B What are the most likely ways of finding Dark Matter?

[FelixH]

What will the scientific experiment look like that finds Dark Matter? Before then, what scientific/technological advancements will need to be made in order to find Dark Matter?

 

[Mark Harder]

Sorry if this is a disappointing answer, but I don't think we're going to find dark matter in amounts sufficient to account for the excess gravity that is attributed to it. We may discover "dark" particles, I suppose. But is that even interesting? Would such a finding add to our knowledge of the 4 fundamental forces? AFAIK, a dark particle's relevance is confined to explaining motions on the cosmic scale and gravitational lensing observations; but if we can't somehow detect actual particles out there and measure their aggregate masses, then how will we know if finding a "dark" particle in a terrestrial setting has anything to do with the clouds of stuff affecting galactic motions or gravitational lensing? Just a hunch, but personally I don't like notions of unobservable stuff permeating matter and space, like the ether and phlogiston. These have been discredited, and I suspect that eventually dark matter will be, too; although I don't know how or when.

 

[Keith_McClary]

If dark matter only interacts gravitationally with regular matter then our existing observational methods may be all there is.

 

[Mark Harder]

Forgive the highly speculative nature of this comment, but it's something I've often worndered about: Matter is sufficient to curve spacetime. But is matter necessarily required for observable and measurable curvature? If spacetime could be curved by some other mechanism, then what we are attributing to humongous amounts of dark matter is excessive curvature and no more. What one would then require is a theory that proposes such a mechanism and predicts its magnitude and structure and other observables. Could spacetime curvature have been given a non-smooth distribution at the moment of its creation, or shortly thereafter? Might the uneven, stringy distribution of visible matter over large scales be attributable to such primal curvature?

 

[Drakkith]

What will the scientific experiment look like that finds Dark Matter? Before then, what scientific/technological advancements will need to be made in order to find Dark Matter?

We don't know. We won't know until we have an experiment that finds dark matter. Until then all we can do is try various methods and see if they work.

Sorry if this is a disappointing answer, but I don't think we're going to find dark matter in amounts sufficient to account for the excess gravity that is attributed to it. We may discover "dark" particles, I suppose. But is that even interesting? Would such a finding add to our knowledge of the 4 fundamental forces?

Yes. It will make it clear that our current theory of gravity (General Relativity) works very well and doesn't need to be modified. It would mean that we have found a particle which only interacts via one of the 4 fundamental forces, which follows a trend of particles which interact via fewer and fewer forces. For example, quarks interact through all 4 forces, electrons through all but the strong force, and neutrinos through only the weak force and gravity. Dark matter would sit at the end of this list as the only known particle to interact solely through one force.

AFAIK, a dark particle's relevance is confined to explaining motions on the cosmic scale and gravitational lensing observations; but if we can't somehow detect actual particles out there and measure their aggregate masses, then how will we know if finding a "dark" particle in a terrestrial setting has anything to do with the clouds of stuff affecting galactic motions or gravitational lensing?

While there's no way to no for sure, finding a particle which only interacts through gravity would be of immense support to dark matter models. Depending on the details, we could constrain its mass and other properties and rule out some of the models we currently have or even develop new or modified models.

Just a hunch, but personally I don't like notions of unobservable stuff permeating matter and space, like the ether and phlogiston. These have been discredited, and I suspect that eventually dark matter will be, too; although I don't know how or when.

Dark matter isn't unobservable. It is readily observable through its influences on the surrounding normal matter. This is little different from neutrinos, which are only seen by their rare interactions with our detectors. The difference is only one of degree. A neutrino can be localized on the atomic scale since it interacts with single particles at a time. Dark matter is only observable on cosmic scales since gravity acts on all types of matter and is very weak, requiring the large masses of galaxy-size clumps and large time scales to be observed.

A truly unobservable particle is exactly that. Unobservable. It wouldn't affect anything because if it did, then it would be observable through those effects.

Could spacetime curvature have been given a non-smooth distribution at the moment of its creation, or shortly thereafter? Might the uneven, stringy distribution of visible matter over large scales be attributable to such primal curvature?

