Posted on 03/17/2004 7:44:05 AM PST by PatrickHenry
Gamma rays streaming from the centre of our galaxy could be the signature of elusive dark matter, astrophysicists claim. The rays support an exotic theory about dark matter: that it consists of very light particles.
Physicists know that a large proportion of the universe's mass cannot be accounted for by objects we can see, such as stars and planets. In galaxies such as our own, there could be as much as ten times more dark matter than normal matter.
One popular idea suggests that the 'missing' dark matter consists of as yet unidentified subatomic particles that are much heavier than entire atoms of normal matter but that hardly interact with it, except through gravity. They are called weakly interacting massive particles, or WIMPs.
But Céline Boehm of Oxford University in England and her colleagues think that dark matter particles need not be massive at all. Instead, they think they could be between ten and a thousand times lighter than a hydrogen atom.
Integral research
Their evidence comes from data collected by a satellite called INTEGRAL, operated by the European Space Agency, which searches the skies for gamma rays. The satellite has mapped out the cosmic sources of gamma rays with an energy of 511,000 electronvolts (511 keV). Such rays are about 200,000 times more energetic than visible light rays and are produced when electrons and their antimatter equivalents, positrons, annihilate one another.
Last January, the INTEGRAL team announced that the 511 keV gamma rays come from a source that is evenly distributed throughout the central bulge of our galaxy1.
Intriguingly, dark matter is known to be concentrated in our galaxy's central bulge, thanks to observations of how the missing mass affects the orbit of stars.
Boehm's team says that if dark matter were made up of particles with a low mass, these particles could generate positrons and electrons when colliding with antimatter. When these products collide, they generate gamma rays.
The researchers calculate that the number of such particles needed to produce the intensity of 511 keV gamma rays seen by INTEGRAL fits well with the amount of dark matter that the galactic bulge is estimated to contain. "The numbers are really reasonable," says Boehm. They report their findings in the current issue of Physical Review Letters2.
"It would be very exciting if it turns out to be real", says gamma-ray astronomer Jürgen Knödlseder of the Centre d'Etude Spatiale des Rayonnements in Toulouse, France, who works with INTEGRAL data.
But Knödlseder cautions that it is not yet clear if Boehm's dark-matter theory is really needed. The source of the positrons could be exploding stars called supernovae, rather than exotic particles. "They are still the most plausible source," he says.
He suggests that very accurate measurements of the distribution of the 511 keV gamma-ray emissions might enable researchers to work out whether the source is dark matter or exploding stars.
A related thread was posted a few months ago, but Nature just published this article, so presumably it's newsworthy:
Astronomers claim dark matter breakthrough.
So9
I'll make the same comment that I made back when someone claimed that a 100 MeV dark matter particle was responsible for the 511 keV emissions. If these things are so light, and can annihilate and produce electron-positron pairs, why don't we see them produced in electron-positron colliders? Whether it's a 100 MeV particle (as was claimed before) or a 1 MeV particle (as is claimed here), we should see GOBS of them produced by every e+e- collider, but we just don't.
Either the dark matter particles are extremely heavy, or they don't couple to electrons. I don't see any way around that.
That would bode well for Kerry.
The more recent articles I've read make a huge distinction between the two with regard to gravity. Dark matter, like black holes, has the property of high positive gravity (center of the galaxy, etc.) Dark energy, OTOH, has the property of negative gravity, e.g. the "vacuum" of space between galaxies, causing the acceleration of the universe.
I don't think they were ever treated as the same thing.
The more recent articles I've read make a huge distinction between the two with regard to gravity. Dark matter, like black holes, has the property of high positive gravity (center of the galaxy, etc.) Dark energy, OTOH, has the property of negative gravity, e.g. the "vacuum" of space between galaxies, causing the acceleration of the universe.
Well, there isn't any negative gravity; it's the same old gravity that causes that repulsive effect when the vacuum itself has an energy density. Analogously, the bubbles in your glass of Yuengling Lager aren't accelerated away from the center of the Earth because of anti-gravity, but because of gravity. It's not the same effect, but it gives you the idea. Mmm, Yuengling.
