Do we really need Dark Energy?

[Bruce Wilson]

If you measure the red-shift of a star 10 million light years away, you measure the velocity of the star 10 million years ago when the stars were traveling faster (if we are willing to accept that the expansion of the universe is slowing down). This not such an outrageous idea since firstly it satisfies the conservation of energy principle and secondly we already accept that the planets and moons slow down with time. The slowing down of the expansion of the universe may simply be the sum of the slowing down of all the celestial bodies.
Hence the existing red-shift data does not automatically prove that expansion of the universe is accelerating and there is therefore no need for Dark Energy. Any thoughts?

 

[PeterDonis]

If you measure the red-shift of a star 10 million light years away, you measure the velocity of the star 10 million years ago

No, you don't. What the cosmological redshift actually measures is how much the universe has expanded since the light was emitted. The "velocity" that is often quoted as being associated with the redshift is just a calculated number used for the convenience of cosmologists, because they happen to prefer velocity units to redshift units. It doesn't have the physical meaning that an ordinary velocity would have.

Hence the existing red-shift data does not automatically prove that expansion of the universe is accelerating

Yes, it does, because we can look at the pattern of redshifts over all galaxies, from small redshifts to large redshifts, and work out a curve for how the universe expanded over time. That curve shows decelerating expansion until a few billion years ago, and accelerating expansion since then.

 

[Chalnoth]

The thing is, redshift-distance information is only part of the evidence.

The CMB, for example, provides extremely accurate estimates of the normal and dark matter content of the universe, as well as the overall geometric shape*. The shape is essentially flat, but the matter content is much too small to make it flat. So we need something else to make up the balance.

There's also an amplification of the CMB power spectrum at very large scales which is caused by the Integrated Sachs-Wolfe effect:
https://en.wikipedia.org/wiki/Sachs–Wolfe_effect#Integrated_Sachs.E2.80.93Wolfe_effect

The ISW effect is the most direct evidence of dark energy.

* Technically, we also need a measure of the current expansion rate to glean this information from the CMB. So another way of stating this is that the current expansion rate is much too fast (about twice as fast) as we would have with the same amount of matter but no dark energy, given the CMB data.

 

[Bruce Wilson]

Thanks everybody for your help. It has been very useful. I will get back to you when I have done some more thinking.