On the traditional balloon model, the surface of the balloon represents space itself. Those dots on the surface of an expanding balloon are not moving relative to space (the surface of the balloon), and so despite the distance increasing between us, the rate of increase of that distance is not bound by the speed of light, which only limits passage through spacetime.

The wikipedia page on redshift describes well how redshift applies to objects which are in space that is expanding: https://en.wikipedia.org/wiki/Redshift#Expansion_of_space

For any still confused, there is also a 'raisin bread' model (which I prefer because it's 3D, unlike the surface of a balloon). The raisins are galaxies, and the bread is the space between them. As it bakes, the raisins get further apart, even though the raisins aren't moving through the dough.

(Although galaxies also have movement through space, this isn't what is causing them to get further apart.)

Raisin bread, oh I love thee even more today.

The balloon is a 2D expansion in 3D space, which is a better analogy for our 3D universe expanding in 4D space. From the viewpoint of a particular dot you see the exact same thing no matter what 2D direction you look. There is no center.

I agree that the balloon is more confusing, but it is more correct in that it doesn't lead to ideas like "the point where the big bang happened".

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

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.

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.

https://news.ycombinator.com/item?id=24424626

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.

(thinking emoji)

(thinking emoji)

(mindblown emoji)

> 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.

No, there is no scaling of matter or light. It just has a bigger space to play in.

I have a terrifying theory on this (granted, the Earth will be long gone). What if the universe is not actually expanding in the sense that it is being propelled by an initial explosion, but instead falling towards a much greater mass - the end of time so to speak, where all mass ends. Considering that gravity is a "weak force", the scale of such a mass makes me uncomfortable to think about.

Perhaps what we observe as the fabric of space-time itself "stretching" is actually a similar process to what happens to matter as it enters a black hole?

We can see the past, but we can’t go there, like an observer inside event horizon looking out, they can see light coming in, but can’t leave.

We can’t see the future, but when we are going there, like an observer falling into the event horizon. They can’t see what’s inside, but they can fall inside.

As we experience time, it’s like we are falling into a black hole made of time.

Carlo Rovelli and others propose that black holes actually only appear as they do to us due to the extreme time dilation, and similar to how a photon's "perspective" is that it's experiencing all of time at once, a black hole's perspective is that it's basically a massive explosion that occurs at essentially the same moment it forms. So under this hypothesis, a black hole never reaches a true singularity, but as close to a singularity as physically possible, and then it almost instantaneously bounces back like a taut rubber band that's stretched back as far as it'll go. From our perspective on the outside, it's an intense explosion occurring in very slow motion, and from the perspective on the inside, it's an intense explosion happening all at once.

Rovelli posits a Planck star, not a singularity, exists past the black hole's event horizon, and the Planck star is the state of maximal compression between the black hole and white hole phases. And somewhat related to this, if I understand the theories correctly, our universe might be a supermassive white hole, and we might hold the perspective of being inside of a super(super)massive black hole that's turned into a white hole and is exploding and jettisoning everything inside of it. Or some stage after that. The ultra-compressed energy before the Big Bang would be an (extremely massive and dense) Planck star.

I have no idea how likely the Planck star theory or white hole universe theory might be, but my blind speculation is that if it's true, maybe dark energy is whatever's fundamentally responsible for gravity/curvature causing our white hole explosion to bend some outer medium - the hypothetical potential medium which our black hole/universe originally formed in - and interact with other extreme contractions and explosions also happening in that medium, in complex and multivariate ways. Maybe a white hole being pushed into or prodded or yanked by many other black holes and white holes from different directions and angles could be perceived from the inside as accelerating expansion of space at certain scales.

Maybe this is happening recursively, even; maybe that outer medium is itself within another even greater medium, which also is within another greater medium, etc. Maybe the nesting depth is small, or near-infinite, or infinite. Or maybe there is no outer medium to our universe, in which case that whole idea's shot.

And I'm not sure if this could be related, but Rovelli suggests the hypothetical Planck star cycle of black hole to white hole applied to primordial black holes that formed in our early universe could explain dark matter, so even without the outer medium idea there could be something here that helps us understand our large-scale observations. Maybe there could be some relation to the "dark fluid" and negative energy idea, too: https://en.m.wikipedia.org/wiki/Dark_fluid

I have absolutely no clue if your or my speculation is reasonable or not (there's currently zero evidence of any sort of outer medium, as far as I know), but the general concept of our universe being the result of a black and/or white hole does definitely seem to interest a lot of physicists.

I’ve been yelling like a crank for a while that a black hole is actually a massive explosion we see in (ultra) slow motion, so it’s really cool to hear I’m not alone, and that other people call it a “planck star”. I haven’t heard anyone connect that to the big bang being a white hole, but it does make a lot of sense, even if we currently have no direct observable evidence or even any theoretical predictions.

