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.

[julius2]

Thanks for the explanations above. I understand that there is quantum mechanics which deals with the things of very small. And general relativity (dealing with gravity?) to explain things of the cosmos. I take it Newtownian mechanics is kind of in between?

I still come back to my first assertion, is that how do we know there was not "something" before the current universe?

From my viewpoint, current science has been heavily reliant on a "discovery" method. Going from the Renaissance, through the Industrial Revolution period and 20th century. It is as if the universe exists waiting for us to discover everything.

If we postulate that there was "something" before the BB, then we may go to a "SEARCH" position. We search for how we can access previous time etc.

Anyway, the world is going towards AI. Perhaps scientists can use AI to do this? (claim / disclaim).

Or even to find the "grand unifying theory" that unites quantum mechanics and general relativity. ?????

[Markus Hanke]

4 hours ago, julius2 said:

I still come back to my first assertion, is that how do we know there was not "something" before the current universe?

We don’t know yet what kind of initial conditions were present to give rise to the universe, since our current best models don’t extend as far back as the actual BB event itself.

Note though that “before the universe” is pretty much a meaningless concept, much like saying “north of the North Pole”.

[julius2]

1 hour ago, Markus Hanke said:

We don’t know yet what kind of initial conditions were present to give rise to the universe, since our current best models don’t extend as far back as the actual BB event itself.

Note though that “before the universe” is pretty much a meaningless concept, much like saying “north of the North Pole”.

Thanks.

I guess what I am thinking if we could solve a lot of science's perplexing problems, by extending the arrow of time backwards.

The one thing your statement doesn't accept is "before the universe", but there was "before the universe".....

 

 

 

[Genady]

2 hours ago, Markus Hanke said:

“before the universe” is pretty much a meaningless concept, much like saying “north of the North Pole”.

When I've mentioned this metaphor to my neighbor, an experienced dentist, he replied that there is nothing wrong with the “north of the North Pole”: you just keep moving up. 

[studiot]

Just now, julius2 said:

Thanks.

I guess what I am thinking if we could solve a lot of science's perplexing problems, by extending the arrow of time backwards.

The one thing your statement doesn't accept is "before the universe", but there was "before the universe".....

 

 

 

Here is some explanation that may help.

The Earth is not 'all there is'.

In other words there is space and time along with contents and the stuff that makes it all up beyond or outside the Earth.

However the Universe is defined as 'all there is'.

In these circumstances both space and time may be part of this 'all there is'.

We don't know if either space or time are finite or infinite.

We don't even know the nature of this infinity if it is there.

We do know of other infinities which which have no boundary but yet can't be reached.

For instance absolute zero of temperature.

As you approach AZ it becomes harder and harder to take the next smaller and smaller temperature step towards it such that it can never be reached.

Another example, if you know any maths, is exhibited by numbers.

Take any  real or rational number you will find there is no such thing as a nearest number to it.

You can always find a nearer number than any propest 'nearest' and can go on indefinitely getting closer and closer to the proposed number, without ever actually reaching it.

 

So if our finite is bounded in space, why not also finite in time ?

 

Nature has shown itself more varied and unwilling to fit into our human conceived boxes and is therefore constantly suprising us with new stuff.

[swansont]

If there was anything before the BB, evidence of it did not survive the event. The only evidence that could would be details of the hot dense state, and we don’t have physics that works to explain the behavior, as Markus said. Conjecture about anything prior has no basis in existing physics. No evidence and not testable.

[zapatos]

10 hours ago, julius2 said:

I guess what I am thinking if we could solve a lot of science's perplexing problems, by extending the arrow of time backwards.

 

What perplexing problems can be solved by extending the arrow of time backwards?

10 hours ago, julius2 said:

The one thing your statement doesn't accept is "before the universe", but there was "before the universe".....

 

Remember that you are having this discussion on a science forum and not having it with some buddies down at the pub. According to science 'before the universe' is a meaningless concept and can be shown to be meaningless. If you are going to deny the science then you should understand the science, and give evidence that the science is flawed. If you do not understand how it could be true it may be useful to ask why before you claim it is false.

[julius2]

Using one of the AI query engines, scientists have in fact examined various models where the singularity is avoided (as expected):

- Loop Quantum Cosmology

- Imaginery Time (Hawking) - time becomes complex avoiding singularity

- Mirror Universe (CPT symmetry)

- Quantum Tunneling Models

Without going in to detail, it looks like the possibility has been examined.

In terms of testability, my thoughts are to build a model where there is a something "before the universe" giving rise to new equations. Giving rise to changed "designs" for experiments. (Not a discovery approach.)

Remember, we have to account for "living things". If you need empirical evidence, just walk in a forest.

I am expecting a model / set of equations that can explain A to Z.

 

[Phi for All]

12 hours ago, julius2 said:

Using one of the AI query engines, scientists have in fact examined various models where the singularity is avoided (as expected):

It doesn't matter if you avoid the singularity. The density and heat of the early universe destroyed any evidence of what might have happened "before", as swansont said. 

If I give you a puddle of molten steel and tell you it used to be a forged item, how would you go about figuring out what it used to be? Is there anything about the molten metal that could tell you whether it used to be a sledgehammer or part of a car frame or a manhole cover? You can guess all you want, but you can't know what that steel used to be because melting it destroys all the evidence.

13 hours ago, julius2 said:

Remember, we have to account for "living things". If you need empirical evidence, just walk in a forest.

In the early universe? Why?

[swansont]

On 2/3/2025 at 8:42 PM, julius2 said:

Using one of the AI query engines, scientists have in fact examined various models where the singularity is avoided (as expected):

Are you suggesting that these were the result of AI, or is this just worded poorly?

Quote

 

Loop Quantum Cosmology

- Imaginery Time (Hawking) - time becomes complex avoiding singularity

- Mirror Universe (CPT symmetry)

- Quantum Tunneling Models

Without going in to detail, it looks like the possibility has been examined

 

None of those give us physics that works earlier than the BB. Some of these aren’t even confirmed for after the BB.