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------------ 2758 - DARK ENERGY - a mystery?
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- We know a lot about gravity but almost nothing about what causes the dark energy expansion of the Universe?. But, there is a new test that confirms dark energy is causing this expansion of the Universe. Astronomers do not know what it is but they do know what it does.
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- In the standard model of cosmology, dark energy fills the universe. It causes the universe to expand at an ever-increasing rate, and it makes up more than 70% of the
mass / energy in the Universe. . But there’s a problem. When we measure the rate of cosmic expansion in different ways, we get results that disagree markedly.
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- There are several ways to measure cosmic expansion. One is to measure the relative motion of distant galaxies. This is done by looking at the light from supernovae within these galaxies.
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- Type 1a supernovae have a fairly uniform standard brightness, so by measuring their observed brightness we know the distance of their home galaxy. This can be compared with the observed redshift of the galaxy to determine the Hubble expansion rate. It is this method that first discovered cosmic expansion was accelerating.
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- Another method is to look at the “cosmic microwave background“. While this faint afterglow of the big bang has an almost uniform temperature of about 3 degrees Kelvin, there are small variations in temperature across different regions of the sky.
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- The scale of these fluctuations depends upon the rate of cosmic expansion. Careful observations from the Planck spacecraft have given us a good measure of this Hubble constant rate of expansion. And this is completely independent of the supernova measure. In principle, these two results should agree, but they don’t.
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- The Planck results give a Hubble parameter of about 67 – 68 (km/s)/Mpc, while supernova observations give a value of about 71 – 75 (km/s)/Mpc. The uncertainty of these measures was large enough that they overlapped, but they are now so precise that they outright disagree. This doesn’t mean that dark energy is wrong, but it does mean there are things we don’t understand about it.
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- One of the difficulties with these results is that they are model dependent. Each depends on certain assumptions about the universe. One of these is that the universe is “spatially flat“. In other words, the light we see from distant galaxies has traveled in basically a straight line. It isn’t deformed by some overall cosmic warping of space.
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- But, there is some evidence in the Planck data that space might have a small overall curvature. This would help account for the difference in results.
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- To settle the issue, astronomers have looked at other ways to measure cosmic expansion. One way is to measure how galaxies cluster at large scales. Galaxies clusters have formed because of small variations in the density of the early universe known as Baryon Acoustic Oscillations.
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- As gravity tries to pull galaxies closer together, dark energy tries to drive them apart. As a result, galaxies have formed into dense super-clusters separated by large voids. The size of these voids allows us to measure the Hubble constant expansion parameter.
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- Recently, a team made the most comprehensive measure of galaxy clustering. They found several interesting things. To begin with, since the structure of the voids depends on both dark energy and the overall shape of space, the team confirmed that the universe is spatially flat.
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- Cosmic curvature can’t account for the different values. For the Hubble parameter, they got a result of about 70 – 74 (km/s)/Mpc, which agrees with the larger supernova result.
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- Their observations focused mainly on galaxies with a redshift of z < 2, or within about 9 billion light-years. When the team added data from more distant galaxies, their result shifted to 68 – 70 (km/s)/Mpc, which better agrees with the Plank result.
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- The upshot of all this is that the universe is flat, dark energy is very real, and there is an oddness to it we still don’t understand.
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- All the revolutionary discoveries that we've made about the Universe, this is the most unexpected and surprising. A great cosmic race has been taking place ever since the Big Bang: between the initial expansion, working to drive everything apart, and gravitation, which works to pull everything back together. For billions of years, the Universe behaved as though these two opposing influences were in perfect balance.
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- Then, about 6 billion years ago, the expansion all of a sudden started speeding up again, causing distant objects to accelerate. Dark energy is the name we give to the unknown cause of this unexpected phenomenon, does dark energy. Does the increase in dark energy as space expands also create more gravity?
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- The mathematics governing General Relativity is quite complicated, and General Relativity itself offers many possible solutions to its equations. But it's only through specifying the conditions that describe our Universe, and comparing the theoretical predictions with our measurements and observations, that we can arrive at a physical theory.
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- Every form of energy in the Universe, no matter how weird, exotic, or unfamiliar it is, obeys the same law of gravity: Einstein's General Relativity.
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- Most of the types of energy we're used to come in the form of quanta: tiny little point-like packets of energy that move through the fabric of spacetime. Some of those quanta are radiation-like, meaning they move at the speed of light (or indistinguishably close to the speed of light). Others are matter-like, meaning they're moving slow compared to the speed of light.
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- Some good examples are photons, which always act like radiation, normal matter and dark matter, which always act like matter, and neutrinos, which behave like radiation in the early Universe (or today, when they're emitted by stars or other nuclear processes at high energies) but behave like matter later on, when the Universe has expanded and cooled sufficiently.
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- All massless particles travel at the speed of light, including the photon, gluon and gravitational waves, which carry the electromagnetic, strong nuclear and gravitational interactions, respectively.
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- Any particle with a non-zero rest mass will travel slower than light, and as the expansion of the Universe causes it to lose kinetic energy, eventually it will become non-relativistic, behaving as matter rather than radiation.
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- The reason for this dichotomy is that every particle has two types of energy it can possibly possess: rest-mass energy, which is the amount of energy inherent to the particle itself, via Einstein's most famous equation, E = mc2, and kinetic energy, which is the energy due to the particle's motion through the Universe.
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- As the Universe expands, the number of particles remains the same but the volume that they occupy, the size of the Universe, increases. That is the density is decreasing.
