On hadron colliders, dark matter and black holes

[Drazi]

So, I was watching Nova Science Now on PBS and they had a special on Dark Matter. They mentioned in it that it is likely that a mass of dark matter in space could act like a lens and magnify the space beyond it. Is it possible to compensate for the added distortion when calculating things like rate of universe expansion and distance between galaxies?

[torstan]

Absolutely. The lensing effect is well understood and is actually one of the ways of looking for dark matter. The reason for this is that a lens distorts as well as magnifying. Here's a galaxy acting as a lens on the light from a galaxy behind it:



You can see that there are a number of long curving images in the picture, around a central bright galaxy. Now they are normal galaxies that are behind the bright galaxy. The light is bent around it, and the lensing effect distorts the image. Incidentally, as it is the mass of the galaxy that is distorting the light, you can use this effect to weigh the galaxy that is acting as a lens. Once again you come up short if you only consider the visible matter, and have to assume that there's a lot of dark matter too.

The point of all this is that if dark matter is acting as a lens, it doesn't just magnify an object, it also distorts the image of it. As long as we're really careful about checking for distortion, it's possible to precisely measure when an object is being lensed, and by how much. This allows astrophysicists to accurately place lensed objects, no matter what it is that is causing the lensing.

The science of gravitational lensing is now very precise and can be used to measure not just the mass of a lensing object, but also the precise distribution of mass throughout the object. It's a very useful tool.

[Drazi]

Is it very likely for us to be able to study Dark Matter here on Earth without being able to create it ourselves?

And also, do we know if the presence of Dark Matter effects the way light behaves? I saw a special on the Science Channel about the Big Bang that mentioned that if the Universe was as old as today's scientific models claim than we shouldn't be able to see as much light as we do (I'm butchering the theory here so apologies for that). So they supposedly have a problem where the only solution was to change the speed of light from being a constant to being variable over time. Now they theorized that perhaps an outside force was acting upon the light just after the Big Bang that made it move faster than it traditionally should. So, the question is could the large concentration of Dark Matter/Energy at the time of the Big Bang affect the Speed of Light?

[torstan]

Your last question first - there are a couple of fairly odd theories that incorporate a variable speed of light. You definitely don't need it, but I believe that some things fall out a little more neatly if you have it. Unfortunately my knowledge of those theories is pretty much non-existent. One of the main proponents of the theory is a guy called Joao Magueijo who's based in either Cambridge or Imperial at the moment - I forget which. If you are interested in this, google his name and VSL. Just don't believe everything that you read. It made a big splash a few years ago and then pretty much disappeared. I wouldn't put a lot of money on it.

Models of dark matter/energy generally don't affect the speed of light, just the way it moves. However the early universe is a weird place and there's a lot still to be understood about it that is still up in the air.

We do know that dark matter affects the way that light travels. One thing dark matter must have is mass. That's because it needs to have a gravitational pull to explain the rotation of galaxies that we observe, as well as the behaviour of clusters of galaxies. Now we also know that anything that gravitates also bends light. This was shown in the early 20th century when it was observed at an eclipse that astronomers could see stars that were behind the sun. The sun had bent the light from those stars around it so that they could be observed on earth. This was a key prediction of relativity over Newtonian gravity and was seen as one a proof of Einstein's relativity.

If dark matter has a mass - which it must - then it has to bend light as well whenever there is a lot of it in one place. This can be observed in so called dark galaxies. These are galaxies in which almost all of the luminous matter has been stripped away by collisions or some catastrophic event, but has left the dark matter galactic halo behind. This object is dark, but has an enormous gravitational pull. It shows up when you look at the bending of light.

Incidentally, the behaviour of light in a gravitational field is the reason balck holes are called that. The gravitational pull of a blakc hole is so strong that light cannot escape. Near the event horizon of a black hole, light is bent so strongly that it will curve in a perfect circle around the blak hole and never escape.

