Timeline for How is the expansion of space distinguishable from objects moving away?
Current License: CC BY-SA 4.0
37 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Jul 12 at 4:38 | comment | added | Sten | As explained in the comments there, the Hubble force in non-dark energy cosmologies is a coordinate artifact. | |
| Jul 11 at 18:59 | comment | added | Yukterez | Your intuition led you to the wrong conclusion that without dark energy there would be no Hubble force on a static box though, which doesn't hold when you plug in the numbers, so if that is a fruit from the tree of nonexpanding space i would regard that as a red flag that your own intuition led you astray as well. | |
| Jul 10 at 20:28 | comment | added | Sten | I agree that the usual FLRW coordinates are convenient. I use them for calculations all the time! The point remains, however, that they are a coordinate choice and not a physical reality -- and that mistaking them for physical reality often leads to an erroneous intuition for how kinematics interact with cosmic expansion. | |
| Jul 10 at 5:31 | comment | added | Yukterez | therefore in those coordinates where the local clocks and rulers are defined in an intuitive way the description of expanding space fits, while the coordinates in which it is transformed into curved spacetime are exotic and use nongeodesic local clocks and rulers. | |
| Jul 10 at 5:27 | comment | added | Yukterez | you can transform the expanding space into curved spacetime, but then you have to use some Droste-style coordinates where the local observers that carry your clocks and rulers are no longer on geodesics but need proper acceleration and they won't work behind the Hubblesphere, so you won't win anything by doing that since the event and particle horizons are much farther away, so we need to work in coordinates that can handle that. In those the g⁰⁰=1, so no curved time there. You can transform the g⁰⁰ to something else than 1, but you can also with Minkowski if you chose accelerated local clocks | |
| Jul 10 at 4:56 | comment | added | Sten | @Yukterez The reason you need to small subintervals is not "expanding space", it's just that spacetime is curved. | |
| Jul 10 at 4:46 | comment | added | Yukterez | of course that integral is correct, but it is d'accord with an expanding space, otherwise you wouldn't need to integrate from one point to its neighbor. that's because the points themselves are receiding from each other, so the relative velocities along the path are that of space itself. | |
| Jul 10 at 4:38 | comment | added | Sten | @Yukterez Davis & Lineweaver address a "SR model" that never made sense in the first place. For example, of course SR Doppler is incorrect if you use the Hubble-flow velocities (e.g. they're defined in the present day -- how could the light arriving now know the present-day behavior of the source?). But if you special-relativistically add up the relative velocities along the spacetime path of the light, the SR Doppler formula is correct, as analyzed in detail by arxiv.org/abs/0808.1081 | |
| Jul 10 at 4:28 | comment | added | Yukterez | Sten wrote: "that misconception, especially in the early days" - to the contrary: your special relativistic claim to use the velocity addition formula to add up the galaxy velocities is old and outdated, while the Davis & Lineweaver PDF where they show why you can't do that in an expanding space is from 2003 and still considered standard literature on that subject, so that's the other way around, see page 5 where it says: "the velocity is due to the rate of expansion of space", and the math says the same. | |
| Jul 10 at 4:25 | comment | added | Sten | @Yukterez Many physicists had that misconception, especially in the early days. I can't access right now, but I would be surprised if Einstein didn't. Sorry if I suggested you don't understand relativity. My intention was to not to say "everyone who understands relativity agrees with me" but rather that scientific arguments may work for that population but will not work for the pop sci group. | |
| Jul 10 at 4:22 | comment | added | Yukterez | If you think i don't understand relativity that's fine, but if you want to take it from the grandmaster who not only understood but invented relativity listen to Albert Einstein himself as he introduces the expanding space: click - in an infinite universe it might be harder to understand, but the arguments become clear if you take a closed universe first and then take the limit to make it infinite. | |
| Jul 10 at 4:07 | comment | added | Sten | @Yukterez The point of the "take it from eminent cosmologists" portion is that the "expanding space" misconception is so pervasive in pop science that people who don't understand relativity are predisposed to believe it and are unlikely to be convinced by relativistic arguments to the contrary. That section is aimed at that population. | |
