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---------------------- 2297 - Dark Energy and Dark Matter?
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- Dark energy is incredibly strange. Dark energy appears strong enough to push galaxies around the entire universe, yet, its source is unknown, its location is unknown and its physics are unknown. But “something” is causing the universe to expand.
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- Physicists have found that for the last 7 billion years galactic expansion has been accelerating. This would be possible only if something is pushing the galaxies, adding energy to them. Scientists are calling this something “dark energy,” a force that is real but is beyond our detection.
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- One of the most speculative ideas for the mechanism of an accelerating cosmic expansion is called quintessence, a relative of the Higgs field that permeates the cosmos.
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- Cosmologist Freeman Dyson’s famous 1979 paper “Time Without End,” looked at life’s ability in the far, far future to act on an astrophysical scale in an open universe that need not evolve into a state of permanent quiescence. Where life and communication can continue for ever. This is where the idea of Quintessence came from.
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- There are several theories for the identity of dark energy:
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- It may be energy generated by ghostly subatomic particles that appear out of nothing before annihilating.
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- It may associated with the recently confirmed Higgs Field, which gives certain kinds of matter mass.
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- It may be explained by string theory by which extra invisible dimensions of space get compressed into sizes much smaller than atoms.
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- It may be an example of fine-tuning that demands the existence of a multiverse to explain it.
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- Almost all physicists agree that if the amount of dark energy in the universe were slightly different, life could never have emerged. The amount of dark energy is astoundingly small compared to the theoretically large range it could be. It has been measured to be about one-hundred-millionth of an erg per cubic centimeter. An erg is a unit of work equal to 1 gram centimeter^2 per second.
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- We happen to live in a universe with a small dark energy value, allowing for expansion rather than contraction, and for the emergence of life. It is an extremely delicate balance.
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- Scientists have established that the universe is expanding at a rate 20 percent faster than it was 5 billion years ago. In 1929, Edwin Hubble first demonstrated that the universe was expanding by showing that galaxies outside the Milky Way, in which Earth’s solar system resides, were moving away from each other.
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- Only about 5 percent of the universe is composed of planets, stars and gaseous structures, tht we can see with the remaining 95 percent comprising dark matter and dark energy.
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- Hubble’s law is the first observational basis for the expansion of the Universe, and provides strong evidence for the Big Bang model. The value of the expansion rate is called the Hubble constant. Hubble constant is a critical parameter in cosmology and the measurement of Hubble constant is a key task for the astrophysicists and cosmologists.
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- The galaxies are kind of like the raisins, and as the loaf rises, the raisins, which are far apart to begin with, rush apart even faster. It doesn’t matter which raisin you are in this loaf; everything looks like it’s moving away from everything else.
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- The physics for the discovery that the universe is expanding at an accelerating rate used images of very distant supernovae to derive what was considered the most accurate measurement to date of speed of receding galaxies.
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- The Hubble Constant of rate of expansion is 49,000 miles per hour per every 1 million lightyears of separation.
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- We know there is gravity because apples fall from trees. We can observe gravity in daily life. If we could throw an apple to the edge of the universe, we would observe it accelerating. So dark energy could not be seen until we could measure things very, very far away.
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- Dark matter research is unsettling as well. Scientists were unnerved when they first noticed that galaxies don’t rotate by the same physics as a spinning plate. The stars at a galaxy’s edge rotate faster than expected. And their motion can only be explained by a lot of invisible matter that we can’t see that is surrounding the galaxy.
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- That was exciting more than unsettling when the field was new and ideas were plentiful and had yet to be proven wrong. Researchers consolidated the possibilities into two main camps, complete with clever acronyms: MACHOs (Massive Compact Halo Objects) and WIMPs (Weakly Interacting Massive Particles).
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- MACHOS Aren’t the Answer;
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- Brown dwarfs are an example of a MACHO, but they don’t exist in large enough numbers to solve the dark matter problem.
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- MACHOs are the less exotic possibility. It is perfectly reasonable to suggest that space and galaxies contain lots of stuff, planets, stars not quite big enough to turn on and light up, world-gobbling space worms. We simply can’t see them because they are literally dark.
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- Except we can detect some of those objects out there because they’re so massive that they bend light around them. They do exist, and we know they’re there despite their darkness. And yet there’s just not enough of them to make the galaxy-rotation math work.
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- The same problem occurs if we imagine a universe littered with black holes. We would need to see these light-bending gravitational lenses everywhere and we don’t, even when we look very hard.
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- The search for WIMPs:
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- So the astrophysics community mostly moved on to WIMPs. Rather than big objects, maybe the universe is full of little things we can’t see. These would be swarms of objects like atoms that just don’t reflect or absorb light or any other kind of electromagnetic energy, unlike all the matter we can touch and measure and see around us on Earth.
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- This concept is more unsettling. We do know that neutrinos exist: tiny, mostly mass-less particles that barely interact with the universe around them. The problem there is that they’re mostly mass-less. We can’t figure out how there are enough of them to make up the 84 percent of the universe’s matter that we can’t see.
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- So maybe dark matter is a different object we haven’t observed at all yet, something called a neutralino. Researchers have come up with a plausible description of such a particle, how the Big Bang as we know it might have created them, and how they would fit into the standard model of particle physics without breaking everything else along the way.
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- Is Dark Matter the New Ether?
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- Researchers hope the XENON experiment will directly detect dark matter particles.
But, we’ve been looking for them for a while. We’ve built incredibly sensitive, bizarre instruments to look for them.
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- These include vats of liquid xenon stored miles underground, and telescopes looking for dark matter particles decaying into things we can see and measure, like gamma rays. It includes the Large Hadron Collider, one of the most expensive science experiments ever built. And we haven’t found them. We haven’t found the WIMPs themselves, and we haven’t found convincing evidence that they exist.
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- Except, of course, for the persistent evidence we can’t ignore that says the universe is heavier than what we can see.
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- Decades ago, scientists were confident about the existence of the “luminiferous aether” as a medium to carry light. Scientists persisted because they were sure that light, like sound, required a medium to move through in spite of the evidence piling up against that concept. Having been fooled once, scientists have to ask: Is dark matter the new ether?
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- There is a theory called Modified Newtonian Dynamics, or MOND. Essentially, it says that physics doesn’t work as we know it at the largest scales. It says we’ve been drawing the wrong conclusions, and dark matter isn’t required to explain the universe.-
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- No one has managed to develop a theory of MOND that adequately explains the universe around us, but it occasionally gains converts simply because the competing theory of dark matter has a glaring flaw: we can’t find it.
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- Perhaps we’re wrong about something in the standard model that defines how the tiniest particles in the universe behave and interact, and dark matter exists, but in a very different form than we’re expecting. Or perhaps we are wrong about the laws of gravity.
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- Or perhaps, maybe even tomorrow, an experiment will turn up a neutralino exactly where researchers say it should be. A particle will strike a tank of supercooled xenon. The Large Hadron Collider team will discover a new particle.
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- Science is hard, and seen against the long story of scientific progress, we only started looking for dark matter yesterday. Until something changes, we’ll have to rest uneasy with the unsettling possibility that physics as we know it might be very wrong, or definitely incomplete. Stay tuned , here is still more to learn. We are only picking up pebbles of knowledge on the beach with the whole ocean of the unknown in front of us.
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- March 6, 2019
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