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The Reason We Haven’t Directly Detected Dark Matter

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Re: The Reason We Haven’t Directly Detected Dark Matter

#241

Post by isaachier »

thx4allthestuff wrote: post #190

Super weird food for thought, but I used to think about the universe a lot as a kid, and back then we were under the assumption that the expansion of the universe was slowing. At some point that viewpoint changed and we now believe it is accelerating, hence the emergence of so called dark matter. Anyway, this led me to envision a fourth dimension, a sphere. Imagine that our universe began at any arbitrary point on th…

I used to be a bit interested in astrophysics, so I can't vouch 100% for this being accurate, but it's my understanding:

The universe isn't the penny, it's the balloon. Physicists believe we are living on the surface of a hypersphere. One important consequence of this idea is that the big bang didn't occur at a specific point in our 3D space, but at the center of the sphere.

Furthermore, the concept of a balloon expanding vs. deflating is a bit of a misconception. The argument used to be that whether or not the balloon is expanding, depending on the rate of the expansion, gravity could eventually win out and cause the matter to collapse back together (big crunch scenario). The problem with that theory is that we now know that galaxies are speeding away from us at a growing speed that (not sure the exact details, probably based on red shift in light from nearby galaxies). So the idea of gravity winning out was not based on evidence, just one of a number of possibilities, but the evidence proved it wrong beyond a doubt.

Re: The Reason We Haven’t Directly Detected Dark Matter

#242

Post by vbezhenar »

dbt00 wrote: post #171
angel_j wrote: post #160

How do you square the following notions? a) dark matter does not interact with measurable matter b) proof of dark matter's existence derived by the measurable, relative, interaction of mass and energy

Summarizing on the fly, so necessarily wrong in details: Regular matter interacts via the four forces: strong interaction, weak interaction, electromagnetism and gravity. Strong interaction: binds fundamental particles together to make atoms. In every day terms, it’s what makes mass, mass. Weak interaction and electromagnetism: causes radioactive decay and EM radiation (photons). In every day terms, it’s how we get l…

How is it even possible to detect dark matter, if it doesn't have EM interaction, weak interaction and strong interaction? Probably some ultra-sensitive gravity detector which could detect a single particle? Sounds like impossible for current technology. So we could have those dark matter particles orbiting with crazy speeds around our planet, sun and everything else without any chance to reliably detect them. But still there are experiments to detect them, does it mean that scientists expect some kind of interaction, may be rare?

Re: The Reason We Haven’t Directly Detected Dark Matter

#243

Post by golemotron »

> But that’s indirect; we know there’s supposed to be a particle associated with it, and that’s what the hunt is all about.

I think is the is the most important line in the article. Why does it have to be a particle? That's a large assumption. We've conceptualized everything in our models so far as waves/particles. Maybe we need a different concept.

Re: The Reason We Haven’t Directly Detected Dark Matter

#244

Post by cwmma »

chongli wrote: post #72
tomp wrote: post #54

The simplest “proof” of dark matter - Kepler’s Laws dictate that objects with smaller orbits (closer to center) move faster (in terms o fangular velocity). Our solar system works like that. Our galaxy doesn’t - its spiral shape indicates that the angular velocity varies minimally with radius.

This carries the implicit assumption that all forces fall off monotonically with distance. Maybe gravity doesn't? Maybe it has a curve similar to a meteor impact crater: raised in the middle, a deep valley, and raised at the outer edge? Of course, with more evidence, such as the observations of the Bullet cluster, these simple explanations fall apart. But at any rate, it's not enough to take one observation and assum…

It's not just galaxies it's also the interactions between galaxies and while you can tweak gravity to handle galxey rotations it doesn't work for those bigger structures, though dark matter does work for both.

Re: The Reason We Haven’t Directly Detected Dark Matter

#245

Post by gmjoe »

vbezhenar wrote: post #242
dbt00 wrote: post #171

Earlier quoted context omitted.

