Black Holes and Dark Matter

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FlatEarth

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MeteorWayne":3rkhdvrq said:
Who suggested it is made up exclusively of WIMPS?
Sorry, I concluded that you were suggesting that to be the case. If anything, at least you know I am reading your comments (if not understanding them). :lol:
 
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FlatEarth

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csmyth3025":3sn9nkn9 said:
Are you just voicing your opinion, or are there observations (or at least a logical argument) supporting your statement that: "...Much of the missing matter is most definitely hidden in black holes or is dark in the sense that it has eluded detection, but is still baryonic matter..."?

Chris
I originally speculated that perhaps dark matter was comprised exclusively of MACHOs, and I believe this was the opinion of many cosmologists in the 90's (okay, so I'm a little slow...). Because insufficient numbers of black holes and brown dwarfs have been discovered, WIMPs are now considered the primary component of dark matter. However, we know there are undetected black holes and brown dwarfs out there, so it's a logical conclusion that a good percentage of dark matter is comprised of "normal" matter (if you consider black holes to be normal, that is).
 
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ramparts

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We can set upper limits on how many brown dwarfs there are - there need to be a lot more brown dwarfs out there in order to account for the missing mass, and if they were that plentiful, we would see a lot more of them. We've run all-sky surveys aplenty which simply do not miss these things, within certain distance and brightness limits. Unless you want to claim that our part of the galaxy is for some reason bereft of brown dwarfs, or that all the missing mass is concentrated in really really really small brown dwarfs. In which case, be my guest.
 
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FlatEarth

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ramparts":2dez135i said:
We can set upper limits on how many brown dwarfs there are - there need to be a lot more brown dwarfs out there in order to account for the missing mass, and if they were that plentiful, we would see a lot more of them. We've run all-sky surveys aplenty which simply do not miss these things, within certain distance and brightness limits. Unless you want to claim that our part of the galaxy is for some reason bereft of brown dwarfs, or that all the missing mass is concentrated in really really really small brown dwarfs. In which case, be my guest.
ramparts, the point I make is that dark matter is not comprised exclusively of non-baryonic matter. You seemed to imply that in an earlier statement.
 
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ramparts

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That depends what we mean by "dark matter". Do you mean all the matter that we can't see? Clearly there's plenty of that in brown dwarfs, black holes, dim stars, faraway stars, faraway galaxies, and all sorts of other things. But what is usually meant by dark matter - a particular component of the universe - is almost certainly non-baryonic. When we talk about dark matter (and when I talk about dark matter), I'm referring to the stuff that flattens galactic rotation curves, and especially important, the stuff that provides gravitational potential wells for galaxies to form in - before brown dwarfs and the rest even exist. Almost certainly baryonic, and not in the form of brown dwarfs or black holes.
 
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MeteorWayne

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ramparts, I think in your last line "Almost certainly baryonic, and not in the form of brown dwarfs or black holes." you are missing a not or non, unless I'm reading you wrong. MW
 
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FlatEarth

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ramparts":2rsng6i0 said:
That depends what we mean by "dark matter". Do you mean all the matter that we can't see? Clearly there's plenty of that in brown dwarfs, black holes, dim stars, faraway stars, faraway galaxies, and all sorts of other things. But what is usually meant by dark matter - a particular component of the universe - is almost certainly non-baryonic. When we talk about dark matter (and when I talk about dark matter), I'm referring to the stuff that flattens galactic rotation curves, and especially important, the stuff that provides gravitational potential wells for galaxies to form in - before brown dwarfs and the rest even exist. Almost certainly (non)baryonic, and not in the form of brown dwarfs or black holes.
The dark matter I refer to is all the missing matter. "...MACHOs may still constitute 20% of the dark matter in the Milky Way Galaxy." is a quote from Wiki. Dark matter is the primary category and WIMPs and MACHOs are sub-categories in this usage of the term. Dark matter is used more and more to describe nonbaryonic matter, but the original meaning covers both.
 
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ramparts

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Well, if Wiki said it, then I have no reply :lol: At this point we're arguing semantics. Yes, there is plenty of baryonic matter we don't see. Some of it certainly contributes to the flat rotation curves, though how much is an area of debate (the answer is probably not much). Just be aware that when most people (and in particular, anyone who actually works in the field) say "dark matter", they're referring to the likely non-baryonic component that appears to be the second biggest contributor to our universe's mass/energy content. Just how the naming works. It's not all that useful to use the word "dark matter" to describe brown dwarfs anymore.
 
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darkmatter4brains

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ramparts,
Do you know what the leading dark matter candidates are today? I know WIMPS have always been an idea. Are there any new ones?

Anybody know what the heck WIMPS are anyhow? I know it stands for Weakly Interacting Massive Particles, but what exactly are they? Are they a predicion of some certain QFT-like theory?
 
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ramparts

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Oh, there's all sorts of kinds. Not having done much particle physics I forget exactly which particles they are (they have funny names like "axions" and "supersymmetric particles" and the like), but they're pretty much all WIMPs. I mean, that's not very specific - a WIMP is, as you said, a weakly interacting massive particle. We know dark matter has mass (because of its gravitational effects), and interacts weakly in the other forces (otherwise it would, for example, emit lots of light). We do have more restrictions - for example, a lot of evidence suggests that dark matter is most likely "cold" (i.e., slow) rather than "hot" (i.e., relativistic/fast), so that suggests neutrinos, which are massive but not by much, so move very quickly, are probably not the solution.

I believe all DM candidates are predicted by some theory or other ;)
 
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darkmatter4brains

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Thanks ramparts! You answered the main thing I was curious about - if WIMPS correspond to supersymmetry theories, etc. So, this sounds like another thing LHC could help us out with. I believe it operates in the range where some of these particles could be discovered. And, yeah, you gotta love the funny names: sparticles, sleptons, squarks, etc.
 
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MOMBODOGFACE

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One theory seems to see gravity not as a force but as a measurable distortion of space-time caused by mass.

I also understand that the graviton particle associated with gravity being a force has not been detected yet.
Not detected yet does not mean to me that it won't be.

Why can't Dark Matter actually be a distortion or characteristic of space-time?
A distortion of space-time that appears to be gravity from an undetected mass.
But is actually a distortion or characteristic of space-time that exists due to something other then undetected mass
or particals.

A distortion of space-time during the expansion that contained the formation of galaxys and galactic grouping.
This distortion of space-time is detectable but does not exist now due to mass in our universe or another.
Maybe its more like what has happened to our space-time in our universe.
 
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FlatEarth

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MOMBODOGFACE":v918qowb said:
One theory seems to see gravity not as a force but as a measurable distortion of space-time caused by mass.

I also understand that the graviton particle associated with gravity being a force has not been detected yet.
Not detected yet does not mean to me that it won't be.

Why can't Dark Matter actually be a distortion or characteristic of space-time?
A distortion of space-time that appears to be gravity from an undetected mass.
But is actually a distortion or characteristic of space-time that exists due to something other then undetected mass
or particals.

A distortion of space-time during the expansion that contained the formation of galaxys and galactic grouping.
This distortion of space-time is detectable but does not exist now due to mass in our universe or another.
Maybe its more like what has happened to our space-time in our universe.
I like your idea. We don't have a complete understanding of the properties of gravity, and perhaps there is a space-time wave distortion associated with galactic masses that magnifies the effects of gravity.

There is another explanation for why galaxies rotate faster than expected. It is called Modified Neutonian Dynamics, or MOND, and is an alternate to the dark matter explanation. It adds a "new constant of nature (a0) to physics..." that says gravity decays at a different rate above vs. below the constant. See the link below for more info.

http://www.universetoday.com/2008/04/08 ... rk-matter/

EDIT: Added MOND info.
 
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MOMBODOGFACE

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Thank you very much Flat.
I have been wondering for a long time what other "ideas" are being worked on.
And thank you for the data link.
It seams to me that the common thread between the Quantum camp and General / Special Relativity
is space / time.
If I could do it myself I would look there for unification.
I had to muster up the courage to ask that question above....lol.
Thank you all for the data made avalible to those of us on the fringes.

Mombo
 
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FlatEarth

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MOMBODOGFACE":22klm76o said:
Thank you very much Flat.
I have been wondering for a long time what other "ideas" are being worked on.
And thank you for the data link.
It seams to me that the common thread between the Quantum camp and General / Special Relativity
is space / time.
If I could do it myself I would look there for unification.
I had to muster up the courage to ask that question above....lol.
Thank you all for the data made avalible to those of us on the fringes.

Mombo
You are welcome, Mombo. I don't want to leave you with the impression that I am a guru on the subject. I only found the link recently and thought I'd share it here. I'm on the fringe as much as anyone, and that's where I like it. ;)
Don't ever be hesitant to post a question or throw out a new idea. That should be the point of a forum, and usually you will get good answers from some pretty knowledgeable people. Even if your ideas get lambasted, there's almost always something to be gained from the exchange.
 
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csmyth3025

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Thanks FlatEarth for that link on the MOND theory. Cobbling dark matter/dark energy onto the Standard Model seems to me to be, at least, an arbitrary an ineligant way of explaining new observations. The properties assigned to dark matter/dark energy remind me of the rather improbable properties of the so-called cosmic aether once proposed as the medium through which electromagnetic radiation propagates.

It may well be that dark matter/dark energy do, indeed, exist. It's good to know, however, that re-examining our fundamental assumpions is part of the serious effort to explain these new observations.

I'm reminded that Einstein changed physics for the better by breaking with the traditional thinking of his time that space and time are invariant.

Chris
 
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ramparts

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Oh, they're certainly inelegant (although many of the candidates for dark matter exist in the standard model or in typical extensions to the standard model, so they do have some independent motivation). In fact, physicists have put a lot of work into studying how modifying gravity can reproduce the effects of either dark matter or dark energy - which, remember, seem to have absolutely no relation (so it's very possible that one is an actual type of matter and the other is an effect of gravity, say).

The problem is that none of these seem to work as well as the more common "inelegant" solutions. This is particularly the case for dark matter; the observations are piling on evidence which makes MOND and similar theories look rather unlikely. I'd say on the whole, physicists "believe" in dark matter far more than dark energy. Most proposed modifications to gravity which would replace dark energy have failed, but it's an elegant solution, and versions of it do still work.
 
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