Sky & Telescope - February 2024 - 27

TYPE IA SUPERNOVA: NASA / ESA / THE HUBBLE KEY PROJECT TEAM / THE HIGH-Z SUPERNOVA SEARCH
TEAM; FADING SUPERNOVA: PETER GARNAVICH (CENTER FOR ASTROPHYSICS, HARVARD & SMITHSONIAN) /
THE HIGH-Z SUPERNOVA SEARCH TEAM / NASA / ESA
A quarter century ago, data
collected from far-off supernovae
turned up something strange -
a discovery that upended the fate
of the universe.
To address these big questions, scientists
tory
S
turned to exploding white dwarfs, which create
Type Ia supernovae. The researchers could calculate
how intrinsically luminous such an event
becomes, compare that against the observed
brightness, and deduce its distance. Pairing
the distance with a measure of how rapidly
the supernova's host galaxy is receding from
us (that is, its redshift) reveals the expansion
rate at that point. By gathering enough such
measurements, astronomers could see how the
expansion rate has changed over cosmological
history, shedding light on the matter density
and the fate of our universe.
ometimes, astronomy starts with
philosophy. Beyond the pretty pictures
and complex math, the motivating
questions that drive astronomers are things like,
" Are we alone? " , " Why are we here? " , or " Will
the universe last forever? "
When Saul Perlmutter (now at University of
California, Berkeley) was a graduate student in
the 1980s, it was the fate of the universe that
was keeping him up at night.
Since the early 20th century, astronomers
had known the universe was expanding - but
would it expand forever? Or would gravity eventually
pull it all back together in a Big Crunch?
There simply weren't enough data to decide.
The answer came down to the universe's density,
which would be reflected in the " shape " of
space itself. In a closed universe, density is high,
and the mutual gravity of galaxies will eventually
recollapse the cosmos, pulling everything
back together again. In such a universe, parallel
lines ultimately converge. In an open universe,
on the other hand, there is not enough mass to
put expansion in reverse, so the space between
galaxies just keeps growing. In this universe,
parallel lines diverge.
A third option lies in between: In a flat
universe, in which parallel lines stay parallel,
there's just enough mass to slow and eventually
halt its expansion - but only after infinite time.
Yet, even though theory suggested the universe
was indeed flat, astronomers didn't see
enough matter to make it so. Without enough
matter, and hence gravity, expansion would
continue forever. Astronomers thus thought
that determining how much matter there really
was could help match observations with theory
and thus predict our universe's future.
Tricky Observations
Planning observations of supernovae is difficult.
The explosions are rare and their locations
unpredictable. To evade these problems, two
groups of scientists - the Supernova Cosmology
Project (SCP), cofounded by Perlmutter,
and the High-Z Supernova Search Team,
led by Brian Schmidt (now at the Australian
National University) and Nicholas Suntzeff
(now at Texas A&M) - masterfully coordinated
telescope time on several telescopes around
the world. The two teams imaged pieces of sky,
collectively measuring tens of thousands of
galaxies. A few weeks later, they'd snap another
picture of the same sky patches. Comparing the
before and after, the astronomers looked for
points of light that weren't there before.
The observers would then follow up on
newly discovered light sources with other
ground-based telescopes, and then finally with
Hubble. Feverishly, the two groups worked to
find as many viable supernovae as possible, in a
heated race with each other.
One of the astronomers on the High-Z team
was Adam Riess (now at Johns Hopkins University),
who was helping analyze the data coming
in. If the universe's expansion were slowing
down due to the mutual gravitational attraction
of many galaxies, then the distant supernovae
should be relatively bright. But that's not
what Riess found. Instead, distant supernovae
were much dimmer than expected - even more
so than anticipated for an open universe.
" The answer I got from my computer was
[that the universe had] negative mass, " he says.
" Now, that doesn't make sense. " Of course the
FADE AWAY The light from Type Ia supernovae, such
as SN 1997cj pictured here, fades in a characteristic
way, so astronomers can determine the intrinsic brightness
without already knowing the object's distance.
sk yand tele scope .o r g * FEBRUARY 2024 27

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Sky & Telescope - February 2024

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