[maaaaattttt]

On a somewhat related note I'm wondering what the expansion of the universe means for our bodies and matter in general? I think, like the accepted answer suggests, the forces on the atomic level make it so that larger structures get back to a certain equilibrium even if constantly streched equaly in all directions. But I have a hard time imagining what the universe expanding really means on a human/solar system scale. I know of the inflatable balloon analogy, but to me, matter is not on the ballon, rather it is the rubber the ballon is made of.

I have never seen this really explained in details to the general public which I belong to. Maybe that's a sign I'm completely misunderstanding the subject though.

[spauldo]

Matter isn't pinned to the space it's in (source: try walking around). As space expands, the other forces which are orders of magnitude stronger than the expansion of space slide matter along so that distances don't change. You can only detect the expansion of space by measuring the distance between things that are so spread apart that the other forces between them are essentially zero.

[maaaaattttt]

That's what I understood from the explanation on stackexchange. But given what you said, if we take the plank length as the shortest length unit, and we consider two theoretical "objects" placed at one plank length away from each other. Does the universe expanding for these two objects mean: 1. the plank length is becomming bigger, 2. more plank lengths are added in between the two objects, 3. Something else and I'm completely off

[spauldo]

The Plank length thing is just going to add confusion here. Quantum stuff is weird. That's where analogies go to die.

But generally speaking, the answer is 2. That's assuming there's no forces between the two objects.

Space doesn't expand like the outside of a balloon or a rubber sheet - I hate those analogies because they give you the wrong idea.

I'm not aware of any major current theories that say space is quantized, or any theories that have a way of pinpointing a "piece" of space, so the following analogy is flawed. But it should at least point you in the right direction.

Draw a line and put eight dots on it. Draw arrows pointing to the fourth, fifth, and eighth dot. We'll call those dots A, B, and C respectively.

We're going to measure distance by dots. Dots A and B have a distance of 1. Dots A and C have a distance of 4.

Now for each dot, add a new dot before and after it. Measure the distances. Dots A and B now have a distance of 3. Dots A and C now have a distance of 12.

Repeat. Measure the distances. Repeat again. Measure the distances. You'll notice that the distances between the dots you've marked is increasing faster with each repetition, and that the distance between A and C is increasing faster than A and B. An object at any of those points would not be experiencing any force - nothing is pushing or pulling on them - but an observer at any of those points would observe the objects at the other points to be accelerating away from them.

That's sort like how space expands. Of course, space doesn't have "points" as far as we can tell, so there's all kinds of problems with the above analogy, but hopefully it helps.

[mr_mitm]

It's 3), I'm afraid. There is no shortest length unit (as far as we know), the Planck length is a constant, and its only significance is that at these length scales, we need a theory of quantum gravity to describe what's happening.

[eggn00dles]

Matter is very much pinned to the space its in. If the space between two galaxies expands, the distance between the two galaxies grows. If matter wasnt pinned the distance between the two galaxies would remain the same despite the expansion.

Things dont fall to the ground because the earth pulls on them. Earth is pulling in the space around it, and those things come with it. See the river model of general relativity for a more thorough explanation.

[spwa4]

I think the point is that the force of gravity is so much stronger than the expansion that the (equivalent) force a normal-sized star exerts on your body from the other side of our galaxy is greater than the force expanding space between you and that star. One of them, not all of them together.

All of them together are so much stronger it's not even funny. And that's for the "underdense" region that we are in. Not a void, but about half of our galaxy's environment does count as a void.

[spauldo]

You apparently didn't try walking around. Give it a shot - you'll find that the matter you are made of isn't pinned to the space it's in.

Gravity ensures that structures at the cluster level and below don't expand as the space they're in expands. The space they're in is expanding just like it is everywhere (assuming a cosmological constant) - gravity just holds them together. Which is what I mean when I say matter isn't pinned to space - it just slides through it.

Gravity is too weak to affect distant objects, so we see the effects of the universe's expansion when we look at them.

[eggn00dles]

i walked around and it involved force and energy.

i will clarify. in the absence of other forces, matter is indeed pinned to the space it is in.

you talk about gravity as if its something distinct from what is driving the expansion of the universe.

we have that model, its what newton proposed.

[spauldo]

I believe all of the current theories being seriously considered hold that whatever is causing expansion isn't gravity, so yes I do talk as if gravity is distinct from what is driving the expansion of the universe.

Regarding being "pinned," that still fails to account for inertia. The idea that there's a specific piece of space that we're stuck to implies there's a rest state at which there is no motion independent of any observer. We know that's not the case.

