How Far Beyond Earth Could Humanity Spread?

Physics and cosmology set a hard limit on how far humanity can ever spread, even in an eternal universe, because dark energy accelerates cosmic expansion and shrinks the cosmological event horizon that bounds our reachable future. The maximum “affectable universe” is a sphere with a comoving radius of about 16.5 billion light-years, containing roughly 20 billion galaxies and 10²¹ stars, which could support up to 10³⁰ future human descendants if expansion happens immediately and at near-light speed. Delay is costly: every year of waiting permanently loses about three galaxies, and after 100 billion years only the local cluster remains. Travel speed matters just as much as timing, since halving speed reduces the reachable volume eightfold, though even at 20% light speed over a billion galaxies stay accessible. Ultimately, far-flung settlements become causally disconnected and can barely communicate with Earth, meaning early launches would spread an imperfect foundation as humanity’s cosmic legacy. The video concludes that while our future could be staggeringly large, we should expand strategically over the next billion years—and first take care of Earth before leaving it.

• ▶ 0:00 The central question is posed: “How big can humanity get?” — and it is argued that physics and cosmology place a hard limit on how much of the universe we can ever expand into.

• ▶ 1:07 A previous result is recalled: our potential view from the Milky Way maxes out at about 63 billion light-years, because dark energy will collapse cosmological horizons — but physically traveling outward is the key workaround.

• ▶ 1:45 To compute the maximum possible extent, the experiment assumes intergalactic travel is possible, travelers can survive extremely long journeys, civilization spreads recursively to new galaxies, and humanity encounters no competing civilizations.

• ▶ 0:00 The provided source includes no timestamp metadata, so real [MM:SS] positions cannot be extracted.
• ▶ 0:00 Even if the universe lasts forever, accelerating expansion withdraws distant galaxies, capping the size of humanity’s bubble.
• ▶ 0:00 Spacetime diagrams and future lightcones in comoving coordinates show that entire spacetime regions remain forever inaccessible.
• ▶ 5:11 Cosmic expansion initially slows due to gravity but later accelerates due to dark energy; in a simplified constant-expansion model, the Hubble horizon (where recession hits light speed) moves outward at light speed and cannot be crossed from inside.
• ▶ 6:25 Switching to comoving coordinates and applying a conformal transformation (so light travels at 45° angles) makes the entire cosmic past and future easier to analyze.
• ▶ 7:24 The cosmological event horizon is the ultimate boundary of our observable universe: it acts like a reverse black-hole horizon—you can escape from inside, but nothing outside can ever reach you, and its current distance is about 16.5 billion light-years.
• ▶ 8:53 The "affectable universe" is the theoretical maximum reach of civilization: a sphere with a 16.5 billion light-year comoving radius, containing ~20 billion galaxies and ~10²¹ stars.
• ▶ 9:46 The full potential — up to 10³⁰ living human descendants — assumes immediate, near-light-speed expansion in all directions; waiting shrinks the accessible universe as the cosmological event horizon recedes.
• ▶ 10:09 Every year of procrastination costs roughly 3 galaxies permanently, and after ~100 billion years only the local cluster remains ("era of isolation"); there's still time, though — only 20% is lost after 1 billion years.
• ▶ 10:48 Travel speed is just as important as timing: the affectable universe's radius scales with speed, so at half light speed the volume drops to one-eighth — but even at 20% light speed, over a billion galaxies remain reachable.
• ▶ 11:24 The far-future observable universe will reach a maximum radius of about 64 billion light-years (from 46.5 billion today), measured in comoving units.
• ▶ 12:25 The ultimately observable universe—the union of all possible light cones for light-speed explorers—spans nearly 160 billion light-years, yet those regions become causally disconnected and unable to communicate.
• ▶ 14:00 Olson's model shows that at 20% light-speed expansion, over half of settled volume gets only 2 or fewer messages home; near light-speed expansion, the vast majority of settlers never hear anything back from Earth.
• ▶ 14:35 Humanity's future could be "very, very large," potentially filling 20 billion galaxies with life and mind—but expanding outward may not be the only worthwhile path.
• ▶ 14:49 If we do expand, we should do so strategically, not by flinging colony ships immediately; we have about a billion years to refine our plans.
• ▶ 15:07 Because any mission will have very limited contact with home, the state of humanity at launch becomes the "seed" of our cosmic legacy—so launching too early risks spreading an imperfect foundation.
• ▶ 15:42 The host announces Earth Month across PBS, using the challenges of interstellar travel to argue why we should take care of Earth before we’re ready to leave it.
• ▶ 16:09 He spotlights the new PBS Terra series Untold Earth, a collaboration with Atlas Obscura and PBS NATURE, exploring North America’s strangest natural wonders.
• ▶ 16:22 Viewers are directed to links for the first episode on redwoods and the full Earth Month playlist, ending with a playful request to tell other PBS channels "Space Time sent you."

• ▶ 0:00 Introduction and Key Assumptions (0:00 - 2:48) - - Frames the big question, long-term perspective, and core assumptions for humanity's future scale.
• ▶ 2:48 The Growing Bubble and Spacetime Tools (2:48 - 5:11) - - Introduces the expanding civilization bubble and the spacetime-diagram framework used to analyze cosmic limits.
• ▶ 5:11 Expansion, Conformal Diagrams, and the Event Horizon (5:11 - 8:53) - - Explains cosmic expansion, conformal diagrams, and why a cosmological event horizon caps light-speed travel.
• ▶ 8:53 The Affectable Universe and the Cost of Waiting (8:53 - 11:12) - - Defines the affectable universe's maximum size and shows how waiting or slower travel shrinks it.
• ▶ 11:12 Observable Universe and Communication Limits (11:12 - 14:35) - - Contrasts the ultimately observable universe with the affectable one and explains causal disconnection limits.
• ▶ 14:35 Strategic Expansion and Cosmic Legacy (14:35 - 15:42) - - Considers strategic choices that could let humanity fill billions of galaxies and leave a lasting legacy.
• ▶ 15:42 Earth Month and PBS Terra Promotions (15:42 - 16:40) - - Promotes PBS Earth Month, the Blue Marble theme, and the Untold Earth series.