As I understand it, the Milky Way galaxy is $\sim 13.6\,{\rm Gyr}$ old, when the universe was "only" about $200\,{\rm Myr}$ old. If we were on a planet orbiting a star in that early Milky Way, what would the night sky (i.e. the visible universe) look like?

Would the CMB be different than it is today? Would the sky be less full of stars? Would there be a large patches of sky with no galaxies or stars?

Would we be in better position to see if there is anything beyond that visible universe?

  • 1
    $\begingroup$ Surely the CMB was much hotter. And if we neglect young stars and galaxy formation, the entire sky was more densely stuffed of objects. Though visually it could have not too dissimilar from now as it was already huge. Waiting for a more technical answer too $\endgroup$
    – Alchimista
    Dec 12, 2017 at 10:25
  • $\begingroup$ 13.6 Gya there was no Milky Way and stars had only just started to form. $\endgroup$
    – hdhondt
    Sep 18, 2018 at 10:06
  • $\begingroup$ @hdhondt spaceplace.nasa.gov/galaxies-age/en $\endgroup$ Aug 6, 2020 at 6:05

1 Answer 1


The time when the Universe was $200\,{\rm Myr}$ old corresponds to a cosmological redshift (relative to today) of about $z\sim19$ (I calculated this assuming the WMAP7 cosmology). From this you can work out several interesting facts about the Universe at that time. One is that the average density was a factor of $(1+z)$ higher than it is now. This applies to the average across the entire Universe, so the local density it not necessarily enhanced by the same factor, e.g. you can't estimate the number density of stars in the proto-MW this way. However, there is plenty of evidence that early galaxies had much higher densities of gas and star formation (based on observations at $z\sim2-3$, not $z=19$!), so there were likely many more young, bright, blue stars around your hypothetical planet than there are around Earth.

Another easy calculation is the temperature of the CMB, which scales as $T(1+z)$, and so would have been about $60\,{\rm K}$ rather than the $3\,{\rm K}$ or so that it is today.

The biggest difference I can think of is that $z\sim20$ is thought to be before the so-called epoch of re-ionization. Most of the gas in the Universe at this time would be neutral and thus largely opaque to optical photons. There would be pockets of ionized gas around stars and proto-galaxies, but with the exception of perhaps a handful of nearby proto-galaxies whose ionization regions might overlap with that of the proto-MW, other galaxies would not be visible.


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