So, imagine the box is filled with hydrogen gas. The space inside the box gets bigger. But protons don't, they're little packets of quarks that hold each other very powerfully. You'd have to expand space very, very, very fast to rip those quarks apart from each other, and once you did, you would have a bunch of bare quarks, not hydrogen. The quarks aren't getting bigger, since they appear to be indivisible and essentially pointlike, and even if they were made of something smaller, those things will hold tight to each other, too. A nice gentle expansion will just expand the space in between the protons.

This scales up to forces in between atoms, molecules, and bits of matter. It's only at the very large scales (larger than galaxies) that all the forces that are holding the stuff together starts getting overwhelmed by the expansion of space and it starts to pull things apart. The expansion is so gentle that even relatively weak gravity can hold galaxy clusters together even as space grows.

In other words: space gets bigger, but matter doesn't. It more or less stays the same size.

Thanks for this reply. I guess I now understand what people mean by "space is expanding".

If I understand correctly, space is what supports matter? Like a table supports dishes for instance (well, except that space is also "inside" particules) — and so if the table grows, the dishes don't.

I never actually even thought of what "space" means before. I thought that when we talked about space, matter was included in that (in the sense that quarks or whatever is the smallest — known? — particle take some "space" ; they're not points that can't be measured — or are they? ^^').

Anyway, it sparkled back an old interest of mine for astrophysics (well, I read one book from Hubert Reeves when I was a kid — and loved it).

Thanks again :)

(oh, as an added bonus: I guess we have no idea _why_ space is expanding, right? Because, I mean, it's a bit mind boggling...)

Edit: to be a bit clearer; I thought space was just void that "always" existed and extended indefinitely. Like, there was nothing except a small point with all the energy in the universe ; "everywhere" else a bit nothingness. Then, bim, the big bang, and matter was just booming away and making stars and planets and then galaxies and all that was just floating in this void/nothingness that was always there, never changing, still extending indefinitely. And that was why we were saying the universe was expanding — because galaxies were still speeding away from each others due to the big bang ; like an explosion with no friction perpetuing indefinitely. Now I guess I'm really confused about what "space" is...

Quarks and electrons act like they're actually points. Nobody can be totally sure, but the standard math says they are, and nobody has managed to smash them apart into something smaller or measure them as anything other than "smaller than anything we can measure." They might be small loops of energy instead (this is part of string theory), but if they are, they'd be so small it would be beyond known science to prove it. It's easier to prove something has a size (by measuring it) than to prove the opposite. Every instrument has some margin of error, so you'd never be able to exclude there being some size.

The weird thing about the Big Bang is that it's really all of space that banged, and to a first approximation, space and time as we know it didn't exist before a singularity at the beginning of the expansion. It's very possible that's wrong (there may have been a "parent universe" or a "big bounce") but to a first approximation it seems to be that there may not have been a "before" the Big Bang.

https://en.m.wikipedia.org/wiki/Expansion_of_the_universe

Space is filled with virtual particles that are constantly popping into and out of existence. As space expands, do these particles keep appearing in a given volume at the same rate? Or is the density of virtual particles declining as the volume of space increases?

Well so in other words "space" is NOT expanding. It's particles of matter that are moving away from each other. Right?

So fine, but then why would we say that space is expanding, that would seem to imply we know there is some edge of space which is moving away from us, but what evidence do we have that space has such an edge?

Saying that "space is expanding" seems to hold the assumption that space has a (limited) width and height.

No space really is expanding. Think of it this way: You have a rubber band and you pull it apart, if you place two beads on the rubber band the distance between the beads will increase (even though they don't move relative to the rubber). But if you connect the two beads by a strong spring, then their distance will stay the same, despite space expanding. The force of the spring makes them move (relative to the rubber), counteracting the expansion of space between them.

> relative to the rubber

This makes me uneasy about your analogy. It’s like saying “relative to space” which is nonsense. Also, why would space limit itself to only expanding outside a physical body?

It does not. It expands at every point in spacetime. This effect is quite literally negligible on all length scales smaller than far extra-galactic distances.

It is equivalent to asking why a car driving towards me does not turn blue due to doppler shifting. Except even more negligible than that.

I never said it is only expanding outside. Also moving relative to space is not nonsense. Motion is not just relative to other objects in space. This what Newton's bucket thought experiment showed.

Edit: the pearl is made from around bound by electric forces, so you can repeat the same argument for inside the pearl. You can repeat it all the way down until you have to admit that the approximations inherent in the concepts useful to describe the world at distances from nano meters to mega parsec eventually break down.

