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If the first stars in the universe were formed 400 million years after the big bang, and the universe is expanding, should that stars at that time not been in our cosmic backyard? And should their light not have reached earth much sooner?

user17081
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  • I think it's important to note that the universe is likely infinite and always has been. – Asher Jun 04 '17 at 01:41
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    @Asher : That's a sweeping statement which I'd describe as contentious at best and just wrong at worst. It greatly depends on which cosmological model you employ and precisely what you mean by "infinite". – StephenG - Help Ukraine Jun 04 '17 at 02:22
  • Also it has nothing to do with the question. – pela Jun 04 '17 at 07:09
  • We actually have more evidence to suggest the universe is infinite than that it isn't. And given that the question seems to assume that everything in the universe started near us and only became distant through expansion, I think it's important to point out that some things started far away from us even near the beginning. – Asher Jun 04 '17 at 08:24
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    @Asher : we don't have any evidence to suggest the universe is infinite. Whilst people say the universe is flat therefore it must be infinite, it's a non-sequitur. IMHO the expanding universe is evidence that it isn't infinite. Noting pela's remark, why don't you ask a question on this? – John Duffield Jun 04 '17 at 12:30
  • @RobJeffries no such criticism from you on pela's answer though, which makes the same assertion. Excuse me if I feel a bit picked on and wonder why. – Asher Jun 04 '17 at 17:17
  • @JohnDuffield the universe is apparently isotropic and homogenous. We can either assume that we just happen to be in the center of a finite universe, or assume that it is in fact the same everywhere, which would imply a spatially infinite or at least unbounded universe. And you must introduce an additional, unevidenced boundary set as an assumption to make it finite. Making the least and simplest assumptions gives us an infinite universe. – Asher Jun 04 '17 at 17:23
  • @asher I see what you mean. But my statement is correct and Pela is specifically not referring to things we can see now. Neither are you, so I withdraw my comment. – ProfRob Jun 04 '17 at 19:10
  • Define "cosmic back yard". How many light years in a CBY? How big was the Milky way 400 million years after the big bang? What did the Milky way look like? How close was Andromeda at that time? Certainly the distant galaxies were closer at that time and some of the light from early stars in distant galaxies reached us long ago, depending on how far and how early. I think the question needs better specifics. – userLTK Jun 05 '17 at 08:17
  • @Asher : we shouldn't assume that we're at the center of a finite universe, nor should be assume that it's the same everywhere. Would you do that if you were in some forest and all you could see was trees? No. Nor should you assume your forest is infinite because of some "least and simplest assumptions". – John Duffield Jun 06 '17 at 18:21

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Yes, and indeed it did. For some stars.

Some first stars were close to us, some were far away. The light from the ones that were very far away has yet to reach us, while the light from the ones that were close to us, reached us in the past; if we look today at their location, we no longer see first stars, but instead see evolved galaxies.

On the other hand, the light from the ones that happened to lie at the particular distance of roughly 2.7 billion lightyears (Gly) when the Universe was 400 Myr old reaches us today. An example of this is the galaxy GN-z11.

"2.7 Gly?", you say, "Shouldn't that light have reached us after 2.7 Gyr, then?"

No, because the Universe expands, it takes longer. In fact at that time, the part of the Universe that was at the distance of GN-z11 expanded at four times the speed of light. That means that even though light traveled in our direction (locally at the speed of light), the distance from us$^\dagger$ to the photons increased. Eventually, however, as the photons reached regions that expanded less rapidly, they "overtook" the expansion of the Universe, and slowly started decreasing their distance to us.

$^\dagger$The hypothetical "us"; our Sun wasn't even born at that time, and probably the Milky Way was just a small density fluctuation in the background density.

pela
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The very first stars were probably, massive, ultraluminous, and very short-lived. So if they were formed 400 million years after the big bang, then they would have ceased to exist only a few million years later.

Given that, if we wish to observe the first stars, then we have to look at the light from galaxies that has been travelling for 13.3 billion years to get to us. Hence we must look at very, very distant galaxies.

If we consider very old, but second generation stars (known as "Population II stars) - those born from material that incorporates the detritus of the first stars, then we can see those closer to home. That is because some of those stars were long lived and survive to the present day in our own and other nearby galaxies.

ProfRob
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