Why Isn’t Our Universe Perfectly Smooth?

When we talk about the smoothness of the Universe, we’re talking about how different one place is from another regarding “amount of stuff,” or density. We call this property homogeneity, or “sameness” in different locations. In fact, our Solar System is incredibly inhomogeneous. The densest places have something like 10³⁰ times as much stuff as the least dense places. But when we talk about the Universe as a whole, we need to look at scales significantly larger than just our paltry Solar System. You might imagine that if you look on much larger scales, maybe the Universe is significantly smoother. And it turns out that, on these scales (a million light years or so), the Universe is still very inhomogeneous. If you took a bubble a million light years in diameter and put it around a very dense region (like a large galaxy), and compared it with an identical bubble placed around the emptiness of intergalactic space, the densities you’d find would indeed be very different. But they’re only different by about a factor of a million. That’s right. Only a million. When you consider that looking at an Earth-sized bubble gave a factor of 10³⁰ difference, a million (10⁶) doesn’t seem so big, does it? So what if we start looking at very, very large scales? Credit: ScienceBlogs.

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https://www.forbes.com/sites/startswithabang/2018/03/15/why-isnt-our-universe-perfectly-smooth/#1ec899d47bc6