- 3142 - UNIVERSE - is it also rotating? This Review tackles the question. Einstein’s theory of general relativity is our only validated theory of the universe as a whole. Without it we would have difficulty explaining where the universe came from and where it is going.
- ----------------------- 3142 - UNIVERSE - is it also rotating?
- There is an earlier Review 3101 - “UNIVERSE - Is it spinning, or, is it just me?” An initial spinning Universe could cause parity-violating asymmetry where gravity is allowing matter to dominate over anti-matter. Bold theories still need a preponderance of evidence. A spinning Universe is a new idea. What does it all mean?
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- One of the earliest predictions of Einstein’s theory was the universe is expanding. The universe expands everywhere all at once like the surface of a balloon being blown up. We can see it expanding by looking at distant galaxies.
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- Based on what we know about the composition of those galaxies, we have a good idea of what wavelengths of light they should be producing. But what we actually see is that those wavelengths are extended. Thus, waves in the visible spectrum that the human eye can see are shifted towards the red.
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- This effect, called the “Doppler effect“, is familiar from the sound of a police or ambulance siren as it passes. When it is approaching, the sound is a higher pitch as the wavelengths are compressed. As it moves away the sound is lower pitched because the wavelengths are elongated.
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- The expansion of the universe means that it is “curved” in the time dimension because the space dimensions are all getting bigger while the time dimension, as far as we can measure with clocks is not expanding. Therefore, when we measure how the universe’s expansion changes over time, we are measuring the universe’s “time curvature“.
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- There are a number of features we can measure about the universe. We can look at its oldest light to see how correlated the universe was. The oldest light in the universe, called the “Cosmic Microwave Background“, comes from a time when the whole universe filled with light when the universe was about 300,000 years old.
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- Because the CMB filled the whole universe, we continue to see it today even as the size of the observable universe grows larger. When we correlate different parts of the CMB, we are looking for signs of statistical correlation. Many patches of the CMB resemble other patches and so we compare them.
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- This is like looking at a big TV screen full of static and trying to understand how likely it is that one patch of static resembles another. If the similarity is unlikely to be purely random, then we say that they are correlated.
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- The existence of correlations across the CMB is called the “horizon problem” because we don’t understand why they are so closely correlated even when they are in completely different parts of the night sky, billions of lightyears apart. Light has not had enough time to reach the farthest parts of the visible universe.
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- We seem to be able to measure so much about the universe it seems like we should be able to tell if the universe is rotating and indeed we can, and what we see is that it is not rotating at all.
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- In this sense, the universe expanding is not like clay spinning on a potter’s wheel, but more like the balloon blowing up. Not only is it not rotating (or if it is very slowly), but some believe that it cannot rotate even though Einstein’s theory would allow it. The reason why goes deep into how we understand space and time.
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- The origin of the question about a rotating universe goes all the way back to Sir Isaac Newton. Newton imagined spinning a bucket of water around, as if it had been placed on our potter’s wheel. As the bucket spins, the water is pushed to the sides making a concave shape in the top of the water.
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- Newton imagined what it would be like for an ant sitting on top of the bucket and spinning with it. From the ant’s perspective, the universe around it, not the bucket, would be spinning. This would be obvious from seeing all the stars and other things whirling around.
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- But suppose that none of those things were there. Suppose the ant and bucket were spinning in an empty universe? Newton surmised that the concave top of the water would tell them that they must be spinning anyway. Thus, he concluded the universe has a preferred state of motion.
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- Fast forward 200 years or so to the end of the 19th and beginning of the 20th centuries. The problem of what to do with Newton’s bucket very much bothered two men. The first was a physicist named Ernst Mach from whom we get our unit of the speed of sound. He believed that the universe could surely have no such preferred state of motion. Rather, he surmised all the matter in the universe creates a preferred state by its relationship with one another.
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- In other words, all those stars whizzing around must somehow reach out and make the surface of the water concave through a mysterious force that they collectively exert on the cosmos. Thus, in an empty universe, the bucket could never rotate or have the telltale concave surface because it would be the sole measure of that universe’s state of motion.
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- While Mach’s principle was appreciated in its day, nobody could quite figure out how to encode it in the mathematical language of physics. Indeed, the idea that the stars could somehow instantaneously reach out and affect a bucket of water here on Earth didn’t make a lot of sense.
