This sounds like a measurable (in principle) pressure. But you say it cannot be locally observed. So how do we actually know it is happening?
I have heard for many, many years that "space itself" is expanding, but never got a sense of what that meant from all the explanations. It's always sounded to me like doubletalk. Probably because I'm dumb.
What I have heard about observations of the expansion of the universe is that objects appear to be moving away, and I've never been able to understand how we know it is not "ordinary" motion.
This just sounds weird, like I thought that Newtonian or Einsteinian physics explained objects with velocities regardless of what those velocities are. I thought there were somehow complete, you know?
I don't really understand any of it, but I read a popular account of relativity by Einstein, and you know, the examples were very 100 years ago, trains, light, geometry and stuff. No magic.
Space literally expanding seems like this star trek thing jumping out of nowhere, and it's confusing to me where it comes from, even though I appreciate that it's orthodoxy nowadays.
Maybe the point is the magic has something to do with general relativity?
So the I wonder if everything expands along with the universe, does it mean that from outside, things moving at the speed of light in our universe, are potentially moving at speeds greater than the speed of light within our universe? If i was watching from outside, a rectangle moving at the speed of light in our universe, would one of its edges be moving away from me at the speed of light + the speed of expansion? If it makes sense.
I know you want me to cheat and set aside all of these complications and just imagine a rectangle moving at the speed of light, but such an answer would be nonsensical.
But the answer to the more specific question, "does the expansion of space make something move faster than the speed of light?", is "no", but it does cause the distance between objects to increase faster than the speed of light. To an external observer (which is inherently difficult to reason about, for the record), these objects would appear to "move" away from one another faster than lightspeed. However, the very idea of "movement" is rooted in the context of spacetime, and is bounded by the speed of light, so it's a confusing/nonsensical question.
e.g. after 10^18 seconds, would the diameter of a hydrogen atom still be the same? or is it scaled to be huge (relative to now) ?
Maybe this is the ‘wind’ preventing the electron from reaching the nucleus :)
However, that questions a different future I’ve seen in popular science talks: That the future becomes dark because of expansion. That makes less sense to me now, since it would seem that a merged set of galaxies would keep plenty of local stars in the skies. Assuming that stars are born as fast as they die (but I don’t recall seeing an answer to the ratio of star births vs star deaths at a local level).
Also, the age of star formation will eventually end. Stars will stop forming somewhere between 1 trillion and 100 trillion years in the future.
If I understand what you’ve written already, the essence of the answer in terms of mekkkkkk’s question (for reference: “ Sorry for also asking a space 101 question, but since the fabric of space is expanding, does that mean that things are getting bigger as well? Naively, it would seem that way. In your balloon example, the dots would get bigger.”), I think the answer would be:
Anything held together (for example by gravity, or the force that keeps out atoms together) is not growing along with the universe simply because those forces are (for now) stronger than the expansion.
This does produce an interesting side question though: aren’t we (humans) shrinking in relation to the universe? Or, without the click-baity phrasing: isn’t the universe growing without us?
Yep.
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> However, local forces like gravity and the strong and even the weak nuclear force are more than sufficient to hold everything together despite this, so the expansion cannot be locally observed.
After some time, the scale factor of the universe grows, so that 1 new meter is 1.05 old meters. But, by hypothesis, as this happens, our meter stick contracts, so that it ends up being just 0.952 new meters (= 1 old meter) long. Why? Did the physical laws governing iron atoms change so that they pack more densely than before?
And if so, why do all particles pack at exactly the same new density? Shouldn't it differ from molecule to molecule?
The dominant factor on this scale is the strong nuclear force, which governs interactions at the scale of quarks - much smaller than molecules.
In the sense that it did not become 0.952 meters long. It didn't "become" anything - it did not change at all. The universe grew around it, or perhaps "underneath" it. More space appeared for it to exist in. It's like putting an iron rod in a 10 meter square house, then moving the walls out another meter.
Edit: let me phrase this another way. Let's imagine that you and I were pulling on the bar at either end. We could pull it forever and ever and it would never come apart (unless it rusted and became brittle or something, but let's imagine it doesn't). That's because we aren't putting in enough force to overcome the forces which are holding it together. Well, the expansion of the universe is putting like 10^-20 times less force into it than our arms would be.
This is a very different idea from "it's the same amount of space, but bigger", which is what I understand by the claim that a scale factor is increasing.
Space itself is getting larger, everywhere, all at once. The scale factor of the universe is getting larger. New space is constantly being "made". Every second, every kilometer in the universe grows by 2.2×10^-18 centimeters. If no other forces were in play, it'd carry everything along with it and pull everything a bit further apart. However, local forces like gravity and the strong and even the weak nuclear force are more than sufficient to hold everything together despite this, so the expansion cannot be locally observed.
That's a small number, but the universe is very big. The hubble constant is usually represented in km/s/Mpsc (kilometers per second per megaparsec), and in those units it comes out to 68. So, every second, every Megaparsec in the universe grows by 68 kilometers. The more parsecs between us and some distant thing, the more space is growing between us and them every second.
For reference, our nearest galactic neighbor is Andromeda, which is 67 kpc away, so about 100 meters of new space is created between our galaxies every second. That being said, Andromeda is heading towards the Milky Way at 110 km/s, so it easily overcomes this and will crash into us any day now^W^W^Win 4.5 billion years.