Grav­i­ta­tional waves hint at black hole se­crets

Why do some black holes ex­ist as pairs? No one knows but re­searchers are con­fi­dent the an­swer is not far away.

Cosmos - - Digest -

The de­tec­tion of grav­i­ta­tional waves has launched “an en­tirely new sub­field of as­tron­omy” that will even­tu­ally ex­plain how black holes join up into pairs.

That’s the pre­dic­tion of Steinn Sigurdsson, from Penn­syl­va­nia State Univer­sity, com­ment­ing in the jour­nal Na­ture on re­search ex­am­in­ing three grav­i­ta­tional waves em­a­nat­ing from bi­nary black hole sys­tems.

The re­search in ques­tion, led by Will Farr from the Univer­sity of Birm­ing­ham, fo­cused on four black hole pairs – three dis­cov­ered in 2015 and one found this year – and com­pared mod­elled grav­i­ta­tional waves against the ac­tual data recorded.

The ob­ject of the ex­er­cise was to an­swer a com­pelling astro­nom­i­cal ques­tion: how do bi­nary black hole sys­tems form?

There are two pri­mary the­o­ries. The first is that they are cre­ated when two pre­vi­ously ex­ist­ing black holes fall into each other’s grav­i­ta­tional field. The sec­ond is that they arise from bi­nary star sys­tems: paired stars that re­main in each other’s or­bit in death as in life.

One key piece of ev­i­dence that will even­tu­ally solve the mys­tery is the an­gu­lar dis­tri­bu­tion of each hole’s spin in re­la­tion to its or­bit. As early as 1993 Sigurdsson sug­gested that spin an­gle was a vi­tal clue – if only it could be de­tected with suf­fi­cient pre­ci­sion.

If the black holes ex­isted in­de­pen­dently be­fore merg­ing, the the­ory sug­gests, then the dis­tri­bu­tion of the mea­sured spin should be “iso­tropic”. That is, the spins of each hole should be aligned at ran­dom, with no re­la­tion to the di­rec­tion of their or­bit around each other.

If, on the other hand, the black holes arose from the death of an al­ready paired star sys­tem, then the spin should be pref­er­en­tially aligned with the or­bit.

Farr and col­leagues re­port that data gleaned from grav­i­ta­tional wave de­tec­tions as­so­ciated with the black hole pairs dubbed GW150914, LVT151012, GW151226 and GW170104 set the odds very slightly in favour of iso­tropic re­sults – in­di­cat­ing that pairs are cre­ated when in­di­vid­ual black holes col­lide with each other.

Avail­able in­for­ma­tion, how­ever, is in­suf­fi­cient to make a de­fin­i­tive call. The study au­thors and Sigurdsson agree that the real ben­e­fit of the re­search lies in the pre­ci­sion of the anal­y­sis, and the con­se­quent re­duc­tion of the amount of new ev­i­dence re­quired be­fore a de­fin­i­tive con­clu­sion can be made.

The Farr team es­ti­mates that only an­other 10 grav­i­ta­tional waves as­so­ciated with bi­nary sys­tems will be needed. At that point, the au­thors say, “the ex­ist­ing pref­er­ence for ei­ther an iso­tropic spin dis­tri­bu­tion or low spin mag­ni­tudes for the ob­served sys­tems will be con­firmed (or over­turned) con­fi­dently in the near fu­ture”.

Sigurdsson adds that the find­ings are “im­por­tant be­cause they tell us how many data are needed to test the main for­ma­tion the­o­ries, and show that the num­ber of re­quired ob­ser­va­tions is likely to be achieved in the near fu­ture.”

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