Summarizing on the fly, so necessarily wrong in details: Regular matter interacts via the four forces: strong interaction, weak interaction, electromagnetism and gravity. Strong interaction: binds fundamental particles together to make atoms. In every day terms, it’s what makes mass, mass. Weak interaction and electromagnetism: causes radioactive decay and EM radiation (photons). In every day terms, it’s how we get l…

How is it even possible to detect dark matter, if it doesn't have EM interaction, weak interaction and strong interaction? Probably some ultra-sensitive gravity detector which could detect a single particle? Sounds like impossible for current technology. So we could have those dark matter particles orbiting with crazy speeds around our planet, sun and everything else without any chance to reliably detect them. But st…

That's exactly it. Nobody knows if it's impossible or not, but there's no way to find out unless experiments at least try, to whatever effort/cost we deem reasonable.

Re: The Reason We Haven’t Directly Detected Dark Matter

#246

Post by WhitneyLand »

isaachier wrote: post #241
thx4allthestuff wrote: post #190

Super weird food for thought, but I used to think about the universe a lot as a kid, and back then we were under the assumption that the expansion of the universe was slowing. At some point that viewpoint changed and we now believe it is accelerating, hence the emergence of so called dark matter. Anyway, this led me to envision a fourth dimension, a sphere. Imagine that our universe began at any arbitrary point on th…

I used to be a bit interested in astrophysics, so I can't vouch 100% for this being accurate, but it's my understanding: The universe isn't the penny, it's the balloon. Physicists believe we are living on the surface of a hypersphere. One important consequence of this idea is that the big bang didn't occur at a specific point in our 3D space, but at the center of the sphere. Furthermore, the concept of a balloon expa…

Wouldn’t that be not the best fit you can imagine with the holographic principle?

Re: The Reason We Haven’t Directly Detected Dark Matter

#247

Post by pas »

bordercases wrote: post #240
thx4allthestuff wrote: post #190

Super weird food for thought, but I used to think about the universe a lot as a kid, and back then we were under the assumption that the expansion of the universe was slowing. At some point that viewpoint changed and we now believe it is accelerating, hence the emergence of so called dark matter. Anyway, this led me to envision a fourth dimension, a sphere. Imagine that our universe began at any arbitrary point on th…

Observations of "cosmic inflation" are not uniformly distributed, so this could very well be true.

Is that the current consensus? Do you happen to have sources for that?

Re: The Reason We Haven’t Directly Detected Dark Matter

#248

Post by WhitneyLand »

I see a lot of comments on mass, but I thought not too long ago there wasn’t even consensus this mass existed.

For example the guy who tried to modify or extend gravitational theories to account for it.

Maybe this was a fringe idea or has lost momentum.

Re: The Reason We Haven’t Directly Detected Dark Matter

#249

Post by pas »

saagarjha wrote: post #236
mikekchar wrote: post #75

Earlier quoted context omitted.

But even then, there's no reason to assume that there is any in that space. For example, imagine that all the dark matter was in a disk around the solar system, way out farther than the asteroid belt. If you could see through it, you would never know. Or maybe it rings galaxies. Or maybe it hangs out in clumps between galaxies. Or... It literally could be anything because we can only measure it on the scale of mind b…

I believe it’s generally believed that dark matter clumps near galaxies because it interacts through gravity. So would it be reasonable to say that it should clump together near massive objects in the solar system?

DM usually forms a halo as it orbits the center of mass of whatever it orbits (galaxy, globular cluster), and because it doesn't interact otherwise, it cannot slow down, cannot shed momentum, so it is likely mostly not a disk, but a big sphere, and probably a shell around galaxies.

Re: The Reason We Haven’t Directly Detected Dark Matter

#250

Post by D-Coder »

gbrown wrote: post #86
krapp wrote: post #56

I like how we have an article explaining the rationale behind dark matter and the scientific principles that lead to the conclusion that it exists, but most of the threads here are simple declarations that no, they must all simply be wrong or not considering some simple alternative. Can people maybe, just maybe not assume that scientists are idiots or, as once dead commenter puts it, charlatans? If you disagree with…

People somehow feel uniquely qualified to speculate wildly about theoretical physics. It's bizarre in a forum full of technical-minded people.

We're tired of debating double-linked lists versus hashed sets.
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