Posted on 10/02/2003 12:55:26 PM PDT by Ernest_at_the_Beach
The identity of the Universe's dark matter may finally have been discovered. In what seems to be the most convincing claim for dark matter so far, researchers in England and France say gamma rays coming from the centre of our galaxy show hallmarks of these ghostly particles.
The research has only just been made public, so the team is still waiting for a response from other dark matter experts. But though the researchers are cautious, there is no hiding their excitement. "I've dropped everything else to work on this," says Dan Hooper of the University of Oxford. "We're really excited," adds his colleague Céline Boehm, also of Oxford. "I'm cautious but it's surprising everything fits so well."
The identity of the Universe's dark matter, which outweighs the visible stuff by at least a factor of seven, is the outstanding mystery of modern astronomy. Scientists think it must exist because its gravity affects the way galaxies hold together. But the particles do not emit any electromagnetic radiation so they have never been detected directly. No one knows what the particles are like, or exactly how they are distributed.
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Mystery cloud |
However, because dark matter "feels" gravity like ordinary visible matter, it is a fair bet that it clumps in the centre of our galaxy. So the team turned their attention to a distinctive pattern of gamma rays coming from the centre of the Milky Way (see graphic). The sharp signal, which has an energy of 511 kiloelectronvolts (keV), is believed to be due to the annihilation of electrons and positrons the antimatter equivalent of electrons.
Virtual standstill
But where did the electrons and positrons come from? People have speculated that the source is anything from the blast waves of a "hypernova" a super-powerful supernova to a neutron star or black hole. "But none of the explanations have seemed satisfactory," says Hooper.
(Excerpt) Read more at newscientist.com ...
I think they just don't fully understand the rotational physics of (for lack of a better word)"Chaotic" Spiral galaxies so they are inserting dark matter as a cure all.
One reason is because in more uniform and understandable (in regards to rotation) elliptical galaxies they do not find any of this dark matter
So why would there be dark matter in one type of galaxy but not the other? I could be wrong but I have heard that Spiral galaxies eventually turn into elliptical galaxies. If that's true then where does this dark matter go?
Right. A million years from now, all those galaxies might even turn into pork chops or something. We don't really know. However ...
I believe that many galaxies, of various ages (judged by the composition of their stars) have been observed. There seems to be none of the rotational disparity you suggest. Thus the assumption that such things remain somewhat steady over time. But you're right. We don't really know.
Right. A million years from now, all those galaxies might even turn into pork chops or something. We don't really know. However ...
I believe that many galaxies, of various ages (judged by the composition of their stars) have been observed. There seems to be none of the rotational disparity you suggest. Thus the assumption that such things remain somewhat steady over time. But you're right. We don't really know.
Pork Chops? Come on now, Cut me some slack
I am not talking about things that are necessary age related, Things like
As the galaxy rotates do the spiral arms remain consistant or do they expand and contract?
Do Spiral arms remain consistant forever? i.e. Do new arms form while others disapate?
Do Eddies at times form during the rotation of galaxies?
At this time we don't and can't know if any of those factors or others we can't even image have any effects on our measurements. Until we do know for sure it seems kind of "Pre-mature" to invent this invisable matter that can do or be anything the observer wants.
There is no dark matter. Dark matter is a mere intellectual tool for mentally resolving why Newton's gravitational equations don't completely explain distant behavior.
Maybe, but that 511 Kev emission needs an explanation.
I don't see the relevance of these issues to the role of dark matter. We see the light from a galaxy. It's from the past, of course, but we can determine its rotation at that time. We can also (given a good telescope) estimate the amount of mass contributed by the galaxy's visible stars. This is a snapshot of one moment, true, but at that moment the galaxy should behave in accordance with its mass, as we see that mass. But many galaxies behave as if there were much more mass involved. That's the situation. Thus the hypothesis of dark matter.
It wouldn't matter if, over time, the spiral arms contract, expand, or wave around. We see a galaxy's behavior at a specific time, and it appears not to have anywhere near enough visible mass to account for its behavior. That's the deal.
Measuring Galactic Rotation Curves
When we determine the rotation curves for galaxies, we actually measure the velocities of two gases in those galaxies, Hydrogen and Carbon Monoxide. The reason we measure gas velocities is that gas produces spectral lines that shift in frequency as the gas moves. ...
Such as?
Spectrum-shifting might
simply be non-uniform
distribution of
silver-wrapped kisses
versus gold-wrapped kisses in
interstellar space.
Pork Chops? Come on now, Cut me some slack
Actually, I think a great Monty Python sketch could be made out of that.
Yes it would, If the spiral arms were say in the process of expanding that would have an effect on the reading of the doppler shift. The problem making measurements with the spiral galaxies is they aren't symetrical and uniform so what is happening on one side isn't necessaraily happening on the other to balance it out. So when you compare the rotating away side with the rotating towards you side without knowing what else is going on the results will be off.
Yes it would, If the spiral arms were say in the process of expanding that would have an effect on the reading of the doppler shift. The problem making measurements with the spiral galaxies is they aren't symetrical and uniform so what is happening on one side isn't necessaraily happening on the other to balance it out. So when you compare the rotating away side with the rotating towards you side without knowing what else is going on the results will be off.
However, remember that we have access to millions of spirals in the universe through optical observations. Statistically, if a spiral galaxy is behaving in some odd way, we would should see it, unless the timescale of the odd behaviour is extremely short.
Not exactly because in order to get correct measurements the galaxy has to be perfectly oriented toward us so there is not that many.
Statistically, if a spiral galaxy is behaving in some odd way, we would should see it, unless the timescale of the odd behaviour is extremely short.
Actually, It would be the opposite being that the timescales are extremely long. If a spiral arm was expanding, contracting, or there are stellar eddies or whatever else could be happening that we don't/wouldn't know about due to the extremely long timespans we wouldn't be able to percieve it for millions of years.
Spiral Galaxies are very complex and there is nothing anywhere near comparable to them for a reference. So until we can get a really good computer model we don't know. Meanwhile Elipitical Galaxies are more uniform and they behave in an understandable "Kepler" fashion so there is no need for scientist to invent this mysterious Dark Matter with them. Spiral galaxies evolve into elipitical galaxies so where does all this matter go? Are we to believe that 90% of the mass that some scientist claim dark matter makes up in spiral galaxies just goes *poof* and vanishes into the void of space?
Also it should be noted that in elipitical galaxies they use planatary nebula instead of Hydrogen clouds to measure rotation. Planatary nebula might be a better choice because also do we really know how exactly a Hydrogen cloud behaves over time?
Using Occam's Razor which is more plausible.
A) There is this mysterious, invisable, undetectable Dark Non-Periodic table matter with multiple properties in spiral galaxies that magically vanishes when the galaxy evolves into an eliptical one.
or
B) Our measurements of spiral galaxies are wrong
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