Seth G. answered • 06/14/21
Physics PhD student with experience teaching Math and Physics
The universe is expanding in all directions. Redshift is a measure of how quickly an object is moving away from us. This is similar to how we can hear the pitch of an ambulance's siren change as it approaches and then moves past us (doppler shift). A star's distance from us is a measure of the amount of space between it and us, and if all of space is expanding then the further away something is from us, the faster it is moving away from us. The key point here is that space itself is moving. The star may be moving towards us with respect to the space around it, but space itself is expanding away from us. Think of passengers on a train. If a passenger is in the front of a train and begins walking towards the back of the train, they are moving backward with respect to the train, but the train is moving so quickly that with respect to the ground they are still moving forward.
