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Thread: Dark matter and galaxies

  1. #1 Dark matter and galaxies 
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    From appearance, why do galaxies look like they are made of normal matter only?

    Why don't we see lots of funny shaped galaxies that cannot be accounted for by the matter we see?

    I know about the Bullet Cluster, one of endless trillions of galaxies and clusters.


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    Quote Originally Posted by MikeinSpain View Post
    From appearance, why do galaxies look like they are made of normal matter only?
    We don't. If you simulate the growth of galaxies with only "normal" matter then you don't get anything like the galaxies we see.

    Why don't we see lots of funny shaped galaxies that cannot be accounted for by the matter we see?
    The galaxies we see are exactly these "funny shapes"!

    I know about the Bullet Cluster, one of endless trillions of galaxies and clusters.
    There are many similar examples.


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    But we are told that the DM is in the halo of the galaxy, rather than in the galaxy itself and outmassing normal matter by a factor of five, why would not the stars migrate to the halo, where the gravity is?

    Why are doughnut ring galaxies not the norm?
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  5. #4  
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    “Halo” means a sphere of dark matter all around (and inside) the galaxy, with increasing density towards the centre of the galaxy.
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    Halo to me means halo.

    In an astronomy term it is a spherixcal area surrounding a galaxy or whatever.

    Not a "solid" sphere.
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    Quote Originally Posted by MikeinSpain View Post
    Halo to me means halo.

    In an astronomy term it is a spherixcal area surrounding a galaxy or whatever.

    Not a "solid" sphere.
    So you need to brush up your art history, as well as physics. :-)

    The dark matter density drops off with distance from the galactic center.
    https://en.wikipedia.org/wiki/Dark_matter_halo

    That page also has a couple of the equations used to model the density of dark matter. (Unfortunately, I have never been able to find a diagram of dark matter density.)
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    Quote Originally Posted by MikeinSpain View Post
    Halo to me means halo.

    In an astronomy term it is a spherixcal area surrounding a galaxy or whatever.

    Not a "solid" sphere.
    As was already pointed out by Strange, "Halo" in astronomy doesn't mean what you think it means. It doesn't matter what it means to you, as this is not the meaning used in working out the dynamics of galaxies.

    Words have different meanings depending on the context in which they are used and you have to use the definition applicable. For example, take the word "prove". You've heard the saying; " The exception proves the rule". What most people don't realize is that "prove" had a different usage at the time the phrase was coined than it has now. At that time "prove" meant "to test and find wanting", or pretty much the opposite of what we take it to mean now. So a proper modern translation of that saying is "The exception disproves the rule". as a result most people misuse that phrase and think it means something other than it was meant to.

    In addition even when referring to what you see in religious works of art, the "halo" depicted in them is just the artist's attempt to represent an aura or full sphere of light entirely enveloping the head, it is is only a more modern stylized version that depicts it as a ring that hovers over the head never touching it. The Astronomical use of the word actually is more representative of its original usage than the modern image the term generally invokes today.
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    I did check up the physics definition of halo before writing the last post.

    Anyway, let's say normal matter embedded in a sphere of dark matter.

    It is a SPHERE of dark matter, spinning.

    So why is the galaxy a spiral galaxy instead of a globular cluster?

    If the DM is a sphere, then why are not the stars a sphere also?

    If the galaxy is a spiral, why is the DM not a disk?

    Maths bothers me. To give a famous example. Hawking came up with numerous ideas, then proved them using maths. He later admitted that (despite the maths being sound) some of the ideas were wrong.

    My problem with dark matter is that it is basically particles of gravity.

    We have the strong atomic force at quantum level. We have gravity at solar sytem level. Why not a new force of attraction at galactic levels and beyond?

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    Janus. The exception BREAKS the rule.
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    Quote Originally Posted by MikeinSpain View Post
    Janus. The exception BREAKS the rule.
    Full explanation here: https://www.phrases.org.uk/meanings/...-the-rule.html and here: https://en.wikipedia.org/wiki/Except...roves_the_rule.
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    Quote Originally Posted by MikeinSpain View Post
    I did check up the physics definition of halo before writing the last post.

    Anyway, let's say normal matter embedded in a sphere of dark matter.