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- Then again, Newton’s theory of gravity had the exact same problem. It too proposed that gravity worked instantaneously. This action at a distance had bothered physicists for centuries, but Newton’s laws never seemed to fail, except when they did with the orbit of the planet Mercury.
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- Mercury’s orbit, we knew, was elliptical and like all orbits, it had a perihelion, a place where it was closest to its star, the Sun. Like all orbits, this perihelion moved around in its orbit because of the tug of other planets in the Solar system.
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- Unfortunately, the change in Mercury’s perihelion was just a little bit off and nobody knew why. Some suspected that Newton’s laws weren’t quite right and needed fixing.
Einstein too had a problem with Newton’s bucket experiment, and originally he was a follower of Mach’s idea. He even managed to create a theory of gravity, which he later rejected, that seemed to include a weak additional force that might be the one that Mach proposed.
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- Einstein later published his theory of general relativity at age 36, having worked out all the details of curved space time and formulating them in the mathematics of differential geometry, the theory of geometry on curved shapes.
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- His theory did several things: it explained the anomaly in the orbit of Mercury, it did away with Newton’s action at a distance and replaced it with a force that moves at the speed of light like electromagnetism, and it described gravity as the curvature of time and space. In doing so, Einstein believed that he had actually succeeded in proving Mach’s principle and disproving Newton’s assertion.
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- Mach disagreed, believing that Einstein had invented something wholly different from what he had intended, and Einstein gradually came to realize that what he had discovered was different than what he thought it was though he probably never understood how different.
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- Einstein disavowed Mach’s principle by the end of his life, saying that it didn’t make sense in the context of relativistic theory. After all, Mach’s idea was about matter influencing each other directly. Einstein’s theory was about matter influencing time and space and time and space influencing matter back.
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- Einstein’s theory solved Newton’s bucket problem in a very ingenious way. All it said is that all the matter in the universe essentially aligns space with itself. Thus, space naturally has a preferred state of rotation because of the matter in it.
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- When you spin a bucket, you are spinning in space that all the matter in the universe has worked together to create. Therefore, the concave surface of the water is the result of spinning in a space that has a different state of motion than the water.
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- However, that Einstein’s theory does not obey Mach’s principle. The reason is because Mach’s ideas are only about space. In Mach’s universe, matter influences everything simultaneously, ensuring that the global, average state of motion of matter is always the “reference” state. Indeed, in Mach’s universe space doesn’t even exist without matter. It has no meaning at all.
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- Einstein’s universe treats space and time as real entities that exist separate from matter. Space and time come together to create a four dimensional shape and you can have universes in Einstein’s theory that are completely devoid of matter, empty, yet somehow still there.
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- Because of the relationship between space and time, space can be twisted as if the universe were rotating. This is true even when there is no “center” of rotation, but rather all matter in this universe have the property of rotation in a particular direction.
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- Einstein’s friend Kurt Gödel discovered in Einstein’s equations one such universe in 1949. In it, galaxies are modeled as particles that twirl around one another in a spiral dance through time. In this universe, light beams follow spiral paths and you can even go back in time.
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- Almost everything in the universe that is gravitationally bound but not stuck together rotates in some way. The Earth spins and orbits the Sun. The Sun spins. The Sun orbits the galactic center. The galaxy itself rotates as do billions of other galaxies. If this property of rotation is so common, might the universe itself have some such property of its own rotation?
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- Stephen Hawking conjectured and it was later proved that no matter exists that can bend space and time to allow us to travel back in time. That means that the Gödel universe may be impossible. If that’s impossible, maybe rotating universes and empty universes are impossible too and Einstein’s theory is too broad for reality. Time will tell:
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- 3135 - UNIVERSE - what is it expanding in to? - If the Universe is infinite, how can infinity be expanding? Astronomers believe the Universe is finite and that eventually it comes back on itself. To explain this we have to decide the shape of the Universe.
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- 3106 - UNIVERSE - The Universe you live in? Space is completely silent. Sound needs an atmosphere to travel through, and since space has no atmosphere, it has no sound. The biggest, most awe-inspiring exploding star wouldn’t even make a peep. Astronauts are able to communicate up there thanks to radio waves, which can travel through space.
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--------------- 3106 includes an index of many more reviews on this topic:
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- April 29, 2021 UNIVERSE - is it also rotating? 3142
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--------------------- --- Saturday, May 1, 2021 ---------------------------
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