When it's thought that we can only see a few percent of it anyways?
Are we assuming translational symmetry and assume that it's same everywhere, even places we can't see?
Are we assuming translational symmetry and assume that it's same everywhere, even places we can't see?
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We don't guess. We experimentally observe and try to deduce the mass from observation and from physics. We're getting pretty accurate as time goes on. As for why we want to measure the mass of the universe, well because it's an extremely important value to know. The mass and composition of the universe tells us literally everything about how the universe behaves and evolves as a whole. The composition determines the shape of our universe, the acceleration speed, the ultimate fate of it, how it started, how galaxies came to be, etc. The list goes on and on.
Secondly, we don't need to see everything to deduce how much there is of it. And we don't even need to know quantities, we need to know ratios. As in, what percentage of the universe is dark matter. We don't need to know how much of it there is, just how much of everything is dark matter. That is a much simpler thing to measure.
I wouldn't say we assume translational symmetry. The proper term in astronomy is to say the universe is isotropic and homogenous. Essentially that means that the universe looks the same in every direction and no matter where you are in the universe it will look the same. This is known as the cosmological principle. It comes with a small caveat. You have to consider a large enough region of the universe. Obviously you can't just look at a region the size of a galaxy because the universe can be very different in one place and not so different in another. The length scale that this applies to is about 500 Megaparsecs (1.6*10^9 light years). We have taken measurements of the universe on this scale and we have seen that it looks the same.
Here is a picture of the large scale structure of our universe. It graphs millions and millions of galaxies when looking in two separate directions. Each blue dot is a galaxy. Overall, it is relatively the same. You may notice that it fades the farther out you go, but that is just because it is harder to see galaxies at such distances.
http://www.ifa.hawaii.edu/~barnes/ast110…
Secondly, we don't need to see everything to deduce how much there is of it. And we don't even need to know quantities, we need to know ratios. As in, what percentage of the universe is dark matter. We don't need to know how much of it there is, just how much of everything is dark matter. That is a much simpler thing to measure.
I wouldn't say we assume translational symmetry. The proper term in astronomy is to say the universe is isotropic and homogenous. Essentially that means that the universe looks the same in every direction and no matter where you are in the universe it will look the same. This is known as the cosmological principle. It comes with a small caveat. You have to consider a large enough region of the universe. Obviously you can't just look at a region the size of a galaxy because the universe can be very different in one place and not so different in another. The length scale that this applies to is about 500 Megaparsecs (1.6*10^9 light years). We have taken measurements of the universe on this scale and we have seen that it looks the same.
Here is a picture of the large scale structure of our universe. It graphs millions and millions of galaxies when looking in two separate directions. Each blue dot is a galaxy. Overall, it is relatively the same. You may notice that it fades the farther out you go, but that is just because it is harder to see galaxies at such distances.
http://www.ifa.hawaii.edu/~barnes/ast110…
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1. It's a fun game.
2. That number, at least in part, predicts is future behavior.
2. That number, at least in part, predicts is future behavior.