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Continuum spectrum emitted by the accretion disk around quasars hold a wealthof information regarding the strong gravitational field produced by the massivecentral object. Such strong gravity regime is often expected to exhibitdeviations from general relativity (GR) which may manifest through the presenceof extra dimensions. Higher dimensions, which serve as the corner stone forstring theory and M-theory can act as promising alternatives to dark matter anddark energy with interesting implications in inflationary cosmology,gravitational waves and collider physics. Therefore it is instructive toinvestigate the effect of more than four spacetime dimensions on the black holecontinuum spectrum which provide an effective astrophysical probe to the stronggravity regime. To explore such a scenario, we compute the optical luminosityemitted by a thin accretion disk around a rotating supermassive black holealbeit in the presence of extra dimensions. The background metric resembles theKerr-Newman spacetime in GR where the tidal charge parameter inherited fromextra dimensions can also assume negative signature. The theoretical luminositycomputed in such a background is contrasted with optical observations of eightyquasars. The difference between the theoretical and observed luminosity forthese quasars is used to infer the most favoured choice of the rotationparameter for each quasar and the tidal charge parameter. This has beenachieved by minimizing/maximizing several error estimators, e.g., chi-squared,Nash-Sutcliffe efficiency, index of agreement etc. Intriguingly, all of themfavour a negative value for the tidal charge parameter, a characteristicsignature of extra dimensions. Thus accretion disk does provide a significantpossibility of exploring the existence of extra dimensions through its closecorrespondence with the strong gravity regime.

Decoding signatures of extra dimensions and estimating spin of quasars from the continuum spectrum