> Also, why would space limit itself to only expanding outside a physical body?

It doesn't, it's just that the forces within the body (the spring in the analogy) are stronger than the expansion.

The "relative to space" bit is the nonsense part of their analogy, but I'm not sure of a clearer way to describe things than what they did. It's not my field.

Maybe a better way to phrase it would have been moving over the rubber/space? Or through the rubber/space? It really is motion relative to the underlying manifold though. Let's say there was such a thing as a gravitational soliton, that is, a stable ripple in spacetime, then if you are on the same spot as this wrinkle as expansion starts you will move away from it. You move relative to a structure made from spacetime. This was my field for many years. :-)

The mathematically precise statement is that you are not moving on a geodesic. You are therefore experiencing acceleration (the objects that are not bound and have increasing distance are on geodesics and do not experience acceleration).

You can't make an electron bigger, so it can't be stretched out. Other point particles are the same. The forces that hold physical stuff together are much more powerful than the gentle expansion of space, so we don't see people and planets dissolving.

Indeed you can, if you realize that the place occupied by the "point particle" is described by its wavefunction.

I know! But when you poke it, it still acts like a particle and the wavefunction localizes again, right? It doesn't exactly "puff up" like a billiard ball. Electrons don't really get bigger when they are put into states where their position is less localized.

But the expansion of space does "stretch" light- or at least, the wavelength gets longer, so it's not not happening.

Such counteract action will make lot of side effects, which can be measured. For example, trajectories of separating or oscillating bodies will be slightly curved. Do you have any experimental support for this theory?

This isn’t Certhas’s personal theory. It’s part of standard cosmology, called the metric expansion of space. That term should get you pointed in the right direction if you want to learn more about the evidence for it.

It's too complex to explain, I will not understand it. Right?

It's not, but it might take more work than reading a few HN comments.

The effect of the expansion of the universe on bound systems is far to small to be observed. Two objects 1 meter apart would move at a speed of 10^-17 m/s. Utterly unobservable. But we know space is expanding by looking at things that are super far away from us and that are not bound.

Any objects? So it's independent of mass?

It's independent of mass because it's just a little extra space being created everywhere all the time. It's not exactly a force. First, there was 1 meter between them. Now there's 1 meter and a tiny bit more. The amount of space being created is, in theory, the same everywhere. It's not a repulsive force- space is just being created.

It can actually end up making things very far away appear to move faster than light away from us, since there's a lot of space in between.

Yes, any two bound objects will do so, irrespective of their mass or anything else.

We can directly measure distance to stars which are near us, using trigonometry and waiting half of year for Earth to make half of distance around Sun. Distance to thousands of stars is already measured with ±5% precision.[0] Yes, it's true that at least some stars, which are near to us, are moving away from us. However, it can be explained in number of ways, without inventing of Bing Bang or other epic events.

We cannot measure distance to super far away stars directly. Period.

10^-17 m/s is very high speed. Distance to Moon is 0.385E15 mm, and it can be measured at sub-millimeter accuracy, so this effect can be spotted. However, it will violate Conservation of Energy principle: no force applied, but job is done.

0: https://www.youtube.com/watch?v=i6GhsYrU5WQ

What's your point? Do you want me to explain the basics of GR and Cosmology, including the vast amounts of evidence we have for everything in HN comments? Do you think you are pointing out subtle errors in reasoning that none of the tens of thousands of physics students since the early twentieth century spotted?

You don't have the decency to try to learn the basics but presume to lecture me/expect explanations? Take a course. Show a little humility. Even reading Wikipedia thoroughly would have informed you that it's galaxies, not stars that confirm the expansion of the universe. The expansion (not the acceleration of the expansion) is beyond doubt. Conservation of Energy is not a priory defined for the question at hand because in GR you can not just add the energy at different points in space.

Measuring the perturbation to the moon's trajectory from expansion would require knowing all parameters that enter the trajectory to this accuracy, not just the average distance.

This is all basic if you want to learn, but you seem to have a different agenda...

I'm trying to point out, that current evidence can be explained in different way: by kind of Tired Light Theory. TLT plays well with Pilot Wave Theory and it doesn't need epic events in the past just few galactic hours ago. I'm familiar with evidence used by expanding Universe theories and with problems in them.

TLT predicts that value of "Speed of expanse" will be different when measured using different methods or frequencies, and it is: https://en.wikipedia.org/wiki/Hubble%27s_law#Measured_values... .

TLT predicts that there is much more stars, but we cannot see them yet because they are too dim, but more powerful telescopes will be able to pickup them. https://en.wikipedia.org/wiki/Tolman_surface_brightness_test https://www.nasa.gov/feature/goddard/2016/hubble-reveals-obs...

