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Several
unexplained measurements are threatening to upend
scientists’ understanding of the universe’s origin
and fate
Back
in the mid-1990s, cosmologists—who study
the origin, composition and structure of
the universe—were beginning to worry
that they were facing a crisis. For
starters, two astronomers had observed
that a huge swath of the cosmos, a
billion light-years or so across, was
moving in a direction inconsistent with
the general expansion of the universe.
Worse, astrophysicists using the Hubble
Space Telescope, then relatively new,
had determined that the cosmos was
between eight billion and 12 billion
years old. The problem: even the high
end of that range couldn’t account for
stars known to be closer to 14 billion
years old, leading to the nonsensical
implication that the stars existed
before the universe did. “If you ask
me,” astrophysicist
Michael Turner told Time magazine at
the time, “either we’re close to a
breakthrough or we’re at our wits’ end.”
But the first observation was never
confirmed. And the impossibly old stars
were explained a few years later with
the discovery
that a mysterious, and still unknown,
dark energy had
turbocharged the expansion of the
universe, making it look younger than it
actually is.
Now,
however, cosmologists are facing a
brand-new problem—or rather a couple of
problems. The Hubble constant (named, as
the telescope is, for Edwin Hubble, who
discovered the expansion of the universe
in the 1920s) is the number that shows
how fast the cosmos is expanding; it’s
been measured with greater and greater
accuracy over the past few decades. Yet
there’s still some uncertainty because
two independent methods of calculating
it have come up with different answers,
giving rise to what’s called the “Hubble
tension.” Although the numbers
aren’t dramatically different, they’re
enough at odds to worry theorists. “In
particle physics,” said David
Gross of the Kavli
Institute for Theoretical Physics at the
University of California, Santa Barbara,
at a conference in 2019, “we wouldn’t
call it a tension or a problem but
rather a crisis.”
Another
issue is that the tendency of matter to
clump together in the early universe is
inconsistent with how it clumps together
today. Known as the sigma-eight,
or S8,
tension, it is like a “little
brother or sister of the Hubble
tension.... So [it is] less significant
but worth keeping an eye on,” says Adam
Riess of the Space Telescope Science
Institute, who shared of the 2011 Nobel
Prize in Physics for his co-discovery of
dark energy.
Both
problems could signal that scientists
are misunderstanding something big about
physics, and a recent paper in the
journal Physical
Review Letters adds
to the suspicion that this might be the
case—for
the S8 tension,
at least. In the so-called
standard model of cosmology, the
universe started off almost but not
quite uniformly dense. We know that
because the oldest light we can see,
known as the cosmic microwave
background, shows
only tiny variations in temperature from
one point on the sky to the next,
reflecting variations in the density of
energy and matter in the cosmos. As the
universe expanded, gravity, as described
by Einstein’s general theory of
relativity, amplified those variations
to create the huge variations we see
today in the form of clusters and
superclusters of galaxies. That process
is somewhat suppressed, however, by dark
energy—the still mysterious force
causing the expansion of the universe to
accelerate rather than slow down—which
pushes matter apart before the density
variations can get even greater.
In
the new paper, scientists argue that
this suppression of clustering is too
large to explain with the standard
model. Not only that, says Robert
Caldwell, a cosmologist at Dartmouth
College, who did not participate in the
new study, “it seems like the timing of
whatever’s causing the acceleration is
not in synchrony with the effect on the
clumpiness,” he explains. That is to
say, the suppression of the growth of
the so-called large-scale structure of
the universe—the web of galaxies,
clusters and other cosmic structures
that are bound by gravity—begins to kick
in later than you’d expect to see from
dark energy alone. This observation
suggests that some theory of gravity
other than general relativity might
conceivably be at play, the authors
argue. “It’s a thought-provoking
analysis,” says Benjamin
Wandelt of the
Lagrange Institute in France, who also
wasn’t involved in the study. “Exciting
if true—but changing general relativity
is a high price to pay.”
