- 4466 - DARK ENERGY - expanding the Universe? Some 13.8 billion years ago, the universe began with a rapid expansion we call the Big Bang. After this initial expansion, which lasted a fraction of a second, gravity started to slow the universe down. But the cosmos wouldn’t stay this way. Nine billion years after the universe began, its expansion started to speed up, driven by an unknown force that scientists have named “dark energy”.
------------------------------- 4466 - DARK ENERGY - expanding the Universe?
- But what exactly is dark energy? We don't know. But we do know that it
exists, it’s making the universe expand at an accelerating rate, and
approximately 68.3 to 70% of the universe is
is made of dark energy.
-
- Dark energy wasn't discovered until the late
1990s. But its origin in scientific study stretches all the way back to 1912
when American astronomer Henrietta Swan Leavitt made an important discovery
using Cepheid variables, a class of stars whose brightness fluctuates with a
regularity that depends on the star's brightness.
-
- All Cepheid stars with a certain period (a
Cepheid’s period is the time it takes to go from bright, to dim, and bright
again) have the same absolute magnitude, or luminosity, or the amount of light
they put out.
-
- Leavitt measured these stars and proved
that there is a relationship between their regular period of brightness and
luminosity. Leavitt’s findings made it possible for astronomers to use a star’s
period and luminosity to measure the distances between us and Cepheid stars in
far-off galaxies and our own Milky Way.
-
- Around this same time in history,
astronomer Vesto Slipher observed spiral galaxies using his telescope’s
spectrograph, a device that splits light into the colors that make it up, much
like the way a prism splits light into a rainbow. He used the spectrograph, a
relatively recent invention at the time, to see the different wavelengths of
light coming from the galaxies in different spectral lines.
-
- With his observations, Silpher was the
first astronomer to observe how quickly the galaxy was moving away from us,
called “redshift”, in distant galaxies. These observations would prove to be
critical for many future scientific breakthroughs, including the discovery of
dark energy.
-
- Redshift is a term used when astronomical
objects are moving away from us and the light coming from those objects
stretches out. Light behaves like a wave, and red light has the longest
wavelength. So, the light coming from objects moving away from us has a longer
wavelength, stretching to the “red end” of the electromagnetic.
-
- The discovery of galactic redshift, the
period-luminosity relation of Cepheid variables, and a newfound ability to
gauge a star or galaxy’s distance eventually played a role in astronomers
observing that galaxies were getting farther away from us over time, which
showed how the universe was expanding.
-
- In 1922, Russian scientist and mathematician
Alexander Friedmann published a paper detailing multiple possibilities for the
history of the universe. The paper, which was based on Albert Einstein’s theory
of general relativity published in 1917, included the possibility that the
universe is expanding.
-
- In 1927, Belgian astronomer Georges Lemaître
published a paper also factoring in Einstein’s theory of general relativity.
And, while Einstein stated in his theory that the universe was static, Lemaître
showed how the equations in Einstein’s theory actually support the idea that
the universe is not static but, in fact, is actually expanding.
-
- Astronomer Edwin Hubble confirmed that the
universe was expanding in 1929 using observations made by his associate,
astronomer Milton Humason. Humason measured the redshift of spiral galaxies.
Hubble and Humason then studied Cepheid stars in those galaxies, using the
stars to determine the distance of their galaxies (or nebulae).
-
- They compared the distances of these
galaxies to their redshift and tracked how the farther away an object is, the
bigger its redshift and the faster it is moving away from us. The pair found
that objects like galaxies are moving away from Earth faster the farther away
they are, at upwards of hundreds of thousands of miles per second. This observation is now known as Hubble’s
Law, or the Hubble-Lemaître law. The universe, they confirmed, is really
expanding.
-
- Scientists previously thought that the
universe's expansion would likely be slowed down by gravity over time, an
expectation backed by Einstein's theory of general relativity. But in 1998,
everything changed when two different teams of astronomers observing far-off
supernovae noticed that at a certain redshift the stellar explosions were
dimmer than expected.
-
- While dim supernovae might not seem like a
major find, these astronomers were looking at “Type 1a supernovae”, which are
known to have a certain level of luminosity. So they knew that there must be
another factor making these objects appear dimmer. Scientists can determine
distance and speed using an objects' brightness, and dimmer objects are
typically farther away though surrounding dust and other factors can cause an
object to dim.
-
- This led the scientists to conclude that
these supernovae were just much farther away than they expected by looking at
their redshifts. Using the objects’
brightness, the researchers determined the distance of these supernovae. And
using the spectrum, they were able to figure out the objects’ redshift and,
therefore, how fast they were moving away from us.
-
- They found that the supernovae were not as
close as expected, meaning they had traveled farther away from us faster than
ancitipated. These observations led scientists to ultimately conclude that the
universe itself must be expanding faster over time.
-
- What could be driving the universe to
stretch out faster over time? Right now,
“dark energy” is just the name that astronomers gave to the mysterious
"something" that is causing the universe to expand at an accelerated
rate.
