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----------------- 1739 - How fast is the Universe Expanding?
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- Hubble’s Constant not only places galaxies in space, it places them in time as well. Edwin Hubble discovered that galaxies were all receding away from us. The further away the galaxy was the faster it was receding. His Constant was a velocity per every million lightyears of space between us and the galaxy.. The more space there was between the galaxies the faster their velocity of separation. His Constant was:
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---------------- 47,000 miles per hour for every million lightyears.
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- This idea that galaxies are flying away from each other is a revolutionary idea in all astronomy. If we simply run time backwards into the distant past galaxies would be flying toward each other. Eventually they come together at one point. If we know how fast they are moving and how far apart they are now we can calculate when the galaxies are all clumped together in the Big Bang. Amazing!
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-If we run the clock backwards to 13,700,000,000 years ago then there was no space between all the galaxies, and in fact, between all the bits of matter that made up the galaxies. The clump had to be exceedingly hot and dense, to say the least.
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- An important concept with this idea of an expanding Universe is that the galaxies are not flying through space, rather, the space between galaxies is expanding. Somehow out of the vacuum there is more space being added.
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- The effect we observe is that the galaxies are moving away from each other. We can measure how fast they are moving. This velocity is 47,000 miles per hour for each million lightyears of space between the galaxies. That is how fast space is expanding or more space is being added.
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- Let’s say we measure the separation of two galaxies and they are 98 million lightyears apart. So, the velocity that these galaxies are moving away from each other is :
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--------------- 47,000 * 98 = 4,606,000 miles per hour.
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- If this velocity was constant for the period of their separation we can work backwards and calculate how long they have been traveling apart. That is how long ago when they were not separated at all.
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----------------------- Time = Distance / velocity
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--------- 98 million lightyears * 5.88 * 10^12 miles / lightyear = 575 *10^18 miles
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------------------ Time = 574 * 10^18 / 4.606 * 10^6 miles per hour
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----------------- Time = 125 * 10^12 hours
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------------------ Time = 125 * 10^12 hours / 8,760 hours per year
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------------------- Time = 14 billion years.
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- That means if the velocity of separation was a constant then these two galaxies were together 14 billion years ago. That must be when the Universe first stated.
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- Let’s do the same calculation for two more galaxies that are 196 million lightyears apart.
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----------------- velocity = 47,000 * 196 = 9,306,000 miles per hour
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--------------- Time = 196* 10^6 LY * 5.88*10^112 mi/LY / 9.306 *10^6 mph = 1152*10^18 / 9.306 *10^6 mph = 124*10^12 hr / 8,760 hr/year = 14* 10^9 years
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----------------- Time = 14 billion years.
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- If the galaxies are twice as far apart their speed of separation is twice as fast, because there is twice as much spaces expanding at the constant Hubble velocity. So, the more separated the galaxies the faster they are moving apart.
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- Therefore, we can conclude that if a galaxies were far enough apart they would be separating at faster that the speed of light. And, that the galaxy’s light would never reach us . Galaxies would be disappearing over the light horizon.
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- Further, the velocity of separation at 47,000 miles per hour is not constant. Astronomer have learned that it is accelerating faster and faster into the future. It is only constant at this point in time.
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- And, it has not always been a constant velocity. In the first 5 billion years after the Big Bang gravity was slowing the expansion down. The mass was concentrated and gravity was stronger. The force of gravity falls off as the square of the distance between masses. As the masses separated their mutual gravity became less and less.
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- In the last 5 billion years the expansion has been speeding up. Today, the 47,000 miles per hour is a good average that we can use to calculate the age of the Universe to be 14 billion years.
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- The ratio is known as the Hubble Constant , designated “ Ho” .
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------------------- Ho = 47,000 miles per hour / million lightyears
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------------------- Ho = Distance / Time * Distance
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------------------- Ho = 1 / Time
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------------------ Time = 1 / Ho
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