I'm just looking for conceptual clarification re: the relationship between matter density and the Hubble parameter in the Friedmann equation. Just for quick reference, the equation I'm looking at is H2 = 8πGρ/3 - ka-2 (I'm working through Liddle's Intro text, and for now we're ignoring the cosmological constant.) My question is pretty broad and basic. Looking at this equation, it seems that the Universe will expand at a faster rate if the density ρ of matter within it increases. Why would that be? Isn't it true that more matter density would generate more gravitation? And doesn't that slow the expansion down, rather than speeding it up? In Liddle's book, he writes in a couple places that this model essentially works just like a gas-and-piston system. Is that to say that the Hubble parameter increasing with density is analogous to how a volume of gas might expand more quickly if we could somehow inject more gas particles into it, because more particles would be colliding with the piston and pushing it out? But for the gas-piston system, there's no gravity force between the particles that might bring them closer together and contract them. I'm just not clear as to how it can be that more matter density would lead to faster expansion. But that seems to be what the equation says. Thanks for your help!