- 4292 -
JAMES WEBB DISCOVERIES!
- Finding the universe's first
galaxies is an extremely difficult task and one of the main motivations behind
building the JWST. Light from these ancient objects is red-shifted into the
infrared, which the JWST excels at sensing. By performing deep-field
observations in the infrared, the space telescope has located some of the
earliest galaxies.
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4292 - JAMES WEBB
DISCOVERIES!
- Can Webb find the
first stars in the universe? The
universe's very first stars had an important job. They formed from the
primordial elements created by the Big Bang, so they contained no metals. It
was up to them to synthesize the first metals and spread them out into the
nearby universe. In astronomy a “metal”
is anything heavier than helium.
-
- But the first
stars are more ancient than the first galaxies. The first stars formed roughly
50 to 100 million years after the Big Bang, and their light brought an eventual
end to the universe's Dark Ages. Astrophysicists think that these stars were
extremely large, with up to 1,000 solar masses.
-
- Due to the lack of
efficient coolants and fragmentation in the chemically unenriched gas at these
early epochs, the resulting metal-free
Population III stars are believed to be characterized by extremely high
masses (characteristic masses 10–1000
solar masses).
-
- To see these early,
massive stars, the JWST will need some help from gravitational lensing.
"Gravitational lensing may render individual high-mass stars detectable
out to cosmological distances, and several extremely magnified stars have in
recent years been detected out to redshifts z = 6". At z = 6, the light has taken over 12.7
billion light-years to reach us.
-
- Gravitational
lensing takes advantage of situations where a massive foreground object, like a
galaxy cluster, is between us and an object we want to observe. As the light
from the target passes by the foreground object, a gravitational lens, the
light is magnified. That makes the otherwise invisible object visible.
-
- The first stars are
at about z = 20 in terms of redshift, and the JWST should be able to see that
light if it can make use of gravitational lensing. If it can, then the powerful
telescope will start to give us observational evidence for a period of time in
the early universe that so far we understand mostly through theory, the Epoch
of Reionization (EoR).
During the Reionization, the universe was dominated by a
dense, obscuring fog of hydrogen gas. When the first stars formed, their
ultraviolet light reionized the gas, allowing light to travel. This is a
critical step in the life of the universe, so finding some of the ancient Pop
III stars that were responsible is an important goal.
-
- These first stars
are compelling in other ways they shaped our universe. They were massive,
millions of times brighter than the sun, and lived for a short time compared to
a star like our sun. They either exploded as supernovae or collapsed into black
holes. The ones that became black holes swallowed gas and other stars and
became the universe's first quasars.
These quasars grew through accretion and mergers to become the
supermassive black holes that anchor the centers of galaxies like our Milky
Way.
-
- The ones that
exploded as supernovae also played an important role. They forged the elements
heavier than hydrogen and helium, then spread those metals back out into space
when they exploded. The stars that came later contained some of these metals,
and the metals also formed rocky bodies.
-
- Prior to Population
III supernovae, there were no rocky planets and certainly no possibility of
life. So these massive, ancient stars, whether they ended as supernovae or
black holes, helped set the stage for the universe we see around us today.
-
- It's difficult to
determine metal-enriched stars from metal-poor Pop III stars
spectro-scopically. One reason is that most of these massive stars are likely
in binary pairs, and that complicates the light signal. Another reason is that
if the stars are still relatively young, they can be surrounded by nebulous
hydrogen, and that also makes the light signals difficult to interpret.
-
- 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. Now,
the James Webb Space Telescope has only entrenched the mystery.
-
- 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, an alarming hole has been growing in this picture: Depending on where
astronomers look, the rate of the universe's expansion, the Hubble constant,
varies significantly. JWST has cemented
the discrepancy with stunningly 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 fierce 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.
-
- Left to their own
devices, 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.
-
- Meanwhile, 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 (which interacts 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, according to this picture, 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. To this day, it shows no
signs of stopping.
-
- 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. To this day, it shows no signs of
stopping.
-
- The simplest and
most popular explanation for dark energy is that it is a “cosmological
constant”, 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 the latter 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.
-
- Many of the
model's predictions have been proven to be highly accurate, but here's where
the problems begin: despite much searching, astronomers have no clue what dark
matter or dark energy are. Most agree
that the universe's present composition is 5% ordinary, atomic matter; 25%
cold, dark matter; and 70% dark energy.
Depending on what method astrophysicists use, the universe appears to be
growing at different rates , a disparity known as the Hubble tension. And
methods that peer into the early universe show it expanding significantly
faster than Lambda-CDM predicts. Those methods have been vetted and verified by
countless observations.
-
- The cosmic
microwave background is the universe's 'baby picture'. The CMB is 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 spat out 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 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.
-
- It 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.
-
- It's one of the
most accurate means that astronomers have today for measuring distances.
-
- 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. In 2019, the method was used by the
Hubble Space Telescope trained on one of the Milky Way's closest neighbors, the
Large Magellanic Cloud. Their result
was explosive: an impossibly high expansion rate of 74 km/s/Mpc when compared
to the Planck measurement.
-
- So when JWST
launched in December 2021, it was poised to either resolve the discrepancy or
cement it. At 21.3 fee 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, arrived
at a nearly identical result: 73 km/s/Mpc.
-
- Other measurements
including one made by Freedman with the Hubble Space telescope 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.
-
- Cosmology in
crisis? The new result leaves the
answer wide open, splitting cosmologists into factions chasing staggeringly
different solutions. How things can be
fixed is unclear. A tweak to the
Lambda-CDM model assumes dark energy (the lambda) isn't constant but instead
evolves across the life of the cosmos according to unknown physics.
-
- It could be
possible to add some extra dark energy before the emergence of the cosmic
microwave background, giving some additional expansion that needn't make it
break from the standard model.
-
- Another group of
astronomers is convinced that the tension, alongside the observation that the
Milky Way resides inside an underdense supervoid, means that Lambda-CDM and
dark matter must be thrown out altogether.
-
- What should replace
it? A theory called Modified Newtonian
Dynamics (MOND).
This 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.
-
- Proponents of the
theory argue that our Milky Way's presence near the center of the
2-billion-light-year wide underdensity of galaxies is skewing our measurements
of the Hubble constant.
-
- It's possible
Lambda-CDM just needs a tweak, or maybe
dark matter and dark energy are the modern-day equivalent of epicycles, the
small circles ancient Greek astronomers used to model planets orbiting
Earth. But once astronomers placed the
sun in the center of the solar system in newer models, epicycles eventually
became irrelevant.
-
- But maybe also
there is dark matter and dark energy and it's just not been discovered
yet. 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. Although it now seems less
likely, it's also still possible the Hubble tension could be resolved by
figuring out some unseen systematic flaw in current measurements.
-
- A solution, or
possibly further riddles, will come from the JWST. Using the telescope’s powerful eye to make
ultradetailed measurements of Cepheid variables; tip-of-the-red-giant-branch
stars; and a type of carbon star called JAGB stars all at once distance. We'll see how well they agree and that will
give us a sense of an overall systematic answer,.
-
-
December 29, 2023
JAMES WEBB DISCOVERIES! 4292
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