Three Cosmic Journeys

If we can reach another planet by traveling through space, then Earth and that planet are in the same universe. Thus, we cannot enter another universe by traveling through space. However, in:

Tau Zero by Poul Anderson

The Triumph Of Time/A Clash Of Cymbals by James Blish

Into Deepest Space by Fred Hoyle & Geoffrey Hoyle -

- intergalactic travel does become intercosmic.

Anderson's characters are in flight long enough to survive this universe;

Blish's characters fly to a place where they can survive a cosmic collision and create new universes;

the Hoyles' characters fly to a quasar where they are diverted through a black hole-white hole system into the inverse universe.

How can anyone remain alive long enough to make an intergalactic journey?

Blish's characters use an FTL (faster than light) drive and anti-agathics;
the Hoyles' characters use time dilation and induced hibernation;
Anderson's characters use extreme time dilation.

FTL would make an early return journey possible although that is not relevant in this case. However, it does enable Blish's characters to reach their destination in time for their purpose. Anderson and the Hoyles describe the relativistic effects of slower than light travel.