Dark matter

[Daniel-K]

ok so i know this has probably already been talked about but i thought i would share my view. basically looking for any information on the subject as it fasiantes me, i find hard to grasp so if anyone has another theory i would love to hear it!

why do you think they have not discovered dark matter? they have been searching for 10years but have know'n about some MISSING MATTER since the 1920's maybe it's not missing maybes its already there but they just don't know what there looking for. All matter has atoms so for there to be any force to be applied then it must have atoms right? now there saying dark matter is invisible and doesn't interact with anything so WHY are they trying to detect it? abit of a contradiction in its own terms don't you think? me personally think that neutrinos are the MISSING MATTER after all evidence says they have some mass so with about 50 million of them passing through your body every second i dont what that would work out as a cosmic scale with relation to them in the universe would be but i would imagine that all of there mass put together flying out of all the stars across the universe every second would amount to something. what are your views on this, if any?

Danny

[AWR]

This video from the Perimeter Institute (which seems to be THE place to be for astro-physicists at the moment) is a good introduction.

http://www.perimeterinstitute.ca/Perimeter_Explorations/The_Mystery_of_Dark_Matter/The_Mystery_of_Dark_Matter/

Andrew

[Daniel-K]

thanks Andrew very good site

[JulianO]

why do you think they have not discovered dark matter? they have been searching for 10years but have know'n about some MISSING MATTER since the 1920's maybe it's not missing maybes its already there but they just don't know what there looking for.

Well I think its nearer the 1930's when Fritz Zwiky started on the dark matter trail, but really it didn't take off until many years later. They've been searching for Dark Matter signals since the 1980s - so thats more than 30 years.

All matter has atoms so for there to be any force to be applied then it must have atoms right?

No - that's baryonic matter. There are lots of other forms of matter. Anti-matter, photons, neutrinos - depends really on what your definition of matter is.

now there saying dark matter is invisible and doesn't interact with anything so WHY are they trying to detect it?

Well it does interact - it interacts gravitationally, thats how we know its there. It also probably interacts via the weak interaction, but only very occasionally, and thats one of the ways of detecting it - a bit like they do with neutrinos, but no where nearly as strongly.

abit of a contradiction in its own terms don't you think? me personally think that neutrinos are the MISSING MATTER after all evidence says they have some mass so with about 50 million of them passing through your body every second

They were an early candidate - but they move too fast - they would be hot dark matter, and simulations show galaxies dont form in the right way with them present.

There are other reasons they are out of favour too as candidates.

The current favourites are (in approximate order of possibility)

• The neutralino - a SUSY particle that is stable and has probably the right sort of characteristics
  
• The sterile neutrino - a slower bigger sort of neutrino
  
• The axion - comes out of QCD theory
  
• The gravitino - another SUSY particle
  
• A stable higgs particle.
  

[K Nixon]

Hi Danny,

Im learning as I go with astronomy but as for astrophysics, it is more my field (I know, you'd think the two go hand in hand!)

I've always had an issue with dark matter. Well I have and I haven't!

The reason we need dark matter is that the galaxy rotates faster than it should http://en.citizendium.org/images/3/31/GalaxyRotationCurve.png

If you look at planetary motion, the strength of gravity drops off as you get further from the sun, so the centripetal force acting on the planets also decreases hence the speed of rotation is lower.

The same should occur to objects as you move further from galactic centre, however it appears that objects all move at the same speed.

This observation tells us that the centripetal force does not drop off with distance.

Basically that just means there is more gravity than there should be.

It is therefore sensible to assume there is more mass providing this gravity. As we cannot see it, if it exists, it does not interact with electromagnetic radiation thus we name it dark.

My issue is that there are other explanations for this increase in gravity. To be honest it may reveal to us a lot more about how gravity functions - something we do not fully understand yet. For one simplistic example (something I have looked at but to be honest lack the maths to check thoroughly - I'm only a physics teacher) the relativistic mass increase of rotating objects such as neutron stars and black holes would cause an apparent increase in gravitational effect of the visible matter, however I imagine this may be observable.

Anyway point being dark matter is just one theory and there is not a huge amount to back it up yet.

However... as you mention neutrinos, these were arrived at theoretically to make the maths work in nuclear interactions. Mathematically it was predicted that billions must be streaming out of the sun every second that pass straight through you and I imagine that this ridiculous idea was met by scepticism when it was calculated. Then we found them and they are an important part of particle theory and are being detected all the time. Maybe the same will be true with dark matter.

Anyway, hope I haven't told you loads of stuff you already know. Ah well If I have hopefully someone else will enjoy it...

All the best

Kieron

[sologuitarist61]

Thanks Kieron a full and concise description (and it saved me reading an entire book for the same information!).