My original point was that gravity and the other forces that hold us together are so much stronger than whatever is causing expansion that the expansion of space doesn't affect us at small scales. The space we're occupying is expanding. We're not dragged along with it. The Triangulum galaxy doesn't move away from us because gravity keeps the Local Group together. We do see the expansion of space between us and distant objects, but that's because there's no force strong enough to hold those distant objects to us. That's not because we're "pinned" to our location, but because the space between us is getting larger.

[eggn00dles]

Not so long ago, noone knew about the weak force, and even the most brilliant minds would never think electricity and magnetism are two sides of the same coin. Now we have a theory that unites all three under the same umbrella.

In many interpretations of GR it is believed that mass deforms spacetime, but is also influenced by the spacetime it moves through. In the absence of mass, spacetime expands. Carrying along any mass with it. When mass is present this default expansion is overpowered by its influence which causes a contraction. Hence black holes.

Gravity is still colored by classical definitions so most assume its power is unidirectional. However there is a quantitative factor for expansion in the very same Einstein equation which defines GR.

We need to stop thinking of gravity as a force acting on objects, and rather something that acts on spacetime. Many people advocate this but stop at the rubber sheet analogy, which is completely devoid of the idea that the sheet actually moves as well.

[rantallion]

> Things dont fall to the ground because the earth pulls on them. Earth is pulling in the space around it, and those things come with it.

Gravity wells aren't pulling surrounding space toward their centre. They're only pulling other masses that occupy the surrounding space.

[CGMthrowaway]

That's an interesting point. The expansion of space still impacts matter on an atomic level though. The space between atom core and electrons influences its bonding abilities and other properties

Analogy, take two attracted magnets, or two opposing electrodes, and expand the space between them. Things change

[spauldo]

The magnet analogy works here, so let's give it a try. Put one magnet on a table and then put one under the table so that it's held up by the magnet above it. Then observe what happens. This will take a while, so maybe go get a 2^128 cups of coffee.

The space between the magnets is expanding, just like space everywhere. Assuming the table and magnets are immune to deterioration over time, you can come back after several billion years and the distance between the magnets will have stayed the same. Space expanded, sure, but the stuff occupying the space didn't.

The forces that hold an atom together are significantly stronger than what's holding the two magnets in the example above.

[mr_mitm]

Expansion happens only in the Lemaitre-Friedmann-Robertson-Walker walker metric, which is a solution to the Einstein equations in a homogeneous universe. That's a fine approximation to our universe at the largest scales, but not on the scale of a solar system. Spacetime locally around the earth looks much more like a Schwarzschild solution. So we're not experiencing expansion.

If you are asking hypothetically, if a human body were floating in the intergalactic medium, then yes, the accepted answer you quoted would apply.

[oersted]

A translation would be appreciated, OP was asking for an explanation for the general public. Even highly technical people will struggle to understand this if you use such insider jargon without clarification.

Surely expansion is happening at every scale but locally other factors dominate right? To what degree? Is it mainly gravity? Electromagnetic attraction between atoms and/or molecules?

The naive mental model that I have is of two balls tied with a rubber band, each on a treadmill going in opposite directions. Since the rubber band attraction dominates, they slip on the surface of the treadmill and their distance is barely affected even if the surface underneath “expands” outwards quite quickly. Is this a reasonable analogy or is it too simplistic?

Of course the rubber band force is proportional to the distance, while the attraction forces we are talking about are inversely proportional. And I have no idea if the expansion of the universe can be reasonably modeled as an outwards “drag force” on matter. How “sticky” is matter with respect to space?

[Kranar]

No expansion does not happen at every scale. Expansion is a phenomenon that applies only when you average out unbounded systems at very large scales. By unbounded I mean systems whose kinetic energy is greater in magnitude than its potential energy.

Even galaxies themselves don't expand over time.

One question would be what do you hope to gain from a potential analogy. If you want a very down to Earth, practical and somewhat physical understanding, then the simplest and best explanation is that there are systems of objects in our universe, at very very large scales, that have a group velocity that is greater than the escape velocity needed to attract it to any other system of objects, and those systems are observed to be accelerating away from each other. No analogy is needed for this, it's just a fact presented plain and simply.

There is no theory that predicts a cause for this, but the best theory of gravity, general relativity, is a very flexible and open-ended framework that allows one to plug all kinds of different and imagined scenarios into it and see the results, even if those scenarios have no actual physical interpretation. Some people did play around with imagined results like an expanding universe, a contracting universe, an infinitely large universe, a closed but unbounded universe etc etc... General relativity doesn't predict any of these universes but it does let you explore these possibilities.

When Edwin Hubble observed that galaxies are moving away from each other then this observation was made to fit into the existing theory of general relativity. In order to take the raw observation and fit it with general relativity which interprets gravity as a purely geometric phenomenon, it did so by reframing this behavior not exactly as an intrinsic motion belonging to these large scale systems but rather as if these systems are stationary but there is more and more space filling up the universe in-between these large scale objects which gives them their apparent motion.

