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Dark matter scaffolding of universe detected for the first time

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Re: Dark matter scaffolding of universe detected for the first time

#21

Post by drostie »

kamaal wrote: post #14
tb wrote: post #5

Earlier quoted context omitted.

> Some other particle may interact with nothing we are familiar with -- and thus we could never study it. Is it even "real" then? Philosophically, this is equivalent to the question of whether other universes, which do not interact with ours and therefore we cannot study, exist or are "real". It is not a question that Science can answer.

Oh well, That question really has far reaching implications. Because if we say Science is what we observe and describe as per our interpretations of logic(And the language of logic - 'Math') then our science is really broken. Because what we can observe doesn't often turn out to be true and what is true is not often observed. Because look at it this way. We are now saying Dark Matter doesn't interact anyway with ligh…

It is only "broken" if you assume incorrectly that the goal of science is to Discover Truth. Moreover it is broken in a more direct way: there exist certain models which are mathematically equivalent but which describe contradictory states of being. A reasonably good example of this is heliocentrism vs. geocentrism: classical mechanics allows you to say "the Earth is at the center of the Solar System, there are gravitational, Coriolis and centrifugal forces around it affecting all of the stuff in space", but it also allows you to say "The Sun-Jupiter barycentre is at the center of the Solar System, and the only force we need is gravity." There is no experiment which can distinguish between those two; they are mathematically equivalent.

(A slightly better example comes from quantum mechanics. In the "Schrodinger picture" there is a "wavefunction of the universe" which changes from moment to moment, while the definitions of space and momentum stay the same. In the "Heisenberg picture" the wavefunction stays the same while the definitions of space and momentum change. You would think there would be an ontological difference to the question, "is the state of the universe different from the state of the big bang?" but, in fact, on this description there is no observable difference, and science could never settle the question.)

This does not reduce science to a religion; science simply studies the observable differences and must be content with not knowing everything -- which most scientists are already content with, since they have to deal with matters of uncertainty and the distinctions between correlations and causations.

Dark Matter does interact with other things, but it does so indirectly, because it has mass and therefore warps spacetime. This is not actually the first use of gravitational lensing to observe dark matter; in fact, earlier it had been used to settle the question of whether dark matter felt any electromagnetic force at all, by looking at galaxy collisions. The prediction would be that the dark matter clouds of two galaxies would more or less "go through each other" in a collision while the luminous stuff would "bump into each other". This was observed as early as 6 years ago, see http://chandra.harvard.edu/photo/2006/1e0657/ .

We certainly can show you, and we can explain to you what it is. The only problem is the same problem that neutrinos have: it's just very hard to detect these particles because they don't have an electric charge and therefore don't care about the electrons which make all the rest of chemistry happen. Our best tool for understanding dark matter is still gravity; its a force which we know the dark matter feels.

Re: Dark matter scaffolding of universe detected for the first time

#22

Post by lutusp »

drostie wrote: post #21
kamaal wrote: post #14

Earlier quoted context omitted.

Oh well, That question really has far reaching implications. Because if we say Science is what we observe and describe as per our interpretations of logic(And the language of logic - 'Math') then our science is really broken. Because what we can observe doesn't often turn out to be true and what is true is not often observed. Because look at it this way. We are now saying Dark Matter doesn't interact anyway with ligh…

It is only "broken" if you assume incorrectly that the goal of science is to Discover Truth. Moreover it is broken in a more direct way: there exist certain models which are mathematically equivalent but which describe contradictory states of being . A reasonably good example of this is heliocentrism vs. geocentrism: classical mechanics allows you to say "the Earth is at the center of the Solar System, there are grav…

> ... there exist certain models which are mathematically equivalent but which describe contradictory states of being. A reasonably good example of this is heliocentrism vs. geocentrism: classical mechanics allows you to say "the Earth is at the center of the Solar System, there are gravitational, Coriolis and centrifugal forces around it affecting all of the stuff in space", but it also allows you to say "The Sun-Jupiter barycentre is at the center of the Solar System, and the only force we need is gravity." There is no experiment which can distinguish between those two; they are mathematically equivalent.

How do heliocentrism and geocentrism represent "contradictory states of being"? They are trivially related to one another, and are mathematically equivalent as you point out. They represent a simple example of geometric relativity.

Consider a gravitational slingshot maneuver, a way to harvest some of a planet's orbital momentum to accelerate a passing spacecraft. If you make the planet the frame of reference, or the sun, or the spacecraft, the math and physics come out the same. No "contradictory states of being".

