Some Dark is still dark, it's still good and black and it always will be best going down.
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.
For a moment I thought I had entered a time warp and this was being posted on April first...
Sure, the speed of light is 299,792,458 meters per second, but what's the speed of dark?
Woof. Tha's a whollotta particles.
Some Dark is still dark, it's still good and black and it always will be best going down.
The cosmological constant problem predates the recent evidence for dark energy. However, dark energy raises a new puzzle, the so-called coincidence problem. If the dark energy satisfies (can’t type it, but it would be critical density of dark energy at 0.7), it implies that we are observing the universe at the special epoch when (critical density of matter) is comparable to (critical density of dark energy), which might seem to beg for further explanation. We might rephrase these two problems as follows: (a) why is the vacuum energy density so much smaller than the fundamental scale(s) of physics? and (b) why does the dark energy density have the particular non-zero value that it does today? If the dark energy is in fact vacuum energy (i.e., a non-zero cosmological constant), then the answers to these two questions are very likely coupled; if the dark energy is not due to a pure cosmological constant, then these questions may be logically disconnected.
In recent years, a number of models in which the dark energy is dynamical, e.g., associated with a scalar field and not a fundamental cosmological constant, have been discussed… These models, sometimes known as “quintessence” models, start from the assumption that questions (a) and (b) above are logically disconnected. That is, they postulate that the fundamental vacuum energy of the universe is (very nearly) zero, owing to some as yet not understood mechanism, and that this new physical mechanism ‘commutes’ with other dynamical effects that lead to sources of energy density. This assumption implies that all such models do not address the cosmological constant problem. If this simple hypothesis is the case, then the effective vacuum energy at any epoch will be dominated by the fields with the largest potential energy which have not yet relaxed to their vacuum state. At late times, these fields must be very slight.
Irrelevant yet topical aside: on the TV show "Malcolm in the Middle" this week, Malcolm gets grilled by the family of his new girlfriend. They end up debating about the role of scalar fields in inflationary cosmology. For once in the history of network television, the "science mumbo-jumbo" actually seems to have been written by somebody who knows something about the subject.
I'm predicting bondbonds. Yummy hypothesis.
Yes. It's affected by gravity. Recall the famous observation of the position of stars near the sun during an eclipse, which was considered confirmation of Einstein's theory.
Experiments performed in a uniformly accelerating reference frame with acceleration a are indistinguishable from the same experiments performed in a non-accelerating reference frame which is situated in a gravitational field where the acceleration of gravity = g = -a = intensity of gravity field. One way of stating this fundamental principle of general relativity is to say that gravitational mass is identical to inertial mass. One of the implications of the principle of equivalence is that since photons have momentum and therefore must be attributed an inertial mass, they must also have a gravitational mass. Thus photons should be deflected by gravity. They should also be impeded in their escape from a gravity field, leading to the gravitational red shift and the concept of a black hole. It also leads to gravitational lens effects.Source: Principle of Equivalence.
Ignoring the effect of the force that would remove half the mass? Like the mass just suddenly changed? Well, yes. I think the trajectory would change. Appreciably? That's a relative term. I think it would change predictably. Please don't ask me to do the math.
It's an instructive question. Consider the earth's mass to be effectively so much larger than a satellite's mass that the satellite's mass becomes insignificant in comparison. If the satellite splits into two parts, would each part not continue on effectively the same trajectory as the entire satellite assuming the two parts, each of half the mass as the entirety, stay within a few feet of each other?
Ouch. E2 = m2c4 + p2c2, where p is the momentum. Since for photons, E = pc, the inertial mass m is zero.
Yup, that's where I was going with this, except that I imagine the two halves connected by a wisp of thread under no tension. Should the thread be severed, would the orbit change?
Excellent sequence of questions. My answers illustrate the folly of going with the intuitive answer. Now you've got me in back-pedaling mode. Ah, wait ... suppose the only mass reduction was the gentle ejection of the thread itselt. What would happen to the thread? I'm fairly confident that in a straight-line course (whatever that means), the thread would remain with the mother ship. But in orbit around earth, with no atmosphere to produce friction? Arrrgh. I think you've stumped me, Phys. Now I'm in full-reverse mode. My original answer was wrong. (Hey, don't come back and tell me that my first response was correct.)
The reason this works is because the gravitational force is proportional to the mass, while according to Newton's 2nd Law of Motion, the acceleration is equal to the force divided by the mass. The masses terms cancel, so the acceleration depends only on the mass of the Earth.
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