A tale of the unending hunt for dark matter | Hacker News Reader

A tale of the unending hunt for dark matter

wired.co.uk

46 pointsdevy28 comments

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.

That's how we discovered that neutrinos oscillate: "missing" neutrinos. Turns out there are three types of them and they swap around.

https://en.m.wikipedia.org/wiki/Solar_neutrino_problem

"Early attempts to explain the discrepancy proposed that the models of the Sun were wrong, i.e. the temperature and pressure in the interior of the Sun were substantially different from what was believed" but in fact, it was the Standard Model that was wrong.

To be fair, physicists are quite aware of that. I'm no expert but it seems to me that theories that remove the need for dark energy are researched, and welcomed rather than mocked. Nobody is satisfied with this dark matter/energy need (especially when we know for a fact that our current fundamental theories are wrong)

The primary difference between the scientific method and religion / dogma

Even as an atheist this wound me up. Please don't sidetrack the conversation.

> it seems to me that theories that remove the need for dark energy are researched, and welcomed rather than mocked

I don’t think that clarifying this 1) is structural to their process, 2) what the process is and 3) a contrasting example of “knowledge” that also covers the same topics but without this process is a sidetrack.

This isn’t an outlying case and all scientists _should_ have this view of contrasting hypothesis. This is obviously in contrast to religion.

Given how many laymen love to make this comment on every dark matter article, I wish even a few mentioned the opposite historical anecdote: neutrinos were originally postulated as invisible essentially non-interacting particles who's only functional role, at the time, was to conserve momentum and energy in interactions where it seemed to be violated. And that turned out to be correct.

https://en.wikipedia.org/wiki/Neutrino#Pauli's_proposal

Big difference between dark matter and ether: Ether was required conceptually, because people could not conceive of a wave of light without a medium for it to propagate in. Getting rid of ether required a huge conceptual leap, and so the idea of ether hung around longer than it should have. As a mistaken concept, ether is a "sticky" in that it has intuitive appeal and that makes it dangerous.

It wouldn't require the same kind of conceptual leap to get rid of dark matter, though. Either there's stuff there we can't measure as of 2018, or we'll need a better theory of gravity. Many physicists recognize dark matter as a hack and hold on to it only because it's the best guess we currently have. Dark matter isn't "sticky": people would rather get rid of it than keep it!

https://medium.com/starts-with-a-bang/what-astronomers-wish-...

"Among the general public, people compare it to the aether, phlogiston, or epicycles. Yet almost all astronomers are certain: dark matter and dark energy exist"

"When someone puts forth the hypothesis that “dark matter and/or dark energy doesn’t exist,” the onus is on them to answer the implicit question, “okay, then what replaces General Relativity as your theory of gravity to explain the entire Universe?” As gravitational wave astronomy has further confirmed Einstein’s greatest theory even more spectacularly, even many of the fringe alternatives to General Relativity have fallen away. The way it stands now, there are no theories that exist that successfully do away with dark matter and dark energy and still explain everything that we see. Until there are, there are no real alternatives to the modern picture that deserve to be taken seriously."

For all the details read the rest of the article.

> "Yet almost all astronomers are certain: dark matter and dark energy exist ... [and it explains] everything that we see."

Siegel's cheerleading overstates the case, let's check wikipedia for a more sober assessment on the consensus:

Although the existence of dark matter is generally accepted by the scientific community, some astrophysicists,[15] intrigued by certain observations that do not fit the dark matter theory ...

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.

Except we have evidence in favor of dark matter.

Or like Neptune. They predicted that from orbital perturbations they couldn't explain before they found it. Much better analogy than aether. Aether was trying to fix a theory, not a measurement.

> This is the epic tale of the unending hunt for dark matter ... Despite huge pots of money being poured since the 1970s into dark matter experiments on, under or above Earth, despite endless late nights spent doing calculations, and despite plenty of media coverage, researchers keep getting nowhere

It's ridiculous to refer to about 50 years of work so far as an "unending" hunt, and it's a sign of the very short-term thinking of our age.

Well we’ve found a lot of other things in that same timespan, for reference.

And a lot of things took much longer, thus my statement.

Also, just because something's been going on for some amount of time says nothing about how much longer it might take to get the answers.

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 holes could only have been created within the first milliseconds of the Big Bang as regions of the universe with a concentrated mass tens or hundreds of times that of the sun collapsed into objects a hundred kilometers across.”

“An as-yet unpublished reanalysis by the same team using an updated list of 1,048 supernovas cuts the limit in half, to a maximum of about 23 percent, further slamming the door on the dark matter-black hole proposal.”

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.

Previously: https://news.ycombinator.com/item?id=18126808

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

Uh what. That’s what black holes are.

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.

We measure black hole mass by their gravitational effect on stars or light (through gravitational lensing). Their density doesn't matter in these calculations since their mass is pretty much the first thing we know about them.

That doesn't actually solve dark matter.

We know that the rotational velocity of a stellar object is proportional to the mass inside its orbit and inversely proportional to its distance from the center.

From measuring the velocities of stars in other galaxies, we find that the stars distant from the core are orbiting much faster than we'd expect them to. The velocity as you go out from the center should drop off quickly; it doesn't.

Therefore we hypothesize that there is a lot more matter at the outer reaches of a galaxy than what we're seeing.

So... you need not just more mass, but mass that isn't clumpy; mass that is mostly at the outer reaches of a galaxy and not the center.

A massive black hole at the center just doesn't cut it.

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