Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Universe's Missing Matter and Black Holes

  1. Jan 9, 2009 #1
    I'm a non-scientist - that watches way too many TV science programs - that is having trouble putting the pieces together.

    I am always hearing about how much visible matter is missing from the universe. I am also hearing about how (it is now believed) that every galaxy contains a massive black hole at it's center. I also recently read where it's believed that the black holes were simultaneouly created along with the galaxies. Here's what I'm not getting:

    1. Billions of years ago, the galaxies were much closer to each other. Also, the galaxies were much more compact - the universe and galaxies hadn't expanded as greatly as today.

    2. Since everything was so much closer, it would have been much easier for the black holes to feed on visible matter.

    3. And, these black holes have been feeding on visible matter for billions of years.

    4. Plus, how many invisible black holes are floating around the universe, of unimaginable mass, that totally consumed their galaxies billions of years ago.

    So, ... how can scientists be certain that the visible matter that is missing hadn't been gobbled up by massive black holes over billions of years? I can't understand how one could know.

    I also read where the missing matter is in the form of gases in the voids of the universe. ???
  2. jcsd
  3. Jan 9, 2009 #2

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    At distances greater than a stellar radius, the gravitational field of a star-sized mass in a black hole is no stronger than the field of a star of the same mass. So the amount of "gobbling" that can happen is no larger than the number of collisions between stars - which isn't much.

    Furthermore, the distribution of dark matter is different than the distribution of stars, which is not what one would expect for black holes.
  4. Jan 9, 2009 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    (bold added)

    There are two parts to your (first) question; I'll address only one aspect (for now).

    Through a variety of independent techniques, using different aspects of physics, astronomers have been able to make estimates of the total mass of objects such as galaxies (of many different kinds) and clusters of galaxies*.

    Estimates of the contribution to the total masses of these same objects from 'baryons' (gas, stars, dust, etc) have also been made, again using several independent methods.

    Put the two estimates together and you get the conclusion that (most) galaxies are comprised of matter that is mostly non-baryonic, and (all?) clusters of galaxies are similarly comprised.

    The super-massive black holes (SMBH) that seem to live in the centre of the nucleus of (almost all) giant and normal galaxies^ have masses that can be reasonably easily estimated (or at least an upper bound on the mass established).

    The estimated combined mass of SMBHs in a rich cluster of galaxies (say) is 'trivial' compared with the estimated total mass of the cluster.

    So, in conclusion, the 'missing mass' in (clusters of) galaxies, in 'the local universe' cannot be in the form of SMBHs (there are a few loopholes to be plugged, but they have been).

    Does that answer at least this part of your question?

    * this is not straight-forward, and simple explanations of how it's done, and why the results are so compelling, not so easy to write
    ^ dwarf galaxies seem to lack these, though far more research needs to be done before we can be reasonably confident of this
    Last edited: Jan 9, 2009
  5. Jan 9, 2009 #4

    So even if the stars were closer, the amount of attraction that the black hole would exert would not be that much greater than what is happening today. Are the reports of black holes gobbling up galaxies just hype.
    Last edited: Jan 9, 2009
  6. Jan 9, 2009 #5
    Thanks Nereid

    You and Vanadium 50 have both been helpful.
  7. Jan 9, 2009 #6


    User Avatar
    Science Advisor

    Well, this has been answered, but I thought I'd mention another independent check. This comes from the very early universe, before the Cosmic Microwave Background was emitted. During the epoch, the universe was dominated by an absurdly uniform and rather dense and hot plasma. By "absurdly uniform", I mean that the universe had a uniform density throughout barring some tiny density fluctuations that were down by a factor of, typically, 1/100,000 from the overall density. This means that instead of the density in an area being 1.0, a typical deviation might be in the regime of 0.99999 - 1.00001. Though some variations would be bigger than others, this would be the typical variation. There may have been a few black holes in extremely overdense regions, but they would have been few and far between.

    These small underdensities and overdensities are what later became the structure we see in the universe around us: as the universe expanded and cooled, matter tended to fall into the overdense regions, creating galaxies, galaxy clusters, etc.

    But what does all this have to do with dark matter? Well, here's the kicker: the normal matter, while it was in this plasma state, didn't want to get very clumpy, as whenever the plasma compressed, it would tend to bounce back (it's only later, after the normal matter cooled from its plasma state that it started to collapse). The dark matter, however, experienced no pressure, and so would just continue to collapse. This difference between the bouncing normal matter and non-bouncing dark matter provides us with the most accurate measurement we have of the ratio between normal matter and dark matter, as it provides an easily-detectable signature in the cosmic microwave background radiation.

    Because it is incredibly difficult to reconcile the lack of observed black holes in the early universe* with the detection that there is around five times as much dark matter as normal matter, this provides, to me, rather definitive evidence that black holes don't make up a significant fraction of the dark matter.

    *Note: if black holes existed in the early universe, and weren't produced by stars, they would be what we call "primordial black holes". Primordial black holes are special because there isn't any lower limit on their masses, so we would expect many of them to have very low masses indeed. Because black holes evaporate, and because low-mass black holes evaporate more quickly (and with a very, very distinctive signature), the fact that we haven't seen any black holes evaporating in any of the supernova searches seems to be a rather clear indication that if there are any primordial black holes, they're pretty rare.

    P.S. Sorry if this is too basic. I never know what audience I'm talking to online.
  8. Jan 9, 2009 #7
    Thanks Chalnoth

    This is all pretty much over my head. I better stick to my paleontology hobby (fighting over whether something is a blastoid, crinoid, or geode that just looks like one).

    I guess I also shouldn't loose sleep over the universe being ripped apart by dark energy many billion years from now - but it still kind of facinates me.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook