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1. I don't think there's much of a consequence of this particular "structure" for our understanding of the Universe. The theory that describes large-scale struc
by DiogenesKynikos 6y ago
1. I don't think there's much of a consequence of this particular "structure" for our understanding of the Universe. The theory that describes large-scale structure in the Universe is fairly well understood, and discovering this particular structure doesn't change the theory, as far as I can tell.
2. What happens at very large scales is actually quite simple, from a physical and mathematical point of view. That's because as you zoom out, the Universe eventually becomes smooth and homogeneous. You can model it as a constant-density fluid with slight density perturbations. General Relativity then gives you a set of equations to work with, which describe both how spacetime evolves and how the density perturbations grow over time. Things only get complicated once the perturbations grow large enough that the equations become non-linear. This happens on the scale of galaxies and galaxy clusters, but on the largest scales, things are still linear. Then, you can use numerical simulations to understand the formation of galaxies and galaxy clusters (these simulations do a really good job of reproducing what we see in the real Universe), or you can use heuristic arguments about how over-densities collapse (there are some heuristic arguments that get you pretty close to what the numerical simulations predict), or you can use perturbation theory to try to analyze the non-linear regime (this involves some pretty hard-core mathematical analysis, and it doesn't get you as accurate an answer as the numerical simulations, but it might help you understand what's going on).
TL;DR: The title of this article is very click-baity. A lot of people will read it and think that this discovery overturns what we previously knew about cosmology. It doesn't, as far as I can tell. Cosmology predicts that structures like this should exist. Since the initial conditions of the Universe were random, cosmology doesn't predict where we'll find these structures.
1. A visualization of the formation of a galaxy cluster, from a numerical simulation called Illustris: The Next Generation: https://youtu.be/Rg12ub0PbeA https://youtu.be/Rg12ub0PbeA
- deanCommie 6y agoThanks for that explanation, I have a dumb follow up. How can we possibly measure this at the scales you're talking about? I understand we can take a look at the scale of let's say background radiation across multiple galaxies, but how can we really infer anything about the universe at all other than to know that this is how things work at this exact moment in human-scale time?
- DiogenesKynikos 6y agoThe scales I'm talking about - above which one can consider the Universe to be smooth - are a few hundred Megaparsecs (at the present time - they were smaller earlier on). For scale, the Milky Way is several kiloparsecs across, and the Andromeda galaxy is a little less than one Megaparsec away. These scales are readily measurable. Large surveys of galaxies observe galaxies that are Gigaparsecs apart from one another. You can trace out the structure of the observable Universe by looking at where the galaxies are, and there's clumping on "small" scales. But above a certain scale, it becomes smooth. The Cosmic Microwave Background (CMB) is also much larger than this scale. If you measure the CMB temperature, you don't see any correlations in the temperatures of points on the sky separated by more than about one degree. Take a look at this figure, which compares real observations to the Millennium Simulation: [1]. Each point is a galaxy. As you go farther away from the center of the figure, you're going back in time, which is why there are fewer galaxies and less clumping (gravity has had less time to do its work). If you want to see a large slice of the Universe fixed at a certain time (so there's no look-back effect), you have to go to simulations. Here's what you get: [2]. Above a few hundred Megaparcecs (Mpc), there's no more structure. 1. https://www.mdpi.com/galaxies/galaxies-07-00081/article_deploy/html/images/galaxies-07-00081-g004.png https://www.mdpi.com/galaxies/galaxies-07-00081/article_depl... 2. https://wwwmpa.mpa-garching.mpg.de/galform/virgo/millennium/seqB_063a_half.jpg https://wwwmpa.mpa-garching.mpg.de/galform/virgo/millennium/...