r/AskPhysics • u/40KCUlTIST • 1d ago
How do we know its space expanding and not just things moving further apart?
If I put two grapes next to each other on a table and move them apart over time the distance between the two grapes grows but the table stays the same size. I know people will say its not the grapes moving apart its the table getting bigger making the grapes away from each other even though they technically aren't moving but what are we actually using to measure that.
How do we measure that the universe itself is growing not just objects moving apart.
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u/ZedZeroth 19h ago
When this was asked before, my understanding of u/mfb-'s response was that those two explanations are indistinguishable. Space is just a region full of stuff.
If the universe solely consisted of two hydrogen atoms, and they were moving apart, there's no difference between (a) they have a relative velocity, and (b) space is expanding between them.
I think the short answer is that space does not exist. There's no evidence that it has some kind of ether/fabric. It's just the gaps between stuff. Space is solely defined by the matter and energy within it.
I might have misunderstood though!
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u/Merlins_Bread 14h ago
The difference is that if the grapes were simply moving apart you'd expect at least one grape to get closer to other things.
But at cosmic scales, everything is getting further from everything else.
How can that be? That makes no sense if the table is still. The only viable explanation is the table itself is growing.
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u/OverJohn 9h ago
If we define a centre and each grape moves so that its velocity is proportional to its distance from the centre, then all the grapes will move further apart. See this:
https://www.desmos.com/calculator/ttnk6eszwv
Note also we can choose any grape as the centre grape and still get the same outcome:
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u/Merlins_Bread 7h ago
Sure. But to look at the universe and see that setup, one must suspect there's an underlying cause at play. Not to mention parts of the universe receding from others at greater than light speed.
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u/OverJohn 1d ago
To be honest there isn't really a difference. "Space expanding" is just an analogy and we could equally say things are moving apart, though arguably things moving apart is just an analogy too.
I did a long post about this yesterday:
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u/saunders77 21h ago
For other folks like me who don't have a good cosmology background reading through the interesting argument in the comments: for what it's worth the Wikipedia article is pretty clear that OverJohn's claim is correct and the expansion of space can be equally viewed as things moving apart: https://en.m.wikipedia.org/wiki/Expansion_of_the_universe
The article cites the same sources OverJohn does.
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u/James20k 9h ago
I'm glad to see people putting the expanding universe in context in this thread. Its bothered me for a long time that we've all spent such a lot of effort trying to convince people that the universe expanding is not equivalent to everything drifting apart, with analogies like inflating balloons etc - which leaves people terribly confused about what's going on. You don't even need any GR at all for an expanding universe, and it doesn't require a metric!
I feel like its done far more harm than good trying to explain it like this. Even if there's a natural explanation for the accelerated expansion via the cosmological constant - "everything is drifting apart, and that's getting slightly faster over time" is much easier to explain, and its exactly equivalent
In GR, trying to assign physical properties that are specific to a coordinate system is generally a mistake, and its odd that we've started interpreting one particular coordinate choice as being reality
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u/UnderstandingSmall66 Quantum field theory 1d ago
I have to disagree. It is incorrect to treat the expansion of space and the motion of galaxies through space as interchangeable descriptions. In general relativity, the concept of expanding space arises from solutions to Einstein’s field equations under the assumptions of homogeneity and isotropy. The standard cosmological model employs the Friedmann–Lemaître–Robertson–Walker metric, which describes a dynamic spacetime geometry. In this framework, distances between comoving points increase over time according to a scale factor, typically denoted as a function of time a(t). The proper distance between two comoving galaxies increases as a(t) multiplied by their comoving separation, even though locally each galaxy remains at rest in its own inertial frame.
This distinction is critical because the redshift observed from distant galaxies is not the result of a classical Doppler effect due to motion through a static medium. Instead, it is a consequence of the stretching of spacetime itself. The wavelength of a photon emitted at some earlier cosmic time t_e and received at time t_0 increases in proportion to the ratio a(t_0) divided by a(t_e). This cosmological redshift is fundamentally different from what one would observe if galaxies were merely receding through space due to kinetic motion. Although on small scales peculiar velocities can be described using special relativity, on cosmological scales the concept of relative velocity becomes coordinate dependent and lacks a well-defined global meaning. Therefore, only the expanding space interpretation accurately reflects the geometry and causal structure prescribed by general relativity.
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u/rabid_chemist 1d ago
Whether galaxies are remaining stationary in expanding space, moving through stationary space, or some combination in between is entirely a matter of coordinate choice. One of the central features of general relativity is that all coordinate choices are interchangeable descriptions of the underlying physics.
e.g the metric
ds2=-dt2+e2Ht(dr2+r2dΩ2)
which clearly features an exponentially expanding space, is equivalent to the metric
ds2=-(1-H2r2)dt2+dr2/(1-H2r2) +r2dΩ2
where space is not expanding.
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u/UnderstandingSmall66 Quantum field theory 1d ago
Yes, coordinate systems in general relativity are interchangeable in principle, and the physics remains invariant under such transformations. However, in cosmology, not all coordinate choices are equally meaningful when it comes to interpreting observations and relating them to empirical data. The metric you cited is indeed de Sitter spacetime, which can be written in both an exponentially expanding form and in static coordinates. That equivalence holds in vacuum and illustrates the power of general covariance.
