According to the Colonial Marines technical manual (p. 144), the technology they used involved "tachyon shunting" (e.g., for the Narcissus, a Romberg 100C tachyon shunt). I think tachyons are particles that can move faster than the speed of light without speeding up time. That probably means you move faster than the speed of light but also start aging faster.
This might explain why in the second movie the crew and personnel had to undergo cryosleep even though the journey would take only three weeks.