Is dark matter real?

debugas
debugas
Joined: 11 Nov 04
Posts: 170
Credit: 77,331
RAC: 0

RE: So Mc^2 is the

Message 38312 in response to message 38310

Quote:

So Mc^2 is the timelike component of the 4-vector that has MV as its three spacelike components, (Mc^2, MVx, MVy, MVz)


i assume you meant here four-momentum vector (Mc,p)
and that is exactly what i think is more fundamental than mass itself.
four-momentum defines the quantity of motion in all directions.
As to the proper mass it characterizes the object.

ah well what i mean is that in energy-momentum equation
E^2=(m*c*c)^2+(p*c)^2
the rest mass m is due to some internal motion and ultimately full energy can be expressed in terms of quantity of motion only , without using mass at all

and i could only speculate that dark matter is such an internal motion at large which does not express itself as particles with mass

Is there a distinction between
a) some motion from A to B and simultaniously the same amount of motion from B to A
b) no motion from A to B and B to A at all

Chipper Q
Chipper Q
Joined: 20 Feb 05
Posts: 1,540
Credit: 708,571
RAC: 0

Cosmic collision reveals dark

Cosmic collision reveals dark matter
It looks like dark matter does indeed exist. By measuring the effect of gravitational lensing of a collision of huge clusters, in addition to x-ray observations, direct evidence of the dark matter is observable; furthermore, there are no modified Newtonian gravity theories that would work to satisfactorily explain the observation. Dark matter looks real, whatever it is...

Mahray
Mahray
Joined: 11 Nov 04
Posts: 43
Credit: 783,499
RAC: 0

RE: Cosmic collision

Message 38314 in response to message 38313

Quote:
Cosmic collision reveals dark matter
It looks like dark matter does indeed exist. By measuring the effect of gravitational lensing of a collision of huge clusters, in addition to x-ray observations, direct evidence of the dark matter is observable; furthermore, there are no modified Newtonian gravity theories that would work to satisfactorily explain the observation. Dark matter looks real, whatever it is...

http://www.newscientistspace.com/article/dn9809-cosmic-smashup-provides-proof-of-dark-matter.html
Also on NewScientist

Chipper Q
Chipper Q
Joined: 20 Feb 05
Posts: 1,540
Credit: 708,571
RAC: 0

Is it possible that the

Is it possible that the predominance of matter over antimatter is symmetric with a predominance of negative charge over positive charge? Do galaxies have a net negative charge? If so, could that be what's responsible for an accelerated expansion? Could dark matter be vast quantities of electron plasma?

Solomon
Solomon
Joined: 6 Nov 05
Posts: 28
Credit: 19,976
RAC: 0

RE: Is it possible that the

Message 38316 in response to message 38315

Quote:
Is it possible that the predominance of matter over antimatter is symmetric with a predominance of negative charge over positive charge?

Not in any serious quantity. Otherwise, there would be measurable effects that we would already be aware of.

Quote:
Do galaxies have a net negative charge? If so, could that be what's responsible for an accelerated expansion?

The expansion of the universe is not a motion of objects through space. So, things that would cause a net force between galaxies aren't relevant. Additionally, for such a force to be large enough to have significant effect, there would need to be enough extra charge that we would see its effects locally.

Quote:
Could dark matter be vast quantities of electron plasma?

Electrons interact with light. Dark matter does not.

Chipper Q
Chipper Q
Joined: 20 Feb 05
Posts: 1,540
Credit: 708,571
RAC: 0

RE: RE: Is it possible

Quote:
Quote:
Is it possible that the predominance of matter over antimatter is symmetric with a predominance of negative charge over positive charge?

Not in any serious quantity. Otherwise, there would be measurable effects that we would already be aware of.


Not so much a measurable effect as an observation: there's more matter than antimatter in the universe; seems like either a violation of symmetry (or conservation); I understand they're created in equal quantities in the lab, but I was thinking about initial conditions prior to inflation.

Quote:
Quote:
Do galaxies have a net negative charge? If so, could that be what's responsible for an accelerated expansion?

The expansion of the universe is not a motion of objects through space. So, things that would cause a net force between galaxies aren't relevant. Additionally, for such a force to be large enough to have significant effect, there would need to be enough extra charge that we would see its effects locally.


I thought the rate of expansion was determined by measuring how fast galaxies are receding from each other, by measuring respective red shifts, and distances (i.e., motion of objects through space). Doesn't a gas expand to the size of its container? Expansion isn't the same as inflation, is it? What does it mean, then, to say the universe is expanding?

Quote:
Quote:
Could dark matter be vast quantities of electron plasma?

Electrons interact with light. Dark matter does not.


I thought it might be possible that a very cold electron plasma might be a nearly perfect medium for transmission of light, in regards to being transparent to the extent of appearing as if there's nothing there (except for the mass, of course).

debugas
debugas
Joined: 11 Nov 04
Posts: 170
Credit: 77,331
RAC: 0

RE: What does it mean,

Message 38318 in response to message 38317

Quote:
What does it mean, then, to say the universe is expanding?


Imagine that you are inflating a baloon and the universe is the 2D surface of that baloon.
As you can see universe in the example does not have borders (wherever you move on the surface you will meet no edge of the world) but at the same time its total size is finite but as you inflate it the size is increasing.

It is like new space is appearing everywhere from nowhere every second, but i bet atoms manage to constantly readjust to it (shrink back to their normal size ) whereas objects that are far apart (like distant stars or galaxies) cannot re-adjust (gravity is too small to keep them together) to stay at the same distance

---
Huh i've just thought of this - while earth is moving around the sun the new space is appearing all the time between them so the actual gravity force must be a bit stronger than we measure (measured gravity force) because it has to compensate for space enlargement that we do not take into consideration in our measurements (we think of constant space between earth and sun when measuring the force).

