Depending on where we look, the universe is expanding at
different rates. Now, scientists using the James Webb and Hubble space
telescopes have confirmed that the observation is not down to a measurement
error.
Astronomers have used the James Webb and Hubble space
telescopes to confirm one of the most troubling conundrums in all of physics —
that the universe appears to be expanding at bafflingly different speeds
depending on where we look.
This problem, known as the Hubble Tension, has the potential
to alter or even upend cosmology altogether. In 2019, measurements by the
Hubble Space Telescope confirmed the puzzle was real; in 2023, even more
precise measurements from the James Webb Space Telescope (JWST) cemented the
discrepancy.
Now, a triple-check by both telescopes working together
appears to have put the possibility of any measurement error to bed for good.
The study, published February 6 in the Astrophysical Journal Letters, suggests
that there may be something seriously wrong with our understanding of the
universe.
"With measurement errors negated, what remains is the
real and exciting possibility we have misunderstood the universe," lead
study author Adam Riess, professor of physics and astronomy at Johns Hopkins
University, said in a statement.
Reiss, Saul Perlmutter and Brian P. Schmidt won the 2011
Nobel Prize in physics for their 1998 discovery of dark energy, the mysterious
force behind the universe's accelerating expansion.
Currently, there are two "gold-standard" methods
for figuring out the Hubble constant, a value that describes the expansion rate
of the universe. The first involves poring over tiny fluctuations in the cosmic
microwave background (CMB) — an ancient relic of the universe's first light
produced just 380,000 years after the Big Bang.
JWST's infrared cameras allow it to look at the universe in
more precise detail than any telescope before it. (Image credit: NASA, ESA,
CSA, J. Diego (Instituto de FÃsica de Cantabria), B. Frye (University of
Arizona), P. Kamieneski (Arizona State University), T. Carleton (Arizona State
University), and R. Windhorst (University of Arizona), A. Pagan (STScI), J.
Summers (Arizona State University), J. D’Silva (University of Western
Australia), A. Koekemoer (STScI), A. Robotham (University of Western Australia),
and R. Windhorst (University of Arizona)) |
Between 2009 and 2013, astronomers mapped out this microwave
fuzz using the European Space Agency's Planck satellite to infer a Hubble
constant of roughly 46,200 mph per million light-years, or roughly 67
kilometers per second per megaparsec (km/s/Mpc).
The second method uses pulsating stars called Cepheid
variables. Cepheid stars are dying, and their outer layers of helium gas grow
and shrink as they absorb and release the star's radiation, making them
periodically flicker like distant signal lamps.
As Cepheids get brighter, they pulsate more slowly, giving
astronomers a means to measure their absolute brightness. By comparing this
brightness to their observed brightness, astronomers can chain Cepheids into a
"cosmic distance ladder" to peer ever deeper into the universe's
past. With this ladder in place, astronomers can find a precise number for its
expansion from how the Cepheids' light has been stretched out, or red-shifted.
But this is where the mystery begins. According to Cepheid
variable measurements taken by Riess and his colleagues, the universe's
expansion rate is around 74 km/s/Mpc: an impossibly high value when compared to
Planck's measurements. Cosmology had been hurled into uncharted territory.
"We wouldn't call it a tension or problem, but rather a
crisis," David Gross, a Nobel Prize-winning astronomer, said at a 2019
conference at the Kavli Institute for Theoretical Physics (KITP) in California.
Initially, some scientists thought that the disparity could
be a result of a measurement error caused by the blending of Cepheids with
other stars in Hubble's aperture. But in 2023, the researchers used the more
accurate JWST to confirm that, for the first few "rungs" of the
cosmic ladder, their Hubble measurements were right. Nevertheless, the
possibility of crowding further back in the universe's past remained.
To resolve this issue, Riess and his colleagues built on
their previous measurements, observing 1,000 more Cepheid stars in five host
galaxies as remote as 130 million light-years from Earth. After comparing their
data to Hubble's, the astronomers confirmed their past measurements of the
Hubble constant.
"We've now spanned the whole range of what Hubble
observed, and we can rule out a measurement error as the cause of the Hubble
Tension with very high confidence," Riess said. "Combining Webb and
Hubble gives us the best of both worlds. We find that the Hubble measurements
remain reliable as we climb farther along the cosmic distance ladder."