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- Unobservable universe and CMBR
How can there be galaxies in the unobservable universe when The CMBR which precedes all galaxies is observable?
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 said: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).
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.PhanthomJay said: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?
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).PhanthomJay said: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?
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.PhanthomJay said:How can there be galaxies in the unobservable universe when The CMBR which precedes all galaxies is observable?
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.PhanthomJay said: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.PhanthomJay said:so that means the answer is YES?
To the extent that there's a meaning to "the exact same time", yes.PhanthomJay said: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
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.PhanthomJay said:because of spacetime expansion where the other one formed further apart in that expansion?
It's worth noting that there are galaxies that are receding faster than the speed of light, and yet we can observe them.Ibix said: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 said: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.
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 said:It's worth noting that there are galaxies that are receding faster than the speed of light, and yet we can observe them.
Or that, at this moment, the galaxy is between the Hubble sphere and the event horizon.Ibix said: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, seeIbix said: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).
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 said:To clarify about horizons and all that, see
https://arxiv.org/abs/astro-ph/0310808
https://doi.org/10.1071/AS03040
Oh yes, I did mean that they recede faster than the speed of light.vanhees71 said:This "moving away faster than the speed of light" is, of course, misleading.
The unobservable universe refers to the portion of the universe that is beyond our ability to observe or detect. This includes regions that are too far away for light to reach us, as well as regions that are hidden from view by other objects or phenomena.
The CMBR, or cosmic microwave background radiation, is the leftover radiation from the Big Bang that permeates the entire universe. It is the oldest light in the universe and is a key piece of evidence for the Big Bang theory.
The CMBR provides evidence for the Big Bang theory because it is a uniform, isotropic (the same in all directions) glow of radiation that is predicted by the theory. It also has a blackbody spectrum, which is a characteristic of radiation that has been traveling through the universe for billions of years.
The unobservable universe is important to study because it can provide insight into the early stages of the universe and help us understand the fundamental laws of physics. It can also give us a better understanding of the structure and evolution of the observable universe.
Scientists study the unobservable universe and the CMBR through various methods such as using telescopes and satellites to observe the CMBR, analyzing data from experiments like the Planck satellite, and using mathematical models and simulations to understand the behavior of the universe. They also look for indirect evidence, such as the effects of gravitational lensing, to study the unobservable universe.