Sunday, March 26, 2023

3930 - UNIVERSE - how big is it, really?

 

-   3930 -   UNIVERSE  -  how big is it, really?  That may be something that we actually will never know.  The size of the universe is one of the fundamental questions of astrophysics. It also might be impossible to answer.

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------------  3930  -  UNIVERSE  -  how big is it, really?

-    Just look out at the night sky; the glimmers you see are really pictures of the distant past. That's because those stars, planets and galaxies are so far away that the light from even the closest ones can take tens of thousands of years to reach Earth.

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-    The closer an object is in the universe, the easier its distance is to measure. The sun?  The moon?  All scientists have to do is shine a beam of light upward and measure the amount of time it takes for that beam to bounce off the moon's surface and back down to Earth.

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-   But for the most distant objects in our galaxy reaching them would take a very strong beam of light. You would have to wait thousands of years to wait around for the beam to bounce off the universe's distant exoplanets and return back to us?

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-    Stars change color as they age, and based on that color, scientists can estimate how much energy, and light, those stars give off. Two stars that have the same energy and brightness aren't going to appear the same from Earth if one of those stars is much farther away. The farther one will naturally appear dimmer. Scientists can compare a star's actual brightness with what we see from Earth and use that difference to calculate how far away the star is.

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-   What about the absolute edge of the universe? How do scientists calculate distances to objects that far away?   The farther an object is from Earth, the longer the light from that object takes to reach us. Imagine that some of those objects are so far away that their light has taken millions or even billions of years to reach us.

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-   Now, imagine that some objects' light takes so long to make that journey that in all the billions of years of the universe, it still hasn't reached Earth. That's exactly the problem that astronomers face.  We can only see a tiny, little “bubble” of the universe. And what's outside of that? We don't really know.

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-  By calculating the size of that little bubble, scientists can estimate what's outside of it.   Scientists know that the universe is 13.8 billion years old, give or take a few hundred million years. That means that an object whose light has taken 13.8 billion years to reach us should be the very farthest object we can see.

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-   You might be tempted to think that gives us an easy answer for the size of the universe: 13.8 billion light-years. But keep in mind that the universe is also continuously expanding at an increasing rate. In the amount of time that light has taken to reach us, the edge of the bubble has moved out.

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-   Scientists know just how far it's moved: 46.5 billion light-years away, based on calculations of universe’s expansion since the big bang.   Some scientists have used that number to try and calculate what lies beyond the limit of what we can see.

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-   Based on the assumption that the universe has a curved shape, astronomers can look at the patterns we see in the observable universe and use models to estimate how much farther the rest of the universe extends. One study found that the actual universe could be at least 250 times the size of the 46.5 billion light-years we can actually see.

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-    There's no saying for sure whether the universe is finite or infinite. Because the universe is expanding at an increasing rate, the outer edges of our observable universe are actually moving outward faster than the speed of light. That means that our universe's edges are moving away from us faster than their light can reach us.

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-   The universe may be flat, but could still be shaped like a doughnut, weird patterns in leftover light from the Big Bang suggest.   All observations so far suggest the universe is flat. In geometry, "flatness" refers to the behavior of parallel lines as they go out to infinity. Think of a tabletop: Lines that start out parallel will remain that way as they extend along the table length.

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-   In contrast, look at Earth. Lines of longitude begin perfectly parallel to each other at the equator but  eventually converge at the poles. The fact that parallel lines initially intersect reveals that Earth is not flat.

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-   The same logic applies to the 3D universe. The cosmic microwave background (CMB) which is light released when the cosmos was only 380,000 years old, now sits over 42 billion light-years away and features tiny fluctuations in temperature across the sky.

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-   Astronomers have calculated the predicted size of those fluctuations compared with observations. If their measured size differs from predictions, that means those rays of light, which started out parallel, changed directions over space-time, indicating that the geometry of the universe is curved.

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-   But those same measurements have revealed that, ignoring small-scale deflections from galaxies and black holes, the overall geometry of the universe is “flat”.  But, there's more than one kind of flat.

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-   Draw parallel lines on a piece of paper. Then wrap one end of the paper to connect with the other, forming a cylinder. The lines remain parallel as they circle the cylinder. In the language of mathematics, any cylinder is geometrically flat but is said to have a different  ”topology”. Close up both sides of the paper, and you make a “torus”, or doughnut shape.

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-    To get another example of a weirdly flat shape, wrap a thin strip of paper in a circle, but make a 180-degree twist in one end. The end result is a “Möbius strip”, which is still geometrically flat, because parallel lines stay parallel, even when they flip over each other.

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-  Mathematicians have discovered 18 possible “geometrically flat”, 3D topologies. In each one, at least one dimension wraps up on itself, and sometimes, they flip over like a Möbius strip or make partial rotations.

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-    In such a twisty universe, if we looked far away, we would see a (maybe upside-down) copy of ourselves from a much younger age.  If the universe were 1 billion light-years across, astronomers would see a version of the Milky Way galaxy as it was 1 billion years ago and, behind that, another copy from 2 billion years ago, and so on.

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-    If the universe were a giant doughnut, astronomers could look in two directions to see such copies.  Astronomers have measured the topology of the universe in multiple ways, from looking for duplicates of patterns of galaxies to matching circles in the CMB. All evidence suggests the universe is both “geometrically flat” and has a simple “unwrapped topology”.

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-     Observations have assumed that the universe wraps around itself in only one dimension and does not have a more complicated topology. Also, observations of the CMB have revealed some strange, unexplained anomalies, like large patterns appearing where they shouldn't.

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-    A universe with a complicated topology could explain at least some of the anomalies in the CMB.   There may be a mirror image of us somewhere in our twisty universe.  Your twisted comprehension of this situation may be comtirbuting to this confusion.

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                   March 25, 2023           UNIVERSE  -  how big is it, really?                 3930                                                                                                                          

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