Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: PmbPhy on 28/07/2018 11:57:27
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Is Dark Matter Real?
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Way too many times physics amateurs have posted claiming dark matter isn't real and we should listen to them etc. I've ignored those threads because they weren't credible. However there's an article on this subject in Scientific American this month entitled Is Dark Matter Real by two astrophysicists.
See: http://www.newenglandphysics.org/other/SciAmDarkMatter.pdf
Let me know what you think.
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Hi Pete, the link doesn’t work I get a 404 error. I think there is a bit missing from the url
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Try it now.
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Nice one, Pete.
I’ve been impressed by the small amount of Hossenfelder’s work I’ve had the opportunity to read. A quick skim through this leaves me feeling that she and McGaugh have given the subject a balanced treatment. Hopefully, I’ll be able to read it more thoroughly, and will, no doubt, have some questions.
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Many dark matter candidates have been cancelled over the years. I may be becoming biased, but am minded to think dark matter does not exist, and is nothing more than a mathematical fix to make observations fit predictions based on a set of equations.
I have been reading up on some of the MOND ideas, and am attracted to Verlindes entropic gravity ideas. Verlindes and the various MOND ideas do not require arbitrary amounts of Dark matter adding, which suggest they may be very close to the truth. DARK MATTER DOES NOT EXIST maybe :)
https://en.wikipedia.org/wiki/Entropic_gravity
I had wondered how time dilation could be included in entropic gravity and pulled this up https://arxiv.org/pdf/1602.05707.pdf which is also interesting
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MOND does not really deal with variations in the Cosmic Microwave Background Radiation, which Dark Matter does try to explain.
See: https://en.wikipedia.org/wiki/Lambda-CDM_model#Historical_development
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MOND does not really deal with variations in the Cosmic Microwave Background Radiation, which Dark Matter does try to explain.See: https://en.wikipedia.org/wiki/Lambda-CDM_model#Historical_development
Is it not still just adding random amounts of dark matter when other effects might also be the cause. Verlinde appears to be adamant we dont need dark matter, https://motherboard.vice.com/en_us/article/xwgk3j/theoretical-physicist-erik-verlinde-says-we-dont-need-dark-matter-to-explain-the-universe
You might find this linkl interesting in support of Verlindes claims www.scirp.org/journal/PaperInformation.aspx?PaperID=85930 4. Summary and Conclusions
The purpose of this paper has been to show how the CMB temperature anisotropy pattern could be a map of gravitational entropy as defined by Roger Penrose in his book entitled, “Fashion, Faith and Fantasy in the New Physics of the Universe.” This is particularly relevant with respect to Erik Verlinde’s theory that gravity is an emergent property of cosmic entropy. Verlinde’s theory dovetails nicely with the July 2018 Journal of Modern Physics paper entitled, “Clues to the Fundamental Nature of Gravity, Dark Energy and Dark Matter.”
In the present paper, the rationale for FSC calculations of gravitational entropy in the form of
S
−
−
√
S
is presented. These calculations indicate a tight correlation with the COBE DMR measurement showing CMB RMS temperature variations of 18 micro Kelvins. The COBE dT/T anisotropy ratio of 0.66 × 10−5 falls within the FSC gravitational entropy range calculated for the beginning and ending conditions of the recombination/decoupling epoch. Thus, the FSC model incorporating gravity as an emergent property of entropy suggests that the CMB temperature anisotropy pattern could simply be a map of gravitational entropy, as opposed to a magnified “quantum fluctuation” event at a finite beginning of time.
EDIT I found this lecture by Verlinde talking about his work which takes about an hour to watch. https://insidetheperimeter.ca/a-new-view-on-gravity-and-the-dark-side-of-the-cosmos-erik-verlinde-public-lecture/?__hstc=261081490.b20e624ce3bbf82fcde9c8f1a30c5bc1.1533466440658.1533466440658.1533466440658.1&__hssc=261081490.1.1533466440658&__hsfp=2495466239
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I hadn't responded to this thread since I have not studied this topic in depth. I am just watching with interest. If dark matter is the answer then that is fine but so is an alternative. If it matches observation.
