# Expansion of the Universe and the fourth dimension

• B
The perimeter of a circle increases by radius, the surface area of a ball increase by radius(which is height which is the third dimension if the ball is a planet like the Earth), and the universe is expanding by time, can we say that the fourth dimension is time by this ?

phinds
Gold Member
The perimeter of a circle increases by radius, the surface area of a ball increase by radius(which is height which is the third dimension if the ball is a planet like the Earth), and the universe is expanding by time, can we say that the fourth dimension is time by this ?
No. That would not even make sense. First of all, you are separating space and time, but in general that's not valid. It's all space-time.

berkeman
Ibix
2020 Award
The perimeter of a circle increases by radius, the surface area of a ball increase by radius(which is height which is the third dimension if the ball is a planet like the Earth), and the universe is expanding by time, can we say that the fourth dimension is time by this ?
I don't think so. Leaving aside how the radius of your circle is increasing without you defining time beforehand, you can imagine a rubber sheet being stretched in its own plane. Its area increases with its radius, which would imply that one of its spatial directions was also time, by your argument.

Time is a fourth dimension in our current best models, yes. But we deduced this from Minkowski's interpretation of Einstein's maths, which in turn followed from detailed mathematical study of electromagnetism. I don't think you can really do science by analogy, which is what you appear to be trying to do.

sophiecentaur
The question is if the universe is creating Spacetime as it expands, which is the older more common view. But some cosmologists think that it is expanding into a 4d space. We have two rather confusing point of view. 1. It is expanding into nothing or 2. What is the 4d space doing; expanding into a 5d space? And so on! Don’t expect final answers as long as we do not know what about 95% of our universe actually is (dark matter and dark energy means dark understanding).

PeroK and weirdoguy
Drakkith
Staff Emeritus
The perimeter of a circle increases by radius, the surface area of a ball increase by radius(which is height which is the third dimension if the ball is a planet like the Earth), and the universe is expanding by time, can we say that the fourth dimension is time by this ?

No. Time is not a spatial dimension. Note that it is not required that there be a 4th spatial dimension into which the universe expands. The idea that the universe is a finite volume of some shape (such as a 2-sphere, or ball) that is somehow embedded into higher dimensions is not believed by cosmologists to be accurate.

But some cosmologists think that it is expanding into a 4d space.

Do you have a reference for this?

russ_watters
phinds
Gold Member
... some cosmologists think that it is expanding into a 4d space.
I suspect that you have misunderstood something you read or more likely you are basing that statement on pop-science, which is not accurate.

berkeman
We have two rather confusing point of view. 1. It is expanding into nothing or 2. What is the 4d space doing; expanding into a 5d space? And so on!

It's rather weird that you wrote something like this since in your "new member introduction" you said that you are an astrophysicst. Anyway, it's a standard knowledge that there are no "two confusing points of view" but only one and very clear that an "expansion of the Universe" is a wording for the existence of the scale factor in metric tensor of FLRW spacetime. A scale factor which is increasing and makes gravitationally non-bound objects "move apart". Knowing what it really means (that is: knowing the maths) makes it rather clear that there is no need for questions like "what does Universe expand into".

davenn, russ_watters, phinds and 2 others
It's rather weird that you wrote something like this since in your "new member introduction" you said that you are an astrophysicst. Anyway, it's a standard knowledge that there are no "two confusing points of view" but only one and very clear that an "expansion of the Universe" is a wording for the existence of the scale factor in metric tensor of FLRW spacetime. A scale factor which is increasing and makes gravitationally non-bound objects "move apart". Knowing what it really means (that is: knowing the maths) makes it rather clear that there is no need for questions like "what does Universe expand into".

I taught a course in general relativity and cosmology from 1970 until 2014 and have followed its progress closely. That was always my favorite course. My research was primarily on uv spectroscopy of interacting binary stars Using various spacecraft capabilities.

weirdoguy
I taught a course in general relativity and cosmology from 1970 until 2014 and have followed its progress closely.

Then you should know that there are no "two rather confusing point of views".

davenn
Then you should know that there are no "two rather confusing point of views".
What about branes? Is it really inconceivable that our universe is enveloped by a higher dimensional Spacetime? The problem is that we conceive of many strange ideas. Without observational or experimental data these concepts are almost invariably wrong.

We are talking about classical, experimentaly well established general relativity. Branes are off-topic here and if that is what you had in mind all the time then you should say that in your very first post.

phinds and PeroK
Okay, no more metaphysics here. The

weirdoguy
PeterDonis
Mentor
2020 Award
The question is if the universe is creating Spacetime as it expands

Spacetime doesn't get "created". Spacetime is a 4-dimensional geometric object that already contains all of the history of the universe. It just "is".

What about branes? Is it really inconceivable that our universe is enveloped by a higher dimensional Spacetime?

Discussions of speculative hypotheses like branes, if they are based on published literature on them, belong in the Beyond the Standard Model forum, not this one.