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How does the universe expand?

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Quick answer

The universe expands because space itself is stretching, not because galaxies are flying outward through empty space. General relativity allows spacetime to grow, and observations show distant galaxies receding faster, with their light redshifted. This expansion is uniform on large scales and is currently accelerating due to dark energy.

The universe expands because space itself stretches between galaxies, carrying them apart like raisins in rising bread. Distant galaxies recede faster, following the Hubble-Lemaître law, not because they move through space but because more space is being created between them.

Step by step

  1. General relativity treats spacetime as a flexible fabric that can expand or contract.
  2. As space stretches, the distance between galaxies increases, even if they are not moving through space.
  3. Light traveling through expanding space gets stretched, shifting toward longer, redder wavelengths.
  4. Observations show that more distant galaxies have greater redshifts, confirming the expansion.
  5. Gravity clumps matter into galaxies and clusters, but on the largest scales, expansion dominates.

Key things to understand

  1. General relativity describes spacetime as a dynamic fabric that can stretch or contract, rather than a fixed stage.
  2. Observations of distant galaxies show their light is redshifted, meaning their wavelengths are stretched as space expands.
  3. The farther away a galaxy is, the faster it appears to recede, a relationship called the Hubble-Lemaître law.
  4. This expansion is not galaxies flying through space; it is the space between them growing over time.
  5. Gravity holds together local groups of galaxies, so expansion is only visible on very large scales.

Labelled diagram

  1. Early universe

    Space is dense and hot; galaxies are close together.

  2. Space expands

    The fabric of space stretches uniformly in all directions.

  3. Galaxies recede

    Distant galaxies move apart as more space appears between them.

  4. Light stretches

    Wavelengths of traveling light lengthen, producing redshift.

  5. Acceleration

    Dark energy causes the expansion rate to increase over time.

How space itself stretches over time

Real-world examples

  • Raisins in rising bread dough move apart as the dough expands, with no center of expansion.
  • The afterglow of the Big Bang, the cosmic microwave background, is stretched and cooled by expansion.
  • Distant supernovae appear dimmer than expected, revealing that expansion is accelerating.
  • The Andromeda galaxy is moving toward us because local gravity overcomes expansion.

Common mistake

✗ Myth: The universe expands because galaxies are flying through space away from a central explosion point.

✓ Fact: Their recession is not mainly motion through space; space itself is expanding between them. There is no center or edge to the expansion, and every observer sees the same recession of distant galaxies.

People also ask

What causes the universe to expand?
The universe expands because space itself is stretching, a consequence of general relativity. The initial expansion began with the Big Bang, and it is now accelerating due to dark energy, a mysterious force that pushes space apart. Gravity slows expansion locally but cannot stop it on the largest scales.
Is the universe expanding faster than light?
Yes, distant regions of space can recede faster than light because they are not moving through space; space itself is expanding between them. This does not violate relativity, which limits motion through space, not the stretching of space. We can still see ancient light from those regions because it was emitted when they were closer.
How fast is the universe expanding?
The expansion rate is described by the Hubble constant, roughly 70 kilometers per second per megaparsec. This means a galaxy one megaparsec away recedes at about 70 km/s, and one two megaparsecs away at 140 km/s. The exact value is still debated because different measurement methods give slightly different numbers.
What is the universe expanding into?
Nothing. The universe is not expanding into empty space; space itself is stretching. There is no outside or center to the expansion, because space is all there is. Every point sees other galaxies receding, like dots on an inflating balloon.
Does the expansion of the universe affect the solar system?
No. Gravity within the solar system, the Milky Way, and even the Local Group of galaxies is far stronger than the expansion. Space expands only on scales larger than galaxy clusters, where gravity is too weak to hold things together.
How do we know the universe is expanding?
In the 1920s, Edwin Hubble and others measured that distant galaxies are redshifted, with their light stretched to longer wavelengths. The farther a galaxy is, the greater its redshift, exactly as expected if space is expanding uniformly.
Will the universe expand forever?
Observations suggest the expansion is accelerating, driven by dark energy. If dark energy remains constant, the universe will likely expand forever, growing colder and emptier. But the nature of dark energy is still an open question in physics.

Part of the guide: Space & Astronomy →