## From astrobites: “Bald Black Holes Wearing Wigs”

Jul 27, 2017

Lisa Drummond

Title: *No-Hair Theorem for Black Holes in Astrophysical Environments*

Authors: Norman Gürlebeck

First Author’s Institution: University of Bremen

Status: *Physical Review D*, open access

The No-Hair Theorem is a conjecture about the simplicity of black holes. They are called “bald” to reflect the paucity of information necessary to characterise their spacetime – only three parameters at most are needed. The theorem is formulated to describe isolated black holes. A realistic black hole will almost invariably be distorted by its astrophysical environment, e.g. a binary companion, surrounding plasma, accretion disks, or nearby jets. It seems possible that in these cases, the black hole could grow hair and the No-Hair Theorem would break down.

However, this paper shows that the distortions from the surrounding environment do not imply the black hole now has hair. Although our observations of the spacetime an infinite distance from the black hole do change, this is solely due to the external neighbourhood around the black hole, not the distorted black hole itself. In the words of Norman Gürlebeck, the author of this paper:

“Thus, even though the black hole might put on a wig it still looks bald.”

The No-Hair Theorem remains valid in this more general context: even when engulfed by a complex environment, the black hole resists growing hair.

The No-Hair Theorem: physical information that enters a black hole is lost forever. No matter what matter contributed to the black hole forming in the first place, and no matter what kind of matter falls into the black hole, the black hole still has no hair! Source: http://gravitation.web.ua.pt/index.php?q=node/362

See the full article here .

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