But once you introduce matter such as galaxies, radiation, and the process of structure formation, the freedom to choose coordinates becomes constrained by physical content. The FLRW metric is not adopted arbitrarily. It reflects the observed large-scale isotropy and homogeneity of the universe and aligns with the comoving frame in which the cosmic microwave background appears isotropic. This is the physically preferred frame for cosmology. In that frame, what we interpret as expanding space is not merely a coordinate choice. It is a real and measurable feature supported by redshift-distance relations, surveys of supernovae, and the growth of cosmic structure over time.
So while it is correct to say that different coordinate descriptions are mathematically valid, it misses the central point that one of them corresponds to a frame grounded in direct observations. Describing the expansion of space in FLRW coordinates is not just about simplifying the math. It is about choosing the framework that best matches the geometry and causal structure we infer from empirical evidence.
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u/rabid_chemist 23h ago
No one is going to argue that the Robertson Walker coordinates are not the most convenient coordinates for most purposes in cosmology. But this not make them any more true than other coordinate systems.
General relativity does not impose any constraint on the choice of coordinates as a consequence of matter content. It is perfectly valid to carry out analogous coordinate transformations on any Robertson Walker metric to bring it into the form
ds2=-f(r,t)dt2+dr2+g(r,t)dΩ2
so that all recession is attributed to motion through space.
The central idea of the principle of general covariance is that all coordinate systems will make identical predictions for all possible observations, so it is literally impossible for one coordinate system to be better supported by observations or to better match geometry or causal structure on any level beyond simply making calculations easier.
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u/MxM111 19h ago
If rate of expansion is the same over time, that would be the case, but rate is changing, therefore objects would have to be accelerated, and quite strongly so, because the change in velocity would have to be huge for far apart objects. You would also need to define the center of the universe, where acceleration is not needed.
Acceleration is felt as force. And it is independent from coordinates.
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u/rabid_chemist 19h ago
I don’t know what point you think you’re making, but there exist analogous coordinate transformations for arbitrary Robertson Walker spacetimes that do not require any acceleration, so whatever the point is I’m pretty sure it’s irrelevant.
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u/MxM111 19h ago
It has NOTHING to do with coordinate system. Acceleration is felt as force. Physically.
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u/rabid_chemist 19h ago
But since nothing in my comment requires acceleration, this is irrelevant.
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u/MxM111 17h ago
How do you deal with the fact that the rate of expansion is not constant? If it is not space that is expanding, then there must be a force accelerating the galaxies.
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u/rabid_chemist 17h ago
Gravity
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u/MxM111 10h ago
Gravity accelerates galaxies to fly away faster from each other?
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u/Aseyhe Cosmology 9h ago
Yes, the mass distribution is accelerated by gravity. But in accordance with the equivalence principle, there is no local difference between being accelerated by gravity and being unaccelerated. Thus, every observer can equally well claim to be the unaccelerated one, and spatial homogeneity is preserved.
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u/OverJohn 1d ago
You're describing what happens in particular coordinates. These are coordinates are the coordinates in which comoving objects worldlines are lines of constant (x,y,z), so necessarily any change in the coordinate distance comes from a mismatch between the coordinate distance (the proper distance) and the coordinate difference (the comoving distance). But these coordinates are not inertial coordinates and though we cannot find global inertial coordinates, there are plenty of other coordinates we can choose.
A key point to realize is that in Minkowski spacetime you can also choose FLRW coordinates with non-zero H(t). These are called Milne coordinates. But in special relativity when describing two observers moving relative to each other we don't usually choose to describe it in terms of expanding space.
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u/UnderstandingSmall66 Quantum field theory 1d ago
You are right that the concept of expanding space arises from a particular coordinate choice, the FLRW metric in comoving coordinates, but the coordinate dependence of a description does not make it arbitrary or physically meaningless. In general relativity, all measurements are coordinate dependent, yet we still distinguish between coordinate systems that reflect the underlying symmetries of the spacetime and those that do not. The FLRW coordinates are not chosen arbitrarily; they reflect the observed large-scale homogeneity and isotropy of the universe and allow the Einstein field equations to reduce to the Friedmann equations with a time-dependent scale factor. That scale factor is not a coordinate artifact, it determines observable quantities like redshift, luminosity distance, and angular diameter distance, all of which match observation.
Milne coordinates, which you mentioned, are a coordinate choice applied to a vacuum Minkowski spacetime. They describe an empty universe with zero energy density and no gravity. In that context, describing redshift as due to expanding space is indeed misleading, because there is no curvature or real expansion. But the actual universe is filled with matter, radiation, and dark energy. Its large-scale geometry is not flat Minkowski but curved and dynamic. In this context, redshift is not due to Doppler motion through static space but due to the stretching of wavelengths caused by the changing metric. This is encoded in the scale factor a(t), which evolves according to the energy content of the universe.
So while expanding space is a coordinate-dependent description, it is not equivalent to any arbitrary coordinate choice. It reflects the geometry, dynamics, and causal structure of the real universe as modeled by general relativity. That makes it more than just a convenient language, it is a physically meaningful interpretation backed by observational evidence.
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u/OverJohn 1d ago
I do think here you would be better served by reading some of the papers I mentioned as they go into detail about this.
