- 4499 - DARK ENERGY - expanding the Universe? - This Review 4499 is about the dark energy part of the universe that we are trying to understand. 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.
------------------------------- 4499 - DARK ENERGY - expanding the Universe?
- See Review 4498 about Dark Matter. We call it “dark” because we don't know what
it is. Dark “Energy” is overcoming
gravity and expanding the Universe at an accelerating rate. We don't what that is either. Together dark energy and dark matter
represent 95% of the known universe. We
live in the 5% of the universe that is not so dark to us.
-------
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- The universe wouldn’t stay the same
expansion rate. 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 don't know what dark energy is, but we
do know that it exists because it’s making the universe expand at an
accelerating rate, and approximately 68.3 to 70% of the universe is dark
energy.
-
- Our discovery of dark energy all started
with “Cepheids”. 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.
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.
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- “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.
-
- 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). -
-
- 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.
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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.
-
- While other possible explanations for these
observations have been explored, astronomers studying even more distant
supernovae or other cosmic phenomena in more recent years continued to gather
evidence and build support for the idea that the universe is expanding faster
over time, a phenomenon now called “cosmic acceleration”.
-
- 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.
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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.
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- 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. 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.
-
- 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.
-
- 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."
-
- 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.
-
- 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.
-
- 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. 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. “Nancy Grace
Roman Space Telescope”, set to launch by May 2027, is designed to investigate
dark energy. Roman's resolution will be
as sharp as the 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
-
- The Vera C. Rubin Observatory, 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.
-
- 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.
-
- 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. Maybe you can be the be the one who figures
all this out?
-
-
June 13, 2024 DARK
ENERGY - expanding the Universe? 4499
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--------------------- --- Friday, June 14, 2024
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