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---------------------- 2312 - Universe - how fast is it expanding?
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- In the beginning, all of space rang like a bell.
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- The ringing was caused by the big bang, and the universe was filled with a high energy plasma. Plasma is energetic , charged particles and radiation. Although the plasma was remarkably smooth, it wasn’t completely smooth. There were slight density and pressure gradients that pushed material around which amounted to be sound waves in the medium of expanding space.
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- The ringing happened everywhere, so intensely that we can still detect it 13.8 billion years later. It has been detected directly in the cosmic microwave background, the afterglow left over from the big bang’s release of the first light.
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- This primordial reverberation is so well measured and modeled that it has been used to deduce the precise rate at which the universe is expanding, a number known as the Hubble constant. That constant which is the number 67 or 73 is the cornerstone of our modern understanding of the size, age and structure of the universe.
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- The units for 67 and 73 are kilometers per second per mega parsec distance. A mega parsec is 3,262,000 lightyears distance. Or, in more familiar units 49,306 miles per hour for every million lightyears distance.
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- The value derived by extrapolating from the ancient sound waves should match up exactly with the value derived from independent studies of the light from distant stars and galaxies.
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- In reality, the two approaches yield a vexing disagreement, and, the more diligently researchers attack the problem, the more definitive this conflict in expansion rate seems to be.
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- Following the big bang, sound waves produced by the intermingling of light and matter traveled freely through the hot, plasma-filled universe. After some 380,000 years matter cooled enough to form atoms, decoupling from light and dampening the sound waves. Suddenly, the ringing stopped, impressing a final, frozen pattern of waves into the escaping light, which we see today in the cosmic microwave background.
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- The sound horizon defines the size of those final waves. How far the sound disturbances propagate by the time the plasma disappears is called the “sound horizon“.
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- Just as you can measure the qualities of a bell from the way it rings such as a small glass bell sounding entirely different than a large brass one, researchers can infer the precise properties of the universe from its sounds as recorded in the microwave background radiation.
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- That is how they can confidently declare the cosmos consists of 4.8 percent ordinary matter, 26 percent of the unseen dark matter and a full 69 percent dark energy which is a mysterious anti-gravitational force that stretches empty space apart.
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- In 2015 microwave measurements from the European Space Agency’s Planck spacecraft revealed the universe is expanding at a rate of 67.8 kilometers per second per mega parsec. Cosmologists typically drop that mouthful if units at the end and simply say that the Hubble constant is between 67 and 68.
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- Meanwhile competing groups of astronomers have been studying the expansion of the universe in a distinctly different way, by seeking out variable stars or supernova explosions of known distance and then directly measuring how quickly they are moving away from us.
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- This “distance ladder” method measuring distances across many millions of light-years is a subtle, time-consuming task plagued with the possibilities for many kinds of systematic errors. Get the location of a star wrong, and the entire calculation is wrong.
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- Every time you improve the accuracy by steadily beating down on the uncertainties and drawing on the latest observations of variable stars the astronomers get an answer for the constant of 73.2.
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- To ferret out any non-obvious problems astronomers are developing a new type of distance measurement using red giant stars as reference points. A third way of measuring the expansion rate.
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- Cosmologists on both sides are also looking to outside groups for guidance. So far, those referees are only deepening the mystery. A University of California, Los Angeles, study that looks at how light is bent by distant galaxies gives a Hubble constant of 72.5, close to the distance-ladder result.
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- Meanwhile an equally convincing study looking at how primordial sound waves affect the distribution of galaxies in the universe gives a constant of 67.
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- Calculations of the Hubble constant anchored to the sound horizon consistently give a lower number than ones based on observations of stars and galaxies.
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- There is one way all of the measurements can be correct, and that is if something is wrong with scientists’ interpretations of those measurements. Everything we know about the origin of the sound horizon depends on a theoretical model of how the universe behaved during its “unseen” initial 380,000 years.
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- If the models are wrong and the size of the sound horizon is different than what they predict, that adjustment would change all of the numbers derived from it, including the Hubble constant.
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- A smaller sound horizon, by just 7 percent, and all of the studies happily agree with one another. The problem is, it is not at all clear what could account for such shrinking. In almost every other way, the model and the observations fit together tightly.
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- It is much easier to tick off the things that do not work: Undiscovered special kind of neutrino? Nope. New type of interaction between photons? Nope. They all conflict with the data.
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- The most convincing explanation is that the very early universe was expanding slightly faster than expected. If so, it would have cooled more quickly and frozen the sound horizon in place a little sooner. Then the sound horizon would be smaller than the one theorists have plugged into their models, and problem solved!
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- Or, the problem is kicked down the road again, because now we need some explanation for what made the early cosmos take off more quickly.
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- Potentially where this is leading us is to a new ingredient in the arena of Dark Matter and Dark energy. Those invisible components of the universe that do not interact with radiation in any way. One is holding galaxies together and the other is spreading galaxies apart
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- Researchers already invoke dark matter to explain galactic motion and dark energy to account for the universe’s accelerating expansion. The divergent measurements of the Hubble constant may be the first sign of the existence of a third dark component, a “dark turbo“, that added to the energy of the early universe, hastening its expansion and changing the pitch of its sounds.
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- A related possibility is dark energy has more than one form, or changes over time in complicated ways. A recent study of 1,598 distant quasars using NASA’s Chandra X-Ray Observatory offers intriguing preliminary evidence for the interpretation that dark energy changes over time.
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- It may seem like cheating to invoke something new and unseen to explain away a confusing result. The Hubble constant conflict may be bringing into view an aspect of the universe that had completely eluded detection until now.
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- New observations of the early universe by the South Pole Telescope in Antarctica and the Atacama Cosmology Telescope in Chile will further probe the sound horizon.
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- A next-generation ground-based project called CMB-S4 intends to map the polarization of the microwave sky with greater sensitivity.
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- Studies of gravitational waves will provide a completely independent way to assess the true Hubble value as well.
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- Soon enough, data will settle whether scientists have been chasing errors or advancing on an undiscovered sector of the universe. It may turn out to be fundamental new physics. Now that would be interesting. Stay tuned , there is still more to learn.
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- Other reviews on this subject:
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- 2292 - Accelerating the Universe from unknown force.
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- 2263 - The Universe as we know it.
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- 2262 - The Universe, how fast is it expanding? This review list 2 other reviews on this subject. All available upon request.
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- March 17, 2019
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-------------------------- Sunday, March 17, 2019 --------------------------
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