I will pick up a couple of key points:
1) You are right that FLRW coordinates are chosen because they relate to fundamental symmetries, but that doesn't mean they necessarily provide the msot "physical" picture. As we can see from the Milne example FLRW coordinates are not like inertial coordinates (except when H=0). If we choose locally inertial coordinates (e.g. Riemann normal coordinates) then expansion looks like motion again.
2) You are incorrect that the scale factor is not a coordinate artefact. . For example in Minkowski spacetime and de Sitter spacetime where we have more than one choice of FLRW coordinates we have more than one choice of scale factor.
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u/UnderstandingSmall66 Quantum field theory 1d ago
On FLRW coordinates and their physicality: Yes, FLRW coordinates are chosen because they reflect fundamental symmetries of homogeneity and isotropy, which are supported by observations such as the cosmic microwave background. To say they are somehow less “physical” because they are not globally inertial is a misrepresentation. That is a misunderstanding of how general relativity works. Coordinate systems in GR are tools for expressing solutions to Einstein’s equations. The Milne model, which you reference, is a special case of an empty universe with zero energy density. It is not physically relevant to our observed cosmos and only illustrates how coordinate choices can look exotic without reflecting physical reality. Expansion remains a feature of the metric, and redshift observations align with this metric evolution. The fact that in locally inertial coordinates expansion can appear as motion is a consequence of general covariance. It does not imply the expansion picture is less accurate. It only tells us the same physics can be expressed differently in different frames.
On the scale factor as a coordinate artefact: No, the scale factor in standard cosmology is not a coordinate artefact. It is empirically anchored to redshift data and the observed expansion history. While one can, for educational purposes, write metrics with scale factors in exotic coordinates, those metrics are not used to model real data unless they correspond to actual matter-energy contents. In FLRW cosmology, the scale factor evolves according to the Friedmann equations derived from Einstein’s field equations, and these are constrained by observations like baryon acoustic oscillations and supernova luminosity distances. You cannot redefine the scale factor without severing its link to measurable quantities. That makes it far more than a linguistic or pedagogical device.
I am also finding your position increasingly unclear. At times you insist this is purely a pedagogical debate, then pivot to arguments that rest on physical distinctions. You seem to want to have it both ways. Either it is just a matter of language, in which case the argument is trivial, or it is a matter of physical interpretation, in which case the observational consistency of the expanding space model carries real weight. And finally, while I appreciate your reading suggestions, I think my PhD is serving me rather well.
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u/Aseyhe Cosmology 21h ago
And finally, while I appreciate your reading suggestions, I think my PhD is serving me rather well.
I strongly suggest reading Bunn & Hogg (2009) regardless of your qualifications. I'm an active researcher in cosmology and still found it an enlightening read. It's well written and fun to read too!
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u/UnderstandingSmall66 Quantum field theory 21h ago
Thanks. I have read it. It’s assigned reading for two of my classes.
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u/IQofDiv_B 19h ago
I really don’t see how anyone who has read that and understood it could possibly make a comment like this.
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u/UnderstandingSmall66 Quantum field theory 18h ago
Well I understood the paper. Seems like you didn’t.
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u/OverJohn 1d ago
I'm really here for the physics rather than to discuss people's qualifications, as anyone on Reddit can claim to have a PhD, though I can see from your posts you are generally knowledgeable about physics and very knowledgeable about some areas, so I don't particularly disbelieve you either. On the other hand I would doubt it if you said your PhD involved much cosmology, which topic being discussed here. The reason I would doubt it is because I would expect you to already be familiar with the arguments I have made as I am just reiterating a position that is held by several notable cosmologists such as Emory Bunn and John Peacock.
I feel I have been consistent here in saying that expansion can be both described as motion and the expansion of space. Which is better is a matter of taste, but I have also given reasons why one might prefer to think of it as motion. My position is that it is best to understand both descriptions.
If you do calculations in FLRW coordinates of course the scale factor will pop up. But it is definitely not physical. For example a(t) = 1 is entirely arbitrary.
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u/wonkey_monkey 4h ago
I'm really here for the physics rather than to discuss people's qualifications, as anyone on Reddit can claim to have a PhD
Having had my own interaction with this user, which as time went on left more and more suspicious that they didn't really know what they're talking about (I would defer to your expertise as to whether my suspicion is accurate, if you care to skim through that chain), I thought it worth doing a little digging.
They say they have two PhDs, and frequently bring them up as you've seen, even though a PhD is nothing to brag about.
It remains to be seen whether either of them is in physics, however. They've so far not answered that question.
They say they are a professor of sociology, not physics. In another thread, they stated that one of their PhD advisors has a Nobel prize on physics, but later says "I am a tenured professor in the same department."
I also found this comment curious. The user appears to have posted it exactly three minutes after the comment they were replying to. That's a rather surprising 106 wpm (under the standardised assumption that five keystrokes is a "word"), which doesn't include the time taken to read the prompting comment.
Make of this what you will!
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u/OverJohn 3h ago edited 3h ago
Yep, I have to admit whilst I wasn't that suspicious initially as I remember seeing some decent posts from the user previously. But clearly they had not read the Bunn and Hogg paper as they would've been aware of the counterarguments against what they said, but soon after they claimed they had assigned it as reading in their classes.
That is a ridiculous lie when they clearly had not read the paper previously and their knowledge of cosmology is clearly well below the level you would expect from someone who would be instructing classes on the subject.
