6 ms·

Anomalous contribution to galactic rotation curves due to stochastic spacetime

143 points by kergonath 3y ago

bloopernova 3y ago
Related article from The Guardian: https://www.theguardian.com/science/2024/mar/09/controversial-new-theory-of-gravity-rules-out-need-for-dark-matter https://www.theguardian.com/science/2024/mar/09/controversia...
- dbacar 3y ago
  Thank you, this is much more accessible for me.
- boxed 3y ago
  That sounds very unconvincing. Afaik there are several found galaxies with more than normal amounts of dark matter, and less. How would these fit into ANY modified gravity theory? This type of theory seems to only handle the common case, but the universe is full of edge cases...
  - scotty79 3y ago
    If the base of this theory is some form of chaos you can always fit the distribution of the chaos to explain outliers with the probability they actually occur at.
    - BlueTemplar 3y ago
      Can you ? How is this different from, say, anything involving pressure and temperature, which under current theories, are statistical phenomena arising from quantum behaviour, fundamentally based on our lack of information about what is going on (aka entropy) ?
      - scotty79 3y ago
        I think technically you can. Although it's just moving the question to "why is the chaos distributed like that?". Current explanation for existence of galaxies is those are quantum fluctuations that grew large. So apparently fluctuations can explain everything if you do enough of hand waving in between. I don't think any particular quality of galaxies as we see them today can be traced to specific quantum property. But that doesn't stop us from believing in that explanation.
        data_maan 3y ago
        Isn't that already enough to know that it is some form chaos that underlies many of the current phenomena? For any explanation X there will always be the question, "now what explains X"?
      - Vecr 3y ago
        I don't think entropy can be defined as your lack of knowledge, at least straight out. You have to be careful as under strict Bayesianism knowledge can't ever decrease, implying that entropy is monotonically non-increasing, and that's clearly wrong.
        BlueTemplar 3y ago
        Then I'll side with Bayesianism being wrong rather than statistical physics being wrong : http://www.av8n.com/physics/thermo/entropy.html#sec-s-not-knowing http://www.av8n.com/physics/thermo/entropy.html#sec-s-not-kn... http://www.av8n.com/physics/thermo/entropy-more.html#sec-s-context http://www.av8n.com/physics/thermo/entropy-more.html#sec-s-c... http://www.av8n.com/physics/thermo/s-relevance.html#ch-s-relevance http://www.av8n.com/physics/thermo/s-relevance.html#ch-s-rel...
        Vecr 3y ago
        Bayesianism is not wrong (it more or less can't be), the main problem is exactly what you mean by "knowing" and "entropy". Obviously, an embedded agent knowing from within physics is limited, but that's not a problem with theoretical Bayesianism, and it's not a problem if you're dealing with cards.
        BlueTemplar 3y ago
        Wrong when applied in the general context of fundamental physics ?? (See also how entropy can often be approximated as an objective and property of a system, its density intensive, but in the general case that's wrong.) Speaking of, I'm somewhat puzzled, you said that "under strict Bayesianism knowledge can't ever decrease", but how does it then deal with something as simple as card shuffling ??
        Vecr 3y ago
        Depending on how you want to do it, the hypotheses that get probabilities can be represented as computer programs that predict the input the agent receives. If done correctly (perfectly if you want it to be absolute) the probability-weighted sum of the calibration metrics won't ever go down.
    - boxed 3y ago
      That makes no sense. You can't form entire stable galaxies and have them rotate cleanly in a disk for millions of years with "chaos".
  - floxy 3y ago
    If a new theory fits 199,999,999,985 galaxies, you deal with the remaining 15 edge cases one at a time.
    - boxed 3y ago
      I hope that was sarcasm...
- layer8 3y ago
  The Guardian headline stands in contradiction to this statement from the paper’s abstract: “We caution that a greater understanding of this effect is needed before conclusions can be drawn”.
  - bloopernova 3y ago
    The byline specifically says: "Exclusive: Paper by UCL professor says ‘wobbly’ space-time could instead explain expansion of universe and galactic rotation" "Could instead explain" is not a definitive statement. Their headline is "Controversial new theory of gravity rules out need for dark matter" "Controversial new" tells me that this isn't settled science, nor is the Guardian trying to say otherwise. "Rules out" by itself, out of context, could imply proven science, but not within the context of the headline and byline.
ramshanker 3y ago
[flagged]
- deleted 3y ago
  [deleted]
quantum_mcts 3y ago
Good. Now explain the Bullet Cluster.
