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If I stood on a theoretical earth at the time that galaxies were just starting to form, what would the sky have looked like?

Due to the smaller size of the universe would it have been densely packed with objects? If yes, what might I have seen?

[Edit added by Roger Wood]

What would the night sky have looked like in 12,900,000,000 years BCE?

Prompted by Hubble's discovery of Earendel, what would the night sky have looked like back 12.9 billion years ago? (ignoring details like the Earth and humans not existing back then),

Apparently this star existed at 7% the age of the universe. Does this also mean the universe was 14 times smaller and 143=2700 times more dense than today? And the night sky would have been 142=200 times brighter? So nearby Galaxies would be much bigger and brighter with an apparent size of several degrees in the sky. Or would distinct, well-formed galaxies even be apparent at that time? What structures would be seen? What would the population of stars be like - lots of big blue ones or a bunch of little red ones? They'd obviously all be very young.

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    $\begingroup$ From which viewpoint - obviously the Sun and Earth didn't exist? From within our galaxy or from intergalactic space? $\endgroup$
    – ProfRob
    Apr 8 at 17:32
  • $\begingroup$ @ProfRob Good point. It's unclear whether our galaxy even existed. I think some discussion of the distribution of stars at that time is part of the question. If galaxies were 2700 times more densely packed, they'd be pretty much all smushed together. $\endgroup$
    – Roger Wood
    Apr 8 at 20:37

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In early times of galaxy formation you would have seen a sky similar to this Hubble Extreme Deep Field image. A sky filled with small, irregularly shaped, young galaxies and galaxy predecessor fragments (sub-galaxies). Galaxies probably formed bottom-up from smaller fragments.

Further back in time it would have been dark. Hydrogen clouds would begin to contract, as predecessors of first stars (population III) and clusters of stars.

Even further in the past (redshift between 300 and 400) the sky would have been glowing red by what we now call cosmic microwave background.

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As @Gerald stated in his excellent answer, the Hubble XDF photo might describe the night sky - many irregular, blue galaxies. As the redshift at 12.9 billion light years is about $z=6.5$, the universe was 7.5 times smaller in radius at the time compared to what it is now.

To answer @RogerWood's questions:

More detailed description of the appearance of the night sky in 12.9 billion years BCE -including star-types, magnitudes, colors, apparent motion, distribution, etc. - with some solid explanation.

This rendition of a young Milky Way from Wikipedia might give a realistic impression as to how the night sky may have looked in the early universe. enter image description here As seen, there is much more star formation hence more red (H-II regions) and blue areas (open clusters and nebulae), the central bulge of our galaxy is not as well-formed, perhaps because of a less-massive SMBH.

As you would expect in the young universe, most stars would be bluer, early type stars with very low metallicity (maybe population III stars?). Late-type stars that formed at that time would now be the old Population II stars we see today in galactic cores and old globular clusters. Given this distribution, the color of the night sky still would be black, but with more blue and red from hot, luminous stars and the red from star-forming regions and warm hydrogen clouds.

Apparent motion would essentially make no difference in the change of the night sky as everything still would be pretty far away (of the order of a few hundred to a few thousand light years) but still closer than the star-forming regions of today.

I hope this addresses some more detail than before, if you have any comments or concerns, let me know.

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  • $\begingroup$ Maybe I’m mistaken, but with a current R value of 1, wouldn’t the R value at a red shift of 6.5 be .129 or roughly 7.75 times smaller since $R=1/(1+z)$? $\endgroup$
    – Justin T
    Apr 8 at 17:56
  • $\begingroup$ @justin I admit, this is a topic I really don't understand. It seems like they're saying the light has stretched by a factor of 7.5. But how does that reconcile with the statement about 7% the age of the universe. Is the expansion non-linear with time? $\endgroup$
    – Roger Wood
    Apr 8 at 20:45
  • $\begingroup$ @fasterthanlight Thanks for the answer. Can I assume that inter-stellar spacing might be quite similar? But In the current universe, galaxies are fairly distinct with large spaces between them (10x their diameter?). I assume this was not at all true 12.9 billion years BCE? Were both the galaxies and their spacings both a lot smaller. Maybe there would be an autocorrelation function that would look similar but have a much smaller characteristic length? $\endgroup$
    – Roger Wood
    Apr 8 at 20:54
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    $\begingroup$ @RogerWood Yes, expansion isn't linear in time. Its rate decelerates at early times due to the mutual attraction of matter, and accelerates at later times due to dark energy. A lookback time of 12.9 Gyr corresponds to a redshift of z = 6.2, meaning that the scale factor was a = 1/(1+z) ≈ 0.14 at that time, i.e. a factor 7.2 smaller. This is true for cosmological distances; inside galaxies, where the interstellar medium is dominated by local physical conditions, it's more complicated. The gas was hotter then, and galaxies hadn't condensed as much, so the ISM wasn't necessarily any denser. $\endgroup$
    – pela
    Apr 8 at 21:12
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    $\begingroup$ @RogerWood In the early universe there was more gas and less metals, so larger stars were more common - you are correct that star formation was high. $\endgroup$ Apr 9 at 0:13

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