It would also be interesting to know if the same amount of new space appear near gravity sources and far away from any gravity sources ?

Ben Owen
Ben Owen
Joined: 21 Dec 04
Posts: 117
Credit: 33,817,954
RAC: 17,033

To get back to the topic in

To get back to the topic in the title of this thread:

Tullio, yes it is shocking to suddenly discover that all the stuff we knew about is only 5% of the universe. That's why everyone is so excited trying to figure it out!

I forgot to mention another piece of evidence for dark matter: Gravitational lensing. If you want to know how the matter is distributed in a galaxy cluster, you can look at the images of galaxies behind the cluster. As these images go through the cluster on their way to us, they get distorted a little. From the pattern of distortions you can work out the distribution of matter. So far that's always tracked the galaxies and the hot gas in the cluster. You can see the hot gas in x-rays, and it's a lot more mass than the galaxies. But the numbers aren't right: The lensing acts as if there's a lot more mass than what you get by adding up the galaxies and the gas. Thus dark matter.

This is especially troublesome for modified gravity theories because the way you have to modify gravity to make the lensing work (without dark matter) is different from the way you modify it to make the galaxy rotation curves work (without dark matter).

Which leads to the neat result ("cosmic collision") that Chipper Q posted. There are these two galaxy clusters with most of their visible matter in the form of gas between the individual galaxies in each cluster. The clusters collided. The galaxies sailed right past each other since they're small compared to the distances between them, but the gas clouds in each cluster filled the spaces between galaxies so they whacked each other and got stripped away from the galaxies. So now you've got these galaxy clusters moving off, but with with their gas more or less left behind at the collision site. These guys looked at the lensing and found that most of the mass is where the galaxies are, not where the gas got left behind.

The simplest interpretation of this is that it's the dark matter halos moving off with the galaxies. (Whatever dark matter is, it's been known for ages it has to interact very weakly with visible matter. So it's not surprising it didn't get whacked by the gas.)

If you try to avoid dark matter by avoiding gravity, you need a modification that is out of whack with the modifications needed to explain other stuff like galaxy rotation curves. And you still can't do it without dark matter, so if you want modified gravity you have to have something like three different (big) modifications PLUS you still need some dark matter, although it could have different properties from what you infer using normal gravity (Einstein's gravity).

In principle you probably could still make a consistent modified gravity theory, but since it was originally proposed to get rid of dark matter Occam's razor says forget it. For that matter, since modified gravity now needs three extra things tacked on to explain different observations, that's more for the razor.

Ordinary gravity with dark matter explains all of those with a single, simpler picture. The properties of the matter we infer from that picture are odd, but not completely crazy. Like it's electrically neutral - and so is matter in general, as Solomon pointed out.

It may be time to dig up some "ancient history" of dark matter in another post.

Back soon,

Ben

Ben Owen
Ben Owen
Joined: 21 Dec 04
Posts: 117
Credit: 33,817,954
RAC: 17,033

OK, some "ancient" history -

OK, some "ancient" history - as in 15 years ago when I was an undergrad.

People knew about the rotation curves of galaxies implying dark matter. If you didn't modify gravity (which no one did at the time), you could infer from the rotation curves that the dark stuff was spread out around each galaxy in a more or less spherical shape.

Particle physicists liked the idea of WIMPs - Weakly Interactive Massive Particles. Like if neutrinos had mass, they might provide what was missing to make the galaxy rotation work. Later we found out neutrinos are indeed massive, but they're too light and there aren't enough of them. No problem, particle theorists predict lots of potential WIMPs. These days the favorite seems to be supersymmetric partners to ordinary matter particles, which would have to be VERY heavy explaining why we haven't seen them yet in accelerators on Earth.

But some astronomers said ha - we've known for ages that our galaxy has a "halo" of old, mostly dim stars spread out in a sort of spherical shape. Maybe it's got a lot more really old, really small, really dim ones we haven't seen. Maybe down to Jupiter size. And we'll one-up the particle physicists by calling these MACHOs, or Massive Compact Halo Objects. They can beat the WIMPs because it's relatively easy to test for them: Look for microlensing events.

What is microlensing? Again it's the idea that light from a far off object bends around one that's in the way, distorting the image. The galaxies I was talking about in the last post have shape distortions, but those are hard to detect. And in those days they couldn't detect that many far off galaxies. But there were plenty of stars outside in neighboring galaxies whose light would have to pass through the MACHOs to get to us. If a MACHO went briefly in front of one of those stars, the lensing would momentarily brighten it.

So they proposed to observe a bunch of stars in a nearby galaxy and look for microlensing. The project was funded, partly because even if they didn't find MACHOs they'd find a lot of intrinsically variable stars, and those are neat for all sorts of reasons. Long story short, they did indeed find a bunch of neat stuff about ordinary variable stars, but nowhere near enough MACHOs to account for the missing matter.

So the dark matter had to be stuff less massive than some limit, I forget but it's something Jupiter-ish. There's wiggle room in that constraint, but basically it looks like the WIMPs beat the MACHOs.

And of course these days we know that even that still-mysterious dark matter is a minority of the universe. Interesting times indeed.

I hope that amused,

Ben

tullio
tullio
Joined: 22 Jan 05
Posts: 1,994
Credit: 31,556,858
RAC: 11,412

Ben, thanks again. A dumb

Ben, thanks again. A dumb question: if dark matter exists, which is its temperature? Does dark matter obey the laws of thermodynamics?
Tullio

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.