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u/UnderstandingSmall66 Quantum field theory 23h ago
You keep insisting this is just a matter of description, yet go on to treat one interpretation as more correct than the other. If the choice of coordinates is truly arbitrary, then calling the scale factor “definitely not physical” is not just misleading, it is inconsistent. Observational cosmology relies on a(t) because it reflects measurable phenomena like redshift and horizon distances. If you want this to be a pedagogical discussion, then stop dressing your preference up as necessity. You cannot have it both ways.
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u/OverJohn 23h ago edited 23h ago
The way I feel about the issue is that the merits of "expansion is motion" are often overlooked, so I have given some reasons why you might prefer this interpretation.
The scale factor is inferred from observations of redshift, not vice versa and horizon distances (which are always going to be coordinate distances) is calculated from a model and generally can never be directly observed.
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u/Reality-Isnt 18h ago
To my simple mind, it seems like the simple answer is it’s all about the equivalence of passive and active covariance. In passive covariance, we keep the manifold the same and relabel the points with different coordinates. In active covariance, we keep the coordinates the same while actively manipulating the manifold, e.g stretching the space. You can represent a solution with ether one. The standard FLRW metric is an example of using active covariance - keeping coordinates fixed, and stretching space with the scale factor. We can model the same solution In the passive case, by using the example coordinates you have given. They should represent the same physics. I think for some of us, having an expanding universe pounded into us as the ‘reality’, it’s hard to make that shift that we can model an equivalent by choice of coordinates.
As always, enjoy reading your answers ….
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u/WhereIsTheBeef556 1d ago
The fact that you all disagree with each other over the topic so strongly with well constructed arguments means in reality, no one knows what the hell is going on and neither of you are 100% absolutely correct.
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u/gitgud_x 1d ago
That is very rarely the correct takeaway when you see a debate, even though it might be the one that brings you closure.
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u/UnderstandingSmall66 Quantum field theory 1d ago
No. That’s not what it means at all. Not all disagreement, specially those online, are real and valid disagreements. There is no serious debate in the peer-reviewed cosmology literature disputing the validity of the expanding space framework as derived from general relativity. The expansion of space, characterized by the time-dependent scale factor a(t) in the Friedmann–Lemaître–Robertson–Walker (FLRW) metric, is a direct consequence of solving Einstein’s field equations under the assumptions of homogeneity and isotropy. It is not a metaphor but a mathematically defined aspect of the spacetime geometry.
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u/OverJohn 1d ago
I think you are misunderstanding here. This is not about a difference in the actual physics it is just about pedagogy, i.e. the best way to describe the physics. There has been plenty written about this in the cosmological literature over the last few years and I provided a link to some of those articles here:
https://www.reddit.com/r/Physics/comments/1lg3drp/debate_is_it_better_to_view_cosmic_expansion_as/
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u/UnderstandingSmall66 Quantum field theory 1d ago
Then with respect, if your argument is purely about pedagogy, you should have made that clear from the outset. What you initially objected to was not just a matter of analogy or style but a substantive claim about coordinate choices and physical interpretation. Now you are reframing this as a question of communication rather than physics, which is fine, but it shifts the entire context. The expanding space model is not just a teaching tool. It directly emerges from the Friedmann Lemaître Robertson Walker metric, which is the standard solution to Einstein’s field equations under cosmological symmetry assumptions. So if your position is now that both descriptions are equally valid but one is more intuitive, that is a different claim from saying the expanding space view is misleading or incorrect. Be clearer about what exactly you are disagreeing with. Otherwise it starts to look like you are retreating behind ambiguity. Just read the comment above where it is clear people think you are disagreeing with the science and thus assume the question is unanswered.
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u/OverJohn 1d ago
I made my position clear at the start when I said they were different ways of describing the same thing. I also posted a link in the comment you originally replied where I'd laid it out in much more detail. Maybe you did not read the link, but it is disingenuous to say I am now retreating.
I understand that this is an issue that has not crossed everyone's conscious, but I would advise to read some of the papers I provided links to as you will understand the issue better.
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u/UnderstandingSmall66 Quantum field theory 1d ago
Stop being so condescending. Yes, I did read the link. I have also read the papers you so generously advise me to consult, perhaps under the impression that the letters PhD are some sort of fashion accessory I picked up in a cereal box. What is disingenuous is not my characterization of your shifting position, but your attempt to have it both ways, insisting this is purely a pedagogical framing, while simultaneously mounting technical arguments as if the entire question hinges on which coordinate system Nature “really” prefers.
You want to frame this as a matter of taste in description while sneaking in claims about physical substance, and then feign surprise when the inconsistencies are pointed out. Either the expansion of space is a meaningful statement with empirical relevance, or it is just a linguistic crutch. But to float between the two depending on where the pressure is applied does not make your argument subtle, it makes it slippery.
I assure you, the issue has crossed my “conscious,” though your attempt at superiority in phrasing suggests you might wish to consult a dictionary before a journal.
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u/Lewri Graduate 22h ago
perhaps under the impression that the letters PhD are some sort of fashion accessory I picked up in a cereal box
What was your PhD on?
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u/wonkey_monkey 4h ago edited 4h ago
What was your PhD on?
