Posted on 01/15/2002 7:02:17 AM PST by PatrickHenry
Once upon a time -- a bit more than 100 years ago -- many scientists believed that seemingly empty space wasn't empty at all, but was filled with a substance called luminous ether. This mysterious stuff, never seen in any laboratory on Earth, was thought to explain how gravity from one celestial body could affect another.
By the end of the 19th century, though, luminous ether had gone the way of countless other scientific misconceptions. Today, another mysterious substance beguiles astronomers, and this one isn't going away. In fact, it's been at the forefront of cosmological theories for decades. It's called dark matter, and it is now widely accepted by astronomers as the stuff most of the universe is made of.
"We've known that it exists for more than 25 years," says astronomer Virginia Trimble of the University of California Irvine. "But we don't know what the hell it is."
How can astronomers be so certain of something they have never seen? The answer comes from observations of how stars and galaxies move, studies that have been going on for more than 50 years. Within spiral galaxies, individual stars and clouds of gas are orbiting faster than they should if they were only being affected by the gravity of the galaxy's visible matter. The same is true for clusters of galaxies: The motions of individual galaxies can't be explained by the gravity of what astronomers can see.
To explain these observations, astronomers have deduced that galaxies are surrounded by vast halos of a different, unseen kind of matter.
This so-called dark matter is invisible to us because it does not radiate energy. But it does have mass, and that means it can supply the extra gravity necessary to hold galaxies, and clusters of galaxies, together. Even in the bizarre world of cosmology, it's a strange proposition.
But is dark matter the only explanation?
Perhaps scientists don't entirely understand the way gravity works; perhaps Isaac Newton's famous law of gravitation needs some revising. But that idea, says the University of Arizona's Chris Impey, is not very popular.
"Definitely most astronomers are extremely unwilling to give up Newton's law," he says. "So it's essentially a choice of two evils: You either hypothesize that Newton's law is wrong, and that our knowledge of the gravity theory is incomplete. Or, you hypothesize a fundamental microscopic particle that has never been detected in any physics lab, whose properties are only constrained by these astronomical observations. Which is a pretty uncomfortable position for physicists to be in."
Still, as Trimble explains, dark matter is the lesser of the two evils, simply because it requires fewer departures from accepted physics.
To explain the observations by revising the theory of gravity, astronomers would have to identify a few different effects, each of which would operate at a different distance scale. But with dark matter as the explanation, Trimble says, "You only need one Tooth Fairy."
[The rest is omitted, but you can visit the source and read it all.]
Antimatter Clouds and Fountains - NASA Press Release 97-83
Antimatter cloud over the galactic bulge
;-)
Thanks for popping in; as I said previously, we should have pinged you much earlier on this thread.
I doubt it, without more data. There are probably several models floating around.
It's misleading to call it a "cloud of antimatter". What is seen is gamma rays that come from electron-positron annihilation (positrons being antimatter electrons). There is no indication that there are antiprotons present. This tells me that what we are seeing is a jet of electrons and positrons being emitted from the center of the galaxy.
I'm particularly interested in anything concerning why the plume would be coming from just one side of the center of the galaxy.
I can offer you a conjecture (knowing nothing more about it than the links you gave, so take it with a shaker of salt). A good way of creating a beam of mixed electrons and positrons is to start with a beam of gamma rays--high energy photons--and shoot it through matter. The gamma rays interact with electrons in the matter and convert into an electron-positron pair (which may in turn create more electron-positron pairs). The resultant electron and positron will travel along the direction of the incident photon's travel.
So my guess is that what we are seeing started out as a gamma ray beam. It stands to reason that the astrophysical source is also sending out a gamma ray beam in the other direction. The observed asymmetry could be as simple as the possibility that one beam has, for the last 3500 years, been pointing at a comparatively denser region of space than the other beam.
