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---------------------------- - 2357 - Cosmology - What is Physics Telling Us?
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- Cosmology is that branch of physics that studies the evolution, structure, and nature of the Universe as a whole.
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- Isaac Newton started the study of cosmology with his vision in 1660 that the Universe was static and all matter was uniformly spread throughout infinite space.
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- In 1826 Olber’s paradox brought this vision in to question. If there is an infinite number of stars in all directions, “why is the sky dark?” Good question.
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- Olber’s paradox says that if we had infinite number of stars uniformly distributed in space the night sky should glow with uniform brightness. The explanation that came later is that we see the Observable Universe so some of the stars are so far away their light has not reached us yet. And, the galaxies of stars farthest away are receding so fast their light is redshifted causing their brightness in the optical wavelengths to be greatly diminished.
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- Albert Einstein proposed an even more dramatic vision in 1915 with his general theory of relativity which gave the Universe four dimensions, in a continuum of space-time.
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- In 1929 Hubble finally changed the vision of a static universe. He discovered that the Universe was actually, dynamically, expanding and that the further away a galaxy was the greater the speed of recession.
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- This expansion is often misinterpreted as the result of a giant explosion, the Big Bang, causing galaxies to still be flying apart from each other. This is not the case. The correct interpretation is that space itself is expanding and the galaxies are embedded in this expanding space.
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- Except, there is no center. No galaxy is at a center of the Universe. The view from all galaxies is the same. This condition is why the Universe is said to be homogeneous and isotropic.
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- Homogeneous means the Universe looks the same in all directions regardless where the observer is standing.
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- Isotropic technically means the Universe’s permittivity, susceptibility, and elastic constants do not vary with direction. In other words the speed, or rate of recession of expanding space is the same in all directions.
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- Here is what cosmology has taught us about the Universe:
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- In one measurement astronomers had determined the rate of recession of space, known as Hubble’s Constant, to be 65 + or - 7 kilometers/second per megaparsec.
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- This is how fast the space is expanding between the galaxies. For every megaparsec, which is 3.26 * 10^6 lightyears, you go out into space the galaxies are receding 65,000 meters per second faster. That is 145,400 miles per hour faster as you go deeper into space every 19,175,700 million million-miles.
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------------------ The Hubble Constant = 65 kilometer/second/megaparsec
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------------------ A megaparsec = 10^6 parsecs = 3.26 million lightyears
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------------------ A parsec = 3.09 * 10^13 kilometers = 3.26 lightyears
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------------------ A parsec is an astronomical unit of length that is equal to a baseline of an astronomical unit subtends an angle of one second of arc. An astronomical unit is the average distance between the Earth and the Sun, 149,600,000 kilometers.
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------------------ Hubble Constant = 65 * 10^-6 km * pc / 3.09 * 10^13 km * sec
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------------------ Hubble Constant = 21.04 * 10^-19 / second
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- By inverting the Hubble Constant we collapse backward to determine the age of the Universe.
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------------------ Age = 1 / Hubble Constant = 1 / 21.04 * 10^-19 seconds
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------------------ Age = 0.0475 * 10^19 seconds * year / 3.16 * 10^7 seconds
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------------------ Age = 15.5 * 10^9 years
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------------------ Age = 15.5 billion years.
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- But, this simple equation assumes the Universe is not influenced by gravity slowing down the expansion. If the Universe is nearly flat and the actual density is nearly equal to the critical density then the age of the Universe is about 2/3 the Hubble Time.
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------------------ Hubble Time = 15.5 billion years
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------------------ 2/3 Hubble Time = Age = 10.35 billion years.
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- But, this does not include the effects of Inflation that occurred between 10^-34 seconds and 10^-32 seconds after the Big Bang. This is when the Universe went through an extremely accelerated expansion caused by the separation of the fundamental force into the three individual forces: electromagnetic, strong and weak forces. If we take Inflation into account we get to an:
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----------------------------- Age of the Universe = 13.7 billion years.
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- In the year 2019 Astronomy has 2 disputed measurements that put the expansion rate at 67.4 and 73.5. Several different techniques are currently trying to reconcile the results to come up with the most accurate rate of expansion for the Universe. This Review uses just one of the early techniques used.
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- If this is the case what does it tell us about the past, present, and future of our Universe?
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- The Big Bang is a unique, hot, dense phase in our past that accounts for our present phase of an expanding universe.
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- Einstein’s General Theory of Relativity is the right theory of gravitation. It says that space is curved by the presence of matter. Therefore the density of matter in the Universe determines its curvature. The critical density is the amount of matter per cubic meter that would just stop the expansion due to gravity over an infinite time.
