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The possibilities for dark matter have shrunk

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Re: The possibilities for dark matter have shrunk

#61

Post by throwawaymaths »

ajross wrote: post #37
throwawaymaths wrote: post #32

Earlier quoted context omitted.

To a certain floor. Of course a particle which only interacts gravitationally is still possible but that's the cosmologist's nightmare. It'd be basically impossible to prove it's existence and anything could happen (like a small but fast fairly massive ~asteroid mass clump of dark matter explaining poltergeists)

> like a small but fast fairly massive ~asteroid mass clump of dark matter explaining poltergeists This is clever, but I have to believe the back of an envelope would show that a dark-matter scale distribution of "dark blobs" big enough to throw things around the room and frequent enough to be given names in human experience would be many, many orders of magnitude higher than the observed gravitational effects.

> have to believe

A lot of weird shit gets floated around, like planet X is dark matter, or at one point someone suggested sag x-1 was a dark matter blob. At that point Why not poltergeists?

Re: The possibilities for dark matter have shrunk

#62

Post by rdlw »

AlbertCory wrote: post #39

Some day, "what is dark matter?" arguments will be recalled with the same amusement with which we view "what kind of substance is the ether?" And then, people will say, "It was so obvious!" So let DM be a lesson the next time anyone claims that some breakthrough 50 years ago was obvious.

Sure, although I do think scientists are asking better questions than that. In the public perception, it seems like we're trying to find information about something that's basically matter, but we can't detect or measure it. In reality, 'dark matter' is just a placeholder for "Well, it looks like there should be a lot of matter here, but we measure a lot less than expected. There must be some kind of phenomenon we don't understand, one of the effects of which is that it makes galaxies that seem not to have enough matter to sustain their structure"

Re: The possibilities for dark matter have shrunk

#63

Post by excalibur »

AlbertCory wrote: post #39

Some day, "what is dark matter?" arguments will be recalled with the same amusement with which we view "what kind of substance is the ether?" And then, people will say, "It was so obvious!" So let DM be a lesson the next time anyone claims that some breakthrough 50 years ago was obvious.

I think it's been fairly obvious for a long time that dark matter and dark energy are fairy tales invented to make our flawed theories fit the data.

Re: The possibilities for dark matter have shrunk

#64

Post by kouru225 »

mppm wrote: post #4

By the way, apart from the fact that dark matter predictions have been contradicted by increasingly sensitive experiments over and over and over, there is another inconvenient piece of observational evidence: An analysis of stellar velocities at the edge of the Milky Way, published last year [1] shows that they are "Keplerian", i.e. following classical behavior without the influence of any purported dark matter. So e…

This is an old article (2023 yea I know that's recent but it's still out of date) and the crisis in cosmology (AKA dark matter) came back harder than ever with subsequent studies: https://www.youtube.com/watch?v=T1JuCPhONlg

Re: The possibilities for dark matter have shrunk

#65

Post by mr_mitm »

kouru225 wrote: post #64
mppm wrote: post #4

By the way, apart from the fact that dark matter predictions have been contradicted by increasingly sensitive experiments over and over and over, there is another inconvenient piece of observational evidence: An analysis of stellar velocities at the edge of the Milky Way, published last year [1] shows that they are "Keplerian", i.e. following classical behavior without the influence of any purported dark matter. So e…

This is an old article (2023 yea I know that's recent but it's still out of date) and the crisis in cosmology (AKA dark matter) came back harder than ever with subsequent studies: https://www.youtube.com/watch?v=T1JuCPhONlg

The hubble tension points to issues with our understanding of dark energy, not dark matter

Re: The possibilities for dark matter have shrunk

#66

Post by AlbertCory »

excalibur wrote: post #63
AlbertCory wrote: post #39

Some day, "what is dark matter?" arguments will be recalled with the same amusement with which we view "what kind of substance is the ether?" And then, people will say, "It was so obvious!" So let DM be a lesson the next time anyone claims that some breakthrough 50 years ago was obvious.

I think it's been fairly obvious for a long time that dark matter and dark energy are fairy tales invented to make our flawed theories fit the data.

I'd just quibble with the phrase "fairy tales."

"Some kind of particle" is a placeholder. They are trying to figure it out, and questioning "our flawed theories" seems to be acceptable, AFAICT.

(I think "dark energy" IS more of a fairy tale, but maybe that's just me)

Re: The possibilities for dark matter have shrunk

#67

Post by nonameiguess »

AnimalMuppet wrote: post #15

Summary: An experiment that looked for WIMPs (weakly interacting massive particles) found none with masses above 9 GeV. So either they exist but are too rare for the experiment to see, or they are massive but not that massive*, or dark matter isn't WIMPs (or dark matter doesn't actually exist at all).