TTL predicts stars and galaxies older than BB (because of no BB), and they are found: https://en.wikipedia.org/wiki/List_of_the_most_distant_astro...

TTL predicts that Cosmic Background Radiation is just radiation of distant objects with Z=1000 .

And so on.

In short, mainstream theories doesn't hold against new data.

Number of red dwarfs is really astonishing: https://youtu.be/LOJ1XmbSKhM?t=1125

Tolman surface brightness test now looks much brighter. ;-)

So yeah, your agenda is not to learn or discuss but to push falsified fringe theories, that were exhaustively discussed until the eighties, when better data ruled them out...

Yep. I was too young in eighties to participate in these discussions, so I can read about them only, and watch some lectures, which is not satisfying enough for my curiosity.

Can we discuss something easier?

Why you think that photon is immortal? Why we have rule of right hand in EM? Why we see star formations older than BB? WTF is "physical vacuum"? What is waved by gravitational waves? What is happen in linear Sagnac interferometer? How photon is formed (it requires FTL to form)? What happens in double slit experiment? And so on.

Have you tried reading the papers from the period? for the rest, enroll in a physics course and try to not fall into the conceptual traps that even many that have studied and taught physics have fallen into.

Nothing is hidden, but it does take time and persistence.

Photons are not "immortal" they can decay if they have enough energy. Rule of right hand is a convention, could be left hand if you define the sign of charge differently. We don't see star formation older than BB but early universe physics is not easy to understand and there are lots of model uncertainties. Physical vacuum is what remains when no excitations are present. Gravitational waves are ripples in space itself. I don't understand what you mean by photons being formed.

You can not expect the concepts and intuitions that you formed by interacting with the macroscopic world to serve you well when thinking about the microscopic and fundamental. People used to think that you need some type of material aether that carries the waves of light. But that turned out not to be concepts that map well to reality. It took decades to learn to unthink these concepts, there is no shortcut to doing so yourself.

If things were moving away from each other, there would have to be something that set them in motion away from each other. What made all far apart galaxies appear to move away from each other?

Expanding space doesn't necessarily imply edge of space. What if your universe isn't flat, what if it's a torus? It could hypothetically have a finite volume but no edge.

There's also no reason an infinite universe couldn't have local expansion everywhere.

Space is expanding but gravitational attraction and other forces are many orders of magnitude stronger on short distance. While the space within the box expands the forces that attract molecules of the box are such that it will not change its shape until the expansion gets much, much stronger.

Think of the changing pressure. If the box is tight and rigid and the pressure outside the box increases, the box will roughly stay the box until the pressure overcomes the the forces between the molecules that build the walls.

Same with space. If you could measure, the increase in the volume of space would put a constant pressure on every molecule to try to rip it from other molecules. The fact that the rate of change of space and the distances between molecules are so small means that it is not evident that it will ever be possible to devise an experiment to show this on very short distances.

We can only see this on very long timescales and between objects that are very far from each other because all other forces on those scales have diminished so much that only then the increase of volume of space can dominate.

Mandatory: https://xkcd.com/895/

Every cosmologist since Hubble who has held that space is expanding, no, they don't assume the universe is finite. The opposite, in fact.

Specific volume is increasing. If space was finite (but still without boundary - think eg surface of a sphere), so would total volume.

Your interpretation of the expansion is incorrect. Basically, it looks like all points are moving away from each other over large distances.

It is easier to understand in 2D: Imagine a deflated balloon with two dots drawn on it. When you inflate the balloon, its 2D surface increases, which results in the two points "moving away" from each other, without either of them actually moving. It‘s just that the surface on which they lie grows. This is also the reason why distant galaxies are getting more distant faster than the speed of light: They are not actually moving, but the space in between us and them grows.

Notice that the farther two points on the surface of the balloon are apart, the faster they will appear to be moving away from each other, which is why the effect is unnoticeable over small (in astronomical terms) distances. Gravity pulls things together much faster than space expansion pulls them apart.

Bringing this back to your example: Imagine "blowing up" the box. Things inside it are growing a little bit (but gravity pulls them back together), but everything seems to be moving away from everything else.

What's still not clear is how I know the other point on the surface of the ballon is moving away from me? I measure it as 9 tick marks away. After inflation it's still nine tick marks.

That is to say, what is my ruler that is untethered to the surface of the balloon?

Your ruler is absolute distances, like the speed of light. You can inflate or deflate the balloon and the number of marks on its surface won't change. But the time required for the light to travel between them will.

