can anyone explain inflation ???

[dark knight]

Spot on Themos, dark matter is just a term we use to make a theory work, a bit like a missing link that is required to explain something, we couldn't explain something so we invented/presumed something (dark matter) to make the theory work.

[acey]

Yes, a bit like air, heat, gravity, time....

[Ben Ritchie]

I think dark matter evidence comes from other galaxies, I don't think there is any local evidence for it.

There are interesting signs from PAMELA and Gran Sasso, but it's a rather hard thing to find locally - much easier remotely, there's a ton of stuff that's extremely hard to explain without dark matter (not least the recent z=8.2 GRB that's in the news, nobody has any idea how you form stars that early if dark matter isn't present).

[dark knight]

Must admit it is a very speculative topic,, but so interesting. If something is beyond our comprehension, just make something up to explain it and before long we will just accept it lol. Are we still really that naive,,or are we just not as intellectual as we like to think we are. What a conundrum.

[Ben Ritchie]

just make something up to explain it and before long we will just accept it lol. Are we still really that naive,,or are we just not as intellectual as we like to think we are. What a conundrum.

Lol? Not really, no. Who's "we", by the way?

There are huge incentives to disprove dark matter Cosmology - a Nobel prize, for one - and it's a testable Physical theory with explicit predictions. It's just proven to be rather hard to find fault with it.

[dark knight]

ok, ok I give in, you've convinced me, just not sure what of.

[themos]

There are huge incentives to disprove dark matter Cosmology - a Nobel prize, for one - and it's a testable Physical theory with explicit predictions. It's just proven to be rather hard to find fault with it.

As soon as one model of DM is ruled out, another one pops up. It's a bit like string theory. The problem is it's not just one testable Physical theory, it's many.

[Ben Ritchie]

You're confusing candidates for dark matter (generally non-baryonic subatomic particles, as primordial nucleosynthesis constraints strongly rule out all of the implied dark matter being baryonic) with the dark matter cosmology itself - i.e. the theory that a significant fraction of the Universe is comprised of non-radiative, gravitating matter.

That is a single, testable theory that provides a wide range of predictions (from the overall distribution of galaxies to formation of the first stars and the lensing of light as it passes through clusters of galaxies). So far it has stood up to the test.

As to what it is, nobody knows. But don't confuse that issue with the general reasons to believe something is there.

[narrowbandpaul]

there is no doubt in my mind that dark matter exists. As ben says there is gravitational lensing and also galactic rotation curves point to a heap of missing mass. Once the rotation of the galaxy enters the keplerian regime, the velocity should fall of proportional to 1/sqrt®. However the velocity is constant for large r, outside the galaxy, this implies that m is proprtional to r outside the visible extent of the galaxy. Further, this implies that the density falls as 1/r^2.

There has to be mass to allow for the higher than predicted star speeds.

So there is a form of matter which does not interact with EM force or we would see it.

[Ben Ritchie]

Well there's the MOND argument to rotation curves that suggests that Newtonian gravity does not apply in the rotation curve case - not that i'm particularly convinced by MOND. But things like high-redshift star formation and weak gravitational lensing are, to me, much more persuasive because tweaking gravitation a la MOND doesn't help - you need non-radiative, gravitating matter there. Unless you fundamentally disbelieve modern Cosmology of course, in which case i'm probably wasting my time

[narrowbandpaul]

I am a firm believer in modern cosmology. I cant think that NG wouldnt apply to rotation curves. A very successful theory (fundamentally wrong of course) that has a special case of rotational motional where Newtonian gravity breaks down.

And yes NDynamics requires an inertial reference frame for the laws to be valid, but I hardly think we could send probes to mars and out the solar system, using equations that didnt work when rotational motion is considered.

MOND i believe (dont know alot about it) is a preposterous theory trying to explain observations that are already explained better by Dark Matter.

paul

[Ben Ritchie]

Sorry Paul, I phrased my message poorly - by "unless you fundamentally disbelieve modern Cosmology of course" I wasn't referring to you in particular (or anyone here). My clumsy attempt at a point was that if you (in the general - again!) don't believe that redshifts are cosmological, or don't believe that there was a hot big bang at some point, then a high-redshift GRB doesn't imply early-epoch star formation and the question of how exactly you form stars at high redshift evaporates.

[narrowbandpaul]

i thought you were using the general 'you', but thought I would state my belief in science, which may have been obvious anyway!

[dark knight]

Wow, sorry guys I did not mean to post in such a manner as to antagonise anyone. I just don't know enough about the subject to debate this topic adequatley, and therefore find by questioning things helps me gain a better perspective. A lot of what has been said is way beyond my learning ability, I'm just a guy with lots of questions, that said I am starting to read about Branes. I have heard of the missing mass. Is there a chance it's not missing at all and that we (mankind) have just not found a means to detect it yet?

[acey]

Either there's a lot of missing mass in the universe or else the theory of gravity is wrong. Dark matter deals with the first option (with no general agreement on what sort of particle(s)/field(s) it is), MOND etc deals with the second option, and is currently a minority view. As Ben has said, a new theory of gravity that actually works would bring accolades to anybody who could prove it - but MOND is an ad hoc solution tailored to making galaxy rotation rates come out right, with no fundamental explanation of why it ought to work.

The gravity tinkerers look back to Einstein and the precession of Mercury. The present-day equivalent just might turn out to be this:

Pioneer anomaly - Wikipedia, the free encyclopedia

Andrew

[Ben Ritchie]

Wow, sorry guys I did not mean to post in such a manner as to antagonise anyone. I just don't know enough about the subject to debate this topic adequatley, and therefore find by questioning things helps me gain a better perspective.

Not antagonised

The problem with this stuff - in general, not just dark matter - is that a lot of what people read on this is the sort of popular New Scientist-type article, and they're wildly speculative. New Scientist is to science reporting as The Sun is to journalism. Next week it's forgotten and they're onto the next thing. But it means that people get exposed to all sorts of rubbish. Neutralinos, Axions, Axalinos, LSPs, ignore all that stuff - one of them may turn out to be right on the money for dark matter, most won't. Branes, for example, are purely speculative. Fun idea, but there's no evidence for it.

Dark matter cosmology - the bit i'm defending (in part because my doctoral thesis was directly related to it and i'm kind of familiar with the topic ) - is just the simple idea that we can't directly see a proportion of the matter in the Universe because doesn't experience the electromagnetic force, but we can see its gravitational influence on matter we can see. Forget what it is, right now nobody knows - although there are hints as to what it may be - it's the basic concept that's a very powerful, predictive theory. There's really no other theory close to it, which is why the great majority of people in active research accept it.

Not that that couldn't change, maybe tomorrow there will be a preprint out with a great bit of observational evidence that'll make people stop and say heck, dark matter can't explain that ... but since Fritz Zwicky first raised the issue in the 1930s it hasn't happened.

[dark knight]

Thanks Ben, I can now grasp (just about) some of the theory of what maybe going on. Trouble is just when I think I grasp it, a new can of worms opens that has me questioning everything all all over again.

[narrowbandpaul]

thats cosmology for you......

the same could be said of quantum mechanics.....

if I hear 'perturbed hamiltonian' one more time, I will scream...:-)

paul

[dark knight]

Kind of reminds me when I was a child learning about magnetism, it wasn't until the magnet was under a sheet of paper and the iron filings were applied that I understood what was happening, mainly because I could see the magnetic field.

[themos]

That is a single, testable theory that provides a wide range of predictions

Help me out then, how many free parameters does the theory have and how many have been fixed by experiment?