- naasking 3y ago
  No one can explain the Bullet Cluster.
  - quantum_mcts 3y ago
    Cold dark matter made of weakly interacting massive particles can.
    - naasking 3y ago
      No, LCDM can't explain the observed velocities.
      - quantum_mcts 3y ago
        Yes it can. https://arxiv.org/abs/1410.7438 https://arxiv.org/abs/1410.7438
- raattgift 3y ago
  > Good. Is it, or isn't it, and why? You don't say. The authors asked themselves how their quantum gravity theory differs from General Relativity, and whether the successes of General Relativity in astrophysical settings would be fatal if their theory has strong differences, and that's the basis for this paper. The tl;dr is that their theory predicts different trajectories outside of large central masses, but that might not conflict with evidence from galactic-dynamics astronomy. This is the second paper released in the past few days by the University College London Oppenheim group. It's a preliminary investigation of the longer length scale features of their classical stochastic theory. The central question is how its version of Schwarzschild-de Sitter (SdS) differs from standard General Relativity. The first paper, and I think the more interesting one, is about the short length scale aspects of their asymptotically free theory, in which the gravitational interaction weakens as distances between interacting sources decreases. The asymptotic freedom means the theory is amenable to renormalization, unlike perturbative quantum gravity and a number of other approaches. That paper is at <https://arxiv.org/abs/2402.17844 https://arxiv.org/abs/2402.17844>. Note that they do not know how to make the gravitational part quantum mechanical without introducing problems (i.e., it is haunted by "bad" ghosts in the sense of <https://en.wikipedia.org/wiki/Ghost_(physics) https://en.wikipedia.org/wiki/Ghost_(physics)>); their classical and stochastic gravitational sector is ghost-free (a point also made at the end of Appendix A in the large-scale paper), and it is reasonable for them to believe that could be good enough that it's worth continuing to investigate what the theory predicts and how its parameters are set. The second paper was motivated by the first: "The theory was not developed to explain dark matter, but rather, to reconcile quantum theory with gravity. However, it was [noted] that diffusion in the metric could result in stronger gravitational fields when one might otherwise expect none to be present, and that this raised the possibility that gravitational diffusion may explain galactic rotation curves". That MOND-like effects might arise in their approach to the problem of small-scale quantum gravity is at least interesting. It was not the starting point. Moreover, they did not start with the idea of modifying General Relativity to get rid of the need for (some or all) cold dark matter. As they say: "While this study demonstrates that galactic rotation curves can undergo modification due to stochastic fluctuations, a phenomenon attributed to dark matter, it is important to acknowledge the existence of separate, independent evidence supporting ΛCDM. In particular, in the CMB power spectrum, in gravitational lensing, in the necessity of dark matter for structure formation, and in a varied collection of other methods used to estimate the mass in galaxies." > Now explain the Bullet Cluster This paper does not seek to do so. "To make it tractable analytically, we have restricted ourselves to spherically symmetric and static spacetimes, with metrics of the form of Eqs. (17)." Eqn 17 describes out an adapted Schwarzschild-de Sitter spacetime and leans on an argument that Birkhoff's theorem applies (in particular that their model spacetime is stable against certain perturbations, notably those concentric upon the source mass). There is further detail in Appendix B. Studying this restricted model, the de Sitter expansion of the spacetime and MOND-like anomalous Kepler orbits at some remove from the Schwarzschild central mass are in their theory driven by entropic forces generated by the fluctuations in the gravitational field of the central mass (and they do a good job in Appendix D explaining this). In GR's Schwarzschild-de Sitter the free-fall trajectories of test particles around the central mass are totally determined by the mass; the gravitational field doesn't fluctuate. The (Boltzmann) gravitational entropy of the region outside the central mass is everywhere very high. In GR-SdS we can consider adaptations where with M=const. we turn the pointlike central mass into a spherically symmetric shell, or a concentric set of such shells, or even a ball of fluid, or a ball of dust, or a ball of stars and other galactic matter. None of these symmetry-preserving adaptations changes the free-fall