-
- Dark energy has been described by some as
having the effect of a negative pressure that is pushing space outward.
However, we don't know if dark energy has the effect of any type of force at
all. There are many ideas floating around about what dark energy could possibly
be.
-
------------------ Vacuum Energy:
-
- Some scientists think that dark energy is a
fundamental, ever-present background energy in space known as vacuum energy,
which could be equal to the “cosmological constant”, a mathematical term in the
equations of Einstein's theory of general relativity. Originally, the constant
existed to counterbalance gravity, resulting in a static universe. But when
Hubble confirmed that the universe was actually expanding, Einstein removed the
constant, calling it “my biggest blunder,” according to physicist George Gamow.
-
- But when it was later discovered that the
universe’s expansion was actually accelerating, some scientists suggested that
there might actually be a non-zero value to the previously discredited
cosmological constant. This additional force would be necessary to accelerate
the expansion of the universe. This theorized that this mystery component could
be attributed to something called “vacuum energy,” which is a theoretical
background energy permeating all of space.
-
- Space is never exactly empty. According to
quantum field theory, there are virtual particles, or pairs of particles and
antiparticles. It's thought that these virtual particles cancel each other out almost
as soon as they crop up in the universe, and that this act of popping in and
out of existence could be made possible by “vacuum energy” that fills the
cosmos and pushes space outward.
-
- Scientists have calculated how much vacuum
energy there should theoretically be in space. They showed that there should
either be so much vacuum energy that, at the very beginning, the universe would
have expanded outwards so quickly and with so much force that no stars or
galaxies could have formed, or… there should be absolutely none.
-
------------------------------ Quintessence:
-
- Some scientists think that dark energy could
be a type of energy fluid or field that fills space, behaves in an opposite way
to normal matter, and can vary in its amount and distribution throughout both
time and space. This hypothesized version of dark energy has been nicknamed
“quintessence” after the theoretical fifth element discussed by ancient Greek
philosophers.
-
- It's even been suggested that quintessence
could be some combination of dark energy and dark matter, though the two are
currently considered completely separate from one another. While the two are
both major mysteries to scientists, dark matter is thought to make up about 85%
of all matter in the universe.
-
------------------ Space Wrinkles:
-
- Dark energy could be a sort of defect in
the fabric of the universe itself; defects like cosmic strings, which are
hypothetical one-dimensional "wrinkles" thought to have formed in the
early universe.
-
------------------- A Flaw in General Relativity:
-
- Dark energy isn't something physical that
we can discover. Rather, they think there could be an issue with general
relativity and Einstein's theory of gravity and how it works on the scale of
the observable universe. Within this explanation, scientists think that it's
possible to modify our understanding of gravity in a way that explains
observations of the universe made without the need for dark energy.
-
- Einstein actually proposed such an idea in
1919 called “unimodular gravity”, a modified version of general relativity that
scientists today think wouldn't require dark energy to make sense of the
universe.
-
- Dark energy is one of the great mysteries
of the universe. For decades, scientists have theorized about our expanding
universe. Now, for the first time ever, we have tools powerful enough to put
these theories to the test and really investigate the big question: “what is
dark energy?”
-
- NASA plays a critical role in the ESA
(European Space Agency) mission “Euclid” (launched in 2023), which will make a
3D map of the universe to see how matter has been pulled apart by dark energy
over time. This map will include observations of billions of galaxies found up
to 10 billion light-years from Earth.
-
- NASA's Nancy Grace Roman Space Telescope,
set to launch by May 2027, is designed to investigate dark energy, among many
other science topics, and will also create a 3D dark matter map. Roman's
resolution will be as sharp as NASA’s Hubble Space Telescope's, but with a
field of view 100 times larger, allowing it to capture more expansive images of
the universe.
-
- This will allow scientists to map how
matter is structured and spread across the universe and explore how dark energy
behaves and has changed over time. Roman will also conduct an additional survey
to detect Type Ia supernovae
-
- In addition to NASA’s missions and efforts,
the Vera C. Rubin Observatory, supported by a large collaboration that includes
the U.S. National Science Foundation, which is currently under construction in
Chile, is also poised to support our growing understanding of dark energy. The
ground-based observatory is expected to be operational in 2025.
-
- The combined efforts of Euclid, Roman, and
Rubin will usher in a new “golden age” of cosmology, in which scientists will
collect more detailed information than ever about the great mysteries of dark
energy.
-
- NASA's James Webb Space Telescope (launched
in 2021), the world’s most powerful and largest space telescope, aims to make
contributions to several areas of research, and will contribute to studies of
dark energy.
-
- NASA's SPHEREx (the Spectro-Photometer for
the History of the Universe, Epoch of Reionization, and Ices Explorer) mission,
scheduled to launch no later than April 2025, aims to investigate the origins
of the universe. Scientists expect that the data collected with SPHEREx, which
will survey the entire sky in near-infrared light, including over 450 million
galaxies, could help to further our understanding of dark energy.
-
- Results from a pioneering cosmic-mapping
project hints that the repulsive force known as dark energy has changed over 11
billion years, which would alter ideas about how the Universe has evolved and
what its future will be.
