The Mechanism
The universe's expansion is accelerating, driven by dark energy. Because of that acceleration, there is a distance — the cosmic event horizon — beyond which the space between us and a galaxy is stretching so fast that light the galaxy emits *now* will never reach us, no matter how long we wait. Galaxies are steadily crossing that line and vanishing from our future forever; their light doesn't blink out, it just red-shifts toward nothing as it strains against the growing gap. In 2007 the physicists Lawrence Krauss and Robert Scherrer worked out the consequence: in the far future, roughly a hundred billion years on, every galaxy outside our own gravitationally bound Local Group will have receded past the horizon. Astronomers of that era, doing careful and honest science, would see a single island of stars in an empty black sky — no other galaxies, no cosmic microwave background, no redshifts to measure. They would have no way to discover that space is expanding, or that there ever was a Big Bang. The evidence for the universe's own origin is expiring, quietly, on a clock.
Why It Matters
The surprising part is that galaxies are not being destroyed in a fireworks sense. They are becoming unreachable because the expansion of space is outpacing the light they emit. This makes the universe's history less observable over time. In the very distant future, observers could live in a real galaxy and still have no direct evidence for other galaxies, the cosmic microwave background, or the Big Bang. The cosmos is not hiding its past by accident; the information is simply running out of time to get here.
Wait — That's Not Quite Right
A common mistake is to think galaxies disappear because they burn out or get swallowed by a black hole. That is not what is happening here. The galaxies keep existing, but the space between them and us stretches so much that their light can no longer arrive. They vanish from our future observations, not from the universe itself.
Vocabulary
- dark energy
- accelerating expansion
- cosmic event horizon
- redshift
- local group
- big bang
- cosmic microwave background
- observable universe
- gravitationally bound
- Lawrence Krauss
- Robert Scherrer
Quick Quiz
5 questions · For classroom or kitchen table
The Experiment
Model a Vanishing Galaxy
Use a balloon to model how space can stretch while objects are not moving through space in the usual way. Draw several small dots on the balloon to stand for galaxies, then slowly inflate it and watch the dots separate. Pick one dot to be 'us' and notice that the farther dots seem to move away faster, because the whole surface is expanding.
Now imagine one dot crossing a line that marks 'too far to ever see again.' In the real universe, that boundary is the cosmic event horizon. The galaxy is still there, but its light has no way to bridge the growing gap. This is a good reminder that disappearance in cosmology can mean losing contact, not being destroyed.
If you want, do a second round with a paper map on a stretchy surface or a loose sheet of gum wrapper and compare which dots become hard to keep track of first. Talk about how the local group stays together, while faraway galaxies eventually fade from reach.
balloon, marker, optional string or tape to mark a horizon line, adult supervision for younger children
Where this came from
- Krauss & Scherrer, "The Return of a Static Universe and the End of Cosmology," *General Relativity and Gravitation* 39, 1545 (2007) — https://www.researchgate.net/publication/1884780_The_Return_of_a_Static_Universe_and_the_End_of_Cosmology ; popular account, "The End of Cosmology?", *Scientific American*, March 2008 — https://pages.uoregon.edu/imamura/123cs/exercises/end_of_cosmology_sciam.pdf
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