| Jul 10 at 3:45 | comment | added | Yukterez | Sten wrote: "velocities in relativity add in a special way; see the relativistic velocity addition formula" - the special relativistic addition is not the proper way to add the recessional velocities, that misconception has been debunked in Expanding Confusion. If the velocities were just added like in special relativity light from everywhere could reach you. Sten wrote: "If you are not sure, then, whether to believe random people on Stack Exchange, take it from eminent cosmologists instead" - 1) pop science and 2) argumentum ad verecundiam | |
| Sep 6, 2023 at 17:27 | comment | added | Sten | @Gintaras Perhaps not! I thought you were referring to angular diameter turnover. | |
| Sep 6, 2023 at 17:25 | comment | added | Gintaras | @Sten you didn't understand my statement. | |
| Sep 6, 2023 at 17:11 | comment | added | Sten | @Gintaras In the context of general relativity, angular sizes grow at large distances due to gravitational lensing by the universe's mass distribution. It converges light from distant sources, making them appear bigger. In the standard FLRW cosmology, you can check explicitly that this effect diminishes as you bring down the density of the universe (raise $\Omega_k$). When $\Omega_k = 1$ (negligible energy density), angular sizes never grow with distance. | |
| Sep 6, 2023 at 16:31 | comment | added | Gintaras | @Sten there is one way to distinguish actually, angular diameter. Expanding space expands: 1) distance 2) angular diameter. Objects moving away expands: 1) distance. In non-expanding space angular diameter doesn't change. There are observations like:"Giant young galaxies appear to be too compact", "packing as many stars as the Milky Way, but in a size 30 times smaller". It favours non-expanding space. There are not compact, object's size is model dependant. In the end, space does not expand. | |
| Sep 6, 2023 at 16:21 | comment | added | Gintaras | Atif, superluminous argument is completely bogus. You can't take distances from one model and compare redshifts with another model. That doesn't work. Distance is model dependant and derived variable, not an observation. Locally we do measure distances by angular velocities in the sky. However in cosmology we don't do that. What we measure is "luminosity distance" which is very different thing. For expanding space hypothesis and for objects moving away hypothesis, both theories predict the same luminosity distance - redshift relationship, and as Sten mentioned, it's indistinguishable | |
| Jul 7, 2023 at 23:27 | comment | added | Sten | @Atif To be clear, I am answering (as stated at the outset) within the context of general relativity. If you wish to depart from that paradigm, I'll not stop you! | |
| Jul 7, 2023 at 23:13 | history | edited | Sten | CC BY-SA 4.0 |
deleted 281 characters in body
|
| Jul 7, 2023 at 23:13 | comment | added | Atif | For example, what happened to "Energy can neither be created nor be destroyed but can only be converted from one form to another"? What is being converted to Dark Energy? Where do the Dark Energy come from? No where. The above theory is proved wrong by observation that galaxies are accelerating away from us. Net energy is being added to this universe. "See relativistic velocity addition" Why? Observation proved the theory wrong. Why apply it? Its very dangerous to use a wrong theory in your calculations because its not a matter of degree. Its a matter of fundamentals. You can be hit anywhere. | |
| Jul 7, 2023 at 23:13 | history | edited | Sten | CC BY-SA 4.0 |
deleted 281 characters in body
|
| Jul 7, 2023 at 23:06 | comment | added | Atif | Nobody knows what Dark Energy is. It just a term invented to fill hole in theory of relativity. Look at history of the term. As science advance more and more data come into observation because of more precise equipments. Theory of Relativity is unable to explain the data. Instead of correcting the theory or making another, professors went the lazy way or making up things willy nilly. They are loose variable because they aren't defined beyond "the thing I need here to plug this hole". They cannot be defined because they aren't known. | |
| Jul 7, 2023 at 23:03 | history | edited | Sten | CC BY-SA 4.0 |
add paragraph explain recession rates
|
| Jul 7, 2023 at 22:55 | comment | added | Sten | @Atif See relativistic velocity addition. If two particles are moving in opposite directions, each at 0.9 the speed of light, then they are moving at (0.9 + 0.9)/(1 + 0.9*0.9) = 0.994 the speed of light relative to each other. | |
| Jul 7, 2023 at 22:51 | comment | added | Sten | @Atif Regarding photons not reaching us: that's because of the gravitational influence of the dark energy. (Just like why photons can't reach us from inside a black hole's horizon.) Note that this doesn't correspond to the galaxy reaching a recession rate equal to the speed of light. As you have been arguing yourself, we already observe galaxies with recession rates greater than the speed of light. It's really just a function of how much dark energy there is and how large the distance is. | |
