Posted on 03/17/2004 7:44:05 AM PST by PatrickHenry
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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