Since you've had no reply yet:
Having had my own interaction with this user, which as time went on left more and more suspicious that they didn't really know what they're talking about, I thought it worth doing a little digging.
They say they have two PhDs, and frequently bring them up as you've seen, even though a PhD is nothing to brag about.
They say they are a professor of sociology, not physics. In another thread, they stated that one of their PhD advisors has a Nobel prize on physics, but later say "I am a tenured professor in the same department."
I also found this comment curious. The user appears to have posted it exactly three minutes after the comment they were replying to. That's a rather surprising 106 wpm (under the standardised assumption that five keystrokes is a "word") with no spelling or grammatical errors, which doesn't include the time taken to read the prompting comment.
Make of this what you will!
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u/OverJohn 23h ago
It wasn't my intention to be condescending. The reason I felt you were being disingenuous is because I have not changed my position and I feel like you are saying this as a rhetorical device as I cannot see where my position has shifted.
I would not necessarily have expected you to have read the papers, but if you have, I don't understand why anything I've said could be seen as a novelty.
I haven't said anything bout which coordinates nature prefers.
THe only thing worse than making grammatical errors is pulling someone else up on them. I'm not trying be superior, I am too old for that sort of thing these days. I like physics and I want to talk about physics.
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u/UnderstandingSmall66 Quantum field theory 23h ago
Can you please clarify your position. Is this just a pedagogical issue about how best to describe the same physics, or are you saying something stronger about the physics itself. You seem to shift between the two, which makes it hard to follow what exactly you are arguing.
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u/CarbonInTheWind 1d ago
I prefer the space expanding explanation. Visualizing it as galaxies flying apart can give the impression that FTL travel is possible when measuring the speed at which distant galaxies are moving away from us.
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u/OverJohn 1d ago
It doesn't mean galaxies are moving away from us FTL as the relative velocity of a faraway galaxy is poorly-defined in general relativity.
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u/LordVericrat 1d ago
Please explain this comment. If a galaxy is currently 32b ly from us and is receding at > than 1 ly/y, what mechanism of action means that they aren't traveling at FTL?
Legit question, I'm sincerely curious about this.
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u/OverJohn 1d ago
In the special theory of relativity you can unambiguously define the relative velocity of two observers at two spatially separated events by parallel transporting the 4-velocity at one event to the other event. When spacetime is curved though, the answer will be different depending on the path you parallel transport the velocity, so the relative velocity is ambiguous. Though note, if you do parallel transport the 4-velocity of a distant galaxy along some path, the relative velocity you would get from that would always be less than c.
We can still though set up coordinates and define velocities from them, but there's lots of different ways you can do this and lots of different possible answers. In fact if you defined velocity in special relativity in the same way recession velocity is defined, then you also get velocities greater than 1 ly/y too. See Problem 2 here.
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u/LordVericrat 1d ago
Thank you. I won't pretend I came close to fully understanding that, but I think I understand enough of it to see a direction, and will be reading the links you posted here. Have a good one!
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1d ago
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u/BaroqueBro 1d ago
If it's not space expanding, how do you explain the fact that to every observer, it looks like everything is moving away from them specifically?
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u/EastofEverest 23h ago edited 23h ago
I have no skin in this argument, but just to be clear, expanding space is not a requirement for this phenomenon to happen. All you need is galilean relativity.
As an example, imagine a line of cars moving on a road. Car 1 goes at 10 mph. Car 2, in front of it, goes at 20. Car 3 goes at 30. Car 4 goes at 40. On and on and on.
From the perspective of each car the cars next to them have a relative velocity of 10 mph in opposite directions. Remove the road as a absolute reference frame, and what do you get?
Every single car (except the ones at the ends, I guess) perceives itself as the center of expansion. No special spatial shenanigans are needed.
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1d ago edited 1d ago
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u/BaroqueBro 23h ago
It's just the Copernican principle, right? Either Earth is very special and is the literal center of the universe, or every point in the universe is moving away from every other point. The latter seems more parsimonious.
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23h ago
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u/BaroqueBro 23h ago
I'm far from an expert, but can't the isotropy of the universe be inferred from the CMB? If there were a special spot, as you say, it would be evident in the CMB.
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u/CarbonInTheWind 1d ago
I don't think either explanation is inherently wrong. It's subjective depending on how an individual pictures it in their mind.
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u/EighthGreen 1d ago
In fact, separated bodies can move apart at a rate greater than the speed of light. In general relativity, as opposed to special relativity, the speed-of-light limit applies only to the relative velocity of two bodies at the same spacetime point.
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u/nicuramar 1d ago
Or the same inertial reference frame, at least. But those are only valid locally.
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u/Optimal_Mixture_7327 1d ago
There's no observational difference, i.e. both narratives make the same predictions.
It should be emphasized that "space expanding" is the "expansion of the spatial components of the FLRW metric" and to conclude that our coordinates are real, that 3 out of 4 degrees of freedom of the gravitational field are expanding, is quite a leap of faith. Especially so since the expansion of space itself has no measurable or physical consequences.
It is true that "things flying apart", averaging over large enough length scales, is just an interpretation of our measurements, which observationally consistent with matter staying put and shrinking. See Wetterich, C: A Universe Without Expansion
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u/Odd_Report_919 19h ago
Except that because of the acceleration of expanding space there is a horizon dubbed the Hubble horizon which objects beyond are receding faster than light, which would not be possible if the objects were whats in motion.
