- 4301 - COSMOLOGICAL CONSTANT? - The cosmological constant is a form of matter or energy that acts in opposition to gravity and is considered by many physicists to be equivalent to dark energy. Nobody really knows what the cosmological constant is exactly, but it is required in cosmological equations in order to reconcile theory with our observations of the universe.
------------------------- 4301 - COSMOLOGICAL CONSTANT?
- In the 1990s,
researchers used supernovae to identify dark energy's existence, bringing
science back to Einstein's once-discarded cosmological constant.
-
- Who came up with
the cosmological constant? Albert
Einstein, came up with the cosmological constant, which he called the
"universal constant," in 1915 as a means to balance certain
calculations in his theory of general relativity.
-
- At the time,
physicists believed the universe was static, neither expanding nor
contracting, but Einstein's work
suggested that gravity would cause it to do one or the other. So, to mesh with
the scientific consensus, Einstein inserted a fudge factor, denoted by the
Greek letter “lambda”, into his results, which kept the cosmos still. No explanation.
-
- A little over a
decade later, the American astronomer Edwin Hubble noticed that galaxies were
actually moving away from us, indicating the universe was expanding. Einstein
called lambda his "greatest mistake."
-
- Hubble's
observations negated the need for a cosmological constant for decades, but that
changed when astronomers examining distant supernovas in the late 1990s
discovered that the cosmos was not only expanding, but “accelerating in its
expansion”. They named the mysterious anti-gravity force required to account
for this phenomena "dark energy."
-
- In the 1920s,
Russian physicist Alexander Friedmann developed an equation, now called the
“Friedmann Equation”, which describes the properties of the universe from the
Big Bang onward.
-
- Using Einstein's
lambda and plugging it into the Friedmann equations, researchers could model
the cosmos correctly with an accelerating expansion rate. This version of the
Friedmann Equation now forms the backbone of contemporary cosmological theory,
which is known as ΛCDM (Lambda CDM, where CDM stands for cold dark matter) and
accounts for all the known components of “reality”.
-
- So, what is this
magic number then? No one truly
understands what lambda is. Most physicists consider it interchangeable with
the concept of dark energy, but that doesn't make things any clearer because
dark energy is simply a placeholder describing some unknown anti-gravity
substance.
-
- We've essentially
reverted to using Einstein's fudge factor.
One potential explanation for the cosmological constant lies in the
realm of modern particle physics. Experiments have verified that empty space is
permeated by countless “virtual particles” constantly popping in and out of
existence. This ceaseless action creates what is known as a "vacuum
energy," or a force arising from empty space, inherent in the fabric of
space-time that could drive apart the universe.
-
- Connecting vacuum
energy to the cosmological constant is not easy. Based on their observations of
supernovas, astronomers estimate that dark energy should have a small value,
just enough to push everything in the universe apart over billions of years.
-
- Yet when
scientists try to calculate the amount of energy that should arise from virtual
particle motion, they come up with a result that's 120 orders of magnitude
greater than what the supernova data suggest.
The worst theoretical prediction in the history of physics
-
- To add to the
mystery, some researchers have proposed that the cosmological constant might
not be a constant at all, but rather changes or fluctuates with time. This
theory is called “quintessence” and some projects, such as the Dark Energy
Survey, are currently making precise observations to see if it has any
observational support.
-
- Cosmologists will
continue to use “lambda”. They may not know what it is, but they know that they
need it to make the universe make sense.
Something is awry in our expanding universe.
-
- Nearly a century
ago when astronomer Edwin Hubble discovered the balloon-like inflation of the
universe, the accelerating rush was
pushing 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 (a value called the
Hubble constant) varies significantly.
-
- Now, on the second
anniversary of its launch, the James Webb Space Telescope (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.
The physics in the1998 discovery of dark energy is the mysterious force
behind the universe's accelerating expansion.
-
- The expansion of
the Universe, “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.
-
- It started all with
a bang. Then 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. You are made of this
left over matter? Thank God.
-
- 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 (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 was another invisible and mysterious entity known as “dark energy”.
-
- 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 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”.
-
- 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.
-
- 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. Those methods have
been vetted and verified by countless observations.
-
- The first method to
measure this growth 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 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
(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”, 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. But it is 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 this discrepancy.
-
- 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, astronomers 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. Cosmologists were
left reeling.
-
- 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.
-
- The new result
leaves the answer wide open, splitting cosmologists into factions chasing
staggeringly different solutions.
-
- How things can be
fixed is unclear. Maybe 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 oomph to the universe’s 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.
-
- 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.
The orbits of planets were described very accurately by epicycles. It was a good model! It fitted the data. But once astronomers placed the sun in the
center of the solar system in newer models, epicycles eventually became
irrelevant.
-
- 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 tuned, I still
have more work to do before I can explain myself.
-
-
January 1, 2023
COSMOLOGICAL CONSTANT? 4301
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