trajectories of test particles outside the outer surface, or the gravitational entropy at any outside point. In the author's theory, the spacetime is stochastic. It fluctuates. Close to the central mass fluctuations are unnoticeably small; the gravitational entropy is very low. Far from the central mass the gravitational entropy is very high, and gravitational fluctuations are noticeable. A sort of thermodynamics leads to a diffusive flow outwards from the central mass, from the low entropy near there to the high entropy at increasing radial distance. This diffusion is carefully constructed so that the outwards flow is only really appreciable at large-scale distances. The effect is that large-radius orbits are statistically pulled inwards by something describable as stronger gravity at larger radiuses (see around Eqn (21)). This is an "entropic force", very roughly analogous to squashing a sponge ball in your hand then releasing the pressure and watching the sponge ball expand, where the material of the sponge represents the gravitational field. Their stochastic fluctuations are still generated by the spherically-symmetric central mass. These fluctuations break the spherical symmetry of the outside metric. Consequently they have to do some work to make the outside metric look appropriately Schwarzschild-like in their "diffusion regime", and to keep that stable against the stochastic perturbations. The authors contend that with reasonable choices of parameters, and restricted to static spherical symmetry of the central mass (and no additional dynamics), this effect comes close to duplicating MOND's low-acceleration regime. They don't go into anything like a backreaction upon the Schwarzschild metric by large fluctuations. (They do have an idea about how to get the de Sitter trajectories though, but that doesn't fit very naturally into this comment, which is already long.) > Bullet cluster The authors know full well that the metric for a gravitationally bound cluster of galaxies isn't well-represented by their choice of SdS-like metric. A galaxy cluster is too lumpy for the Schwarzschild part. Two gravitationally bound galaxy clusters having passed through each other (trailing collided gas and dust, and tidally stripped stars and other matter) is even less like Schwarzschild. This is because SdS solutions of the Einstein Field Equations do not linearly superpose. So their metric is a poor description of any sort of "close call" interaction between galaxies or galaxy clusters, even if the individual components are "close enough" to Schwarzschild from the perspective of an observer sufficiently large (as in cosmologically large) distances. They do not (and within this initial paper should not really be expected to) offer a more suitable metric. I'm sure they'd love to look into things like that though. The non-linear superposeability of useful solutions of General Relativity is a problem for asking how astrophysics differ in most theories that preserve the equivalence principle (this one does, it's a metric theory of gravitation). As the replacement for the Einstein Field Equations lose symmetries (sphericity, staticity) they tend to become analytically intractable and non-numerical approximations become unreliable. The authors -- imho in a strikingly principled way -- call attention to various difficulties in using this work to describe astrophysical systems, particulary from the middle of the fifth page of the PDF. They are not obviously worse off than the Verlinde programme of emergent-entropic gravity, where the gravitational field is generated by entropic forces rather than vice-versa.
indeyets 3y ago
But… WHY is it wobbly? Is there a particle which represents this wobbliness? :)
- SideburnsOfDoom 3y ago
  Is that the "postquantum" bit? i.e. not that spacetime is quantized as such, but that it's not a uniform continuous medium at all scales, that classical mechanics would assume.
- scotty79 3y ago
  I can imagine that wobbliness comes from chaotically passing strong gravity waves of very high wavelengths coming from outside of the observable universe and from before times of Big Bang. There's no specific reason that forces us to believe that Big Bang was the actual beginning of everything. It might have been "local" high energetic event happening in much larger structure. For example try to imagine how the collision of two dense clusters of trillions of black holes each traveling at nearly light speed would look like from the point of view of the dust swirling in-between and around them. I can't imagine it would be very different from what we observe in our universe.
  - MauranKilom 3y ago
    Don't you think that we would be able to observe some anisotropy in the data if that were the case? (Note: I have no clue, but it seems unlikely...)
