Dark Matter

[EdgePenguin]

I want to start a non-FTB related thread, as I'm worried if I spend too much long in the big thread I might end up being too damn negative all the time. So lets try science.

I do research into dark matter, and am aware that some people are skeptical of the concept (as in, they think its a lazy fudge on the verge of being overturned). I've been blogging about the subject, and would appreciate some feedback:

http://edgepenguin.com/content/darkmatter.html
http://edgepenguin.com/content/darkmatter2.html

[16bitheretic]

I'm not a physicist, but given that dark matter is a placeholder that's basically "we don't know what this is yet", wouldn't we need to know a lot more than we currently do to either prove or disprove dark matter?

Anyone is free to call me a moron on this subject, but my uber-layperson level understanding is that we're not close at all to validating or debunking dark matter and dark energy.

[LMU]

Is it correct that the Higg's Boson mass being at or near 126GeV rules out Supersymmetry and therefore a large number of theoretical dark matter candidates? Or is that overstating it? What are current dark matter candidates?

[EdgePenguin]

I'm not a physicist, but given that dark matter is a placeholder that's basically "we don't know what this is yet", wouldn't we need to know a lot more than we currently do to either prove or disprove dark matter?

Anyone is free to call me a moron on this subject, but my uber-layperson level understanding is that we're not close at all to validating or debunking dark matter and dark energy.

Dark matter isn't truly validated until we get a detection, but its a very strong hypothesis. The MOND stuff I have read is either a) astoundingly bad or b) modified gravity but still requires the presence of some dark matter - even if it is Baryonic. What we know for certain is that there is a very large amount of excess gravitation going on, at all levels of the universe, and we have no other explanation for gravitation besides mass.

Dark energy is, despite the name, quite a separate issue from dark matter. Its also IMHO a fair bit more contentious, but I don't do large scale cosmology/early universe stuff so I'm no more of an expert than you.

Is it correct that the Higg's Boson mass being at or near 126GeV rules out Supersymmetry and therefore a large number of theoretical dark matter candidates? Or is that overstating it? What are current dark matter candidates?

I believe it only rules out some versions of supersymmetry, but likely yes that will have thinned the field of dark matter candidate particles a bit. I am not a particle physicist, and unlike a certain sky-pointer popular in the UK I don't merrily skip between particle physics and astrophysics.

[LMU]

Thank you for your reply! Your blogs look good and clear to me. For some reason I thought that we had landed probes on Mercury, but apparently not!

[LMU]

I have a physics question, which is: "Why can't information get out of black holes?"

Let me explain a little what I mean. I know that the escape velocity of the black hole is equal to the speed of light at the event horizon (if this is wrong, please tell me, it might explain my confusion), so I know you can't just send a light signal from inside a black hole. However the escape velocity is the velocity an object would need to escape to infinity, it should still be possible to escape to some finite smaller distance. For example on Earth if I toss a piece of bread into the air I know that it has to fall down to Earth again because I can't throw it that hard, but while it is in the air things can still happen to it. It could be snatched by a bird that would eat it, or by a person in a balloon who could throw it again, but starting from a higher altitude. Light that tries to leave from the event horizon will either be bent back around to the black hole, or be red shifted to nothing if it is traveling directly away (right?). But as it is being bent or before it is red shifted away completely, why can't it be intercepted by something near but outside the event horizon? Imagine a space ship near a black hole that drops a series of probes into the event horizon. The light signals from the probes cannot escape to infinity, but can they be spaced so that each probe is able to signal to the one above it and ultimately to the space ship? Why doesn't this work? I assume I am thinking about this too classically and not appreciating the effects of GR or maybe my mistake is more basic than that. Thank you for any reply!

[EdgePenguin]

Thank you for your reply! Your blogs look good and clear to me. For some reason I thought that we had landed probes on Mercury, but apparently not!

It would be extremely difficult. Getting into Mercury orbit is hard enough, because as you fall in towards the Sun you pick up a lot of speed, which means you need lots of fuel to brake when you get to Mercury. So your payload is much more limited than a Mars or Venus mission, and thus far this hasn't allowed a lander to be added.

I have a physics question, which is: "Why can't information get out of black holes?"

Let me explain a little what I mean. I know that the escape velocity of the black hole is equal to the speed of light at the event horizon (if this is wrong, please tell me, it might explain my confusion), so I know you can't just send a light signal from inside a black hole. However the escape velocity is the velocity an object would need to escape to infinity, it should still be possible to escape to some finite smaller distance. For example on Earth if I toss a piece of bread into the air I know that it has to fall down to Earth again because I can't throw it that hard, but while it is in the air things can still happen to it. It could be snatched by a bird that would eat it, or by a person in a balloon who could throw it again, but starting from a higher altitude. Light that tries to leave from the event horizon will either be bent back around to the black hole, or be red shifted to nothing if it is traveling directly away (right?). But as it is being bent or before it is red shifted away completely, why can't it be intercepted by something near but outside the event horizon? Imagine a space ship near a black hole that drops a series of probes into the event horizon. The light signals from the probes cannot escape to infinity, but can they be spaced so that each probe is able to signal to the one above it and ultimately to the space ship? Why doesn't this work? I assume I am thinking about this too classically and not appreciating the effects of GR or maybe my mistake is more basic than that. Thank you for any reply!

Yes, its a relativistic thing. I shall try to explain (but my GR is a bit rusty and I might not be strictly accurate, I don't generally deal with stuff where its needed)

In GR, gravity is a change in the shape of space time. Starting from a single point in space time (an event) there is a set of events you can move to next in your personal timeline. All of these events are in your personal future, and you can't move faster than light, so you end up with a 4-dimensional analogue of a cone in space time, called your future light cone.

The intense gravity near a black hole changes the shape of this, so that the moment you cross the event horizon, all the events you can visit next - all the directions you can move regardless of how fast - lead further down into the black hole. The singularity is no longer a possible place you can visit, its a certain event in your future.

The first diagram in this wiki page illustrates this, sort of.

http://en.wikipedia.org/wiki/Event_horizon

[LMU]

Thank you for answering my question! So I guess if you could lower a rod across an event horizon (without falling in yourself), then you wouldn't be able to pull it back? Part of the reason I ask is because I thought that you weren't supposed to be able to tell if you had crossed the event horizon, or maybe that is only true if you are in free-fall?

[EdgePenguin]

Thank you for answering my question! So I guess if you could lower a rod across an event horizon (without falling in yourself), then you wouldn't be able to pull it back? Part of the reason I ask is because I thought that you weren't supposed to be able to tell if you had crossed the event horizon, or maybe that is only true if you are in free-fall?

The force holding the rod together is electromagnetism, and is carried by photons. a physical objected can't straddle the event horizon in the way you describe, as this would allow photons and information to exist the black hole. Yes, not noticing you've passed the event horizon is only theoretically possible when free falling into a supermassive black hole. A stellar black hole's tidal forces would tear you apart before you got there.

[LMU]

Thanks again for the reply! Sorry the questions were so inane.

On topic, do you have any thoughts or opinions on "dark force"? Basically does dark matter interact with other dark matter particles? I'm talking about this sort of thing: http://www.newscientist.com/article/dn2 ... nline-news

[EdgePenguin]

Self-interacting dark matter (SIMD) is a contender for explaining certain properties of dark matter haloes i.e. the difference between the ones predicted by large scale cosmology and the ones we actually infer from galaxies. I've honestly not read much into it, but I don't get the impression many people take it that seriously at the moment. Its something I've been meaning to read up on, though - I think its more credible than things like MOND certainly.