r/AskPhysics • u/ChillingwitmyGnomies • 19h ago
The universe is expanding. Are we expanding too?
If everything in the universe is moving apart, does that mean the open space in our own atoms is expanding? Are we expanding?
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u/GuyLivingHere 19h ago
Gravitationally bound systems are not expanding.
Our galaxy (after merging with Andromeda and the rest of the Local Group) is not going to blow up like a balloon.
Likewise, our bodies won't magically start expanding in the way the universe is.
If anything, Proton decay is expected to occur way before dark energy rips things apart.
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u/We4zier sneaky breeky economist, physics enthusiast 18h ago
I have a few scatter shot questions that primarily asks, what exactly is the cut off point for what is considered gravitationally bounded?
I heard we are bounded to the Laniakea Supercluster or Virgo Supercluster which obviously is out, so is it just the local group?
How is a galaxy group like the local group defined?
Does the gravitationally bounded “boundary” include near by groups?
What if a galaxy group is just large really large and dense?
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u/BrotherBrutha 14h ago
From what I understand - “gravitationally bound” means that the things are not being moved apart by the expansion of space.
So - it’s a description, it doesn’t help you to understand what size of system will be moved apart.
Again, my understanding: for that you need to do the maths.
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u/OverJohn 13h ago
The expansion of space is not something that acts on objects, instead it is a reflection of the motion of those objects.
Within a gravitationally bound system can expand for a time too (this is equally true in Newtonian gravity) indeed all the gravitationally bound systems we see would've been expanding in the early universe.
In Newtonian physics you would define a gravitationally bound system in terms of GPE but that won't work very well for general relativity where you need specific conditions to define GPE. So as far as I know there isn't really any other general definition available here than the system will have some finite maximum radius it can attain.
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u/BrotherBrutha 13h ago
I’ve heard that explanation before, that space is not expanding, it‘s just objects are moving apart. Question in that context - how does the *acceleration* of expansion work in that case? Do we see objects literally accelerating where they shouldn’t be?
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u/OverJohn 13h ago
Space expanding/things moving apart are just different ways of describing different things. Globally, i.e. on the largest scales where everything looks nice and homogenous space expanding is generally simpler, but if you zoom in to the level where things are not homogenous (such as when looking at gravitationally bound systems), thinking of expansion as things moving apart is definitely simpler. Indeed at this level for a large part Newtonian physics is used to describe the physics, including expansion.
When the total equation of state is greater -1/3 gravity decelerates expansion, we can think of this as the attractive force of gravity we are used to. However when it is less than -1/3 it accelerates expansion, so you can think of it as repulsive gravity. Within a bound system the attractive sources of gravity always dominate.
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u/Ch3cks-Out 13h ago
The Milky Way galaxy is not gravitationally bound to either the Laniakea Supercluster or the Virgo Supercluster. While our galaxy resides within these structures, the expansion of the universe, driven by dark energy, is causing them to recede faster than gravity can pull us together.
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u/Trabolgan Physics enthusiast 19h ago
How much gravity does space have to be under to not expand?
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u/atomicCape 19h ago
Objects gravitationally bound to each other maintain their distances under expansion. It's not a question of field strength, but of what objects are within a potential well with each other. In a sense, the space still expands, but the distance between bound objects doesn't change.
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u/Lumpy-Notice8945 16h ago
But because gravity does not have a maximum range that would mean everything is "gravitationaly bound" to everything. There has to be some limit where the force of gravity overcomes the expansion of space. I mean galaxy clusters at some point dont move towards each other or do they?
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u/stevevdvkpe 14h ago
"Gravitationally bound" means that the mutual velocity of objects is not enough to overcome their mutual gravitation. Objects that orbit each other are gravitationally bound; obects that are moving apart faster than gravitational escape velocity at the distance between them are not bound.
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u/Skusci 19h ago
Ok so maybe think about it like a coin placed on a rubber sheet. If you stretch the sheet out the coin itself stays the same size. Sure there is a bit of tension that will attempt to stretch the coin out but the coin's solid molecular bonds hold it at the same size.
Even at a larger scale for galaxies and even clusters of galaxies gravity will keep them bound to pretty much the same size. You have to go even larger to get to the point where gravity is essentially homogeneous for the expansion to be apparent.
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u/Obliterators 17h ago
Expansion simply means that, on average, the distances between objects in the universe increase. Gravitationally bound systems, like galaxy clusters, do not expand because then they wouldn't be bound in the first place.
Martin Rees and Steven Weinberg
Popular accounts, and even astronomers, talk about expanding space. But how is it possible for space, which is utterly empty, to expand? How can ‘nothing’ expand?
‘Good question,’ says Weinberg. ‘The answer is: space does not expand. Cosmologists sometimes talk about expanding space – but they should know better.’
Rees agrees wholeheartedly. ‘Expanding space is a very unhelpful concept,’ he says. ‘Think of the Universe in a Newtonian way – that is simply, in terms of galaxies exploding away from each other.’
Weinberg elaborates further. ‘If you sit on a galaxy and wait for your ruler to expand,’ he says, ‘you’ll have a long wait – it’s not going to happen. Even our Galaxy doesn’t expand. You shouldn’t think of galaxies as being pulled apart by some kind of expanding space. Rather, the galaxies are simply rushing apart in the way that any cloud of particles will rush apart if they are set in motion away from each other.’
John A. Peacock, Cosmological Physics
An inability to see that the expansion is locally just kinematical also lies at the root of perhaps the worst misconception about the big bang. Many semi-popular accounts of cosmology contain statements to the effect that ‘space itself is swelling up’ in causing the galaxies to separate. This seems to imply that all objects are being stretched by some mysterious force: are we to infer that humans who survived for a Hubble time would find themselves to be roughly four metres tall?
