Astronomy / Space Dark matter proof or concept?

[Enride]

This must be a good place to ask some questions since people here seem both friendly and full of knowledge.

I am not educated in astronomy of physics and while I have no problem understanding math, i have no formal education there either (aside from highschool and some relating to electricity due to work).

Ive never been interested in physics and astronomy up until a few years ago which began when I for some reason wanted to understand what relativity was all about.
Well that took me down a path that im not regretting at all. Since that day, astronomy and physics has become one of my biggest interests and somewhat of a hobby!
It has given me an understanding of the world and the universe that I had no clue to earlier, and since much of the physics touch on subjects of electricity and magnetism, its even given me some better knowledge in my own line of work!


Anyway, im rambling. The above is just to tell you guys that while im very interested in these things, Im not in any way "down to details" on much of it.
On to my question:


Physicist treat the subject of Dark Matter as a confirmed fact.
And I understand as far as due to observations and calculations done on galactic mass that the galaxies would rip apart since their stellar mass isnt enough to gravitational bind them into orbit.

But dark matter was originally just a concept to establish that there is "something" out there behaving as matter or else the galaxies wouldnt be…

And sometimes theres some talk about matter/antimatter relation and how it could potentially be possible to convert matter completely to energy by matter/antimatter collisions.

How do we know this?
It feels like science has gone long way past "something unknown acting as matter" but I cant find what evidence and confirmations that would be.
I accept it might be outside of my understanding, but bottom line:
Is there concrete and obvious proof of the existence of dark matter particles?


Sorry for the long post, as I said i might not know alot but im very interested

Thanks

 

[metatheurgist]

Until a physicist turns up I'll attempt to slake your thirst for knowledge.

When the concept first came up it was assumed that there was some kind of invisible mass out in the universe. There were 2 ideas, MACHOs (Massive Astronomical Compact Halo Objects) asteroids, rocks, dust stuff that didn't produce light or WIMPs (Weakly Interacting Massive Particles) large sub-atomic particle that don't affect the environment much. After some observations they worked out it was unlikely to be MACHOs, so they concentrated on detecting WIMPs.

Most detectors seem to be about neutrinos. There are a few neutrino traps around the world. I seem to recall they've registered hits. They work usually by detecting the particle's passage through a medium.

I think the science is at the stage that we are pretty sure WIMPs exist but we don't know enough about them to be certain.

 

[Enride]

Oh im embarrassed to say that I have confused two different concepts.

Matter / antimatter is not related to dark matter, forgive me for mixing the two together. This what happens when a layman like myself try to get a grasp of things =)

(though if someone would like to explain how we can be certain antimatter exist, then by all means do! But it might be another thread)

Meta, thanks for your reply.
But neutrinos are a part of the radiation emitted by a star, correct?
So if we know the mass of a star, we should know the mass of everything in it, including any unemitted neutrinos.
Shouldnt most of the combined neutrinomass still be within the stars, and by that be a part of the calculation already?

And if not, how are we certain that the hits is in fact this dark matter, and not something else?

 

[Zieman]

though if someone would like to explain how we can be certain antimatter exist, then by all means do! But it might be another thread

In experiments, lots of antimatter particles (like positrons = antielectrons) have been produced, also some anti-hydrogen and anti-helium atoms.

Wikipedia is always your friend: http://en.wikipedia.org/wiki/Antimatter

 

[JeffRyan]

Is there concrete and obvious proof of the existence of dark matter particles?

Absolutely, but it might be difficult for a layman to accept (no offense) because it seems a bit like magic.

• We can detect the gravitational effects of something (we can see this because of the effect it has on galaxies. But we can't see it.
• It's big enough that it if it was behaving as ordinary matter it would be creating stars.
• It isn't doing that.
• It must have strange properties that mean that it doesn't behave as ordinary matter.

There's a lot of non-logical (like the above) evidence that also shows that it exists. Things like gravitational lensing and inferences from the cosmic background radiation. But that's what I know, just off the top of my head.

 

[metatheurgist]

(though if someone would like to explain how we can be certain antimatter exist, then by all means do! But it might be another thread)

Anti-Matter has been successfully created and observed, so we can say we are certain about that one.

But neutrinos are a part of the radiation emitted by a star, correct?
So if we know the mass of a star, we should know the mass of everything in it, including any unemitted neutrinos.
Shouldnt most of the combined neutrinomass still be within the stars, and by that be a part of the calculation already?

And if not, how are we certain that the hits is in fact this dark matter, and not something else?

