Stretching light
As space expands, light from distant galaxies is shifted toward longer wavelengths. This measured shift helps relate a signal to cosmic time.
NASA · cosmological redshiftDEEP UNIVERSE
Light arrives
with a history.
Light from distant galaxies takes time to reach us, so we observe them as they were when that light was emitted.
Gravity as a lens
Mass bends light. A foreground galaxy or cluster can magnify and distort the view behind it, making otherwise inaccessible sources measurable.
NASA · gravitational lensesEstimating distances and masses
Distances, masses and early structures are constrained by multiple observations, models and comparison—not by a single beautiful image.
NASA · measuring galaxy distancesTHE DISTANT LIGHT