Night skies in a contracting universe, would they look blue?

[Gerinski]

We live in an expanding universe (at least in our epoch) so radiating sources in the cosmos look redshifted, the more the farther they are from us, with the limit being the CMBR which is redshifted to 3ºK when it was actually around 3000ºK when emitted.

Were we to live in a contracting universe, the cosmological sources would look blueshifted, with the limit being the CMBR with the highest blueshift. Does that mean the night skies would look blue, or even ultraviolet or even more, actually being deadly radiation such as X or gamma rays?

And in a related note, what would have to be the expansion or contraction rate at which the red / blueshifting would be actually zero, so we would see every cosmic source with its true frequency and the CMBR (the night skies) would look glowing with their actual 3000ºK colour as when it was emitted?

 

[Chalnoth]

If the universe had been contracting all along, then there wouldn't be any CMB at all: the CMB was emitted when the plasma in the early universe cooled to become a transparent gas.

The only way contraction makes sense is if it was a recent thing. The universe would still have been expanding for most of its history. So the CMB would still be cold, and very far-away objects would still have high redshifts. It would only be the relatively nearby objects which emitted their light since the contraction began that would be blueshifted. Exactly how that blueshift changes with distance would depend upon how quickly the contraction accelerated.

In the far future, the CMB would become hot again, which would destroy all life everywhere.

 

[Gerinski]

Thanks. Yes of course I meant that such hypothetical contraction would have to have started at an age of the universe so that Earth could have still have formed to its present state, but I presume that Earth could have still formed even if the start of contraction happened a few billion years ago (just guessing).

 

[Orodruin]

3ºK

First off, there is no unit which is "degrees Kelvin", the unit is "Kelvin".

Were we to live in a contracting universe, the cosmological sources would look blueshifted, with the limit being the CMBR with the highest blueshift. Does that mean the night skies would look blue, or even ultraviolet or even more, actually being deadly radiation such as X or gamma rays?

There would be no CMBR. The CMBR originated in a time when the universe was hot and dense. If it contracted to a habitable state, it would not have been hot and dense earlier.

And in a related note, what would have to be the expansion or contraction rate at which the red / blueshifting would be actually zero, so we would see every cosmic source with its true frequency and the CMBR (the night skies) would look glowing with their actual 3000ºK colour as when it was emitted?

If you are talking about a universe that first expands and then contracts, the shift is not dependent on the expansion rate directly. It is proportional to the ratio of the scale factor of the universe now and the scale factor when the light was emitted.

 

[Gerinski]

If you are talking about a universe that first expands and then contracts, the shift is not dependent on the expansion rate directly. It is proportional to the ratio of the scale factor of the universe now and the scale factor when the light was emitted.

Thanks Orodruin. I'm afraid I do not understand fully. Do you mean that as long as the scale factor between present and the CMBR emission time was >0, we would not observe any CMBR blueshifting? So that in practice blueshifting of the CMBR would not be observed until the size of the contracting universe would be the same or less as when the CMBR was emitted? Sorry not sure if that's what you meant, thank you in advance for clarifying.

 

[Orodruin]

I'm afraid I do not understand fully. Do you mean that as long as the scale factor between present and the CMBR emission time was >0, we would not observe any CMBR blueshifting?

> 1. Otherwise correct.

So that in practice blueshifting of the CMBR would not be observed until the size of the contracting universe would be the same or less as when the CMBR was emitted?

Right.

 

[Chronos]

If the universe had contracted in the past, only objects nearer than the turnaround age would be blueshifted. The really far away stuff would remain redshifted.

 

[Chalnoth]

If the universe had contracted in the past, only objects nearer than the turnaround age would be blueshifted. The really far away stuff would remain redshifted.

A bit further than that: nearer than when the scale factor was the same as it is now.

 

[Chronos]

I assume we agree the universe would look very different than it does.