The coalescence¹ of two black holes -- a very violent and exotic event -- is one of the most sought-after observations of modern astronomy. But, as these mergers³ emit no light of any kind, finding such elusive⁴ events has been impossible so far. Colliding black holes do, however, release a phenomenal amount of energy as gravitational waves. The first observatories⁵ capable of directly detecting these 'gravity signals' -- ripples⁶ in the fabric⁷ of spacetime first predicted by Albert Einstein 100 years ago -- will begin observing the universe later this year. 

 

When the gravitational waves rolling in from space are detected on Earth for the first time, a team of Northwestern University astrophysicists predicts astronomers⁸ will "hear," through these waves, five times more colliding black holes than previously⁹ expected. Direct observations of these mergers will open a new window into the universe.

 

"This information will allow astrophysicists to better understand the nature of black holes and Einstein's theory of gravity," said Frederic A. Rasio, a theoretical astrophysicist and senior author of the study. "Our study indicates the observatories will detect more of these energetic events than previously thought, which is exciting."

 

Rasio is the Joseph Cummings Professor in the department of physics and astronomy in Northwestern's Weinberg College of Arts and Sciences. 

 

Rasio's team, utilizing¹⁰ observations from our own galaxy¹¹, report in a new modeling study two significant findings about black holes:

 

 Globular clusters (spherical collections of up to a million densely¹³ packed stars found in galactic haloes) could be factories of binary¹⁴ black holes (two black holes in close orbit around each other); and 

 

 The sensitive new observatories potentially could detect 100 merging¹⁵ binary black holes per year forged in the cores of these dense¹² star clusters. (A burst of gravitational waves is emitted whenever two black holes merge².) This number is more than five times what previous studies predicted. 

 

The study has been accepted for publication by the journal Physical Review Letters and is scheduled to be published today (July 29).

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