Ultra-light bosons such as axions or axion-like particles (ALPs), are promising candidates to solve the dark matter problem. A unique way to detect such ALPs is to search for the periodic oscillation feature of the position angles of linearly polarized photons emitted from the galaxy center. In this work, we use the high-resolution polarimetric measurements of the radiation near the super-massive black hole (SMBH) in the center of the Milky Way, i.e., Sagittarius A⋆ (Sgr A⋆), by a sub-array of the Event Horizon Telescope to search for the ultra-light ALPs. We derive upper limits on the ALP-photon coupling of ∼ 10-12  GeV-1 for ALP masses of ma∼ (10-19-10-18) eV, with a solitonic core + NFW dark matter density profile. Our results are stronger than that derived from the observations of SN1987A and a population of supernovae in the mass window of (10-19-10-17) eV. Improved polarimetric measurements with the full Event Horizon Telescope can further strengthen the constraints.

We calculate the deflection angle, as well as the positions and magnifications of the lensed images, in the case of covariant f(T) gravity. We first extract the spherically symmetric solutions for both the pure-tetrad and the covariant formulation of the theory, since considering spherical solutions the extension to the latter is crucial, in order for the results not to suffer from frame-dependent artifacts. Applying the weak-field, perturbative approximation we extract the deviations of the solutions comparing to General Relativity. Furthermore, we calculate the deflection angle and then the differences of the positions and magnifications in the lensing framework. This effect of consistent f(T) gravity on the lensing features can serve as an observable signature in the realistic cases where f(T) is expected to deviate only slightly from General Relativity, since lensing scales in general are not restricted as in the case of Solar System data, and therefore deviations from General Relativity could be observed more easily.

We consider the effective field theory formulation of torsional gravity in a cosmological framework to alter the background evolution. Then we use the latest H 0 measurement from the SH0ES Team, as well as observational Hubble data from cosmic chronometer and radial baryon acoustic oscillations, and we reconstruct the f(T) form in a model-independent way by applying Gaussian processes. Since the special square-root term does not affect the evolution at the background level, we finally summarize a family of functions that can produce the background evolution required by the data. Lastly, performing a fitting using polynomial functions and implementing the Bayesian information criterion, we find an analytic expression that may describe the cosmological evolution in great agreement with observations.

We calculate the quasinormal modes (QNM) frequencies of a test massless scalar field and an electromagnetic field around static black holes in f(T) gravity. Focusing on quadratic f(T) modifications, which is a good approximation for every realistic f(T) theory, we first extract the spherically symmetric solutions using the perturbative method, imposing two ansätze for the metric functions, which suitably quantify the deviation from the Schwarzschild solution. Moreover, we extract the effective potential, and then calculate the QNM frequency of the obtained solutions. Firstly, we numerically solve the Schrödinger-like equation using the discretization method, and we extract the frequency and the time evolution of the dominant mode applying the function fit method. Secondly, we perform a semi-analytical calculation by applying the WKB method with the Pade approximation. We show that the results for f(T) gravity are different compared to General Relativity, and in particular we obtain a different slope and period of the field decay behavior for different model parameter values. Hence, under the light of gravitational-wave observations of increasing accuracy from binary systems, the whole analysis could be used as an additional tool to test General Relativity and examine whether torsional gravitational modifications are possible.