Basic science questions

[julius2]

According to research, the universe is expanding. We "know" this from observation.

How does this work in practice?

Is the universe expanding "in" to something?

Or is the universe itself perpetually pushing a "boundary" ?

 

[Genady]

2 minutes ago, julius2 said:

According to research, the universe is expanding. We "know" this from observation.

How does this work in practice?

Is the universe expanding "in" to something?

Or is the universe itself perpetually pushing a "boundary" ?

 

No, and no. Instead, the distances between unbounded parts of the universe increase.

[julius2]

9 hours ago, Genady said:

No, and no. Instead, the distances between unbounded parts of the universe increase.

I see. This sounds like spacetime to me.

So (if) the universe is made up of spacetime, you can't apply a simple Newtonian mind to it.

So the concept of our very existence is weird?

[swansont]

5 minutes ago, julius2 said:

So (if) the universe is made up of spacetime

Spacetime is not a substance.

[zapatos]

1 hour ago, julius2 said:

So the concept of our very existence is weird?

No, not at all.

[julius2]

6 hours ago, zapatos said:

No, not at all.

According to a source, the expansion of the universe is due to space itself stretching...

 

Looking at the First Friedmann equation for explaining the expansion of the universe,

the right includes terms:

- energy density

- curvature of space

- dark energy

Is the equation some kind of "balance"?

[Markus Hanke]

30 minutes ago, julius2 said:

Is the equation some kind of "balance"?

The Friedmann equation is just the 00-component of the Einstein equations for the case of FLRW spacetime with a perfect fluid taken as the source term; essentially it gives a relationship between expansion rate and its second derivative, which needs to be fulfilled in order for the model to be consistent with the laws of gravity. Thus, it describes what form the scale factor a(t) can have.

Or to put it differently - this equation states that spacetime in the interior of an isotropic, homogenous perfect fluid has an intrinsic tendency to metrically expand, unless counterbalanced by just the right kind and amount of background curvature. This is a direct consequence of the laws of gravity, and not some idea that got inserted into the model post-hoc.

Here is a good non-technical overview over where the Friedmann equations come from.