this post was submitted on 29 Sep 2024
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Good visualization but inaccurate. Space between galaxies in a cluster and even the stars in a galaxy is also growing. The difference is in scale. There's so much distance between galactic clusters and the largest structures of the universe that added up that expansion amount is so much bigger. The balloon analogy with galaxies as dots on the surface is closer since the dots also do grow some, but the balloon would have to be huge to capture a good scale comparison.
Are you sure that galaxies are growing? They're gravitationally bound enough to have organized orbits, do those orbits get larger over time?
My understanding was that in a gravitationally bound system like that, the orbits would be slightly larger (or slower for the same distance) based on the rate of expansion and the distance, but not grow any unless the rate of expansion increases. Like maybe the earth is a few angstroms farther from the sun than in a not expanding universe, but that number doesn't change as long as the expansion keeps going the same. Same for galaxies and clusters.
At the planetary scale, such a change would be completely overpowered by other orbit defining effects, like resonance, mass flow/loss, and even drag.
At the cluster scale, I can absolutely see spacetime expansion overpowering gravity.
At the galaxy level, I can't tell. Does spacetime expansion limit the size of galaxies? Is that limit shrinking due to the acceleration of expansion? Are galaxies under that limit larger than otherwise expected? Is this effect large enough to effect the speed of galaxy rotation and does it need to be taken into accout when measuring the effects of dark matter?
At the cluster level it will depend on the velocities and distances. For example, using very rough numbers the current expansion rate means that space between us and the Andromeda galaxy is expanding at 55 km/s. Seems fast until you realize the distance needed to see the effect build to this level. For perspective I found someone's calculation to reduce it to solar system level to end up with ~10 meters/AU/year. But of course at this distance gravity dominates so we can't measure that directly and it may not even be large enough to consider.
A larger and slower moving galactic cluster would be more affected than a tighter one. I don't know what our Local Group would be considered to be, but there are a hundred or so galaxies around us that appear blue shifted, so they are moving towards us even with the expansion.
That wouldn't significantly affect most galaxies though, would it? The rasin bread model might insinuate that the space in a galaxy isn't expanding (which is wrong), but it is accurate in thst gslaxies themselves are not growing larger.
Correct, the differences make the analogy good enough to visualize the concept. It does however suffer from the same problem as the balloon one, in which someone can get the impression the expansion has a center. The wiki for the expansion of the universe goes through the various analogies and where they break down.
I would suggest Dr Becky's Youtube channel for a number of excellent videos on the expansion as well as the current problem of getting an accurate measurement of the correct Hubble expansion rate. The James Webb telescope was hoped to solve that dilemma, but we still aren't sure.
I see where this is diverging a little bit.
But everything is expanding. Including matter. But the mass isn't chaning.
But this also includes the space in between the objects.
So objects are getting further apart, but so are the objects getting bigger at the same rate.
The mind bend for me was realsing it's not space that expanding really, it's distance.
This is why distant light is red shifted. Because what started out as white, has had the wavelength expand with the universe, making it appear more red.
Yes, all distances are expanding, but not everything in space is expanding. Atoms aren't expanding because atomic forces are far stronger than expansion is, for example.
Yet the distance between galaxies is increasing, so there must be a crossover point where one structure can stay structured but a slightly bigger structure is torn apart.
My question was if this size is larger or smaller than galaxies, and it seems to be quite a bit larger than galaxies at the moment.
The interesting thing is that the expansion is increasing, so this size limit is shrinking. Unless some change in forses happens (like inflation or some kind of false vacuum collapse) the limit will eventually be smaller than galaxies and they'll get ripped apart. Eventually star systems will be ripped apart too, then stars (if any remain at that point) then planets, molecules, atoms, and bosons; and if if that continues to quarks funny things start happening that kind of look like the big bang.
That last part is still speculation of course, but I do still wonder if the expansion of the universe affects galaxy formation and dynamics, and if ancient galaxies were different in part because of this.
So yes attoms are expanding. everything is expanding. I mean that very literally.
Let me put it this way.
If you had a million year old meter stick. It would always be a meter. Accurate to the definition of a meter using the wavelength of I don't remember what off the top of my head. It would always be a meter exactly.
But.
If you magically placed the meter stick next to itself from a million years ago, they would not measure the same. Even though they started with the same definition.
Like I said. Space isn't expanding. Distance is.
EDIT I don't mean the distance between things is expanding. The definition of what a distance was is expanding. So yes, attoms, when measured by size (the distance from one edge to another) has also expanded.
