Preposterous Universe

Wednesday, May 19, 2004
Darker and darker

More evidence for an accelerating universe, this time from the Chandra X-ray satellite observatory. They observe X-rays from the hot gas in distant clusters of galaxies. A cluster is just a set of galaxies bound together by their mutual gravitational pull; but in addition to the galaxies themselves, the cluster is full of hot gas between the galaxies, not to mention dark matter. This picture is of the cluster Abell 2029; in blue you see the galaxies (visible in ordinary light) and in red the hot gas (reconstructed from the X-ray image). Knowing the properties of the gas, they can figure out the distances to the clusters. Comparing these with the redshifts (which tell us by how much the universe has expanded since the light we see left the cluster), we can reconstructe the expansion history of the universe.

The answer they get for the acceleration is consistent with our recent consensus model for cosmology, including substantial dark energy that seems to be nearly (or exactly) constant as the universe expands. So, not a dramatic overthrowing of what we already knew, but a nice confirmation. Which is very important, when the thing you're confirming is as surprising and ill-understood as the acceleration of the universe. Our previous evidence (from distances to supernovae, temperature fluctuations in the cosmic microwave background, and the dynamics of galaxies and large-scale structure) was very good, but every extra piece of evidence bolsters the case for this preposterous universe.

Chandra is named after Subrahmanyan Chandrasekhar, one of the leading theoretical astrophysicists of the 20th century. Also a longtime University of Chicago faculty member, and part of a tradition at NASA of naming its satellite observatories after famous scientists with UofC connections -- Chandra was preceded by the Hubble space telescope, named after a prominent alumnus, and the Compton gamma-ray observatory, named after another former faculty member. This tradition ended with the Spitzer infrared observatory, but that's okay because Lyman Spitzer was my grand-advisor (the Ph.D. advisor of George Field, my adviser). After that things went dramatically downhill, with the successor to Hubble being named after James Webb, a former NASA administrator.

Ideas on culture, science, politics.
Sean Carroll

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