301
the large-scale structure of the
universe have since revealed that
billions of galaxies are clustered
together around vast empty voids
(p.296). This structure corresponds
to minute ripples in the CMB that
show how observable matter—the
stars and galaxies—emerged in
anomalous regions in otherwise
empty space. However, the future
of the universe was uncertain. It
was unknown whether it would
expand forever or one day collapse
under its own gravity.
Decelerating universe
Throughout the 20th century,
cosmologists assumed that the rate
of expansion was slowing down.
Following a rapid initial expansion,
gravity would start decelerating.
It seemed there were two main
possibilities. If the universe was
heavy enough, its gravity would
eventually slow the expansion to
a stop and begin to pull matter
back together in a cataclysmic
Big Crunch, a kind of Big Bang
in reverse. The second possibility
was that the universe was too
light to stop the expansion, which
would therefore continue forever,
gradually slowing down. This
process would result in heat death,
where the material of the universe
had broken up, become infinitely
dispersed, and ceased to interact
in any way at all. A measurement
of the deceleration of the universe’s
expansion would tell cosmologists
which possible future the universe
was heading for.
By the mid 1990s, two programs
were under way to measure the
rate of expansion of the universe.
The Supernova Cosmology Project
was headed by Saul Perlmutter
at Lawrence Berkeley National
Laboratory, while Brian Schmidt,
based at the Australian National
University, led the High-Z Supernova
Search Team. Adam Riess, of
the Space Telescope Science
Institute, was the lead author
for the latter project. The project
leaders considered merging, but
had different ideas about how
to proceed, and so opted instead
for a healthy rivalry.
Both projects were using a
discovery made by the Calán/
Solodo Supernova Survey, carried
out in Chile between 1989 andSee also: The theory of relativity 146–53 ■ Spiral galaxies 156–61 ■ The birth of the universe 168–71 ■ Beyond the Milky
Way 172–77 ■ Searching for the Big Bang 222–27 ■ Dark matter 268–71 ■ Redshift surveys 274–75
THE TRIUMPH OF TECHNOLOGY
The Chandra X-ray Observatory took
this image of the remnant of type 1a
supernova SN 1572 in Cassiopeia.
It is also known as Tycho’s nova,
as it was observed by Tycho Brahe.
- The survey found that type
1a supernovae could be used as
standard candles, or objects that
can be used to measure distances
across space. A standard candle
is an object of known brightness, ❯❯
If you’re puzzled by what
dark energy is, you’re
in good company.
Saul Perlmutter