    It is a SPHERE of dark matter, spinning.

    So why is the galaxy a spiral galaxy instead of a globular cluster?

    If the DM is a sphere, then why are not the stars a sphere also?

    If the galaxy is a spiral, why is the DM not a disk?
    Matter interacts electromagnetically; that is why solid materials feel solid, it is why fluids flow and so on. This means that clouds of gas or dust will gradually "cool" and share momentum as they bump into one another. They all end up with *roughly* the same speed in the same direction. This interaction means that matter eventually collapses from a sphere to a flattened sphere and then to a disk.

    Dark matter doesn't do that.

    We have the strong atomic force at quantum level. We have gravity at solar sytem level. Why not a new force of attraction at galactic levels and beyond?
    Attempts to explain dark matter by using new forces or modified gravity just don't fit all the data. People are still trying, though.

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    Just to add a bit to Strange's post. As he said, what we consider "normal"* matter interacts electromagnetically. It's the interaction between the electromagnetic fields that allow object to collide with each other and either stick together or bounce off. The other aspect of this is that when this happens, the matter involves accelerates, which due to the electromagnetic nature of the interaction creates electromagnetic waves which radiate away. These waves carry energy, which come at the cost of kinetic energy from the matter involved. Thus the matter ends up moving a bit slower after a collision than it was before. This is the "cooling" strange mentioned. The matter cools, the more likely it is to clump together forming structures, If the original material which formed a galaxy has a net rotation, the end end result tends to be a rotating disk.

    Dark matter does not react electromagnetically at all as a result, it not only doesn't collide with itself or "normal" matter, but passes right by, neither can it radiate electromagnetically. So even if it were to accelerate (by gravitational interaction) it still has no mechanism to radiate away or lose energy the way "normal" matter will. It's only effect will be gravitationally, and that is more of a combined effect of all the DM in the region of the galaxy. This is enough to modify the rotation curves of the galaxy disk, but not enough to overcome, the Matter to matter interactions that created the disk shape.

    In terms of the data mentioned, two particular observations come to mind:

    There is the Bullet cluster. This is the aftermath between a collision on the galactic scale. What makes this stand out is that we can see the effect of large enough gravity fields by the way they bend the path of light passing through them. This is termed Gravitational lensing. Normally, when we look at a galaxy, the gravitational lensing we see is a combination of both the visible matter and dark matter both centered around the same point. From this alone, you can't tell with certainty whether the extra lensing is due to DM or, as you suggest, the fact that gravity just behaves differently at large scales.
    However, the Bullet cluster offers us an answer to this question. If as alluded to above, the effects of collision for visible matter an DM are different, then we should expect the aftermath of such a collision to be different. The visible matter involved would tend to radiate away energy in the collision and thus separate again moving a bit slower, the DM however should just continue on it merry way with no slowing. In other words, with a collision at the scale of the Bullet cluster, the DM should be "knocked free" of the visible matter is normally paired with. And while we can't see this DM directly, we can see its gravitational lens "footprint".
    This is exactly what we see when we look at the Bullet cluster. We see the visible matter clumps separating after the collision, and ahead of them a gravity lens in a spot where there is no visible matter to cause it, and right where we would expect the Knocked free DM to be. This cannot be explained just assuming a "different set of rules" for gravity, As this would still need some visible matter in that spot to apply these rules to.

    The second observation is the more recent discovery of galaxies that show little to no signs of dark matter at all. These galaxies look just like other galaxies that do show evidence of DM. They are about the same size and have the same amount of visible matter, etc, yet the one rotates as if there is no more extra mass than what we see there, and another rotates as if there iwas extra mass in the form of DM. (It is also important to note that it isn't enough to just add extra mass, that mass has to have the same spherical distribution as a DM halo.)

    If you are trying to invoke new rules for gravity, they would have to be applied to both galaxies equally. You can't just say they work this way for galaxy A and a completely different way for galaxy B. The only way to explain it is that one galaxy contains something we don't see that the other galaxy doesn't.

