Evidence for the expansion of the universe comes from the observation of
light from distant galaxies. What is this observation?
Light from distant objects is red shifted
More distant pulsars have a lower orbital frequency
Light from other galaxies is focused by gravitational lensing
The number of stars per volume drops at greater distances
Light from distant galaxies is redshifted, providing direct evidence for the universe’s expansion as wavelengths stretch due to space itself expanding between us and those galaxies. This phenomenon, first noted by Edwin Hubble, shows spectral lines shifting toward the red end of the spectrum, with more distant objects exhibiting greater redshift, aligning with Hubble’s Law.
Why Redshift is Correct
Redshift occurs because the universe’s expansion stretches light waves en route, increasing their wavelength like a Doppler effect for receding sources. Observations confirm nearly all distant galaxies recede from Earth, faster with distance, supporting the Big Bang model’s ongoing expansion. This cosmological redshift differs from local Doppler shifts, as it affects all wavelengths uniformly across vast scales.
Incorrect Options Explained
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More distant pulsars have a lower orbital frequency: Pulsars are neutron stars in our galaxy; their frequencies relate to spin or orbits but show no systematic drop with distance tied to cosmic expansion. This confuses local binary systems with universe-wide effects.
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Light from other galaxies is focused by gravitational lensing: Lensing bends light around massive objects like clusters, creating arcs or multiples, but reveals mass distribution, not expansion. It can magnify but doesn’t indicate recession.
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The number of stars per volume drops at greater distances: Galaxy density relates to structure formation and evolution, not direct expansion proof. Early universe views show denser packing due to youth, but this supports hierarchical models, not redshift’s recession evidence.
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Redshift remains the cornerstone observation, verified by telescopes like Hubble and JWST measuring spectra from billions of light-years away.
