18 - Starmans Quest by Robert Silverberg

In a universe where time bends and space stretches, twin brothers embark on an extraordinary journey. One twin, aboard a starship traveling at near-light speeds, experiences time differently than his brother on Earth. As years pass and their lives diverge, the starship twin makes the bold decision to leave the safety of the ship and venture back to Earth, determined to uncover the mystery of his brothers accelerated aging. What he discovers will challenge everything he knows about time, love, and the bond between siblings.