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 Originally Posted by OngBonga
We see expansion in every direction. But we're looking at the past, so what we're observing is past expansion, not present expansion.
This is one reason why we know how much the universe was expanding in the past compared to now.
This is one piece of evidence that the expansion is accelerating.
We're looking into the past, but the closest things to see (which are, admittedly, pretty far out there) are still relatively not that far in the past compared to the CMBR. So we have a long record of what the expansion was over a wide span of time.
Additionally, we've been watching the sky with decent instruments for a while now. We compare the past images to the present images with spectroscopy and we can see the red shift has changed.
Originally Posted by OngBonga
In a 3D model, sure. But this is 4-dimensional expansion. We're talking about spacetime, not space. The same acceleration in all directions is because we're always looking in the same direction... the past.
If we're seeing expansion at the very edge of the observable universe, then we're witnessing the expansion of the early universe, not the now universe. We know that was insane expansion. I mean, if we're seeing an acceleration of expansion, then surely what's actually happening is expansion is slowing down, because we're witnessing faster acceleration in the distant past to what we observe locally.
I'm just confusing myself here. I'm sure that made sense before I read it back.
We're looking into the past, but it's still moving in chronological order.
The scientists working on this know the GR much better than you and I. I'm certain the many measurements of the Hubble constant and the cosmological constant haven't all accidentally made some mistakes that end up all getting the sign (+/-) of their estimate wrong.
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