Stupid question - How do you know for certain it isn’t just that light is slowing down?

Relativity probably doesn’t work this way at all, but if light did actually slow down, would it be perceived by us as light moving at the same speed across a greater distance?

I asked Ann Nelson (look her up) this when I audited her cosmology class in grad school, thinking I'd come up with a great new idea. She said yes, it's equivalent to the speed of light changing, and then moved on.

Bear in mind that time is not universal. For the speed of light to change over time, it would change at different rates for observers in different reference frames, and this difference would be measurable.

As to the previous question about relative size. If the universe doubled in size, the objects within it such as atoms don’t also double in size. Gravity and the atomic forces don’t change, so eg the size of stable electron orbits or planetary orbits don’t double in size.

Not exactly. Remember, we're not timing the light's transit. We're observing a redshift of it's frequency which could not be caused by c simply decreasing over time.

It's incorrect to think with this analogy. There is no ruler reaching across the universe, growing with it.

It is not the meter that is growing, that's impossible. Instead you must see it as it is, which is that there are more meters continuously appearing in all of space.

I think physical forces (electromagnetism, gravity, strong/weak nuclear) probably become weaker even though your metric (9 ticks) stays the same. I'm just guessing here, that's the only consequence I can think of.

> looks like all points are moving away from each other ...

Do you mean all particles are moving away from each other? I think the question here was what would it mean for "points" to move? What are points anyway?

The analogy was dots on a balloon, and the next sentence used "points" instead of "dots" but meant the same analogy.

A point (in space) is something that could be occupied by an object.

It boils down to this: You take a bowling ball, and put it 1 meter stick away from you at zero velocity compared to you. Assume that at this distance, no forces are exchanged between you and the ball. You wait a billion billion gazillion years, take your meter stick again and measure the distance between you and the ball, and notice it is now 2 meters away. It is the distance between you and the ball that has grown.

Note that the ball has not grown twice its size. The sizes of atoms and molecules have not changed (as they have mechanisms to keep them exactly the size they are, despite space growing). It is only when there are no mechanisms or forces to keep distances between objects fixed, they seem to drift apart very slowly. The question the article addresses, is whether that growth is accelerating or not.

My (limited) understanding is that only space is expanding. Iow there is more space but the space that matter takes up stays constant.

Maybe I'm also confused though

That's basically my lay-person's understanding also. It's because the expansion isn't significant at small scales, so gravity and electromagnetism overpower it. This causes existing objects and clouds to effectively remain the same size [I have to assume that there's an effect, stealing potential energy at very small scales].

Gravity and EM are holding our galaxy, our Laniakea, our Universe. See https://www.youtube.com/watch?v=rENyyRwxpHo .

As I understand it, the forces that keep matter together are strong enough to counteract any expansion.

Unlikely, because expansion of space is equivalent to the slowing of light, as other commenters have mentioned.

Such heuristic explanations are distractions: Locally, gravitational effects (including spatial expansion) manifest as pseudo-forces and hence can be counteracted.

Think about drawing two circles on a balloon, one with radius 1cm, and the other with radius 2cm. the distance between the two perimeters is 1cm, but if we inflate the balloon so that the surfaces double, now the inner circle has a radius of 2, and the outer circle a radius of 4, the distance between the perimeters has grown along side the rest of all the lengths. That's my best understanding of the situation, I mostly do non-linear classical mechanics.

Who pumping the balloon? Where is source of this energy? Can we use this source of energy for our own needs?

We do not know why the balloon is inflating. I think "dark energy" is a form of energy assigned for being responsible for it, but other than the name I don't think we have reached conclusions as to what it actually is.

I am not a physicist.

My understanding is that the fundamental forces are strong enough to resist the local expansion of space.

So the box gets bigger but particles do not get ripped apart or themselves expand. Space is expanding. Not matter. The particles occupy the same space and the fundamental forces keep them together. Space just keeps moving and expanding around them.

This (coupled with the fact that, at least currently, the expansion is happening very slowly) is why the expansion isn't observable unless you're looking at a very large scale.

This makes me wonder whether we would be able to tell the difference between the universe getting larger and matter in the universe getting smaller.

Someone has published that idea a few year ago https://news.ycombinator.com/item?id=19588996

I think it's more a mathematical trick than something deep about the universe. If you tweak the change of the universal "constants" correctly in a universe that doesn't expand, it is equivalent to a universe that expands where the "constants" don't change.

Note that there is some research about the change of the universal constant, but in most models they don't have just the rights values to make the universe look like expanding. https://en.wikipedia.org/wiki/Time-variation_of_fundamental_...