Posted: Mar 25, 2017 9:23 am
by newolder
crank wrote:
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:

Yes, it's obviously more complicated than my simple brain can cope with and I may easily misremember stuff and get ideas conflated. There's the Integrated Sachs-Wolfe effect and Sunyaev-Veldovitch effect that may have something to say on these things. I recall (vaguely) an explanation by Martin Rees (Astronomer Royal) about how a CMBR photon gains energy as it falls into a void but, because the void grows during photon passage, it loses more energy on the way out (it has to climb further out than it fell in). This leads to cold spots on the CMBR. Of course, this could all be irrelevant. :dunno: