# Dark Matters

#### yifan

It is said that most mass in the universe consists of dark matters which don't emmit light. What is the difference between dark matters and planets?
Thank u.

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#### Phobos

Staff Emeritus
Gold Member
Welcome to Physics Forums, yifan!

Dark matter is still a mystery. Astronomers don't know what it is, but they can deduce its existance by it's gravitational influence on the matter we can see.

Some dark matter could be ordinary matter than we just can't see (does not reflect/emit enough light or whatever) and/or some might be a new form of exotic matter that we are unfamiliar with.

#### Nereid

Staff Emeritus
Gold Member
There's a set of resources in the General Astronomy sticky "A&C reference library", including some posts in this thread (not wanting to blow my own trumpet too hard ...)

#### jcsd

Gold Member
Planet-like objects do almsot certainly make up a certain percentage of the dark mass, though it's worth saying that brown dwarfs and even smaller bodies like Jupiter are not completely dark (hence the name) due to the fact they are slowly contracting they do produce infrared waves. There is a chance that a fair amount (or as some would have it all, in the form of MACHO's) of the dark mass is baryonic*, infact it's known that luminous baryonic mass only accounts for 10% of the total baryonic mass needed to explain light elemnet synthesis. That said even though it is dark there are other ways of detecting baryonnic dark matter in it's theorized form and though some has been detected, it's not nearly enough . This leaves the majority view that the bulk of dark matter is non-baryonic (proabably made from some exotic and as yet unobserved particle) and only interacts weakly with other particles.

*The baryons are a divison with the particle zoo. They are particles made from (in almost all cases three) quarks and are fermions. The vast bulk of the mass of everyday objects and observable objects interstellar objects like the planets and the sun is made up by baryons as that family includes the proton and the neutron.

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#### mathman

Current estimates of the makeup of the universe is that it consists of about 5% ordinary (baryonic) matter (most of it not visible), 25% non-baryonic matter (all dark), and 70% dark energy. Dark energy is the stuff thought to be responsible for the increase in the rate of expansion of the universe.

#### jcsd

Gold Member
Dark matter is just any kind of matter that makes up the dark mass (that is the mass whose existance can only be inferred by it's graviational influence), there's certainly a distniction between cold baryonic matter and baryonic dark matter, for example cold gas, is cold and baryonic, but it's not dark matter as it's not particularly difficult to observe.

It's quite frankly absurd to say that MACHO's are not dark matter and are not baryonic, excepting the possibilty that may be primordial black holes.

Memmoe if you want to argue with accpted terminolgy, or put forward you're own theories, Theory Development is the forum that you are looking for.

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#### meemoe_uk

Sorry you feel the need to delete my posts.
I know you dark matter dudes get very upset when anyone opposes your undoubtably correct theory. Oh well, time will tell.

#### mathman

there's certainly a distniction between cold baryonic matter and baryonic dark matter,
The above sounds confusing. The important distinction is between baryonic matter(visible or dark) that we all know about (it's what everything we experience is made of) and non-baryonic matter (known only by its gravitational effect).

#### jcsd

Gold Member
mathman said:
The above sounds confusing. The important distinction is between baryonic matter(visible or dark) that we all know about (it's what everything we experience is made of) and non-baryonic matter (known only by its gravitational effect).
I could be accused of being obtuse, because to be honest it doesn't matter wheehr or not you include cold gas as part of the dark mass as gas could only make up a very small amount of the dark mass, as gas in general is pretty easy to observe.

Despite a few naysayers some who do understand the problem, some who don't, the majority view is overwhelmingly that most dark mass is non-baryonic, this is for the simple reason that this explanation fits in best with what is observed.

#### Nereid

Staff Emeritus
Gold Member
meemoe_uk said:
Sorry you feel the need to delete my posts.
I know you dark matter dudes get very upset when anyone opposes your undoubtably correct theory. Oh well, time will tell.
I'd be very surprised if anyone deleted any of your posts meemoe_uk! Do you happen to recall which one(s) you think got deleted? Perhaps you kept a copy of them?? AFAIK, only Phobos, Janus, Greg, and chroot can delete posts here in the Astronomy and Cosmology area.

