I just finished an introductory astrophysics course$^1$ and I have a lingering question that I can't seem to resolve.

We learned that for the first few hundred million years, the universe was pretty boring and not much interesting$^2$ happened. We also learned, correctly or not, that the first stars started forming somewhere between 500 million and 600 million years after the Big Bang.

We also learned how to use redshift values to calculate age, and we talked about how the highest $z$-number we've discovered is some galaxy (GN-z11) at roughly $z=11$.

Using this calculator, we calcultaed that this galaxy apparently formed about 410 million years after the Big Bang.

So, this galaxy seems to be older than when astrophysicists think star formation happened. How can this possibly be? Clearly one of my assumptions is wrong, so is it:

  • Stars actually started forming before 500 million years post-Big Bang.
  • Using the UCLA calculator to calculate age is technically incorrect.
  • A galaxy doesn't need stars to be considered a galaxy.
  • Some other assumption I made is wrong which makes this post invalid.

As a follow-up question, what happens if we keep finding galaxies at higher $z$-numbers? At what point do we need to reconsider our theory about what happened in the "early" universe?


$^1$I'm not an astrophysics major, so forgive any blatant falsehoods in this post.

$^2$On a macro-scale, at least.

  • I don't think you can describe the first 100million years as boring!! I think the first stars were about 150 millon years, and the galaxy GNz11 would have been made this population of stars, plus lots of still partly neutral hydrogen gas. – James K May 9 '16 at 19:04
  • I think it is number 1. If there's a galaxy, then stars have formed. – Rob Jeffries May 9 '16 at 19:46
  • I agree with Rob. The first stars are thought to have formed at a few 100 Myr after BB, earlier than the 5-600 Myr you quote. The first structures had masses of $10^5$–$10^6$ $M_\odot$ or so, and later merged to form larger galaxies, so whether you want to call these structures "galaxies" is a matter of opinion, I think. – pela May 9 '16 at 20:59

There is evidence that this assumption is incorrect.

Stars actually started forming before 500 million years post-Big Bang.

There is some evidence that Population III stars formed as early as 200 million years after the Big Bang. The oldest known star, the Methuselah Star, dates back to 200 million years after the Big Bang. It is a second generation star (judging from its metal content), so that means a generation of very large stars must have lived and died before this one was born.

The Universe might have expanded at changing rates. $\Omega_M$ can also influence the redshift. It is not precisely known whether the universe is really flat or open / closed. This can also influence the redshift calculations. (Note: It is also believed that the Hubble constant changed over the evolution of the Universe, decreasing or increasing). Someone else has mentioned before me, and I learned the same at my astronomy classes: massive stars could have formed 0,1Gyrs after TBB. They must have had extremely short lives though. In the first billion years quasars were very trendy, usually they have the great z values.

And don't take this the hard way, but astronomers tend to work with ridiculously big margins of error. Also, some functions can be used in a specific value interval only. The farther the data is out of that interval (meaning it's too big/small), the more probable it is that it won't yield a realistic result. Extreme data like this should be taken with a grain of salt.

  • The first part of this answer is unhelpful. The evolution of the Hubble parameter is of course taken account of when inferring an age from a redshift. – Rob Jeffries Sep 5 '17 at 20:21

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