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One thing which we can, probably, predict is that if the "Heavies" are questioning the existence of dark matter, there will be some lively discussion.
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i dont know if anyone else is interested in Verlindes theories. The little i know about quantum entanglement suggests only information is transferred. Can forces(gravitational) be transmitted between entangled particles / qubits.
Edit this might be the paper Verlindes references in his lecture posted above.
https://arxiv.org/pdf/1611.02269.pdf
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Never liked 'gravitons' and 'dark matter', then again, that's a purely personal opinion. What is missing in your link though is a explanation of how they propose this 'modified gravity' is supposed to work, also how well it will fit Einsteins definitions of gravity
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Actually it makes me think of 'phonons'
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Never liked 'gravitons' and 'dark matter', then again, that's a purely personal opinion. What is missing in your link though is a explanation of how they propose this 'modified gravity' is supposed to work, also how well it will fit Einsteins definitions of gravity
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Actually it makes me think of 'phonons'
Remember its all just maths, and it may not explain the real world accurately. A curve fit can be made to make predictions based on a set of observations, once the model is stretched beyond its limits it develops errors. Einsteins equations have worked successfully for a long time, but they apply no intrinsic properties to space time except curvature, and arbitrary amounts of dark matter are required to make the model work. Einstein and others also wrote papers on wormholes EPR bridges to explain entanglement and quantum tunneling effects. The modern theories are trying to incorporate quantum effects to explain how gravity works.
What many of the new theories of gravity are exploring is that spacetime has properties arising from quantum theory and string theory. Verlindes approach and the Holographic approach both are exploring the nature of space time, and incorporate a wormholes. Verlinde claims his model does not require dark matter also I understand the Holographic model does not require it either.
The graviton is a undetectable spin 2 boson virtual particle that might stretch across a universe or might not and it might not exist either just like dark matter which could be anything you want it to be explain away the errors in equations, or observations that dont make sense. ;D
Edit. Verlinde views space time geometry arises from the entanglement structure of the quantum state. Gravity emerges from this quantum information as a result of changes in entanglement caused by matter. For further info read Verlindes paper posted above.
Does Verlindes model look a little like a lattice of phonons also, are you maybe thinking down the same lines
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What have I missed does this link claim to put an end to the holographic universe idea or Verlindes version of emergent gravity.
3 days ago https://phys.org/news/2018-08-flaw-emergent-gravity.html
Edit If I am correct the holographic principle relies on an artificial anti de sitter space, which Verlindes version of imergent gravity does not.
A simpler link on Verlindes ideas https://darkmatterdarkenergy.com/2016/12/30/emergent-gravity-verlindes-proposal/
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Interesting dead cat, but I don't know what to think there. Just take this statement "If space is defined by the connectivity of quantum entangled particles, then it becomes almost natural to consider gravity as an emergent statistical attribute of the spacetime. After all, we learned from general relativity that “matter tells space how to curve, curved space tells matter how to move” – John Wheeler."
It seems you can get a mathematical model that allow space to exist without matter, and you can 'bend it' etc etc. All in accordance with relativity. But it makes sense to think of the universe as in some way entangled. There a key question could be how many entanglements a 'object' can have. If we presume only one at a time then a new 'bump' should replace the old, right?
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Now that would be a good theoretical test of Verlindes proposal, wouldn't it? If he also can create a mathematical description of how a empty space/Universe can 'bend' and 'rotate', in accordance with the equations of relativity naturally.
"So in the AdS model, gravity is emergent and its strength, the acceleration at a surface, is determined by the mass density on that surface surrounding matter with mass M. This is just the inverse square law of Newton. In the more realistic dS model, the entropy in the volume, or bulk, must also be considered. (This is the Gibbs entropy relevant to excited states, not the Boltzmann entropy of a ground state configuration)." from your link.
What he says here should make it possible "Verlinde posits that the information and entropy content of space are due to the excitations of the vacuum state, which is manifest as dark energy."