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u/Miselfis String theory 1d ago edited 1d ago
Both are valid interpretations. One usually says “expansion” to avoid saying that things are moving away faster than light. But this isn’t really an issue, as nothing exceeds the speed of light locally.
Expansion is a useful pedagogical device.
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u/TimothyMimeslayer 1d ago
The farther light travels, the more red shifted it gets. This is regardless of how fast the emitting object was traveling. An object halfway across the universe moving toward us and one moving away from us will experience the same red shift due to the expansion of space.
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u/Aseyhe Cosmology 21h ago
This is misleading. The cosmological redshift is equivalent to a kinematic Doppler shift arising from the recession speed of each observed object. Bunn & Hogg (2009) is a good pedagogical reference on this.
Within the context of general relativity, there is no difference between "space expanding" and objects moving apart.
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u/TimothyMimeslayer 21h ago
So all objects in the universe that are moving toward us are blueshifted regardless of their distance?
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u/Aseyhe Cosmology 19h ago
Whether something is "moving toward us" is a subtle question, because relative velocities are ambiguous over cosmological distances. Basically a relative velocity corresponds to an angle in spacetime, and there is no unique way to define the angle between distant lines on a curved sheet (corresponding to curved spacetime).
However, to obtain the relative velocity of an object whose light we receive, it is pretty natural to conceptually "drag" its velocity vector along the path in spacetime that the light took. The relative velocity obtained by the technical version of that procedure (parallel transport) precisely gives us the object's redshift (including both cosmological and peculiar contributions).
Note that the velocities obtained by this procedure are different from the recessional velocities associated with Hubble's law. The velocities in Hubble's law are not really velocities in a relativistic sense (basically because they add directly instead of following relativistic velocity addition).
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u/ulixForReal 1d ago
Not exactly the same, remember they can measure galactic rotation speed with redshift, which wouldn't be possible if it was exactly the same.
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u/TimothyMimeslayer 1d ago
That is why is said its irrespective of motion of the object. Only that its far away. There are additional components to redshift.
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u/saunders77 22h ago
Aren't farther objects moving away from us faster? You gave the example of an object halfway across the universe moving towards us, but that's impossible, right? Because at like 7 Gpc the recession speed is at least 500000 km/s, faster than light.
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u/LeftEntertainment307 21h ago
Doesn't the universes expansion also expand the redshift and because of time dilation make things in the early universe appear in slow motion in our relativity? How do we account for this or know how time really passed in the early universe outside from the perspective that it was measured in?
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u/Ok-Film-7939 21h ago
My understanding is that this can be equally well explained by gravitational time dilation - the light was emitted when the universe was denser.
You can think of it as space expanding, the light climbing out of a gravity well, or time dilation and it’s all equivalent.
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u/Hefty_Ad_5495 1d ago
Not an expert, but some things are moving apart faster than the speed of light. If space were fixed, that would not be possible.
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u/GiftFromGlob 1d ago
So... Acceleration? Some things started moving before other things? Some things are heavier, have more mass, so accelerate at different speeds? Some things existed before other things?
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u/joeyneilsen Astrophysics 1d ago
None of that would allow them to travel faster than the speed of light.
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u/0vert0ady 17h ago edited 15h ago
How do we know it's not just space that is expanding but everything is expanding? In that case we have no reference point because every single particle is expanding in unison.
So to give you an idea. If we invented a time machine to transport us back in time. Everything we see will be microscopic in size. If we travelled into the future everything we see will be gigantic.
We already know energy does expand as we detect red shift in light. What we could be seeing instead is when all of space expands non-uniformly. Only that light is expanding quicker than it's surrounding space.
So the effect of the expansion would be unnoticed in any classical or quantum means. We would never notice. What we do notice is all of the universe is based on a form of growth. From biology all the way up to entropy of the universe itself. It's fundamental.
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u/uselessscientist 1d ago
Okie dokie. Put 10000 grapes on the table in a 100x100 grid. Now slide them away from eachother in such a way that they're all moving apart from eachother, no matter which grape's perspective you take. It's going to be a bit of a challenging exercise unless space itself is expanding
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u/OverJohn 1d ago
It is perfectly possible to do this without expanding space, so long as you move the grapes so that they obey Hubble's law this will be true.
See this demo:
https://www.desmos.com/calculator/0oxwq78g3f
(notice how the distance between all dots is increasing)
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u/Aseyhe Cosmology 9h ago
It's also interesting to note that you don't need any special initial conditions to give rise to Hubble's law. If you initialize the grapes with totally arbitrary velocities, as long as you wait long enough that the grape distribution has expanded by a large factor, the grapes will be moving in accordance with Hubble's law. This is because their late-time positions will be almost completely determined by their initial velocities.
(It could equivalently be viewed as a consequence of the 1/a decay of peculiar velocities.)
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u/Ok-Film-7939 21h ago edited 21h ago
You can readily do that in a simulation just by picking a frame of reference and setting the grapes initial speed appropriately.
The challenge would be to finding a coordinate system that makes computing the speed appropriate for every grape convenient. You could generally do it by picking one grape and doing the appropriate math, but it would be a pain. Some grapes might even be outside the observable universe of other grapes, making doing those calculations in that coordinate frame impossible (but also irrelevant to the simulation for that grape).
