<p>I'm asked first to calculate the critical energy density $\rho_{crit}$ of the universe given certain constants. The value I got was $5500\,\mathrm{MeV}\,\mathrm{m}^{-3}$.</p>

<p>Then I'm asked to calculate various number densities of particles assuming the universe is made up of a single type. To do this, I'm told I need to set the universe's matter density equal to $\rho_{crit}$.</p>

<p>I'm not sure how to do this since they are in different units. My expression to find $\rho_{crit}$ had a $c^2$ in it, and if I take that out I would have units of matter density. Is that what I'm supposed to do?</p>

<cosmology><density>
Equating critical energy density to matter density
Spuds
# Equating critical energy density to matter density

(Asked by: username_0 on 2017-02-17)

I'm asked first to calculate the critical energy density $\rho\_{crit}$ of the universe given certain constants. The value I got was $5500\,\mathrm{MeV}\,\mathrm{m}^{-3}$.

Then I'm asked to calculate various number densities of particles assuming the universe is made up of a single type. To do this, I'm told I need to set the universe's matter density equal to $\rho\_{crit}$.

I'm not sure how to do this since they are in different units. My expression to find $\rho\_{crit}$ had a $c^2$ in it, and if I take that out I would have units of matter density. Is that what I'm supposed to do?

**Tags:** cosmology, density

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## 0 answers

