What Actually Expands In An Expanding Universe?

The video explores the concept of redshift, explaining how light from distant galaxies shifts to longer wavelengths, indicating the universe's expansion. It discusses three types of redshift: Doppler, gravitational, and cosmological, and explains how they are interconnected. Through experiments and thought experiments, it demonstrates that redshift is not an intrinsic property of photons but depends on the observer's frame of reference. The video concludes that while the universe is expanding, this does not mean that objects like atoms, stars, or humans are expanding with it.

What is the significance of co-moving observers in the context of cosmological redshift?

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How does the concept of expanding space relate to the redshift of photons?

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What is the take-home message regarding the expansion of molecules, stars, and galaxies?

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In what extraordinary circumstances might one expand with the universe?

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