On hadron colliders, dark matter and black holes

[torstan]

That's a low down, dirty, and excellent idea.

As for the question of the galactic wind - I believe they do. I'm not an expert on that, but it does seem to be the case that there's an overall wind of charged particles throughout the galaxy. Now they'll be both electrons and protons as all stars emit both. In general that means that the overall galaxy is still neutral, however at the particle level the wind is charged. So yo have a movement of charged particles. You also have an overall magnetic field in the galaxy - so charged particles that come into the galaxy from elsewhere are deflected by the magnetic field and don't travel in straight lines. Initially it was thought that the galaxy has a very disordered magnetic field, but more recently evidence has been emerging that the galaxy might have quite a structured magnetic field on large scales. This means we might be able to see a very high energy cosmic ray hit earth (that must come from something outside out galaxy) and track it back, plotting the expected bend from it's passage through the magnetic field and figure out where it came from.

So yes, there is a galactic equivalent of the solar wind. There's certainly an interstellar medium of charged particles and the solar system plows through it with a bow shock as our solar wind hits the interstellar galactic wind (just like a boat forging through a lake). There's also a large scale galactic magnetic field that is intimately connected to the galactic wind. This affects all charged particles to travel through the galaxy, making it hard (but not impossible) to track a charged particle that hits earth back to its origin.

@Jaxilon: You're right that people believe the earth's magnetic field flips over time. The field is caused by the spinning iron core of the earth. Apparently that changes over time which causes the poles of the earth to wander. However there's certainly no fusion at the core of the earth. If there were then we would be a star.

[RPMiller]

However there's certainly no fusion at the core of the earth. If there were then we would be a star.

And I'm pretty sure that would be a Very Bad Thing...

[Scotia]

Thanks for the answer.

Another thing that has me somewhat confused based on the discussion earlier in the thread...

If I understood what you said earlier correctly, dark matter is its own anti-matter and will annihilate when it comes in contact with itself. There's also speculated to be rather a lot of it kicking around -- in the order of 5x the mass of all other particles in the universe -- with most of it nicely clumped along with the visible structures (galaxies and galaxy clusters). I think you mentioned that it's neutrally charged (so presumably it doesn't repel itself) but gravitationally active/attractive (hence Einstein lensing techniques can be used to get an idea of its shape and distribution). Assuming I've got all that right...wouldn't these massive clusters of dark matter have a tendency to destroy themselves over time? I realize that space is unimaginably vast and even this massive amount of dark matter is a drop in the bucket compared to space's emptiness; but one would think that if all of the universe's other (visible) structures have been able to form in the 14 billion years since the big bang, surely dark matter would have been able to do much the same. How is it possible that there's still so much of it kicking around? Does that imply that the early universe had a much, much greater amount of dark matter and it's gradually been whittling itself down to the current 5% figure (and spitting out lots of non-dark-matter particles in the process, thus providing more stuff to make visible structures with)? It just seems a little counter-intuitive for there to be so much of it collected into galactic halos when one would expect that clumping to result in its destruction.

[Juggernaut1981]

Usual options for "Why something sticks around..."

#1 Entropy likes it. Those things with the least energy and the most chaos tend to stick around for the longest.

#2 Maybe we don't know everything about it. Potentially Dark Matter is smashing with (anti-) Dark Matter and popping, then making more, then popping, then making more... and so on...

#3 Maybe a big pile of other matter becomes Dark Matter... and Dark Matter can become 'normal' matter.

#4 Option 4: Physicist have no clue, not even the sniff of a clue and have their fingers crossed that something really awesome and strange happens. Cause the stranger it is, the more PhDs you can get AND the more you're likely to actually learn about the stuff.

[Juggernaut1981]

Awesome - Sort of reminds me of the fusion engine at the core of the earth. They believe it flips every so many years.

Also, and I'm sure PLENTY of Scientists have made this jump too... the nice big spinning bar magnet (iron core) to our planet will waggle around chaotically because of the fluxing of the Sun's Magnetic Field. So as it flips every X years, it probably causes the iron core to go a little off-kilter (just think... HUGE magnet a moderate distance from a weak magnet is likely to do SOMETHING noticable... considering we find new planets and moons by measuring disturbances in orbits using some really shiney energy-based mathematics... [voice=sexy and drooly] mmmm Langrangian Dynamics... Calculus... partial differentials...[/voice])

But no fusion engine. Fusion requires the kind of heat that outside a really well built laboratory would result in the melting of half the planet... just before it gets sucked in and crushed.

[Jaxilon]

@Jaxilon: You're right that people believe the earth's magnetic field flips over time. The field is caused by the spinning iron core of the earth. Apparently that changes over time which causes the poles of the earth to wander. However there's certainly no fusion at the core of the earth. If there were then we would be a star.

Yeah, now that you point it out that doesn't make sense. I guess I got my wires crossed on what was going on in the center of the earth and what goes on in stars. Too much Discovery channel as I'm falling asleep at night most likely.

[torstan]

No problem at all.

@Scotia: There are two things at work. First is the question of how much is in any one place - essentially how dense the cloud of dark matter is. Now we mostly have great big lumps of ordinary matter separated by large tracts of empty space. Dark matter is spread over a much larger volume and much more evenly, so there's never any concentrations as large as we have for ordinary matter. If our ordinary matter were spread out in the same way, the chances of two particle hitting would be relatively. The dark matter also extends way out into the space far from a galaxy so the volume it occupies is much lower.

