What exactly is cosmological redshift?

To grasp the concept of redshift, it's helpful to take a step away from light (pun intended) and think of a similar auditory experience we all know. Picture a car, more specifically, its horn. No need to hop in and drive; imagine you're a kid playing stickball in your neighborhood. Your friend notices a car coming down the street, and you step aside onto the sidewalk. The driver is honking as they pass by. What do you hear? As the car approaches, the sound is higher in pitch, but as it speeds away, the tone drops. This phenomenon is known as the Doppler effect. When sound moves toward you, the waves are shorter and have a higher frequency. When it moves away, the waves stretch and the pitch decreases.

Light behaves in a similar way. As light gets closer to us, the waves compress, raising their frequency. As it moves away, the light waves stretch, lowering the frequency. Light that is approaching us is called blueshift, while light moving away is referred to as redshift. And that's your introduction to redshift – light moving away from you. Our job here is done. Time for happy hour.

Alright, maybe not just yet. But why do we call the redshift 'red' and the blueshift 'blue'? It's not merely for poetic flair or to call the short waves 'Stan' and the long waves 'Harry.' In the electromagnetic spectrum, red light has a lower frequency, and blue light has a higher frequency. So when light is moving away from us, it shifts toward the red part of the spectrum. When it's moving toward us, it shifts toward blue.

So, what does the cosmological part have to do with all of this? It’s just redshift on a grand scale. The Big Bang explosion was so enormous that nearly everything we observe in the universe is still moving away from us. (Some nearby objects like planets or stars may be moving toward us.) The farther away they are, the faster they seem to be moving. Thus, cosmological redshift occurs because light stretches as space itself expands. In fact, it stretches so much that the visible and ultraviolet light from distant galaxies eventually shifts into the infrared spectrum. Infrared telescopes like the James Webb Space Telescope, which was set to launch in 2018, will help us peer deeper into the universe and study the far-off galaxies that are speeding away from us.