eli5: when a star leaves the observable universe due to expansion, does it get fainter over time from our perspective, or it just appears to turn off, like a flashlight?
In: 30
The light undergoes something called a “redshift.”
Ever have an ambulance or something go past you, where it sounds different coming toward you than it does going away from you?
That’s the Doppler affect at work. When the siren is coming toward you, the sound waves are compressed and the pitch changes. When the siren is moving away from you, the same thing happens except this time the sound waves are stretched out, and it sounds lower-pitched.
The same thing happens with light. With light, when something is moving toward you, the wavelength of the light is compressed. This makes it turn more blue, and the phenomenon is called “blueshifting.” When something is moving away from you, the wavelength of the light is stretched and the light turns redder. That’s called “redshifting.”
When something moves out of the observable universe, it’s not crossing some magic boundary where it’s instantly either in or out of the observable universe. It moves away from us at some rate, and with the universe expanding that rate gets faster and faster over time.
So the light from, say, a star that’s moving away from us toward the edge of the observable universe will undergo more and more extreme redshifting over time. This means the light will get redder and redder (as well as dimmer and dimmer as it gets further away) , shifting into the red, then infrared, then radio spectrum, until we can no longer see it.
Once it crosses the border of the observable universe, no new light that it emits will be able to reach us. But the light it has been emitting as it travels will be able to, and it’s that light that undergoes the redshifting.
Latest Answers