Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Cliff on 11/02/2015 15:11:58
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There are a number of things I don't understand about Dark Matter etc., but one which is at the top of the list is:
If dark matter is thought to constitute the majority of the matter int he universe - so much so that it is thought to be the reason that galaxies don't pull apart when they spin - then what form/size does this matter take? Are there planet-sized lumps of it? Surely not, as this would block out the light from stars etc. behind it. So are we talking about dark matter particles? If so, could this have such a profound effect as that described above...and why does it not, then, interact with itself and 'clump' together....
It always seems that one questions leads to many more, but any help would be appreciated....
Thanks....
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If dark matter is thought to constitute the majority of the matter int he universe - so much so that it is thought to be the reason that galaxies don't pull apart when they spin..
That's not what Dark Matter is exactly. It's the matter responsible for describing the shape of the galaxies rotation curve, i.e. the speed of stars as a function of their distance from the center of the galaxy.
More later.
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Ever since evidence for the existence of dark matter started to build up, physicists and astronomers have proposed various theories that might account for it. A range of these is presented here: http://en.wikipedia.org/wiki/Dark_matter#History_of_the_search_for_its_composition
A large number of Black holes wandering the galaxy (and other galaxies) were quickly ruled out, as this would have caused microlensing effects. It is now thought that there are a large number of free-floating planets wandering around the galaxy, but not enough to account for the "missing mass".
Most currently popular theories focus on some subatomic particles which are predicted by (some) theories, but which do not interact strongly with matter, except via gravitation. Unfortunately, no sightings have yet been confirmed; some physicists hope that the supercharged LHC may be able to generate some of these.
While most theories focus on weakly-interacting massive particles (or WIMPs), a recent speculation suggested that the particle may in fact be a strongly-interacting light particle. This particle would be blocked by the Earth's atmosphere, explaining why it has never been detected on Earth. The authors propose to put a detector on a space probe, exposed to the vacuum of space.
There are also a number of theories that say that dark matter does not exist, and propose various alternatives like adjustments to Newton's (and Einstein's) laws of gravity at large distances.
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Personally, I prefer the notion that we have gravitation wrong over the notion of dark matter. That's not to say it couldn't be dark matter, but I am not convinced by WIMP theories. I have also wondered why dark matter wouldn't coalesce under gravitational attraction with itself. Also, why don't massive objects (like the Earth) attract it gravitationally? Wouldn't dark matter fall into the gravitational fields, adding to the mass of objects, or bringing it here into range for observation? Wouldn't dark matter be orbiting in galaxies and solar systems too, and couldn't we see the effect on other orbiting bodies (tidal forces/rotational damping etc.)
Some people have mapped out where higher and lower densities of dark matter "should be" to account for gravitational effects observed, so there could be some truth there, but I wonder what else could account for the discrepancies between prediction and observation. I wouldn't be terribly surprised if in 100 years we could do away with both dark energy and dark matter by tweaking (or totally replacing) current gravitational theory...
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If you think about a galaxy rotating around a large central black hole then there is a gradient of time dilation that slows things down nearer the centre. As this dies away with radial distance velocities will speed up. The time moves faster therefore the objects move faster. If we have gravitation wrong then this is the answer.
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One of the currently most popular theories about Dark Matter is that it is "Cold" Dark Matter (http://en.wikipedia.org/wiki/Cold_dark_matter). According to this theory...
Wouldn't dark matter be orbiting in galaxies and solar systems too
It is thought that the velocity of the Dark Matter Particles is smaller than the escape velocity of a galaxy, and so "clumps" of orbiting dark matter occur in the same locations as galaxies, and led to the formation of galaxies.
However, when something has this high a velocity, it far exceeds the escape velocity of our solar system, so it won't be orbiting in our solar system, merely passing through. So it won't provide a consistent gravitational tug (eg how Pluto was discovered).
why don't massive objects (like the Earth) attract it gravitationally? Wouldn't dark matter fall into the gravitational fields, adding to the mass of objects, or bringing it here into range for observation?
Earth's gravity would slightly accelerate the Dark Matter Particles as they approach the Earth, and slightly slow them down as they move away from the Earth.
A meteorite approaching the Earth will be stopped by the atmosphere or (if it is big enough) by hitting the land or the sea. The hypothetical Dark Matter does not interact strongly with matter, so it passes straight through the atmosphere, the ocean and the Earth. They have a velocity which far exceeds Earth's escape velocity, so they merely pass on through.
