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In a stationary electrovacuum, asymptotically flat black holes in scalar-tensor theories of gravity are described by the Kerr-Newman family of solutions, just as in general relativity. We show that there exist two mechanisms which can render Kerr black holes unstable when matter is present in the vicinity of the black hxole, as this induces an effective mass for the scalar. The first mechanism is a tachyonic instability that appears when the effective mass squared is negative, triggering the development of scalar hair-- a black-hole version of "spontaneous scalarization." The second instability is associated with super-radiance and is present when the effective mass squared is positive and when the black-hole spin exceeds a certain threshold. The second mechanism is also responsible for a resonant effect in the superradiant scattering of scalar waves, with amplification factors as large as 105 or more. [ABSTRACT FROM AUTHOR]

We perform an analysis of the cosmic reionization in the standard cold dark matter (CDM) paradigm and in alternative dark matter scenarios. Building upon the work of Corasaniti et al., we predict the reionization history for CDM, for warm dark matter, late-forming dark matter and ultralight axion dark matter models which reproduce state-of-the-art measurements of the galaxy luminosity function at very high redshifts 6≤z≤10. To this purpose we adopt a reionization model parametrized in terms of the limiting UV magnitude of galaxies contributing to the reionization Mlim and the average effective escape fraction of UV photons reaching the intergalactic medium f. For each DM model we compute the redshift evolution of the Thomson scattering optical depth τe(z) and the comoving ionization fraction QHII(z). We find the different DM models to have similar reionization histories. Differences with respect to the CDM case increase at fainter limiting UV magnitudes and are degenerate with the effect of varying the reionization model parameters. Using Planck's determination of the integrated optical depth in combination with measurements of the neutral hydrogen fraction at different redshifts, we infer constraints on f and Mlim. The results are largely independent of the assumed DM scenario; in particular for Mlim≳-13 we obtain that the effective escape fraction lies in the range 0.07≲f≲0.15 at 2σ. [ABSTRACT FROM AUTHOR]

In a stationary electrovacuum, asymptotically flat black holes in scalar-tensor theories of gravity are described by the Kerr-Newman family of solutions, just as in general relativity. We show that there exist two mechanisms which can render Kerr black holes unstable when matter is present in the vicinity of the black hxole, as this induces an effective mass for the scalar. The first mechanism is a tachyonic instability that appears when the effective mass squared is negative, triggering the development of scalar hair--a black-hole version of "spontaneous scalarization." The second instability is associated with superradiance and is present when the effective mass squared is positive and when the black-hole spin exceeds a certain threshold. The second mechanism is also responsible for a resonant effect in the superradiant scattering of scalar waves, with amplification factors as large as 105 or more. [ABSTRACT FROM AUTHOR]