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--------------------- 2632 - DARK ENERGY - some form of anti- gravity?
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- See Review 2631 about Dark Matter. Dark Energy is totally different and even more mysterious to astronomers. Dark Matter is 23% of the Universe and Dark Energy is 72% of the Universe. Remember that mass and energy are 2 forms of the same thing:
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------------------------ Energy = Mass * (speed of light)2
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- Of all the revolutionary discoveries that we've made about the Universe, the most unexpected and surprising has to be Dark Energy. Ever since the Big Bang the force between the initial expansion, working to drive everything apart, and gravity working to pull everything back together, the Universe has behaved as though these two opposing influences were in perfect balance.
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- Then, 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, but suddenly things don't add up.
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- Does dark energy produce gravity? 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 an understandable 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, that is “Einstein's General Relativity“. Most of the types of energy we are used to come in the form of quanta: tiny little point-like packets of energy that move through the fabric of spacetime.
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- Some of those quanta are “radiation-like“, meaning they move at the speed of light. Others are “matter-like“, meaning they're moving slower compared to the speed of light.
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- 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.
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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.
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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.
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- How the matter density drops over time 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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- 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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- But 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. 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: which are long, thin, two-dimensional sheets of energy that stretch across the Universe. Domain walls can span the whole Universe in two dimensions, but will still dilute in the one other dimension.
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- Cosmological constant: which is a form of energy that's inherent to the fabric of space itself. 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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- Is “dark energy” the “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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- But 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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- You would 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. 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.
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- Why does the presence of dark energy, either in the form of a cosmological constant 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 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. Einstein first put forth his greatest theory of all, General Relativity, in 1915.
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- Using this theory Karl Schwarzschild worked out the solution for a non-rotating black hole. Other solutions soon followed: for an empty Universe; for gravitational waves; for a cosmological constant all by itself.
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- 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, and fortunately we only need to examine the first one to learn how the Universe expands dependent on what forms of energy are in it. The first term in the equation is the Hubble expansion rate (squared): a measure of how quickly the fabric of space is stretching at any moment.
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- All of the other terms in the equation represent a combination of:
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--------------------------------- all the matter,
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-------------------------------- all the radiation,
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-------------------------------- all the neutrinos,
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-------------------------------- all the dark energy, if it is a cosmological constant
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- Because the dark energy density remains a constant, the expansion rate will never drop below a certain amount if dark energy is real. When the Universe expands enough that the density of everything else becomes negligible, the expansion rate will asymptote to a constant as well.
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- For our Universe, this means that the expansion rate will never drop below about
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-------------------- 55 kilometers / second / Megaparsec, about 80% of its present value ,
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------------------- which is: ------------- 74.2 km / sec/ Mpc
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------------------- which is: ------------------- 49,300 miles per hour / million light years distance
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- Whether the expansion of the Universe accelerates or decelerates depends not only on the energy density of the Universe , but also on the pressure of the various components of energy. For something like dark energy, where the pressure is large and negative, the Universe accelerates, rather than decelerates, over time.
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- For matter, the pressure is negligible so long as it's moving slow compared to the speed of light. For radiation, the pressure is positive, meaning the expansion rate slows more quickly than it does for matter alone.
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- But for dark energy, the pressure is not only negative, it's three times as powerfully negative as radiation pressure is positive. For dark energy, the pressure is actually equal to the negative of the energy density. Instead of decelerating, the Universe accelerates when dark energy dominates.
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- There is a large suite of scientific evidence that supports the picture of this expanding Universe and the Big Bang, complete with dark energy. 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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------- Other reviews about Dark Energy and see Review 2631 to learn about Dark Matter.
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- 2337 - Dark Energy - When the Universe began, gravity soon separated away from the three other forces. This happened at 10^-43 seconds and at a temperature of 10^32 degrees Kelvin. Gravity is the force that pulls masses back together. But, there is another force that is expanding the Universe.
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- 2297 - Dark Matter is the gravity that is holding each galaxy together. Dark Energy is the anti-gravity that is hurling separated galaxies away from each other at an ever accelerating rate. Astronomers and physicists cannot explain either one. These two unknowns occupy 23% and 72%, totaling 95%, of all the matter-energy in the Universe. Everything we know and understand is in the remaining 5%. We have a lot to work on.
