f(T)f(T) Theories and Varying Fine Structure Constant

In analogy to $f(R)$ theory, recently a new modified gravity theory, namely the so-called $f(T)$ theory, has been proposed to drive the current accelerated expansion without invoking dark energy. In the present work, by extending Bisabr's idea, we try to constrain $f(T)$ theories with the varying fine structure "constant", $\alpha\equiv e^2/\hbar c$. We find that the constraints on $f(T)$ theories from the observational $\Delta\alpha/\alpha$ data are very severe. In fact, they make $f(T)$ theories almost indistinguishable from $\Lambda$CDM model.Comment: 12 pages, 4 figures, 1 table, revtex4; v2: discussions added, Phys. Lett. B in press; v3: published versio

Rotating and twisting charged black holes with cloud of strings and quintessence as a particle accelerator

In this paper, we study the effects of the rotation parameter $a$, the twist parameter $n$, the string cloud parameter $b$, the quintessence state parameter $\omega _{q}$ and the charge parameter $q$ on the horizons and ergosphere of rotating and twisting charged black holes with cloud of strings and quintessence, and obtain the equations of motion and effective potential of the particle on the equatorial plane of black hole. We find that a particle with the critical angular momentum $L = L_C$ falling from infinity reaches the event horizon($u^r$=0) and satisfies the circular orbit condition $V_{e f f}={V_{e f f}}'=0$. We derive the expression of the centre-of-mass (CM) energy of two particles with different masses from the equations of particle motion. We show that the CM energy can be arbitrarily large for extremal black holes when the particles reach the event horizon by adjusting the angular momentum of the incident particles. However, for non-extremal black holes, the CM energy of particles that reach the event horizon cannot diverge by adjusting the angular momentum of the incident particles.Comment: 17 pages, 7 figures, 2 Table

Gravitational lensing by a stable rotating regular black hole

Recent observational data from the Event Horizon Telescope (EHT) collaboration provide convincing realistic evidence for the existence of black hole rotation. From a phenomenological perspective, a recently proposed stable rotating regular (SRR) black hole circumvents the theoretical flaws of the Kerr solution. For the purpose of obtaining observational signatures of this black hole, we study its gravitational lensing effect. In the strong field limit, we calculate the deflection angle of light, the radius of the photon sphere, and other observables. The observables include the relativistic image position, separation, magnification, and time delays between different images. Then, by modeling M87* and Sgr A* as the SRR black hole, we compute their observables and evaluate the deviation of the observables from the Kerr case. In the weak field limit, we calculate the light deflection angle of M87* and Sgr A* via the Gauss-Bonnet theorem (GBT). With the growth of deviation parameter $e$, the gravitational lensing effect in the weak field limit intensifies monotonically, and the gravitational lensing effect in the strong field limit changes dramatically only at high spins. Our research may contribute to distinguish between SRR black holes from Kerr black holes under higher-precision astronomical observations.Comment: 24 pages, 10 figures, 4 table