Would you be so kind as to tell us who you consider to be 'dark matter dudes' who 'get very upset when anyone opposes [our] undoubtably correct theory'?

Leaving aside the question of whether I'm a 'dude' or not, I'm not married to any 'dark matter theory'! However, I do rather insist that those with a different view be able to account for the observations that are posited as evidence for dark matter.

Apart from MOND, do you have any alternatives to propose? As for MOND, how do you reconcile your support for the idea with observations that are clearly inconsistent with it?

#### jcsd

Gold Member
Nereid said:
I'd be very surprised if anyone deleted any of your posts meemoe_uk! Do you happen to recall which one(s) you think got deleted? Perhaps you kept a copy of them?? AFAIK, only Phobos, Janus, Greg, and chroot can delete posts here in the Astronomy and Cosmology area.

Would you be so kind as to tell us who you consider to be 'dark matter dudes' who 'get very upset when anyone opposes [our] undoubtably correct theory'?

Leaving aside the question of whether I'm a 'dude' or not, I'm not married to any 'dark matter theory'! However, I do rather insist that those with a different view be able to account for the observations that are posited as evidence for dark matter.

Apart from MOND, do you have any alternatives to propose? As for MOND, how do you reconcile your support for the idea with observations that are clearly inconsistent with it?
I think it's fair enough to argue that dark matter is baryonic (it may be a fringe view, but it's hardly one that would mark someone out as a crackpot) or to even argue for MOND (which looks though to be little more than a dead end), but I don't think it's fair to argue against accepted terminology (which is why I assume his post was deleted) as it's just a shorthand used to describe concepts.

#### meemoe_uk

Hi All,
Not an expert in physics, so I cant give precise analytic reasons for why I support MOND and not DM.
But I can give common sense reasons...
DM been studied for at least 30 years, still no direct evidence for existance, still very imcompatible with observations. MOND gives far better predictions.

If you want to see why DM theory is so outclassed, the best webpages are at

http://www.astro.umd.edu/~ssm/mond/

which is in easy layman language, and is a very good summary of the problems in modern theorectical cosmology.
Most of the arguments I'd hold against DM are expressed properly there.

MOND cannot be considered anything other than a stop-gap theory, and that why so many love to hate it! For the many romantics, a theory must be all or nothing! But physics is physics, the best theory is the one that gives best results, no matter how ugly it is. Having said that, MOND isn't that ugly.

After they've digested everything on those webpages, anyone who still goes for DM, must be a fanatical DM dude, because, as it stands, it simply isnt logical to stick with it.

#### jcsd

Gold Member
I'd be slightly wary of that page meemoe as really all MOND does is (or originally did) is replicate the results that originally led CDM to be proposed in galaxies (in this manner DM and MOND can be seen as equally ad hoc), it runs into trouble when you look at the larger scale indicators of DM which have been found since it's creation and when you start to reconcile it with general relativity.

Also there has only been one attempt to vigourously test MOND against DM theories by observation, by the the Chandra x-ray telescope 2002. The observations of NGC 720 are difficult to reconcile with MOND and indicate the presecnce of some form of DM.

#### shrumeo

Yea, ever since I heard of "dark matter" and "dark energy" I thought it was bunk.

Look, we ASSUME whole truckloads of stuff about the universe and our theories about it. We ASSUME that the redder the light is from an object, the farther away it is, but no one has difinitively proven that, really. We ASSUME that the universe is expanding based on the first assumption. We ASSUME that the expansion is accelerating based on the second. We build on assumptions that sound so right, and MUST be true because they sound so right and fit nifty models.

Then we say, "this part of our theory matches pretty well with observation, so the other part MUST be true also. Our theory, based on this observation here, says that there must be 10 times more of something we've never detected than all the rest of the matter in the universe." When something like that happens, why the hell arent' we questioning the theory? Why aren't we doing our damnednest to rework the theory to fit observation? (I know, I know, that's what it feels like we're doing with this dark matter garbage.)