Right, finally we get to your first question. Is it very likely we can study dark matter here on earth if we can't produce it? Well there are two answers to that. Firstly, we might produce it on earth when the LHC turns on, so in that sense yes - hopefully we'll be able to create dark matter and study it directly. If we aren't able to then we can still study it in two ways. We can't create stars on earth, but we can get a really good idea of how they work by studying their existence in the universe around us. We look at the light they throw out and the effect of their presence on the objects around them and work back to a detailed understanding of what they do. We can do the same with dark matter. We already know what it does and doesn't do - it does have mass and a gravitational pull, it is electrically neutral and doesn't interact through the strong nuclear force. It is massively abundant and it is stable on lifetimes similar to those of the universe. So yes, we can study it without producing it. We'd get a better idea of what it was if we could produce it, but we're doing pretty well so far. Finally, even if we can't produce it, we can still find it - because it is already here. It keeps the galaxy together and is like a diffuse gas throughout the whole galaxy. If our theories of dark matter are correct then roughly 1 billion particles of dark matter are passing through each person's body every second. It hardly interacts at all, and when it does, we'd never notice it. So we build detectors that would notice such an interaction. In mines in Asia, the States and Europe there are detectors in operation and yet more being constructed in deep mines behind layers of shielding trying to find elusive interactions of the dark matter that permeates the universe around us. They will be able to test the theories of dark matter independently from the collider at CERN. If we find signals of dark matter in both places then we will have two separate windows on one of the great mysteries of the universe - and both sets of experiments should be reporting important data in the next few years.

[Drazi]

Thanks for that response. As you might have guessed already I'm a bit of an armchair enthusiast when it comes to theoretical physics. I find the logic behind new theory to be very exciting. I've always equated theoretical physics to philosophy, in a way it seems to satisfy the same part of me that demands a logical system for things to work in. At a time I considered theoretical physics as a possible career choice, but I find the math involved with it to be tediously boring, I'm much more fond of the ideas and how they relate to on another. Anywho, thanks for the great info, I'll be posting again if I come up with more questions.

Looking forward to hearing about the collider in action.

-Drazi

[torstan]

Well, you're preaching to the converted on that one. I did a joint honours in physics and philosophy. We did the maths in the physics course and read the original papers of Einstein, Bohr, Newton et al in philosophy. There is a lot of interesting philosophy going on in physics at the moment, especially while we wait for the new data.

I'm more than happy to keep answering questions - and will keep the guild updated on the collider as long as there is an interest

[RPMiller]

Incidentally, the behaviour of light in a gravitational field is the reason balck holes are called that. The gravitational pull of a blakc hole is so strong that light cannot escape. Near the event horizon of a black hole, light is bent so strongly that it will curve in a perfect circle around the blak hole and never escape.

WARNING: Thread derailment imminent...

Just curious if these were intentional misspellings because of this:
http://cityhallblog.dallasnews.com/a...-turns-ra.html

Yea, it is really ridiculous and I would suggest not reading the comments that follow it. I was just curious since they are all different.

[Torq]

Well, you're preaching to the converted on that one. I did a joint honours in physics and philosophy. We did the maths in the physics course and read the original papers of Einstein, Bohr, Newton et al in philosophy. There is a lot of interesting philosophy going on in physics at the moment, especially while we wait for the new data.

One of the other boards that I visit a bit has a guy that has the following in his sig:

"Observer-dependent physics undermines the gods' decision 3000 years ago to ban cats from straddling the borders of the netherworld. We won't have it!". I laugh every time I see it.

Torq

[torstan]

Oh dear

White holes are hypothesised to exist - they would be the opposite of a black hole - spewing out and almost endless stream of light and matter. They are hypothesised as the 'other end' of a black hole. But there again we head out of what I know and into the dark depths of wikipedia.

@Torq: That's an excellent sig. That should bait pretty much everyone on the science/religion divide.

[Turgenev]

Just wanted to say this has been a great thread. I'm an armchair science nut. I might not understand it all but I love reading up on the stuff (especially astrophysics, astronomy, paleontology, archaeology, and biology).

When I hear about White Holes, I always think of the Red Dwarf TV show where Lister shoots pool with planets to close a White Hole.