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machine: light from a distant galaxy may have taken billions of years to
reach us, so we are seeing it as it was far in the past. Looking back in time,
astronomers saw the universe expanding at speeds that ware lower, rather
than higher. At first they were mortified, since this was exactly the opposite
of what had been expected. The statistical quality of the data was also not
good enough to constute ironclad proof, and there were worries about
systematic errors. The case for an accelerating expansion has however been
nailed down by high-precision mapping of the dim, sky-wide afterglow of
the Big Bang, known as the cosmic microwave background. Some theorists
have proposed reviving Einstein’s cosmological constant to account for the
acceleration, while others believe it is evidence for a mysterious form of
matter which exhibits gravitational repulsion. The generic term for this
unknown stuff is “dark energy.” Some recent ideas on this topic can be
found in the January 2001 issue of Scientific American, which is available
online at
http://www.sciam.com/issue.cfm.issueDate=Jan-01 .
Note added February 15, 2003
The microwave background measurements referred to above have been
improved on by a space probe called WMAP, and there is no longer much
room for doubt about the repulsion. An article describing these results,
from the front page of the New York times on February 12, 2003, is
available online at
http://www.nytimes.com/2003/02/12/science/12COSM.html
(free registration required).
Astronomers consider themselves to have entered a new era of high-
precision cosmology. The WMAP probe, for example, has measured the age
of the universe to be 13.7
+0.2 billion years, a figure that could previously
be stated only as a fuzzy range from 10 to 20 billion. We know that only
4% of the universe is atoms, with another 23% consisting of unknown
subatomic particles, and 73% of dark energy. It’s more than a little ironic to
know about so many things with such high precision, and yet to know
virtually nothing about their nature. For instance, we know that precisely
96% of the universe is something other than atoms, but we know precisely
nothing about what something is.
Section 10.6*Evidence for Repulsive Gravity
The WMAP probe’s map of the cosmic microwave background is like a “baby pic-
ture” of the universe.