When you are trying to solve one of the biggest conundrums in cosmology, you should triple check your homework. The puzzle, called the "Hubble Tension," is that the current rate of the expansion of the universe is faster than what astronomers expect it to be, based on the universe's initial conditions and our present understanding of the universe’s evolution.
Scientists using NASA's Hubble
Space Telescope and many other telescopes
consistently find a number that does not match predictions based on
observations from ESA's (European Space Agency's) Planck mission. Does resolving this discrepancy require new physics? Or
is it a result of measurement errors between the two different methods used to
determine the rate of expansion of space?
This image of NGC 5468, a galaxy located about 130
million light-years from Earth, combines data from the Hubble and James Webb
space telescopes. This is the farthest galaxy in which Hubble has identified
Cepheid variable stars. These are important milepost markers for measuring the
expansion rate of the universe. The distance calculated from Cepheids has been
cross-correlated with a type Ia supernova in the galaxy. Type Ia supernovae are
so bright they are used to measure cosmic distances far beyond the range of the
Cepheids, extending measurements of the universe's expansion rate deeper into
space.
Hubble has been measuring the
current rate of the universe’s expansion for 30 years, and astronomers want to
eliminate any lingering doubt about its accuracy. Now, Hubble and NASA’s James
Webb Space Telescope have tag-teamed to produce definitive measurements, furthering the
case that something else – not measurement errors – is influencing the
expansion rate.
“With measurement errors negated,
what remains is the real and exciting possibility we have misunderstood the
universe,” said Adam Riess, a physicist at Johns Hopkins University in
Baltimore. Riess holds a Nobel Prize for co-discovering the fact that the
universe’s expansion is accelerating, due to a mysterious phenomenon now called
“dark energy.”
As a crosscheck, an initial Webb observation in 2023 confirmed that Hubble measurements of the
expanding universe were accurate. However, hoping to relieve the Hubble
Tension, some scientists speculated that unseen errors in the measurement may
grow and become visible as we look deeper into the universe. In particular,
stellar crowding could affect brightness measurements of more distant stars in
a systematic way.
The SH0ES (Supernova H0 for the
Equation of State of Dark Energy) team, led by Riess, obtained additional
observations with Webb of objects that are critical cosmic milepost markers,
known as Cepheid variable stars, which now can be correlated with the Hubble data.
“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.
The team’s first few Webb
observations in 2023 were successful in showing Hubble was on the right track
in firmly establishing the fidelity of the first rungs of the so-called cosmic distance ladder.
Astronomers use various methods to
measure relative distances in the universe, depending upon the object being
observed. Collectively these techniques are known as the cosmic distance ladder
– each rung or measurement technique relies upon the previous step for
calibration.
But some astronomers suggested
that, moving outward along the “second rung,” the cosmic distance ladder might
get shaky if the Cepheid measurements become less accurate with distance. Such
inaccuracies could occur because the light of a Cepheid could blend with that
of an adjacent star – an effect that could become more pronounced with distance
as stars crowd together and become harder to distinguish from one another.
The observational challenge is that
past Hubble images of these more distant Cepheid variables look more huddled
and overlapping with neighboring stars at ever farther distances between us and
their host galaxies, requiring careful accounting for this effect. Intervening
dust further complicates the certainty of the measurements in visible light.
Webb slices though the dust and naturally isolates the Cepheids from
neighboring stars because its vision is sharper than Hubble’s at infrared
wavelengths.
At the center of these side-by-side images is a special class of star used as a milepost marker for measuring the universe’s rate of expansion – a Cepheid variable star. The two images are very pixelated because they are a very zoomed-in view of a distant galaxy. Each of the pixels represents one or more stars. The image from the James Webb Space Telescope is significantly sharper at near-infrared wavelengths than Hubble (which is primarily a visible-ultraviolet light telescope). By reducing the clutter with Webb’s crisper vision, the Cepheid stands out more clearly, eliminating any potential confusion. Webb was used to look at a sample of Cepheids and confirmed the accuracy of the previous Hubble observations that are fundamental to precisely measuring the universe’s expansion rate and age. NASA, ESA, CSA, STScI, Adam G. Riess (JHU, STScI)
“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,” said Riess.
The new Webb observations include five host galaxies of eight Type Ia
supernovae containing a total of 1,000 Cepheids, and reach out to the farthest
galaxy where Cepheids have been well measured – NGC 5468 – at a distance of 130
million light-years. “This spans the full range where we made measurements with
Hubble. So, we've gone to the end of the second rung of the cosmic distance
ladder,” said co-author Gagandeep Anand of the Space Telescope Science
Institute in Baltimore, which operates the Webb and Hubble telescopes for NASA.
Hubble and Webb’s further
confirmation of the Hubble Tension sets up other observatories to possibly
settle the mystery. NASA’s upcoming Nancy Grace Roman Space Telescope will do wide celestial surveys to study the
influence of dark energy, the mysterious energy that is causing the expansion
of the universe to accelerate. ESA's Euclid observatory, with NASA contributions, is
pursuing a similar task.
At present it’s as though the
distance ladder observed by Hubble and Webb has firmly set an anchor point on
one shoreline of a river, and the afterglow of the big bang observed by
Planck’s measurement from the beginning of the universe is set firmly on the
other side. How the universe’s expansion was changing in the billions of years
between these two endpoints has yet to be directly observed. “We need to find
out if we are missing something on how to connect the beginning of the universe
and the present day,” said Riess.
These finding were published in the
February 6, 2024 issue of The Astrophysical Journal Letters.
The Hubble Space Telescope has been
operating for over three decades and continues to make ground-breaking
discoveries that shape our fundamental understanding of the universe. Hubble is
a project of international cooperation between NASA and ESA. NASA's Goddard
Space Flight Center in Greenbelt, Maryland, manages the telescope. Goddard also
conducts mission operations with Lockheed Martin Space in Denver, Colorado. The
Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts
Hubble and Webb science operations for NASA.
The James Webb Space Telescope is
the world's premier space science observatory. Webb is solving mysteries in our
solar system, looking beyond to distant worlds around other stars, and probing
the mysterious structures and origins of our universe and our place in it. Webb
is an international program led by NASA with its partners, ESA (European Space
Agency) and the
Source: NASA's Webb, Hubble Telescopes Affirm Universe's Expansion Rate, Puzzle Persists
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