I think my problem with it is that it is called 'dark'. From what I understand there is at least the same amount of 'whatever' out there, as there is observable matter, probably much more, so where the hell is it? Just calling it 'dark' seems a total cop-out. It seems to me that what they are really saying it 'we have no idea what it is'. Dark matter seems to have been invented purely to make the observations make sense and fit what is seen.

I wonder if 'matter' is the wrong description as it makes it sound as if there is in fact something there to be found - something tangible - when in all probability it is another 'force field' that works on such huge distances that we just cannot observe it with our present stage of technology. Could it in fact be 'super gravity' - we know what gravity does over reletively small scale (the solar system etc) but does that hold true for greater distances - probably not.

This is what I truly love about astronomy, all the questions that are raised.

[K Nixon]

yes, the 'dark' is a cop out.

I just love that it is called dark because it does not interact with radiation or indeed matter itself. But we will build a detector for it just the same...

I think they calculated the visible matter makes only something like 5% of the matter they think is there.

Then of course there is dark energy....

yes thats why i love the subject too!

K

[sologuitarist61]

Don't get me started on 'dark energy' ..........!!

[K Nixon]

Ha ha!

I like the idea of it, but it is a little unnecessary!

K

[sologuitarist61]

I think Keiron, it has got into the psyche of the astronomy scientists to have to name something - they seem afraid to admit that they just don't know. I can understand that to a degree, but in this particular case calling it 'dark matter' gives it a credence that it shouldn't have and because of the very name they have called it, forces others along those particular lines of investigation. Whereas if you admit you have no idea then people will feel free to pursue all manner of ideas as to what it might or might not be.

[Grunthos]

we know what gravity does over reletively small scale (the solar system etc) but does that hold true for greater distances - probably not.

I always thought gravity works over vast distances.

e.g. Arp 87 (Interacting galaxies)

[K Nixon]

excellent picture!

yes it does, it is the only force that has any effect at long (cosmological) distances. however it follows the 'inverse square law'. Basically the effect drops by a factor of four as you double your distance from the object.

K

[JulianO]

My issue is that there are other explanations for this increase in gravity. To be honest it may reveal to us a lot more about how gravity functions - something we do not fully understand yet. For one simplistic example (something I have looked at but to be honest lack the maths to check thoroughly - I'm only a physics teacher) the relativistic mass increase of rotating objects such as neutron stars and black holes would cause an apparent increase in gravitational effect of the visible matter, however I imagine this may be observable.

There are other theories that try and explain things, such as MOND, which tinkers with F=ma equation over big distances. There is also teves which is general relativity like framework.

Anyway point being dark matter is just one theory and there is not a huge amount to back it up yet.

Well actually there are quite a lot of lines of evidence for dark matter.

There are the galactic rotation curves as you mention. However there is also:

Evidence from gravitational lensing. The way galaxies bend light (called gravitational lensing) shows that they bend light as though there is a lot more mass present than we can account for by observation. You can weigh cluster of galaxies by a variety of techniques, some based on matter present, and some using things like X-ray emission as a proxy, and they don't add up. A particularly good example is the bullet cluster which shows how when two galaxies collide a lot of the gas collides, but most of the mass passes straight through (and gas is the majority of the visible mass of a galaxy).

There is also a component of the CMB radiation that lets us work out how much stuff is regular atoms and how much is dark matter. This comes out as ~2% - whereas the total matter comes out at around 23%.

If you run simulations without dark matter - galaxies don't form, or at least not by the present day. Throw in the appropriate ratio of dark matter and galaxies form nicely, in a way that is statistically equivalent to the way they look in observational terms.

There is also evidence from big bang nucleosynthesis, which gives various ratios.

Dark matter isn't 100% proven, but its pretty well accepted in academic circles as the most likely possibility. No - we haven't detected it directly yet, but as a theory it works very well for nearly all cases.

[AWR]

I think Keiron, it has got into the psyche of the astronomy scientists to have to name something - they seem afraid to admit that they just don't know. I can understand that to a degree, but in this particular case calling it 'dark matter' gives it a credence that it shouldn't have and because of the very name they have called it, forces others along those particular lines of investigation. Whereas if you admit you have no idea then people will feel free to pursue all manner of ideas as to what it might or might not be.

I think you're being quite harsh. Most astro-physicists are quite happy to admit that they don't know what dark matter is (or if it even exists). What they are trying to do is to come up with a feasible model which explains the rotation speed of galaxies (among other things). The most popular idea at the moment is that there could be more mass than we can currently detect. The name dark matter comes from the traditional method of estimating the mass of a galaxy - by looking at it's brightness, ie observing the bright matter. But as said, it's just a suggetsion. Varying groups have come up with differing models as to what this mass may be like and have built detectors hoping to get lucky. On the other hand, thare are groups who believe that instead of more mass, our theory of gravity may be incorrect.

Andrew

EDIT: The above response explains my point much better!

[K Nixon]

cool, the CMB bit is new to me!

