What Actually Expands In An Expanding Universe?

Distant galaxies show dark lines in their light spectra shifted toward longer wavelengths, appearing red-shifted compared to the sun's spectrum.

As light travels through expanding space, photons become stretched, with short wavelengths becoming longer. This is called cosmological redshift.

• Does expanding space stretch atoms and molecules? • Does it stretch stars and galaxies? • Are you expanding with the universe?

• Doppler Redshift: observed when moving relative to each other • Gravitational Redshift: observed at different locations in gravitational fields • Cosmological Redshift: observed over vast cosmic distances

The 1959 Pound-Rebka experiment sent photons up and down a 22-meter tower at Harvard, finding photons at the top were red-shifted by the amount predicted by General Relativity.

The redshift happens gradually as the photon climbs the tower, losing energy with each millimeter traveled, appearing green in the middle.

Being at rest on Earth's surface is indistinguishable from being in a rocket in deep space accelerating at 1g, producing identical redshift results.

A free-falling observer would measure no redshift and see the photon as blue throughout, demonstrating that wavelength is not intrinsic to the photon itself.

When a rocket at rest emits blue photons and then accelerates, external stationary observers initially see blue photons, but observers inside experience different colors.

• Interior observers see green photons • Stationary observers outside still see blue photons • This difference resolves the equivalence principle

As the rocket speeds up relative to the source, interior observers experience Doppler redshift because they move away from the light source quickly.

Wavelength and energy are not intrinsic photon properties but depend on the photon-observer system and relative motion.

At the largest observable scales, galaxies behave like molecules in a fluid, and the universe appears homogeneous and isotropic without preferred orientation.

• The cosmic fluid spreads out uniformly in all directions • Matter density decreases over time • Objects move apart regardless of observation direction

Co-moving observers maintain fixed coordinates as the cosmic fluid expands. Earth is not a co-moving observer, moving at 600 km/s relative to cosmic microwave background radiation.

When co-moving observers exchange photons across large distances, wavelengths stretch by the amount the universe expanded during the photon's journey.

Multiple co-moving observers along a photon's path each absorb and re-emit identical photons, with each observer measuring progressively longer wavelengths.

Each observer sees neighboring co-moving observers moving away in locally flat spacetime and attributes wavelength increases to relative Doppler shift.

Cosmological redshift can be viewed as a long series of Doppler shifts without requiring the concept of expanding space.

• All three redshift types reduce to one underlying physics • They differ only in frame of reference • Wavelength depends on emission and absorption observer conditions

• The universe is not homogeneous at human scales; matter condenses into objects and Earth • The universe is not isotropic; looking up differs from looking down • Local spacetime curvature is dominated by Earth

Your body is held together by electromagnetic forces, which prevent expansion even if placed in deep space away from massive objects.

Only if electromagnetic forces were turned off and particles didn't interact would you expand over time, driven by dark energy dominance in the universe.

• Photon redshift does not mean space stretches and pulls things apart • Molecules, stars, galaxies, and humans do not expand • Expansion occurs only under extraordinary circumstances