4341 - DARK ENERGY SURVEY? - In 1998, astrophysicists discovered that the universe is expanding at an “accelerating rate”, attributed to a mysterious entity called “dark energy” that makes up about 70% of our universe. At the time, astrophysicists agreed that the universe’s expansion should be slowing down because of gravity. Surprise!!!!
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- This revolutionary discovery, which
astrophysicists achieved with observations of specific kinds of exploding
stars, called “type Ia supernovae”.
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- The scientists working on today's “Dark
Energy Survey” have released the results of an unprecedented analysis using the
same technique to further probe the mysteries of dark energy and the expansion
of the universe. They placed the strongest constraints on the expansion of the
universe ever obtained with the “DES supernova survey”.
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- Cosmology results with 1,500 new
high-redshift type Ia supernovae using the full 5-year dataset, DES
astrophysicists report results that are consistent with the now-standard
cosmological model of a universe with an accelerated expansion.
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- An example of a supernova discovered by the
Dark Energy Survey within the field covered by one of the individual detectors
in the Dark Energy Camera. The supernova exploded in a spiral galaxy with
redshift = 0.04528, which corresponds to a light-travel time of about 0.6
billion years. This is one of the nearest supernovae in the sample.
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- DES mapped an area almost one-eighth the
entire sky using the Dark Energy Camera, a 570-megapixel digital camera. DES
scientists took data for 758 nights across six years.
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- Type Ia supernovae occur when an extremely
dense dead star, known as a white dwarf, reaches a critical mass and explodes.
Since the critical mass is nearly the same for all white dwarfs, all type Ia
supernovae have approximately the same actual brightness and any remaining
variations can be calibrated out. So, when astrophysicists compare the apparent
brightnesses of two type Ia supernovae as seen from Earth, they can determine
their relative distances from Earth.
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- Astrophysicists trace out the history of
cosmic expansion with large samples of supernovae spanning a wide range of
distances. For each supernova, they combine its distance with a measurement of
its redshift which how quickly it is moving away from Earth due to the
expansion of the universe. They can use that history to determine whether the
dark energy density has remained constant or changed over time.
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- As the universe expands, the “matter
density” goes down. But if the dark
energy density is a constant, that means the total proportion of dark energy
must be increasing as the volume increases.
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- The standard cosmological model is
ΛCDM, “Lambda Cold Dark Matter”, a model
based on the dark energy density being constant over cosmic time tells us how
the universe evolves, using just a few features, such as the density of matter,
type of matter and behavior of dark energy.
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- According to the standard cosmological
model, the density of dark energy in the universe is constant, which means it
doesn’t dilute as the universe expands. If this is true, the parameter
represented by the letter “w” should equal –1.
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- The results found “w = –0.80 +/- 0.18”
using supernovae alone. Combined with complementary data from the European
Space Agency’s Planck telescope, “w” reaches –1 within the error bars.
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- DES scientists combined data about the
spectrum of each supernova to determine their redshifts and to classify them as
type Ia or not. They then used images taken with different filters to identify
the flux at the peak of the light curve, a method called “photometry”.
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- Redshift is the term used to describe the
stretching of wavelengths of the light with the expansion of the universe; the
greater the object’s distance, the greater the redshift. The detailed history
of the expansion of the universe is determined with a precise relation between
the distances to galaxies, or supernovae, and their redshifts.
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- The new study uses the new approach,
photometry, with an unprecedented four filters, to find the supernovae,
classify them and measure their light curves. The use of these additional
filters enabled data that is more precise than previous surveys.
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- They used advanced machine-learning
techniques to aid in supernova classification. Among the data from about two
million distant observed galaxies, DES found several thousand supernovae. They
used 1,499 type Ia supernovae with high-quality data, making it the largest,
deepest supernova sample from a single telescope ever compiled.
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- Even if we measure dark energy infinitely
precisely, it doesn’t mean we know what it is.
Dark energy is still out there to be discovered.