Let's refrain from speculation and stick to mainstream topics please.

 

[Mark Harder]

Dark matter isn't unobservable. It is readily observable through its influences on the surrounding normal matter. This is little different from neutrinos, which are only seen by their rare interactions with our detectors. The difference is only one of degree. A neutrino can be localized on the atomic scale since it interacts with single particles at a time. Dark matter is only observable on cosmic scales since gravity acts on all types of matter and is very weak, requiring the large masses of galaxy-size clumps and large time scales to be observed.

A truly unobservable particle is exactly that. Unobservable. It wouldn't affect anything because if it did, then it would be observable through those effects.

That's an interesting idea: large-scale structures in the universe as instruments for some observations.

"Let's refrain from speculation and stick to mainstream topics please.

OK.

 

[Chalnoth]

What will the scientific experiment look like that finds Dark Matter? Before then, what scientific/technological advancements will need to be made in order to find Dark Matter?

It's hard to say for sure until we detect the dark matter particle. There is ample evidence for dark matter's existence at this point through a variety of gravitational observations. But the particles seem to interact weakly enough with matter that they're very difficult to detect.

There are many different attempts to detect dark matter particles. You can read about some of them here:
https://en.wikipedia.org/wiki/Weakly_interacting_massive_particles#Direct_detection

Some of these experiments have recently claimed some detections, but they aren't consistent with one another. If these detections are real, then we're pretty close to nailing down dark matter, but dark matter has some pretty exotic properties. It's more likely, unfortunately, that these detections are not real, but due to some unaccounted for systematic error.

 

[nikkkom]

Could spacetime curvature have been given a non-smooth distribution at the moment of its creation, or shortly thereafter?

General Relativity is actually a very simple theory, it has just one equation, which in very simplified form is G = kT: "at every point of spacetime, curvature tensor is proportional to stress-energy tensor".

It means that curvature can't be arbitrary. Non-smooth curvature means non-smooth stress-energy, which usually means there must be also some matter or energy. Thus, we'd see non-smooth mass distribution, but...

Might the uneven, stringy distribution of visible matter over large scales be attributable to such primal curvature?

Observed CMB rules that out.

 

[Mark Harder]

General Relativity is actually a very simple theory, it has just one equation, which in very simplified form is G = kT: "at every point of spacetime, curvature tensor is proportional to stress-energy tensor". It means that curvature can't be arbitrary. Non-smooth curvature means non-smooth stress-energy, which usually means there must be also some matter or energy.

Nikkom, thank you for an explanation that is both educational and satisfying. However, I'm not sure what you mean by 'arbitrary', I wasn't proposing that the non-smooth distribution was without cause. As you know, there can be chaotic systems which follow well defined dynamic equations, yet are irreproducible because of extreme sensitivity to initial conditions that can never be absolutely determined. To my way of thinking, the curvature is/was not arbitrary. I was guessing that the Big One was such a chaotic event. From which followed rank speculation on my part, which I won't attempt to reproduce without further education.

Observed CMB rules that out.

This brings up another matter. I've seen it more than once. Someone points out a map of the CMB temperatures, which are not uniform. They don't even look randomly distributed. And then the accompanying text makes a statement to the effect that the CMB distribution is uniform! Drives me nuts. How can you say a blotchy distribution is uniform? It isn't. It's blotchy, with patches and strings of temperatures both higher and lower than average. Maybe it's not as blotchy as some models would predict, but it certainly doesn't look uniform, either. My discontent on this particular point isn't with the science per se, but with an interpretation that I, though a layman, find very difficult to swallow.

 

[PeterDonis]

How can you say a blotchy distribution is uniform? It isn't. It's blotchy, with patches and strings of temperatures both higher and lower than average.

Higher than lower by 1 part in 100,000, or less. The "blotchiness" is greatly exaggerated in illustrations so that it can be easily visualized; but it's still very small in absolute terms. So to a good first approximation, the CMB is uniform, which is why that's the word that's used when a simple description is needed.

 

[Khashishi]

I guess our best bet is to keep observing colliding galaxy clusters and nebulae. If only there was a way to speed up time so we could watch a collision unfold.