(Why 3500 years? Because the "plume" reaches out 3500 light years. In order to travel that far in the face of the galactic magnetic field, a charged particle would have to have a very appreciable energy, so they must be travelling close to the speed of light. So 3500 years is the relevant timescale.)
I guess we're talking about materialism in the philosophical sense, because I doubt very few quality scientists would believe that there are no unknown unknowns. I think misunderstandings occur when philosophical materialism gets mixed up with materialist methodologies, which is popular with scientists because it yields the most defensible answers, as well as raising many good questions. After all, if there really were a stubborn materialist philosophy underpinning science I doubt that we ever would have heard of mass-energy equivalency, wave-particle duality, time dilation, dark matter, dark energy, and such.
I was trying to picture a high energy gamma ray beam skewering the Milky Way to create such a plume.
It seemed to me that the gamma ray beam would have to be beyond the (center of the galaxy black hole) event horizon based on current theory and that there would have to be more renewing matter on the plane of the Milky Way.
But now I'm wondering why we wouldn't pick up the originating gamma ray beam on the other side?
Anyway, I went looking to visualize all this for myself - and only ended up more bewildered. But I did find two more good links on the subject for anyone following this:
Well, it would be two beams originiating within the Milky Way. There's one source, I imagine, sending out two beams in opposite directions.
It seemed to me that the gamma ray beam would have to be beyond the (center of the galaxy black hole) event horizon based on current theory and that there would have to be more renewing matter on the plane of the Milky Way.
I don't follow you.
But now I'm wondering why we wouldn't pick up the originating gamma ray beam on the other side?
Because it isn't pointing at us. Gamma rays are photons; if they don't strike your detector (eye), you can't see them...unless, of course, they shine on something and you see the result. In this case, I imagine that one beam is striking matter and producing electron-positron pairs, and the other beam is simply shining off into the void.
My bad. I was taking your example literally – a gamma ray beam shooting through matter creating the electrons and positrons which annihilate each other in the massive plume – that’s why I was looking for a larger, renewing source of matter on the Milky Way plane to be skewered.
On the other point, I assumed gamma rays would surrender to the gravity of a black hole, so such a gamma ray beam couldn’t be coming from inside the event horizon. OTOH, if the source were above the event horizon on the one side, that also might explain why it appeared to be one-sided in the article.
Here are some additional resources for those interested in this subject -
The 21cm sky
A brief on the ISM
Lecture notes on the ISM
H clouds are not simple
21 cm H density varies rapidly
AGNs probed via 21cm H line
An AGN 21cm anamoly
What about ionized hydrogen?
A flock of abstracts
On the last, see especially the Couchman abstract (third on the list), to get a sense of how far the mathematical models to date are from reproducing the observed data. And notice how the repelling forces created by stellar dynamics (supernovas in this case) are needed to get anything like the realistic matter spread we actually see.
What I thought you meant at #126 was that a gamma ray beam, originating from one side of an object on the opposite side of the Milky Way plane, was shooting through matter on the plane creating the electrons and positrons that form the 3500 light year ‘antimatter’ plume out of just the one side of the galaxy center.
After a reread, I can see that you are suggesting the object is on the same side as the plume.
To visualize how that might be, I surmise that the opposing side positron and electron stew might be ‘eaten’ by the black hole, or perhaps that it never found sufficient matter to propagate gamma rays, or that it does but we can’t see it.
I'm still not comfortable that I understand what's going on there (it is such an enormous anomaly.)
I picture a source somewhere near the center of the galaxy, say a neutron star or a black hole, perhaps even the giant black hole at the center of the galaxy. By some unspecified interaction of local matter with the source, it produces two beams of gamma rays, shining in opposite directions. Surely these beams would be aligned with the magnetic poles of the object; one beam would come out from the north pole, and the other from the south.
