The discussion revolves around the phenomenon of distant celestial objects appearing to move away from us faster than the speed of light, as indicated by their redshift. Participants explore the implications of this observation in the context of the universe's shape, expansion, and the interpretation of Hubble's law.
Participants express differing views on the interpretation of redshift and its implications for understanding cosmic expansion. There is no consensus on the implications of superluminal velocities or the shape of the universe.
There are limitations in the discussion regarding the definitions of key terms and the assumptions underlying the measurements of redshift and distance. The complexity of General Relativity and the frame-dependence of simultaneity are also noted but remain unresolved.
Sparer said:I heard somewhere that the amount of redshift in the most distant stars show that they are moving away faster than light and its due to the shape of the universe. I was wondering how do we even know what she shape of the universe is, I thought we couldn't see any boundaries.
abitslow said:?. They don't "seem" to be. You seem to be confusing what you heard, that or what you heard is wrong. We can't see (normal) stars at cosmological distances, they're too small/dim. We do see galaxies and supernovae. We are able to measure the redshift of the light we detect from them. IF you were to (incorrectly) plug these redshifts into the equation for Doppler shift, we would NOT get a velocity greater than the speed of light.
For instance given a redshift, z, of 8.6, if γ=v/c the doppler equation 1+z = √((1+γ)/(1-γ)) gives 97.85%. A redshift of 100 would give 99.98%. It IS reasonable to ASSUME that if we measure an object with a cosmological redshift of 8.6, which would mean that the light we are NOW measuring was emitted 13+ billion years ago, it IS reasonable to assume that that object is NOW beyond our ability to ever see again, and its distance is increasing at faster than the speed of light, c. In fact, I don't know of any theories of cosmology which would not accept that as being 'correct' ("true"). This is a difficult subject, especially since the words we use are often used in different ways. Distance, speed, velocity, even universe all have various DIFFERENT meanings depending on context. Most of the confusion arises in thinking that there is a single (intuitive) meaning of these concepts. For instance, most cosmologists accept the fact that the Universe is much larger than our Observable Universe, and many accept the assumption that it is infinite. The Observable Universe is what we can (or could or will ever) see. Speaking about what goes on outside of that region is more philosophy/religion than science (at least the way I define science).
I should note that a cosmological redshift of the 'most distant' object ever measured is 8.6, although I am not sure if that has been accepted by the consensus yet. We'll NEVER see something with a cosmological z of 100!
This quantity is called β. We have β = v/c, and γ = 1/√(1 - β2).abitslow said:if γ=v/c the doppler equation 1+z = √((1+γ)/(1-γ))
There is no meaningful way to define the word NOW in relativity, because which distant event happens at the same time as your present time is frame dependent. The oft-quoted example is that if you start walking toward the Andromeda Nebula, the definition of NOW for events there jumps forward by 4 days.it IS reasonable to assume that that object is NOW beyond our ability to ever see again, and its distance is increasing at faster than the speed of light, c. In fact, I don't know of any theories of cosmology which would not accept that as being 'correct' ("true"). This is a difficult subject, especially since the words we use are often used in different ways.
Wikipedia lists four objects with greater redshift than this, up to z = 11.9. The redshift of the CMB is estimated to be about 1000.I should note that a cosmological redshift of the 'most distant' object ever measured is 8.6, although I am not sure if that has been accepted by the consensus yet. We'll NEVER see something with a cosmological z of 100!