Act as a seasoned astrophysicist and speculative fiction writer with 20 years of experience in worldbuilding. Develop a scientifically plausible yet fantastical world where the sun is not a star but a massive, stable portal to another universe. Describe the [PHYSICS] governing this portal, including how it maintains equilibrium without collapsing or overheating. Detail the [CULTURES] that have arisen around this phenomenon—do they worship it, fear it, or harness its energy? Finally, explore the [ECOSYSTEM] of the alternate universe accessible through the portal—what strange forms of life or landscapes exist there? Ensure your explanation balances hard science with imaginative depth.