Dark energy

[GaryJohnson]

Neutrinos are matter and interact very much like dark matter does. The two are not so dissimilar.

My real point there was that even if someone doesn't think there is something identifiable as matter out there in the regions where dark matter is thought to exist, that pocket of unidentifiale nothingness is still what is being called dark matter.

 

[Locut0s]

1. OP: No, dark matter does not interact with EM radiation. Also, dark energy has an (apparently) constant density throughout the universe, while dark matter clumps up gravitationally.

2. Gravitational wave observatories like LIGO won't detect dark matter. They look for strong waves from supernovae and the rapidly orbiting binary systems like SphinxnihpS says.

3. Dark matter isn't just made up BS. It can be measured. We know it's location, the total mass, its effects, just not what it is. We don't fully understand all of the inner workings of the sun, but we know it's gonna rise in the morning, that it's hot, and that it gives us light.

4. You can measure the mass of a black hole using kinematics. This is how they measure the mass of stars and planets too, not by knowing their size.

5. Gravitational wave detectors don't pick anything up right now because they have to be extremely sensitive. They measure contractions of the length of the arms of the apparatus, something which can change by many more orders of magnitude by background vibrations.

6. Yes, there most likely is dark matter in and around your body right now. It is fairly smoothly distributed within galaxies. As we are in a galaxy, we are traveling through the Milky Way's dark matter halo as we speak.

What about the possibility that general relativity needs to be reformulated at large distance scales to account for dark matter observations. This doesn't seem to be a very popular theory but it's one that could potentially explain both dark matter and dark energy. Of course this opens a whole different can of works, which is why guess it's not that popular.

 

[silverpig]

What about the possibility that general relativity needs to be reformulated at large distance scales to account for dark matter observations. This doesn't seem to be a very popular theory but it's one that could potentially explain both dark matter and dark energy. Of course this opens a whole different can of works, which is why guess it's not that popular.

Eh, that's a bit of fitting the theory arbitrarily. It's easy to modify any theory by adding in a few generic terms, giving them coefficients, and then doing a fit. The thing is it doesn't tell you anything about the physics.

With GR you say: "The stress energy tensor curves spacetime in such a way: <x>. This curvature than has the effect: <y>."

That's great. You can do something with it. You understand where this curvature comes from and why it looks like it does.

It's easy enough to then modify it and say: "For all distances greater than z, divide the force by 2."

Okay, that's simple, but it doesn't tell you WHY you divide by two, or WHY gravity falls off at a certain distance. It's just some arbitrary addition.

 

[Locut0s]

Eh, that's a bit of fitting the theory arbitrarily. It's easy to modify any theory by adding in a few generic terms, giving them coefficients, and then doing a fit. The thing is it doesn't tell you anything about the physics.

With GR you say: "The stress energy tensor curves spacetime in such a way: <x>. This curvature than has the effect: <y>."

That's great. You can do something with it. You understand where this curvature comes from and why it looks like it does.

It's easy enough to then modify it and say: "For all distances greater than z, divide the force by 2."

Okay, that's simple, but it doesn't tell you WHY you divide by two, or WHY gravity falls off at a certain distance. It's just some arbitrary addition.

Yeah my bad, I wasn't actually talking about a kludge but the fact that this may point to the fact that General Relativity may not be correct at large distance scales and that we therefore need to seek an alternative superseding theory.

Most of the dark energy and dark matter candidates assume that General Relativity holds up at large distance scales and hence postulate things like negative energy WIMPS and MACHOS. While these would seem to be the best explanations assuming that GR continues to hold as these scales I think it's just as likely that they are indications that perhaps GR isn't the whole picture. In the same way that pertubations in the orbit of mercury are explained by General Relativity superseding Newtonian gravity, as opposed to say a mystery planet we haven't discovered.

I know some scientists have postulated this as well but it doesn't seem to be all that popular. I guess the main issue is where the fuck do you start if you go down this road.