[AnkitaA (OP)]

Richard asks

"Detecting black holes is really hard unless they are accreting material. Supermassives will be at the centre of galaxies therefore easy to find, but there are many stellar sized black holes within our galaxy’s disc.  Because they have a temperature of very nearly absolute zero, they are 3 degrees cooler than the background temperature of the universe.  So with the right telescope looking at the right temperature a non accreting black hole could be spotted by detecting a cold spot in the field being observed.  However, stellar black holes are tiny so the problem is telescope resolution and distance.

With today’s equipment how close would a stellar black hole need to be to is to us in order for a black hole to be spotted using this method?"

Do you have an answer?

[pensador]

Richard asks

"Detecting black holes is really hard unless they are accreting material. Supermassives will be at the centre of galaxies therefore easy to find, but there are many stellar sized black holes within our galaxy’s disc.  Because they have a temperature of very nearly absolute zero, they are 3 degrees cooler than the background temperature of the universe.  So with the right telescope looking at the right temperature a non accreting black hole could be spotted by detecting a cold spot in the field being observed.  However, stellar black holes are tiny so the problem is telescope resolution and distance.

With today’s equipment how close would a stellar black hole need to be to is to us in order for a black hole to be spotted using this method?"

Do you have an answer?

How could you detect an object in space colder than the CBR, the CBR would swamp the signal would it not?.

You can detect a black hole via the lensing effect on light, or by the movement of other visible objects around it.

Edit the Boomerang Nebulae is about 1.5 Kelvin, the temperature was detected like this http://adsabs.harvard.edu/abs/1997ApJ...487L.155S. I don't think this method would work for your black hole.

Edit 2 noddy explanation of boomerang nebulae temperature measurement http://www.sci-news.com/astronomy/science-boomerang-nebula-01493.html

[Halc]

You can detect a black hole via the lensing effect on light, or by the movement of other visible objects around it.

Lensing and disturbance are indeed the best way to detect things nearby.  I want to add that these methods do not identify the object as a black hole unless the mass is measured large enough for one.  There are plenty of objects that are cold enough to emit effectively no light, including MaCHO dark matter.  Such objects can be discovered by deviations of orbits of large distant stars like S2, but I don't think a mass measurement can be taken of such things at that distant if they're not part of a binary pair.

[Astrogazer]

We have seen pictures of the CMB which are created by ‘measuring’ the temperature of the sky.  The thermal fluctuations that needed to be resolved were ‘tiny’ and certainly less than 1 deg Kelvin.  Surely therefore detecting a 3 deg Kelvin difference should be easy, if only it was large enough to appear as an ultra cold spot.
But what is the best angular resolution obtainable from the equipment that measured the CMB?  From that we could estimate how close a stellar black hole would need to be for it to be detected by this method.  (Probably as close as Jupiter, lol)

[evan_au]

The Gaia spacecraft is currently in the process of mapping the positions of a billion stars in our galaxy, and measuring the velocity of the closer ones.

It does this by repeatedly measuring the position of stars with extreme accuracy, about 70 times over 5 years. It is able to detect stars down to astronomical magnitude 20.

If there is a "dark" black hole lurking in our galactic neighborhood, Gaia may be able to detect this as anomalous position readings for stars in one of the measurements. This would show up as an inconsistent reading, perhaps affecting several stars in a part of the sky.
See: https://en.wikipedia.org/wiki/Gaia_(spacecraft)

[pensador]

We have seen pictures of the CMB which are created by ‘measuring’ the temperature of the sky.  The thermal fluctuations that needed to be resolved were ‘tiny’ and certainly less than 1 deg Kelvin.  Surely therefore detecting a 3 deg Kelvin difference should be easy, if only it was large enough to appear as an ultra cold spot.
But what is the best angular resolution obtainable from the equipment that measured the CMB?  From that we could estimate how close a stellar black hole would need to be for it to be detected by this method.  (Probably as close as Jupiter, lol)

No the CBR is measured from satellites, that measure the temperature in different directions relative to the satellites, they can not discriminate depth or distance, in the measurements. If something measuring 0Kelvin located a light year away existed it could not be detected because the surrounding CBR at 2.75Kelvin +/- a fraction of a kelvin would swamp, the lower reading. The only way of extracting the temperature would be to make a local measurement, or for the source to be emitting radio waves like the boomerang nebulae. The black hole in question emits nothing, except maybe Hawking radiation, which would be at a lower energy level than the CBR and again would be undetectable.

[evan_au]

what is the best angular resolution obtainable from the equipment that measured the CMB?

The Planck satellite has produced our best map of the CMB; it has an angular resolution of 5-10 arc minutes (0.08°).
But it ( also has a temperature resolution of about a millionth of a degree.
See: http://planck.mpa-garching.mpg.de/Planck/planck.html

From that we could estimate how close a stellar black hole would need to be for it to be detected by this method.

A stellar black hole with the mass of the Sun would have a diameter of about 6km, and a black-body temperature of nanoKelvins (ie Planck could not distinguish it from absolute zero).

A back-of-the-envelope calculation: At 5 arc-minutes (.001 radians), this black hole would cover 1 pixel at a distance of 4000km, which is about 1% of the Earth-Moon distance.

However, it would make a detectable change in temperature in 1 pixel at a distance of around 70 AU = 70 times the Earth-Sun distance.

I suggest that a far more sensitive detection would come from measuring the motions of the dwarf planets in the outer Solar System.

The positions of these is measured to far greater accuracy than Planck can measure angles.
- In fact, the search for the hypothetical "Planet 9" is examining the orbits of these very carefully
- And suggestions are that they are perturbed some object with the mass of the Earth or greater
- This could detect a stellar-mass black hole at far greater distances than we could detect with measurements of the CMB.
- Sedna, one of the dwarf planets being observed, has an average orbital radius of 86 AU, so it would be sensitive to black holes at >10 times this distance.
See: https://en.wikipedia.org/wiki/Planet_Nine#Evidence

[evan_au]

Another factor is the direction that the black hole approaches from, and how quickly.

It is suggested that a black hole produced in an asymmetrical supernova could be ejected from the disk of the galaxy. If the black hole is moving rapidly, that reduces the chance that it would be detected before it got quite close.

If it approached along the Solar System axis, it would not produce such an obvious deflection of planetary orbits, compared to the situation where it is in the plane of the Solar System, and would be attracting closer minor planets more than others on the other side of the Solar System.