Journalism of Courage
Researchers from Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune and the University of KwaZulu-Natal (UKZN), Durban have shown that for a black hole’s horizon to stay unstretched while it absorbs matter, it must emit classical “Vaidya Radiation”.
A new research paper published in the journal Physical Review D (Letters), Rituparno Goswami of the University of KwaZulu-Natal (UKZN), Durban, and Naresh Dadhich of Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune have provided this new finding.
Professor Dadhich explained that the Vaidya radiation is a result of the work by eminent Indian physicist, relativist and mathematician professor Prahlad Chunnilal Vaidya in the 1940s. “Our current research gives a new and novel prediction using it,” professor Dadhich said. He added that it shows the phenomenon of accreting black holes giving out classical Vaidya radiation, paving the way for Hawking radiation to propagate out even when matter is falling in. This enables the black hole to “evaporate” quantum mechanically.
In a statement issued on Tuesday by IUCAA, the researchers said that “Hawking radiation is a well-known concept in Astrophysics, but something critical is required for it to come out so that a Black Hole can always remain ‘black’. Till now it was not understood how this sensitive condition was possible to achieve, with all the wobbles that may be caused by all the matter falling into the Black hole. A recent result addresses this issue showing that it must radiate out the classical “Vaidya Radiation” for this condition to be achieved.”
The present research shows that the Vaidya radiation in the case of an accreting Black hole is generated by heat produced due to tidal deformation of the in-falling matter. Goswami and Dadhich have shown that the necessary and sufficient condition for the horizon to remain unstretched and photon-like while the matter keeps falling in is that the heat must be eliminated outward as Vaidya radiation. Remarkably, the amount of energy to be radiated out is the same as required for the horizon to remain unstretched. That is, as matter reaches the horizon it attains the photon-like character of the horizon. It is perfectly in consonance with the horizon, and hence there is no need for it to get stretched.
“For a black hole to remain a black hole, the infalling fluid must be in consonance with fluid on the black hole horizon. For that, it has to undergo tidal deformations giving out heat flux which manifests as classical Vaidya radiation emanating from the boundary of the accreting zone. It is amazing that an accreting black hole not only radiates quantum Hawking but also classical Vaidya, and the latter paves the way for the former to reach out to infinity,” Dadhich added.
