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 tried to explain this concept to my girlfriend in the way you did and have met a full-court press in short time. Do you think I need more field-effect?
In the case of a nuclear decay, you start with a precisely known amount of energy. Three particles come out: a positron, a nucleus (with an atomic number one less than you started with), and a neutrino. You can't measure the neutrino, and you generally can't measure the final momentum of the nucleus, but you can measure the positron energy very precisely, and from an ensemble of decays, you can measure that there's an unseen particle recoiling against the positron. (If you look at the angular momentum, you can see right away that a spin-1/2 particle escapes.) One of the things that makes this analysis much easier is that there's one particle missing, and that one is massless. The missing energy thus equals the missing momentum, which is a valuable constraint.
In the case of an electron and positron colliding to form a neutrino pair, the input energy is very well known. Unfortunately, you don't see anything coming out, because the neutrinos can't be measured, and there's nothing else to see. You have to get lucky and observe a radiated photon or two. If you know the final state is a neutrino pair, that's all well and good, but if you don't, you're up a bit of a creek, because the problem is underconstrained. Two massless particles can give the same result as a single massive particle...or seventeen particles of different masses, for that matter.
The problem gets worse if you're trying to sift these unknown particles out of hadronic events, where there are many particles in your detector. For one thing, the particles are not perfectly measured. The more particles there are, the more the uncertainties add up. For another thing, you can't always tell what the "flavors" of the particles are. Pions and K-mesons are notoriously difficult to distinguish, but the K is three times heavier than the pi.
That's not to say that these difficulties can't be sorted out. What it means is that some hairy statistical analysis is required to pin down such a beast. You would never be able to look at an event and say, "Aha! Two 100 MeV particles are missing from this event!" But with enough events, you probably could say something like, "Look at this shoulder in the missing PT plot! That's not supposed to be there. Damn it! Now I have to spend my weekend chasing down the bug in the analysis code. Are you sure there's nothing wrong with the pedestal subtraction circuits? Howard was screwing around with them, last shutdown..." Then, a year later, when you've decided that there is no bug in the code after all, you say, "Maybe something's there."
Same thing with the Hubble Constant,
And don't even get me started on the Asteroid that supposedly killed off the Dinosaurs" nonsense
Annihilating themselves into what?
Long as I remember The rain been comin' down.
Clouds of myst'ry pourin' Confusion on the ground.
Good men through the ages, Tryin' to find the sun;
And I wonder, Still I wonder, Who'll stop the rain.
I went down Virginia, Seekin' shelter from the storm.
Caught up in the fable, I watched the tower grow.
Five year plans and new deals, Wrapped in golden chains.
And I wonder, Still I wonder Who'll stop the rain.
Heard the singers playin', How we cheered for more.
The crowd had rushed together, Tryin' to keep warm.
Still the rain kept pourin', Fallin' on my ears.
And I wonder, Still I wonder Who'll stop the rain.
Yes, sorry I was
scatter-brained yesterday. I
started thinking I
was disagreeing
with you, but then realized
I agreed with you.
Maybe little things
will draw attention back to
Hoyle's "little" big bangs:
Cosmologists Fred Hoyle, Geoffrey Burbidge, and Jayant Narlikar have recently developed a detailed "quasi-steady state" model of the universe. As in the original model, they propose that the universe has always existed, but they abandon the idea of the continuous creation of matter, suggesting instead that a series of large creation events, or little big bangs, occurred 10 to 15 billion years ago, which caused our part of the universe to expand. Since then smaller creation events have continued to occur, producing energetic objects such as quasars and radio galaxies. However, in the future the expansion of our part of the universe will weaken, allowing the formation of new creation centers and another episode of large creation events. ... ["Cosmology and the Big Bang," By David Pratt]
Positrons created by photon-photon collisions can be a source of both rest mass and opacity. They can become dominant in astrophysical systems where the energy densities are large and particles are accelerated to relativistic energies. Examples include accreting black holes and neutron stars, relativistic jets, pulsars, gamma-ray bursts and the cosmological universe. ["Positrons in Charge: e^+e^- in Astrophysics," by E. S. Phinney (California Institute of Technology)]
That sounds vaguely racial. Wesley Clark, Howard Dean and Al Sharpton should be beginning protests calling for the firing of astronomers any minute now.
