- 4316 - WEBB BROKE COSMOLOGY? - After 2 years in space, the James Webb telescope has broken cosmology. For decades, measurements of the universe's expansion have suggested a disparity known as the “Hubble tension”, which threatens to break cosmology as we know it. A new finding by the James Webb Space Telescope has only entrenched the mystery.
-------------------------- 4316 - WEBB BROKE COSMOLOGY?
- Nearly a century
ago, the astronomer Edwin Hubble discovered the balloon-like inflation of the
universe and the accelerating rush of all galaxies away from each other.
Following that expansion backward in time led to our current best understanding
of how everything began, the Big Bang.
-
- But over the past
decade depending on where astronomers look, the rate of the universe's
expansion (the Hubble constant) varies significantly. The James Webb Space Telescope (JWST) has
cemented the discrepancy with precise new observations that threaten to upend
the standard model of cosmology.
-
- The new physics
needed to modify or even replace the 40-year-old theory is now a topic of
debate. It's a disagreement that has to
make us wonder if we really do understand the composition of the universe and
the physics of the universe.
-
- In an instant, the
young cosmos was formed: an expanding, roiling plasma broth of matter and
antimatter particles that popped into existence, only to annihilate each other
upon contact.
-
- The matter and
antimatter inside this plasma should have consumed each other entirely. But
scientists believe that some unknown imbalance enabled more matter than
antimatter to be produced, saving the universe from immediate self-destruction.
-
- Gravity compressed
the plasma pockets, squeezing and heating the matter so that sound waves
traveling just over half the speed of light, called “baryon acoustic
oscillations”, rippled across their surface.
-
- The high energy
density of the early universe's crowded contents stretched space-time, pulling
a small fraction of this matter safely from the fray. As the universe inflated like a balloon
ordinary matter interacting with light congealed around clumps of invisible
dark matter to create the first galaxies, connected together by a vast cosmic
web.
-
- Initially as the universe's
contents spread out, its energy density and therefore its expansion rate
decreased. But then, roughly 5 billion years ago, galaxies began to recede once
more at an ever-faster rate. The cause
was another invisible and mysterious entity known as “dark energy”.
-
- The Big Bang is
immediately followed by a rapid expansionary period called inflation. Then, as
protons and electrons combine to form atoms, light can travel freely; leaving
the cosmic microwave background imprinted upon the sky. The universe's
expansion slowed around 10 billion years ago, and it began to fill with
galaxies, stars and giant black holes. Around 5 billion years ago, dark energy
caused this cosmic expansion to rapidly accelerate. It shows no signs of
stopping.
-
- The simplest and
most popular explanation for dark energy is that it is a “cosmological
constant”. It is an inflationary energy
that is the same everywhere and at every moment; woven into the stretching
fabric of space-time. Einstein named it “lambda” in his theory of general
relativity.
-
- As our cosmos grew,
its overall matter density dropped while the dark energy density remained the
same, gradually making dark energy the biggest contributor to its overall
expansion.
-
- Added together the
energy densities of ordinary matter, dark matter, dark energy and energy from
light set the upper speed limit of the universe's expansion. They are also key
ingredients in the “Lambda cold dark matter” (Lambda-CDM) model of cosmology,
which maps the growth of the cosmos and predicts its end with matter eventually
spread so thin it experiences a heat death called the “Big Freeze”.
-
- Astronomers have no
clue what dark matter or dark energy are.
Most people agree that the universe's present composition is 5%
ordinary, atomic matter; 25% cold, dark matter; and 70% dark energy.
-
- But an even greater
threat to Lambda-CDM has materialized: Depending on what method astrophysicists
use, the universe appears to be growing at different rates. This is a disparity known as the “Hubble
tension”. And methods that peer into the early universe show it expanding
significantly faster than Lambda-CDM predicts.
-
- The “cosmic
microwave background” is the universe's 'baby picture' taken by the European
Space Agency's Planck satellite. The
first method to measure the universe's expansion rate looks at the “cosmic
microwave background” (CMB), a relic of the universe's first light produced
just 380,000 years after the Big Bang.
-
- The imprint can be
seen across the entire sky, and it was mapped to find a “Hubble constant” with
less than 1% uncertainty by the European Space Agency's (ESA) Planck satellite
between 2009 and 2013. In this cosmic
"baby picture," the universe is almost entirely uniform, but hotter
and colder patches where matter is more or less dense reveal where baryon
acoustic oscillations made it clump.
