Posted: Mar 25, 2017 12:43 am
by crank
newolder wrote:I haven't heard of any problems with energy conservation before. The stretching of photon wavelengths seems (to me at least) would cost energy. Perhaps that supplies the apparent energy loss. :dunno: I'll have a rootle about and/or wait for a more informed reply.

ETA Physics Stack Exchange has this: ... -conserved

The linked paper starts:
A common belief about big-bang cosmology is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must be viewed in terms of the stretching of space. We argue that, contrary to this view, the most natural interpretation of the redshift is as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that it is always valid to choose a coordinate system that is locally Minkowskian. We show that an observed frequency shift in any spacetime can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a suitable family of observers along the photon's path. In the context of the expanding universe the kinematic interpretation corresponds to a family of comoving observers and hence is more natural.

arxiv link
So my explanation is duff. Heigh ho. Learned something new today. :thumbup:

The paper may be a great explanation, but it doesn't change a basic fact, a red shift is about the relative velocities of the source at emittance and receiver at reception, the separation/travel time is irrelevant. The cosmic red shift doesn't much care about relative velocities, they will generally be fairly insignificant, the shift is actually determined by travel time/separation. That seems kinda fundamental to me, even if the actual physics involved allows for a different view point. In other words, there's a categorical difference that the physics is blind to. Or maybe I'n saying it wrong, or I'm full of shit. :scratch: