Brenda wrote:
Don't you think that the Doppler is to measure things closer by? Sound takes time to come back, and if you want to find out where the universe ends, we need time.
But where does it expand to? And why was there nothing before? And what is nothing? How far away is "the end"? Why? What changes things?
Every answer triggers more questions, and so does every question

Doppler effect also is used to describe light wave shift. Unfortunately all that changes at the event horizon of a black hole.
Also if the device receiving the light is moving (such as planet earth) the relative measurement becomes difficult.
The end is twice as as far as from the the start to the middle.
You will find the answer to nothing in a bottomless bucket with no sides.
Finding the bucket is known as quantum mechanics.