How can the observable universe shrink in a Big Rip?
I've found the answer to this is actually very interesting and (to me, at least) surprising — it is not just the light cone that shrinks, but the cosmic event horizon as well! To explain this, let us first look at a de Sitter universe. The scale factor $a$ of a de Sitter universe can be expressed as a function of time with:

$$a(t) = e^{kt}$$

where $k$ is some constant, depending on the specifics of our universe (in a de Sitter universe, $k = H$, the Hubble parameter). A universe dominated by phantom energy is actually pretty similar to a de Sitter universe, but the expansion rate gets exponentially faster. Thus, in a Big Rip universe $k$ is time dependent ($\dot{k} &gt; 0$), so the scale factor can be written as:

$$a(t) = e^{k(t)t}$$

Why is this relevant? Well, the comoving distance to the event horizon is actually inversely proportional to $a$ and $k$:

$$d_{eh}(t)\propto\frac{1}{a(t)k(t)}$$

In a de Sitter universe (and our own universe, assuming $w = -1$), $k$ is equal to the Hubble constant, so the event horizon’s distance will remain constant (in a de Sitter universe, the $\dot{H} = 0$ because the deceleration parameter $q=-1$). In a Rip universe, $k$ is time dependent; as $k$ increases, the distance to the horizon decreases.

From this formula, we can see that the event horizon’s distance is inversely proportional to the scale factor; thus, if $a(t)$ reaches infinity, the event horizon’s distance will reach zero. This means that the Big Rip can be defined as either the scale factor reaching infinity, or the observable universe’s diameter reaching zero — these two events happen at the same time.