| HN Mirror

I'll get to your questions below. If you want any of the preliminaries or the non-tl;dr answers explained a bit more simply, say so in a reply and I'll do my best in response.

Your questions (if you pardon the expression) poke holes in the balloon analogy. The latex or other stretchy balloon material is denser and under less tension around dimples. Local experiments by a (spatially) 2d observer could determine these features experimentally, and is likely to determine that there is a shear force that is not confined to its 2d "world". The 2d observer could in principle also do geometry and discover the amount of large-scale positive spatial curvature of its "world"; when we do that at cosmological scales we find flat or even slightly negative spatial curvature. The 2d observer could also discover the gravitation of our world: put a drop of water somewhere on the surface of an inflated ballon, and that drop will tend to roll downwards. We haven't found anything like that.

Humanity has looked for forces that give even the slightest evidence in favour of extra spatial dimensions, but there is no experimental evidence that favours having more than our familiar three. We've also looked at many many ways in which space could be some sort of medium comparable to the balloon latex, and practically none of them has survived contact with experiment (and those that survive are mostly hard to analogize with stretchy latex, even when entropic forces -- those are why you can scrunch or inflate a balloon and when you release the scrunching-pressure or internal air pressure the balloon relaxes to pretty much its original shape -- are relevant gravitationally).

> Do we know if dimples on the balloon expand as fast as the rest of it

tl;dr: yes: the material of galaxy clusters collapses gravitationally; galaxy clusters expand away from each other.

The scale of cosmology is such that galaxies are considered so small that you can treat the entire collection as a set of fluids or a dust that dilutes with the expansion of the universe. The part of any given galaxy that's mostly protons is a mere "dust mote" that floats in free-fall. And the entire dust expands, we don't capture local gravitational collapse.

However, physical cosmologists can also take gravitational collapse into account, for instance to study structure formation ("why are there filaments of galaxies"?). Typically we would take the cosmological expanding spacetime and embed within it "vacuoles" which are collapsing spacetimes, i.e., where the dusts tend to concentrate to a single point over cosmological times. These would typically represent a galaxy cluster. We have some mathematical techniques to figure out what happens at a "junction" between the collapsing spacetime and the expanding spacetime, and the junction is usually at the point where the influence of the collapsing mass is very small. This approach accords well with a lot of observations of how radiation leaves galaxy clusters, and how matter might fall into galaxy clusters from "the great beyond" represented by the expanding matter fluids. It also lets us use much more complicated models of matter ("enriched chemistry" is the jargon) within the vacuole while ignoring it in the mostly-diffuse-hydrogen extragalactic space, which is useful for figuring out how galaxies assemble and how their first stars ignite.

> Do we know if the dimples on the ballon expand as fast as the rest of it?

> Do the areas around the dimples expand faster?

tl;dr: (1) yes, we know, and are improving accuracy and precision (2) space expands between clusters of galaxies, and matter out there dilutes away; matter within clusters of galaxies tends to concentrate into stars, black holes, and the like, so the behaviour is really opposite.

The "areas around the dimples" are analogous to the expanding cosmological spacetime. The dimples themselves are analogous to a gravitationally bound galaxy cluster, best represented with a collapsing spacetime. So, it's practically a question of the sign of the expansion changing near galaxy clusters, rather than the magnitude.

> Do we know that the balloon has always been inflating at the same rate?

We know it hasn't been.

The universe's expansion history isn't uniform. For illustrative purposes there are two interesting "eras", while I'll take in reverse:

The dark matter dominated area, which we are in, has an relatively quick expansion rate, which appears to be getting quicker. This is captured for most practical purposes by the cosmological constant, although we're looking for more complicated representations of the increase of the rate of expansion during this era.

The matter dominated era, which ended about 4 billion years ago, had a relatively slower expansion because the universe's matter was dense enough to overwhelm the acceleration of the expansion.

The ESA article linked at the top is essentially about improving our understanding of the expansion of the universe in these two eras.

The Cosmic Microwave Background formed fairly early in the matter dominated era, then there's a gap of a few hundred million years before we get stars and galaxies. That gap is the "dark ages". We have very little data about the expansion history during the "dark ages", but good data from after them and good data from before them. The early and late data imply slightly different things about the expansion history of the universe, and that presents everyone with an interesting puzzle with lots of ways it might be solved.

One possible solution was, "The Hubble space telescope (HST) data was wrong or misleading because of the instrument's history or what part of the spectrum it looks at". That solution (like similar ones) now seems much less likely since the JWST (newer, not known to have ever broken down or been in need of repairs, sensitive to longer wavelengths than HST, and farther away from Earth) data supports the HST results.