Posted on 08/28/2007 11:51:06 AM PDT by SunkenCiv
Radiation was used to pinpoint the normal matter, while the observation of gravitational lensing was used locate dark matter. Gravitational lensing allows matter to be oberved, even when it does not emit or absorb light, by examining the movement of galaxies as our line of sight passes through the area of interest. Massive objects will distort the image and cause it to move in unexpected directions. Because the normal matter could interact through electromagnetic radiation, it was found to have slowed violently during the collision while the dark matter sailed on through... In the meantime, other astronomers began using gravitational lensing to make a map of dark matter in clusters and it hasn't taken too long to find results that cast doubt on our understanding of dark matter -- though it does not cast doubt on its existence... Dark matter only interacts gravitationally, which means it should not have been slowed up much during the collision -- there is no way to dissipate the energy as it slows and condenses. Each blob of dark matter from the clusters should have continued along the direction of motion of the cluster before the collision, with the odd tweak in direction thanks to gravity. However, something has slowed the dark matter so that it can conglomerate into a dense mass.
(Excerpt) Read more at arstechnica.com ...
It is. It is what your cat sees and you can't.
The relevant theories are gravitational theory and field theory. They are theories, because they are supported by the evidence. In the case of dark matter, it was both a matter of field theory and gravitational theory, showing some particle(s) was missing observing something new, that fit the missing particle.
In gravitational theory, their was missing mass. Mass that must be there, but could not be observed as an otherwise interacting particle, by way of the electroM, weak, or strong force. In field theory, specifically with regard to strong interactions, there was a lack of explanation for the theory's prediction the charge*parity(CP) would be violated and the observation that CP is not violated experimentally. Parity is the symmetry of a particle(s) when it's direction is reversed. This strong CP problem can be understood by the existance of a particle called and axion. It has a very small mass, but does not interact through the strong, weak, or EM force. That's because, in an overly simple pic for this particle, all fields cancel due to symmetry and all that's left is mass. The particle was found over ~a year ago and reported on FR.
The axions would have been created shortly after the Big Bang, at the time the matter/antimatter imbalance was established and can explain both the CP problem and the missing mass required by GR.
"Galaxies are rotating too fast to fit the theory of angular momentum."
It's not that the spiral galaxies are rotating too fast, but the fact that the radial distribution of angular momentum is not what it should be, if all that was present is the visible mass. Instead of having a fast rotating core and a slow outer region, as would be expected from the distribution of what could be seen, the galaxies rotate with a high speed from the center out. The existance of massive, but otherwise non-interacting particles flying around the galaxy explains the motion. Axions fit.
"distant galaxies are expanding faster then they should according to the theory of relativity."
That's due to dark energy, not dark matter. Dark energy can be seen as the effect of the vacuum. It has a small negative gravitational constant. THe energy density of the universe is zero. It consists of mass and that contained in the EM, weak, and strong particle fields, which are denoted positive energy, and the negative(potential) energy of the gravitational field. The big bang doesn't consist of stuff flying out from and explosion. Space itself is expanding. As the space expands, the energy density decreases, and the relative effect of the vacuum increases. That means the gravitaitonal "constant" gets smaller, and a higher acceleration results.
"Now there is something that is causing light to bend, but is not observable by either absorbing or reflecting energy. Since this behavior is neither like regular matter nor a black hole, and the fact that something is generating enough gravity* to cause light to bend they have dubbed it ‘Dark Matter’.
In that way they IDed the dark matter in this case. Normally the dark matter flies at around the speed of light, but always below it. In spiral galaxies the trajectories for the axions can be calculated and they appear more concentrated at the outskirts. In this case the particles are all concentrated at the center and trajectories haven't been calculated. They 'll need to do that to explain the absence of observed angular momentum. Some questions are raised. The kinetic energy of those axions must have gone somewhere and gravity is qite a small force to accomplish the disipation of all that kinetic energy w/o resulting in a net angular momentum. they'll have to examine the possibilities of creating such a formation that way. If it's impossible to do it, they'll be a new discovery made in the physics involved.
They're called WIMPS-weakly interacting particles. The sun puts out tons of neutrinos and they go right through you. So do axions. They only have an effect on the large scale by their combined gravitational field, like in spiral galaxies and other large scale dynamics. On small scales, they are WIMPS.
If they don’t even know what it is, how can they even claim to know how it’s “supposed” to behave?
Science is supposed to be more than guessing.
Or making stuff up...
L
According to Newton, that's not possible. If there are just a few of these 'wimps' then they'd get trapped in Earth gravity or even my gravity. Eventually there would be a huge 'mass' of these 'wimps' all coalescing around Earth, me, and my carpet.