But in the same breath, the measured distance never changes. Because the way you use to measure distance has also expanded by the same amount. So nothing ever changes in reality, but everything is just constantly bigger.
Physics is full of hard to explain paradoxes.
What is expanding in this scenario? If atoms are expanding, then either atomic forces have also scaled to match the expansion, or atoms are getting more radioactive?
I don't understand how atoms are supposed to be expanding in this model. The size of atomic nuclei and electron clouds are governed by the strength and range of the fundamental forces. If everything was expanding in lockstep such that atoms expand but don't change their properties, then there would be no observable effects. Yet we can see the distance between galaxies not just getting larger, but speeding up.
If orbits, matter, and even the fundamental forces were expanding to match, no such change in "distance" should be possible, beyond the normal movement of matter.
If our metre stick was measured as 1/299,792,458th of a light second, then a million years later it was measured as exactly the same length but was somehow dimensionally larger, then lightseconds must have become larger is lockstep.
If that were true, this expansion could not explain the redshifting of light, as c would increase in lockstep with space, leaving light the same wavelength. Redshifting only happens when the distance between waves increases in relation to the speed of light, and so a universe with redshifting must have a difference in the rate of expansion and the rate of c scaling. Such a difference should be visible as increasing distance or an increase in the flow of time, at minimum.
In your model, everything is expanding equally. Literally everything, including the speed of light, the elementary charge, and even the plank constant, are expanding in lockstep, to the point of unobservability. Is this right?
Yes everything is expanding like that.
Look back to the red shifted light.
When a white star starts with white light, has the literal wavelength expanded to be more red looking.
https://www.esa.int/Science_Exploration/Space_Science/What_is_red_shift
It's the literal light getting shifted. So the speed of light is the same, but the distance it travels in a given time is not. Making it red shifted.
I am aware of what redshift is. What I don't understand is how you think a metre bar can expand and the speed of light increase in lockstep with it such than we can't measure the change.
Let's say we have a metre bar that's currently one unit long, and we measure it to be one metre long. There's also a galaxy a billion light years away.
Let's say the universe doubles in size after a billion years. The metre bar is two units long, but we still measure it to be one metre long, because the speed of light has doubled (presumably). We measure the light as the same length. The light from the galaxy has now reached us, and is twice as long, but is also moving twice as fast, so the wavelength stays the same. We measure the light as the same length.
Do you see my issue with this situation? How can the measured length of light change (redshift) while the measured length of light also stay the same (metre bar)?
Either redshift isn't caused by expansion, the fundamental forces and constants are changing as we expand, or space is expanding but matter isn't. We have good corroborating evidence that redshift is caused primarily by expansion. We also have evidence that the laws of physics haven't changed significantly in at least the last 2 billion years or across the universe. And lastly, we can measure the acceleration of expansion by several corroborating methods, including redshift.
I'd love to be proven wrong here, the implications of gluons being streched by expansion is fascinating.
So you assume the speed of light is the same between references frames. There not. It's always the same. The definition of a second changes such that the speed of light is always the same.
That's relativity.
Yes, relatively means that light appears to move at c in every inertial reference frame. That doesn't change how we measure distance in a single reference frame.
How can a metre bar be measured as a metre when it's one unit and two units long? We're measuring the bar in it's own reference frame each time, so relatively causes no change. Either c increased, or time slowed down to match the expansion of space. Either way, light doesn't get redshifted by expansion.
Help me understand, how does light appear to change speed over time in the same reference frame? How do we see a change of distance affect light between galaxies, but not between atoms?
The reason the speed of light doesn change is because rthe universe bends the rules of time to make it the same. So as the universe expands, the speed of light stays the same because the definition of time changes.
Like I said. The expansion of the universe isn't space expanding, it's the definition of distance that's expanding. Yes time is being fucked with as part of the expansion. But the universe doesn't hold distance or time as constant frames to compare to. As speed is only calculated with a frame of reference. Where distance is a little more fundamental to the universe.
Because the scale is so so much less. Like 73 km/s/Mpc.
So the rate of something to the scale of 10^-9m, would be somewhere in the order of 10^-25m/s. Which is much much smaller than anything with the attoms itself.
But the distance is always the same. A meter is still a meter in all points of time. But it's still bigger.
And yet you said that a metre bar would be larger yet measure the same. If all the aspects of the universe are expanding in lockstep such that any distance appears constant, then redshift caused by expansion is impossible.
If the increasing distance between atoms is unmeasurable, then so too must be the increasing distance between galaxies be undetectable.