This is not so different from neutrinos; I've seen a claim that if you built a lead wall from here to Alpha Centauri, half of the Sun's neutrinos would still come out the other side. The fraction which are stopped by the Earth is miniscule.
I have also wondered why dark matter wouldn't coalesce under gravitational attraction with itself.
There are some puzzles about the way a planetary system forms, but it is thought to occur by the atoms of a dust cloud hitting each other at high velocity, which heats them up, radiating the energy away into space, cooling them down, and averaging out their angular momentum until it forms a cool, flat, spinning disk from which planets coalesce. Electrons and atoms interact strongly with each other, and with the electromagnetic spectrum.
The hypothetical Dark Matter would not interact strongly with itself, so it passes straight through other Dark Matter particles. It does not emit electromagnetism, so the particles can't radiate away their kinetic energy and stick together, so they just keep independently orbiting the galaxy.
Of course, at this time, this is all purely hypothetical....
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Thanks evan, those are helpful answers. I guess "heavy" neutrinos (WIMPS) can explain the observations pretty well.
Would it make sense to place a lower limit on the mass of the particle by assuming a Boltzmann-type velocity distribution given the temperature of interstellar (intragalactic) space and comparing to the escape velocity of a galaxy? Or would the "weakly interacting" part inhibit thermal equilibration?
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No idea if this is a valid analysis (please let me know the many ways in which it might be wrong), but:
If we assume dark matter to behave as an ideal gas, and to be in thermal equilibrium at 2.7 K, and if we take the escape velocity from the milky way to be 500 km/s (which is about what it is from where we are), then in order to have a rms speed less than that escape velocity, I calculate the particles couldn't have a mass less than about 3.8x10–34 kg (this would be about 2.3x10–7 times the mass of a proton or 700 times the average mass of a neutrino)...
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thermal equilibrium at 2.7 K
To be in thermal equilibrium with the microwave background radiation assumes that the dark matter particles interact with the electromagnetic force.
To be in thermal equilibrium with each other assumes that the dark matter particles interact with each other.
would the "weakly interacting" part inhibit thermal equilibration?
This is all speculation at this time, but the hypothetical sterile neutrino and axions would not interact in either of these ways, leaving them with the original momentum that they had when the galaxy formed. I expect this would make thermal equilibrium unlikely.
Of course, any particles that exceeded the escape velocity of the galaxy would by now be somewhere in intergalactic space, and perhaps swallowed by the general expansion of the universe.
Perhaps like an expanding gas (and the cosmic microwave background), this means that they would be cooling over cosmic timescales, if temperature means anything in a gas of non-interacting particles...
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If dark matter is thought to constitute the majority of the matter int he universe - so much so that it is thought to be the reason that galaxies don't pull apart when they spin - then what form/size does this matter take?
Nobody knows for sure.
Are there planet-sized lumps of it? Surely not, as this would block out the light from stars etc. behind it.
We have no evidence of any planet-sized lumps of dark matter.
So are we talking about dark matter particles?
Not necessarily. Though this is the presumption that is most prevalent. I rather think it's something that's encouraged by particle physicists and the people who have spent twenty years searching for dark matter particles.
If so, could this have such a profound effect as that described above...and why does it not, then, interact with itself and 'clump' together...
Because gravity is weak.
The thing that's hardly ever mentioned in dark-matter articles is relativity. Relativity is the Cinderella of contemporary physics. See The Foundation of the General Theory of Relativity (http://einsteinpapers.press.princeton.edu/vol6-trans/197?highlightText=%22gravitatively%22) where Einstein says "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy". It’s energy that causes gravity, not matter per se. A gravitational field is a place where there's more spatial energy than usual. This causes more gravity. And note that a gravitational field isn't made out of WIMPs. It's just space. And space, of course, is dark.
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what form/size does dark matter take?
Nobody knows for sure.
Some astronomers are hoping that if dark matter decays, or interacts with matter with some characteristic energy (even if infrequently), then a large space telescope looking for this specific energy signature may eventually be able to map the distribution of dark matter in our galaxy, and nearby galaxies.
But for now, we have to rely on computer simulations, which can render the (simulated) dark matter visible to the programmer. Some of these models end up with a distribution of visible matter that is similar to what we see in space; this suggests that the model also represents a distribution of dark matter which is similar to what occurs in space.
On the large scale, these models seem to point to a structure like soap bubbles or a sponge, where the galaxies and dark matter are clustered in the walls of the bubbles, surrounding large amounts of seemingly empty space.
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Some astronomers are hoping that if dark matter decays, or interacts with matter with some characteristic energy (even if infrequently), then a large space telescope looking for this specific energy signature may eventually be able to map the distribution of dark matter in our galaxy, and nearby galaxies.
