- 4325 - COSMOLOGY CHANGED ? - Studying the universe involved 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.
------------------------- 4325 - COSMOLOGY CHANGED ?
- The glue that held
the cosmology stories together had been discovered a few years earlier, in 1998
called “dark energy”. This mysterious
force 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 this model
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.
-
- As astronomers
made more precise observations of the universe across the sweep of cosmic time,
cracks began to appear in this 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. 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 universe. And even more subtle
mismatches come from detailed studies of the universe’s earliest light.
-
- 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.
-
- To characterize
our universe, scientists use a handful of numbers, which cosmologists call
parameters. 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. 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”.
-
- 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.
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.
-
- 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.
-
- If this S8 tension
is really true, there’s something which we do not understand yet.
Cosmologists are now poring over the details of the
observations to take out sources of uncertainty. The Subaru team estimated the distances to
most of their galaxies based on their overall color, which could lead to
inaccuracies.
-
- 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. Plus, blurring from Earth’s atmosphere can
slightly alter the shape of a galaxy, which is one of the reasons why
astronomers are leading a weak-lensing
analysis using NASA’s James Webb Space Telescope.
-
- 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.
-
- Measurements of
how fast the universe is expanding today, 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”.
-
- 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.
-
- 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. 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. A fraction of a second into
cosmic history, some occurrence, perhaps a period of extreme acceleration known
as inflation, sent jolts oj 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 called 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.
-
- 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 discordant
values, roughly 67 versus 73, have
ignited a fiery debate in cosmology that is still unresolved.
-
- A type of old, red
star that typically lives in the outer portions of galaxies where fewer
overlapping bright stars and less dust can lead to clearer measurements. Using
those stars astronomers have measured an expansion rate of around 70 km/s/Mpc.
-
- JWST’s powerful
infrared eye is measuring distances to 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.
-
- 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.
-
- Data coming in the
next year, 2025, 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, 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 is still in its
infancy. “Now, “Everything has changed”.
-
-
January 21, 2023
COSMOLOGY CHANGED ? 4325
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