Unobservable Universe and the CMBR

[PhanthomJay]

TL;DR

Unobservable universe and CMBR

How can there be galaxies in the unobservable universe when The CMBR which precedes all galaxies is observable?

 

[PeroK]

The CMBR we detect today was emitted about 13 billion years ago. That region of the universe has evolved for those 13 billion years and presumably is full of galaxies "today" (in comoving coordinates).

 

[PhanthomJay]

The CMBR we detect today was emitted about 13 billion years ago. That region of the universe has evolved for those 13 billion years and presumably is full of galaxies "today" (in comoving coordinates).

Thanks. So are those galaxies that are unobservable actually the same distant galaxies that we see today as they existed 13 billion years ago in their formative state, but now aged to their present mature state of 13 billion years old now?

 

[PeroK]

Thanks. So are those galaxies that are unobservable actually the same distant galaxies that we see today as they existed 13 billion years ago in their formative state, but now aged to their present mature state of 13 billion years old now?

The cosmological principle assumes that the universe is homogeneous and isotropic on the largest scale. In the appropriate "comoving" coordinates, therefore, it is the same everywhere today. Where to be precise, we emphasise that "today" means in the common comoving time coordinate.

The light from further away takes longer to reach Earth, so we see the Sun as it was 8 minutes ago, Alpha Centauri as it was 4 years ago, the Andromeda galaxy as it was 2 million years ago; distant galaxies as they were 13 billon years ago; and the CMBR emitted 13.7 billion years ago, approximately.

Then there are galaxies beyond that from which the light will eventually reach us. And, if the current model is correct, galaxies beyond that from which the light will never reach us.

 

[Ibix]

So are those galaxies that are unobservable actually the same distant galaxies that we see today as they existed 13 billion years ago in their formative state, but now aged to their present mature state of 13 billion years old now?

No, because those galaxies are closer to us than whatever has formed from the plasma that emitted the CMB photons now arriving at Earth. But I think you have the principle correct - the galaxies we see at a long distance from us appear very young because the light from them has taken so long to get here. We expect those young-looking galaxies to be more or less the same as our galaxies now. (You do need to watch for some subtleties around the definition of "now", which is why PeroK is alluding to comoving coordinates).

 

[PeterDonis]

How can there be galaxies in the unobservable universe when The CMBR which precedes all galaxies is observable?

We have only seen part of the CMBR, the part that was emitted from within our observable universe. We haven't seen the part of the CMBR that is outside our observable universe any more than we have seen galaxies there.

 

[vanhees71]

Have a look at this very illuminating paper concerning the various horizons in cosmology:

https://arxiv.org/abs/astro-ph/0310808

 

[PhanthomJay]

Thank you all for your responses. Without doubt, you are the VERY best in your field.

 

[PhanthomJay]

Ok another question to further my understanding.

Is the unobservable universe that part of the universe that developed from the hot plasma within the first 380,000 years after the Big Bang?

 

[Vanadium 50]

That describes our neck of the universe.

 

[PhanthomJay]

so that means the answer is YES?

 

[jbriggs444]

so that means the answer is YES?

No. That means that the answer is "no". The point being made is that our portion of the universe developed from the hot plasma that existed approximately 380,000 years after the big bang.

Similarly, portions of the universe that are unobservable developed from the hot plasma that existed approximately 380,000 years after the bit bang.

Whether some portion of the universe developed from the hot plasma has absolutely nothing to do with whether it is observable (from here).

 

[Ibix]

so that means the answer is YES?

No. All of the universe evolved from that plasma. The unobservable universe is the part of the universe that is so far away that we can't yet see it or will never see it.

We can, in principle, see things that happened before the universe became transparent by looking at gravitational waves.

 

[PhanthomJay]

Oh so it seems you are saying that two galaxies may both have formed at the exact same time say 12 billion years ago, but one is observable and the other is not because of spacetime expansion where the other one formed further apart in that expansion?

 

[Ibix]

Oh so it seems you are saying that two galaxies may both have formed at the exact same time say 12 billion years ago, but one is observable and the other is not

To the extent that there's a meaning to "the exact same time", yes.

because of spacetime expansion where the other one formed further apart in that expansion?

Sort of. Even in the absence of expansion, there's only a finite distance we can see in a universe with a beginning because there's only been so much time for light to travel. But in an expanding universe there can be some parts that we can never see even if we wait forever, because the expansion can make the distance light has to travel to reach us grow faster than the light can close the distance.

 

[Jaime Rudas]

But in an expanding universe there can be some parts that we can never see even if we wait forever, because the expansion can make the distance light has to travel to reach us grow faster than the light can close the distance.

It's worth noting that there are galaxies that are receding faster than the speed of light, and yet we can observe them.

 

[PhanthomJay]

To the extent that there's a meaning to "the exact same time", yes.

Sort of. Even in the absence of expansion, there's only a finite distance we can see in a universe with a beginning because there's only been so much time for light to travel. But in an expanding universe there can be some parts that we can never see even if we wait forever, because the expansion can make the distance light has to travel to reach us grow faster than the light can close the distance.

Got it, thanks!

 

[Ibix]

It's worth noting that there are galaxies that are receding faster than the speed of light, and yet we can observe them.

Indeed. But unless I've got my cosmological horizons confused again we will never see them looking 14 billion years old, even if we wait for all eternity (unless the universe is closed and collapses).

 

[Jaime Rudas]

Indeed. But unless I've got my cosmological horizons confused again we will never see them looking 14 billion years old, even if we wait for all eternity (unless the universe is closed and collapses).

Or that, at this moment, the galaxy is between the Hubble sphere and the event horizon.

 

[vanhees71]

Indeed. But unless I've got my cosmological horizons confused again we will never see them looking 14 billion years old, even if we wait for all eternity (unless the universe is closed and collapses).

To clarify about horizons and all that, see

https://arxiv.org/abs/astro-ph/0310808
https://doi.org/10.1071/AS03040

 

[Jaime Rudas]

To clarify about horizons and all that, see

https://arxiv.org/abs/astro-ph/0310808
https://doi.org/10.1071/AS03040

There it's explained why the galaxies that are currently between the Hubble sphere (14.4 Gly) and the event horizon (16 Gly) are moving away faster than the speed of light and that we will be able to see them in the future as they are today.

 

[vanhees71]

This "moving away faster than the speed of light" is, of course, misleading.

 

[Jaime Rudas]

This "moving away faster than the speed of light" is, of course, misleading.

Oh yes, I did mean that they recede faster than the speed of light.