Learn how mass curves spacetime, why gravity is geometry, and how this shapes motion, light, and time.

Learn how telescopes detect the ancient universe, from redshift to the cosmic microwave background, and why different observatories reveal different eras.

Learn how hydrogen, gravity, and dark matter turned a hot, dark universe into the first stars and galaxies.

Learn what zero gravity means, why astronauts float, and how weightlessness is created in space, aircraft, and laboratories.

Learn why microgravity is not the same as zero gravity, how orbit creates weightlessness, and what astronauts actually experience in space.

Learn what the observable universe is, why it has a cosmic horizon, and how astronomers estimate its true scale.

A clear look at the biggest unknowns in dark energy, from cosmic acceleration to the role of the vacuum.

Discover how gravity affects astronauts’ bodies, performance, and health during spaceflight, from muscle loss to fluid shifts and bone density changes.

Explore the leading scientific theories for how the universe may end, from heat death to a Big Crunch, and what current cosmology says.

Dark energy drives the universe’s accelerated expansion. If it weakens, cosmic history could change in ways that range from subtle to dramatic.