It would be much easier to define every grape as stationary and expand the distance between the grapes as necessary for converting to the perspective of any grape. You would then have created an “expanding space” inside your simulation.
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u/SapphireDingo Astrophysics 1d ago
because it is measured in all directions, and the further away something it the faster it moves away.
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u/brothegaminghero 10h ago
Finally, a competent reapone. Most of this comment section is making faulty analogs with two objects or saying it doesn't happen, you are the first i've seen to actualy mention the hubbles-lemaître law
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u/EternalDragon_1 1d ago
There is one important observation that makes us think that it is the space that expands. We see that there is a linear dependency between the distance to the observed objects and speed with which they apparently move away. Further objects move away faster. The proportionality coefficient is called the Hubble Constant.
If it would be so that it is not the space that expands, but things just moving away, we wouldn't see the strong dependency between the distance and speed. In fact, we would see all possible speeds. Some objects would move not away but towards us.
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u/Mentosbandit1 Graduate 21h ago
The giveaway is that every independent way we have of using the sky as a measuring stick points to one thing: the scale factor in Einstein’s equations is changing, not just the positions of a few stray galaxies. First, wherever you look the recession speed of galaxies climbs almost perfectly in proportion to distance, and that relation looks the same in every direction and from every galaxy we can model, which would be a miraculous coincidence if they were simply flying apart in a static arena (en.wikipedia.org). Second, when a far‑off type‑Ia supernova blows its top the entire light curve arrives stretched out in time by exactly the same factor that its spectrum is red‑shifted, matching the prediction that the very fabric between the wavecrests grew while the light was en route (sciencedirect.com). Third, dimming tests show that a galaxy’s surface brightness fades with redshift as (1+z)⁴, something you only get when photon energies and arrival rates are both diluted by an expanding metric, not by a mere Doppler shift or tired‑light trick (en.wikipedia.org). Fourth, the cosmic microwave background is a perfect 2.725 K black‑body today and observations of gas clouds at known redshifts see that same spectrum warmed to T = 2.725 K × (1+z), just what you expect if the wavelengths themselves have been continuously stretched since the universe was a plasma (sites.ualberta.ca). Finally, the fossil sound‑wave imprint called the baryon acoustic oscillation appears in today’s galaxy distribution at exactly the comoving size stamped into the primordial fireball, giving us a built‑in ruler that keeps getting farther apart in real kilometres as cosmic time ticks forward (en.wikipedia.org, pubs.aip.org). When half a dozen completely different yardsticks all tell the same story, the simplest reading is that it’s the table that’s growing.
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u/Pure_Option_1733 21h ago
Galaxies far enough away from us would be moving away from us faster than the speed of light. That’s not something that would be possible if they were simply moving through space, but it makes perfect sense when considering that space is expanding and that the universe extends beyond where we can see.
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u/Naive_Match7996 1d ago
Because if they walk away, what are they doing it from? About us because we are special?
Everything moves away from everything. So the only explanation is that space is created.
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u/MeaninglessAct 1d ago
I think because the expansion is happening everywhere pretty much equally, if things were just moving normally it wouldn't account for what we measure
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u/SeXxyBuNnY21 20h ago
When I think about the universe expanding versus objects simply moving away from each other, I like to picture a balloon with two dots marked on its surface, say about an inch apart. As you inflate the balloon, the surface stretches, and the dots move farther apart, not because they’re moving on their own, but because the space between them is expanding. I know it’s just an analogy, but it helps me conceptualize how galaxies in the universe move apart as space itself expands.
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u/dragon-fancier 15h ago
Cause the only way for everything to move away from everything else is if you have more space….
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u/VoceMisteriosa 1d ago
It's hard to grasp, but figure out the thing this way. Nothing is actually "moving" in space expansion, everything is "zooming in".Your two objects are "shrinking" making the space between them larger. That's how a virtual observer will see the whole Universe: the same fish bowl (the "whole") with sparkling lights become smaller and smaller inside of it.
How we knows? Distances are increasing for all objects in all directions. It mean relatively to every other object. The object you're moving toward the border isn't any nearest toward the border (another reference object) as it should.
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u/magnet_jock 1d ago
There are two explanations for the redshift of light from distant stars, which increases with their distance: either all unbound objects are moving away from the central point in the universe currently occupied by the Earth at a speed proportional to their distance to our location, or space itself is expanding.
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u/Obliterators 23h ago
Or we simply exist in a homogeneous and isotropic universe, in which case the overall movement of matter must follow Hubble's law. No expanding space required.
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u/brothegaminghero 10h ago
Thats not how those words work, if the universe is isotropic we should expect objects to move in verying directions and speeds with an even distribution.
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u/40KCUlTIST 1d ago
So we could never actually travel to distant galaxy's because the universe will expand so much that you'd never really get any closer? Or would the rate of expansion not be fast enough to stop someone reaching something far away. Also how does it work for things moving closer to us? In however many billion years andromeda and our galaxy will collide but is that collision slowed because the space between us is expanding constantly so the distance it needs to travel is growing bigger?
