  Continuum spectrum emitted by the accretion disk around quasars hold a wealth
of information regarding the strong gravitational field produced by the massive
central object. Such strong gravity regime is often expected to exhibit
deviations from general relativity (GR) which may manifest through the presence
of extra dimensions. Higher dimensions, which serve as the corner stone for
string theory and M-theory can act as promising alternatives to dark matter and
dark energy with interesting implications in inflationary cosmology,
gravitational waves and collider physics. Therefore it is instructive to
investigate the effect of more than four spacetime dimensions on the black hole
continuum spectrum which provide an effective astrophysical probe to the strong
gravity regime. To explore such a scenario, we compute the optical luminosity
emitted by a thin accretion disk around a rotating supermassive black hole
albeit in the presence of extra dimensions. The background metric resembles the
Kerr-Newman spacetime in GR where the tidal charge parameter inherited from
extra dimensions can also assume negative signature. The theoretical luminosity
computed in such a background is contrasted with optical observations of eighty
quasars. The difference between the theoretical and observed luminosity for
these quasars is used to infer the most favoured choice of the rotation
parameter for each quasar and the tidal charge parameter. This has been
achieved by minimizing/maximizing several error estimators, e.g., chi-squared,
Nash-Sutcliffe efficiency, index of agreement etc. Intriguingly, all of them
favour a negative value for the tidal charge parameter, a characteristic
signature of extra dimensions. Thus accretion disk does provide a significant
possibility of exploring the existence of extra dimensions through its close
correspondence with the strong gravity regime.
