20 billion is a long time, and the universe is very big. And we're not the first galaxy to form within that 20 billion year time frame.
It's very likely we're not the first in the universe and it's also likely we're not alone in coming to sentience at this very moment in the universes life.
What I don't understand is how we reasonably quantify the probability of sentient life evolving. Sure the universe is very big, but the probability could be very small. All the discussions I've seen make the assumption that there's nothing unusual about life on Earth and given similar conditions elsewhere we would expect to see life, but how does one conclude that from a sample size of 1?
I guess the argument is something like 'if life is unlikely to evolve, then it is surprising to find ourselves existing so early in the history of the universe. If life is likely to evolve, then how do we explain being apparently alone?' and then the grabby aliens theory tries to answer the second question.
I don't find the argument convincing though (per my other comment)
But we are not "so early" in the history of the universe. Something like 95% of all the stars that will ever be formed have already been formed. It's actually rather late in the day.
> Something like 95% of all the stars that will ever be formed have already been formed.
Sure, but clearly civilizations don't exclusively develop around newly formed stars: we didn't. Most stars are red dwarves and will continue shining for trillions of years.
That's true, but the star formation rate peaked around Z = 1.5, which was long before the solar system formed. By the time our solar system formed the rate was maybe 1/4 of the peak, and has fallen since.
I don't think it's a reasonable assumption that the rate of life (or civilization) formation would remain constant on a planet around a red dwarf over extended periods of time. I don't think Earth could survive for trillions of years as a life bearing planet, even with a more long lived star. Without radioactivity and primordial heat, plate tectonics would cease and the magnetic field would go away. The lack of magnetic field would mean the atmosphere would be lost as well.
It's very likely we're not the first in the universe and it's also likely we're not alone in coming to sentience at this very moment in the universes life.