Thanks for sharing, now I can follow along the research.

Maybe the downvotes (which seem to no longer outweigh the upvotes?) are because you said "is actually" rather than "might actually be". Humility and uncertainty is crucial in science. Even for a field that's much more testable, and even for a leading scientist in a field, let alone some random speculator on the internet. It's fun to think and talk about these things, but especially as total amateurs, we should never claim anything with even a little bit of confidence.

My own speculative stuff I posted above is probably all dead wrong and hopelessly confused; I'm just throwing it out there as stuff that's popped into my head. And Rovelli may be wrong, too, though he's a highly-respected theoretical physicist and has published several papers about it. And even if the Planck star and black hole/white hole theory is true, it may not necessarily mean it has anything to do with how our universe started; that's just additional unsubstantiated speculation by me and some other people, and not anything Rovelli has claimed, though his theory seems to have increased discussion about that topic.

Here're some resources:

https://phys.org/news/2014-02-astrophysicists-duo-planck-sta...

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

https://en.wikipedia.org/wiki/White_hole#Big_Bang/Supermassi...

The situation you described would result in a game of universal pong, in which matter is passed from one side of the universe to the other before reaching a critical point where it bounces back. Obviously I have no means of validating the "Plank star beyond the horizon" theory, but nor can anyone else can prove differently at this point.

Kind of like a graph of 1/x, where everything behaves predictably, but there's that one point that doesn't fit?

Rovelli hypothesizes this never actually happens and is not physically possible, and rather, when the most physically extreme possible level of curvature does occur, it "snaps back". If true, this ensures black holes do actually behave predictably, and physics doesn't actually break down; it just gets as extreme as it can possibly get.

So under this hypothesis, x gets really high, but the universe prevents it from ever reaching infinity. This would also potentially resolve some paradoxes related to black holes.

I really appreciate your comments! If you're interested in hypothesizing about physics, I think it would do you a lot of good to learn at least a bit of math to go with it.

Not disagreeing, I just wanted to suggest that the ideas come first, right? The math simply frames the ideas in a way that we can quantify and discuss, while also providing a "line in the sand" that other ideas must be compatibile with in order to be accepted among other the ideas that have already crossed that line. Those are very important things, but I would suggest that they are merely supporting an original idea that does not require math to intuit.

I almost never really think or talk about physics in general. This thread just caught my eye.

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

For the expansion to have a significant impact on relative distances, the distances involved have to be so extreme that there's no real other interaction between the objects you're measuring from.

That doesn't contradict strict conservation of energy - it just adds another energy source that has to be accounted for when balancing everything.

It might contradict the aphorism "energy is never created or destroyed", but I'm not sure that was ever science.

It's more fun to think about it in terms of thermodynamics. Can that extra energy be used to reverse the second law of thermodynamics? I'm guessing not - it's so diffuse that it's probably impossible to use it to reduce entropy. The math for actually doing those calculations is well beyond what I know, though, so I'll just say that one's no more than a guess.

Like other gravitational effects, locally, spatial expansion would manifest as a pseudo-force, but be too weak to overcome the nuclear and electromagnetic forces that keep matter bound.

However, Friedmann cosmology is only a large-scale approximation. A more realistic model is swiss-cheese cosmology, which comes with a different metric within the 'holes', and spatial expansion might indeed not act on galactic scales (though I would image it to have an indirect effect as boundary conditions at the edge of the hole have to be matched; I'm no expert on this part of the story, though).

The usual response of “it only affects space between galaxies not inside galaxies” is deeply unsatisfying, and feels like it must be an oversimplification.

What I think is meant, but I absolutely want someone wiser to confirm or refute, is that the rate of expansion is proportional to distance, and therefore so small inside a single galaxy it can be ignored.

What I don’t know is: can any orbits still be stable in an expanding universe?

(In theory I could simulate the orbits question easily; in practice, last time I tried to do astrophysics maths I made an elementary mistake).

It obviously affects all space, inside galaxy or outside of it. Let's just check the rough math, not even bothering about the fact that expansion was not linear. The universe expanded to around 100BLY in about 10BY, i.e. got ten times bigger than its potential light cone. So, for every "initial" 1km, there is additional 9km in 10BY timespan, or 0.0009mm per year, or 0.00003nm per second. The general ratio is around 1e-17 per second per length unit.

Proton size is around 1e-15m, Planck size is around 1e-35m. A thousand Planck units per second is not that much, really. And on sub-milliseconds it just loses its effect completely, due to that Heisenberg guy. (If we take my bullshit math seriously, I mean.)

Of course, if you calculate out far enough time, you would eventually end up with the observable universe getting smaller (because the stretching of spacetime would take away more space than was added by light reaching us from further away), down to our galaxy, and eventually everything getting torn to shreds, even down to protons...