-
- The fate of the Universe might not be as
dark and empty as cosmologists have long suspected. That’s one potential implication emerging
from an innovative project that has produced some of the biggest maps ever made
of the Universe.
-
- Results from the “Dark Energy Spectroscopic
Instrument” (DESI), based at the Kitt Peak National Observatory near Tucson,
Arizona. DESI started mapping the Universe in 3D in 2020 and was designed to
measure the elusive force, dark energy, that is pushing galaxies apart. The surprising early results suggest that
dark energy could be weakening over time.
-
- Although the study was based on only the
first of the five years planned for data collection, it is already one of the
largest maps ever made of the Universe, and it reveals the effects of dark
energy across an unprecedented 11 billion years of cosmic history
-
- The hints that dark energy might be
weakening would bring the first substantial change in decades to the generally
accepted theoretical model of the Universe. And if dark energy is not constant,
that would hold implications for theories of how the Universe has evolved and
for what its future might hold.
-
- At the largest scales, the cosmos is ruled
by gravity, and Einstein’s general theory of relativity allows for gravity to
be repulsive as well as attractive. Whereas ordinary forms of energy, which
includes the mass of matter, result in an attractive force, general relativity
also predicts that some more-exotic forms of energy could produce “repulsive
gravity”.
-
- Dark energy, discovered in 1998, when
astronomers used supernova explosions in distant galaxies to measure how the
rate of cosmic expansion has changed. Their results indicated that the rate has
accelerated over time, pushed by some unseen repulsive force that would later
be dubbed dark energy. The name was intended to echo the equally mysterious
entity known as dark matter,which is invisible but can be measured by its
gravitational influence on galaxies.
-
- A consensus emerged around a theory called
Λ cold dark matter (ΛCDM), in which cosmic history is largely the result of a
struggle between the pull of dark matter and the push of dark energy.
-
- Save for small deviations that remain
unexplained, all of the evidence cosmologists have collected so far has
strengthened this “ΛCDM model”. The gold standard was set in 2013 by the Planck
space mission of the European Space Agency (ESA), which mapped the relic
radiation from the early Universe, called the “cosmic microwave background”.
-
- The data from that mission are in
“exquisite” agreement with the model.
The current Universe, Planck found, is about 70% dark energy, 25% dark
matter and 5% ordinary matter, the stuff of stars, planets and people.
-
- The standard assumption of ΛCDM is that the
expansion of the Universe will continue to accelerate, and that most galaxies
would ultimately disappear from view. But theorists have developed hundreds of
other models of dark energy; many posit that dark energy could be getting
slowly diluted, and the Universe’s expansion will start to slow down. Another
possibility is that dark energy is getting stronger and will ultimately rip
galaxies apart.
-
- For a long time, the hints from
observations were too vague to answer even the most basic questions about dark
energy: exactly how strong is it, and is it constant or slowly changing? DESI
is the first in a new generation of experiments aimed at providing some
answers.
-
- Others include “ESA’s Euclid mission”, which
launched into space last year; the massive,
8-meter telescope of the Vera C. Rubin Observatory nearing completion in
the Chilean Andes; and NASA’s Nancy Grace Roman Space Telescope, scheduled to
launch in 2027. Another telescope, called eROSITA, part of a Russian–German
space mission, has mapped the Universe in the X-ray spectrum
-
- All of these efforts rely on mapping the
distribution of matter in the Universe over vast distances, which — because of
the time that light takes to reach Earth — also means over vast stretches of
time. DESI does not take pictures of the sky in the way that an ordinary
telescope camera does, but instead collects light from selected locations in
its field of view.
-
- It does so by pointing optical fibres at
objects, typically galaxies or quasars, with its 5,000 robotic arms, and
routing that light to sensitive spectrographs. The spectrum of each object
reveals its distance, because the farther away the object is, the faster it
moves away, and the more its spectrum has ‘redshifted’ towards longer
wavelengths.
-
- To reconstruct the history of cosmic
expansion from its 3D data, the DESI team uses one of the most well-established
techniques in cosmology. It looks at the relic of what used to be sound waves
in the primordial Universe.
-
- As space expanded and matter cooled over
time, the waves became frozen in the distribution of protons and neutrons
(known collectively as baryons) across the Universe. That imprint, called
“baryon acoustic oscillations”, or BAO, is still detectable today in how
galaxies are scattered across space.
-
- The BAO features are the largest structures
in the Universe. DESI doesn’t just see
the BAOs in the current Universe. Its 3D map stretches back in time, and by
measuring how the average size of the features has grown over time,
cosmologists can reconstruct the rate of expansion, the strength of dark
energy. The instrument’s results are in principle still compatible with all the
options — a dark energy that is constant, weakening or even strengthening.
-
- The standard model was created as the
simplest possible theory for the Universe, but the actual physics of its
contents is probably more complicated.
-
-
May 10, 2024 DARK
ENERGY - expanding the Universe? 4466
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--------------------- --- Saturday, May 11, 2024
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