Sky & Telescope - February 2024 - 28

A Look Back
universe has mass, he thought. There had to be some sort of
bug in the code.
Or maybe he had stumbled onto something bigger.
Riess sent an email to Schmidt. It contained a plot of
supernova data and a simple subject line: " What do you think
of this? "
" I could only think of what he might have done wrong, "
Schmidt recalled in a 2006 essay. As far as he knew, Perlmutter's
SCP team was finding that the universe's expansion was
decelerating. " It is one thing to get a different answer than
the competition, it is quite another to get a different answer
and have your answer be crazy. "
Meanwhile, unbeknownst to Schmidt, the SCP team was
actually finding the same odd result.
Perlmutter recalls, " We had the main job of a scientist . . .
97% of your time is trying to figure out how you're going
wrong. " They checked if the way supernovae evolve changed
over cosmological time. They checked if there was some weird
" gray dust " that scatters all wavelengths of light, dimming
distant supernovae without being detectable itself. They even
checked if the gravity of intervening galaxies had bent the
Closed
Curvature
Flat
Dark energy is a big unknown . . .
The universe may continue to
accelerate or, if dark energy proves
changeable, it may yet recollapse.
light of some supernovae in such a way as to make it fainter.
Or perhaps there really was just a bug in the code.
But if the data held up, then there seemed to be a choice
between two possibilities: Either the universe contained a
negative amount of mass (which was obviously untrue), or
something was working against gravity, pushing the cosmos
apart at an ever-faster clip.
The idea of a mysterious energy working against gravity
wasn't new. Albert Einstein had suggested that a cosmological
constant (denoted lambda, Λ) could hold the universe motionless
and keep it from collapsing under its own gravity. However,
once it was discovered that distant galaxies were moving
away from one another, Einstein discarded the idea. James
Peebles (Princeton) invoked the constant again in the 1980s
to up the energy density of the universe and allow space to
Open
Flat
Fate
Big Crunch
Expansion
slows
Expansion continues
forever
Uncertain fate, due to the
uncertain nature of dark energy
Present
FATE AND CURVATURE Before the discovery of dark energy, astronomers did not know if the universe was
closed, flat, or open. The fate of a closed universe (leftmost) would end in a Big Crunch, while a flat universe
(center left) would expand more and more slowly, stopping after an infinite amount of time. In an open universe,
expansion would continue forever. But observations show that expansion is actually speeding up (rightmost
scenario). If dark energy is the cosmological constant, then this accelerated expansion will continue, with space
growing at a faster and faster rate - even though observations now confirm the theoretical idea that our universe
is flat. Dark energy thus works with curvature to decide the universe's fate.
28 FEBRUARY 2024 * SKY & TELESCOPE
Past
Future
BEATRIZ INGLESSIS / GREGG DINDERMAN / S&T

Sky & Telescope - February 2024

Table of Contents for the Digital Edition of Sky & Telescope - February 2024