Six billion years into the Age of Dark Energy

When I first studied cosmology in school, the textbooks all started out by talking about Friedman-Robertson-Walker models with zero cosmological constant. The expansion of such a universe always decelerates as the matter in the universe tugs on itself. It either comes to a stop and collapses again (if the matter density is above some critical threshold) or expands forever in an ever-slowing manner. If you assume the simplest possible topology, the finite-in-time models are also finite spatially, and the infinite-lifetime ones are also infinitely large. The deceleration is a little different in the early part when radiation abounds, and by that time many favored inflation scenarios in which radically different things happened even earlier than that, but that's basically the story.

Then, the texts went on to... the other possibilities. The cosmological constant—a strange, hypothetical kind of energy pervading the universe, introduced by Einstein as a pure fudge term, with a density that remained constant no matter the size of the universe, and a tendency to make the universe expand—was "Einstein's greatest blunder" in the sense that you couldn't get out a static universe that way, as he wanted to do; and we don't live in a static universe anyway. But you could also do other crazy things with cosmological constants, plugging various numbers into the equations. You could have an Eddington-Lemaitre-style universe that began as an infinitely old, quasi-static "cosmic egg" that hatched at some finite time in the past, like a puzzler from St. Augustine. You could have a universe that contracts from infinity ever since forever, decelerates, then turns right around and expands again. In the limit of very low matter density compared to the cosmological constant, you got the de Sitter universe: one that just expanded forever in an exponential manner, a little like Hoyle's steady-state universe, only emptier—of course, without continuous creation of matter like Hoyle had, at some point in the past any nonzero matter density would have been large enough to ruin the uniformity.

And you could have "hesitation universes", in which any observational problems you had about relative lifetimes of the universe and the things in it could be papered over by arranging the cosmological constant just so: the universe would expand from a Big Bang and then decelerate, slow down to some minimum expansion speed, and then speed up again as the inflationary gravitation of the cosmological constant overpowered the self-attraction of matter. This kind of theory generally seemed like an antique kludge with a no-longer-extant rationale.

Ladies and gentlemen, guess what kind of universe we live in.

Distances to stars

People often ask me, "This is probably a stupid question, but do you know how astronomers know how far away stars are?" Actually, it's not a stupid question at all. It is very hard to find astronomical distances and astronomers use a collection of different techniques, most of which have large margins of error and only apply to certain types of objects. Fortunately I don't have to describe them all, because Ned Wright has a page describing one for each letter of the alphabet.

Attention chicken_cem and <user site="livejournal.com" user="bu

The Massachusetts state Senate voted overwhelmingly to repeal the 1913 ban on marriages by out-of-staters whose marriages would not be legal in their home jurisdictions. The repeal probably won't pass the House or the governor's veto, though. Several towns will undoubtedly keep issuing licenses to out-of-staters for now, but Romney is threatening court orders. (I have a feeling that some more fun litigation is ahead, especially as regards Rhode Island.)