    * I really dislike using the word "normal" here. It strikes me as being "bary-centric". Just because we and everything we interact with is predominately made of baryonic matter, we tend to think of this as being the "normal" state of matter. But that is just due to our own self-centered viewpoint ( This not something new, The Mediterranean sea, translates to "Middle Earth" Sea as it namers thought it was located at the middle of the Earth's surface. Later we thought the Earth was at that center of the universe, then the Sun was thought to be at the center of our galaxy...). If Dark matter accounts for more mass than visible mass by many-fold in the universe, then maybe it is the "norm" and we are made from "outside of the norm" matter. Sure, baryonic matter allows for all the interesting stuff like planets, stars, life etc, but that doesn't necessarily mean that it is normal.
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    Janus: " ‘exception confirms the rule in the cases not excepted’"

    Someone playing with words, and not very well.
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    Strange. The problem is that no one knows what dark matter is, or excactly where it is.

    Just what it supposedly does.

    This leaves the possibility that it is something else entirely that we do not know about or where that something is.

    That "sign" demonstrates the doubts of many people over DM.

    The idea that we have (inert) particles that are nothing more than a gravitational force does not sound right.
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    Janus. Well, we have the Bullet Cluster, and the Bullet Cluster, and ah....the Bullet Cluster, followed by the Bullet Cluster. You cannot pick one cluster out of trillions and say that proves what we say (while the rest do not).

    No visible matter as in we cannot see any in the very tiny blobs we call photos. And:

    The Bullet Cluster: Evidence for Dark Matter or Not

    Gas refracting light? Who'd have believed it? And with two clusters ripping each other apart, there would be plenty of gas.

    Another point with DM is that though it is said to react gravitationally and produce structures light years across, etc, it does not clump (as far as I know). It does not produce any dense structures like star masses. It is a force of gravity and yet repulsion. And if DM is repelling other DM, so it does not clump....?
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    Quote Originally Posted by MikeinSpain View Post
    Strange. The problem is that no one knows what dark matter is, or excactly where it is.
    We don't know what it is (yet). But we do know where it is.

    This leaves the possibility that it is something else entirely that we do not know about or where that something is.
    Absolutely. But, apart from matter, the the only "something else" I can think of is either a modified form of gravity or a new force. All the models along these lines do not fit all the evidence. But people are still searching for other explanations.

    The idea that we have (inert) particles that are nothing more than a gravitational force does not sound right.
    We already know of neutrinos. They only interact via gravity and (very occasionally) the weak force. So dark matter is not very different. Some of the attempts to detect them are based on the idea that they may also interact via the weak force.
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    Quote Originally Posted by MikeinSpain View Post
    Janus. Well, we have the Bullet Cluster, and the Bullet Cluster, and ah....the Bullet Cluster, followed by the Bullet Cluster. You cannot pick one cluster out of trillions and say that proves what we say (while the rest do not).
    It is not just one example. The Bullet cluster was the first found and so the most well known, but there are others.

    https://www.forbes.com/sites/startsw...sicists-think/

    Another point with DM is that though it is said to react gravitationally and produce structures light years across, etc, it does not clump (as far as I know). It does not produce any dense structures like star masses. It is a force of gravity and yet repulsion. And if DM is repelling other DM, so it does not clump....?
    There is no repulsion involved (part from your repulsion by dark matter!). The reason it doesn't clump is because it does not bounce or stick, so it doesn't achieve a common momentum or form small scale structures.
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    Quote Originally Posted by MikeinSpain View Post
    Janus. Well, we have the Bullet Cluster, and the Bullet Cluster, and ah....the Bullet Cluster, followed by the Bullet Cluster. You cannot pick one cluster out of trillions and say that proves what we say (while the rest do not).

    No visible matter as in we cannot see any in the very tiny blobs we call photos. And:

    The Bullet Cluster: Evidence for Dark Matter or Not

    Gas refracting light? Who'd have believed it? And with two clusters ripping each other apart, there would be plenty of gas.

    Another point with DM is that though it is said to react gravitationally and produce structures light years across, etc, it does not clump (as far as I know). It does not produce any dense structures like star masses. It is a force of gravity and yet repulsion. And if DM is repelling other DM, so it does not clump....?
    As Strange pointed out, there are other examples besides the Bullet cluster. And for this particular effect to show you have to looking at collisions between clusters sometime after the collision and not just any random cluster. You would only expect this effect to show up under these circumstances. Gases produced in a collision would not likely be ahead of the separating components but rather spread out between them. In addition, refraction by gases or any other material would produce chromatic aberration due to the fact that different frequencies of light refract by different amounts. Gravitational lensing does not produce such an effect.

