How do we know that the universe is expanding when it could be simply stretching?

[Achilles]

Stretching within itself. How do we know the universe is expanding? What is it even expanding into?

[Endy0816]

We observe that the distance between clusters of galaxies is increasing.

It doesn't expand into anything.

Think in terms of the path length increasing.

Edited August 16, 2018 by Endy0816

[beecee]

24 minutes ago, Achilles said:

Stretching within itself. How do we know the universe is expanding? What is it even expanding into?

Because in places of higher mass-energy density, spacetime expansion is negated by gravity. If it was stretching in the manner that you seem to be proposing, it would be stretching everywhere.The BB also describes the evolution of space and time [as we know them] so asking what was before t=0, is like asking what is north of the north pole to quote the late great Stephen Hawking.

[Strange]

2 hours ago, Achilles said:

Stretching within itself. How do we know the universe is expanding? What is it even expanding into?

What do you mean by "stretching"? Depending what you mean, that might be an accurate analogy for the metric expansion of space. Or not...

It isn't expanding "into" anything. Because the universe is all there is. It might be easier to think of it as getting less dense, instead.

[Airbrush]

"Expanding" and "stretching" mean about the same thing right?

[beecee]

On 8/19/2018 at 10:01 AM, Airbrush said:

"Expanding" and "stretching" mean about the same thing right?

In common speak, probably yes, but as an analogy, firstly recognising that analogies do have limitations.

 

On 8/17/2018 at 6:27 AM, beecee said:

Because in places of higher mass-energy density, spacetime expansion is negated by gravity. If it was stretching in the manner that you seem to be proposing, it would be stretching everywhere.The BB also describes the evolution of space and time [as we know them] so asking what was before t=0, is like asking what is north of the north pole to quote the late great Stephen Hawking.

The highlighted part of my previous answer, now seems full of holes. I withdraw that part.

Edited August 20, 2018 by beecee