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- The theory is that early on, the Universe
behaved as though it were made solely of matter and radiation, with no effects
from dark energy appearing from the first fractions-of-a-second to the first
several billion years. As the Universe expanded and cooled, the densities of
matter and radiation began to plummet, eventually dropping so low that a new
form of energy, dark energy, became important to the Universe's evolution.
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- As dark energy rises to prominence, the
expanding Universe's behavior changes dramatically, and structure on the
largest scales stops gravitationally growing and instead gets torn apart.
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- Imagine looking out at the Universe: beyond
the stars of the Milky Way and -the nearest galaxies to us, all the way to the
most distant objects we can find. When we do exactly that, examining the
galaxies, quasars, and other forms of matter that appear billions of
light-years away, we’re seeing those objects not as they are today, but as they
were in the distant past: back when their light was first emitted.
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- At those earlier times, the Universe was
hotter, denser, and filled with smaller, younger, less-evolved galaxies. The
light we see from way back in our Universe’s history only arrives at our eyes
after journeying across these vast cosmic distances, and only after that light
has been stretched by the expanding
space.
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- It is these early signals, and the process
of how that light gets stretched to longer wavelengths, redshifted,
more severely as we look to more and more distant objects, that teach us
how the Universe has expanded throughout its history.
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- We have learned that the Universe wasn’t
just expanding, but that distant objects appear to speed up, faster and faster,
as they mutually recede from one another: the discovery of the accelerated
expansion of the Universe. That’s how we discovered dark energy and measured
its properties, changing our conception of the Universe forever.
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- At the start of the hot Big Bang, the
Universe was rapidly expanding and filled with high-energy, very densely
packed, ultra-relativistic quanta. An early stage of radiation domination gave
way to several later stages where radiation was sub-dominant, but never went
away completely, while matter then clumped into gas clouds, stars, star
clusters, galaxies, and even richer structures over time, all while the
Universe continues expanding.
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- Initially after the Big Bang no light
signals would be able to freely propagate from one region to the other; you’d
have to wait until the Universe reached an age of 380,000 years old, as that
was when the Universe became transparent to light. The cosmic microwave
background, initially visible, would fade to infrared and then microwave
wavelengths that we see today.
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- As time continued to pass molecular clouds
form and contract, followed by stars and black holes forming in a slew of early
nebulae, followed by the mergers of star clusters, leading to proto-galaxies.
Those proto-galaxies would then merge, gravitate, and grow, evolving into
full-fledged galaxies that came to exist within groups and clusters.
Eventually, those early galaxies would evolve into the more familiar modern-day
galaxies, going through long, quiet eras punctuated by bursts of
star-formation.
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- Galaxies comparable to the present-day
Milky Way are numerous, but younger galaxies that are Milky Way-like are
inherently smaller, bluer, more chaotic, and richer in gas in general than the
galaxies we see today. For the first galaxies of all, this effect goes to the
extreme.
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- That’s what the Universe itself would be
like, not just where the Milky Way and a random distant galaxy would be
located, but all across the vast expanse of space. One of the great properties of the Universe
is that the laws of physics appear to be constant: unchanging throughout time.
This means that atoms always have and always will absorb and emit light at very
specific wavelengths: wavelengths that are the same everywhere, and determined
by the energy levels that the electrons within the atom occupy.
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- By identifying series of atomic absorption
or emission lines that correspond to the same element, we can then measure how
our observations are shifted in wavelength relative to the wavelengths we’d
observe if that same atom were at rest in a laboratory.
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- The shift, known as a redshift if it gets
stretched to longer wavelengths or a blueshift if it gets compressed to shorter
wavelengths, is almost always observed to be a redshift for distant galaxies,
with redshift generally increasing with distance. By measuring both of those
properties, redshift and distance together
across a wide enough array of objects with different properties, we can
use those measurements to reconstruct the history of the expanding Universe.
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- When we observe the light from a
relatively nearby galaxy, its light is redshifted by an amount that corresponds
to the amount the Universe has expanded from when that light was emitted until
we observe it. The light from a slightly more distant galaxy is redshifted by a
slightly greater amount: by the same amount as the nearer galaxy plus the
additional amount of expansion that occurred due to its greater distance.
