Constraints on Barrow Entropy from M87 and S2 Star Observations*

Barrow holographic dark energy with Granda-Oliveros cutoff

A study on the effects of implementing the Granda-Oliveros infrared cutoff in the recently introduced Barrow holographic dark energy model is presented, and its cosmological evolution is investigated. We find how the deformation parameter, Δ , affects the evolution of H(z) and that from this model it is possible to obtain an accelerated expansion regime of the universe at late times. We also observe that increasing Δ causes a transition of the EoS parameter from quintessence to phantom regimes. In addition, we show that the model can be used to describe the know eras of dominance. Finally, after studying the stability of the proposed model, a fit of the corresponding parameters is preformed, utilizing the measurements of the expansion rate of the universe, H(z). The best fit of the parameters is found to be ( α , β , Δ ) = 1 . 00 - 0.02 + 0.02 , 0 . 69 - 0.02 + 0.03 , 0 . 000 - 0.000 + 0.004 at 1 σ C.L., for which the Bekenstein-Hawking relation is favored.

Constraining MOdified Gravity with the S2 Star

We have used publicly available kinematic data for the S2 star to constrain the parameter space of MOdified Gravity. Integrating geodesics and using a Markov Chain Monte Carlo algorithm, we have provided the first constraint on the scales of the Galactic Centre for the parameter α of the theory, which represents the fractional increment of the gravitational constant G with respect to its Newtonian value. Namely, α≲0.662 at 99.7% confidence level (where α=0 reduces the theory to General Relativity).