LIGO can detect changes of distance on the order of 10⁻²¹, and it should be increasing in effective length by 2×10⁻¹²m/s, yet I don't see any mention of any large interferometer measuring anything but gravitational waves, and I don't see any large time-dependent components of LIGOs systemic error data.
We also can measure the increasing distance of galaxies via redshift, so unless you can explain how light from galaxies is different from the light in a large interferometer, I must conclude that the interferometers aren't expanding at the same rate as the observed expansion of the universe.
We aren't expanding like the universe is expanding.
We can still measure the red/blue shift to find the star, but if you don't correct for it, it will be wrong.
Also I don't know enough about gravitational waves wo know how it would be effected by the expansion of the universe.
But remember when LIGO measures, it's not measuring absolute values that we would see drift in. It's all relative measurements from a short time period prior. It would follow in lockstep with the expansion.
Also gravitational waves arent particles. They're disturbances in the fabric of the universe. So they don't behave like standard waves do. They have their own wave mechanics that I haven't studied.
And light is having its wavelength stretched. Speed of is not proportional to frequency in a vacuum only the permittivity and permeability of free space. So it's wavelength is getting expanded without
But again. Space isn't expanding. Distance is.
Also that's not how informeters work.
They compare distance across two lines. They can only detect the differences between those lines. Because expansion is universal in all directions, it's not detectable on informeters.
Actually that's a good point about interferometers, the only detectable change whould be in the difference between each arm's length.
Gravitational waves do behave like EM waves, we've seen a neutron star merger simultaneously in gravity and light. If there was a difference, one observation would lag behind.
How exactly would we measure an absolute value of distance? The whole thing about general relativity is than everything is relative. If everything was scaled up such that the fine structure constant stayed the same, we wouldn't be able to measure a difference.
Which brings us back to the question I have with your model: How can a changing distance be measured by light to be the same (metre bar) but also different (redshift)? If light is scaling with the rest of the universe, it shouldn't get shifted. This in the crux of my confusion.
The answer is there's no such thing as absolute distance. Because there's no such thing as absolute position. Quantum garuntees inaccuracies in position.
And your right. We can't actually measure the expansion of the universe directly. It's actually because of the red shift we do.
The reason we can see the red shift is because the universe holds the speed of light in a vacuum constant.
So if the universe is expanding, and the speed of light is expanding with it, in-order for the speed of light to stay the same, it has to travel more distance in a time. Meaning it's stretching it's wavelength as it moves. Just like something moving away from us does. IIRC it's because of observations that everything is constantly moving further from us, the further out you go, the faster it's moving away.
But everything is moving from everything, including itself.
I do apologize if I'm a little muddy, I did my physics degree about a decade ago.
Edit as for why gravitational waves travel at the same as E&M waves is because "information" is what travels at the speed of light. For an electro magnetic wave that's disturbances in E&M. For gravity that's ripples in the fabric of space-time. For quantum there's experiments showing that entangled particles will collapse together, if sperated by distance, the lag time is also the speed of light.
EDIT 2:
The only thing faster than the speed of light, is actually the expansion of the universe beyond a certain distance. Don't remember what it is. But because distance istself is expanding, that's proportional to distance. So the expansion rate is actually faster than the speed of light far enough out. But no SINGLE point is expanding faster than the speed of light.
If there's no such thing as absolute distance, then how can you say that a metre bar (and the metre) is larger than it used to be?
If distance is relative, and matter isn't expanding relative to anything else, then matter isn't expanding.
We ultimately define distance in terms of c, and the fundamental forces agree with this. We do not observe atoms expanding, but we do observe the space between galaxies expanding. Presumably the space we occupy is also expanding, but it's such a small effect as to be irrelevant.
Back to my original question, is the boundary between irrelevantly small and detectable above or below the galactic scale?
I think the trouble is also partly based around thinking of then universe as a volume, which implies a centre. And that's where this analogy falls apart.
Because everything is expanding from everything, there is no centre. YOU are always centre. So you are "expanding" but you don't change volume.
This is why I keep saying space isn't getting bigger, distance is.
It's not that a sheet of paper becoming bigger so the grid paper becomes larger,. It's changing it's distance of something, not it's size and shape.
We don't observe galaxies getting bigger. We observe them constantly moving away from us. Even. When they're moving to us, but it's done at a slower pace than expected. The further away you are, the faster you move away. And it's a universal constant of 73km/s/Mpsc.
Notice that is a speed per distance. It's not saying space is getting bigger, it's saying things are moving faster away from you the further you go away.