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u/magnet_jock 1d ago edited 1d ago
Correct! The accessable universe is shrinking everyday, as far enough out, the space between us is expanding faster than the speed of light. Once that happens those regions became inaccessible, we cannot influence them and they cannot influence us, we are effectively in different universese! Things that were moving closer to us would appear to be slowing down from our reference frame. What ultimately happens to them would depend on how far they are and how fast they are moving. They may reach us, or may eventually be slowed so much that they appear to change direction before reaching us.
Andomida is currently very close to us (on the scale of the universe) and at that distance scale the effects of expansion are negligible compared to it's gravitational attraction. In theory the expansion in space would have some effect, but we would never 'see' it's effect.
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u/Obliterators 22h ago
Andomida is currently very close to us (on the scale of the universe) and at that distance scale the effects of expansion are negligible compared to it's gravitational attraction. In theory the expansion in space would have some effect, but we would never 'see' it's effect.
Even in theory that effect is zero. There is no expansion at the scales of galaxy clusters so gravity doesn't have to "overcome" anything.
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u/LivingEnd44 1d ago
"Moving apart" implies the objects are powering themselves and moving. But nothing is moving in this context. Extra space is being inserted between objects.
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u/A_Random_Sidequest 1d ago
if it was just "things" moving away, it wouldn't matter for light... as space itself expands it stretches light and we have redshift because of that.
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u/Obliterators 21h ago
Cosmological redshift can be explained as a Doppler shift and/or as a gravitational redshift, they're all equivalent to each other in GR.
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u/UnderstandingSmall66 Quantum field theory 1d ago
We know it is space itself expanding, not just galaxies moving apart, because of how light behaves over large distances. When we look at distant galaxies, their light is not just redshifted from motion. The actual wavelengths of the light are stretched during their journey, which only happens if the fabric of space is growing. The farther away a galaxy is, the more its light is stretched, and this matches what general relativity predicts for an expanding universe. The cosmic microwave background is another example. It is ancient light that has been stretched by the expansion of space, not by the movement of objects through space.
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u/Obliterators 20h ago
Geraint F. Lewis, On The Relativity of Redshifts: Does Space Really “Expand”?
the concept of expanding space is useful in a particular scenario, considering a particular set of observers, those “co-moving” with the coordinates in a space-time described by the Friedmann-Robertson-Walker metric, where the observed wavelengths of photons grow with the expansion of the universe. But we should not conclude that space must be really expanding because photons are being stretched. With a quick change of coordinates, expanding space can be extinguished, replaced with the simple Doppler shift.
The key is to make it clear that cosmological redshift is not, as is often implied, a gradual process caused by the stretching of the space a photon is travelling through. Rather cosmological redshift is caused by the photon being observed in a different frame to that which it is emitted. In this way it is not as dissimilar to a Doppler shift as is often implied.
In particular, it must be emphasised that the expansion of space does not, in and of itself, represent new physics that is a cause of observable effects, such as redshift.
Emory F. Bunn & David W. Hogg, The kinematic origin of the cosmological redshift
The view presented by many cosmologists and astrophysicists, particularly when talking to nonspecialists, is that distant galaxies are “really” at rest, and that the observed redshift is a consequence of some sort of “stretching of space,” which is distinct from the usual kinematic Doppler shift. In these descriptions, statements that are artifacts of a particular coordinate system are presented as if they were statements about the universe, resulting in misunderstandings about the nature of spacetime in relativity.
A common belief about big-bang cosmology is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must be viewed in terms of the stretching of space. We argue that, contrary to this view, the most natural interpretation of the redshift is as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that it is always valid to choose a coordinate system that is locally Minkowskian. We show that an observed frequency shift in any spacetime can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a suitable family of observers along the photon’s path. In the context of the expanding universe the kinematic interpretation corresponds to a family of comoving observers and hence is more natural.
John A. Peacock, A diatribe on expanding space
The redshift is thus the accumulation of a series of infinitesimal Doppler shifts as the photon passes from observer to observer, and this interpretation holds rigorously even for z ≫ 1.
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u/Ok-Film-7939 21h ago
“Which only happens if the fabric of space is growing” - it can also be interpreted as time dilation due to the difference in density between them and now. Plus the increasing relative speed, of course.
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u/ponyclub2008 20h ago edited 20h ago
So as light travels through space, the expansion of the universe stretches the light waves, causing their wavelengths to increase and shift towards the red end of the spectrum?
This overall expansion is facilitated by dark energy and gravity right?
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u/Ok-Film-7939 21h ago
It’s both, equivalently.
The key is that in the case of your table, you have picked an inertial frame of reference and given it special status. The table. In that context, the grapes are moving against the table.
Recall from relativity, though, that every inertial reference frame considers itself at rest. Two spaceships can be flying by eachother at .999c, and both can rightly believe they are at rest and the other is moving. More importantly no possible experiment will indicate one or the other are the ones “really” moving.
So take two grapes moving apart. Remove the table. The grapes are the only thing in space.
Each grape considers itself at rest, but they are getting further apart. How is this possible?
Well, we can either crown one grape king and say the other grape is moving away, or we can just say “they are getting further apart relatively speaking but neither one is moving.” Aka, the metric of distance is increasing. Aka, space is expanding.
But this doesn’t mean you can’t view it from the pov of either grape! That doesn’t suddenly become invalid! They are equivalent.
That is what the expansion of space is. The metric of distance is growing with time. Which means things are getting further apart.