One of the beams, I conjecture, is pointing at a region of space that happens not to contain much matter. It shines out into intergalactic space and is undetectable. The other beam, for the time being, happens to be pointing at a relatively dense region of space, say a molecular cloud. As the beam passes through the cloud, some fraction of the gamma rays converts into electron-positron pairs, and these travel along the direction of the gamma beam. As the positrons fly out of the galaxy, some fraction of them, all along the beam, encounter electrons (which are common) and annihilate them. This interaction emits gammas (photons) of a characteristic energy, 511 keV. If we look in the right direction at that frequency, we can see the path of the beam.
Then the same process of positron production, collision with ordinary electrons, and release of the gamma rays, would occur very close to the original source. The released gamma rays would in turn still hit other ordinary matter in the area, heating it, and making it glow - emitting a widening cascade of photons less than gamma ray in intensity, but more numerous.
Just like the gamma rays emitted during nuclear fusion in the core of the sun, by the time their energy has bounced through millions of collisions, what we see out the other end (the sun's surface) is visible light photons, far more numerous than the original gamma rays, but individually less energetic. Provided the area of matter the second beam were shining on were dense enough, we might only see overall brightness, rather than the pure high energy gamma rays.
If all of the transfer from high energy gamma rays to more numerous low energy light takes place in the first few LY, we wouldn't see any long beam structure thousands of LY in length. The problem with this alternative is trying to imagine how the matter in the path of the beam avoids getting "blown away" by the gamma ray energy, clearing a path out away from the source. The difficulty with the "hit nothing" explanation is imagining a channel clear of all dense matter for thousands of LYs.
It is possible the clear channel idea is correct - but rather than just by happenstance, reflects the gamma ray beam having already "boiled away" all the gas cloud along that line. Meaning that like a galactic pressure hose, it has already punched clear a path through the ISM on one side, though we still see it lighting up the gas in the way, on the other side.
I hope this helps.
As I understand this, the OSSE is detecting a 3500ish light year annihilation fountain from one side which we are able to see because they are gamma rays – so if that’s the case, wouldn’t we see the offsetting clear channel ‘boiled away’ gamma ray beam on the other side? It seems like it would have a longer reach unobstructed that would come into a line of sight somewhere, but is it too narrow to be seen?
Conversely, if the high energy gamma rays transmute (or whatever) into visible light because of the very near mass on the far side – would this be seen as a ‘glow’ near the galactic center?
It's entirely possible that the other beam is being blocked, but I don't think it would be by something close to the black hole. Things that are very close to the black hole might block the line of sight for a little while, but the relevant timescale here is 3500 years (because the positron plume we see is that long). If the sources are blocked intermittently, we'd still see the plume. We'd need a really dense object to remain in place close to the black hole for a long while, but in the vicinity of the galactic black hole things move around very quickly. On the other side, I'm only asking for a large, diffuse cloud some significant distance (tens or hundreds of LY from the black hole) to cover the business end of the beam for that long.
So instead of your nearby dense object to block the "missing" beam, I'd prefer a more distant, more diffuse cloud that is nevertheless optically thick enough to convert all of the gammas to electron-positron pairs, and furthermore to scatter or absorb the telltale annihilation radiation from the positrons. That seems less likely to me than a clear sky, as I'll explain below.
As for striking the accretion disk, the beams coming from the poles typically come out perpendicular to the accretion disk (unless there is a large amount of precession), and so won't intersect it.
The difficulty with the "hit nothing" explanation is imagining a channel clear of all dense matter for thousands of LYs.
I don't see that as a problem. Once you get past the immediate environment of the black hole, most of the sky will still be black. If anything, it's a stretch for me to ask for a dust cloud for the positron-producing beam to hit; clouds of dust and gas are rarer in the galactic bulge than in the galactic disk. The only other thing to hit would be Pop II stars, each of which would remain in the beam line but briefly.
It is possible the clear channel idea is correct - but rather than just by happenstance, reflects the gamma ray beam having already "boiled away" all the gas cloud along that line.
I would think that you'd still have a lot of matter from the cloud drifting into the path of the beam, resulting in a large amount of electron-positron pairs.
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