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

## TL;DR

This paper investigates how extra dimensions influence the continuum spectrum of quasars by comparing theoretical models with observations, suggesting signatures of extra dimensions through a negative tidal charge parameter.

## Contribution

It introduces a method to analyze quasar spectra to detect signatures of extra dimensions via the tidal charge parameter in a Kerr-Newman-like spacetime.

## Key findings

- Theoretical luminosity models favor a negative tidal charge, indicating extra dimensions.
- Comparison with 80 quasars constrains the black hole spin and tidal charge.
- Results support the possibility of detecting extra dimensions through accretion disk spectra.

## Abstract

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.