As far as I know, the expansion of the universe only has a correlation to distance. If something 13 billion years away is receding at 80% the speed of light, the "Big Rip" implies that the hubble volume would shrink and after a million years, something that's only 13 billion years away would then recede at 90% the speed of light instead, whereas normal space expansion would say that it doesn't matter how much time passes, something 13 billion light years away is always going to appear to be receding at 80% the speed of light. So why wouldn't the expansion have a constant acceleration in proportion to distance and simply lead to a big chill? Why does anyone think there's a big rip?

Edited April 30, 201411 yr by SamBridge

This depends on the equation of state for the dark energy (typically parametrized by w, the ratio between its pressure and energy density). If the dark energy is simply a cosmological constant the equation of state will be w = -1 and we would simply have a constant Hubble parameter. However, we do not know the equation of state for the dark energy and there are models which could accommodate w < -1, leading to a Big Rip where the Hubble parameter increases and eventually rips the Universe apart. With today's experimental observations of the acceleration of the expansion, we do not know whether or not w < -1, w = -1, or w > -1.

The big rip has largely been discounted, The simple reason is that the energy-density of the cosmolical constant per m³ is miniscule compared to the strong force and gravity. Its easily overpowered. As the cosmological constant is in fact constant, it will remain easily overpowered. Instead were heading towards heat death (big chill)