Nothing can 'weakly' interact gravitationally. Gravity either affects something or it does not. And if these 'wimps' have mass, they have gravity and therefore would be subject to Newtons Laws.
Ergo, 'dark matter' is a bunch of made up hokum so astrophysicists can 'explain' something they can't explain.
It's bunk.
L
They're flying way too fast-close to the speed of light. Since they are non-interacting otherwise, they fly right through the Earth unhindered. They can fly right through a neutron star unhindered. They would only be captured by black holes. Otherwise there orbits are around the black hole, or around objects the size of galaxies.
Neutrinos, which come from nuclear reactions in the sun interact sometimes when they hit the Earth. They can do that deep in the Earth, because they interact via the weak force. Since their mass and x-sections are similar to axions, they otherwise fly right by unnoticed
"Nothing can 'weakly' interact gravitationally."
A dimensionless coupling constant for the gravitational force between 2 electrons is 2*10-45. The electromagnetic coupling constant is 1/137. So the force of gravity is 43 orders of magnitude smaller than the EM force. The force of gravity is only large on Earth, because of the mass of the Earth. Notice space walkers need ropes, else they'd easily wander off into space. their ships gravity is not large enough to bring them back anytime soon, even from a small push. The astronaut and his ship are still a sprung system, with gravity as the spring, but the escape velocity is very low. If they don't make that escape velocity, they obit each other at huge distances, or the period of oscillation will be huge. If the Earth wasn't there, If the Anaut forgets his rope, I could imagine an Anaut coming back to bounce off his old ship every 500 years.
Have they 'caught' any?
The force of gravity is only large on Earth, because of the mass of the Earth
So this 'dark matter' stuff should be orbiting, or falling to, Earth and landing on my carpet. Eventually after billions and billions of years at least 'some' of the stuff should have been captured by Earth or Jovian or solar gravity.
I just don't buy the argument that solar gravity is too weak to trap this 'dm' stuff. After all this time there should be some of this crap relatively close to us. It can't only be 'out there'.
L
"So this 'dark matter' stuff should be orbiting, or falling to, Earth and landing on my carpet."
They fly at the spped of light. The escape velocity from Earth's surface is ~11000m/s. The speed of light is 3*108m/s. They'll always fly back off into space. Neutrinos can interact with nuclear matter, so they can interact. Their small x-section means they have a low probability of doing so. So, if they do inteact, it's always deep in the Earth as they fly through. Axions just fly through.
"or Jovian or solar gravity."
Larger objects are required to capture them into orbit. It's the collection of large objects in a galaxy, or black hole that can and does.
So the gravity of the Sun is powerful enough to capture Jupiter, but it's not powerful enough to capture a sub-atomic particle. Do I have that right?
And even if these 'wimps' are traveling at near light speed, they should still be deflected by gravity the same way a bullet is deflected by Earths gravity. Newtons Laws apply to everything. Don't they?
L
"And even if these 'wimps' are traveling at near light speed, they should still be deflected by gravity the same way a bullet is deflected by Earths gravity. Newtons Laws apply to everything. Don't they?"
Newton's law applies where the space can be approximated as flat. That is a good approx in the space throughout the solar system beyond say mercury.
Consider making a 200yd shot with a 22. It will be more of an artillery shot, than the same shot made with a 223. That's because the time they spend in reaching the target is smaller and the acceleration toward the Earth will be much smaller with the 223. A missile at twice the speed of the 223 will be deflected even less. The deflection of something flying near the speed of light will be unnoticeable.
What's really important is the x-section. I'm just going to pull rough numbers out of my head... The unit of area used for the x-section, or area of targets and particles in neclear physics is the barn. ...after broad side of the barn... A barn is 10-24cm2. Normal nuclear targets and particles have target sizes of barns and fractions of barns. The size of the target will depend on the interaction involved. If the interaction is weak(not refering to weak force), the x-section will be small and larger it's strong. The size difference is because the targets are components with processes involving different forces. Even the binding force, or particle is part of the target. Targets for neutrino interactions are much smaller, in the range of 10-15 barns I think. With axions, the x-section is about zero and for simplicity undefined, because there are no interactions.
That means these WIMPS fly right through stuff. They'd be slightly accelerated on the way in, and retarded back to their original speed on the way out.
Keep in mind, that the distance between these targets, which are nuclei are at least 2000+ times larger than the target. So even though it looks dense, the volume of most stuff is empty space/vacuum. So the Earth really looks like a light cloud to a neutrino, and an axion see the Earth as nothing more than the vacuum with a tiny bump in the gravitational field.