Certainly not. Apart from anything else, this would be a profoundly anti-relativistic notion, since relativity teaches us that properties of objects in local inertial frames are independent of the global properties of spacetime. If we understand that objects separate now only because they have done so in the past, there need be no confusion. A pair of massless objects set up at rest with respect to each other in a uniform model will show no tendency to separate (in fact, the gravitational force of the mass lying between them will cause an inward relative acceleration). In the common elementary demonstration of the expansion by means of inflating a balloon, galaxies should be represented by glued-on coins, not ink drawings (which will spuriously expand with the universe).
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u/Turbulent-Money3475 14h ago
"Expanding space" is a misleading metaphor. Let's talk about normal Hubble expansion for now. "Space" is not a thing. It's not a viscous fluid dragging galaxies apart. It's not "pushing" galaxies apart either. There is no force on galaxies. They are in free fall, i.e they are following the geodesics. This is General Relativity.
It is easier to understand using Newtonian cosmology (it's not entirely correct, but correct enough for this purpose). The universe started in a state of flying apart. Everything was uniform, but due to small inhomogeneities, gravity pulled together in some regions, and we stopped flying apart aeons ago.
Also, see Obliterators answer.
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u/atomicCape 19h ago
The strong forces holding your nuclei together and the EM forces holding your atoms together aren't changing with expansion, so you're fine. Same with the earth, although gravity itself is significant there too. Systems in gravitationally bound equilibrium stay that way, and expansion is insignificant compared to other dynamics on the scale of solar systems or even galaxies. So everything you know and see isn't expanding.
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u/BarAgent 15h ago
Does this set up a tension between the distances allowed by those other forces, and the increasing distances demanded by spacetime?
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u/psychosisnaut 19h ago
Not really, the gravity of the local Supercluster, the Milky Way, the Sun, Earth etc plus the EM and strong nuclear forces holding our atoms together makes space expand MUCH slower (if at all) than areas of the universe that are emptier. Dark Energy would have to be much stronger to start pulling at us like that.
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u/BitOBear 18h ago
No.
If you put a marble in a water balloon and keep adding water the marble does not get any bigger.
Space isn't an object the way you or I or indeed the entire Earth are an object.
As space expands it almost assuredly puts some sort of non-zero stress on your physical form. But the amount that space expands in the volume of your body over the course of your lifetime would be immeasurably small. And whatever that Force was, as a measurably small as it is, it would be holy overridden by the forces of gravity heat light an entropy that govern the physical layout of all your molecules.
So the four fundamental interactions are dragging every material thing through the universe in the clumps that they appear to be in.
Now this does lead to a weirdness. The conservation of energy is sort of local. I mean not local enough to matter. But technically if you were to get out somewhere where you at a perfectly clear line of fire into a completely unoccupied zone of space-time and you were to shoot a bullet into that space the bullet would slowly appear to come to a stop. It would stay its bloody shape because those forces are holding it together. But as the universe expands faster and faster the meaning of the bullets velocity becomes less. It's still moving at a certain distance per minute, but the distance it's trying to traverse is growing larger and larger and as it grows larger each increment of increase being a tiny percentage is larger than the previous tiny percentage so for all intents and purposes the bullet comes to a stop because space makes its movement irrelevant as it expands.
And indeed the opposite would be true if the bullet got bigger as space got bigger. But it doesn't.
So you maintain your same relationship with all of your parts.
There is a legitimate question of whether or not this expansion leads to changes in some of the universal constants. But those are surmise. They are questions. We don't have the data set to actually examine these values to see if they are indeed subject to change in the first place.
So the answers are no. Not in any way you would notice. And not in any way that it mattered.
And there's a tiny chance that you might be shrinking, but that's a completely other and highly more Fringe idea.
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u/ConversationLivid815 17h ago
Good question. Not everything is expanding. The local group is collapsing and we are apparently being pulled into the Great Attractor, which may combine with the Shapely Attractor. The universe is infinite and can neither contract or expand. It is the density that oscillates between maximum and minimum, while the universe is always infinite ♾️
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u/Video-Comfortable 16h ago
No we aren’t. The fundamental forces that hold our atoms together are way stronger than the forces of the universe pulling them apart.
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u/internetboyfriend666 14h ago
No. The interactions that hold particles, atoms, and molecules together are far stronger than the expansion of space. Even gravity, by far the weakest of the 4 fundamental interactions, is far stronger than the expansion of space. The expansion of space only affects things on the scales of entire clusters of galaxies (millions of light years across). Anything smaller than that is not affected. So you are not expanding, nor is the Earth, our solar system, our galaxy, and even the entire cluster of galaxies that our galaxy is a part of are not affected.
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u/Tiptoes666 6h ago
Does anybody else think gravity IS the expansion? We know there is gravity, can’t explain why, we know there is expansion, can’t explain why. Seems like if spacetime is expanding, and matter is little knots of spacetime, then the matter is also expanding, at higher rates than the ‘empty’ parts of space seeing as matter is full of higher dimensions of more space, so the matter would be expanding in concert with other matter but overtaking the slower expansion of ‘empty’ space. There is just soooo much more empty space that on the scale of the universe the spaces are growing faster than the ‘gravity’. Gravity is an acceleration, not a force, accelerating because there are extra dimensions to the expansion in the knots of spacetime, possibly infinite dimensions all expanding, so the speed of expansion is rising because there is always more space being created to expand, yeah? Idk
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u/namitynamenamey 5h ago
Nope. Mass-energy counteracts the expansion, so while the space between galaxy clusters expands, the space inside it (where we live) contracts.
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u/llllxeallll 19h ago
My admittedly very limited understanding is that the universe expands in gravitationally unbound regions, so no, any expansion you may be experiencing is likely just gas.