E5 Galaxies and Cosmology

Topic

Modern cosmology describes a universe that is flat, homogeneous, and isotropic on a large scale—a universe that is also expanding at an accelerating rate.

The Friedmann equation, derived from general relativity, governs the evolution over time of the universe's scale factor, based on its matter and energy density, its curvature, and the cosmological constant that also characterizes it.

Within that same equation, the universe's critical density determines its possible cosmic fates: if the actual density were to exceed this critical threshold, the universe would eventually collapse back in on itself; conversely, if it fell below that threshold, it would expand forever. The scenario actually observed is virtually identical to that critical threshold—a flat universe—yet a dominant fraction of that density consists of dark energy; it is precisely this dark energy that causes the universe's expansion to accelerate over time rather than slow down.

The observable universe is approximately 13.8 billion years old, has a radius of tens of billions of light-years, and contains—within that entire volume—on the order of two trillion galaxies. Thus, from the flat, homogeneous universe with which this text begins to the two trillion galaxies with which it concludes—taking in the Friedmann equation and the critical density that determines whether the outcome is collapse or eternal expansion—modern cosmology reveals that the entire fate of the universe is written in the simple comparison between its actual density and that single critical threshold.

ID:3909