Clashing Cosmic Numbers Challenge Our Best Theory of the Universe
As measurements of distant stars
and galaxies become more precise, cosmologists are struggling to make sense of
sparring values.
https://www.quantamagazine.org/clashing-cosmic-numbers-challenge-our-best-theory-of-the-universe-20240119/
Scientists working to solve the
biggest puzzle of them all — how the universe works — are running into trouble.
Kouzou Sakai for Quanta
Magazine
January 19, 2024
astronomyastrophysicscosmologydark energydark matterHubble constantphysicssupernovasAll topics
Introduction
In the
early 2000s, it seemed that cosmologists had solved the largest and most
complex puzzle of all: how the universe works.
“There was this amazing moment
when all of a sudden, all the pieces in cosmology snapped together,” said J. Colin Hill, a theoretical cosmologist at
Columbia University.
All the ways of studying the
universe — mapping galaxies and their larger structures, catching catastrophic
stellar explosions called supernovas, calculating distances to variable stars,
measuring the residual cosmic glow from the early universe — told stories that
“seemed to overlap,” Hill said.
The glue that held the stories
together had been discovered a few years earlier, in 1998: dark energy, a
mysterious force that, rather than gluing the cosmos together, is somehow
causing it to expand ever more speedily instead of slowing down over time. When
scientists included this cosmic something in their models of the universe,
theories and observations meshed. They drafted what is now known as the
standard model of cosmology, called Lambda-CDM, in which dark energy makes up
nearly 70% of the universe, while another mysterious dark entity — a type of
invisible mass that seems to interact with normal matter only through gravity —
makes up about 25%. The remaining 5% is everything we can see: the stars,
planets and galaxies that astronomers have studied for millennia.
But that moment of tranquility
was only a brief respite between times of struggle. As astronomers made more
precise observations of the universe across the sweep of cosmic time, cracks
began to appear in the standard model. Some of the first signs of trouble came
from measurements of variable stars and supernovas in a
handful of nearby galaxies — observations that, when compared with the residual
cosmic glow, suggested that our universe plays by different rules than we
thought, and that a crucial cosmological parameter that defines how fast the
universe is flying apart changes when you measure it with different yardsticks.
Cosmologists had a problem —
something they called a tension, or, in their more dramatic moments, a crisis.
Introduction
Those
discordant measurements have only become more distinct in the decade or so
since the first cracks emerged. And this discrepancy isn’t the only challenge
to cosmology’s standard model. Observations of galaxies suggest that the way in
which cosmic structures have clumped together over
time may differ from our best understanding of how today’s universe should have
grown from seeds embedded in the early cosmos. And even more subtle mismatches
come from detailed studies of the universe’s earliest light.
Other inconsistencies abound.
“There are many more smaller problems elsewhere,” said Eleonora Di Valentino, a
theoretical cosmologist at the University of Sheffield. “This is why it’s
puzzling. Because it’s not just these big problems.”
To alleviate these tensions,
cosmologists are taking two complementary approaches. First, they’re continuing
to make more precise observations of the cosmos, in the hope that better data
will reveal clues as to how to proceed. In addition, they are finding ways to
subtly tweak the standard model to accommodate the unexpected results. But
these solutions are often contrived, and if they solve one problem they often
make others worse.
“The situation right now seems
like a big mess,” Hill said. “I don’t know what to make of it.”
Warped Light
To characterize our universe,
scientists use a handful of numbers, which cosmologists call parameters. The
physical entities that these values refer to are all gears in a giant cosmic
machine, with each bit connected to the others.
One of those parameters relates
to how strongly mass clumps together. That, in turn, tells us something about
how dark energy operates, as its accelerating outward push conflicts with the
gravitational pull of cosmic mass. To quantify clumpiness, scientists use a
variable called S8. If the value is zero, then the universe has no
variation and no structure, explained Sunao
Sugiyama, an observational cosmologist at the University of
Pennsylvania. It’s like a flat, featureless prairie, with not even an anthill
to break up the landscape. But if S8 is closer to 1, the
universe is like a huge, jagged mountain range, with massive clumps of dense
matter separated by valleys of nothingness. Observations made by the Planck
spacecraft of the very early universe — where the first seeds of structure took
hold — find a value of 0.83.
Sunao Sugiyama of the University
of Pennsylvania led an analysis suggesting that matter could be
distributed throughout the cosmos a bit differently than theories predict.
Koji Okumura/ Forward Stroke
Inc.
Introduction
But
observations of recent cosmic history don’t quite agree.
To compare the clumpiness in
today’s universe with measurements of the infant cosmos, researchers survey how
matter is distributed over large swaths of sky.
Accounting for visible galaxies
is one thing. But mapping the invisible network upon which those galaxies lie
is another. To do that, cosmologists look at tiny distortions in the galaxies’
light, because the path light takes as it weaves through the cosmos is warped
as the light is deflected by the gravitational heft of invisible matter.