IMHO the issue with that is that they've been talking about WIMPs for thirty years now, but there's no evidence for them whatsoever, and meanwhile relativity has had no airtime. I suspect that part of that is because the WIMP guys want everybody to think that theirs is "the only game in town".
But for now, we have to rely on computer simulations, which can render the (simulated) dark matter visible to the programmer. Some of these models end up with a distribution of visible matter that is similar to what we see in space; this suggests that the model also represents a distribution of dark matter which is similar to what occurs in space.
No problem. The problem is with the assumption that dark matter must consist of particles. There's just no justification for that. That's doing physics as if relativity never happened.
On the large scale, these models seem to point to a structure like soap bubbles or a sponge, where the galaxies and dark matter are clustered in the walls of the bubbles, surrounding large amounts of seemingly empty space.
Seemingly is the operative word. Make sure you read this (http://www-history.mcs.st-and.ac.uk/Extras/Einstein_ether.html) where Einstein said this:
"This space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that "empty space" in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gmn), has, I think, finally disposed of the view that space is physically empty."
Space isn't nothing. It's something.
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IMHO the issue with that is that they've been talking about WIMPs for thirty years now, but there's no evidence for them whatsoever,
This isn't actually true. Even if one considers only the evidence from the CMB, that is significant evidence for the existence of something like WIMPs. The evidence from the CMB indicates the presence of mass in a form that does not interact through electromagnetism or self-interact to a great degree. Even if other candidates for dark matter are viable, they have to be very, very much like WIMPs.
and meanwhile relativity has had no airtime.
This is also not true. All the gravitational calculations done by contemporary cosmologists, be they from the WMAP project, the various supernovae projects, the large-scale structure projects, and so on, are relativistic calculations. Only by not actually looking at these projects and their publications can someone be ignorant that they are using general relativity--why someone would want to throw baseless allegations at these scientists, I do not know.
I suspect that part of that is because the WIMP guys want everybody to think that theirs is "the only game in town".
A look at the WMAP results can see that they are open to different kinds of dark matter from their very first official published results (Spergel et al., "FIRST-YEAR WILKINSON MICROWAVE ANISOTROPY PROBE (WMAP)1 OBSERVATIONS: DETERMINATION OF COSMOLOGICAL PARAMETERS", The Astrophysical Journal Supplement Series, 148:175–194, 2003 September). There are many different teams proposing different kinds of dark matter; these teams do not do so in ignorance of the available evidence.
No problem. The problem is with the assumption that dark matter must consist of particles. There's just no justification for that. That's doing physics as if relativity never happened.
Again, this seems like a baseless insult entirely unrelated to the facts. There have been a couple of papers that attempted to show that relativistic effects could account for the phenomena associated with dark matter at the galactic level--all these attempts have failed. All the work of contemporary cosmology is done with general relativity.
Perhaps the poster would like to show us an example of a specific paper that ignores general relativity? Perhaps the poster would like to present an alternative theory to those of contemporary physicists working in the field that one could compare to observations?
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A project attempting to map out the shape of dark matter at large scales can be found here: http://www.cfhtlens.org/
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This isn't actually true. Even if one considers only the evidence from the CMB, that is significant evidence for the existence of something like WIMPs. The evidence from the CMB indicates the presence of mass in a form that does not interact through electromagnetism or self-interact to a great degree. Even if other candidates for dark matter are viable, they have to be very, very much like WIMPs.
There is no evidence for WIMPs, or for something like WIMPs. None whatsoever. If you'd like to dispute that, then provide a link.
This is also not true. All the gravitational calculations done by contemporary cosmologists, be they from the WMAP project, the various supernovae projects, the large-scale structure projects, and so on, are relativistic calculations. Only by not actually looking at these projects and their publications can someone be ignorant that they are using general relativity--why someone would want to throw baseless allegations at these scientists, I do not know.
I'm not throwing baseless accusations at cosmologists or relativists, I'm saying that there are dark-matter particle physicists who wilfully ignore relativity along with "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy".
A look at the WMAP results can see that they are open to different kinds of dark matter from their very first official published results (Spergel et al., "FIRST-YEAR WILKINSON MICROWAVE ANISOTROPY PROBE (WMAP)1 OBSERVATIONS: DETERMINATION OF COSMOLOGICAL PARAMETERS", The Astrophysical Journal Supplement Series, 148:175–194, 2003 September). There are many different teams proposing different kinds of dark matter; these teams do not do so in ignorance of the available evidence.