In the "near" term, the effect is so small that we have much more to worry about from the sun expanding in a few billion years than the Big Rip.

I am not that person. But I feel like this can't work -- if all space is expanding equally, in proportion to its own extent, then we would perceive no expansion at all. If two points start out 1 light year apart, and over time 1 light year expands to be 10% longer than it was before... and also, everything in the universe expands to be 10% larger in all spatial dimensions... then the distance in meters between the two points hasn't changed, because meters grew too.

Really? These structure constants never involve any units of distance? Isn't distance changing at all levels?

If I have a meter stick, and then space expands by 10%, you're saying that my meter stick will end up being 0.91 new-meters long, yes? Wouldn't that require the structure constants to change, to allow for the same molecules to take up less space?

Let’s come at this from the other direction: if everything including the space between atoms and galaxies expands by 10%, how do you tell the difference between an old-meter and new-meter? For this to be observable, something has to still scale by the old-meter, or you wouldn’t be able to observe a difference at all. That something would be the physical constants with units, like the speed of light. This means the electrical force governing the chemical bonds in your meter stick will still try to restore the distance between them to their original old-meter distance, but in new-meter units.

Then if you define the actual meter by the length of your meter stick, or the physical constants, you end up getting the same meter that everything not macro-scale “snaps back” to.

The meter-long stick you put into that space to measure it is still one meter long at some time in the future because the force that pulls the space apart (dark energy) doesn't pull on the matter that makes up the meter stick strong enough to overcome the forces that hold the stick together. But even if it did, the idea of one meter hasn't changed its definition. It's just the tool you used to measure one meter has broken.

Every second, the tiles on the floor push away from all their neighboring tiles by one tile width and new tiles appear in their place. The tiles are small enough that the dancers are easily able to adjust their step/grip on eachother to simply maintain their dance routine.

However, even tho the dancers can stay together, the couples drift apart at an ever increasing rate. They start 10 tiles apart, after one second the couples will be 20 tiles apart from eachother, 40 tiles apart after 2 seconds, 80 tiles apart after 3 seconds (a new tile is being inserted between each pair of existing tiles). My tiling math is probably slightly off, but I think this illustrates the exponential increase in expansion.

The dancers are quarks, atoms, molecules, planets, galaxies, etc. Their grip on eachother is big enough to overcome the relatively small adjustment in the space that was created between them.

A few questions using this analogy arise.

Do the new tiles actually appear between the dancers, or is it more a case of the force of friction the dancers are applying to the floor prevents those tiles between them from moving apart ?

So when 'new space' is being created, is it created around strongly bound things and the things simply slide around through this new space to maintain their current configuration, or do the 'strong' forces prevent new space being formed around them ?

If the new space is not being created, what happens to the energy that would have gone into the creation of the new space ? Does it simply get applied to the nearest point where it can squeeze in more space ?

If the space is being created and the dancers pull themselves together to overcome it, where does this extra energy come from ?

As for the big rip, I always thought it was the equivalent to the dancers finally getting tired and not being able to hold onto eachother hence giving in to the expansion of space. ie. It happens around the heat death of the universe when everything has dissipated its kinetic energy so much that the energy of the expansion is more than is left in the system to keep things together.

“Due to the expansion increasing as distances increase, the distance between two remote galaxies can increase at more than 3×108 m/s, but this does not imply that the galaxies move faster than the speed of light at their present location (which is forbidden by Lorentz covariance).”

But from outside we can move faster than the speed of light? Sorry having trouble getting my head around this.

I think you're getting tripped up on a variation of this idea, e.g. moving a laser pointer across the moon's surface faster than light. https://www.universetoday.com/109147/how-a-laser-appears-to-...

That's unknown. The observable universe is all we know for sure and there's evidence that there is both something beyond what we can observe, which conforms to what we know as space and matter, and something beyond that (whatever existed around the big bang locality).

By definition, there can't be. Evidence implies something we can observe, which would mean it isn't beyond the observable universe.

That's an arbitrary definition. We can't observe our local past events or even non-local events, yet we use them as theories/evidence depending on context (eg Red Shifted galaxies as evidence of expansion). Reasoning about past events would affect other future events is just as much evidence. That's how physics experiments are generally conducted.

Are you saying the observable universe is bounded to our Hubble volume?

Of course, but basic reasoning isn't evidence. Again... "evidence" implies something that can be observed, and something that can be observed, by definition, lies within the observable universe.

>Are you saying the observable universe is bounded to our Hubble volume?

I'm saying the observable universe is bounded to what we can observe, and evidence of something beyond the observable universe is a contradiction in terms.

