Final answer:
Ragnarok in Norse mythology predicts a post-apocalyptic rebirth of the world, which is conceptually similar to modern speculations of Earth's distant future. If the universe continually expands, radiation would weaken, galaxies would drift apart, and life's survival would become precarious due to energy scarcity. The Sun's transformation into a red giant will ultimately engulf Earth, making it uninhabitable.
Step-by-step explanation:
The question regarding Ragnarok and what happens to mankind, as well as the type of world that will emerge after Surt's fires burn out, delves into Norse mythology. In the context of the subject of Ragnarok, it is foretold that the world will experience a series of catastrophic events leading to a great battle, resulting in the death of many gods, heroes, and creatures. Following this apocalypse, it is believed that the world will resurface anew and fertile, repopulated by two human survivors. This mythological end-of-the-world scenario can be likened to more modern discussions of apocalyptic events, such as those described by poems like "Fire and Ice" by Robert Frost or scientific speculations about the long-term fate of Earth and the universe.
Regarding the universe's distant future, if it expands forever, the radiation from the primeval fireball would presumably thin out and lose energy over time. The future evolution of galaxies might involve them drifting apart further and star formation slowing down as the available gas is depleted or becomes too dispersed. Life as we know it would face tremendous challenges in such an expanded and cooled universe, primarily due to energy scarcity and the vast distances between any remaining habitable zones.
Sun becomes a red giant event specifically pertains to the future of Earth within the solar system, where the expanding Sun will eventually engulf the inner planets, including Earth, leaving it uninhabitable.