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- As we accumulate more objects at greater
and greater distances, we can use that data to construct a curve that teaches
us how the Universe has expanded over its cosmic history.
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- Once we know what the Universe is made of,
we can use the information we have about how various forms of energy evolve
with time to answer that initial question: what we would see if we could track
a single, individual galaxy’s distance and redshift (as seen from our
perspective) throughout the history of the Universe.
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- That redshift, if we interpret it as a
Doppler shift, can be made to correspond to an “apparent recession speed”, how
quickly the object in question appears to be moving away from us. In reality, it isn’t that the object’s motion
is causing the redshift, although motion toward (blueshift) or away from
(redshift) an observer can certainly cause that effect. Instead, it’s the fact
that the light is traveling through the fabric of space and that the fabric
expands while the light travels that
causes the redshift we observe.
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- The distances get larger and larger over
time, as the Universe continues to expand. This pushes all objects that aren’t
gravitationally bound mutually away from one another, increasing the measured
distance between them. From the perspective of any one galaxy, all distant,
unbound galaxies continue to move farther and farther away as time goes on.
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- The Universe’s expansion rate changes, and
it changes dependent on the total matter and energy density present in the
Universe. Since an increasing volume means a decreasing energy density, the
expansion rate drops, and the distant galaxy appears to move away from us at a
slower and slower speed, as though the initial expansion drove things apart,
and gravity attempts, however unsuccessfully, to pull them back together.
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- Light may be emitted at a particular
wavelength, but the expansion of the Universe will stretch it as it travels.
Light emitted in the ultraviolet will be shifted all the way into the infrared
when considering a galaxy whose light arrives from 13+ billion years ago. The
more the expansion of the Universe accelerates, the greater the light from
distant objects will be redshifted and the fainter it will appear.
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- This makes sense when you think about the
expanding Universe in the context of the Big Bang. There is a great cosmic race
going on: between gravity, working to pull everything back together, and the
initial expansion rate, working to drive everything apart. The race has been
underway for 13.8 billion years, and the Big Bang was the starting gun.
Everything in the Universe begins moving away from everything else initially,
at an extremely rapid rate to start, while gravity works as hard as it can to
pull everything back together. You can imagine multiple possible fates for how
things would turn out.
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- If there were too much matter in the
Universe, everything would expand only until a point, as the Universe reached a
maximum size and then the expansion reversed, and things began contracting.
Eventually, the Universe would recollapse, and everything would end in a “Big
Crunch”.
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- On the other hand, if there were too little
matter, the expansion would continue forever, with the expansion rate
decreasing but never stopping or reversing, while the apparent recession
velocities would forever decrease but never reach zero.
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- Or you could imagine what we call a
“Goldilocks” case: where the Universe lived on the border between those two
prior scenarios. The Universe would expand forever, but the expansion rate
would approach zero. If there were one more atom in the Universe, it would
recollapse, but without that atom, things just keep expanding, albeit as slowly
as the laws of physics allow.
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- The Big Bang, on its own, offers no
explanation as to why the initial expansion rate at the moment of the
Universe’s birth balances the total energy density so perfectly, leaving no
room for spatial curvature at all and a perfectly “flat Universe”. Our Universe
appears perfectly spatially flat, with the initial total energy density and the
initial expansion rate balancing one another to at least some 20 significant
digits.
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- That last case was consistent with what we’d
see happening for a long time: for billions of years, in the case of our
Universe. An individual galaxy appears to move away from us at an incredibly
fast rate, but then its recession velocity drops as the matter and radiation
densities drop. Since it’s the total energy density at any particular instant
that determines the Universe’s expansion rate, and the expansion rate in turn
determines what we infer the recession speed of a galaxy to be, this all makes
intuitive sense.
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- But after a few billion years, something
fishy begins to occur. Instead of approaching zero, the expansion rate starts
to decrease at a slower rate than one would expect, and a distant galaxy’s
recession speed doesn’t drop in the same fashion anymore.
