- #36
ohannuks
- 32
- 2
Gravity ;)Chronos said:How would you propose to detect tachyonz?
Imho, I'm not confident enough in particle physics to give an intelligent answer to this.
Gravity ;)Chronos said:How would you propose to detect tachyonz?
timmdeeg said:Their calculation is based on the assumption that the expansion occurs "at all scales", thus obviously including the solar system.
timmdeeg said:They don't mention tidal forces at all and presumably their assumption is not related to tidal forces
timmdeeg said:On the other side they mention "external forced"; what is the meaning, if not tidal forces.
Ok, if they say expansion, their have tidal forces in their mind. It's not really important, but remembering "... even if the expansion does actually occur at all scales, we will show ..." they seem to believe that there is no clear theoretical foundation to assume the existence of tidal forces (thereby neglecting the dark energy), otherwise "even if" makes no sense.PeterDonis said:Not really. Their calculation assumes that the effect of the expansion would have to show up as "external forces" (see below).
Hmm, and also induced by the dark energy, right? Otherwise the universe wouldn't expand accelerated.PeterDonis said:The "external forces" are tidal forces. Basically they are trying to compute the tidal forces induced by the rest of the matter in the universe on the solar system.
timmdeeg said:and also induced by the dark energy, right?
timmdeeg said:on this scale the tidal forces should be computed with respect to the effect of the dark energy only, correct?
In this case it seems, they talk expansion (effect of accelerated expansion) but calculate deceleration.PeterDonis said:I don't think their calculation includes dark energy; they seem to be using the matter-dominated FRW model. Including dark energy does change the tidal forces, yes.
I think there are 2 scenarios only , if dark energy is not included.PeterDonis said:Once again, it depends on what assumptions you make. Cooperstock et al. appear to be making the assumption that the individual objects in the solar system are not moving on "comoving" worldlines, which means they are not assuming that the matter in the universe is exactly homogeneous and isotropic--if it were, every single piece of matter everywhere in the universe, on all scales, would be moving on a "comoving" worldline. If that were true, we would be able to see tidal forces on any scale due to all of the matter and energy in the universe, whether it was ordinary matter, dark matter, radiation, or dark energy.
In our actual universe, individual pieces of matter do not move on comoving worldlines; the "comoving" worldlines only describe the average motion of the matter in the universe on very large scales (hundreds of millions to billions of light years). So on much smaller scales, we would not expect tidal forces due to the matter to be significant (and Cooperstock et al.'s calculations appear to show that). But the density of dark energy is constant everywhere, so effectively, dark energy moves on "comoving" worldlines everywhere, and tidal forces due to dark energy should be present on all distance scales. (But on the scale of the solar system, they are still very small.) However, as above, I don't think Cooperstock et al. included dark energy in their calculations.
timmdeeg said:they talk expansion (effect of accelerated expansion) but calculate deceleration
timmdeeg said:Cooperstock use scenario 1. eventually?
This would make sense, in contrast to scenario 2. (universe structured) in my opinion.PeterDonis said:Not explicitly, but I think that's what their analysis amounts to. They are basically trying to see what "comoving" worldlines would look like in local coordinates centered on the worldline of the solar system's center of mass.