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-------------------- 2572 - UNIVERSE - size and age is determined?
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- We have 8 planets in our solar system and we know how much they weigh. The eight planets in our solar system each weigh between 10^24 and 10^27 kilograms .
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- That is 1 followed by 24 or 27 zeros.
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- The Sun weighs in at 10^30 kilograms
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- Our galaxy weighs in at around 10^42 kilograms
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- The entire visible universe reaches 10^53 kilograms.
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- How in the world do we know this?
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- The numbers are so huge no matter how you try to wrap your brain around them. Changing from kilograms to pounds doesn’t make much of an appreciable size or distance at these scales, either. When it comes to imagining such colossal masses, the human mind is completely out of its element.
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- Physicists and astronomers continue to refine their measurements of the seemingly immeasurable and definitely inconceivable. They estimate the mass of the Milky Way galaxy including both the disk of twinkling stars and an invisible sphere of dark matter that presumably surrounds it.
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- The mass of he Milky Way Galaxy is equivalent to 890 billion Suns. Most is dark matter, with just 60 billion Sun masses representing all the stars and gas that we can see.
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- “Weight” describes the gravitational force acting on an object, so a galaxy “weighs” nothing while floating in empty space. Astronomically “weighing” an object comes down to measuring the gravitational force between it and another massive partner.
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- By the 1600s, estimates of Earth’s diameter, and therefore its volume, were pretty good. But , no one was sure of the density. Do we use the density of water or of rock for the average? The estimates of the planets mass were not even close. The planet is actually made up mostly of metal, which is denser than water or rock.
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- To figure out that density and the mass of the Earth British scientist Henry Cavendish measured the overall strength of gravity in 1798. Isaac Newton had shown in the 1600s that all objects pull on all other objects, and those with more mass pull harder.
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- Cavendish hung small metal balls from a wire, placed heavier spheres nearby, and watched the wire twist as the spheres attracted each other. In this horizontal twisting he was able to determine the intensity of the gravitational force in general. And, knowing how hard the Earth’s mass tugged downward on his spheres (that is, their weights), he could use Newton’s equations to calculate the Earth’s composition at metal-like 5.42 times the density of water. Modern physicists have found that he was off by only seven-tenths of one percent.
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- Once Newton and Cavendish had worked out the strength of gravity in general and the mass of the Earth in particular, scientists had the tools they needed to go forth and weigh much of the rest of the universe.
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- The Sun pulls on the Earth just hard enough to swing it around once every 365 days, implying a certain force, and therefore certain mass. Similarly, by considering the Sun as the prime partner of various heavenly body pairings, researchers could calculate the mass of the rest of the planets based on the length of their years.
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---------------------- Mass of the Sun = 4 *pi^2*radius ^3 / period of orbit * Gravitational Constant
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---------------------- Mass of the Sun = 4*(3.14)^2 * (1.5*10^11 meters)^3 / (6.67*10^-11 Newtons * meters^2 / kilograms^2) * (3.15 *10^7 seconds)^2
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--------------------- Mass of the Sun = 2.0*10^30 kilograms
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-------------------- Force of gravity = G*m*M / r^2 = mg = on the surface of the Earth
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---------------------- Mass of Earth = M = g *R^2 / G
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------------------------ Mass of Earth = (9.80 meters / second^2) * ( 6.37 *10^6 meters)^2 / (6.67*10^-11 Newtons * meters^2 / kilograms^2)
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--------------------- Mass of Earth = 5.97 * 10^24 kilograms
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- Watching how moons orbited planets provided another check, as well as a way to weigh the moons. Asteroid mass estimation remains something of a dark art based on guessing plausible densities and sizes.
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- Just as researchers can infer the mass of the Earth by watching how hard it drags down objects on or near its surface, or the mass of the Sun by watching how quickly planets orbit around it, they can read the galaxy’s mass in the motion of the objects that circle it.
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- It was these trajectories of orbiting stars that first flagged the presence of dark matter in the 1970s. In our solar system, Mercury zips around nearly nine times faster than Neptune does because it lies much closer to the source of the vast majority of our solar system’s mass, the Sun,
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- This relationship holds close to the center of most galaxies, but then stops. After a point, no matter how far out they looked, astronomers discovered that stars orbited at surprisingly similar speeds. This would imply that a second, invisible source of mass is also pulling on them.
