ExpandersWill expanding civilizations meet in the universe?

If civilizations keep expanding, will the universe fill up? This page covers hard steps, expansion-front speed, horizons and the grabby aliens model.

If some civilizations expand forever, why have we not seen them? A 2021 model by Robin Hanson and colleagues gives an unusual answer. We appear so "early" because expanding civilizations will fill the universe quickly, leaving little time for latecomers. The "Expanders" channel in the game turns this model into a picture: a front advancing at 80% of the speed of light, heading toward the Milky Way.

This page first covers what the model actually says, then why a front can look "faster than light", how warning time is calculated, and how cosmic expansion isolates civilizations. It ends with an outline of the game's scenario. To be clear, the model is a hypothesis, and no expanding civilization has ever been observed.

Why we appear "too early"

Arriving early: the universe is only 13.8 billion years old, but small red dwarf stars can shine steadily for trillions of years. If life could appear in any era, we showed up remarkably early. Hanson and colleagues explain it this way: expanding civilizations will fill the universe in the not-too-distant future.
Hard steps: Earth's history of life has a few extremely unlikely steps: the origin of life, the eukaryotic cell, sexual reproduction, multicellular animals, intelligence. Each may need a long wait for luck. The model uses about six steps.
Quiet civilizations: the same model also implies that non-expanding "quiet civilizations" are very rare. For even one to exist in the Milky Way, the probability that a quiet civilization turns into an expanding one would have to be below one in ten thousand.

The model's picture of the universe

Half the universe taken: expanding civilizations may already occupy 40% to 50% of the universe by volume, and each will eventually control roughly 100,000 to 30 million galaxies. We would meet them in about 200 million to 2 billion years.
Two soap bubbles: the model assumes expanders stop when they meet another expander. Two bubbles press together and leave a boundary between them that barely moves.
Arc in the sky: if the edges of expanding civilizations are not obvious in the sky, we might find many very long circular arcs.
Harvesting starlight: a civilization controlling a galaxy can use about 10³⁷ watts, which is a Kardashev type III civilization. If it collects starlight and releases the waste heat, the galaxy dims in visible light and brightens in the infrared.

How fast the front is, how short the warning

The slower the expansion, the easier it is to see before it arrives. That no expanding civilization has been seen suggests their fronts move fast, at a large fraction of the speed of light.

Something advancing toward you at close to the speed of light can look faster than light: it arrives almost together with its own light. The Hubble telescope measured motion in the jet of galaxy M87 that appears to be about 6 times the speed of light. So the warning time is extremely short: the front's light arrives earlier than the front itself by only "distance times (the ratio of light speed to the front's speed, minus one)" years. At 80% of the speed of light, you can only see it once it has covered the last quarter of its journey.

Cosmic expansion isolates civilizations

The universe is expanding at an accelerating rate. A signal sent today from beyond about 16.5 billion light-years away can never reach us. The more widely the expanders spread, the more they resemble residents of different universes. As for your own front: traveling at 99% of the speed of light, 1 year passes on the ship while about 7 years pass outside, so the faster the ship, the more galaxies it can claim before the other front arrives.

The idea in the game

What follows is game lore. The channel is discovered by unlocking "Horizon mapping" in the Tree of the Beyond, or when the main civilization completes "Galactic colonization". In the scenario, the horizon mapping data contains a very long circular arc that belongs to no known structure. On the far side of the arc, galaxies dim in visible light and brighten in the infrared, and someone is collecting the light of stars. Their front is advancing at 80% of the speed of light, toward the Milky Way, at a distance of about 1.8 billion light-years.

The mechanic uses a cosmic clock: each time the main civilization completes a main-line event of the interstellar civilization, the universe ages a step and the front gets closer. When the main civilization becomes a "Kardashev type III" civilization, the front reaches the Milky Way. Because it advances at close to the speed of light, the main civilization can see it only a very short time before it arrives. The channel's three endings are Reserve, Merger and Border. All of this is fiction.

Schild's Ladder (Greg Egan, 2002): vacuum decay is real physics (Coleman 1977; Coleman and De Luccia 1980), and a real vacuum bubble would expand at close to the speed of light. In the novel, a physics experiment accidentally creates a new kind of vacuum that expands outward at half the speed of light and swallows everything in its path.

The studies below concern cosmic expansion, the classification of civilizations and the Fermi question. They are related real science, not evidence that expanders exist.

Using distant supernovae as "standard candles", researchers found that the expansion of the universe is accelerating, and dark energy entered physics. The work won the 2011 Nobel Prize (Riess et al., The Astronomical Journal, 1998).
A second team independently reached the same conclusion: some form of energy in the universe drives the accelerating expansion (Perlmutter et al., The Astrophysical Journal, 1999).
A sufficiently advanced civilization might enclose its star, and we could find such civilizations through anomalous infrared radiation (Dyson, Science, 1960).
Civilizations classified into three types by the energy they can use: planetary, stellar and galactic (Kardashev, Soviet Astronomy, 1964).
If extraterrestrial civilizations were common, they should long ago have spread across the Milky Way, so we may be the first (Hart, Quarterly Journal of the Royal Astronomical Society, 1975).

