Running On Empty

The cosmos, like some of the rest of us, is getting old. If we look back in time, to redshift 1, for example, we can see that star formation rates were much higher in the past. Of course if we look back even further, to redshifts of 7 or more, star formation rates again decline. It took a while for star formation to really get underway in the universe. This decline is hardly a mystery. With the passage of time, more and more of the required baryons - hydrogen gas - gets locked up in stars and other things. Moreover, the remaining hydrogen is being diluted by cosmic expansion. So star formation rates have declined by perhaps a factor of ten since those good old days.

That might be a good thing from the standpoint of life. Star formation produces supernovae and perhaps other dangerous stuff that you really might not want in your galactic neighborhood.

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