Redshifts

Astronomers often calculate what is called redshift to determine different properties of celestial bodies, mainly their distance from us.  The redshift is often related to the Doppler effect of the light that is coming to us from that body, say a star.  In the same way that an ambulance siren's pitch varies to our ears when it approaches and leaves us,  the light spectrum of a star varies to our eyes (telescopes) as it travels through the space.  The change in wavelength of  that body's spectrum determines its redshift.

So far so good.  We have a simple and yet powerful concept to work with.  We can effectively derive the age, distance, velocity, temperature, and mass of bodies billions of miles away from us just by looking at their light spectrum's period and amplitude.  You see redshift and Doppler effect in so many astronomy papers and textbooks that even to a non-astronomer like me they become common knowledge. Something as famous and powerful as E = mc^2 or F = ma.  And just as you feel a little comfortable and confident about calculating at least one thing in astronomy, here came our professor and announced the biggest misconception in astronomy textbooks:  The redshift of galaxies is not due to Doppler effect, it is due to the expansion of the universe! Astronomers call this cosmological redshift!  Doppler effect is responsible for redshifts in our solar system and in our own galaxy.  When we look at spectra of other galaxies, their redshift is a result of the Hubble's law!

Ouch!  My summer intern was calculating redshifts for galaxies and he had mentioned Doppler effect in his report.  I emailed him directly from the training.  They cannot get a return on investment from an Astronomy for Engineers training any faster than that!  Of course, it does not affect calculations of the redshift and the results, but just the motivation behind the solution was not stated correctly scientifically.