Again don't quote me, I'm not a physicist: Neutrinos are just one speculated WIMP, there may be others. Stars are supposed to emit neutrinos but they may not be the only source of them. As for how certain we are, we know the properties of the known particles very well. We know their limitations. We know that it's unlikely that they would penetrate one mile underground because unlike WIMPs they will react with the environment. This is why all those detectors are stuck miles underground in clean and controlled environments. The only particles that should successfully pass through all that rock to hit the trap should be a WIMP.

 

[Dinosawer]

..
Again don't quote me, I'm not a physicist: Neutrinos are just one speculated WIMP, there may be others. Stars are supposed to emit neutrinos but they may not be the only source of them. As for how certain we are, we know the properties of the known particles very well. We know their limitations. We know that it's unlikely that they would penetrate one mile underground because unlike WIMPs they will react with the environment. This is why all those detectors are stuck miles underground in clean and controlled environments. The only particles that should successfully pass through all that rock to hit the trap should be a WIMP.

That's not exactly correct. Neutrino's do fly through earth without any problems, and the detectors underground you speak of are neutrino detectors.
The problem with neutrino's is that there don't seem to be enough to explain all the dark matter we're seeing, so we don't really know what it is.

 

[Memnoch]

The problem with neutrino's is that there don't seem to be enough to explain all the dark matter we're seeing, so we don't really know what it is.

But are we really "seeing" it? Or is it simply a neat plug to fill in the gap in our knowledge with regards to a poorly understood area of Physics?

 

[Cosmos]

Absolutely, but it might be difficult for a layman to accept (no offense) because it seems a bit like magic.

• We can detect the gravitational effects of something (we can see this because of the effect it has on galaxies. But we can't see it.
• It's big enough that it if it was behaving as ordinary matter it would be creating stars.
• It isn't doing that.
• It must have strange properties that mean that it doesn't behave as ordinary matter.

There's a lot of non-logical (like the above) evidence that also shows that it exists. Things like gravitational lensing and inferences from the cosmic background radiation. But that's what I know, just off the top of my head.

I think one of the problems is that much of theoretical physics seems counter intuitive. Its a fascinating subject of course and I love listening to the likes of Feynman and Krauss.

The OP might like to listen to this lecture:

https://www.youtube.com/watch?v=sUH77mYBUtM

Its fully of cool humour too

 

[Dinosawer]

But are we really "seeing" it? Or is it simply a neat plug to fill in the gap in our knowledge with regards to a poorly understood area of Physics?

Depends on your definition of 'seeing', really. Not visualy, no (hence 'dark' matter). There are however ways of measuring mass that don't involve direct observation.
One way is to measure movement of stars in a galaxy and using our theories of gravity to calculate the mass distribution that should be there. And we see that it's way more than there are stars to provide it.
Furthermore, there's gravitiational lensing, which provides a way of measuring a mass by determining how much it bends light.
There are other, more statistical ways to do the same, which I won't explain because they are fairly involved (and Ive forgotten the details )
Of course, there are then multiple theories that might also explain the missing mass. And you check those, and they turn out to be false.
Of course, all of this assumes our theories of gravity and general relativity are correct. They might be slightly wrong, obviously, and there are theories that suggest that they are and that there is no dark matter (such as MoND). However, such theories often have 2 problems:
a) They often bring other problems that are hard to solve. (e.g. MoND works non-relativistically but is apparently a horror to make relativistically correct)
b) They aren't always correct everywhere. e.g. MoND explains why galaxies don't fly apart, but fails when calculating galaxy clusters (which have the same problems as galaxies)

And then there's the Bullet Cluster. Alternative gravity theories still have to assume that gravity comes from the stuff we can see. However, in the Bullet Cluster, we see a cloud of gas trailing behind nothing. After looking at gravitational lensing, it turns out there should be a cloud of massive matter it's trailing behind, but there's nothing to see. Many astronomers see the bullet cluster as fairly convincing proof of dark matter.

So, are we sure that there is actually dark matter? Definitely not. But at the moment, it's the most plausible explanation, so we go with it at the moment, until it's either proven or disproven. That's how science works.

 

[MacArthur]

Here is how another non-physicist understands the . . erm . . matter.

Normal, non dark, matter is observable by EM spectrum emission and absorbtion and mass is observed by its gravitational effects. Trouble is there doesn't seem to be enough stuff absorbing and emitting EM energy to account for all the observed gravitational effects. Something else must be out there, surrounding, penetrating, and binding the galaxies together . AFAIK, no experiment designed to detect dark matter particles has succeeded, therefore there is no scientific proof of its existence but there is scientific proof of "something". Dark matter, is really a kinda place holder name/concept for observations which do not fit current cosmological theories.