    DM does "clump", it's just that the mechanism for clumping is limited to gravity alone, while visible matter has the additional mechanism of electromagnetism to help shed kinetic energy and thus clumps faster and more readily. The universe is old enough for visible matter to form galaxies, stars, planet's etc, but DM, limited to only gravitational interaction has only been able to clump into much larger and looser structures. Gravity may draw both DM and visible matter together, but it is the electromagnetic interaction that allows for visible matter to stay together, and not just separate again, once it comes together.
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    Strange. While there are endless zillions of neutrinos, they are of such low mass that as far as any gravitational effect is concerned, they are ignored. They pass through planets without the gravity of planets affecting them.
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    Strange. The link is from pop science icon,. Ethan Siegel (pbuh) who talks of something then comes to the conclusion that it MUST be what he says, without any other thoughts.

    "For the first time, astronomers have seen the image of a distant quasar split into multiple images by the effects of a cloud of ionized gas in our own Milky Way Galaxy. Such events were predicted as early as 1970, but the first evidence for one now has come from the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope system."

    https://www.sciencedaily.com/release...0828173117.htm

    As I have pointed out to others elsewhere, I can show you gases in a lab refracting light. Now show me gravity refracting or bending light in a lab.

    When the best photos we have are a few coloured blobs, an agenda is needed to make decisions like Siegel does, saying that distant light behind a cluster comes from exactly where he says it does.

    The other samples also have superheated gas. Where are told cold dark clusters producing similar results?
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    Janus. If DM clumping works better once DM has got rid of kinetic energy, then DM would indeed mostly be in halos of galaxies away from hot stars rather than spread throughout a galaxy amid the active stars.
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    Quote Originally Posted by MikeinSpain View Post
    As I have pointed out to others elsewhere, I can show you gases in a lab refracting light. Now show me gravity refracting or bending light in a lab.
    Ah, the old "if you can't it in a lab, it isn't real".

    Show me the moon orbiting the Earth in a lab, or I don't it happens.

    (Sorry, but that comment doesn't require a serious response.)
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    Quote Originally Posted by MikeinSpain View Post
    Janus. If DM clumping works better once DM has got rid of kinetic energy, then DM would indeed mostly be in halos of galaxies away from hot stars rather than spread throughout a galaxy amid the active stars.
    Please show us the detailed calculations or simulations that you base this on.

    Or is it just "common sense"?
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    Quote Originally Posted by MikeinSpain View Post

    As I have pointed out to others elsewhere, I can show you gases in a lab refracting light. Now show me gravity refracting or bending light in a lab.
    And when light is refracted in the lab, it displays distinctive characteristics, such as chromatic aberration and absorption lines in the spectrum. Characteristics that don't show up in the Bullet cluster observations. You can't play cafeteria science and just pick out the property you want ( light bending) and toss out everything else that goes with light refraction.

    Quote Originally Posted by MikeinSpain View Post
    Janus. If DM clumping works better once DM has got rid of kinetic energy, then DM would indeed mostly be in halos of galaxies away from hot stars rather than spread throughout a galaxy amid the active stars.
    I have no idea what body orifice you pulled this idea from, but it has no legs to stand on. Again, DM doe not interact electromagnetically, that includes absorbing energy, kinetic or otherwise from the output of stars. As far as DM is concerned, all those active stars are just gravity sources. Now DM can shed energy though gravitational interaction. Either by transferring energy to something else in the interaction or through gravitational radiation. Since there is more matter in the form of visible matter in the galactic disk and bulge, there is more opportunity for this to occur there. As a result, you would expect a slight increase in DM density as you move inward and no tendency for it to be excluded to the outskirts.

    It is becoming apparent that you have a closed-mind when it comes to dark matter and are willing to grasp at any straw, no matter how feeble, in order to deny it.
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    Wise words Janus! You've done your best but you can't educate crackpots...
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    Strange. I did not expect a serious response. I merely used it to point out that gas refracts light. It ain't always gravity or dark matter.
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    Strange. Common sense is uncommon.

