Astrophysics Questions

Zillah

Such a cliche but this stuff has literally kept we awake tonight. A lot of this is probably going to sound very amateurish but if someone could please help me sort some of this out I'd be very happy.

Ok, first of all, a bit about dark matter and blackholes.

Dark Matter is the term we use to describe the apparently sourceless gravity that is holding the galaxies together. There is more gravity than the amount of matter can account for. One problem I have understanding this is that I thought blackholes were incredibly difficult to detect considering that they are essentially non-radiating bodies. And considering that blackholes would account for a huge amount of the matter in the universe how can we conclude there is not enough matter to account for observable gravity when a titanic amount of matter is already invisibly tied up in blackholes?

That's the big question that is bothering me. Now a hypothetical that's been bouncing around my head. If we assume for a moment that Dark Matter is literally a form of matter which is only detectable by its gravity, but behaves normally in terms of mass and gravity, then is it possible for massive amounts of Dark Matter to collapse into a singularity? And if so, once the Dark Matter vanishes into the blackhole would such a Dark Matter blackhole be in any way distinguishable from a normal-matter blackhole? We can't see inside a singularity once it forms, so who's to say what kind of matter is in there, except to say that it is producing gravity, which could be caused by regular or dark matter. Considering blackholes are detected by their gravity (and I think radiation from their accretion disc? -- Which could be present anyway) how could we tell them apart?

In fact, if Dark Matter is as common as we're led to believe, then isn't it likely that most blackholes are at least hybrids?

dbran

Hii Zillah

Some very thought provoking stuff there for a Friday afternoon.:)

I understand that Dark Matter is a type of matter that is totally different from ordinary baryonic matter and that we don't have the remotest clue what it is. It dosen't interact with ordinary matter at all other then with gravity.

Blackholes are made up of ordinary matter although massively compressed into compact objects. But they still retain the ability to interact with ordinary matter, have momemtum, spin, magnetic fields etc. They effectively have a "presence" in space as an area of high mass and high gravity.

When you look at the distribution of matter in the Milky Way you would think that most of the matter is in the center and less out towards the edge. That is after all the distribution of stars, gas and dust that we see in the galaxy. This would make you think that the the stars nearer the center are orbiting significantly faster then those out towards the edge. It turns out that the stars are in fact orbiting radially ie the same speed irrespectve of their distance from the center. This which suggests that there is a massive halo of "stuff" pervading the galaxy which we cannot see but is affecting the gravity of the milky way in a uniform way. The ordinary matter is like the froth or foam on a massive pool of dark matter which is the real player when it comes to gravity in the galaxy. But this dark matter dosent have a "presence" because you cant point to it an say you know where it is and what it is.

If the dark matter or "stuff" was made up of black holes then there would have to be a hell of a lot of them and they would be so pervasive that we would be seeing constant evidence of them such as microlensing events, gama ray bursts etc associated with them. There just isnt enough of this going on that we can see to account for the amount of dark matter needed to explain why the stars orbit the milkey way as they do.

As for dark matter forming a singularity, who really knows as we don't really have a clue what the properties of dark matter are. So this is a question that is unanswerable at present.

Im not a physics expert but this is my understanding of it.

Hope this lets you sleep better at night.

dbran