User:DEBJYOTI1992

How many black holes are known to man that are in the universe? And out of those know black holes what are the names?

The Answer The answer to your first question depends a lot on how strong an evidence you would want to accept something as a black hole.

Astrophysicists generally agree that when the compact object in an X-ray binary system is shown to be more massive than about 3 times the mass of the Sun, then this compact object is a black hole beyond reasonable doubt. These are called "dynamically confirmed black holes." Dr. Orosz maintains a list of such systems (currently 14) on his home page:

If you accept a less strict standard of evidence, then there are many more black holes that (we think) we know of. These include additional X-ray binaries such as Cygnus X-1, the mysterious object at the center of our Galaxy, and the central objects in many (perhaps even most) luminous galaxies.

For example, the Sloan Digital Sky Survey aims to measure the distances to more than a million galaxies and quasars:

A large fraction of these galaxies, and all the quasars, are thought to contain a supermassive black hole. Given such a huge number, there is no plan to individually name these black holes; astrophysicists use designations based on their positions on the sky (you see examples on Dr. Orosz's page). The Question (Submitted September 13, 2001)

Where is the closest black hole and how far away is it?

The Answer It is actually difficult to determine the distance to black holes, but a nearby object believed to be a black hole from observations of strong X-ray emission is Cygnus X-1, located about 8000 light years away. Cyg X-1 is an ordinary star that is believed to be orbiting a black hole. There are other nearby candidates for black holes which include :

GRO J0422+32 =V518 Per 1.39 kpc or 2 kpc A0620-00 = V616 Mon 0.87 kpc or 1.05 kpc XTE J1118+480 0.83 kpc or 1.8 kpc

(kpc stands for kiloparsec, or approximately 3250 light years) The Question (Submitted October 24, 2001)

Why do theories say that time will slow down on the event horizon and then go backwards? Please explain in laymans terms.

The Answer Time does not slow down and then go backwards at a black hole's event horizon, but a lot of strange stuff does occur.

If you (in a space ship, for example) were to approach the event horizon and cross it, to a person watching you from a great distance it would look like you moved slower and slower as you got closer and closer to the horizon. To them it would look like you never quite reached the horizon. But this is an illusion caused by the fact that the light you emit from your space ship is taking longer and longer to reach the outside observer. This is due to the black hole's immense gravity. From your own point of view, you reach the horizon and cross it, with nothing special happening at the boundary. But of course, the gravitational forces of the black hole will crush you do death sooner or later! f light doesn't have a mass, how can it get sucked into a black hole?

The Answer In a Black Hole, light traveling outwards towards an event horizon is pulled back by the very strong gravitational field, because of the warping of space-time inside the event horizon, regardless of the lack of a photon mass. This prevents light from ever escaping the Black Hole. My question is, what would happen to the all the matter in the universe when it turns into stars, which in turn turn into black-holes, which would suck in more matter, would it just become an "empty space?"

The Answer First off, all the matter in the universe will not end up in black holes. Most stars in the universe don't have enough mass to become black holes at the end of their lives. Neutron stars and white dwarfs are much more numerous; this is what most stars end up as.

Secondly, black holes are not cosmic vacuum cleaners, whatever you may have heard. They will not suck up everything in the universe. They only suck up what crosses their event horizons.

The universe, if it is "open" and keeps on expanding forever, will probably end up as a cosmic graveyard, populated by things like black holes, neutron stars, and white dwarfs. The Question (Submitted October 30, 2001)

Given that the universe may have started from a singularity in a Big Bang, and that it seems that an awful lot of the universe is going to end up at the singularity inside a Black Hole, is it possible there is a connection between the two?

I've seen the depiction of Space-Time as a rubber sheet with each mass causing a distortion to it -(recall Homer Simpson looking into the deep distortion caused by a Black Hole?) - Is it possible for the singularity of a sufficiently massive Black Hole to distort Space-Time to the extent where it ruptures, the singularity exploding into the void beyond giving rise to another Big Bang and the start of another universe?

The Answer Although the Big Bang also represents a spacetime singularity, it is not really a black hole. Actually, the Big Bang has more of a resemblance to the time-inverse of black holes: white holes (that may not actually exist in nature). But the Big Bang singularity is not really a white hole either -- there are technical differences in the natures of their event horizons and their connection to the rest of the universe and its constituents.

It is also not necessarily the case that "an awful lot of the universe is going to end up at the singularity inside a Black Hole," since the universe could very well be Open rather than Closed and simply expand forever (in fact the latest evidence seems to point in that direction).

The Big Bang included all of spacetime and so does not represent a rupturing of some space in which it is embedded (that is, there is no "void beyond" the universe), and there is no danger of supermassive black holes causing any sort of miniature Big Bangs although they are responsible for the super-energetic phenomena know as quasars.

-- Michael Loewenstein and Amy Fredericks for "Ask an Astrophysicist"