I'm sure I sound totally high here, but I must agree with anyone who wholly doubts the existence of the elusive 90% of the matter in the universe, especially if it has to be something exotic (90% of the matter in the universe wouldn't be exotic, it would be us that's exotic.)

:uhh:

#### Nereid

Staff Emeritus
Gold Member
meemoe_uk said:
Hi All,
Not an expert in physics, so I cant give precise analytic reasons for why I support MOND and not DM.
But I can give common sense reasons...
DM been studied for at least 30 years, still no direct evidence for existance, still very imcompatible with observations. MOND gives far better predictions.

If you want to see why DM theory is so outclassed, the best webpages are at

http://www.astro.umd.edu/~ssm/mond/

which is in easy layman language, and is a very good summary of the problems in modern theorectical cosmology.
Most of the arguments I'd hold against DM are expressed properly there.

MOND cannot be considered anything other than a stop-gap theory, and that why so many love to hate it! For the many romantics, a theory must be all or nothing! But physics is physics, the best theory is the one that gives best results, no matter how ugly it is. Having said that, MOND isn't that ugly.

After they've digested everything on those webpages, anyone who still goes for DM, must be a fanatical DM dude, because, as it stands, it simply isnt logical to stick with it.
Thanks mee, an interesting website.

Just a note, for those who perhaps can't quite contextualise the info: MOND's successes (per the website) relate almost exclusively to individual galaxies. While the list of such successes is impressive, the two 'uncertain, but not promising' items (for MOND) - galaxy clusters and gravitational lensing - are pretty horrendous (for MOND), so it's not all black and white ("DM been studied for at least 30 years, still no direct evidence for existance, still very imcompatible with observations. MOND gives far better predictions")

#### Phobos

Staff Emeritus
Gold Member
shrumeo said:
Look, we ASSUME whole truckloads of stuff about the universe and our theories about it. We ASSUME that the redder the light is from an object, the farther away it is, but no one has difinitively proven that, really. We ASSUME that the universe is expanding based on the first assumption. We ASSUME that the expansion is accelerating based on the second. We build on assumptions that sound so right, and MUST be true because they sound so right and fit nifty models.
Granted, it's an odd feeling that most of the universe is completely unknown, but rest assured that Big Bang Theory is founded on more than those simple assumptions. For example, measurements of the cosmic microwave background confirm that the universe used to be hotter & denser. The list of evidences goes on...but this would be getting off on a tangent (start a new topic or look for an older one on this subject).

I'm sure dark matter/energy research will bring us all kinds of surprises & modifications.

#### Nereid

Staff Emeritus
Gold Member
shrumeo said:
Yea, ever since I heard of "dark matter" and "dark energy" I thought it was bunk.

Look, we ASSUME whole truckloads of stuff about the universe and our theories about it. We ASSUME that the redder the light is from an object, the farther away it is, but no one has difinitively proven that, really. We ASSUME that the universe is expanding based on the first assumption. We ASSUME that the expansion is accelerating based on the second. We build on assumptions that sound so right, and MUST be true because they sound so right and fit nifty models.
Actually, the Hubble relationship (distance vs redshift) is pretty well established, with multiple types of observations giving consistent and (mostly) independent results.

You're right of course that 'no one has difinitively proven that, really', but then, this is science, so no such proof is possible (even in principle).

Perhaps you'd like to lay out the basis for your statements in a little more detail? In particular, which of the many methods used to establish the Hubble relationship do you feel is a house of cards?
Then we say, "this part of our theory matches pretty well with observation, so the other part MUST be true also. Our theory, based on this observation here, says that there must be 10 times more of something we've never detected than all the rest of the matter in the universe." When something like that happens, why the hell arent' we questioning the theory? Why aren't we doing our damnednest to rework the theory to fit observation? (I know, I know, that's what it feels like we're doing with this dark matter garbage.)

I'm sure I sound totally high here, but I must agree with anyone who wholly doubts the existence of the elusive 90% of the matter in the universe, especially if it has to be something exotic (90% of the matter in the universe wouldn't be exotic, it would be us that's exotic.)
Yes, but ... while there are lots of questions and not a few inconsistencies, the approach is precisely to make predictions and go test them (through observation).

