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*To*: dinosaur@usc.edu*Subject*: Date for Origin of Universe*From*: keel@bildad.astr.ua.edu (William Keel)*Date*: Wed, 1 Oct 1997 08:42:07 -0500*Reply-to*: keel@bildad.astr.ua.edu*Sender*: owner-dinosaur@usc.edu

Having tried to reply to the original post, but not seen it, I'll try again, with something a little less flip than: "As I know the age of the universe is 20 billion years based on the new data or 6000 years if you ask the Jehovah-guys." (Hey, look, I've got new data and am a "Jehovah guy" - go figure). There is (naturally) still controversy on the age of the Universe, largely because there are several ways to estimate it which take quite different approaches. The one most recently in the news has been through measuring its expansion rate, the so-called Hubble constant, which is the cosmic expansion over unit distance (thus has units of inverse time, except that astronomers tend to use km/s per Mpc for convenience where Mpc = megaparsec ~ 3 x 10^24 cm). This gives an estimate of the age of the Universe, in the simplest case by running the expansion backward at a constant rate until everything was in the same place at once - this time span is known as the Hubble time, and is simply the inverse of the Hubble constant. There has long been an allowable range of values for the Hubble constant from 50-100 km/s/Mpc, giving Hubble times of 10-20 Gyr (billion years). The excitement over the Hubble Space Telescope results was that they allowed accurate distance measurements over a much greater depth than previous work, thus giving a less adulterated measurement of the Hubble constant than earlier nearby observations (which could have the cosmic expansion shift diluted by Doppler shifts from local motions, that will be less important and large distances). Several (but not all!) of these studies are converging on values 68-80 km/s/Mpc, which gives a Hubble time of about 13 Gyr. (One character has been a co-author on studies giving values from 50-80; I suspect that an appropriate amount of beer could be a very interesting investment...) Naturally, we can't take this number as the "age of the Universe" yet, because we are aware that the Universe contains matter which will slow the expansion. The overall effect of matter is still quite uncertain, because the "dark matter" probably dominates the total (and is hard to measure). If the Universe is at critical density (more or less, right at its own escape velocity), its real age is 2/3 of the Hubble time. Now, enter another way to estimate ages, stellar evolution. Models for the lifecycles of stars, which have passed pretty stringent observational tests, suggest that there are old globular clusters with ages uncomfortable close to 13 Gyr (let alone 2/3 of that). Recent work has suggested some wiggle room, involving details of convection in stellar interiors that are difficult to model (being one of those "Grand Challenge" problems taking place over a huge range in size scale at once). Lots of action on this front by now - watch for results from counting white dwarfs (which have completely different physics and possible errors from "live" stars), and re-evaluation of the cosmic distance scale from gravitational lensing, to name a few. The upshot - 13 Gyr won't bother too many people as an age estimate, but few would be surprised at 10 or 15. The apparent discrepancy between the Hubble time and stellar ages has gotten lots of press. Not surprisingly, star people think the Hubble constant's wrong, and galaxy people are suspicious of the stellar-evolution calculations. And then there are people who suspect that the cosmological model is wrong/incomplete, so that the expansion may have even accelerated with time (invoking the so-called cosmological constant, which, while not a priori silly, has a nasty history of showing up at every potential crisis in cosmological understanding and then quickly disappearing). One way I use to help students visualize these distance and time spans is to present, side-by-side, fossils and artifacts of various ages and images of things from which the light-travel time is comparable to its age. I use stone tools, a Miocene reptile coprolite (giggles from a few students), dino bone, ammonite, and trilobite, but one sort of runs out of fossils before getting past the Coma cluster of galaxies. And for the stuff I work on at look-back times of about 2/3 the age of the Universe, just give it up. [Note how slickly I put an on-topic clause in this?] Hope what you were after was in there somewhere... Bill Keel Astronomy, University of Alabama

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