I guess I'm not really seeing why you think dark energy is considered to be behaving differently in this case.
I'm speaking of quintessence. Here are some articles:
Many physicists are uncomfortable with this line of reasoning, and they are seeking the answer in different class of theories known as quintessence, after the Greek word for the fifth element. Modern physics, noted Dr. Paul Steinhardt, a theorist at Princeton, is replete with mysterious energy fields that would exhibit negative gravity. The trick, Dr. Steinhardt explained, is finding a field that would act like the dark energy without a lot of fudging on the part of theorists.
"The observations are forcing us to do this," he said. "Dark energy is an interesting problem. Any solution is quite interesting."
One theory that captured the fancy of the astronomers in Baltimore was a modification of gravity recently proposed by three string theorists at New York University: Dr. Gia Dvali, Dr. Gregory Gabadadze and Dr. Massimo Porrati. In string theory — so named because it describes elementary particles as tiny vibrating strings — the ordinary world is often envisioned as a three-dimensional island (a membrane, or "brane" in string jargon) floating in a 10- or 11- dimensional space. Ordinary particles like electrons and quarks and forces like electromagnetism are confined to three dimensions, to the brane, but gravity is not.
As a result, Dr. Dvali suggested that gravity could only travel so far through conventional space before it leaked off into the extra dimensions, thereby weakening itself. To an observer in the traditional three dimensions it looks as if the universe is accelerating. The cosmological constant, in effect, he said, is a kind of gravitational brane drain. "Gravity fools itself," he said. "It sees itself as a cosmological constant."
Dr. Dvali's theory was welcomed by the astronomers as a sign that string theory was beginning to come down from its ivory tower of abstraction and make useful, testable predictions about the real world. (In another string contribution, Dr. Steinhardt introduced a new theory of the early universe, in which the Big Bang is set off by a pair of branes clashing together like cymbals.)
Afterward Dr. Riess and Dr. Perlmutter pressed Dr. Dvali on what they would see when they looked out past the crossover point where gravity began falling out of the world; would the transition between a decelerating universe and an accelerating one happen more abruptly than in the case of the cosmological constant? Dr. Dvali said he hadn't done any calculations, but he said it was his "naïve guess" that the crossover would happen more smoothly than in a lambda world.
In my understanding, dark matter has always been defined as that matter which we know to exist in the universe but cannot detect by emitted radiation but rather infer by the gravitational effect on visible matter. It was only when the universe was found to be accelerating that it was determined some 3/4 of that matter had the opposite gravitational property and was relabeled as dark energy.
When I first read the article (shame on me for not having a cup of coffee first) - speaking of gamma ray emissions from dark matter - my mind leaped ahead to hot dark matter v cold dark matter v dark energy. I was thinking the author was equating hot dark matter and dark energy. Most of Universe's Matter still MIA reaching back to dark ages (pun intended) before dark energy.
Sigh... Sorry about that.
Dark energy is energy that is associated with the vacuum itself. It interacts gravitationally just like any other energy, but it can't move around like free energy. It has a fixed density. The net effect is that free energy objects are repelled. (Recall the rising bubbles.)
Alan Guth exploited this effect in his inflationary cosmology: the vacuum energy density comes from the "false vacuum". What quenches this expansion is that the false vacuum decays into the true vacuum, and the extra vacuum energy density turns into the free energy that makes up all the galaxies, Pokemon cards, ring bologna, etc. That should convince you that it's the same "stuff" as the energy that comes out of your outlets.
So you see, it's really the same stuff; we call it "dark" energy because it can't shine (i.e., move around) like light can.
How is this energy tied to the structure of the vacuum? That's anybody's guess.
Nope. (will go into it further this evening)
I have read many times that black holes produce enormous Gamma emissions during accretion - wouldn't a centrally located SMBH also produce steady amounts of gamma?
I need to do a bit of research. :-)
Will see what I can find tonight.
There is a property or group of properties that we conceive of as mass. Why would everything have to have that property or that group of properties? Does light, ignoring whether photons are anything but a convenient metaphor, have mass?
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