> A slightly better example comes from quantum mechanics. In the "Schrodinger picture" there is a "wavefunction of the universe" which changes from moment to moment, while the definitions of space and momentum stay the same. In the "Heisenberg picture" the wavefunction stays the same while the definitions of space and momentum change.

No,. this isn't really a "better example" -- Dirac demonstrated the mathematical equivalence of Schrodinger's wave mechanics and Heisenberg's matrix mechanics. Again, the difference is only apparent and superficial.

Re: Dark matter scaffolding of universe detected for the first time

#23

Post by Jarihd »

This is something i thought about Dark Matter: I believe Dark Matter to be the resultant force(and/or field) generated due to the interaction of the forces(and/or fields) of individual moving objects(matter).

consider a magnet(refer here as object) - something which has the property to attract(gravity like) and repel(field like): Now if you were to have 2 magnets(moving objects) come close enough such that they repel(or attract); but due to forces(and/or fields) of other moving objects in their vicinity(or far enough[1]); they get locked or entangled such that their movement(and other properties) is now dependent on the strongest forces or fields of nearby objects. Over time; these other objects also get entangled and tend to form clusters and keep moving(exhibiting other properties like radiation etc). But now their movement(and other properties) seem to be the resultant effect of forces (and/or fields) of all the objects now entangled - giving an illusion of some matter that exists - now known as Dark matter.

I have used magnets as just as an example - one could think of matter having both these properties to attract and repel - such that the area affected by them could vary depending on various properties of the objects(matter).

[1](far enough) - such that their observation is neglected; but these objects tend to have forces(and/or fields) that they affect a particular system under observation.

Re: Dark matter scaffolding of universe detected for the first time

#24

Post by InclinedPlane »

ok_craig wrote: post #10

There's one thing I don't understand about the mystery of dark matter. I don't understand why the simple explanation for it isn't just that it's regular matter that is not stars. Maybe there are just bajillions of planets and dust clouds out there. Matter that isn't directly circling stars, thus not reflecting light. Why is the popular assumption that if the mass isn't stars or things in orbit of stars, that it must…

Science!

We can tell how much dark matter is out there because we can "weigh" it through indirect measures. And then we can take different theories of dark matter (such as, say, the theory that it's all just a bunch of interstellar orphaned planets and "black dwarfs" and what-have-you made up of ordinary matter) and figure out what sorts of implications that would have, make predictions on observable effects of those different models and then test those predictions. And that is precisely what happened about 20-30 years ago. A lot of work was done to pin down what type of dark matter makes up the majority of it out there.

For example, you can point a telescope at a set of neighboring galaxies and look for brightening effects due to gravitational micro-lensing from a chance alignment of a "macho" (e.g. orphaned gas giant planet) along the line of sight. Surveys were set up and indeed found that there were orphaned "macho" objects in our galaxy, but the statistics showed that they were orders of magnitude too rare to make up the bulk of dark matter we know about from other studies. Another line of evidence involves studying the large-scale structure of the Universe (e.g. the layout of galaxies, galaxy clusters, etc.) and comparing it with various computer simulations of models with different assumptions on the composition of the mass of the Universe (e.g. 100% "ordinary" baryonic matter, various percentages of "special" dark matter such as cold and hot dark matter, WIMPs, etc.)

From this and many other lines of evidence we came up with very strong evidence that the vast majority of the mass budget of the Universe is in the form of so-called "cold dark matter" which is composed of weekly interacting massive particles other than neutrinos (neutrinos are dark matter, but we've been able to place an upper limit on how much they contribute to the dark matter budget of the Universe, because they are detectable to a degree, and it's only a fraction).

So that's it, just a simple matter of comparing the predictions of different theories with observations and eliminating the theories that do not predict what we actually see out there in the Universe.

