Sidebar: Pancake or Oatmeal Universe – What's for Breakfast?

The articles about the comic microwave background (CMB) in this edition of Cosmic Times talk about it being lumpy and being smooth, so this sidebar aims to clarify the different definitions as they apply to cosmology.

The first thing to remember is that when astronomers talk about structure in the CMB, they are really talking about structure in the Universe shortly after the Big Bang. By most definitions, the CMB is quite uniform (or smooth). In fact, the level of uniformity caused some problems for cosmologists, which we discuss in the article on Inflation Theory, "Inflation in the Universe." The anisotropies, or "lumps", that astronomers found in the CMB were tiny. The magnitude of the anisotropies was just 1 part in 100,000, which in length scales would be is like the width of a human hair compared to a full-sized school bus.

Most of the matter that we see in the Universe around us now is in the form of "clumps": planets, stars, nebula, star clusters, galaxies, and clusters of galaxies. This is a huge difference from the early Universe as seen in the CMB. It is important for students to understand these distinctions.