- 4528 - DARK
ENERGY - what causes universe expansion? -
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?”
-------------------------------------- 4528
- DARK ENERGY
- what causes universe expansion?
- The “color” of a galaxy tells about its
distanceand 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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- 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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- Although precisely determining the
distances of individual structures and galaxies from us is not always easy, it
is vitally important. 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.
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- In principle, 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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- No two galaxies are the same. Astronomers combined spectroscopic data 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. Analyzing the
observed distortions of the galaxy images, scientists will be able to learn
something about the behavior of cosmic structures today and billions of years
ago and understand them better. This will yield insights into the evolutionary
history of the universe.
-
- 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.
-
- 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. This is because dark energy is
poised to catch up and potentially arrest the formation of larger accumulations
of mass in the universe altogether.
-
- 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”.
-
- We do know that dark energy exists, it’s
making the universe expand at an accelerating rate, and approximately 68.3 to
70% of the universe is dark energy.
Remember matter and energy are two forms of the same thing.
-
- 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. They have the same absolute magnitude, or
luminosity, the amount of light they put out.
-
- Astronomers measured these stars and proved
that there is a relationship between their regular period of brightness and
luminosity. These 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 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.
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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 – an
observation 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.
-
- 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.
-
- 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. Here are four leading explanations
for dark energy. Keep in mind that it's possible it's something else entirely.
-
------------------------ 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”.
-
- 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.
-
- 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 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. -
-
- This means that the amount of vacuum
energy in the cosmos 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."
-
----------------------- 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 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.
-
-------------------------- 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.
-
------------------------ 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. 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.
-
-
July 1, 2024 DARK
ENERGY - what causes universe expansion? 4528
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