Completing a nearly 30-year marathon, NASA's Hubble Space Telescope has calibrated more than 40 "milepost markers" of space and time to help scientists precisely measure the expansion rate of the universe – a quest with a plot twist.
From: NASA
May 19, 2022 -- Pursuit
of the universe's expansion rate began in the 1920s with measurements by
astronomers Edwin P. Hubble and Georges Lemaître. In 1998, this led
to the discovery of "dark energy," a mysterious repulsive force
accelerating the universe's expansion. In recent years, thanks to data from
Hubble and other telescopes, astronomers found another twist: a discrepancy
between the expansion rate as measured in the local universe compared to
independent observations from right after the big bang, which predict a
different expansion value.
The cause of this
discrepancy remains a mystery. But Hubble data, encompassing a variety of
cosmic objects that serve as distance markers, support the idea that something
weird is going on, possibly involving brand new physics.
"You are getting
the most precise measure of the expansion rate for the universe from the gold
standard of telescopes and cosmic mile markers," said Nobel Laureate Adam
Riess of the Space Telescope Science Institute (STScI) and the Johns Hopkins
University in Baltimore, Maryland.
Riess leads a
scientific collaboration investigating the universe's expansion rate called
SH0ES, which stands for Supernova, H0, for the Equation of State of Dark Energy.
"This is what the Hubble Space Telescope was built to do, using the best
techniques we know to do it. This is likely Hubble's magnum opus, because it
would take another 30 years of Hubble's life to even double this sample
size," Riess said.
Riess's team's paper,
to be published in the Special Focus issue of The Astrophysical Journal reports
on completing the biggest and likely last major update on the Hubble constant.
The new results more than double the prior sample of cosmic distance markers.
His team also reanalyzed all of the prior data, with the whole dataset now
including over 1,000 Hubble orbits.
When NASA conceived of
a large space telescope in the 1970s, one of the primary justifications for the
expense and extraordinary technical effort was to be able to resolve Cepheids,
stars that brighten and dim periodically, seen inside our Milky Way and
external galaxies. Cepheids have long been the gold standard of cosmic mile
markers since their utility was discovered by astronomer Henrietta Swan Leavitt
in 1912. To calculate much greater distances, astronomers use exploding stars
called Type Ia supernovae.
Combined, these objects
built a "cosmic distance ladder" across the universe and are
essential to measuring the expansion rate of the universe, called the Hubble
constant after Edwin Hubble. That value is critical to estimating the age of
the universe and provides a basic test of our understanding of the universe.
Starting right after
Hubble's launch in 1990, the first set of observations of Cepheid stars to
refine the Hubble constant was undertaken by two teams: the HST Key
Project led by Wendy Freedman, Robert Kennicutt, Jeremy Mould, and
Marc Aaronson, and another by Allan Sandage and collaborators, that used
Cepheids as milepost markers to refine the distance measurement to nearby
galaxies. By the early 2000s the teams declared "mission
accomplished" by reaching an accuracy of 10 percent for the Hubble
constant, 72 plus or minus 8 kilometers per second per megaparsec.
In 2005 and again in
2009, the addition of powerful new cameras onboard the Hubble telescope
launched "Generation 2" of the Hubble constant research as teams set
out to refine the value to an accuracy of just one percent. This was
inaugurated by the SH0ES program. Several teams of astronomers using Hubble,
including SH0ES, have converged on a Hubble constant value of 73 plus or minus
1 kilometer per second per megaparsec. While other approaches have been used to
investigate the Hubble constant question, different teams have come up with
values close to the same number.
The SH0ES team includes
long-time leaders Dr. Wenlong Yuan of Johns Hopkins University, Dr. Lucas Macri
of Texas A&M University, Dr. Stefano Casertano of STScI, and Dr. Dan
Scolnic of Duke University. The project was designed to bracket the universe by
matching the precision of the Hubble constant inferred from studying the cosmic
microwave background radiation leftover from the dawn of the universe.
"The Hubble
constant is a very special number. It can be used to thread a needle from the
past to the present for an end-to-end test of our understanding of the
universe. This took a phenomenal amount of detailed work," said Dr. Licia
Verde, a cosmologist at ICREA and the ICC-University of Barcelona, speaking
about the SH0ES team's work.
The team measured 42 of
the supernova milepost markers with Hubble. Because they are seen exploding at
a rate of about one per year, Hubble has, for all practical purposes, logged as
many supernovae as possible for measuring the universe's expansion. Riess said,
"We have a complete sample of all the supernovae accessible to the Hubble
telescope seen in the last 40 years." Like the lyrics from the song
"Kansas City," from the Broadway musical Oklahoma, Hubble has
"gone about as fur as it c'n go!"
Weird Physics?
The expansion rate of
the universe was predicted to be slower than what Hubble actually sees. By
combining the Standard Cosmological Model of the Universe and
measurements by the European Space Agency's Planck mission (which
observed the relic cosmic microwave background from 13.8 billion years
ago), astronomers predict a lower value for the Hubble constant: 67.5 plus or
minus 0.5 kilometers per second per megaparsec, compared to the SH0ES team's
estimate of 73.
Given the large Hubble
sample size, there is only a one-in-a-million chance astronomers are wrong due
to an unlucky draw, said Riess, a common threshold for taking a problem
seriously in physics. This finding is untangling what was becoming a nice and
tidy picture of the universe's dynamical evolution. Astronomers are at a loss
for an explanation of the disconnect between the expansion rate of the local
universe versus the primeval universe, but the answer might involve additional
physics of the universe.
Such confounding
findings have made life more exciting for cosmologists like Riess. Thirty years
ago they started out to measure the Hubble constant to benchmark the universe,
but now it has become something even more interesting. "Actually, I don't
care what the expansion value is specifically, but I like to use it to learn
about the universe," Riess added.
NASA's new Webb
Space Telescope will extend on Hubble's work by showing these cosmic
milepost markers at greater distances or sharper resolution than what Hubble
can see.
The Hubble Space
Telescope is a project of international cooperation between NASA and ESA (European
Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland,
manages the telescope. The Space Telescope Science Institute (STScI)
in Baltimore, Maryland, conducts Hubble science operations. STScI is operated
for NASA by the Association of Universities for Research in Astronomy in
Washington, D.C.
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