Real basis

Arc in the sky: In 2021 Hanson et al. predicted that if the boundaries of expanding civilizations are not conspicuous in the sky, we might still find many very long arcs across it.
Arriving early: The universe is only 13.8 billion years old, yet small red dwarfs can shine steadily for trillions of years. If life can appear in any era, we showed up strangely early. Hanson et al.'s explanation: expanding civilizations will fill the universe in the not-too-distant future, leaving latecomers little time.
Hard steps: Earth's history had several very unlikely steps: the origin of life, eukaryotic cells, sexual reproduction, multicellular animals, intelligence... each one may take a long wait for luck. Hanson et al.'s model uses about six steps.
Speed of the frontier: The slower an expansion, the more likely it is seen before it arrives. We have seen no expanding civilization so far, which suggests their frontiers move fast, at a sizable fraction of light speed.
Illusion of superluminal speed: Something advancing head-on at nearly light speed looks faster than light: it arrives almost together with its own light. The Hubble Space Telescope measured apparent motion of about 6 times light speed in the jet of galaxy M87.
Half the universe taken: Hanson et al.'s model says expanding civilizations may already occupy 40% to 50% of the universe's volume, each eventually controlling about 100,000 to 30 million galaxies; we would meet them in about 200 million to 2 billion years.
Warning time: When the frontier is D light-years away and advancing at speed v, its light arrives only D×(1/v−1) years before the frontier itself. At 80% of light speed, you only see it once it has covered the last quarter of the distance.
Quiet civilizations: The same model also implies that non-expanding “quiet civilizations” are very rare. For even one to exist in the Galaxy, the probability that a quiet civilization turns into an expanding one must be below one in ten thousand.
Harvesting starlight: A civilization controlling a galaxy can use about 10³⁷ watts of power: that is Kardashev type III. Collect the starlight and release it as waste heat, and the galaxy dims in visible light and brightens in infrared.
Two soap bubbles: Hanson et al.'s model assumes an expander stops when it meets another expander. Two bubbles pressed together leave an almost motionless boundary between them.
Letters that can't return: The universe is expanding faster and faster. Beyond about 16.5 billion light-years from us, a signal sent today can never reach us. The more the expanders spread apart, the more they live in different universes.
Your frontier: At 99% of light speed, 1 year passes on the ship and about 7 outside. The faster the main civilization's ships, the more galaxies it can claim before the frontier arrives.
Schild’s Ladder (Greg Egan, 2002), the science: the theory of vacuum decay is real physics (Coleman 1977, Coleman and De Luccia 1980); a real vacuum bubble would expand at close to the speed of light.

Related research

These studies relate to this topic but are not direct proof of this page.

Accelerating expansion: Using distant supernovae as “standard candles,” they found the expansion of the universe is accelerating. Dark energy entered the picture, and the work won the 2011 Nobel Prize.
Source: Riess et al., The Astronomical Journal 1998, "Observational evidence from supernovae for an accelerating universe and a cosmological constant"
42 supernovae: Another team independently reached the same conclusion: some energy in the universe is driving the accelerating expansion.
Source: Perlmutter et al., The Astrophysical Journal 1999, "Measurements of Omega and Lambda from 42 high-redshift supernovae"
Dyson sphere: A sufficiently advanced civilization might wrap its star, and we could find it through anomalous infrared radiation.
Source: Dyson, Science 1960, "Search for artificial stellar sources of infrared radiation"
Scale of civilizations: Sorts civilizations into three types by the energy they can use: planetary, stellar and galactic.
Source: Kardashev, Soviet Astronomy 1964, "Transmission of information by extraterrestrial civilizations"
Where are they?: If alien civilizations were common, they should already fill the galaxy. From this Hart argued that we may be the first.
Source: Hart, Quarterly Journal of the Royal Astronomical Society 1975, "An explanation for the absence of extraterrestrials on Earth"

Game lore (fictional)

Node “Arc in the sky”: In the horizon-mapping data, one arc belongs to no known structure.
Node “Speed of the frontier”: It measures out at 80% of the speed of light.
Node “Illusion of superluminal speed”: Something coming at you at 80% of light speed looks like it is moving at 4 times light speed.
Node “Harvesting starlight”: The galaxies they pass through look exactly like this.
Node “Two soap bubbles”: Between them and another expander there is already such a boundary.
Node “Your frontier”: The main civilization has begun to expand too.
Node “Before the meeting”: The frontiers are still several hundred million years from meeting.
Node “Meeting”: The two frontiers meet at the edge of the Milky Way.
Channel ending “Reserve”: The frontier stopped outside the Milky Way and went around. On the Expanders’ star map, the Milky Way is marked as a reserve: a civilization that does not expand is worth leaving alone to watch. This is one version of the “zoo hypothesis”.
Channel ending “Merger”: The two frontiers merge and keep pushing toward the horizon. The main civilization’s history becomes one chapter in a larger civilization’s.
Channel ending “Border”: The frontier stopped at the edge of the Milky Way, like the film between two soap bubbles. The starlight on either side of the border is never touched again.
Summary of the novel Schild’s Ladder: A physics experiment accidentally creates a new kind of vacuum, which expands outward at half the speed of light and swallows everything in its path. Twenty thousand years later, people are still trying to find a way to face it.

See it in the game