Get with the progam!
Dark chocolate has more than
three time the caffeine
as milk chocolate...
Both rot our teeth, but at least
dark keeps us buzzing...
Hershey's Special Dark Chocolate Bar
1 bar (1.5 ounces) 31 Caffeine (mg)
Hershey Bar (milk chocolate)
1 bar (1.5 ounces) 10 Caffeine (mg)
Let me clarify that more with another anology.
Trying to determine how galaxies rotate over the relative short time we've been viewing them is equvilent to looking at pluto then looking at it again 10 seconds later and then look at Mars and look at it again 10 seconds later and then with the only with that tiny bit of information try to explain how all planets and moons orbit.
No, it isn't. Here's
a site about galaxies,
and, specifically,
here's a contained link
that explains what it is like.
Both are good reading.
I think it's done by first finding a galaxy which is appropriately tilted from our point of view, and observing the blueshift of the stars in the arm which is rotating toward us, and the redshift for the part rotating away. If I'm wrong, we'll hear about it soon enough.
However, You would think that if this dark matter stuff can effect the rotation of whole Galaxies if it were present around here it would be effecting the rotation of the planets of the solar system, So unless those perturbsions/anomalies in Uranus and Neptune's orbit that lead to the discovery of Pluto was/is caused by dark matter there isn't any around here.
Personally I think they aren't measuring the rotation of the galaxies accuratly. Considering it takes upwards of 250 million years per rotation and we have only been seriously looking at galaxies for a few decades so we have a lot to learn.
First, I don't think you know how retation curves are measured. The measurements of rotation curves of spirals, ellipticals, and even clusters of galaxies are made with radio spectroscopic measurements of clouds of hydrogen gas. First, in the case of our galaxy, extra care is needed because of the peculiarity of our location inside the disk. Through clever use of trigonometry, it is trivial to measure spectroscopic velocities of objects of known radii within the solar orbit. Note that measuring the redshifts and blue shifts of the doppler shifting of spectral lines gives an instantanous velocity. Astronomers tend to use large clouds because of their large mass. They are hard to gravitationally perturb, and therefore, their velocity is not changed except by the gravitational potential of the galaxy itself.
Outside the solar orbit, it is much different because our geometric relations no longer work. Most early representations of the outer rotation curve used extremely luminous objects to measure the rotation curves, objects like O and B stars, carbon stars, and HII regions. More recently, Merrifield et al (1992) came up with a very clever method of measurement using a relation for the thickness of the HI disk as a function of distance from the galactic center. It is this paper and its followups that lead astronomers to believe that there is dark matter in the galaxy.
For external galaxies, it is advantageous to use edge on galaxies to obtain the rotation curve, or galaxies with a viewing angle of less than 20 degress. Except for a few measurements, nearly every spiral galaxy shows evidence of dark matter.
However, recently, an article was released in Science by Romanowsky, Freeman et al., “A Dearth of Dark Matter in Ordinary Elliptical Galaxies,” Science 301:5640, 1696-1698, 09/19/2003. This paper showed a lack of dark matter as per the rotation curves of the first three elliptical galaxies that they studied in a survey of field elliptical galaxies. It is their belief that this is anomalous because of the location of the galaxies. They were located in sparse clusters, or in clusters without any evidence of intra cluster interactions between members that would have caused the dark matter to be stripped from the cluster. This result is believed by the authors to be important as the main method of formation of ellipticals is through ancient intercluster interaction, where member spirals merge together to create these larger ellipticals. I'm wondering, of course, if this isn't why they don't find much CDM, since any interaction was in the distant past, and the interactions are usually assumed to disperse gas and dust in the elliptical, since most ellipticals contain little or no young stars in them because of the stripping of gas and dust in the past. Perhaps the CDM was stripped in the early interactions? It is tough to say.
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