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Let's pretend that you have a basket with 100 apples. You know apples are about 100g each, because you weighed 10 of the them and all of the apples seem about the same size. You know that basket weighs 1000g. You put the whole thing on a scale and find it weighs 500,000g. You know something else is in that basket. You aren't sure what, and frankly it doesn't make sense, but trying different scales and remeasuring more individual apples gives the same result. So you decide that there must be something you can't see but must exist. That's dark matter/energy.
That's dark matter/energy.
No... it's dark apples
The worst pie - Dark Apple Pie. QED
The scale probably just can't measure the apples all together that way. Maybe it's not calibrated to see all the different ways apples can interact. Maybe time to go back to the scale drawing board.
That's that funny thing, they've tried different scales. They've tried radically different ways of measuring it, and always come up with the same discrepancy.
If summing energy works differently on a large scale, why? Since we don't know what we can do is start measuring the difference between observable energy and the "extra" that appears when we add it up. We could call that "unobservable energy" so we can see if there is a pattern, or if it's actually something else. You know "unobservable energy" is a mouthful, why not just call it dark energy?
We don't know what it is. We have tested lots of theories and dark energy doesn't seem to fit any answer, hence the name. I get thinking that it can't be that hard to reconcile and scientists must be missing an obvious conclusion, but it's likely that your theory has already been tested. Maybe you have the solution and can resolve the discrepancy, but right now all data shows that dark energy is a large part of the universe.
I'm not sure what you're getting at. Dark matter has been proven numerous times, is a predictive model, and is the only explanation that has held up to scrutiny and observations. It's very clearly the right explanation and we know how dark matter generally behaves, we just don't know specifically what it is.
See, for example, the behavior of the bullet cluster merger.
Sorry, but...aren't modified gravity theories gaining some more traction recently? Not enough to say that modified gravity is the most likely explanation for observations, but at least enough to avoid saying that dark matter is "clearly the right explanation"?
edit: I've just realised that some people would describe modified gravity as a specific theory to describe the observational effect of dark matter. Is that what you were doing here?
Maybe it's all because of tiny strings.
Guys
Listen
It's the strings
Ok this gave me a laugh.
But to be serious, isn’t the opposite true? Like, my understanding is that string theory is basically dead, and only getting deader. But I thought modified gravity as explanations for the dark matter observations is seeing a bit of a resurgence lately.
my understanding is that string theory is basically dead, and only getting deader.
Huh... where is this impression from? String theory isn't dead, it's just a very narrow field in which most of the participants specialize in a subset of it that's less concerned with completing it as a whole. It's incredibly difficult work, progress is slow, and it's currently too broad to be applicable to reality (which is important for funding). The tests we can think of to validate the correspondence of math to the physical world are... significantly out of reach due to the energy requirements.
But it's still the leading theory of quantum gravity and there's active work in, say, AdS/CFT correspondence - which shows that string theory can line up to reality and be predictive. It's the best idea we have right now, it's satisfyingly elegant, and it's working as a useful tool at the very least.
There are competing alternatives that get their own research, of course. We should persue them all until a clear victor emerges!
But I thought modified gravity as explanations for the dark matter observations is seeing a bit of a resurgence lately.
Modified gravity, so far, is non-predictive and does not account for things like the bullet merger while also accounting for ultra diffuse galaxies and our observations of the CMBR. All proposed modified gravities have failed to pass experimentation compared to general relativity. Modified Newtonian Dynamics (MOND) fails in the face of light and gravity having the same speed. And even if MOND were to be true, it still requires the presence of (albeit possible baryonic) dark matter to be even considered due to existing mass measurements of galaxies.
So, again, dark matter is simply the best model we have.
I probably overstated the case, but I was mainly going on what this video said about string theory. It’s had no reliable test results, no predictive power. It’s a useful mathematical model, but not actually a good theory to explain the real world.
I don’t even know where I got that idea about modified gravity from. I think I vaguely saw a few headlines about it recently but I didn’t even bother to read the articles they came from, and I somehow allowed that to stick in my brain. My bad.
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Sorry for not responding earlier, I don't seem to be getting notifications! My other reply further down in the thread hopefully answers all of your (wonderful) questions, though. Have a great day!
Yeah no worries. I’ve had the same problem with notifications sometimes not appearing. No idea why. Thanks for the feedback though!
I was working on a machine last week that's supposed to turn on when it's plugged into a battery. And it did that, but the battery it was plugged into was faulty, so we changed it. And then it wouldn't turn on when plugged in. We tried it with several different batteries, and it would only turn on when plugged into that first one. We couldn't figure it out.
The next day, I came into work and told my coworker that battery is magic. Because it was the only explanation that could accurately predict the results we saw.
Then the battery stopped being magic.
You have a fundamental misunderstanding of physics if you think that analogy is even remotely similar to dark matter.
Let’s say I construct an apple orchard with harvesters that run on batteries. The batteries send power to the machines when they are inserted into them, and the harvesters use the battery power to harvest 100g apples. If I weigh the battery before putting it in a machine, and the battery weighs 1,000g, would that be fucked up, or what?
Look, all I know is that when I bake an apple pie with those apples, my family is happy and no longer hungry, and isn't that what dark matter is all about? The things that matter.
I'm happy to talk about this more, but I'm afraid I don't understand your analogy. I'm sorry! If you'd like to rephrase it, I'll make myself available to respond. 🙂
Why isn't it called "cold matter"?
Don't we just not see it because it's not burning?
Couldn't the unseen mass be clouds, planets and black wholes?
As far as we're aware, dark matter only interacts with the universe gravitationally. It doesn't even interact with itself, which is why we don't see dark planets/stars/galaxies popping into existence. It only follows normal matter around.
As for why it's not called cold, is for two reasons:
- Cold gases of normal matter can condense to form stars. Dark matter doesn't interact with itself, which implies it cannot condense into more concentrated forms of itself the way a gas cloud can eventually form a star.
- We just don't know what the stuff is, it could be clouds, planets, black holes, neutron stars, brown dwarfs, etc. But our best observations of dark matter are from very large distances away where we can measure the distortion of spacetime due to dark matter. We can't see these smaller objects at these distances. But we should be able to see other clues that would indicate it's normal matter.
If it happened to be clouds of gas and dust that overall had a net gravitational effect on the background galaxies, we'd be able to detect the spectral lines of these clouds. Same for just about all the other objects in that list. In some cases we do detect intergalactic gas clouds. But in places where there's very clearly unaccounted for gravitational lensing, there isn't any sign of this. So far the only things we can match up to the observations is a mathematical model of the stuff.
That makes sense. Thanks
No, because those would also interact with other things like electromagnetic radiation, light, etc. Dark matter only interacts with normal matter via gravity.
Dark matter is admittedly a bit of a misnomer but that's what everyone's been using for years 🤷🏻♂️