This is a means of reconciling the geometric view of gravity, ie. space-time, with the actual observed data. The specific technical details of how this reconciliation is performed is as OP mentioned, the FLRW metric which is here but as you said is too technical for most people to appreciate:

https://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtr...

You might then ask, what does this solution predict happens to atoms, or our solar system? Perhaps it predicts a very small and imperceptible expansion because other forces dominate, but nevertheless it must predict something, right? This is a tempting position, but it's not quite right.

The key reason is that the FLRW metric, which explains Hubble's observation as the literal stretching of space, literally don't make sense and can't be solved for systems like planets, solar systems, or even galaxies because it can only be used if certain requirements/preconditions are fulfilled.

These requirements are present only on the absolute largest scales where the universe looks fairly even/balanced, there is no center of mass, there is no region of the universe that is more special than any other region. In our solar system the sun is a pretty special center of mass and the solar system is not evenly balanced, same thing goes for our galaxy, and hence none of the models currently studied to describe Hubble's observations work for both the extremely large scale universe as well as for other scales.

[kmm]

Without taking into account dark energy or a cosmological constant (so on scales smaller than a few hundreds of millions of lightyears), in the usual cosmological model you can see the expansion of the universe simply as a remnant of the initial kick all matter got from the Big Bang. There is no active pushing anymore, it's just matter moving apart, constantly slowing down due to the mutual gravitational attraction.

So for our bodies, planets, solar systems, even galaxies and clusters, because these are bound (either electromagnetically or gravitationally), the influence of the expansion of the universe on them is not just negligible, it's non-existent.

It's a little different when wo do include dark energy and other mechanisms more complicated than a simple matter or light content. For your intuition, you can think of this as a constant omnipresent negative pressure. We have no idea how it works on scales smaller than those of the observable universe, but if we imagine it works the same on every scale, then it's an extremely tiny force constantly pulling your body apart.

[biorach]

https://m.youtube.com/watch?v=5U1-OmAICpU

"Brooklyn is not expanding"

[itgoon]

https://www.youtube.com/watch?v=gEyXTQ9do-c

https://en.wikipedia.org/wiki/Big_Rip

[ahazred8ta]

As near as we can tell, space is expanding by about 10 meters per year per astronomical unit. It's 7% per billion years. In practice, it would show up as a very small fudge factor in the tenth decimal place of the gravitational constant.

[freedomben]

Personally I like the raisins in a baking muffin analogy better as it's 3 dimensional

[randallsquared]

The inflatable balloon analogy has the advantage that, two-dimensionally, there's no "edge" beyond which you could travel.

[3cats-in-a-coat]

The whole "expanding Universe" model is repeatedly dealt blows in the past few years, so I'd accept it as a plausible, but not sufficiently demonstrated hypothesis and not waste time bothering about the impact on our bodies.

But if you will, think about it like that. All life adapts to its environment. All life. All the time. Everywhere. And the expansion is not that fast that a single generation of anything from a one day fly to a centenarian turtle, or a millennia old tree has to bother about it. It's invisible at our timescale.

I've always thought why animals were so huge at the beginning. Not just dinosaurs, but insects the size of a large dog. Maybe it's the oxygen rich environment. Maybe it's evolutionary processes shrinking size in time to optimize energy needs and improve survival. Maybe gravity somehow changed in time, or it was the expansion of the Universe or a myriad of sci-fi reasons we can come up with. But life adapted and moved on.

So that's what expansion means for us. One of millions of variables we constantly adapt to. If it's slow enough, no problem. But if it's hitting us fast like the accelerating climate change or technological progress, that... we may have problems with. That's when you see unrest, violence, crime. Wars. Famine. Suffering.

The Universe is not our problem. We... are our own problem.

[TeMPOraL]

Almost agreed, but I feel compelled to correct one thing, because it's a common and annoying misconception "nature lovers" harbor:

> One of millions of variables we constantly adapt to. If it's slow enough, no problem. But if it's hitting us fast(...), that... we may have problems with. That's when you see unrest, violence, crime. Wars. Famine. Suffering.

No. Famine, starvation, disease, suffering, mass deaths - and even wars - are exactly the process through which life adapts to slow changes. That's what it means for ecosystems to thrive, for nature to be in balance - that balance is held dynamically, by constant cycles of excessive slaughter followed by mass starvation.

We brought a lot of new problems on the table, both for ourselves and all other life, the latter of which can't even keep pace. But senseless suffering and comically painful death - that one came from nature, and we're actually successfully reducing it.