic potential. Dr. Mannheim [5] has the opinion that the general stability analysis may be performed considering many body dynamics. Observations on where circular orbits might actually terminate could thus be very instructive.
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(b) 2 FIG. 1: (a) vtotal vs R and (b) f (R) vs R for UGC2885
[1] P.D. Mannheim and J.G. O’Brien, Phys. Rev. Lett. 106, 121101 (2011).
(2009). [4] F. Rahaman et al, Phys. Lett. B 694, 10 (2010).
[2] K. Lake, Phys. Rev. Lett. 92, 051101 (2004). [5] Private correspondence [3] K. K. Nandi et al, Mon. Not. Roy. Astron. Soc. 399, 2079