Seemingly, the most mundane possibility here is they exist, they're not rare at all, but "not rare" on the global scale of all galactic and larger superstructures in the observable universe doesn't mean you're necessarily going to find them in South Dakota in some specific 280 day window.

Re: The possibilities for dark matter have shrunk

#68

Post by pfdietz »

rbanffy wrote: post #54
pfdietz wrote: post #43

Earlier quoted context omitted.

Is it the solar neutrino noise floor, or distant supernova neutrino noise floor? Maybe the justification for interstellar colonization will be so we can run a DM experiment on the near-relativistic ship during the trip. Boost those dark matter particles to some good fraction of c for easier detection.

Considering time dilation, a detector inside a relativistic spacecraft will be hit by a lot of neutrinos in the apparent short duration of the mission. I wonder what is the speed at which interstellar neutrino bombardment becomes lethal to a crew considering it’s essentially impossible to shield against them. You can put a mountain in front of the ship, but neutrinos will just pass through it like is a cloud. OTOH if…

Neutrinos already move at relativistic speeds, so they aren't much of a problem.

The real possible problem would be massive dark matter particles. These would be traveling now at around 300 km/s, so a relativistic spacecraft would boost their speeds by three orders of magnitude and their energies by six orders of magnitude or higher. And, of course, the number passing through a given area in the vehicle per unit of time would also be orders of magnitude higher. If the interaction cross section is high enough (and not so high they could be shielded) then travel at that speed becomes impossible.

Re: The possibilities for dark matter have shrunk

#69

Post by SideQuark »

throwawaymaths wrote: post #60
SideQuark wrote: post #36

Earlier quoted context omitted.

That link also claims that the results you're listing are not likely to hold up, including comments from an author of the paper used for the article. So if one of the paper;s own authors expects these claims to disappear once more data is gathered, then maybe it's too early to claim the end of dark matter. After all, DM has been measured in many different ways. One surprising finding doesn't negate hundreds of surpri…

> DM has been measured in many different ways That is begging the question. Claiming that DM has been measured many ways presupposes that the different ways that "it has been measured" are have the same cause, which itself is the dark matter hypothesis. Until we get a multimodal observation of DM itself we can't claim that DM has been measured many ways. We are very much still in the "guessing that these things are D…

> we can't claim that DM has been measured many ways

Yes we can.

It's shown up in 1) precise measurements of galaxy rotation curves (relevant to this paper, which only addresses this method), 2) velocity spread of bound stars, 3) x-ray emission from hot gas, 4) gravitational lensing, 5) cosmic background radiation measurements in CMBFAST and others, 6) provides solutions to issues in stellar strucutre formation, 7) supernovae behavior, 8) baryon oscillations support DM via empirical evolution compared computed with and without, 9) redshift observations support DM also. There are more.

So yes, we could claim there are 39 different causes, but that the effects of DM would give all these results, historical (and Occam's Razor, a still useful part of physics) means the most likely explanation is the simplest - 1 cause until proven otherwise.

If you dig, you can find papers covering all this with the math and experimental and computational error bounds to see how well all of these (and more) line up.

Next you'll tell me there are 5 types of electrons, despite all experimental evidence being consistent with one type of electron.

Re: The possibilities for dark matter have shrunk

#70

Post by throwawaymaths »

SideQuark wrote: post #69
throwawaymaths wrote: post #60

Earlier quoted context omitted.

> DM has been measured in many different ways That is begging the question. Claiming that DM has been measured many ways presupposes that the different ways that "it has been measured" are have the same cause, which itself is the dark matter hypothesis. Until we get a multimodal observation of DM itself we can't claim that DM has been measured many ways. We are very much still in the "guessing that these things are D…

> we can't claim that DM has been measured many ways Yes we can. It's shown up in 1) precise measurements of galaxy rotation curves (relevant to this paper, which only addresses this method), 2) velocity spread of bound stars, 3) x-ray emission from hot gas, 4) gravitational lensing, 5) cosmic background radiation measurements in CMBFAST and others, 6) provides solutions to issues in stellar strucutre formation, 7) s…

It's basically two major observations

0) Galaxy curves

1) cmb observations

And a few minor ones.

The 0th was the motivation for dark matter.

The 1st is good, but also not predicted a priori by dark matter. In it's defense, it doesn't need fine tuning to work (it works out with ~the estimated amount of DM)

For all the minor ones there's a lot of fine tuning necessary to obtain the results. So it's not surprising that an LCDM model (which can select an arbitrary distribution of DM) can fit some sundry minor observations. Again, you're cherry picking the minor ones.

LCDM has a hard time with: external field effect, renzo's rule, Tully fisher relation (requires tons of fine tuning), early galaxies, why dense elliptical and lenticular galaxies as a rule have no DM, etc. those are all phenomena explained and predicted (except for Tully fisher) by e.g. MOND.

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