How can we detect a black hole accurately?

Question

I know there are many ways in detecting a black hole, but there are odds against them too. So I wanted to know a accurate method to detect a black hole.

Here are the known methods to me and the odds I think

One is by observing a dark region of space where even photon particles are missing and other is by detecting the X rays emitted from the black holes from our reference (say earth)

• If we decide to tell there is a black hole in a specific space by the absence of light, because black hole doesn't allow light to pass over them. The odd is that one can say that there is no nearby star producing enough light or can say that the light is not reaching the reference.
• If we decide to tell there is a black hole due to X rays emitted from it, one can say that the atmosphere in reference place is absorbing the X rays and not allowing them to get detected.

So is there any way other than these two to detect a black hole in space accurately?

Answer 1

Have a look at the paper Experimental Evidence of Black Holes, which describes the experimental evidence that would prove the existance of black holes. There is little point in me reproducing the contents of the paper here, but it's worth giving a couple of examples:

One method is to measure the mass and radius of an object. For example we can measure the mass of the black hole at the centre of the Milky Way, Sagittarius A$^*$, by observing the orbits of stars around it. This works out to be about four million Solar masses. We can also estimate upper limits for the size of Sagittarius A$^*$. These upper limits are considerably greater than the Schwarzschild radius ($r_s$ is about 12 million km) but they are low enough that an object of that density must inevitably collapse into a black hole.

Another method is to measure emissions from gas accreting onto the black hole, and show the emissions match those expected. For example this was done for the archetypal black hole Cygnus X-1, and provides evidence that it is indeed a rotating black hole.

Answer 2

Evidence for black holes are indirect. The absence of light in a certain region is not a proof of the presence of black hole, since other objects can produce the same effect. Also, a black hole does not emit radiation, apart from the hypothetical Hawking radiation, which is nevertheless very weak.

One way to detect a black hole is by detecting its gravitational effect on the lights of objects nearby. A black hole (as any other very massive object) acts as a gravitational lens (see Wikipedia). By studying the "bending" of the light emitted by objects behind the black hole (in our visual perspective) it is possible to measure its mass and to determine whether the gravitational lens effect is consistent with a black hole.

In Wikipedia there is a list of other possible detection methods, which are all indirect.