The universe isn't expanding like a loaf of bread because it has a volume. It's expanding from one volume to another. Where the universe doesn't.
Not thinking of cosmic expansion as a volume expanding is an interesting thought.
It does imply that the changing distance only happens at large distances though. "Faster-than-light" expansion is already non-local (I think), but all expansion being non-local is consistent with it being driven by vacuum energy. That kinda makes the rasin bread analogy stronger, as the rasins don't expand at all.
I wonder if we could detect frame-dragging at large distances. If expansion causes frame-dragging, then it's actually a change in space, not just distance.
I wonder if linear motion can even cause frame-dragging, or if it's just rotation that causes it. I do not know enough about the math to say.
So it does happen on a small local scale though. It happens on ALL scales.
But everything is expanding from everything. Meaning the observer is always centred of the expansion. This is because volume is constant. The rasins themselves do expand, but locally it's such a small scale (10^-23 m/s for our solar system).
This also works for how we understand the change in density. Volume is constant, but we've gone from infinitely dense to almost nothing.
We went over this, we observe the distance between galaxies increasing, but the distance between atoms has not.
The expansion happens everywhere, but subatomic forces massively overpower the expansion, so atoms don't expand.
Likewise, raisins are strong enough to not get pulled apart by the expanding bread. There may be slight force on them, but the bread expanding by a factor of 2 leaves the raisins the same size.
I don't understand how you think a change in distance can be detectable by light between galaxies, but not detectable by like between ends of a metre bar, or between electrons.
You are correct that the raisins would have other constraints to keep it from infitatly expanding into nothing. Not because it's not expanding but because it has external constraints like gravity keeping it there.
They do have expansion applied to them, but gravity and other things effecting space time would be keeping it on place.
As for attoms, I think you picture something solid. But there's not. The electrons are getting further from the nucleas, but it's still bound quantum mechanically to the attoms regardless of its position.
But then the nucleas isn't soldi either. It's made of smaller things yet, and so on and so forth. So inside would also be expanding. But again other forces at play would bind things together.
The expansion is also not a force. It can't overcome other forces so it keeps things in line.
Yes, atoms are made of smaller parts; electron orbitals change chemistry based on their distance from the nucleus, nucleons change the chances of emitting radiation based on their distance from each other, and quarks greating increase their mutual attraction based on distance.
The relative distance between fundamental particles is governed primarily by forces which don't seem to have changed much since nucleosynthesis. If expansion doesn't affect any of this, then saying things governed by forces are expanding is nonsensical.
I can see a perspective where time is slowing down, reducing the effective range of the forces and letting all matter shrink to fit the changing effective distance, and leaving unbound matter to appear to expand. However, I can't see how this would be meaningfully different from an expansion of all space, not how such a difference might be detected.
Regardless, the distances within atoms continue to behave consistently, while the distances within galactic superclusters do not.
Yes, I think they do.
I think that expansion doesn't increase distance, but velocity between objects (or so was my interpretation back when I looked at the formulas). That means that moving objects speed up over time. As such, orbital velocities increase, too, and that lifts their orbit - similar to when a rocket on a closed orbit propulses forward.
But I might be wrong; I feel 70% certain about this one.
You should downgrade your certainty. By a lot.
The expansion is an expansion of space, and therefore explicitly increases the distance between galaxies. It does not, and cannot increase the speed at whoicj those galaxies travel through that space.
Right now, there are galaxies moving away from us at rates higher than the speed of light, a thing which is physically not possible if the expansion is due to an acceleration of the galaxies themselves.
You've misunderstood things completely backwards.
Educate. They said they weren’t sure, so no need to be so critical of their knowledge. You, and all of us reading this, would be better served with a good layman’s explanation rather than you reinforcing how wrong the parent comment was. Twice.
Metaphors are great if you assume they're mostly wrong
Am I getting bigger too?
There's still a center of the balloon and bread
There’s no center of the surface of the balloon (approximating the balloon as a sphere and ignoring the nipple).
If you are standing on the earth, there is no center of the surface of the earth. The actual center of the earth requires movement in the 3rd dimension, but the surface is 2 dimensional.
If our universe is hyper spherical, the “center” would be something unreachable; it would require moving in the 4th spacial dimension.
Yeah I get that, but there is no evidence of a 4th spacial dimension, so it's really just creative writing at this point.
There is evidence of space time being curved
Yes, but just because the discrete distance between 3D points isn't constant does not mean a 4D space exists that allows for what is being proposed.
Yes.
In summary, for the purposes of this analogy, a balloon has no center.