    - scotty79 3y ago
      Maybe? I don't know. I imagine we don't have any good ways to observe gravitational waves, even fairly strong ones with periods of millenia or even centuries which are still very short times on a galactic scale. And apart from that, I think that we can only look at CMB and conclude that it's a bit wobbly. Must be quantum fluctuations, sure, why not, but is it only the quantum fluctuations? Or maybe the spacetime between us and CMB source is a bit wobbly too? Another thing is large scale structure of our universe. Visualizations look like foam. Planty of chaos that wobblines can safely compose into. I think the work of those scientists is very important because it might allowed us to pinpoint how strong distortions of the spacetime would need to be to explain what we see and maybe we could narrow down the range of frequencies which might gives us ideas how to look for them. Simplistic and grandiose assumptions make our current best model of the universe a bit restrictive. To the point that we are starting to find direct counterexamples for our theories derived from it. Mature galaxies way too old. Megastructures way too large. CMB fluctuation not exactly fitting best theoretical models. Unreconcilable differences between Hubble constant measured from CMB and that measured from galaxies. I think accepting a bit of chaos beyond what we currently believe is an inevitable way out.
oneshtein 3y ago
Finally, someone is at right path.
A_D_E_P_T 3y ago
There's something distinctly unsatisfying about this sort of paper, because the number of potential solutions to the problem they're trying to solve is in principle vast, and you can arrive at any number of them if you laboriously work backwards and fit your equations to the data. Without very rigorous efforts towards empirical validation, in novel ways if required, this sort of thing is just another wholly speculative theory to add to the large and growing pile. The interesting thing, as far as I'm concerned, is the size and shape of the answer space. How many theoretical solutions can be coaxed to fit the cosmological data? It can't be infinite, but it seems as though it's a rather large number.
- szundi 3y ago
  This is exactly what Sabine Hossenfelder is talking about for years now.
  - naasking 3y ago
    Indeed, and even she thinks this is at least a fairly novel approach in this space which hasn't seen much new thinking recently (20+ years).
- hnaccount_rng 3y ago
  To be fair, everything that astrophysics comes up with will be massively under constrained. For all intents and purposes we work with a single datapoint and try to infer dynamics.
  - Nevermark 3y ago
    Exactly. A few "data points", i.e. unexplained effects, and then Occamm's razor, parsimony, etc. I really like the parsimony of this paper. If a very small detail of space-time, i.e. stochastic behavior at the Planck scale, results in an emergent effect that explains something on the cosmological scale, that is explaining a lot with a little. Stochastic might not even mean random or non-deterministic (as I understand - and as the paper suggests, i.e. non-quantum), it could just be chaotic. Which would be very parsimonious. So many seemingly different effects we observe are the result of very simple things, under stochastic, statistical, entropy, thermodynamic type situations.
josh-sematic 3y ago
Note that this isn’t a “get out of dark matter free” card, as there are clear signatures of dark matter in the cosmic microwave background. https://physicsworld.com/a/dark-energy-spotted-in-the-cosmic-microwave-background/ https://physicsworld.com/a/dark-energy-spotted-in-the-cosmic...
- willis936 3y ago
  Only if we've constrained ourselves to take inflation as true.
  - Vecr 3y ago
    Inflation in what era? There has to have been some inflation at some point, right, otherwise how could the very hot period that's considered to be right after the start of the universe be low entropy? Sure, maybe you could say there's no big bang and time runs the other way on the other side of that low entropy state, but there has to be some sort of low entropy state.
    - tsimionescu 3y ago
      It could be that it is actually not low entropy, and there are some other processes that explain a recution in entropy at some scale. That is, maybe the second law of thermodynamics is not that universal.
      - Vecr 3y ago
        That would be really weird. I'd be more willing to buy galaxies just coming into existence somehow in the gaps as the universe expands, as that at least drives the arrow of time consistently in the right direction. The arrow of time can survive local entropy reduction, but probably not what you're talking about.
    - willis936 3y ago
      >Inflation in what era? Why the inflationary epoch of course. https://en.wikipedia.org/wiki/Inflationary_epoch https://en.wikipedia.org/wiki/Inflationary_epoch I'm saying the very notion of the big bang warrants more scrutiny. All singularities do.
nabla9 3y ago
>"explain galactic rotation curves without needing to evoke dark matter." I wonder if this makes it incompatible with dark matter? If stochastic spacetime explains the rotation curves exactly without black matter, then black matter can't exist at the same time. The evidence for dark matter is find in, galaxy rotation curves, gravitational lensing, cosmic microwave background, structure formation, bullet cluster and other galaxy cluster collisions, baryon acoustic oscillations, Lyman-alpha forest absorption lines, etc. There is plenty of evidence for dark matter and it's existence is widely accepted. Explaining just one piece of evidence can be evidence against the new explanation if it's incompatible.