Actually I joked about that here somewhere, suggesting that it was 'virtual particles' creating this 'extra mass' we needed looking at galaxies and the universe. So maybe? Then again, he's also using entropy for it, which makes it extremely theoretical to me. He's not easy to assimilate, I'll listen to that link you gave and see if I can make more sense of it , won't promise anything though, :)
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A universe of ideas dead cat :)
Information, I should write more about him
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and yes, we're alike in that.
He do make sense
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What I found frustrating before was the way he always referred to 'entropy', without me seeing exactly how he thought there. But in the 'lecture' he simplifies it to counting the number of bits, giving you a numerical expression of the amount of possibilities any given system can become in, defining that as the 'entropy' of the system. That's simpler and make sense. But if I get it right this numerical evaluation of bits are not what he means, it's just a safe starting stone from where we move to 'information', and uncertainty, itself, he's a deep thinker.
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A universe of ideas dead cat :)
Information, I should write more about him
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and yes, we're alike in that.
He do make sense
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What I found frustrating before was the way he always referred to 'entropy', without me seeing exactly how he thought there. But in the 'lecture' he simplifies it to counting the number of bits, giving you a numerical expression of the amount of possibilities any given system can become in, defining that as the 'entropy' of the system. That's simpler and make sense. But if I get it right this numerical evaluation of bits are not what he means, it's just a safe starting stone from where we move to 'information', and uncertainty, itself, he's a deep thinker.
Mond and other theories like Verlindes seem to be gaining credibility. Hossenfelder seems to support Verlindes ideas in this paper https://arxiv.org/pdf/1703.01415.pdf also in the following later paper ref reddhift https://arxiv.org/pdf/1803.08683.pdf she adds further credence to Verlindes theories.
My conclusion is that Dark Matter is a result of over stretching Einsteins equations, it does not exist. ie It has never been detected because it does not exist.
I am going to reread Verlindes paper above, I may have m i s s ed a lot. ie I did not get the impression ADs was a part of Verlindes ideas, whilst ds was, I also got the impression that his ideas were not derived from the holographic principle.
Edit MOND, fudges the laws of gravity on large scales in such a way as to just give an accurate picture of the galactic rotation anomalies, allowing the maths to fit what is observed without dark matter. Verlinde’s idea gives a actual physical framework to consider modifications to the laws of gravity on very large scales. On very large scales the Holographic universe idea falls down, at least that is what I think Verlinde is banging on about and at very large scales ADs is not included, I also think Hossenfelder doesnot include Anti de sitter either in her analysis of Verlindes work. Clearly I still more reading to do.
@PmbPhy do you have any comments to add to your initial post, on this thread, you are very very quiet :) it is very sTRANGE
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What I found frustrating before was the way he always referred to 'entropy', without me seeing exactly how he thought there. But in the 'lecture' he simplifies it to counting the number of bits, giving you a numerical expression of the amount of possibilities any given system can become in, defining that as the 'entropy' of the system. That's simpler and make sense. But if I get it right this numerical evaluation of bits are not what he means, it's just a safe starting stone from where we move to 'information', and uncertainty, itself, he's a deep thinker.
This
link
tries
to
explains verlindes ideas
on
entropy
https://www.science20.com/hammock_physicist/it_bit_entropic_gravity_pedestrians
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Not really my subject area, but I'd appreciate elucidation from those in the know.
As I understand it, the hypothesis of dark matter is invoked to explain that the outermost stars of some galaxies are orbiting the center more rapidly than can be accounted by the presumed mass of observable stuff in the galaxy.
Given that we only have about 100 years' observations of individual stars in distant galaxies, why do we think that they are in stable orbits and not just rushing towards the galactic centre from deep space? How good is the historic velocity data? Given that 100 years is a fleabite in galactic history, how accurate is the acceleration measurement? How do we know the gravitational centre of the galaxy hasn't moved with respect to the apparent centre if there are several light years between the outer star, say closer to us, and the gravitational centre, further away?
I'm always impressed by how much information astronomers manage to glean from a few ancient photons, but recorded history from geocentric flat earth to nebulae also shows that they are more often wrong than right, so why are they so sure about this year's invisible turtles?