I don't know how I can possibly make it simpler.

great sci fi plot, the earth is the center of the universe because anything out side of our sphere can’t be observed by our consciousness and ‘collapse’ into existence :)

but if the usual laws of physics apply to that outside "observer" then it will not be able to observe photons from those objects.

the usual assumption is that space is basically infinite but we can only observe our own "observable universe" sphere of it. every point in space has its own universe basically.

see also cosmic light horizon: https://www.thedallasgeek.com/single-post/2018/06/11/Our-Cos... (the difference between regions we can communicate with and regions from where a photon coming toward us will eventually reach us)

This is a conflation of two different things. Most of the current evidence points to a universe which, when it began, was already infinite in size, but infinitely dense. In that model, it remains infinite in size, but is also expanding.

The answer to this in the early universe is called inflation, which you can tag onto Alan Guth around 1981 or so. It involves spacetime itself stretching. Originally, everything was nearby and so it was all evened out -- about the same temperature, density, and so on, then it just gets pulled apart and "writ large" in a twinkling.

Physics stack exchange

Its a lot like looking at a number line. That is infinitely long in both directions. And if consider all real numbers, it is infinitely expandable.

First of all, the big bang didn't happen somewhere, it happened everywhere. There's no "center" of the universe that everything is moving away from. The universe did not start from a point, it started from a small scale factor and the scale factor rapidly increased.

Assuming the universe is infinite (we're not sure), you could think of it as expontentially more infinities being created all the time. Say you take a 1-dimensional line and put tick marks 1cm apart, to positive and negative infinity. Then you multiply every tick mark position by 2. Two times infinity is still infinity, but the nature of the infinity has changed: it's scaled up by a factor of two. You could then subdivide it again, adding markings in between, 1 cm apart. You can think of these new markings as representing new centimeters which have appeared on your 1D line. This is happening to the universe in three dimensions.

Now, instead of doing this by discrete intervals, just do it constantly, so the scale factor is always expanding. Any two points are always moving apart from each other at the rate of this expansion, let's say 1cm/cm/sec. Now, introduce an attractive force between objects (e.g. gravity) which obeys the inverse square law. If the objects are sufficiently close to one another, they'll move towards each other at a rate faster than 1cm/sec, which would overcome the expansion force and allow them to remain together. In the real universe, the fundamental forces have this effect, and this is why we don't observe everything flying apart on local scales.

Now let's say that the maximum speed limit is 10cm/sec, and light travels at this speed across our 1D line. If we have two objects 2cm apart, then the distance between them will grow by 2cm in one second. In that time, light from each will have travelled the same distance and met one another. Now, they're 4cm apart, and in the next second will travel 4cm apart, twice as fast. Light can still make this journey. At the next tick, they're 8cm apart, and light still makes it. 2/16th of a second later and they'll be 10cm apart and moving away from each other at the speed of light.

At this point, no future emissions will reach them. However, we haven't been emitting discrete light pulses - galaxies continuously emit staggering amounts of light. So, the space between them is full of light which is constantly making the journey towards the other end. If no other factors were at play, they'd still be receiving old light from each other forever. But there are other factors at play: the inverse square law, combined with the increasing redshift, will eventually reduce the light to effectively nothing.

These factors are sufficient to extend the effective observation duration of distant objects well after they've crossed the lightspeed event horizon. To recontextualize this into our real universe, we can look at extremely distant objects like GN-z11, which is 32 billion light years away. If light were to re-tread this distance, it would require twice the current age of the universe to complete the journey. However, this light only took 13.4 billion years to reach us, because it was emitted when the universe was much closer together, and carried towards us on the "flow" of expanding space time, while the object was carried away from us on that same "flow" - faster than the speed of light.

Oh, and it's also worth noting that the rate of expansion wasn't always consistent. Check out this wiki: https://en.wikipedia.org/wiki/Inflation_(cosmology)

https://news.ycombinator.com/item?id=24424912

But I'll give you another analogy anyway. Say you put two rocks into a mass of water in an environment without gravity (e.g. the ISS), then freeze the water, causing it to expand. The expanding ice will cause the rocks to become more distant from each other, but they're not "moving" in local terms - the manifold in which they exist is itself expanding. That said, the frozen water and the rocks both exist in space, and the expanding ice moved the rocks through space. But, in this metaphor, there is no second, external "space" - the water itself represents space.

And so to for our universe. Space itself isn't a part some other, second "space" that it's expanding into (at least not so far as we're aware). The existence of somewhere for space to expand "into" is not necessary for the expansion to take place. Remember, it's the scale factor which is changing, not a discrete object (i.e. the universe) physically expanding.

But yes, it is accellerating, and if it continues to do so, eventually it will overcome gravity. But like I said, this depends on an incomplete understanding of physics, and the current evidence suggests (inconclusively) that this outcome is unlikely. Check out dark energy if you want to learn more.

all we have is the sphere of light that can reach us. That doesn't imply anything about a global universe frame