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.

[julius2]

On 1/19/2025 at 10:12 PM, Markus Hanke said:

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.

Still reading over the overview of the Friedmann equations, but first,

- a(t) is a kind of "velocity"    i.e. rate of movement between two points in spacetime?

- the BB singularity, is this a "mathematical" construction, or does it have a physical meaning

5 minutes ago, julius2 said:

Still reading over the overview of the Friedmann equations, but first,

- a(t) is a kind of "velocity"    i.e. rate of movement between two points in spacetime?

- the BB singularity, is this a "mathematical" construction, or does it have a physical meaning

Is dark energy a way of explaining the expansion rate of the universe, in other words without this phenomenon the equations just won't work?

7 minutes ago, julius2 said:

Still reading over the overview of the Friedmann equations, but first,

- a(t) is a kind of "velocity"    i.e. rate of movement between two points in spacetime?

- the BB singularity, is this a "mathematical" construction, or does it have a physical meaning

Is dark energy a way of explaining the expansion rate of the universe, in other words without this phenomenon the equations just won't work?

Sorry I didn't do 3rd year uni maths, is a tensor like a matrix?

From memory, multiplying 2 matrices together and expanding can reveal several lines of vectors?

18 minutes ago, julius2 said:

Still reading over the overview of the Friedmann equations, but first,

- a(t) is a kind of "velocity"    i.e. rate of movement between two points in spacetime?

- the BB singularity, is this a "mathematical" construction, or does it have a physical meaning

Is dark energy a way of explaining the expansion rate of the universe, in other words without this phenomenon the equations just won't work?

Sorry I didn't do 3rd year uni maths, is a tensor like a matrix?

From memory, multiplying 2 matrices together and expanding can reveal several lines of vectors?

You don't know of a good article for Einstein's Field Equations?

[Genady]

1 hour ago, julius2 said:

a(t) is a kind of "velocity"    i.e. rate of movement between two points in spacetime?

No, it is not. It is a dimensionless number.

1 hour ago, julius2 said:

the BB singularity, is this a "mathematical" construction, or does it have a physical meaning

To help me understand this question, please answer this one: number 3, is this a "mathematical" construction, or does it have a physical meaning.

[Markus Hanke]

2 hours ago, julius2 said:

a(t) is a kind of "velocity"    i.e. rate of movement between two points in spacetime?

 

It’s an expansion factor. It tells you how the spatial distance between the same two arbitrarily chosen points change over time.

2 hours ago, julius2 said:

the BB singularity, is this a "mathematical" construction, or does it have a physical meaning

Think of it as a placeholder for “we don’t know yet what happens there”. That’s because if you go far enough in time, both gravity and quantum physics become relevant simultaneously, and we don’t yet have a theory of quantum gravity. Hence we don’t know (yet) just how exactly the universe started off, we only know how it evolved after a certain very early point in time.

2 hours ago, julius2 said:

Is dark energy a way of explaining the expansion rate of the universe, in other words without this phenomenon the equations just won't work?

Without DE the rate of expansion would be a constant (but the equations still work). However, the data we have suggests that this rate isn’t constant, but accelerating - so DE is a mechanism that actively pushes the universe apart. The exact physical nature of DE is as of yet unknown.

2 hours ago, julius2 said:

Sorry I didn't do 3rd year uni maths, is a tensor like a matrix?

This is not easy to answer in a non-technical way. Think of a tensor (as it is most commonly found in GR) as a function that takes as input vectors, and produces as output a new vector, a real number, or sometimes another tensor. 

2 hours ago, julius2 said:

You don't know of a good article for Einstein's Field Equations?

That depends what kind of background knowledge you have…? Perhaps the Wiki article on General Relativity is a good starting point.