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Comment by oarla | Hacker News Reader

Not a physicist, but I recall reading that there is enough empirical evidence to support the current model of expanding universe.

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OK. But if that evidence were adequate to exclude this new hypothesis, then one could reject it. And TFA says that one can't. That implies that existing empirical evidence is just as consistent with the new hypothesis as it is with the accepted one.

I always wonder about that. I remember very specifically learning 40(?) years ago that the expansion was slowing, supposedly proven by some sort of red shift in light. That is now out of favor, but a few more new facts just might swing things the other way again.

> That is now out of favor

It's not "out of favor" just as a matter of fashion: a lot more data was collected, extending much further away and therefore much further into the past.

> a few more new facts just might swing things the other way again.

No, it would take a lot more than that, because our current model of accelerating expansion for the past few billion years (note that before that, the expansion was decelerating) is based on a lot more data than we had 40 or so years ago.

Accelerating universe is supported by supernova data. We have incomparably more supernova data compared to the past. Discovery of accelerating universe was mainly due to radically scaling up supernova discovery process.

Yes, but that’s still dependent on a differential velocity model for the origin of redshift. This paper changes that core assumption, meaning the same data can be interpreted to have a radically different meaning.

Afaik, we have no direct observations of galaxies moving away from us - over such enormous distances we have not had enough time since the start of observations to actually see a galaxy recede.

No, the origin of the cosmological redshift is not proposed to be due to differences in velocity, but metric expansion.

... which causes a change in relative velocities

That makes it sound like cosmological redshift is a form of Doppler shift. In reality, the target object emits light towards us with little Doppler shift as it will have experienced little acceleration in its reference frame. The light becomes redshifted as it moves through the universe while the universe is expanding, effectively stretching out the wavelength of the light.

The change in wavelength is exactly proportional to the increase in size of the universe during the time of flight, and it is not directly related to speed of recession except in common and inaccurate discussion. The link to relative velocity (if that concept makes sense for objects in widely separated and unrelated reference frames) is model dependent, while the metric expansion measurement is directly measured.

Ok. I was under the impression that expansion had no effect on local phenomena because local phenomena always counteract expansion. But if you view light more as a wave instead of a particle then could see the argument that the light is locally stretched if there is no force keeping it from being stretched e.g. gravity or energy.

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