![]() While the spatial size of the entire universe is unknown, it is possible to measure the size of the observable universe, which is approximately 93 billion light-years in diameter at the present day. According to this theory, space and time emerged together 13.787 ☐.020 billion years ago, and the universe has been expanding ever since the Big Bang. ![]() The Big Bang theory is the prevailing cosmological description of the development of the universe. The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. Observable universe: 8.8 ×10 26 m (28.5 G pc or 93 G ly) We call this the cosmological principle.The Hubble Ultra-Deep Field image shows some of the most remote galaxies visible to present technology (diagonal is ~1/10 apparent Moon diameter) Therefore, we will assume that our local patch is not special in any way because we have no other option (or at least no option other than giving up). ![]() If the universe isn't homogeneous, though, then we can't make any claims about the matter distribution of the universe as a whole since the properties of the universe that we observe would just be due to the specialness of our own local patch. To be clear, while we can safely say that the universe appears to be approximately isotropic from our point of view, we can't necessarily say that it is homogeneous because we can't observe the universe from other locations. (Note that we are defining homogeneity different from how homogeneous mixtures are defined in chemistry, where mixtures can be homogeneous even if they are in a container.) Therefore anything with edges technically is not homogeneous. If a system of particles has edges, then you would see something different if you stood on one of the edges versus standing somewhere in the middle. ![]() The pool cue ball is homogeneous but not isotropic because if you were standing on the surface and looked up versus looked down, you would see very different things.Ĭan a system of particles be homogeneous if it has edges? Answer The Gobstopper is isotropic but not homogeneous because it does not look the same if you hopped to a location anywhere other than the center. If you were to miniaturize yourself and stand on the surface, the surface would look the same to you no matter where you stood. (Image credit: Wikipedia)Īn example of something that is homogeneous is the surface of a pool cue ball. \): A Gobstopper is a hard candy that consists of spherical layers.
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