Some do so in wilful ignorance of the fact that it's energy that causes gravity rather than matter per se. They perpetuate the myth that matter particles causes gravity.
Again, this seems like a baseless insult entirely unrelated to the facts. There have been a couple of papers that attempted to show that relativistic effects could account for the phenomena associated with dark matter at the galactic level--all these attempts have failed. All the work of contemporary cosmology is done with general relativity.
Provide the references.
Perhaps the poster would like to show us an example of a specific paper that ignores general relativity?
No, but I can give you an example of the way dark matter is hyped: Dark matter may have killed the dinosaurs (http://www.telegraph.co.uk/news/science/dinosaurs/11422695/Dark-matter-may-have-killed-the-dinosaurs-claims-scientist.html). What a soundbite. Here's another example at the BBC (http://www.bbc.co.uk/science/space/universe/questions_and_ideas/dark_matter) which gives a subset of the Wikipedia article saying this "According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle". There is no such consensus. What there is, is a propaganda campaign.
Perhaps the poster would like to present an alternative theory to those of contemporary physicists working in the field that one could compare to observations?
No, but I can refer you to http://arxiv.org/abs/1209.0563 which concerns inhomogeneous and interacting vacuum energy. You will be aware that a concentration of vacuum energy will have a gravitational effect, and that space expands between the galaxies but not within. Given conservation of energy, every galaxy must be embedded in a region of space where the vacuum energy is higher than the surrounding space. That energy would have a gravitational effect. And yet most of the media reports on dark matter do not mention this, because they originate from particle physicists who want to persuade the public that theirs is the only game in town.
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There is no evidence for WIMPs, or for something like WIMPs. None whatsoever. If you'd like to dispute that, then provide a link.
As someone else here noted, you sound very much like a broken record.
For those actually interested in looking at the scientific papers, the ones here, http://map.gsfc.nasa.gov/ , you should find discussions about the evidence for dark matter and the restrictions on the possible nature of dark matter.
I'm not throwing baseless accusations at cosmologists or relativists,
You appear to be ignorant of your own claims. Please reread them.
I'm saying that there are dark-matter particle physicists who wilfully ignore relativity along with "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy".
Can you provide a citation? You have been very demanding of citation, yet here you are, producing claims about people who work in dark matter that are bordering on libelous. I suspect that you cannot produce a single example to support your case (at least, not one that is not a crackpot).
Some do so in wilful ignorance of the fact that it's energy that causes gravity rather than matter per se. They perpetuate the myth that matter particles causes gravity.
Now you appear to be mixing your claims. Can you provide an example of either case?
Provide the references.
Sadly, I don't feel like taking the time to dig up these now obscure papers. I will wait for you to produce a shred of evidence for cosmologists ignoring general relativity.
quote author=PhysBang link=topic=54299.msg450805#msg450805 date=1424440041]Perhaps the poster would like to show us an example of a specific paper that ignores general relativity?
No, but I can give you an example of the way dark matter is hyped:[/quote]
OK, so your entire claim about how cosmologists ignore relativity is based on a single pop science article in the Telegraph? How stupid do you take your readers here for? This is insulting to everyone here, not to mention the many scientists that you insult.
a subset of the Wikipedia article saying this "According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle". There is no such consensus. What there is, is a propaganda campaign.
There is a quite clear consensus among the available papers and in academic conferences. You appear to be harboring a conspiracy theory without bothering to read the science.
Perhaps the poster would like to present an alternative theory to those of contemporary physicists working in the field that one could compare to observations?
No, but I can refer you to http://arxiv.org/abs/1209.0563 which concerns inhomogeneous and interacting vacuum energy. You will be aware that a concentration of vacuum energy will have a gravitational effect, and that space expands between the galaxies but not within. Given conservation of energy, every galaxy must be embedded in a region of space where the vacuum energy is higher than the surrounding space. That energy would have a gravitational effect. And yet most of the media reports on dark matter do not mention this, because they originate from particle physicists who want to persuade the public that theirs is the only game in town.
You appear to be using "given conservation of energy" in a way that is inconsistent with scientific usage and, in particular, inconsistent with the paper you cited. The paper certainly does not have anything to do with dark matter (aside from allowing the possibility that the source of an inhomogeneous vaccuum field could be quintessence-like dark matter). Indeed, the only similarity between your claims and the paper is the use of the word "inhomogeneous". I find fascinating that the very paper you cited is one that presents a homogeneous spacetime as the background for their work, a solution that you seemed to not believe existed.