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The remarkable agreement of general relativity (GR) with a host of experimental tests only adds to its phenomenal
success. However, its failure to explain observations like galactic rotation curves and the accelerated expansion of the
universe makes the quest for a more complete theory increasingly compelling which may either involve modifications
to the gravity sector or additions to the matter sector or both. In this talk I will discuss three possible alternatives
to Einstein gravity having their origin in three distinct paradigms-(a) higher dimensions, (b) higher curvature gravity
and (c) a class of generalized Horndeski theories. In order to test these theories against observations we choose the
near horizon regime of black holes where the curvature effects are maximum. Interestingly all the aforesaid models
exhibit black hole solutions with tidal charge parameter that can assume negative values, in sharp contrast with
the Reissner-Nordstr¨om scenario in GR. The continuum spectrum emitted from the accretion disk around black holes
encodes the imprints of the background spacetime and hence acts as a storehouse of information regarding the nature of
gravitational interaction in the extreme situations. Using the optical data of eighty Palomer Green quasars we illustrate
that the difference between the theoretical estimates of luminosity and the observational results gets minimized for
negative values of the tidal charge parameter. Error estimators, including reduced-χ
2
, Nash-Sutcliffe efficiency, index
of agreement and modified versions of the last two are used to solidify our conclusion and the implications of our result
are discussed.
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