    Hawking could always use maths to prove what he said, even when he later admitted he was wrong.

    How does one do gravity simulations in a lab?

    If DM, aka particles of gravity were near stars, why would they, neutron stars, black holes, etc not just hoover them up over endless billions of years? Isn't that how gravity works?

    Would not every stellar body, and even planets not be full of DM? Yet we have never found it in coliders. There is no process which can make matter lighter by getting rid of its DM.
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    Janus. We are not talking BUllet Cluster observations here but stars beyond it

    "chromatic aberration", with tiny points of light even in our best photos? Have they even looked for that or anything else to compare it, or is it "that's gravitational lensing". Job done. The same with abosrption lines.

    They "knew" it was due to gravity so why bother? Show me anywhere that such tests have been done.
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    PhDemon. A little kid has hijacked your account and is posting baby stuff.
    Last edited by MikeinSpain; August 9th, 2019 at 11:43 PM.
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  32. #31  
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    Quote Originally Posted by MikeinSpain View Post
    I merely used it to point out that gas refracts light.
    And do you know what else gas does to light? It scatters it. It is why the sky is blue and why distant landscape objects appear hazy. It is why nebulae are visible. In other words, large collections of gas are visible. Dark matter is not visible and therefore cannot be a large collection of gas. The fact that dark matter does not scatter light is why it is said that dark matter does not interact electromagnetically.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by MikeinSpain View Post
    Janus. We are not talking BUllet Cluster observations here but stars beyond it

    "chromatic aberration", with tiny points of light even in our best photos? Have they even looked for that or anything else to compare it, or is it "that's gravitational lensing". Job done. The same with abosrption lines.

    They "knew" it was due to gravity so why bother? Show me anywhere that such tests have been done.
    From
    https://pdfs.semanticscholar.org/ffc...a3b4e0b632.pdf

    "Different physical processes leave their signatures on different wavelengths, thus our review is based on a multifrequency analysis."

    The paper then goes on to discuss how these signatures are used to determine what is causing what.
    Just because you tend to jump the gun and claim things simply based on superficial appearance and the fact that you think is supports your view doesn't mean that this is how professional scientists work. In other words, they do know how to determine whether or not their is enough visible matter in a region to cause the effect they see, and they do check to see if it could be the possible cause before publishing. Because they know that if they didn't, someone would call foul on them.

    If DM, aka particles of gravity were near stars, why would they, neutron stars, black holes, etc not just hoover them up over endless billions of years? Isn't that how gravity works?

    Would not every stellar body, and even planets not be full of DM? Yet we have never found it in coliders. There is no process which can make matter lighter by getting rid of its DM.
    DM is not "particles of gravity", they would be particles that have mass, but don't interact other then by gravity. And no, that isn't how gravity works. Gravitationally, DM doesn't act any differently than visible matter, and since neutron stars and black holes haven't "hoovered up" all the visible matter in the universe, there is no reason to expect that it has done so for DM. In fact it is quite the opposite. The collection of visible mass into these objects is aided by the electromagnetic interaction between the matter. Black holes have accretion disks of visible matter, which due to inter-collisions radiate away energy and causes it fall into the black hole. DM doesn't do this, so the only way for DM to be trapped by a Black hole is if the initial path of the DM intersects the event horizon. On top of this these objects form in regions where the visible matter density far overshadows the DM density. There may be more DM in the universe over all, but it tends to be spread out over a much much larger volume, giving it very low local density. Black holes can swallow up some dark matter, but just not as easily or as much as it does visible matter.

    So far all your arguments have been based on a misrepresentation of DM or the physics involved. And while I'm assuming you are being earnest, and this is due to your applying the best of your understanding to the issue, you need to accept that the best of your personal understanding falls far short of what the total amassed knowledge on these subjects is.
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    KJW. As I said before:

    "For the first time, astronomers have seen the image of a distant quasar split into multiple images by the effects of a cloud of ionized gas in our own Milky Way Galaxy. Such events were predicted as early as 1970, but the first evidence for one now has come from the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope system."

    https://www.sciencedaily.com/release...0828173117.htm

    What does dark matter do? Whatever is required of it, since it is easy to make such a claim on something we know nothing about.
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    Janus. I remember about two decades ago reading an astronomer claimed to have found a redshift showing an object was moving away from "us" at greater than light speed. As I guessed when I read the article, he had also measured a gravitational redshift, as someone later pointed out to him. If only scientists were as infallible as some people think they are.