Sorry I didn't want to expand before (for fear of going on at length - I do do that...) but all I was trying to say was that the evidence points to more gravity rather than necessarily more mass, it is just the obvious and therefore most likely explanation that the gravity is created by mass.

It is generally well accepted and by very clever peeps (which usually just means they know more than us laymen can really understand). I'm not against the theory - as I say it smacks very much of the discovery of the neutrino, predicted long before it was detected. But equally as a model there are alternatives which I find interesting.

It is very difficult to theorise and model how something like gravity will act in response to something as we have not thoroughly modelled gravity yet. This makes it one of those fantastic 'spaces to watch' as more and more is discovered. Who knows. in ten years maybe we'll all be rushing out to buy our very own Dark matter scopes so we can have an unimpeded look at the centre of the galaxy.

Man I miss tomorrow's world...

K

[sologuitarist61]

I think you're being quite harsh. Most astro-physicists are quite happy to admit that they don't know what dark matter is (or if it even exists). What they are trying to do is to come up with a feasible model which explains the rotation speed of galaxies (among other things).

You are probably right Andrew that may have been harsh, but having worked in Chemistry labs for much of my working life I know that that sort of attitude is quite prevelant in the scientific community. It may be different in astronomy and if so I retract my statement.

[K Nixon]

Very true. We don't know what it is only what it does, if it exists at all.

But thats true of most things. There is another strand running about what is time.

think on this: what is matter?

do you know what it is or just what it does.

I've studied physics for quite some time and have come to the conclusion that we don't ever really know what something is, rather what it does or how it acts.

But no, sologuitarist, we don't know what it is, you are right. That bugs the heck out of most astrophysicists! but as a theory we have to give it a name.

after all we named gravity

K

[AWR]

You are probably right Andrew that may have been harsh, but having worked in Chemistry labs for much of my working life I know that that sort of attitude is quite prevelant in the scientific community. It may be different in astronomy and if so I retract my statement.

I think in all sciences there are those that take their theories as absolute truth (whatever that may be?) and those who view them as models.

I'm just a teacher but I firmly belong to the second group. My view of the astro-physicists is only based on some excellent lectures and programmes I have seen so I too could be completely wrong.

Andrew

[K Nixon]

I think you're right Andrew. Although scientists tend to present their ideas as truths whether out of a sense of self importance (only human!) or because its easier than trying to explain the alternative to people.

I made the mistake of explaining to a group middle ability students that facts, especially in physics are at any one time a sort of 'best guess' based on the evidence to hand and that in fact you can't 'know' anything to be 100% true. I was hoping to encourage a bit of a philosophical discussion along a how science works idea. What I did was make 36 young teenagers question the point of learning anything else if their teacher basically just told them they can't ever know anything. I have never seen a lesson fall apart so quickly.

K

[sologuitarist61]

I think in all sciences there are those that take their theories as absolute truth (whatever that may be?) and those who view them as models.

I'm just a teacher but I firmly belong to the second group. My view of the astro-physicists is only based on some excellent lectures and programmes I have seen so I too could be completely wrong.

Andrew

Couldn't agree with you more Andrew but we are dealing with human nature here. And no you are not wrong at all and, like me, can only judge from what you know - I am certainly not saying that I am right (more than possibly I am wrong), but we all have our opinions don;t we and discussing them here is great fun as well as informative?

[AWR]

I made the mistake of explaining to a group middle ability students that facts, especially in physics are at any one time a sort of 'best guess' based on the evidence to hand and that in fact you can't 'know' anything to be 100% true. I was hoping to encourage a bit of a philosophical discussion along a how science works idea. What I did was make 36 young teenagers question the point of learning anything else if their teacher basically just told them they can't ever know anything. I have never seen a lesson fall apart so quickly.

K

I've been there and done that!

I now tend to keep my philosophy for my top set. I find that discussing models of the atom works quite well. "Do you think that an atom is really like that?", pointing to some simple shell diagrams on my wall.

Andrew

[JulianO]

No matter what model of the atom you use, you go one level deeper and find its not quite right!

[AWR]

Couldn't agree with you more Andrew but we are dealing with human nature here. And no you are not wrong at all and, like me, can only judge from what you know - I am certainly not saying that I am right (more than possibly I am wrong), but we all have our opinions don;t we and discussing them here is great fun as well as informative?

Definitely!

I just had visions of them locking up all the weirder astro-physicists before the programme began. (That's what we do before open-day:))

Andrew

[AWR]

No matter what model of the atom you use, you go one level deeper and find its not quite right!

Exactly! The question which divides them though is whether we will ever know what an atom is like. Will the model ever be perfect? Can the model ever be perfect?

Andrew

[K Nixon]

I do feel sorry for A level students who find out they get lied to throughout their GCSEs only to discover the A level version of things is not quite right... If only they knew that that was kinda the whole point!

K