So
is it true? The answer so far is that
nobody knows for sure. “It’s an
interesting paper,” says David
Weinberg, chair of the astronomy
department at the Ohio State University,
who wasn’t involved in the study, “but I
wouldn’t say it’s a big deal on its
own.” The investigation does, however,
“fit into a larger set of papers that
are maybe finding a discrepancy between
the level of matter clustering in the
present-day universe, compared to what
we would predict based on what we
observe in the cosmic microwave
background,” he says. These
discrepancies would be small enough to
make theorists wary that they might not
be significant at all, except that they
all tend to point in the same direction,
with modern-day density variations below
what you’d expect, based on the standard
model.
“If
they’re real,” Weinberg says, “the
implications are very profound because
you would probably have to modify the
theory of gravity on cosmological scales
in order to explain it.” And, he adds,
“that’s not easy to do.” (To be clear,
this kind of change would be different
from “modified Newtonian dynamics,” or MOND,
a theory of modified gravity proposed to
explain away dark matter. Here, too, the
idea of tinkering with general
relativity has been tough for
astrophysicists to entertain.)
What
might be different in this case is that
the authors—Nhat-Minh Nguyen, Dragan
Huterer and Yuewei Wen, all at the
University of Michigan—didn’t set out to
solve the problem of the S8 tension.
They were interested in whether the
history of the universe’s expansion was
consistent with the history of structure
growth. “We expected,” says Nguyen, lead
author of the paper, “that they would,
in fact, be consistent.” When the
researchers found this wasn’t the case,
he adds, they went back and rechecked
their analysis to make sure they weren’t
missing something. “But we found that we
weren’t,” Nguyen says. The
inconsistency, it turned out, might be
explained by some additional force
layered on top of gravity and dark
energy—a force that would add to the
tendency of dark energy to tamp down
structure formation. Or it could suggest
that dark energy itself became stronger
at some point, Caldwell says. “That’s
what excited me about the paper,” he
adds.
Caldwell
doesn’t consider the paper definitive,
though. Jo Dunkley, a physicist at
Princeton University, who also wasn’t
involved with the work, agrees. “This is
interesting,” she says, “but to me, it
is too soon to say that this shows
significant evidence of a problem” with
the standard model of cosmology. And a
few scientists, including David Spergel,
former chair of astrophysics at
Princeton and now president
of the Simons Foundation, think
the argument isn’t very convincing.
“[The authors] ignore recent
measurements that are
consistent with standard theory,” says
Spergel, who wasn’t part of the study.
“And as this
paper argues, analyses of
large-scale structure at [nearby
distances] are probably underestimating
the important role that galaxy winds
play in driving gas out of galaxies. I’m
not sure I would have published this
paper.”
On
Spergel’s first point, Nguyen agrees
that he and his colleagues need to do
more research. “We’re looking into more
datasets from new, presumably
independent experiments of the same
observables,” he says. But Nguyen also
points out that in the “recent
measurements” that Spergel cites, the
latter’s team actually references Nguyen
and his colleagues’ latest work and the
idea of tweaking with general relativity
as a possible solution to the S8 tension.
And, Nguyen argues, “the community is
still divided over the role of [winds]
in reconciling S8.”
In
short, everyone, including Nguyen and
his co-authors, agree that their results
are not definitive. “It’s useful to play
these exercises,” says Nico
Hamaus of the Ludwig
Maximilian University of Munich in
Germany. “That’s exactly how you find
loopholes in the models, and if we can
really substantiate such things, that
really means there’s something going on
that we don’t understand.” But even if
definitive confirmation comes, the
Hubble tension remains, and almost
everyone agrees that problem is a much
bigger deal.
And
“tensions” aren’t even the only things that keep
cosmologists up at night. In a recent op-ed in the New
York Times entitled “The
Story of Our Universe May Be Starting to Unravel,”
astrophysicist Adam Frank of the University of
Rochester and Marcelo Gleiser of Dartmouth College
cite the thorniest issues facing cosmology. They
focus primarily on the Hubble tension (but,
interestingly, not the S8 tension)
and also point to discoveries by the James Webb
Space Telescope of surprisingly
large galaxies that formed
surprisingly soon after the big bang. “We may be at
a point,” they write, “where we need a radical
departure from the standard model, one that may even
require us to change how we think of the elemental
components of the universe, possibly even the nature
of space and time.”
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