Could the Universe's dark matter be made up of black holes?
There is a lot of controvery about what dark matter is, but couldn't some of the missing matter be contained in black holes? Once matter is in a black hole, it cannot be seen or measured, so it's unaccountable.
Once matter is in a black hole, it cannot be seen, but it's not really true to say that its effect cannot be measured. Black holes still exert a gravitational influence due to their mass, just like every other massive object in the Universe. This is how we actually discover and measure the mass of black holes: by watching their effect on the matter around them. For instance, the supermassive black hole at the center of the Milky Way galaxy is so strong gravitationally that the stars very near it orbit at a very, very high rate. Using this and the equations that describe the orbits of these stars, we can actually estimate the mass of the black hole.
We know that dark matter exists in galaxies because the rotation curve is flat at large distances from the center of the galaxy. A "rotation curve" is simply what it sounds like: a graph of how fast the "stuff" in a galaxy is rotating as a function of distance from the center. Gravity predicts that V = sqrt (GM/R). The "M" means all the mass that is interior to the radius R.
A rotation curve is flat when the velocity is constant. If you look at this equation, this means that M/R must also be constant. So that means that as we go farther and farther out in a galaxy, the mass is growing (a LOT) even though the starlight is falling off dramatically. There needs to be some matter that we don't see. A host of other cosmological observations also imply the existence of dark matter, and amazingly, they predict about the same amount!
What's important to realize about this is that our studies of dark matter don't just tell us that "it's out there somewhere"; when we study a galaxy, we learn something about the total distribution of matter within it. This means that we know the dark matter surrounds galaxies and is not a central object, like a black hole, within galaxies.
The problem with your idea is that black holes are nothing that special, gravitationally: they're just accretions of matter. They are centralized in the middle of the galaxy, and according to the laws of gravity, they can't pull very hard on stuff far out at the edge of a galaxy.
Get More 'Curious?' with Our New PODCAST:
- Podcast? Subscribe? Tell me about the Ask an Astronomer Podcast
- Subscribe to our Podcast | Listen to our current Episode
- Cool! But I can't now. Send me a quick reminder now for later.
- What is dark matter?
- What's the difference between dark matter and dark energy?
- Could dark energy come from matter anti-matter pairs forming spontaneously and then annihilating?
- Could neutrinos be dark matter?
- Could photons be dark matter?
How to ask a question:
If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.
This page has been accessed 15144 times since September 12, 2007.
Last modified: September 12, 2007 2:52:17 PM
Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.
Warning: Your browser is misbehaving! This page might look ugly. (Details)