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- Astronomers weigh our galaxy through similar analyses of local stars and groups of stars. The recent research, which extends another estimate from earlier this year, harnesses a database of nearly 3,000 “tracer” objects, such as stars, star clusters, and gas clouds, that orbit the center of the Milky Way. Using the motion of these tracers, they calculated how much mass, visible and dark, the galaxy contains.
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- The absolute size of the universe is unknown, and is constantly expanding, so its mass is similarly undefined. Astronomers can define the volume of the observable universe, however, based on the distance light has been able to travel between the Big Bang and present day.
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- But the density of existence, averaged out over all of the universe of planets, stars, galaxies, and voids, has proved challenging to measure. One estimate came from the Wilkinson Microwave Anisotropy Probe (WMAP), a satellite that measured warm spots and cool spots in the universe’s earliest light from 2001 to 2010.
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- These patches are the remnants of a power struggle from when a dense soup of matter and light filled the young universe. Gravity drew particles together while light pushed them apart, creating sloshing ripples that grew with the expanding cosmos until WMAP picked them up. From the patterns in these variations today, cosmologists can calculate the age and composition of the universe, including its overall density to be about six protons of mass per cubic meter.
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- That number technically represents an energy density (since matter and energy can be converted using Einstein’s famous equation), so it includes visible matter, dark matter, and the unknown dark energy driving the expansion of the universe.
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- WMAP and its successor, the Planck satellite, estimated that by this metric the universe is about 5 percent visible matter, 27 percent dark matter, and 68 percent dark energy.
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- In this way cosmologists can, much as Cavendish did in 1798, combine their estimates of their target’s volume and density to estimate the universe’s overall mass as something like 100,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 kilograms.
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-That’s roughly 100 billion Milky Way galaxies.
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- In April 1920, Harlow Shapley and Heber Curtis argued over the scale of the universe in the great auditorium of the Smithsonian Institution’s Natural History Museum.
This discussion preceded Edwin Hubble’s discovery of the nature of galaxies by just a few years,
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- 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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- Meanwhile, Shapley 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. The age and size of the universe are interrelated, and both depend critically on this Hubble constant.
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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. 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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- If you pick a middle number , say 74.2 km / sec / mpc , that equates to 49,300 miles per hour per million lightyears distance, or 13 miles per second per 1,000,000 lightyears.
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- Mpc is a megaparsec. A parsec is 3.26 lightyears. Megaparsec is 3.26 million lightyears.
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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 yet, astronomers are limited by both assumptions and a lack of adequate data to agree on the true cosmic distance scale.
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- Using today’s most powerful telescopes, astronomers see galaxies located over 13 billion light-years from Earth. (A light-year equals about 6 trillion miles, or 10 trillion kilometers.) 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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- Since the Big Bang, the expansion of the universe has slowed and then sped up.
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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- The Alan Guth idea suggests 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 this inflation occurred, then the universe is much larger than we might expect based on current observations.
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- Here’s where it gets weird: If inflation happened, then it may have occurred in many places (perhaps an infinite number) beyond the visible horizon and the limits of the space-time continuum we are familiar with. 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?
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- Before WMAP and Planck, the best approach for determining the universe’s age relied on the much-debated Hubble constant, a figure that describes the rate at which the universe is expanding. 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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-------------------- Velocity = distance / time = Hubble Constant * distance
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---------------------------- Time = 1 / Hubble Constant
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- But there was a problem. 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 the 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 3.26 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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- 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 technique yields ages of 4.4 billion years for the oldest rocks on Earth (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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- Alternatively, astronomers 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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- Such studies of numerous globulars, based on distance measurements provided by the European Space Agency’s Hipparcos and Gaia missions, suggested an age for many of the oldest stars of around 13 billion years.
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- 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 had to be among the first objects to form.
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- Any discrepancies narrowed significantly with the release of WMAP data, before essentially disappearing when researchers announced Planck’s latest findings in 2015. 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.
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- There you have it. The Universe is 13,800,000,000 years. That is a close as I can get.
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- December 31, 2019 2572
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--------------------- Wednesday, January 1, 2020 --------------------
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