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- Once the Universe reaches an age that’s
7.8 billion years after the Big Bang, things start to get weird: these distant
galaxies stop slowing down in their recession entirely, and appear to “coast”
in the sense that they move away from us at a constant speed from
moment-to-moment, as though the expansion had stopped decelerating.
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- And then, as the Universe continues to age,
the recession speeds no longer remain constant, nor do they go back to
decreasing. Instead, these distant galaxies appear to recede from us (and one
another) more and more quickly. It’s as though some effect is causing the
expansion to neither decelerate nor remain constant, but to actually increase
and accelerate!
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- The expected fates of the Universe all correspond to a Universe where matter and
energy fight against the initial expansion rate. In our observed Universe, a
cosmic acceleration is caused by some type of dark energy, which is hitherto
unexplained. All of these Universes are governed by the “Friedmann equations”,
which relate the expansion of the Universe to the various types of matter and
energy present within it.
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- This data, and these observations, teach us
something profound: the Universe cannot simply be filled with merely matter and
radiation. Even adding in neutrinos, black holes, dark matter and more won’t
allow us to successfully account for everything.
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- In addition to all of those entities, we
also require something novel, known as dark energy: a form of energy inherent
to space itself. As the Universe expands, dark energy doesn’t dilute, but
rather remains at a constant density. Everything else, including all forms of
matter and radiation, dilute as the Universe expands, as the number of
particles remains fixed but the volume they occupy increases: a consequence of
the expanding Universe. Only dark energy, inherent to space itself, remains at
a constant energy density.
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- After 7.8 billion years, the matter density
drops far enough that the effects of dark energy begin to become important. 7.8
billion years after the Big Bang, when the dark energy density has grown to be
as large as half the matter density, it reaches the critical value to cause a
distant galaxy to stop decelerating from our perspective, and begin
accelerating instead. This is a critical time: the repulsive effects of dark
energy on the Universe’s expansion exactly counteract the attractive effects of
matter.
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- But time doesn’t stop here. Instead, it
continues forward, and the matter density continues to drop. Once 7.8 billion
years on the cosmic clock ticks by, dark energy now becomes more important than
matter and radiation as far as the expansion rate is concerned. Distant
galaxies reached their minimum recession speed at that time, but then will
appear to speed up once again.
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- In a Universe that comes to be dominated by
dark energy, there are four regions: one where everything within it is
reachable and observable, one where everything is observable but unreachable,
one where things will someday be observable, and one where things will never be
observable.
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- As time marches forward, distant objects
not bound to one another will recede from each other’s perspective at a faster
and faster rate. By the time the Universe is 9.2 billion years old, right when
our Solar System is forming, the matter density will have dropped below the
dark energy density.
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- By the present day, 13.8 billion years
after the Big Bang, dark energy accounts for approximately 70% of the total
energy in the Universe; when the Universe reaches twice its present age, dark
energy will account for over 95% of the Universe’s total energy. Throughout all
that time, distant galaxies will continue to speed up, faster and faster, in
their apparent recession from our perspective.
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- For the past 6 billion years, the
Universe’s expansion has been accelerating, meaning that any distant galaxy we
monitor appears to recede from us at an ever-increasing speed. Any galaxy
currently at a distance of approximately 18 billion light-years from us now
appears to recede away faster than the speed of light, meaning there’s nothing
we can ever do to reach or contact it again.
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- Given that the Universe is already 46
billion light-years in radius, this means that 94% of the galaxies in the
Universe are already forever beyond our reach.
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- For billions of years, dark energy’s
density would have been tiny compared to the density of matter, meaning its
effects would have been undetectable if humans had arisen too early. Tens of
billions of years from now, dark energy will have pushed everything beyond our
Local Group far away from us; the merged remains of the Local Group will be the
only galaxy remaining.
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- It’s only because you came along when you
did to read this at this golden cosmic time, that you can perceive what the
Universe is actually made of. Dark energy is real, began dominating our
Universe at an age of 7.8 billion years old, and determines our cosmic fate
from here on out! Amazing huh!
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February 3, 2023 DARK
ENERGY SURVEY? 4341
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------ “Jim Detrick” -----------
--------------------- --- Sunday, February 4,
2024
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