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A tale of the unending hunt for dark matter

wired.co.uk

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Re: A tale of the unending hunt for dark matter

#11

Post by whatshisface »

JBReefer wrote: post #2

Dark matter/energy always sounds like a restatement of aether - unknowable and unreachable, conceived to fill gaps in the knowledge of the period. I'm probably wrong, but the parallels are strong.

Dark matter could turn out to be a new particle or group of particles, which would surely be as satisfying to all of us as anything else.

Re: A tale of the unending hunt for dark matter

#12

Post by simcop2387 »

tarboreus wrote: post #3
JBReefer wrote: post #2

Dark matter/energy always sounds like a restatement of aether - unknowable and unreachable, conceived to fill gaps in the knowledge of the period. I'm probably wrong, but the parallels are strong.

My instinct says this as well. "My equation isn't balancing, there must be something sitting on the seesaw!"

It's more like, my equations that work for everything else don't match what we observe in a few cases. But if there's something sitting on the seesaw that I can't see then it works everywhere. And there being a particle (or particles) that doesn't interact with the EM force is a testable theory. We've used particle accelerators to continually exclude areas of energy that the particles themselves would exist in, and eventually we're going to detect everything that we've predicted with the standard model or we'll find something new in an area that we didn't predict and start to know where things are actually wrong. Either in the particle side of things, or possibly the gravitational side of things. We know there's something missing or wrong between the two of them but they appear to work correctly for every case that we've found a way to test for so far, even if they don't seem to be possible to merge together.

Re: A tale of the unending hunt for dark matter

#15

Post by Entangled »

erik_landerholm wrote: post #9
Entangled wrote: post #8

Forgive my utter ignorance but, what if black holes are not "holes" but solid spheres of massive density? There you may have your dark matter to boot.

https://www.sciencedaily.com/releases/2018/10/181002102723.h... “Based on a statistical analysis of 740 of the brightest supernovas discovered as of 2014, and the fact that none of them appear to be magnified or brightened by hidden black hole "gravitational lenses," the researchers concluded that primordial black holes can make up no more than about 40 percent of the dark matter in the universe. Primordial black hol…

So, how dense you think the black hole at the center of the milky way is? Twice the whole galaxy at least? Or just a tiny fraction?

Re: A tale of the unending hunt for dark matter

#16

Post by andrewflnr »

Entangled wrote: post #8

Forgive my utter ignorance but, what if black holes are not "holes" but solid spheres of massive density? There you may have your dark matter to boot.

Similar hypotheses have been considered. The most popular (edit: AFAICT, but I'm not any kind of scientist), gravastars, would appear to have been ruled out by LIGO, as described here https://astronomy.stackexchange.com/questions/25449/gravasta... in the article linked therein. And as my current sibling comment mentions, they're not much good for dark matter either. They would be exactly as good as black holes for dark matter and, since they are still compact enough to do gravitational lensing, are ruled out by the same test

Re: A tale of the unending hunt for dark matter

#19

Post by GW150914 »

Entangled wrote: post #15
erik_landerholm wrote: post #9

Earlier quoted context omitted.

https://www.sciencedaily.com/releases/2018/10/181002102723.h... “Based on a statistical analysis of 740 of the brightest supernovas discovered as of 2014, and the fact that none of them appear to be magnified or brightened by hidden black hole "gravitational lenses," the researchers concluded that primordial black holes can make up no more than about 40 percent of the dark matter in the universe. Primordial black hol…

So, how dense you think the black hole at the center of the milky way is? Twice the whole galaxy at least? Or just a tiny fraction?

If you really meant dense then you have to rememberer that the average density of a black hole enjoys an inverse proportionality to its volume. In other words a supermassive black hole is far less dense on average than a much smaller hole. However it is still far far denser than a whole galaxy, which is mostly empty space and diffuse dust.

I don’t think you meant dense though, I think you meant mass, because that’s sort of how your post seems to be phrased. The answer then is that it is a minute fraction of the mass of the galaxy. Sag A* is about 4 million solar masses, and the Milky Way has 250 billion stars, many of which are far more massive than Sol. Even ignoring other objects like stellar remnants and nebulae, the mass of the central black hole is negligable on the scale of the whole galaxy.

Re: A tale of the unending hunt for dark matter

#20

Post by GW150914 »

Entangled wrote: post #8

Forgive my utter ignorance but, what if black holes are not "holes" but solid spheres of massive density? There you may have your dark matter to boot.

The whole point of a black hole is that nothing can support its own mass past a certain point. A black hole is a region more than it is an object, with a singularity beyond an event horizon. If there really is a singularity, it is a point or ring, not a sphere. There are other conjectures which replace the singularity with something else, like masses of strings, but it doesn’t really matter to the universe outside of the event horizon.

As to it being a source of dark matter, the idea that black holes and other compact objects are dark matter is encapsulated in the MACHO (MAssive Compact Halo Object) theory. It is extremely unlikely to be correct, because observations largely rule this out. Black holes warp spacetime around them and result in detectable phenomena as a result of gravitational lensing. It would appear that such lensing is not common enough to allow for black holes in sufficient quantity to explain dark matter.

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