
Scientists Find Elusive Giant Black Hole Pairs - kizzy55
https://www.nasa.gov/mission_pages/chandra/news/seeing-double-scientists-find-elusive-giant-black-hole-pairs.html
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Very cool, this works uses a lot of observational data from different
telescopes. The LBT [1] is, depending on how you measure, the largest optical
telescope in the world (two 8 meter mirrors on the same mount versus 10.4 m
GCT [2]).

But one of the things that the articles mentions that I think is really cool
and may not be familiar to many people, what with LIGO and gravitational waves
in the news:

> LIGO is not able to detect gravitational waves from supermassive black hole
> pairs. Instead, pulsar timing arrays such as the North American Nanohertz
> Observatory for Gravitational Waves (NANOGrav) are currently performing this
> search. In the future, the Laser Interferometer Space Antenna (LISA) project
> could also search for these gravitational waves.

I've visited the GBT [3,4] several times for observing and have heard some
presentations on the NANOGrav [5] project. It's very cool. The signal
properties of pulsars can be measured so accurately, that they can be used to
accurately constrain very low frequency gravitational waves, ie nanohertz
frequencies (hence the clever acronym), for binary supermassive blackholes
with wide separations. This complements the high frequency gravitational waves
detected by LIGO of merging compact objects, in the 10-100 Hz range. If I'm
remembering correctly, you don't measure any particular pair of supermassive
black holes, but measure the field from all such systems (at the Earth).
Measurements have not been sensitive enough yet to detect this field, but as
they dig to higher sensitivity, they are able to place constraints on
cosmological models that predict how common binary supermassive blackholes are
(we really don't have a good idea) and what the distribution of masses and
separations are.

[1]
[https://en.wikipedia.org/wiki/Large_Binocular_Telescope](https://en.wikipedia.org/wiki/Large_Binocular_Telescope)
[2]
[https://en.wikipedia.org/wiki/Gran_Telescopio_Canarias](https://en.wikipedia.org/wiki/Gran_Telescopio_Canarias)
[3] [http://greenbankobservatory.org/](http://greenbankobservatory.org/) [4]
[https://en.wikipedia.org/wiki/Green_Bank_Telescope](https://en.wikipedia.org/wiki/Green_Bank_Telescope)
[5] [http://nanograv.org/](http://nanograv.org/)

