My blog has received its first mention in another blog, at Telic Thoughts. Since my blog was new I decided not to promote it at all (other than linking to it from my web site, and telling a few friends about it), and to wait and see when it would get some attention. As a result of this mention, the number of hits/day skyrocketed (from a high of 15, to over 100 today). OK I’m sure this isn’t much by major blog standards, but since I’m new at all this, I was impressed.
I received an interesting email from someone who saw the Telic Thoughts post. He pointed out that, while we often think of the universe as being very old, from the standpoint of stellar evolution, the universe is quite young. Fourteen billion years is a long time by human standards, but by the standard of how long it takes stars to form and then die (9 billion years for a star like our sun), we’re pretty early on in the universe — we’re in the first or second cycle of stellar evolution. The emailer then writes:
The Universe is only old enough to be in its first generation of G-type solar systems and there’s already life in (at least) one of them. It appears that at the very first opportunity in time for life to emerge in our Universe, it immediately did. It did not take generations upon generations of G-type stars located in habitable zones for life to finally get started.
This seems amazing and points toward some telic destiny beyond the explanation of chance.
I do think this early-stellar-evolution point is an important point, but I’m nevertheless happy with the chance explanation. The reason I’m happy with the chance explanation is that I lean toward the hypothesis that the universe is spatially infinite, with an infinite number of stars and planets. Even though it would be unlikely for life to arise on some particular planet within the first or second phase of stellar evolution, given an infinite number of planets, we would expect it to happen somewhere.
It’s worth noting that there is a potential doomsday-style argument that could come into play here. If the universe is going to exist for a long time, through many cycles of stellar evolution, then it’s very surprising that we find ourselves at the very beginning. But if the universe is only going to exist through a small number of cycles of stellar evolution (or if life is only possible during the first few cycles of stellar evolution), then it’s not at all surprising that we are in the first or second.
I think this provides evidence for a Steinhart-Turok-style cyclic model, where we are currently in an expansion phase of the universe, but the universe will collapse back on itself in a finite amout of time. If there are only a few cycles of stellar evolution before collapse, then it’s not improbable that we would find ourselves in an early cycle, but if the universe is going to keep expanding, with many cycles of stellar evolution, then it would be quite improbable that we were in an early one. This arguably provides some evidence that there will only be a few cycles.
A final note: doomsday arguments are tricky, and to fully spell out the argument I’ve sketched here would take a lot of care. For some discussion of the trickiness, see my paper “How to Predict Future Duration from Present Age” (and see also the New York Times discussion of my paper here, and the associated blog discussion here). Also see my paper “The Doomsday Argument Without Knowledge of Birth Rank”.