- 4514
- DARK ENERGY
- what is 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. Nine billion years
after the universe began, its expansion started to speed up, driven by an
unknown force that scientists have named “dark energy”. But what exactly is dark energy?
--------------- 4514
- DARK ENERGY
- what is expanding the Universe?
- 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 this “dark energy”, whatever it
is?.
-
- 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, 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 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, 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.
-
- 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. 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.
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.
-
- 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” .
-
- 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. 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.
-
- 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.
-
- Additionally, 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.
-
- 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. Let us know as soon as you figure it out!
-
-
June 29, 2024 DARK
ENERGY - what is expanding the Universe? 4514
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--------------------- --- Saturday, June 29, 2024
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