#### meteor

To all those MOND-fanatics:
http://www.phatnav.com/wiki/wiki.phtml?title=Modified_Newtonian_dynamics [Broken]
"One reason why some astronomers find MOND difficult to accept is that it's an effective theory, not a physical theory. As an effective theory, it describe the dynamics of accelerated object with an equation, without any physical justification"
That is so I get my pen and write an equation that fits the data. But I don't explain why the equation works, what's the physics behind it. Very nice. So 90 years of general relativity go to the trash (even considering that GR has passed all kind of tests, actually passing another with Gravity probe B, and it really has a physical justification, namely the curvature of spacetime). Are you really willing to go so far?

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#### meemoe_uk

actually passing another with Gravity probe B,
dont jump the gun.
The GPB test will be the stiffest test ever for GR. There's a fair chance it wont pass.

#### Nereid

Staff Emeritus
Gold Member
meemoe_uk said:
dont jump the gun.
The GPB test will be the stiffest test ever for GR. There's a fair chance it wont pass.
What do you think the results from GPB will be? What predictions does MOND make about these results?

#### meemoe_uk

I think GPB will detect more space warping than predicted by GR!
MOND? It doesnt predict anything in the GPB test, no galaxys see.

#### Nereid

Staff Emeritus
Gold Member
meemoe_uk said:
I think GPB will detect more space warping than predicted by GR!
MOND? It doesnt predict anything in the GPB test, no galaxys see.
Great! Care to make an estimate of how much more?

#### meemoe_uk

OK, GR is nice it would be nice to keep hold of it.
The best thing that could happen is that GR is correct in everyway 'cept that it greatly underestimates the effect of moving mass on space warping.
So I predict that there will be massive amount of space warping due to moving mass, enough to explain galaxy rotation curves. Yes! the very high amount of space warping observed by GPB due to Earth moving will correlate with the very high amount of space warping needed to account for GRC. MOND will have a physical explaination by this, the old romantics get to hang on to their beloved old theory a while longer, albeit slighty modifed ( untill someone quantitizes it ), so everyone is happy. What a happy ending!

No wonder universe dynamics are so hard to analyse what with all the extra space warping no-ones considering!

Anyone care to do the calculation to give a figure for the space warping GPB will find based on the idea that it'll be enough to account for GRC?

Anyone care to argue why this idea cannot work?

#### Nereid

Staff Emeritus
Gold Member
IIRC, MOND doesn't incorporate relativity in any form. Nor does it have the concept of 'space warping' (things may have changed; it's been a while since I looked).

It's not obvious to me how 'space warping' would account for galaxy rotation curves, and I suspect if there were an obvious way to do this, there'd be dozens of papers on it already. Perhaps there's a simple extension of GR that'll do it?

#### meemoe_uk

IIRC, MOND doesn't incorporate relativity in any form.
correct, but then again it doesn`t incorparate any other bit of physics in any form either. It has no theoretical frame work at all. That is why anyone is free to propose theory for why MOND works. So I am justified in making this theory.

It's not obvious to me how 'space warping' would account for galaxy rotation curves,
simple.
Imagine spacetime in the vincinity of a galaxy is spinning around the centre. Any star whose orbital speed appears to be the same as that of the spacetime it occupies will actually have no orbital speed, the star will freefall into the centre. See? John Gribben goes on about this effect near black holes, in his book 'in search of hte big bang' , but it'll work wherever frame-dragging is significant.
orbital speed is actually v-s where v is the apparent speed and s is the speed of frame draging at that point in space.
Space itself is moving.
Also, any object that has no apparent orbit velocity will acutally have negative orbit velocity ,because any object at rest in moving spacetime is effectively moving through space in the oposite direction.
So, any object with steady orbit in the same direction as the frame dragging, will have to move faster than expected.
That's why galaxy rotation is faster than we expect. When frame draging is taken into account, the stars are actaully orbiting with expected speed.

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