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As I understand it, the hypothesis of dark matter is invoked to explain that the outermost stars of some galaxies are orbiting the center more rapidly than can be accounted by the presumed mass of observable stuff in the galaxy.
Its not simply the outermost stars.
Given that we only have about 100 years' observations of individual stars in distant galaxies, why do we think that they are in stable orbits and not just rushing towards the galactic centre from deep space?
Take a look at a galaxy and notice how the matter looks like its in orbit. Then note that we get the same results no matter how far away (and thus how old) the galaxy is. And this holds for all stars in a galaxy, not just the outermost.
How good is the historic velocity data? Given that 100 years is a fleabite in galactic history, how accurate is the acceleration measurement?
Its the doppler that's measured and from that the speed.
How do we know the gravitational centre of the galaxy hasn't moved with respect to the apparent centre if there are several light years between the outer star, say closer to us, and the gravitational centre, further away?
The centers of the galaxies are all moving. In the centers frame the stars are in orbit of it. It's not accelerating with respect to its local inertial frame.
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Quote from: alancalverd on Yesterday at 22:29:51How good is the historic velocity data? Given that 100 years is a fleabite in galactic history, how accurate is the acceleration measurement?
Its the doppler that's measured and from that the speed.
Yes, but the hypothesis of dark matter derives from acceleration, not speed. So you need two credible Doppler measurements some time apart.
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MOND theory, really.
My one self has a real problem with being squeezed down to nothing but information on the surface of a black hole.
And my other self does not like being shadow of my other self.
I understand that MOND can describe the angular momentum of galaxies. But describing the universe as bits of random information one the surface of a black hole does not sound any better than imaginative matter that we cannot see.
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Given that we only have about 100 years' observations of individual stars in distant galaxies, why do we think that they are in stable orbits and not just rushing towards the galactic centre from deep space? How good is the historic velocity data? Given that 100 years is a fleabite in galactic history, how accurate is the acceleration measurement?
While we don't have observations for any single galaxy that has lasted more than 100 yrs, we do have observations of different galaxies at differing distances from the Earth, and thus viewed from different points of the universe's history spread over billions of years. If galaxies were not relatively stable in terms of star orbits, we should see galaxies in various stages of collapsing in on themselves or flying apart. Closer galaxies should appear, on average to be in the later stages of this than further galaxies. Even a difference of 1 billion ly should produce a noticeable difference in the appearance of galaxies. As it is, It doesn't. That means that over a billion yrs or so, galaxies have not evolved much.
As for the stars rushing in from deep space. When we measure the orbital velocities of those outer stars we are looking at galaxies that are rotating more or less edge-on* to us. We then compare the Doppler shift between the stars on one side Which would be moving away and the stars on the other side, which are moving towards us. The average between these readings should work out to be the recessional velocity of the galaxy as a whole, while the difference between this average and the velocities we record gives us the orbital velocities at along a line perpendicular to the radial line to the center.
we can verify the galaxy as a whole velocity by looking at the center point of the galaxy where the majority of the orbital star motion is at a right angle to our sight-line and doesn't significantly contribute to the Doppler shift readings.
Now if stars wee rushing in or out from the center of the galaxy, then, when looking straight in at the center, we would see a wide variation in Doppler shifts around the average, which we don't measure.
* if we were looking "straight down" at a galaxy, all the orbital velocities would be at a right angle to our line of sight, and we would not be able to get direct Doppler shift measurements of them. Exactly edge on would have us looking through the disk at the stars we are trying to measure the Doppler shifts for, meaning we would get all those other star Doppler shifts mixed in.
A slight angle is best, where we can look directly at the part of the disk we are measuring, but it still has a significant velocity component along the line of sight. We can then use a little trig to work out the actual velocity perpendicular to the radial line. ( for example if the galaxy is tipped towards us at 10 degrees, and we measure an Doppler shift equating to 200 km/sec, we can calculate the actual orbital velocity as being 203 km/sec at a 45 degree tilt, that same 203 km/sec orbital velocity would give us a Doppler shift reading of 143.5 km/sec.)