Re: Dark matter scaffolding of universe detected for the first time

#25

Post by InclinedPlane »

So, interestingly enough the leading theory for the identity of the bulk of dark matter is the "weekly interacting massive particle" (or WIMP) the neutralino, and there has been recent evidence from the Fermi gamma-ray telescope that supports the theory that the neutralino is the primary component of dark matter.

http://arxiv.org/abs/1201.1003

http://arxiv.org/abs/1205.1045

Re: Dark matter scaffolding of universe detected for the first time

#26

Post by mkup »

InclinedPlane wrote: post #24
ok_craig wrote: post #10

There's one thing I don't understand about the mystery of dark matter. I don't understand why the simple explanation for it isn't just that it's regular matter that is not stars. Maybe there are just bajillions of planets and dust clouds out there. Matter that isn't directly circling stars, thus not reflecting light. Why is the popular assumption that if the mass isn't stars or things in orbit of stars, that it must…

Science! We can tell how much dark matter is out there because we can "weigh" it through indirect measures. And then we can take different theories of dark matter (such as, say, the theory that it's all just a bunch of interstellar orphaned planets and "black dwarfs" and what-have-you made up of ordinary matter) and figure out what sorts of implications that would have, make predictions on observable effects of those…

(A bit off-topic question to physics expert on HN)

I've read that supermassive black hole accretion is the most energy-effective process of mass to energy conversion in the Universe (50% efficiency or so).

I'm just curious: Where does all that energy go? Extremely powerful jets of radiation are emitted into the intergalaxy space and then what? Does it just disappear? Isn't this energy responsible for Universe expansion? It must push galaxies away from each other, right?

Re: Dark matter scaffolding of universe detected for the first time

#27

Post by Tloewald »

mkup wrote: post #26
InclinedPlane wrote: post #24

Earlier quoted context omitted.

Science! We can tell how much dark matter is out there because we can "weigh" it through indirect measures. And then we can take different theories of dark matter (such as, say, the theory that it's all just a bunch of interstellar orphaned planets and "black dwarfs" and what-have-you made up of ordinary matter) and figure out what sorts of implications that would have, make predictions on observable effects of those…

(A bit off-topic question to physics expert on HN) I've read that supermassive black hole accretion is the most energy-effective process of mass to energy conversion in the Universe (50% efficiency or so). I'm just curious: Where does all that energy go? Extremely powerful jets of radiation are emitted into the intergalaxy space and then what? Does it just disappear? Isn't this energy responsible for Universe expansi…

Same place sunlight goes.

Re: Dark matter scaffolding of universe detected for the first time

#28

Post by rwmj »

Xcelerate wrote: post #8
pdonis wrote: post #7

Earlier quoted context omitted.

Gluons also interact gravitationally, like anything with energy. So would any other hypothetical particle, since it would have to have energy to exist at all.

I said earlier in the post "there's one interaction between ALL of the particles that is missing: gravity", so it was implied. Perhaps I should have made that a little clearer.

I thought the interaction with the Higgs is supposed to create mass?

Re: Dark matter scaffolding of universe detected for the first time

#29

Post by JonnieCache »

ok_craig wrote: post #10

There's one thing I don't understand about the mystery of dark matter. I don't understand why the simple explanation for it isn't just that it's regular matter that is not stars. Maybe there are just bajillions of planets and dust clouds out there. Matter that isn't directly circling stars, thus not reflecting light. Why is the popular assumption that if the mass isn't stars or things in orbit of stars, that it must…

On top of the stuff other people have posted, we can (apparently) peer at the cosmic microwave background and look for patterns in its anisotropy power spectrum from baryonic acoustic oscillations in the early universe's quark-gluon plasma. The results we see demand WIMPs.

I'm not going to explain what any of that means because A. I've only recently woken up, B. you're perfectly capable of googling for yourself, and C. I'd probably get it wrong anyway. Suffice to say it's all brain-meltingly interesting.

The excellent sean carroll has done a lecture on this: http://online.kitp.ucsb.edu/online/lens06/carroll/

He has also written several long-ass blogposts:

http://blogs.discovermagazine.com/cosmicvariance/2011/02/26/...

http://blogs.discovermagazine.com/cosmicvariance/2012/05/09/...

EDIT: you may also want to look up the Bullet Cluster. Or just read those blogs, it's all in there.

Re: Dark matter scaffolding of universe detected for the first time

#30

Post by aroberge »

rwmj wrote: post #28
Xcelerate wrote: post #8

Earlier quoted context omitted.

I said earlier in the post "there's one interaction between ALL of the particles that is missing: gravity", so it was implied. Perhaps I should have made that a little clearer.

I thought the interaction with the Higgs is supposed to create mass?

Two things, greatly oversimplified: 1) all forms of energy (including mass) are sources for gravity. 2) Interaction with the Higgs gives rise to what we observe as mass of particles; without this interaction, we'd have massless particle, with essentially the same energy, but moving at the speed of light. The net gravitational effect would be very different from what we observe though...
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