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- If the actual density of matter in the Universe is larger than the critical density then the Universe is sphere-like and has a positive curvature. The Universe would in this case eventually slow down, stop expanding, and begin to contract. Hubble’s law would be in reverse until the Universe crushed back into a singularity.
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- If the actual density of matter in the Universe is less than the critical density then the Universe is saddle-like and has negative curvature. The Universe in this case would expand forever.
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- If the actual density of matter in the Universe is equal to the critical density then the Universe is flat.
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- Our conclusion is that the Universe is “nearly flat“. We live in a low-mass universe with a substantial cosmological constant.
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- In the Big Bang, conditions were very hot. It was too hot for elements heavier than lithium to live very long. Higher level elements were almost immediately torn apart by the radiation of photons.
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- Then, with Inflation, the Universe cooled very quickly, over just a few hundred seconds. As soon as it was cool enough for heavier elements to survive, it was too cool to make them. Therefore, the ratio of hydrogen to helium to lithium in the Universe today is a very good supporting data for the Big Bang and Inflation.
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- Space is called space for a good reason, because it is mostly empty. The critical density is extremely small = 9.75 * 10^-24 grams per cubic meter.
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- If the Hubble Constant is 65 Km/sec/Mpc and the critical density is 9.75 * 10^-27 kg/m^3, how much mass would be enclosed in a sphere with the radius of the Earth’s orbit?
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------------------------- Volume of the sphere with radius of one astronomical unit
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------------------------- Volume = 4/3 pi radius
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------------------------- Volume = 4/3 * 3.14 * (1.5 * 10^11 m)^3
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------------------------- Volume = 14.137 * 10^33 m^3
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------------------------- Mass = density * volume
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------------------------- Mass = 9.75 * 1-^-27 kg/m^3 * 14.137 * 10^33 m^3
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------------------------- Mass = 137 * 10^6 kg.
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- The mass enclosed in a sphere of radius 1 AU is 137 million kilograms.
The mass of the Earth alone is 600,000,000,000,000,000 million kilograms. This means that the Universe is a very low density.
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- The Universe is not slowing down detectably and is most likely be speeding up its expansion.
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-------------------------- The Universe is 13,700,000,000 years old.
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- If we observe a galaxy 500 mega parsecs away moving away from us at 35,000 kilometers/second, when does that suggest the Big Bang occurred? This is just one of many of the calculations used.
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-------------------------- Time = Distance / Velocity
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- Time = 500 Mpc * sec * 3.09 * 10^19 km * year / 35,000 km * Mpc* 3.16 * 10^7 sec
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-------------------------- Time = 13,970,000,000 years
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- The density of matter in all forms normalized to the critical density is 30% + or - 10%. 25% is Dark Matter and 5% is ordinary matter.
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- The density of ordinary matter, the sort of matter we are made of, is 4% + or - 1%. This is the type of matter that is the elements in the periodic table from hydrogen to uranium. Most of ordinary matter is the hot gas in and between the clusters of galaxies.
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- We can conclude that we humans are not made of the dominant stuff in the Universe.
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- Einstein showed that mass and energy are the same. Energy = a constant * mass. The constant is the speed of light squared. If the Universe is flat the sum of mass and energy, normalized to the critical density, must be equal to one.
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- Therefore 65% + or - 10% of the Universe must be Dark Energy. This is also called the Cosmological Constant, or sometimes the Vacuum Energy Density. But, since we do not know what it is, let’s call it Dark Energy and it is whatever stuff is causing the Universe to expand at an accelerating rate.
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- The amplitude and slope of the spectrum of primodial bumps and wiggles in the Cosmic Background Radiation = 1 + or - .2, 2 + or - .5 , normalized to the large angular scales found in the Microwave Background Radiation pattern. This pattern grew into the superclusters and voids that accounts for the distribution of galaxies in our Universe.
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- The Hubble Constant is large and the density is small causing the Universe to keep expanding indefinitely. Under this scenario what is the future of our Universe?
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- Eventually binary stars and galaxies clusters will collapse into each other. Stars will gradually burn out and form black holes. On time scales of 10^32 years, protons will decay into radiation. All particles and black holes will eventually evaporate.
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- All the mass in the Universe will disappear. All that will be left is radiation which reddens and weakens forever. Yet, the total energy in the Universe remains unchanged inside this infinite space.
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- Of course, we are basing these conclusions on the 5% of the information of what we know. If we add up Dark Matter 25% and Dark Energy 75% we total 95% which we do not have an explanation for.
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- We and everything we see and know is made up of the remaining 5% of the Observable Universe. No doubt we all have a lot more to learn.
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- May 7, 2019. 50
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--------------------- Tuesday, May 7, 2019 -------------------------
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