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Starts With A Bang

5 Things We Still Don’t Know About Black Holes (And 2 We Do) After LIGO

With a new data run coming in 2019 at unprecedented sensitivity, we might finally get our answers.

Over the past three years, LIGO discovered ten independent instances of merging black holes in our Universe.

A still image of a visualization of the merging black holes that LIGO and Virgo have observed so far. As the horizons of the black holes spiral together and merge, the emitted gravitational waves become louder (larger amplitude) and higher pitched (higher in frequency). The black holes that merge range from 7.6 solar masses up to 50.6 solar masses, with about 5% of the total mass lost during each merger. The frequency of the wave is affected by the expansion of the Universe. (TERESITA RAMIREZ/GEOFFREY LOVELACE/SXS COLLABORATION/LIGO-VIRGO COLLABORATION)

Despite all we’ve learned, five big unknowns still plague scientists.

Of all the merging black holes LIGO has observed, the lowest-mass progenitor is approximately 8 solar masses. Yet black holes as low as ~3 solar masses may exist. This is a limitation of our detectors so far: a gravitational wave’s amplitude is proportional to the merging black hole masses, and LIGO is not yet sensitive to the lowest-end of the mass spectrum. (NASA/AMES RESEARCH CENTER/C. HENZE)

1.) How small are the lowest-mass black holes?

LIGO has yet to detect any low-amplitude binaries, providing no information about this population.

The 30-ish solar mass binary black holes first observed by LIGO are very difficult to form without direct collapse. Now that it’s been observed twice, we can state that black holes of ~30 solar masses are common, but whether they’re more or less common than black holes of ~25 or ~35 solar masses remains to be determined. (LIGO, NSF, A. SIMONNET (SSU))

2.) Is there a pile-up of black holes above a certain mass?

We don’t have enough detections to know what mass of black holes are most abundant.

LIGO and Virgo have discovered a new population of black holes with masses that are larger than what had been seen before with X-ray studies alone (purple). This plot shows the masses of all ten confident binary black hole mergers detected by LIGO/Virgo (blue), along with the one neutron star-neutron star merger seen (orange). While the merging black holes seen are of approximately equal masses, we do not know whether this is universal or just a selection effect among the mergers seen so far. (LIGO/VIRGO/NORTHWESTERN UNIV./FRANK ELAVSKY)

3.) What are the mass ratios in binary systems?

The ones found so far are of nearly equal, 1-to-1 ratio masses. Large mass differences are hitherto undetected.

When you form two very massive stars in a binary star system, they can both become black holes, which may eventually inspiral and merge in an interesting fashion. Where these black holes form in the Universe, and which types of galaxies are most likely to house them, is still an unanswered question. (NASA, ESA AND G. BACON (STSCI))

4.) Where do black hole binaries form?

We haven’t identified whether they’re primarily located in rich clusters or isolated galaxies.

Black holes, when they merge, emit gravitational radiation that travels across the Universe at the speed of light. With enough black hole mergers detected, we should be able to determine if the merger rate increases, decreases, remains the same, or changes in a complex fashion as we go from earlier to later times in the Universe. (AEI POTSDAM-GOLM)

5.) Do merger rates change as the Universe evolves?

A dearth of events, particularly as a function of distance, prevents understanding whether or how merger rates change.

Aerial view of the Virgo gravitational-wave detector, situated at Cascina, near Pisa (Italy). Virgo is a giant Michelson laser interferometer with arms that are 3 km long, and complements the twin 4 km LIGO detectors. With three detectors instead of two, we can better pinpoint the location of these mergers and also become sensitive to events that would otherwise be undetectable. (NICOLA BALDOCCHI / VIRGO COLLABORATION)

On the other hand, we can already draw two amazing conclusions.

Star-forming regions, like the ones inside the Orion Nebula, in visible light (L) and infrared light (R), are where black holes get created. Where binary black holes form, whether in field (isolated) or clustered galaxies, has yet to be determined. But we do know that, of the binary systems we’ve found (and haven’t found), about 99% of them cannot be more massive than a certain threshold, which is around ~43 solar masses. (NASA; K.L. LUHMAN (HARVARD-SMITHSONIAN CENTER FOR ASTROPHYSICS, CAMBRIDGE, MASS.); AND G. SCHNEIDER, E. YOUNG, G. RIEKE, A. COTERA, H. CHEN, M. RIEKE, R. THOMPSON (STEWARD OBSERVATORY, UNIVERSITY OF ARIZONA, TUCSON, ARIZ.); NASA, C.R. O’DELL AND S.K. WONG (RICE UNIVERSITY))

1.) 99% of black holes in binary, merging systems are below 43 solar masses.

A computer simulation, utilizing the advanced techniques developed by Kip Thorne and many others, allow us to tease out the predicted signals arising in gravitational waves generated by merging black holes. Based on the event merger rate we’ve seen so far, we can finally estimate, with some accuracy, how many black holes originating from massive stars merge in the Universe every year: approximately 800,000. (WERNER BENGER, CC BY-SA 4.0)

2.) Our observable Universe contains 800,000 ± 500,000 merging black hole binaries per year.

LIGO’s sensitivity as a function of time, compared with design sensitivity and the design of Advanced LIGO. The “spikes” are from various sources of noise. As LIGO’s sensitivity becomes better and better, and as more detectors come online, our capabilities allow us to detect more of these waves, and the cataclysmic events that generate them, across the Universe. (AMBER STUVER OF LIVING LIGO)

With LIGO’s new data run coming later this year, we hope to obtain superior answers.

Mostly Mute Monday tells the scientific story of a physical phenomenon in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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