| Jul 7, 2023 at 22:48 | comment | added | Atif | "Does the fact that two particles ..." How it don't? Aren't all frames-of-reference equally valid? Is there any data that show that one particle do not see the other moving away from it at superluminal speed? Even if there is it dont prove the theory of relativity. One counter example is enough to disprove a theory. It don't matter how many pro example you being then. | |
| Jul 7, 2023 at 22:37 | comment | added | Atif | Please don't mix things up. Ofcourse photons emitted millions of years ago will continue to reach us for millions of years because then the galaxy wasn't moving away from us with speed of light or above. The discussion is about photons generated after the galaxy went superluminal. Come to think of this, if space is not expanding then even those photons will reach us and normally so. The photons always travel at speed of light ofcourse so whether or not the galaxy went superluminal do not matter. We will continue to see the galaxy. There cannot be a Hubble Sphere then. What do you think? | |
| Jul 7, 2023 at 22:37 | comment | added | Sten | @Atif As explained in one of the linked answers, the cosmological recession rate is what you get if consider a sequence of galaxies leading to your target object and add the relative velocities between all of the galaxies without using the relativistic velocity addition formula. So we should no more worry about the result being superluminal here than we should worry about the two oppositely moving particles having a superluminal relative velocity. | |
| Jul 7, 2023 at 22:30 | comment | added | Sten | @Atif Does the fact that two particles can move in opposite directions each at more than half the speed of light disprove relativity? | |
| Jul 7, 2023 at 22:29 | comment | added | Atif | Agreed that galaxies moving at superluminal speeds away from us is a deduction, not an observation, but its a deduction based on data and there is no data that go against this deduction. "Again, its not a relative velocity" If you mean relative as a term in theory of relativity then don't keep bringing theory of relativity up because it dont agree with data. Its like being a witness in your own case. Theory of relativity cannot prove itself, duh. If you mean relative in normal sense then how? If a thing move away from me then it moved away relative to me, had a velocity relative to me. | |
| Jul 7, 2023 at 22:23 | comment | added | Sten | @Atif To clarify that last point, since I've seen misconception about it elsewhere too: there are galaxies that "leave our horizon" at some point in their history. We won't see the future of those galaxies beyond some point. But they never disappear from our view -- we just continue to see them in the past. This is similar to a black hole's event horizon, if you've heard of what happens there. | |
| Jul 7, 2023 at 22:17 | comment | added | Sten | @Atif We don't have data showing that things move faster than light. The recession rate is inferred theoretically, and we already know it can be faster than light even without observations. Again, it's not a relative velocity. (See the linked answers explaining what the recession rate means!) Also, we don't "stop seeing" galaxies, nor do we expect to. Neither the "expanding space" nor the "galaxies moving apart" conventions (which are again equivalent) predict that. | |
| Jul 7, 2023 at 22:11 | comment | added | Atif | ... superluminal. This is observation. It goes against the speed of light assumption in theory of relativity. It makes the theory a fairytale. The theory do not explain this universe even in approximation because the base of theory is gone. Theories are based on assumptions. Its a matter of fundamentals. The theory now explain a fictional universe. So please don't keep bringing it up. Try to think outside that theory and give an explanation. I don't downvote your answer because its clear and logical. It just don't explain anything outside an anti-observation theory. | |
| Jul 7, 2023 at 22:02 | comment | added | Atif | Theory of Relativity has assumption that nothing that has mass can travel faster than speed of light. The theory is built upon that. If anything massive can be shown to move faster than speed of light then the theory just become a fairytale, may be symmetric and beautiful, even logical but dont explain anything about this universe. We have data that show galaxies - that are obviously massive - moving away from each other at speed of light and since we observe them still accelerating - photons from them continue to come redshifted - AND we stop seeing them after that we can deduce they go ... | |
| Jul 6, 2023 at 18:18 | history | edited | Sten | CC BY-SA 4.0 |
added 18 characters in body
|
| Jul 6, 2023 at 11:53 | history | answered | Sten | CC BY-SA 4.0 |