Question

question: which of the following measurements show(s) that the universe is currently expanding? answer options: select one of four relativistic doppler effect, blueshift, gravitational microlensing, size of spiral galaxies

Explanation:

1. Analyze "Relativistic Doppler effect": The relativistic Doppler effect for light from distant galaxies shows redshift (light shifted to longer wavelengths), indicating galaxies are moving away, supporting universe expansion.
2. Analyze "Blueshift": Blueshift means objects are moving toward us, which would suggest contraction, not expansion.
3. Analyze "Gravitational microlensing": This is used to detect celestial objects (like exoplanets) by gravitational lensing, not to measure universe expansion.
4. Analyze "Size of spiral galaxies": The size of spiral galaxies doesn't directly relate to the universe's expansion (expansion is about distance between galaxies, not their internal size).

So the correct option is the one related to redshift from moving - away galaxies, which is the Relativistic Doppler effect.

Answer:

Relativistic Doppler effect