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- 2257 - What is the fate of our Universe? The key to unlocking our ultimate cosmic fate is dependent on our understanding what we are looking at And then ensuring that our answers aren't biased by the assumptions we're making about the objects we're measuring. Dark energy may not be a constant. Stay tuned, an announcement on the fate of the Universe will follow.
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- 2138 - - The Birth of My Universe. The Universe was homogeneous before Inflation happened. and because it expanded much faster than the speed of light it remained homogeneous. The velocity of the expansion today is 47,000 miles per hour for every million lightyears distance. In 1998 astronomers discovered that this velocity was accelerating. There is some type of Dark Energy in the vacuum of space that is accelerating the expansion of the Universe.
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- 2112 - Cosmological constant , the fudge factor that expanded the universe. The fudge Factor that expanded the Universe. . We know the Universe is not only expanding its expansion is accelerating at an ever faster rate. Now, science is resurrecting the cosmological constant fudge factor in order to use Einstein’s equations to explain an accelerating expansion that began 5 billion years ago.
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- 2105 - Energy Expanding Universe. Universe is expanding at 49,306 miles per hour per million lightyears distance. . Eventually at the edges of the Universe the separation speed exceeds the speed of light. Then light from those galaxies at the edge will never reach us.
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- 1896 - Dark Energy and the expansion of the Universe. Astronomers are trying different methods to measure the accelerating expansion of the Universe. The hope is for new discoveries to point to the source of “Dark Energy” that is creating this repulsive force.
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- 1884 - What we know about the expansion of the Universe? Making accurate distance measurements to stars and galaxies is an amazing challenge for astronomers. This review discusses how we got to what we think we know about the Universe’s expansion and what could be causing it.
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- 1864 - How is Dark Energy expanding the Universe? Why is this expansion rate accelerating? Where does the energy come from? What is the likely end result?
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- 1855 - Cosmology, studying energy flows and statistics. - Cosmic Inflation requires that the Actual Universe to be much larger then the Observable Universe. The observable is 93 billion lightyears diameter.
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- 1800 - Dark Energy - Something is counter acting Gravity. For the first 7 billion years gravity was king. Then, something else took over once expansion reached this point. Now the Universe is expanding at an ever accelerating rate over this next 7 billion years.
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- 1749 - No matter where you go in the Universe your field of reference has everything expanding away form you, an at ever accelerating rate. Dark Energy Expanding the Universe. What is it that is causing the Universe to expand at an ever increasing rate? What force is overcoming the force of gravity?
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- 1594, 1595, 1596, 1597, 1599 - Reviews to study why is 95% of the Universe “dark”.
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- 1516 - If the Universe density were greater then 10^-29 grams / cm^3 then gravity would win over repulsion expansion. We need to find 200 times more Dark Matter in order to reach this “ critical density”.
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- 1499 - Graph of Universe composition versus Redshift expansion rate.
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- 1290 - Calculations of the average density of the Universe. Ours is a dynamic universe unbalanced by the mysterious Dark Energy. Why is Dark Energy Expanding the Universe? Most astronomical data we have today confirms the fact (theory in fact) that the Universe is still expanding at an ever increasing rate. The cause is suspected to be some type of anti-gravity that is named “ Dark Energy”. We know what gravity is and we expected gravity to eventually stop the expansion and reverse the Universe into a “ Big Crunch”. However, we now think that Dark Energy is the predominate force in the Universe and that it will expand the Universe forever into a “Cold , Rarified Space of almost Nothing”
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- 1147 - Dark Energy is not predicted by any physics we know of. Here are 3 theories that are in the works.
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- 1083 - 1084 - 1085 - Dark Energy - Equation for how fast mass would have to decrease to keep the speed of light at 186,600 miles per second.
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- 1047 - Is the Universe like Swiss Cheese?
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- 822 - Things are “Dark” because we can’t see them and we do not know what they are.
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- 715 - Cosmic Dark Ages. First billion years after the Big Bang.
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- 660 - then there are Dark Matter reviews , and Cosmic Inflation reviews
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- 374 - What is dark matter? How do we know it is there?
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- February 24, 2020 2632
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--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
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--- to: ------ jamesdetrick@comcast.net ------ “Jim Detrick” -----------
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--------------------- Tuesday, February 25, 2020 --------------------
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