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-----------------1864 - How is Dark Energy expanding the Universe?
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- Empty space that separates everything is growing wider and wider. Not that we can notice. We need cosmic scales before we can begin to measure it. Edwin Hubble made the first measurements in 1920. His distance measurements for galaxies determined that the farther the galaxy was away from us the faster it was receding away from us. The space between us was expanding.
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- In 1998 measurement of supernovae determined that not only was the Universe expanding it was accelerating in its expansion. 18 years later and all we have is a “name” for this phenomena, “ Dark Energy”, but, no explanation on what is causing this cosmic acceleration.
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- One theory assumes that empty space is not a vacuum, not nothing, but, teeming with pairs of “ virtual particles”. Virtual because these particles and their anti-particles spontaneously appear and disappear so quickly they avoid detection. In the process these particles carry energy. Energy can produce gravity just as mass can produce the effects of gravity.
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- Somehow this energy is negative pressure, a type of anti-gravity. In Einstein’s equation for gravity this term is called the “ Cosmological Constant”. He called it the “ constant energy density throughout space”.
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- Another theory is that the Dark Energy is a “Quintessence“ field, where every point in space counteracts the pull of gravity. If Dark Energy is a quintessence field it might change with time. So it may accelerate the Universe for a while , then, reverse itself and return the Universe into a “ Big Crush”.
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- Still a third theory is that our theory of gravity, the “General Theory of Relativity’, is incomplete. These laws of gravity themselves might need to change. But, where, why, how?
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- None of these theories mathematically, completely match observation. Regardless the vacuum energy must be very tiny. It only is effective on cosmic scales. Still any one of these theories if true could produce very different results in the long run.
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- Is Dark Energy a field? Will it send the Universe into a “ Big Rip” or change direction and collapse the Universe in a ‘ Big Crunch”?
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- Or, is there no Dark Energy at all? Instead we may learn that we need to change the laws of gravity?
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- If space is expanding under the cosmological constant in a trillion years from now all the galaxies will be so far away their light will never reach us. Even the Cosmic Microwave Background will have a wavelength wider that our visible universe.
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- “ Cosmic inflation” suggests the Universe ballooned enormously in the first fraction of a second after the Big Bang. Maybe there are multiple pocket universes that form in isolation from one another doing the same thing?
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- This last theory comes out of “ string theory” and “ M-theory”.
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- To learn the nature of Dark energy we need more measurements of the pressure and the density of space. It the ratio of pressure to density is equal to -1 it could be that the “cosmological constant” is the right theory.
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- If the ratio differs from -1 it may be energy is evolving differently over time. If the ratio is different with different observations maybe the Theory of Relativity needs to be modified.?
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- The new data astronomers need are to learn how galaxy clusters grew over time. A new technique for astronomers is Gravitational Lensing. This new tool allows the most distant measurements of galaxies. The data collected will define how the Universe has expanded over time. The distant measurements rely on “ redshifts’ of light spectrum. The greater the redshift the more distant the source.
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- A new measurement for distant measurement involves “ Baryonic Acoustic Oscillations”. Newly discovered “ Gravitational Waves” may also be a tool to measure cosmic evolution.
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- Another very interesting test involves the equations for gravity being verified measuring the changing distance to the Moon. This is done using laser light off reflectors placed on the Moon by the Apollo astronauts. Astronomers are looking for minuscule discrepancies in the calculations using current laws for gravity.
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- There are many tests being focused on the current model for cosmology. Dark Matter is 23% of the Universe, Dark Energy is 73%. 4% ordinary matter. Matter is using gravity to slow cosmic expansion. Dark Energy is using anti-gravity to accelerate expansion. How has these ratios changed over time and will they remain constant in the future?
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- Measurements of the Cosmic Microwave Background radiation is a snapshot of the Universe expansion 400,000 years after the Big Bang. Using this data to calculate expansion does not agree with today’s direct measurements.
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- The Universe expansion rate is called the “ Hubble Constant” but it is only constant everywhere at this point in time. Measurements are trying to determine the rate existing in the past, and the rate does not appear to be constant going into our future.
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- Direct measurements rely on “standard candles” of brightness of stars in galaxies to determine how fast galaxies are moving away from each other. That is , how fast is space growing in between the galaxies. Only those gravitationally bound together can resist this pressure to spread them farther apart.
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- Brightness calculations depend on the “intrinsic brightness” of these “candles” compared to their “ apparent brightness” measured by the Hubble Space Telescope. This month (April 2016) the data was published for a measured constant with an uncertainty of +-2.4%. But, that answer is 8% faster expansion rate compared to the Cosmic Microwave Background results.
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- Using distance ladders with Cepheid stars and Type1a supernovae astronomers get an expansion rate of 73.02 +-1.79 km/sec/mps That is with an uncertainty error of +-2.45%. Translating these mega parsecs into lightyears we get 47,000 miles per hour expansion rate for every million lightyears of distance. The further the distance the faster the galaxies are receding.
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- Using the CMB to calculate expansion rate we get 67.43 km/sec/mps +-0.7 which is an uncertainty of +-1.04%
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- The bottom line is the Universe looks like it is expanding 8% faster than expected based on looking at its youth (CMB) and how we expected it to evolve from then to now.
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- Three theories surface to try to explain the disparity:
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- (1) Maybe the elementary particles of Dark Matter that we have not yet discovered have different properties than our current thinking.
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- (2) Maybe Dark Energy is not constant. It has been different in the past and it is changing its acceleration into the future.
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- (3) Maybe “ standard candles” measurements were not precise enough to give us accurate results.
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- May your curiosity always reign, and may you always have more to learn” Cosmology is certainly granting us this wish. God has made sure we have plenty to do.
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- Request these Reviews to learn more about Dark Energy:
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- #1749 - When the visible universe was half its current size the density was 8 times higher and the radiation twice as hot.
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- This review lists 5 more reviews about Dark Energy.
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- #1516 - Dark Matter would have to be 200 times the mass of visible matter in order to have the critical density needed to halt expansion.
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- #1499 - Astronomers find about one supernova explosion every night.
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- #1290 - The observable universe has 10^22 sun-sized stars as a measure of ordinary matter.
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- #1083 - The intensity of luminosity of the supernovae follow the Blackbody Curve which has a defined peak as a function of wavelength and temperature.
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- #1047 - Is the Universe like Swiss Cheese?
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- #822 - Does gravity only come in one polarity? The more studying you do the more mysteries you find.
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----- 707-536-3272 ---------------- Monday, April 25, 2016 -----
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