Critical Density: What Is It and Where Does It Come From?

[LSulayman]

What exactly does the critical density mean?
In articles they keep comparing the desity of the universe to the critical density and what the consequences are but nobody explains what exactly the critical density is and where it comes from.

 

[Nabeshin]

The critical density is the density required to have a spatially FLAT universe. It comes from... well, requiring that the universe be spatially flat!

Does that answer your question?

 

[LSulayman]

Yes it does! A rather simple answer, but very logical actually.

 

[Chalnoth]

I think I'd also like to point out that the critical density depends upon the rate of expansion. A larger rate of expansion requires a greater density for spatial flatness. Specifically:

$$\rho_c = {3 H_0^2 \over 8\pi G}$$

 

[sardar]

The critical density refers to the amount of matter and energy required for the universe to have a flat geometry. In other words, if the mass and energy in the universe were evenly distributed, the universe would have a flat, balanced shape. This is important because the overall geometry of the universe has a significant impact on its expansion and evolution.

The concept of critical density comes from the theory of general relativity, which describes the relationship between matter, energy, and the geometry of space-time. According to this theory, the distribution of matter and energy in the universe determines the curvature of space-time, which in turn affects the expansion of the universe.

The critical density is calculated by considering the amount of matter and energy in the universe, as well as the expansion rate of the universe. It is often expressed in terms of the critical density parameter, which compares the actual density of the universe to the critical density.

In cosmology, the comparison of the actual density to the critical density has important implications for the fate of the universe. If the actual density is less than the critical density, the universe will continue to expand forever. If the actual density is greater than the critical density, the universe will eventually stop expanding and collapse in on itself in a "big crunch." And if the actual density is exactly equal to the critical density, the universe will reach a state of equilibrium and stop expanding.

In summary, the critical density represents a crucial factor in understanding the structure and evolution of the universe. By comparing the actual density of the universe to the critical density, scientists can make predictions about the fate of the universe and gain insight into its underlying principles.