Applying general relativity to the universe as a whole reveals something unsettling: the universe cannot be static. Space itself must be either expanding or contracting. Einstein initially resisted this conclusion and added a “cosmological constant” to force a static solution. He later called it his greatest blunder. Observations proved the universe is expanding — and the expansion is accelerating.
Galaxies are not flying apart through space like shrapnel from an explosion. Space between galaxies is growing. Every point in the universe is getting farther from every other point — not because anything is moving, but because the metric of space is changing.
Mental model: draw dots on a balloon. Inflate the balloon. Every dot moves away from every other dot, but no dot is at the “center” of the expansion. There is no center. The expansion is happening everywhere, uniformly.
If galaxies were moving through space, they would be limited to speeds below c. But the expansion of space has no speed limit — distant galaxies can recede faster than light without violating relativity. Nothing is moving faster than light through space. Space itself is growing, carrying galaxies along. The speed limit applies to motion through space, not to the expansion of space itself.
Light traveling through expanding space gets stretched. Its wavelength increases and its frequency decreases — it redshifts. This is not the Doppler effect. Doppler redshift comes from a source moving away through space. Cosmological redshift comes from space expanding while the light is in transit.
A photon emitted with a wavelength of 500 nm (green light) that travels for 10 billion years through expanding space might arrive with a wavelength of 1500 nm (infrared). The photon did not lose energy to anything. The space it traversed grew, stretching the wave along with it.
The redshift of a distant galaxy indicates how much the universe has expanded since the light was emitted. A redshift of z = 1 means the universe has doubled in size since that light left its source. A redshift of z = 10 means the universe was one-eleventh its current size. Redshift is a direct measurement of cosmic expansion history — and by extension, of the age and evolution of the universe.
In 1929, Edwin Hubble observed that distant galaxies are receding from us, and their recession velocity is proportional to their distance. Twice as far away means twice the recession speed. This relationship — now called the Hubble-Lemaitre law — is the observational signature of uniform expansion.
The proportionality constant (the Hubble constant) gives the current expansion rate: roughly 70 km/s per megaparsec. A galaxy one megaparsec away (3.26 million light-years) recedes at 70 km/s. A galaxy ten megaparsecs away recedes at 700 km/s.
Different methods of measuring the Hubble constant give different answers. Measurements from the early universe (cosmic microwave background) give about 67.4 km/s/Mpc. Measurements from the nearby universe (supernovae, Cepheid variables) give about 73.0 km/s/Mpc. This discrepancy — the “Hubble tension” — persists despite increasingly precise measurements and may indicate new physics beyond our current models.
At sufficient distance, the expansion of space carries galaxies away from us faster than light can cross the intervening space. Beyond this distance — the cosmic horizon — light emitted today will never reach us. Those regions are causally disconnected from us. We cannot see them, signal them, or be influenced by them. Ever.
The observable universe has a radius of about 46 billion light-years (larger than 13.8 billion because space expanded while light was in transit). Beyond that boundary, the universe continues — but it is forever inaccessible.
The cosmic horizon is the ultimate network partition.
The observable universe is one partition — the set of nodes that can communicate with one another. Beyond the cosmic horizon, nodes exist but no messages will ever be exchanged with them. The expansion of space is partition growth: the set of unreachable nodes increases over time as space carries more galaxies beyond the horizon.
This is the CAP theorem at universal scale. (The CAP theorem states that when a network is partitioned, a system can preserve either availability — every part keeps working independently — or consistency — every part agrees on a single shared state — but not both.) The universe chose availability — expansion continues, new structure forms, local physics works perfectly — over consistency — causal contact between all parts.
In most distributed systems, partitions are temporary. When the network heals, nodes can synchronize. The cosmic horizon is a permanent, growing partition. Eventual consistency is not achievable — the light-travel time between partitions increases without bound. Every point in the universe has a different observable universe, a different set of accessible information, and there is no global state that any observer can ever reconstruct.
In 1998, two teams independently discovered that the expansion of the universe is not slowing down (as gravity would predict) but accelerating. Something is pushing space apart faster and faster. We call it dark energy and it constitutes roughly 68% of the total energy content of the universe. We do not know what it is.
If the acceleration continues, the cosmic horizon will shrink — more and more galaxies will pass beyond it. In the far future, distant galaxies will redshift beyond detectability. An observer trillions of years from now would see only their local galaxy cluster, surrounded by apparently empty space, with no evidence that the broader universe exists.
This lesson establishes: