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DARK ENERGY - you could discover it? -
New model reveals what the color of a galaxy tells about its
distance. It will be used for measuring
cosmic structures. Our universe is
around 13.8 billion years old. Over the vastness of this time, the tiniest of
initial asymmetries have grown into the large-scale structures we can see
through our telescopes in the night sky: galaxies like our own Milky Way,
clusters of galaxies, and even larger aggregations of matter or filaments of
gas and dust.
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- DARK ENERGY - you
could discover it
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- How quickly this growth takes place depends
on a wrestling match between natural forces: Can dark matter, which holds
everything together through its gravity and attracts additional matter, hold
its own against dark energy, which pushes the universe ever further apart?
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- This is where telescopic observation
projects come in, capturing large swaths of the sky very precisely in images.
For example, there is the Dark Energy Survey with the Blanco telescope in Chile
and the recently commissioned Euclid satellite. LMU scientists have been
involved in both projects, including in leadership roles, for years.
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- Although precisely determining the distances
of individual structures and galaxies from us is not always easy, it is vitally
important. After all, the further away a galaxy is, the longer its light has
been traveling to us, so the snapshot of the universe revealed by its
observation is therefore older. An important source of information is the
observed color of a galaxy, which is measured by ground-based telescopes like
Blanco or satellites like Euclid.
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- The distance of a galaxy can be precisely
determined by means of spectroscopy. This involves measuring the spectral lines
of distant galaxies. As the universe as a whole is expanding, these appear to
have a longer wavelength, the further away from us a galaxy is located. This is
because the lightwaves of distant galaxies are stretched out on the long
journey to us.
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- This effect, known as “redshift”, also
changes the apparent colors that the instruments measure in the image of the
galaxy. They appear redder than they are in reality. This is similar to the
Doppler effect we hear in the apparent pitch of an ambulance's siren as it
passes us and moves away.
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- The combined spectroscopic data from DESI
of a total of 230,000 galaxies with the colors of these galaxies in the
KiDS-VIKING survey and used this information to determine the relationship
between the distance of a galaxy from us and its observed color and brightness.
No two galaxies in the universe are the same, but for each class of similar
galaxies, there is a special relationship between observed color and redshift.
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- If we can combine distance information with
measurements of the shape of galaxies, we can infer large-scale structures from
the light distortions.
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- To be able to observe the course of
structure formation over time, you do not need to wait billions of years; it is
enough to measure the structure at various distances from the Earth. With
images alone, this is almost impossible, as you cannot just tell the distance
of a galaxy to ours from its appearance in an image.
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- Using a model for what the apparent
"color" of a galaxy tells us about its distance from us. The major goal of this precise observation
and distribution of galaxies at various distances is to derive insights into
the great wrestling match between the natural forces of dark matter and dark
energy.
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- Dark energy is poised to catch up and
potentially arrest the formation of larger accumulations of mass in the
universe altogether.
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- 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”.
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- We don't know what Dark energy is. 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 dark energy.
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- 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.
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- 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 – 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.
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- 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).
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- 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.
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- 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.
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- 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.
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- 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.
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- In 1927, Belgian astronomer Georges
Lemaître, who is said to have been unaware of Friedmann’s work, 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.
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- 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, as they called
them).
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- 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 is now known as “Hubble’s Law”, or the
“Hubble-Lemaître law”. The universe, they confirmed, is really expanding.
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- 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.
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- 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).
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- 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.
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- But, as scientists built up a case for
cosmic acceleration. Why? What could be
driving the universe to stretch out faster over time. “Dark energy” is just the name that
astronomers gave to the mysterious "something" that is causing the
universe to expand at an accelerated rate.
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- 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. Here are four leading explanations for dark energy. Keep in mind that it's
possible it's something else entirely.
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--------------------- 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.
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- 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,” .
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- 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”. They suggested that 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.
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- 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.
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- While this theory has been a popular topic
of discussion, scientists investigating this option 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.
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- This means that the amount of vacuum energy
in the universe must be much smaller than it is in these predictions. However,
this discrepancy has yet to be solved and has even earned the moniker "the
cosmological constant problem."
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------------------- 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.
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- It's even been suggested by some scientists
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.
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---------------------- Space Wrinkles:
- Some scientists think that 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.
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----------------------- A Flaw in General Relativity:
- Some scientists think that 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.
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- 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.
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- 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?”
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- 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.
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- 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.
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- 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
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- 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.
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- 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.
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- 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.
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- 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.
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- NASA also supports a citizen science project
called “Dark Energy Explorers”, which enables anyone in the world, even those
who have no scientific training, to help in the search for dark energy answers. I have signed up our Coffee Club. You dont need a telescope, you can use your
computer to log into available telescopes that create a TV image. You could be famous!!!
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September 25, 2024 DARK
ENERGY - you could discover it? 4564
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------ “Jim Detrick” -----------
--------------------- --- Thursday, September 26,
2024
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