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Quote from: alancalverd on Yesterday at 22:29:51How good is the historic velocity data? Given that 100 years is a fleabite in galactic history, how accurate is the acceleration measurement?
Its the doppler that's measured and from that the speed.
Yes, but the hypothesis of dark matter derives from acceleration, not speed. So you need two credible Doppler measurements some time apart.
The magnitude of the speed remains constant since the acceleration is centripetal acceleration and as such acceleration is through changes in direction. As such a single Doppler measurement will give you acceleration. Recall that a = v2/r
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Only if r is constant, i.e. you know that the orbit is stable. So the question is to what degree of accuracy can we say that dr/dt = 0 for any of these objects? Given that r ≈ 105 light years or so, I wouldn't have thought that even 100 years of good observation of the doppler shift of one star would convince anyone that it was in a stable elliptical orbit at r.
But astronomers are known to be clever.
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Only if r is constant, i.e. you know that the orbit is stable.
Not true. The formula is an expellant approximation if the orbit is not perfectly circular. The direction of motion of the stars can be determined by the doppler.
Determining the rotation curve is not a simple matter. Note that the rotation curve is flat too and that says a lot. Its not determined from a single star but many stars. You do know what a galaxy looks like, don't you? What does is appearance tell you about the motion of stars? What do you think Doppler analysis tells us?
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Only if r is constant, i.e. you know that the orbit is stable.
Not true. The formula is an expellant approximation if the orbit is not perfectly circular. The direction of motion of the stars can be determined by the doppler.
Determining the rotation curve is not a simple matter. Note that the rotation curve is flat too and that says a lot. Its not determined from a single star but many stars. You do know what a galaxy looks like, don't you? What does is appearance tell you about the motion of stars? What do you think Doppler analysis tells us? That curve is for all galaxies for the most part.
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Bedtime! Back to basic newtonian physics tomorrow!
But what concerns me intellectually is the invocation of another aether: stuff with no mechanical or electrical properties, but mass. Or at least some gravitational property that is shared by objects with mass, in exact proportion to their inertial mass, which this stuff apparently does not have!
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Bedtime! Back to basic newtonian physics tomorrow!
But what concerns me intellectually is the invocation of another aether: stuff with no mechanical or electrical properties, but mass.
You mean like black holes, brown dwarfs, etc? They do have mechanical properties.
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Bedtime! Back to basic newtonian physics tomorrow!
But what concerns me intellectually is the invocation of another aether: stuff with no mechanical or electrical properties, but mass. Or at least some gravitational property that is shared by objects with mass, in exact proportion to their inertial mass, which this stuff apparently does not have!
What makes you believe that dark matter wouldn't have an equal gravitational and inertial mass?
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Janus
" While we don't have observations for any single galaxy that has lasted more than 100 yrs, we do have observations of different galaxies at differing distances from the Earth, and thus viewed from different points of the universe's history spread over billions of years. If galaxies were not relatively stable in terms of star orbits, we should see galaxies in various stages of collapsing in on themselves or flying apart. Closer galaxies should appear, on average to be in the later stages of this than further galaxies. Even a difference of 1 billion ly should produce a noticeable difference in the appearance of galaxies. As it is, It doesn't. That means that over a billion yrs or so, galaxies have not evolved much. "
Doesn't that depend on what 'origin' one presume for this universe? If I assume it to 'originate everywhere', then the universe should be isotropic and homogeneous, with all galaxies being more or less equivalent.
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ah, ok, you were thinking of it in terms of the time passed since some origin, and now it makes sense to me :)
Need some free time here ::))
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Doesn't that depend on what 'origin' one presume for this universe? If I assume it to 'originate everywhere', then the universe should be isotropic and homogeneous, with all galaxies being more or less equivalent.
"At this moment", yes. However we aren't seeing the galaxies as they are at this moment, but how they were when the light we are seeing left them. Thus, if our galaxy and Andromeda were the same age, we would be now seeing Andromeda when it was 2.5 million years younger than our galaxy. The light from a galaxy that we measure as being 1 billion ly away, would be seen as it was when it was 1 billion years younger than it is "now" As we look further and further way, we are looking at younger and younger versions of the universe.