But no gravitational acceleration is 100% efficient. Eventually some of these wimps would be slowed down somewhere in the universe. If the gravitational field were large enough, they'd be brought to a complete stop. Right?
So what these astro types are trying to tell me is that this hasn't happened anywhere? That makes no sense. Also if the stuff is moving at light speed pretty much randomly, how does it form 'clouds' (for lack of a better word) that causes this lensing effect?
Logic would dictate that they're flying in all directions instead of forming vast clouds that can 'lens' light.
Right?
L
THey'll stop only at the event horizon of a black hole. They can be slowed, only after leaving the region os a galaxy, but in that case they'll be accelerated back again to their original speed when they return to the galaxy.
Let's say they're slowed for that reason. Then they're captured by a relatively slow moving large object. They will collect in and around the center of the object, because they are weakly interacting. They will never sit on the surface, because the surface layers offer no impediment to their motion. Most likely, they will simply be rendered into eccentric orbits that pass through the object.
Regardless, the gravitational field of the nearby galaxy that holds that particle, will also grab the big object and haul it in. Since the WIMP has it's orbit in the galactic field, the other object has it's own orbit in that field and all the WIMP is held by is gravity, the WIMP will be torn away again by the galctic field and return to it's oriignal v in it's orbit. Effectively, no relatively amall and lone object, other than black holes, can grab one. They can only provide the impediment of a particle cloud and temporary gravitational bump.
"Logic would dictate that they're flying in all directions instead of forming vast clouds that can 'lens' light."
Neutrinos are flying in all directions as they are being created. Axions were created around the time quarks condensed after the big bang. They follow the expansion of the universe and such things as black hole and galaxy formation.
"Also if the stuff is moving at light speed pretty much randomly, how does it form 'clouds' (for lack of a better word) that causes this lensing effect?"
the clouds of them in spiral galaxies are understood as orbits around and through the galaxy. THe same would happen around a black hole. Those regions containing many orbits would lens.
Cluster galaxies have little, or no spin. The particles could just be in orbits with no net angular momentum. That would be trivial, but no one's done the calcs and reported them yet. I don't know much about clusters, such as formation, ect... , so I can't say much.
Then shouldn't they be out at the 'edges' of the Universe instead of mixed in willy nilly?
Not giving you a hard time. I'm really interested in this stuff.
L
"Then shouldn't they be out at the 'edges' of the Universe instead of mixed in willy nilly?"
Here's how to think about that. In the beginning, the whole universe was all at a point, or tiny area. The stuff didn't fly out from there, space expanded. It's the space that is doing the expansion, not the stuff. That means, as you look out into space, the things you see, are things that were happening distance/c years ago. The universe looks like a balloon. It is all contained in the surface region of an expanding space-the balloon surface. The speed of light restricts how much of that surface region we can see. It is really quite small, and looks flat from here. If you take a balloon and make 3, or more dots an it, then watch the dots as the baloon is inflated, you'll note that they always get further away from each other.
The universe is expanding at around c. C is usually used for the expansion rate. So it's the balloon's radius that's increasing at c. The surface of the balloon just gets a bit thicker as time goes on.
As far as the edges of the universe go, they are in the future, so they can't be seen, only the past can be seen and it's all in that balloon like surface.
It's quite easy for me to see that as an object like this formed from high density stuff, it collected the orbiting WIMPs and now the fog that appears, is simply the gazillions if complex and ever changing orbits in the cluster. The stars do the same complex orbits, because they're always being purturbed by nearby passing stars, but they're moving slower and can collide. Being an old system though, collisions will be rare, because the orbital paths are relatively stabilized. The axions however just whip through the stars and see little gravity bumps. Gazillions of them can orbit a central black hole, as if the other stars weren't even there.
Things inside a large spherical mass only see the gravity of the mass inside the radius that is equal to the distance of the thing from the center of the entire spherical mass. So the axions are simply orbiting the huge mass in the center. There's high order effects in gravity which result in fine structure. Those and the fact that the field comes from many objects give the gravitational field of the cluster a multipolar configuration. That can be seen in the image above on the thread as the regularly placed and appearing lumps in the pic. So,I'd say the fog of dark matter in these clusters is simply due to the orbits.
the analogy is the electron cloud surrounding the nucleus in any atom, or molecule. The particle orbits, but is observed as aa cloud. That's because, as the electron goes through it's various orbits, at relativistic speeds, the fog density follows where the electron most often is.
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