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--------------------- 2553 - UNIVERSE - why is it homogeneous?
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- Astronomers “ assume” the Universe is homogeneous. From the dictionary something homogeneous is composed of parts or elements that are all the same, having a common property throughout. The Universe is the same in every direction and everywhere we are in it.
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- This further assumes that the laws of physics are the same everywhere as well. Astronomers call this assumption the “Cosmological Principle“.
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- All the observations and calculations that astronomers make must make use of this principle. We do not know anything different and that is the only way the math works out. The age of the Universe, the density of the Universe, the size of the Observable Universe are calculations that all assume the Universe is homogeneous.
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- Astronomers also assume the Universe is isotropic, that is everything looks the same in every direction you look. If you look to the eastern night sky and most of the galaxies are spinning counter-clockwise, and, if you look to the western night sky the same is true, most of the galaxies are spinning counter-clockwise. The mix of spinning galaxies is about the same regardless of which direction you look.
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- Obviously on small scales our smaller pieces of the Universe are neither homogeneous or isotropic. Earth is not, the Solar System is not, the Milky Way Galaxy is not, the Local Group of Galaxies is not. At what scale of dimension does the Universe agree with the Cosmological Principle?
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- The answer is that the size needed is almost the size of the Observable Universe. When we observe the Cosmic Microwave Background radiation for the entire Universe it does appear to be smooth and be the same in all directions up to 1 part in 10,000 of measurement.
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- Astronomers have to make another assumption to explain this smoothness, or homogeneity. the CMB is a consistent temperature of 2.73 Kelvin. Looking east to the galaxies 10 billion light years away that is the temperature of the background, 2.73 Kelvin. Looking west to galaxies 10 billion lightyears away in the opposite direction the temperature is the same, 2.73C. These two regions are 20 billion lightyears apart.
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- The Universe is only 13.7 billion years old. There is no way light or any radiation could have reached both locations by now. How could they be the same temperature without ever being in contact? These two regions must have been in contact sometime in the past for thermal equilibrium to have occurred.
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- So, astronomers have come up with another assumption. That “Inflation” occurred shortly after the Big Bang. All the regions were in contact in the beginning, thermal equilibrium did occur , then the space expanded at faster than the speed of light for a short period of time before returning to its present rate of expansion.
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- If you take a box of space and measure the density of the mass inside the box obviously the density is going to depend where in space you make the box. In the center of the Earth, in the interstellar space, in orbit around the Moon, or the center of a Blackhole.
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- How big does the box need to be before we get the same density regardless of where in the Universe we put the box. The answer is greater than a billion light years on a side. Our universe is like Swiss cheese with voids and bubbles between the webs of galaxies. However, with a big enough slice the average density of the cheese is the same, it becomes homogeneous.
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- So, we have been talking about homogeneity over space. What about homogeneity over time?
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- The Swiss cheese structure of the Universe is caused by gravity. Very small lumps of matter and regions of rarified density were amplified by gravity over time to create the structure we have today. Starting with quantum fluctuations shortly after the Big Bang the math of gravity gives us the Swiss cheese structure we have today. We assume the laws of physics have not changed over that time.
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- Surprisingly, we can test this assumption that the laws of physics have not changed over time. Light photons emitted by atoms of a particular element have a particular energy equal to the energy gap that the atom’s electron jumped when it went to a lower energy level and emitted that photon. What astronomers have found is that no matter how far back in time or where you go in the Universe to find these same atoms the energy levels and the speed of light are consistently the same.
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- The math works which makes us think the laws of physics are the same anywhere in space and any where in time, at least up the first few seconds after the Big Bang. However, Isaac Newton’s math worked good enough to put a man on the Moon and bring him home safely.
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- But, we now know that his math was not accurate enough for high velocities approaching that of light or in the vicinity of enormous mass / gravity. Einstein’s Theory of Relativity offers math that is more accurate and seems to work even at these extreme conditions.
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- Maybe there are some other theories that has math we do not know about yet that could explain Inflation and Dark Energy. Inflation theory would not be necessary if the speed of light were not constant. If , for some reason, the speed of light were faster in the early universe maybe that could be the correct assumption for how the Universe could be homogeneous.
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- Dark Energy is used these days to explain the accelerating expansion of the Universe that we observe. Dark Energy is assumed to be an anti-gravity created in the vacuum of space.
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- Another explanation might be that our Observable Universe is in the center of a humungous hole in the Swiss cheese structure. If we were in the center of a giant void and there was a massive structure and gravity surrounding us that we have not detected then that could explain the accelerating expansion of space that we observe.
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- Still another explanation could be that time itself is slowing down only making us think that expansion is speeding up. If time were really slowing down in the Universe and we were not aware of it. When we calculate speed in space distance per unit time, that is, meters per second, it would appear to us that we were gradually moving faster.
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- So, is the Universe expanding in space or is the Universe slowing in Time? How could we tell?
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- If time were gradually slowing down it might take billions of years of time passage to see the change. We would simply measure everything as getting faster and faster until everything reaches a snapshot and is frozen into one instant in Time.
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- Could this time-slowing be a realistic explanation for Dark Energy? We do not know. See Review 1006 “ Is Time Slowing Down?” for more analysis on this assumption. It gets even weirder.
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- December 21, 2019 2553 1005
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--------------------- Sunday, December 22, 2019 --------------------
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