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- If we ask the question of how the matter density drops over time, it should dilute as the volume does: in proportion to the size of the Universe cubed. But if you have a lot of kinetic energy, or you're something like a massless photon where your energy is defined by your wavelength, not only do you dilute with volume, but your wavelength also gets stretched as your Universe expands. Radiation, therefore, dilutes in proportion to the size of the Universe to the fourth power.
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- Various components of the Universe's energy density, and when they might dominate, changes over time. Radiation is dominant over matter for roughly the first 9,000 years, but remains an important component, relative to matter, until the Universe is many hundreds of millions of years old, thus suppressing the gravitational growth of structure.
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- There are other forms of energy the Universe is allowed to have besides particles. In particular, three different ideas have existed for a long time that all have energy, but all have their own evolution:
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------------------- Cosmic strings: which are long, thin, one-dimensional strands of energy that stretch across the Universe.
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------------------- Domain walls: which are long, thin, two-dimensional sheets of energy that stretch across the Universe.
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------------------ Cosmological constant: which is a form of energy that's inherent to the fabric of space itself.
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- As the Universe expands, cosmic strings can still span the entire Universe in one dimension, but will take up less of the Universe's volume in the other two.
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- Domain walls can span the whole Universe in two dimensions, but will still dilute in the one other dimension.
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- But for a cosmological constant, the fact that space is expanding just means that there's more volume, and it doesn't dilute at all. The energy density will remain a constant.
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- The simplest, most widely used candidate for dark energy, and also, the most consistent with the full suite of data, is that dark energy is a “cosmological constant“. The fact that we see the Universe expanding as it does means that there must be some new form of energy causing these distant galaxies to recede from us faster and faster as time goes on.
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- If the energy present in the Universe is what causes gravity to work, as all the different forms of energy attract all the other forms of energy, then why are progressively more distant galaxies appearing to accelerate away from us as the Universe ages?
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- This is a non-intuitive thing, after all! You'd think that if the Universe possessed a cosmological constant, it would be gaining energy as the Universe expanded, and would gravitate more, slowing the expansion rate down. But that isn't what happens at all.
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- The four possible fates of our Universe into the future; the last one appears to be the Universe we live in, dominated by dark energy. What's in the Universe, along with the laws of physics, determines not only how the Universe evolves, but how old it is.
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- If dark energy were about 100 times stronger in either the positive or negative direction, our Universe as we know it would have been impossible. Why does the presence of dark energy, either in the form of a cosmological constant or something very close to it, mean that distant galaxies are accelerating away from us at faster and faster speeds as the Universe continues to expand?
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- The answer, believe it or not, is because we live in a Universe governed by Einstein's laws, and we have to follow what those laws tell us, even the parts of it that are counterintuitive.
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- Einstein first put forth his greatest theory of all, General Relativity, in 1915. Immediately, people began working out the consequences of that theory. In 1916, Karl Schwarzschild worked out the solution for a non-rotating black hole.
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- Other solutions soon followed: for an empty Universe; for gravitational waves; for a cosmological constant all by itself. But the most important advance came in 1922, when Alexander Friedmann derived the general solution for a Universe filled with energy that was both isotropic (the same in all directions) and homogeneous (the same in all locations in space).
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- The two equations he derived are, even today, still known as the Friedmann equation.
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--------------------All of the other terms in the equation represent a combination of:
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--------------------all the matter,
--------------------all the radiation,
--------------------all the neutrinos,
--------------------all the dark energy (which is the last term if it's a cosmological constant)
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- Dark energy doesn't change in density as the Universe expands, which is why it comes to dominate the Universe at late times. The dark energy density remains a constant, the expansion rate will never drop below a certain amount if dark energy is real.
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- The dark energy density term being a constant, the Universe expands enough that the density of everything else becomes negligible, the expansion rate will asymptote to a constant as well. For our Universe, this means that the expansion rate will never drop below about 55 km/s/Mpc: about 80% of its present value.
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- If dark energy didn't gravitate, it couldn't contribute to the energy density of the Universe or the Universe's expansion. Whether the expansion of the Universe accelerates or decelerates depends not only on the energy density of the Universe, but on the pressure of the various components of energy.
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- For something like dark energy, where the pressure is large and negative, the Universe accelerates, rather than decelerates, over time. For dark energy, the pressure is not only negative, it's three times as powerfully negative as radiation pressure is positive.
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- For dark energy, the pressure is actually equal to the negative of the energy density, so that the second derivative of the scale factor (which determines acceleration vs. deceleration) flips in sign from a matter or radiation-dominated Universe. Instead of decelerating, the Universe accelerates when dark energy dominates.
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- This leads to an even more counterintuitive result: as the Universe continues to expand, dark energy means that the total amount of energy contained within our observable volume always increases.
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- Yet as it does, the Universe doesn't decelerate, but rather speeds up. The most sacred laws in all of physics, the conservation of energy, only applies to particles interacting in a static spacetime. When your Universe expands (or contracts), energy is no longer conserved.
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- There is an amount of energy intrinsic to the fabric of space itself, but the effects of the energy density are overwhelmed by the effects of the negative pressure that arises. The Universe's expansion doesn't slow down due to the presence of dark energy, but rather distant galaxies will speed away faster and faster due to its cumulative effects.
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- For anything beyond our Local Group of galaxies, its fate is already sealed: it will speed away, faster and faster, until we can no longer access it in our accelerating Universe.
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- A mind once expanded never returns back to its previous size. I hope these theories have expanded your mind a little. The whole concept blows my mind.
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- June 8, 2020 2758
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--------------------- Tuesday, June 9, 2020 -------------------------
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