The second factor is the interaction strength of the force that causes the interaction between two particles to occur. So if you throw together two particles that interact via the strong force then they are more likely to interact than two particles that interact through electromagnetism. So in that example, even if you have the same amounts of both stuff the strongly interacting particles will interact more than the weakly interacting stuff. In the case of dark matter, we know it's very weakly interacting. Therefore you can have a reasonably high density of it and the chances of one particle sailing right by another are quite high. Indeed we see this in space. There were two galaxy clusters that collided. The normal matter (mostly hydrogen gas) interacted and slowed down - you can see the shock waves from the interaction. However the overall mass of the galaxy clusters was still in two roughly spherical blobs way out in front of the gas. So the two dark matter halos basically passed right through each other. This restricts the strength of the interaction that causes the particles to interact. It has to be very weak.

This is also the reason that the dark matter is still in big spherical halos. Normal matter tends to bounce around and interact. That means that it loses energy through colissions and moves closer to the mass that it's orbiting - normally the center of the galaxy, or the local solar system, or the local proto-planet. As it loses more and more energy it falls towards the local gravitating object and sticks to it. Thus it goes from a big cloud to individual clumps. As the dark matter doesn't lose energy through interactions (much) it doesn't go from clouds to clumps in this way and stays very diffuse.

These two things mean that even though it can interact and annihilate, it does so very rarely. The PAMELA satellite was very exciting because it might have been evidence that it interacts more than we thought, but if I'm right then it's not dark matter, and we can leave that evidence to one side.

As for the change over time - we see the evidence for dark matter in the very earliest light we can observe from the big bang and it's there in about the same amount as we see today, so we expect that there's a roughly constant (though possibly lightly decreasing) amount of dark matter through the universe's history.

Let me know if that makes sense? I can go into more detail if you're interested. But it's not entropy, or an exchange process, or the fact that we have no clue. We've got a very good idea what it is. We also have a very good idea about the spinning iron core of the earth and the earth's magnetic field, but that's another story (and one I'm not an expert on).

[torstan]

Actually there's an interesting detail that is relevant. I'll post a bit more on it in a second. Have to marinade chicken for dinner first.

Edit: Done.

Okay, so there's an interesting chapter in the dark matter annihilation story. In the very early universe you can have two particles of normal matter annihilating and producing two particles of dark matter. Because the energies of the particles in the early universe are really high, the fact that the dark matter is much heavier is irrelevant. Basically E >> 2mc^2 (where m here is the mass of the dark matter particles). So you can create them from scratch with no difficulty. Equally, the universe is hot and dense enough that dark matter particles annihilate regularly - into normal matter. So you end up with an equilibrium with matter and dark matter sloshing back and forth quite happily. Now cycle forward a little. The universe is cooling down and expanding. This means that the energy in a collision of two particles is getting pretty small. At some point it's too small for two particles of matter to annihilate into two dark matter particles: E < 2mc^2. So you don't get anything going from matter to dark matter. However you do still get dark matter going to matter. Particles collide and annihilate and your dark matter numbers go down fast. This continues as the universe continues to expand and cool. At some point the universe expands so far that the density of dark matter drops to a point where it's really unlikely for a dark matter particle to hit another dark matter particle - this is called freeze out. At that point you sit back and call it done. However much you're left with is now the amount of dark matter in the universe today.

Now you're going to call foul because if you start off with roughly equal amounts of normal matter and dark matter at equilibrium and then allow for the dark matter to annihilate that means there's less dark matter today than normal matter. You're right, and this does not contradict the statement that normal matter makes up only 20% of the mass of the universe where dark matter makes up 80%. The difference is that dark matter is much, much heavier. So for normal matter we can say that the heaviest stable particle is the up or down quark at roughly a third of the proton mass. In contrast, the dark matter is expected to be around 300 times heavier at least. Now remember that a lot of the matter we were talking about earlier is in electrons (1/2000 th of the proton mass) so if you really add up the normal matter, it's not too surprising that there's less mass of it than in the dark matter.

The large mass of an individual particle also means that there's a lower number of dark matter particles in a galaxy than normal matter, (about 1 per 10 cm cube at earth). With that low density, it's not too surprising that they don't annihilate. In fact it's a lot more surprising that they do - hence the surprise from the PAMELA results.

As for the earth's core - I was thinking about it. The average magnetic field from the sun at earth is about a micro-Gauss. This is really weak - note that it doesn't move a nail on the table or cause any noticeable change in the earth's magnetic field (about 10,000 times stronger). As it doesn't affect small magnetised objects, I'd expect it to have very little effect on a magnet the size of the earth's core. Also, the direction of the field from the sun that we feel changes every 27 days so the average magnetic field is really much weaker. However the time it takes is hundreds of thousands of years and there seems to be no clear consensus on the cause so I can't say that the solar magnetic field has no effect, just that due to the numbers the chance is very very small that it does.

[Juggernaut1981]

Let me know if that makes sense? I can go into more detail if you're interested. But it's not entropy, or an exchange process, or the fact that we have no clue. We've got a very good idea what it is. We also have a very good idea about the spinning iron core of the earth and the earth's magnetic field, but that's another story (and one I'm not an expert on).

That's okay Torstan, my sub-atomic physics isn't particularly strong at the best of times. I did mostly theoretical chemistry and anything smaller than an electron is ignored and treated as "one of those irritating physics things". So I may be throwing too much "micro-scale" physics into something that is on a significantly smaller scale.

[torstan]

Ah, that makes a lot of sense. I can see why entropy and transfer processes were the first things to spring to mind then. In the second of the posts above you'll see that when you come to the earl universe - you were dead right. The equilibrium is indeed due to dark matter turning into stuff that then turns back into dark matter. It's just the current state of affairs that is somewhat different.