Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

The possibilities for dark matter have shrunk

sciencenews.org

Post reply on HN
Print view View original

11–20 of 82 posts

Re: The possibilities for dark matter have shrunk

#11

Post by yfontana »

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…

As written in that article, there are questions to be answered before this observation can be considered a serious challenge to dark matter. Such as: why don't we observe the same phenomenon in other galaxies? Why are things orbiting the Milky Way farther out (such as the Magellanic clouds) not affected either?

Re: The possibilities for dark matter have shrunk

#12

Post by mppm »

FactolSarin wrote: post #7
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…

We've also seen instances where visible "clumping" in a galactic disc correspond to differences in the rotation curves, which dark matter can't really explain. But this is still a topic of much debate. However, there's lots of good evidence for dark matter besides rotation curves of galaxies. For instance, models of galactic formation work a lot better with it than without it (without dark matter, as galaxies coalesc…

The Bullet Cluster is indeed a very interesting example, and one that would be rather puzzling otherwise.

Re: The possibilities for dark matter have shrunk

#13

Post by exe34 »

FactolSarin wrote: post #7
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…

We've also seen instances where visible "clumping" in a galactic disc correspond to differences in the rotation curves, which dark matter can't really explain. But this is still a topic of much debate. However, there's lots of good evidence for dark matter besides rotation curves of galaxies. For instance, models of galactic formation work a lot better with it than without it (without dark matter, as galaxies coalesc…

> For instance, models of galactic formation work a lot better with it than without it

I'm guessing gravito magnetism hasn't been included in those models yet.

Re: The possibilities for dark matter have shrunk

#14

Post by yfontana »

westurner wrote: post #9

There is no dark matter in theories of superfluid quantum gravity. Dirac later went to Dirac sea. Gödel had already proposed dust solutions, which are fluidic. PBS Spacetime has a video out now on whether gravity is actually random. I don't know whether it addresses theories of superfluid quantum gravity. >>> "Gravity as a fluid dynamic phenomenon in a superfluid quantum space. Fluid quantum gravity and relativity."…

>>> PBS Spacetime has a video out now on whether gravity is actually random. I don't know whether it addresses theories of superfluid quantum gravity.

It doesn't, it talks about the possibility of randomness allowing gravity to be classical and not quantum. If my understanding is correct, the gist of it is that certain random fluctuations in gravity would prevent it from contradicting Heisenberg's uncertainty principle when interacting with quantum entities.

Re: The possibilities for dark matter have shrunk

#15

Post by AnimalMuppet »

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).

Re: The possibilities for dark matter have shrunk

#17

Post by ben_w »

exe34 wrote: post #13
FactolSarin wrote: post #7

Earlier quoted context omitted.

We've also seen instances where visible "clumping" in a galactic disc correspond to differences in the rotation curves, which dark matter can't really explain. But this is still a topic of much debate. However, there's lots of good evidence for dark matter besides rotation curves of galaxies. For instance, models of galactic formation work a lot better with it than without it (without dark matter, as galaxies coalesc…

> For instance, models of galactic formation work a lot better with it than without it I'm guessing gravito magnetism hasn't been included in those models yet.

I thought gravitomagnetism was extremely weak? Wouldn't normal magnetism and its interaction with environmental plasma (which IIRC is also neglected on the grounds of being a rounding error) be more significant?

Re: The possibilities for dark matter have shrunk

#18

Post by elzbardico »

jajko wrote: post #10
api wrote: post #8

Earlier quoted context omitted.

Hmm... you could probably still do a powered flyby though: https://en.wikipedia.org/wiki/Oberth_effect This is different from a normal gravity assist. Close to a black hole the gravitational field would be quite strong, making this potentially very powerful.

I love this human can-do attitude. Screw black hole radiation, distortion of space time fabric at its core, just lets use it as a mega slingshot!

We learned this from cats, I am pretty sure.

Re: The possibilities for dark matter have shrunk

#19

Post by CorrectHorseBat »

FactolSarin wrote: post #7
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…

We've also seen instances where visible "clumping" in a galactic disc correspond to differences in the rotation curves, which dark matter can't really explain. But this is still a topic of much debate. However, there's lots of good evidence for dark matter besides rotation curves of galaxies. For instance, models of galactic formation work a lot better with it than without it (without dark matter, as galaxies coalesc…

I remember reading/hearing somewhere that the bullet cluster actually fits MOND better than dark matter. I think it was a video by Sabine Hossenfelder but I can't find it right now and I'm not qualified to say whether it made sense or not.

Re: The possibilities for dark matter have shrunk

#20

Post by api »

jajko wrote: post #10
api wrote: post #8

Earlier quoted context omitted.

Hmm... you could probably still do a powered flyby though: https://en.wikipedia.org/wiki/Oberth_effect This is different from a normal gravity assist. Close to a black hole the gravitational field would be quite strong, making this potentially very powerful.

I love this human can-do attitude. Screw black hole radiation, distortion of space time fabric at its core, just lets use it as a mega slingshot!

You wouldn't get close enough to be torn apart. You'd fly by and thrust into the gravity well for an Oberth effect boost. I'm thinking of unmanned probes of course. We could send them on flyby missions to other star systems to do things like take pictures of possibly habitable exoplanets. It's unlikely that humans could survive the acceleration of such a maneuver without ending up like this guy:

https://www.youtube.com/watch?v=AGmTZeiCmJY

There wouldn't be much radiation unless the black hole were actively "eating" something, which a PBH in far solar orbit would not. That's part of what makes it hard to find and what makes PBHs candidates for dark matter: they're dark and don't do much unless something encounters them. There would be no net emission of Hawking radiation since the BH's Hawking temperature would be colder than the cosmic microwave background.

The hypothetical PBH that could be planet 9 would be about 2-3 Earth masses and about the size of a golf ball in terms of event horizon radius. You couldn't get that close to it without being "spaghettified" and added to its mass.

Probably the most valuable thing we could do with it is study it and use it as a lab to learn about quantum gravity. We could chuck small things into it with probes nearby to precisely measure the result, etc. All black holes we know about are far too distant to reach. Having one we could study would be huge.

Post reply on HN