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Presents information on Sagittarius A*, the central black hole of the Milky Way. Mass equivalent; Astronomical observations of Sagittarius A*; Description of the light rays passing near Sagittarius A*.

Focuses on the astrophysical bodies located in the center of the Milky Way galaxy. Description of the astrophysical entities; Interaction among gaseous particles and photons of various wavelengths; Collisions of stellar winds; Information on a circumnuclear disk in the galaxy; Possible existence of a black hole in the galaxy's center.

We consider here some popular f(R) models generally viewed as possible alternatives to the existence of dark energy in General Relativity. For each of these, we compute the redshift zmax at which the angular diameter distance dA(z) is expected to reach its maximum value. This turning point in dA(z) was recently measured in a model-independent way using compact quasar cores and was found to occur at zmax=1.70±0.20. We compare the predictions of zmax for the f(R) models with this observed value to test their viability at a deeper level than has been attempted thus far, thereby quantifying an important observational difference between such modified gravity scenarios and standard Lambda Cold Dark Matter (ΛCDM) cosmology. Our results show that, while the most popular f(R) models today are consistent with this measurement to within 1σ, the turning point zmax will allow us to prioritize these alternative gravity theories as the measurement precision continues to improve, particularly with regard to how well they mitigate the tension between the predictions of ΛCDM and the observations. For example, while the Hu-Sawicki version of f(R) increases this tension, the Starobinky model reduces it. [ABSTRACT FROM AUTHOR]