By studying these distortions
(known as weak gravitational lensing), researchers can trace the distribution
of dark matter along the paths the light took. They can also estimate where the
galaxies are. With both bits of information in hand, astronomers create 3D maps
of the universe’s visible and invisible mass, which lets them measure how the
landscape of cosmic structure changes and grows over time.
Over the past few years, three
weak-lensing surveys have mapped large patches of the sky: the Dark Energy
Survey (DES), which uses a telescope in Chile’s Atacama desert; the Kilo-Degree
Survey (KIDS), also in Chile; and most recently, a five-year survey from the
Subaru Telescope’s Hyper Suprime-Cam (HSC) in Hawai‘i.
A few years ago, the DES and
KIDS surveys produced S8 values lower than Planck’s — implying smaller
mountain ranges and lower peaks than what the primordial cosmic soup set up.
But those were just tantalizing hints of flaws in our understanding of how
cosmic structures grow and conglomerate. Cosmologists needed more data and were
eagerly awaiting the Subaru HSC results, which were published in a series of five papers in December.
Introduction
The Subaru
HSC team surveyed tens of millions of galaxies covering about 416 square
degrees on the sky, or the equivalent of 2,000 full moons. In their patch of
sky, the team calculated an S8 value of 0.78 — in line
with the initial results from earlier surveys, and smaller than the measured
value from the Planck telescope’s observations of the early universe’s
radiation. The Subaru team is careful to say that their measurements only “hint”
at a tension because they haven’t quite reached the level of statistical
significance that scientists rely on, although they’re working on adding
another three years of observations to their data.
“If this S8 tension is really true,
there’s something which we do not understand yet,” said Sugiyama, who led one
of the Subaru HSC analyses.
Cosmologists are now poring over
the details of the observations to suss out sources of uncertainty. For
starters, the Subaru team estimated the distances to most of their galaxies
based on their overall color, which could lead to inaccuracies. “If you got the
[average] distance estimates wrong, you would get some of your cosmological
parameters you care about wrong as well,” said team member Rachel Mandelbaum of
Carnegie Mellon University.
There may be, in fact, multiple pieces of new physics at play.
J. Colin
Hill, Columbia University
On top of that, these
measurements aren’t easy to make, with subtle complexities in interpretation.
And the difference between a galaxy’s warped appearance and its actual shape —
the key to identifying invisible mass — is often very small, said Diana Scognamiglio of NASA’s Jet Propulsion
Laboratory. Plus, blurring from Earth’s atmosphere can slightly alter the shape
of a galaxy, which is one of the reasons why Scognamiglio is leading a
weak-lensing analysis using NASA’s James Webb Space Telescope.
Adding more confusion,
scientists with the DES and KIDS teams recently reanalyzed their measurements together
and derived an S8 value closer to the Planck results.
So for now, the picture is
messy. And some cosmologists aren’t yet convinced that the various S8 measurements are in
tension. “I don’t think there’s an obvious hint of a major catastrophic failure
there,” Hill said. But, he added, “it’s not implausible that there could be
something interesting going on.”
Where Cracks Are Evident
A dozen years ago, scientists
saw the first hints of trouble with measurements of another cosmological
parameter. But it took years to accumulate enough data to convince most
cosmologists that they were dealing with a full-on crisis.
In brief, measurements of how
fast the universe is expanding today — known as the Hubble constant —
don’t match the value you get when extrapolating from the early universe. The
conundrum has become known as the Hubble tension.
The cosmic microwave background,
seen here as measured by the Planck mission, is an imprint of the first light
that traveled freely in the infant universe.
ESA and the Planck Collaboration
Introduction
To
calculate the Hubble constant, astronomers need to know how far away things
are. In the nearby cosmos, scientists measure distances using stars called
Cepheid variables that periodically change in brightness. There’s a well-known
relationship between how fast one of these stars swings from brightest to
faintest and how much energy it radiates. That relation, which was discovered
in the early 20th century, allows astronomers to calculate the star’s intrinsic
brightness, and by comparing that to how bright it appears, they can calculate
its distance.
Using these variable stars,
scientists can measure the distances to galaxies up to about 100 million
light-years from us. But to see a bit farther away, and a bit further back in
time, they use a brighter mile marker — a specific type of stellar explosion
called a type Ia supernova. Astronomers can also calculate the intrinsic
brightness of these “standard candles,” which allows them to measure distances
to galaxies billions of light-years away.
Over the past two decades, these
observations have helped astronomers pin a value on how fast the nearby
universe is expanding: roughly 73 kilometers per second per megaparsec, which
means that as you look further away, for each megaparsec (or 3.26 million
light-years) of distance, space is flying away 73 kilometers per second faster.
If the expansion rate could somehow be increased, just a little bit for
a little while in the early universe, you can resolve the Hubble tension.
Marc
Kamionkowski, Johns Hopkins University
But that value clashes with one
derived from another ruler embedded in the infant universe.
In the very beginning, the
universe was searing plasma, a soup of fundamental particles and energy. “It
was a hot mess,” said Vivian Poulin-Détolle, a
cosmologist at the University of Montpellier.
