Sky & Telescope - February 2024 - 29

GREGG DINDERMAN / S&T
Expansion history of the universe
Today
be flat, even though the matter density seemed to be low. The
teams' data suggested Peebles was right.
Yet, invoking a mysterious and unknown energy is a big
deal. Both groups wanted to be sure. In his book The Extravagant
Universe, High-Z team member Robert Kirshner (Harvard
University) recalls an email he wrote: " I am worried . . . you
might need some lambda. In your heart, you know this is
wrong, though your head tells you [that] you don't care and
you're just reporting the observations. "
Debates, Arguments, and Results
Today, a quarter century later, there is still no clear consensus
about what happened next. But events unfolded something
like this.
In late 1997, Perlmutter and colleagues presented their
work to physicists at various departments. Wanting to be
cautious, the researchers stressed that their results were preliminary.
But at the end of one of Perlmutter's talks, physicist
Joel Primack (University of California, Santa Cruz) stood
up. Barely able to contain himself, he explained to everyone
in the room that these results were amazing, because they
implied that there was a cosmological constant.
The next month was the January 1998 meeting of the
American Astronomical Society, one of the biggest gatherings
in the astronomy community. Both supernova groups
presented there, telling a similar story: Our universe has low
matter density and will continue to expand forever. Ariel
Goobar (Stockholm University, Sweden), a member of the
SCP team, stated in a press release that " astrophysicists may
have to invoke Einstein's cosmological constant " if the results
from the supernova data held up.
At the same press conference, another team, composed of
Ruth Daly and Eddie Guerra (both then at Princeton), looked
at galaxies with extensive jets shooting from their cores. By
comparing the jets' observed lengths with that predicted by
their evolution, the researchers calculated each galaxy's distance.
In their January 1998 press release, they indicated that
their data showed that the universe would not only expand
forever but that its expansion was accelerating.
Other corroborating evidence came out around the same
time, such as work from Neta Bahcall and Xiaohui Fan (also
both then at Princeton) on the evolution of galaxy clusters,
which also indicated a low matter density in the universe.
Both supernova teams met again in February at a conference.
In front of a hushed audience, Alexei Filippenko
(University of California, Berkeley) of the High-Z Supernova
Search Team proclaimed in plainer language that they had
the evidence: There was " antigravity " in the cosmos - the
thing we today call dark energy.
Commotion followed. The media jumped in. By the time of
a workshop in May 1998, a straw poll indicated two-thirds of
the scientists thought the supernova evidence made a strong
case for dark energy. The discovery was named Science magazine's
Breakthrough of the Year, and members of both supernova
teams were awarded the 2011 Nobel Prize in Physics.
Past
Big
Bang
INCONSTANT UNIVERSE Measurements of the universe's expansion
history, based on observations of distant Type Ia supernovae and other
phenomena, now show that our universe's expansion rate did slow at
one time, just not permanently. Matter's gravity decelerated the expansion
during the first half or so of cosmic history, then dark energy took
over. Now expansion is accelerating.
Changing Cosmic Understanding
In retrospect, it may seem surprising that the scientific community
was so eager to accept the existence of a force dubbed
dark energy when even now, 26 years later, we still don't
know what it is. However, the agreement between two highly
competitive and extremely thorough research groups helped
the idea gain acceptance. " All the i's have been dotted, all the
t's have been crossed, " says Riess.
Also, not long after the announcements, an entirely independent
method used observations of the Big Bang's afterglow,
known as the cosmic microwave background, to confirm
both the universe's low matter density and the existence
of dark energy. Additional data from extensive galaxy and
supernova surveys, as well as studies of how galaxy clusters
evolve over time, have all confirmed that dark energy makes
up more than two-thirds of the universe. Energy, not matter,
dominates our cosmos and its fate.
The geometry of our universe is currently flat, but that
doesn't mean it will expand forever. Dark energy is a big
unknown, including how it will evolve with time. The universe
may continue to accelerate or, if dark energy proves
changeable, it may yet recollapse.
Today, dark energy continues to be one of the greatest
discoveries - and mysteries - in astrophysics. The irony isn't
lost on Perlmutter. " The odd thing about this result was that
all the questions I thought we'd get to answer, we didn't get
to answer, " he says. " So we still don't know whether the universe
is going to last forever, and we still don't know whether
it's infinite or not. "
These questions await the next generation of scientists.
¢ ELIZABETH FERNANDEZ is a science writer with a PhD in
astronomy. She writes about science and society and has read
S&T since she was young.
sk yand tele scope .o r g * FEBRUARY 2024 29
Future
e
D
r
o
eler
a
c
n
f
n
or f
o
i
t
A c c e l e r a t i
o
dark matter
m
m
d
a
r
k
e
n
e
r
g
y
http://skyandtelescope.org
Sky & Telescope - February 2024

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