What Actually Expands In An Expanding Universe?

a portion of this video was sponsored by Salesforce more about Salesforce at the end of the show [Music] the first piece of evidence that showed our universe is expanding came in the light from distant galaxies if you look at the Spectrum of the sun you see these dark lines and we see those same lines in the Spectra from galaxies except they're shifted to longer wavelengths towards the red end of the spectrum so we say their light is redshifted now the usual explanation for this redshift is that as the light is traveling through expanding space the photons themselves become stretched so short wavelengths get longer and this is known as cosmological redshift the explanation is fairly intuitively satisfying and most people go on without giving it a second thought but the problem is if you do give it a second thought you think well if expanding space can stretch something like a photon something that's so incredibly tiny does it also stretch atoms and molecules is expanding space stretching stars and galaxies and what about you are you expanding with the universe to answer these questions we've got to take a closer look at what it really means to redshift physicists actually talk about three different types of redshift Doppler redshift where observers moving relative to one another measure photons to have different wavelengths gravitational redshift where observers at different locations in a gravitational field measure different wavelengths and cosmological redshift where observers exchanging photons over vast cosmological distances in an expanding Universe measure different wavelengths these three cases appear very different and they're governed by different equations so how does each redshift actually occur let's start with a photon in a gravitational field there is this famous experiment conducted in 1959 by pound and rebka sending photons up and down a 22 meter Tower at Harvard now they used gamma rays but I'll represent them with a visible light they found that photons detected at the top of the tower were redshifted relative to The Source by the exact amount predicted by general relativity which is a tiny amount I'm dramatically exaggerating the effect just so you can see it now where along the photons path does this redshift take place well it seems to happen continuously the photon loses a little bit of energy each millimeter it climbs up that Tower meaning that the ...