Recent content by nutgeb

I'll let Ich provide a longer answer, I don't have time right now. The 'asymptotically flat' condition required by Birkhoff's means 'spatially flat', not flat spacetime. Our real gravitating universe is considered to be vanishingly close to spatially flat because it is at critical density...

I agree that a complete cosmological model can't be calculated using Birkhoff's. I was suggesting it as a way to take a snapshot (a constant time slice) of a spatial subset of a cosmological model in order to test certain characteristics. What I'm specifically interested in is applying...

The contraction is the same kind of Lorentz contraction that applies in SR. It varies at a hyperbolic rate depending on recession velocity. At recession velocities well below c, the contraction is negligible. But in Minkowski coordinates, as the recession velocity approaches c, the Lorentz...

A couple of additional thoughts about the physical and nonphysical interpretations I suggested for why photons move faster than c in FRW proper coordinates: First, I have to acknowledge that the linear frame dragging idea requires a healthy dose of bootstrapping. It offers a potential answer...

Agreed, but my comments were specifically about the kinematic paradigm, not the 'expanding space' paradigm. The Milne model provides a satisfactory description of a universe without gravity, using Minkowski coordinates and GR. The advantage of FRW coordinates is that they provide a more...

Nobody knows the answer to your question. As I've expained in other posts, both the 'expanding space' paradigm and the 'kinematic' paradigm yield precisely equal mathematical calculations of what the observations would be (such as redshift). And if the galaxy-filled universe is infinite, then...

That's the intuitive answer but it's actually not correct. It turns out that the lightcone is curved in FRW coordinates. For a photon emitted from a distance beyond the Hubble Radius, initially the photon's proper distance from the observer will increase (due to the superluminal recession...

Well first, you have to specify what part of the rope you want to be at rest in its local comoving Hubble flow. If the center point of the rope is at rest in its local Hubble flow, then if the rope remains intact the two ends will respectively have .5c and -.5c velocities in their local...

I agree with you with respect to most coordinate systems, but in the particular case of FRW proper distance coordinates you are dividing change in proper distance (an invariant) by change in proper time (another invariant) to obtain proper velocity. So it seems to me that an invariant divided...

I think modeling the Milne case is a good idea. But it seems like a lot of steps would be required, so the analysis would be convoluted. One might start with Minkowski recession velocities and chart the rope end's velocity increase (acceleration) as a function of Minkowski time. Then transfer...

Well I think that's just casual terminology on my part. Technically, "proper time" is called the "timelike spacetime interval" and "proper distance" is the "spacelike spacetime interval." All freefalling observers will agree on the value of these quantities, regardless of their coordinate...

brainstorm, you're just going to confuse yourself if you try to pose your question as a GR cosmology question and an SR inertial frame question at the same time. You have to ask the question one way or the other. Part of walking before you run is being reasonably specific in how you frame your...

If the .9c relative velocity of the galaxy and Earth in the example is a comoving recession velocity, then the posts in this thread are way off track. If you assume for the purposes of the example that the Earth and the other galaxy are comoving, and the relative recession velocity is .9c...

This is not the thread to discuss the point in detail, but, no, although the proper gravitational acceleration experienced by a freefalling observer is finite at the event horizon, for a hovering observer at the event horizon it is infinite. See http://www.mathpages.com/rr/s7-03/7-03.htm" ...

That depends on the coordinate system you use. In the Minkowski coordinates normally used for SR analysis, the relativistic velocity addition formula applies. But in FRW coordinates, it does not apply to comoving recession velocities. For example, Hubble velocities are simply added straight...