A fraction of a second into
cosmic history, some occurrence, perhaps a period of extreme acceleration known
as inflation, sent jolts — pressure waves — through the murky plasma.
Then, as the universe cooled,
light that was trapped in the elemental plasma fog finally broke free. That
light — the cosmic microwave background, or CMB — reveals those early pressure
waves, just as the surface of a frozen lake holds onto the overlapping crests
of waves frozen in time, Poulin-Détolle said.
Cosmologists have measured the
most common wavelength of those frozen pressure waves and used it to calculate
a value for the Hubble constant of 67.6 km/s/Mpc, with an
uncertainty of less than 1%.
The peculiarly discordant values
— roughly 67 versus 73 — have ignited a fiery debate in cosmology that is still
unresolved.
Astronomers are turning to
independent cosmic mile markers. For the past six years, Wendy Freedman of the University of Chicago
(who has worked on the Hubble constant for a quarter century) has focused on a
type of old, red star that typically lives in the outer portions of galaxies.
Out there, fewer overlapping bright stars and less dust can lead to clearer
measurements. Using those stars, Freedman and her colleagues have measured an
expansion rate of around 70 km/s/Mpc — “which is actually in pretty good
agreement with the Cepheids,” she said. “But it’s also in pretty good agreement
with the microwave background.”
Wendy Freedman, a cosmologist at
the University of Chicago, is using multiple cosmic mile markers to measure how
fast the universe is expanding.
Nancy Wong
Introduction
She has now
turned to JWST’s powerful infrared eye to approach the problem. With her
colleagues, she is measuring distances to these giant red stars in 11 nearby
galaxies while simultaneously measuring the distances to Cepheids and a type of
pulsating carbon star in those same galaxies. They expect to publish the
results sometime this spring, but already, she said, “the data look really
spectacular.”
“I’m very interested to see what
they find,” said Hill, who works to understand models of the universe. Will
these new observations widen the cracks in cosmology’s favorite model?
A New Model?
As observations continue to
constrain these crucial cosmological parameters, scientists are trying to fit
the data to their best models of how the universe works. Perhaps more precise
measurements will solve their problems, or maybe the tensions are just an
artifact of something mundane, like quirks of the instruments being used.
Or maybe the models are wrong,
and new ideas — “new physics” — will be needed.
“Either we haven’t been clever
enough to come up with a model that actually fits everything,” Hill said, or
“there may be, in fact, multiple pieces of new physics at play.”
J. Colin Hill, a theoretical
cosmologist at Columbia University, is trying to resolve a disagreement between
astronomical observations and our standard model of how the universe works.
John Smock/Simons Foundation
Introduction
What might
they be? Perhaps a new fundamental force field, Hill said, or interactions
among dark matter particles that we don’t yet understand, or new ingredients
that aren’t yet part of our description of the universe.
Some new physics models tweak
dark energy, adding a surge of cosmic acceleration in the early moments of the
universe, before electrons and protons glommed onto each other. “If the
expansion rate could somehow be increased, just a little bit for a little while
in the early universe,” said Marc Kamionkowski, a
cosmologist at Johns Hopkins University, “you can resolve the Hubble tension.”
Kamionkowski and one of his
graduate students proposed the idea in 2016, and two years later they outlined some signatures that a high-resolution cosmic
microwave background telescope should be able to see. And the Atacama Cosmology
Telescope, perched on a mountain in Chile, did see some of those signals. But
since then, other scientists have shown that the model creates problems with
other cosmic measurements.
That kind of fine-tuned model,
where an additional type of dark energy surges for a moment and then fades out,
is too complicated to explain what’s happening, said Dragan Huterer, a theoretical cosmologist at the
University of Michigan. And other proposed solutions to the Hubble tension tend
to match observations even more poorly. They’re “hopelessly tuned,” he said,
like just-so stories that are too specific to be in step with the long-held
idea that simpler theories tend to win out against complex ones.
Data coming in the next year may
help. First up will be the results from Freedman’s team looking at different
probes of the nearby expansion rate. Then in April, researchers will reveal the
first data from the largest cosmological sky survey to date, the Dark Energy
Spectroscopic Instrument. Later in the year, the Atacama Cosmology Telescope
team — and researchers making another primordial background map using the South
Pole Telescope — will likely release their detailed results of the microwave
background at higher resolution. Observations on the more distant horizon will
come from the European Space Agency’s Euclid, a space telescope that launched
in July, and the Vera C. Rubin Observatory, an all-sky mapping machine being
built in Chile that will be fully operational in 2025.
The universe might be 13.8
billion years old, but our quest to understand it — and our place within it —
is still in its infancy. Everything in cosmology fit together just 15 years
ago, in a brief period of tranquility that turned out to be a mirage. The
fissures that appeared a decade ago have split wide open, creating bigger rifts
in cosmology’s favorite model.
“Now,” Di Valentino said,
“Everything has changed.”
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