Hopefully Dr Wookie will be along soon so I have a related question for him. I don't remember seeing black holes suggested as a candidate for dark matter. Why is this. Naively, they seem to fit the requirement of massive and hard to see. Large ones dotted about, or a sea of tiny ones everywhere. Could they be small enough to satisy the W of WIMP but still be M enough to hold galaxies together?

 

[Enride]

I think one of the problems is that much of theoretical physics seems counter intuitive. Its a fascinating subject of course and I love listening to the likes of Feynman and Krauss.

The OP might like to listen to this lecture:

https://www.youtube.com/watch?v=sUH77mYBUtM

Its fully of cool humour too

Thanks

Ive listend to/seen alot with mr Krauss and for the mostpart i enjoy it, but imo he has a tendency to often just state that things are a "fact" without trying to explain them.
Granted, you cant always go into details on everything needed to understand a certain subject, but even if i dont understand everything I DO want those details.

 

[Dinosawer]

...
Hopefully Dr Wookie will be along soon so I have a related question for him. I don't remember seeing black holes suggested as a candidate for dark matter. Why is this. Naively, they seem to fit the requirement of massive and hard to see. Large ones dotted about, or a sea of tiny ones everywhere. Could they be small enough to satisy the W of WIMP but still be M enough to hold galaxies together?

Well, I'm definitely not an expert on black holes (I'm only a university student) but you can measure 'clumpiness' of dark matter by measuring small gravitiational lensing effects - a mass that is clumped together in a ball will bend light whereas a uniformly distributed mass will not. It has been measured that way that dark matter doesn't exist of clumps between 0.5 to 100 solar masses, which excludes the so called MaCHO's and I assume regular (relatively light) black holes (the heavier ones should be visibly because of star in their vicinity going really fast).
It doesn't exclude tiny black holes though. I recall seeing something about that recently but I can't immediately remember where.
Edit: Found it! http://www.iflscience.com/space/could-dark-matter-be-made-miniscule-black-holes

 

[Cosmos]

Thanks

Ive listend to/seen alot with mr Krauss and for the mostpart i enjoy it, but imo he has a tendency to often just state that things are a "fact" without trying to explain them.
Granted, you cant always go into details on everything needed to understand a certain subject, but even if i dont understand everything I DO want those details.

No problem. I just watched it again myself. I like his dry sense of humour.

I think the point being that in the lecture I linked he's trying to make the concepts understandable without having to explain formulae which would probably lose many people. If you are interesting in doing some study Udacity might be a good place to start

 

[Memnoch]

So if this dark matter stuff is real, a tangible thing you could touch, what do you think it would look like? Presumably it wouldn't have the same gravitational pull as a black hole, but much denser than other matter. Denser than Neutron star material?

 

[Dinosawer]

So if this dark matter stuff is real, a tangible thing you could touch, what do you think it would look like? Presumably it wouldn't have the same gravitational pull as a black hole, but much denser than other matter. Denser than Neutron star material?

Well it would look like nothing (you can't see it) and you wouldn't be able to feel it either (touching stuff is done via electromagnetic interaction, and dark matter doesn't interact that way either.
It's density is fairly low actually (one study I find says 0.008 sun masses per cubic parsec in our neighbourhood, which is about 5.5*10^(-22) kg/m³, i.e. very little). The reason it can still weigh a lot more that the normal matter is the fact that in stars, all the mass is clumped together but they are very far apart, whereas dark matter appears to be evenly spread out.

 

[JeffRyan]

One of the interesting things about dark matter is we can tell you what it isn't, but can't tell you what it is. It doesn't form stars, it isn't anti-matter (which is identical to ordinary matter really, it's just... opposite...) The only thing we know for sure is that it obeys the laws of gravity (which dark energy doesn't). Dark matter isn't evenly spread out, in fact it's the clumps of dark matter that helped ordinary matter clump together into stars and galaxies.

 

[metatheurgist]

That's not exactly correct. Neutrino's do fly through earth without any problems, and the detectors underground you speak of are neutrino detectors.
The problem with neutrino's is that there don't seem to be enough to explain all the dark matter we're seeing, so we don't really know what it is.

Erm, I thought that was exactly was I said. Maybe I should read my dictionary again to make sure I use words good.

 

[Dinosawer]

Erm, I thought that was exactly was I said. Maybe I should read my dictionary again to make sure I use words good.

No, I'm sorry, it was me who misunderstood what you wrote. I somehow thought that the part about other particles not getting through the ground was about neutrino's.
You were indeed correct.

 

[Memnoch]

Is it at all possible that our current understanding of gravity could be wrong? Can the properties of gravity be altered to explain away the existence of dark matter?