# Is cosmological constant really constant?

As the Universe expands, the dark energy in it also increases. I heard that the cosmological constant $\Lambda$ represents dark energy, so that constant must change as time passes, right? Correct me if I'm wrong...

• The cosmological constant is an energy density, so it could be constant, even if the total amount of dark energy and the total size of the universe were increasing. Is it constant? Probably not. What causes it/dark energy and how does it/dark energy change in time? We don't know. – CuriousOne Jun 18 '16 at 6:30

It is important to be clear that dark energy does not necessarily respond to the expansion in the way matter does. We describe the expansion using a scale factor $a(t)$, where we take the value of $a$ to be one right now. So $a=2$ means the universe has doubled in size. The average density of matter is given by:
$$\rho_m(t) = \frac{\rho_0}{a^3(t)}$$
where $\rho_0$ is the current average density. This should make sense - if the universe expands by a factor of $2$ then the volume of any region within it increases by a factor of $2^3 = 8$ and the density of matter falls by a factor of eight.