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------------------------------- 2551 - UNIVERSE - the age and size explored?
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- In April 1920, Harlow Shapley and Heber Curtis argued over how big the universe was.
In this discussion, which preceded Edwin Hubble’s discovery of the nature of galaxies by just a few years, Curtis argued that the cosmos consists of many separate “island universes,” claiming that the so-called “spiral nebulae” were distant systems of stars outside our Milky Way.
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- Shapley on the other hand argued that spiral nebulae were merely gas clouds in the Milky Way. Shapley further placed the Sun toward the edge of our galaxy, which, in his view, was the entire universe, whereas Curtis believed the Sun to be near the galaxy’s center.
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- Curtis was right about the large size of the universe but wrong about the Sun’s place within it. On the other hand, Shapley was wrong about the small size of the universe but right about the Sun’s location within it.
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- With the advent of many extragalactic distance measurements and two camps arguing for different results on the critical number called the “Hubble constant“, which is the expansion rate of the universe. These astronomers staged a second great debate in 1996. The age and size of the universe are interrelated, and both depend critically on this Hubble constant expansion rate.
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- In the same auditorium used by Shapley and Curtis, galaxy researchers Sidney van den Bergh and Gustav Tammann argued over the same question. Van den Bergh offered evidence supporting a high value of the Hubble constant (about 80 kilometers per second per megaparsec), suggesting a young age and therefore small size of the universe.
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- Tammann argued for a low value of the constant (about 55 km/sec/Mpc), which would indicate an older, larger universe.
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- As was the case with Shapley and Curtis, the antagonists van den Bergh and Tammann each provided crisp, clear-cut arguments and data supporting his side, and neither succeeded in convincing astronomers from the other camp. As of today astronomers are still limited by both assumptions and a lack of adequate data to agree on the cosmic distance scale.
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- Despite this, astronomers can still set some limits on what must be true based on the observations they have collected and refined over the past century. Using today’s most powerful telescopes, astronomers see galaxies located over 13 billion light-years from Earth. Since they see these distant galaxies in all directions, the current “horizon” of visibility is at least 26 billion light-years in diameter.
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- But the universe is probably much larger than the portion we can see. This will be the case in the highly likely event that the inflation hypothesis, put forth in 1980 by MIT’s Alan Guth, proves correct.
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- Guth’s idea suggested that the extremely young universe experienced a brief period of hypergrowth so severe that it ballooned from the size of a subatomic particle to the size of a softball almost instantly. If inflation occurred, then the universe is much larger than we might expect based on current observations.
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- If inflation happened, then it may have occurred in many places (perhaps an infinite number of places) beyond the visible horizon and the limits of the space-time continuum we are able to see. If this is so, then other universes might exist beyond our ability to detect them.
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- Science begs off this question, as by definition science is about creating and experimenting with testable ideas. For now, it’s wondrous enough to know we live in a universe that’s at least 550 billion trillion miles across, and it may be much bigger than that. We have no way of knowing.
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- The launch of NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) in 2001 and the European Space Agency’s Planck satellite in 2009 changed all that. Before WMAP and Planck, the best approach for determining the universe’s age relied on the much-debated Hubble constant, that figure that describes the rate at which the universe is expanding.
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- To find the Hubble constant, astronomers observe distant galaxies and measure their distances (by using Cepheid variable stars or other objects of known intrinsic brightness) as well as how fast they recede from Earth.
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- They then determine the Hubble constant by dividing the galaxy’s speed of recession by its distance. Once they decide on a value for the Hubble constant, they can estimate the maximum age of the universe by calculating the constant’s reciprocal.
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- The units for the Hubble constant is kilometers / second / megaparsec.
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- The megaparsec is a distance equal to 3.0857 * 10^19 kilometers.
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- So the units are km / sec / km or 1 / sec. The reciprocal of time. Therefore the reciprocal of the Hubble constant is time, the age of the Universe. Of course this assumes that the constant was always constant, that is the same rate of expansion since the beginning.
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- If the Hubble constant is 49,300 miles per our per million lightyears distance.
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- Then the age of the universe is 13.86 billion years.
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- But there was a problem with thee calculations.. The values astronomers got for the Hubble constant depended on various assumptions about the universe’s density and composition and the method used to determine distances. So astronomers of different mindsets got different values for Ho, the Hubble constant.
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- They generally divided into two camps, one in the range of 50 kilometers per second per megaparsec and the other up at 80 km/sec/Mpc. A megaparsec equals 3.087 * 10^19 kilometers , or, 3.26 million light-years, or about 20 billion billion miles.
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- Therefore, the two groups estimated a range for the age of the universe of about 10 to 16 billion years. Higher values of the Hubble constant produce younger age values for the universe.
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- Astronomers who were using the Hubble Space Telescope to measure the distances to many galaxies narrowed in on a value toward the faster, and thus younger, end of the scale. But uncertainties still remained.
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- The other series of approaches for determining the universe’s age attempted to directly measure the ages of the oldest objects in the universe. Astronomers can estimate the age of the cosmos by measuring the decay of radioactive elements.
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- This radioactive decay technique yields ages of 4.4 billion years for the oldest rocks on Earth. These were zircons found in Jack Hills, Australia. And, 4.6 billion years for the oldest meteorites, effectively dating the solar system but not the universe.
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- Applying this method to gas in the Milky Way or to old stars is less precise, however, due to assumptions about the primordial abundances of various isotopes. These calculations pointed to a universe between 12 and 15 billion years old, with a large uncertainty of plus or minus 3 to 4 billion years.
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- Astronomers also measured the ages of white-dwarf stars, the shrunken remnants of stars that are as heavy as the Sun but only as large as Earth. By finding the faintest, and thus oldest, white dwarfs, astronomers estimated how long they have been cooling.
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- Comprehensive attempts at cataloging white dwarfs and measuring their ages yielded about 10 billion years for the age of the Milky Way’s disk. The galaxy’s disk formed about 2 billion years after the Big Bang, yielding an age of the universe of about 12 billion years.
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- Measuring the ages of ancient star clusters offers yet another avenue for exploring the age of the universe. By looking at the most luminous stars in a globular cluster, astronomers can determine an upper limit for the cluster’s age. They look at the brightest stars on the so-called main sequence, the primary track on a plot of stellar brightness versus temperatures.
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- The European Space Agency’s Hipparcos and Gaia missions, suggested an age for many of the oldest stars of around 13 billion years. And astronomers think the age of globulars gives a pretty good indication for the age of the universe. That’s because globulars contain hardly any elements heavier than hydrogen and helium, and so they had to be among the first objects to form.
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- Any discrepancies narrowed significantly with the release of WMAP data. Such studies of numerous globulars, based on distance measurements provided by the, before essentially disappearing when researchers announced Planck’s latest findings in 2015.
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- By carefully examining the microwave background radiation, astronomers have pinned down the universe’s age to 13.8 billion years, accurate to better than 1 percent. The results pretty much ended the debate.
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- We think the Universe is 13,800,000,000 years old and its expansion velocity is 49,300 miles per hour per million lightyears distance. That is 13 miles per second per million lightyears. That is pretty fast. Better make the best of the time you have.
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- December 21, 2019 2551
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--------------------- Saturday, December 21, 2019 --------------------
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