    The difference with DM is that unlike "normal matter", it does not form dense structures but instead forms massive structures as in a galaxy would be embedded in it, rather than just lumps of DM here and there.

    This leads to the hoover principle, like if a hoover like device is introduced into a room full of gas and so it starts sucking it in and diluting the gas which continues to spread out to compensate for the gas being removed. So it would be with stars, neutron stars and black holes, and even planets where over billions of years, DM would be hoovered up, because it reacts gravitationally and forming large structures would be its weakness.

    As I just said in the previous post, we have no clue as to what DM is so meanwhilke it is said to do whatever is required of it.
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    Quote Originally Posted by MikeinSpain View Post
    Janus. I remember about two decades ago reading an astronomer claimed to have found a redshift showing an object was moving away from "us" at greater than light speed. As I guessed when I read the article, he had also measured a gravitational redshift, as someone later pointed out to him.
    We can observe galaxies receding at much more than light speed. This is not exactly surprising or unexpected (because separation speed is proportional to distance).

    I would be rather surprised if gravitational redshift could be measured that easily. But without a reference, I can't really comment.

    This leads to the hoover principle, like if a hoover like device is introduced into a room full of gas and so it starts sucking it in and diluting the gas which continues to spread out to compensate for the gas being removed. So it would be with stars, neutron stars and black holes, and even planets where over billions of years, DM would be hoovered up, because it reacts gravitationally and forming large structures would be its weakness.
    I suppose if you invent an unrealistic, fantasy model of gravity you can claim anything should happen.
    ei incumbit probatio qui dicit, non qui negat
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    Are galaxies with the black holes and the dark matter so massive that they can pull on the light causing a relative red shift regardless of where it is observed from?
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    Quote Originally Posted by Freelance Scientist View Post
    Are galaxies with the black holes and the dark matter so massive that they can pull on the light causing a relative red shift regardless of where it is observed from?
    No.
    ei incumbit probatio qui dicit, non qui negat
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    Strange. Is dark matter not a fantasy model of gravity?

    No one has any actual hard evidence of its existence.

    There have an effect as in galaxies holding together when it seems they should not so people are making wild guesses as to the cause of that.

    I know I annoy you though it is not deliberate. I answer questions on the flat earth, a 6,000 year old Earth, etc. It is easy to just call them a bunch of loons but I find that taking time out to answer them makes me think of what I know and comiing up with new angles on it rather than just remembering what I have read on a subject.
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    Quote Originally Posted by MikeinSpain View Post
    KJW. As I said before:

    "For the first time, astronomers have seen the image of a distant quasar split into multiple images by the effects of a cloud of ionized gas in our own Milky Way Galaxy. Such events were predicted as early as 1970, but the first evidence for one now has come from the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope system."

    https://www.sciencedaily.com/release...0828173117.htm
    What point are you making with this reply to what I said about gas scattering light? My point about gas scattering light was that the absence of scattering of light by dark matter excludes the possibility that dark matter is ordinary matter that is too small or too dim to be seen. Also, such phenomena as the Bullet Cluster excludes the possibility that dark matter is an anomaly in the law of gravitation. This doesn't leave much left as possibilities of what dark matter is.


    Quote Originally Posted by MikeinSpain View Post
    What does dark matter do? Whatever is required of it, since it is easy to make such a claim on something we know nothing about.
    It is not true to say that we know nothing about dark matter. We have observations that indicate properties of dark matter as well as eliminate particular possibilities about what dark matter could be. Even if we don't know what dark matter is, we do know what it is not.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Though no one knows what DM is, the claims continue.

    DM may predate the big bang:

    Dark Matter May Be Older Than the Big Bang - SpaceRef

    DM may be warm, not cold:

    https://www.universetoday.com/143083...warm-not-cold/

    In which case surely we would get readings above 2.7K for "space".
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    Quote Originally Posted by MikeinSpain View Post
    In which case surely we would get readings above 2.7K for "space".
    Your levels of ignorance are impressive.
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