-
- As the universe
exploded outward, this soap-bubble structure ballooned into the cosmic web, a
network of crisscrossing strands along whose intersections galaxies would be
born.
-
- By studying these
ripples with the Planck satellite, cosmologists inferred the amounts of regular
matter and dark matter and a value for the cosmological constant, or dark
energy. Plugging these into the Lambda-CDM model out came a Hubble constant of
roughly 46,200 mph per million light-years, or roughly 67 kilometers per second
per megaparsec. (A megaparsec is 3.26 million light-years.)
-
- If a galaxy is at a
distance of one megaparsec away from us, that means it will retreat from us
(and us from it) at 67 kilometers per second. At twenty megaparsecs this
recession grows to 1,340 kilometers per second, and continues to grow
exponentially there onward. If a galaxy is any further than 4,475 megaparsecs
away, it will recede from us faster than the speed of light.
-
- A second method to
find this expansion rate uses pulsating stars called “Cepheid variables” These are dying stars with helium-gas outer
layers that grow and shrink as they absorb and release the star's radiation,
making them periodically flicker like distant signal lamps.
-
- In 1912, astronomer
Henrietta Swan Leavitt found that the brighter a Cepheid was, the slower it
would flicker, enabling astronomers to measure a star's absolute brightness,
and therefore gauge its distance.
-
- t was a landmark
discovery that transformed Cepheids into abundant "standard candles"
to measure the universe's immense scale. By stringing observations of pulsating
Cepheids together, astronomers can construct cosmic distance ladders, with each
rung taking them a step back into the past.
-
- To build a distance
ladder, astronomers construct the first rung by choosing nearby Cepheids and
cross-checking their distance based on pulsating light to that found by
geometry. The next rungs are added using Cepheid readings alone.
-
- Then, astronomers
look at the distances of the stars and supernovas on each rung and compare how
much their light has been redshifted (stretched out to longer, redder
wavelengths) as the universe expands.
-
- This gives a
precise measurement of the Hubble constant.
The result high expansion rate of 74 km/s/Mpc.
-
- When JWST launched
in December 2021, it was poised to either resolve the discrepancy or cement it.
At 21.3 feet wide, JWST's mirror is
almost three times the size of Hubble's, which is just 7.9 feet wide. Not only
can JWST detect objects 100 times fainter than Hubble can, but it is also far
more sensitive in the infrared spectrum, enabling it to see in a broader range
of wavelengths.
-
- By comparing
Cepheids measured by JWST in the galaxy NGC 4258 with bright Type Ia supernovas
(another standard candle because they all burst at the same absolute
luminosity) in remote galaxies the result arrived at a nearly identical result:
73 km/s/Mpc.
-
- Other measurements
on the rapid brightening of the most luminous "tip of the branch" red
giant stars, and another with light bent by the gravity of massive galaxies
came back with respective results of 69.6 and 66.6 km/s/Mpc. A separate result
using the bending of light also gave a value of 73 km/s/Mpc.
-
- The CMB
temperature is measured at the level of 1% precision, and the Cepheid distance
ladder measurement is getting close to 1%.
So a difference of 7 kilometers per second, even though it's not very
much, is very, very unlikely to be a random chance. There is something definite
to explain.
-
- How things can be
fixed is unclear. A tweak to the Lambda-CDM model that assumes dark energy (the
lambda) isn't constant but instead evolves across the life of the cosmos
according to unknown physics.
-
- What should replace
it? A theory called Modified Newtonian
Dynamics (MOND).
-
- The theory proposes
that for gravitational pulls ten trillion times smaller than those felt on
Earth's surface (such as the tugs felt between distant galaxies) Newton's laws
break down and must be replaced by other equations.
The presence near the center of the 2-billion-light-year wide
underdensity of galaxies is skewing our measurements of the Hubble constant.
-
- Cosmologists are
looking for answers in a number of places. Upcoming CMB experiments, such as
the CMB-S4 project at the South Pole and the Simons Observatory in Chile, are
searching for clues in ultraprecise measurements of the early universe's
radiation. Others will look to the dark matter maps produced by ESA's Euclid
space telescope or to the future dark energy survey conducted by the Dark
Energy Spectroscopic Instrument.
-
- Stay tunes we have
more to learn. So do you.
-
-
January 14, 2023
WEBB BROKE
COSMOLOGY? 4316
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