here is a frustrating problem http://www.astro.ucla.edu/~wright/cosmo_03.htm Scroll down to the "Space-Time Diagrams" section and you will see a pear-shape lightcone outlined in red. This is the lightcone in the spatially "flat" case which we believe applies to our universe-----the Ω = 1 case---- the critical density case-----whatever. But this case drawn here is for zero cosmological constant---zero Lambda----zero vacuum energy density. Up to the present this makes hardly any difference because the dark energy has not caused very much acceleration in expansion YET. But in the future that lightcone will no longer look like a pear. BTW each of those worldlines is another galaxy, and they have little triangles on them to show their future lightcones. The side of the pear always matches the slope of the future lightcone of each galaxy it meets. This determines its shape. Suppose the worldlines of the other galaxies started bending out to show accelerated expansion. What new shape would the pear acquire? This chapter in Wright, and the preceding one, is suggesting ways to think visually about spacetime.