- raattgift 3y ago
  > Can dark matter and [the author's theory of gravitation] fit together...? Nobody knows yet. The authors aren't seeking to overthrow the concordance cosmology; this paper is essentially trying to see if their theory is quickly killed by being unable to model a stable spherically symmetric galaxy in an expanding universe. Their theory predicts significant differences from the Schwarzschild-de Sitter (SdS) metric in General Relativity, with SdS serving as a proxy for an isolated galaxy. The paper in part investigates whether those differences can be "hidden" by removing the cold dark matter which otherwise would be the standard source of outer orbit anomalies. They don't go deeper than that, and at this stage just can't: "To make it tractable analytically, we have restricted ourselves to spherically symmetric and static spacetimes". They restrict themselves in other ways too. > The evidence for dark matter From just after Eqn (23) in the preprint: "While this study demonstrates that galactic rotation curves can undergo modification due to stochastic fluctuations, a phenomenon attributed to dark matter, it is important to acknowledge the existence of separate, independent evidence supporting ΛCDM. In particular, in the CMB power spectrum, in gravitational lensing, in the necessity of dark matter for structure formation, and in a varied collection of other methods used to estimate the mass in galaxies"
  - nabla9 3y ago
    If stochastic spacetime explains the rotation curves exactly without black matter, then how black matter can't exist at the same time?
_obviously 3y ago
[flagged]
- clysm 3y ago
  The fuck... I can't believe what I've just read. That's like saying Newton set us back over a hundred years. There was no regression because of Einstein.
  - _obviously 3y ago
    [flagged]
  - oneshtein 3y ago
    Mainstream theories are so good that they dwarfed competition. Lack of competition caused lack of progress. It was easy to collect hundreds of downvotes on HN just by talking about alternative theories or about physical representation of abstract theories. Now such comments will collect mere dozens of downvotes. It also should be obvious that a new successful theory will account for more things, so it will be much more complex than previous theories.
- AnimalMuppet 3y ago
  Yeah, um... if you're going to claim something like that, you need more than the claim. What evidence do you see for your position? What evidence can you give us so that we can evaluate your position?
nukeyoular 3y ago
[flagged]
fallingfrog 3y ago
I wonder how this relates to Erik Verlinde’s entropic gravity? https://arxiv.org/abs/1611.02269 https://arxiv.org/abs/1611.02269 It seems like this theory is proposing some entropic term on top of classical general relativity whereas Verlinde came up with a completely entropic theory of gravity. They both claim to explain dark matter.
kromem 3y ago
I love their terminology of 'postquantum.' There's a certain irony that a hundred years ago Physics was divided between bitterly divided camps declaring light as continuous or as quantized until finally they found it was a duality of both, specifically converting around the inflection point of interactions with persistence (i.e. no quantum eraser). So it's a bit bizarre to watch modern physics repeating the process with spacetime and the commitment to finding a 'solution' to quantum gravity, particularly when including notions of quantized particles like gravitons or dark matter that can't be directly interacted with in persistent ways (in which case, why are they expected to be quantized and not continuous?). I know most physicists proudly state that they only concern themselves with 'how' and not 'why' - but it's refreshing to see work that clearly had the wherewithal to ask themselves 'why' regarding a number of broad assumptions about the fundimentality of quantization as a default mode and I look forward to more 'postquantum' work in the future.
- eigenket 3y ago
  I think there's a bit of language problem here. In the way physicists use the word in 2024, making gravity "quantum" means something very different to just having discrete "quanta" particles. It means a theory with stuff like superpositions of classical states,interference and entanglement. It also means having a theory which can consistently interact with other quantum stuff. The standard through experiment goes something like this. Take a particle with some mass (maybe am electron, maybe a bowling ball), general relativity tells us what its mass does to space-time around it. Now take that particle and put it in a superposition of being located at point A and point B (you can do this if the particle is quantum). What the heck is it doing to spacetime now? GR doesn't tell us, because GR can't deal with quantum particles. Naively it seems like space-time should also be in a superposition, but we tried to make theories like that and they didn't really work.
  - kromem 3y ago
    Yes, I'm familiar with the thought experiments. My favorite to date was Zych, et al Bell’s theorem for temporal order (2019). But it still seems a bit like putting the cart ahead of the horse given the amount of resources dedicated to the search when the smallest gravitational field measurements to date (or at least as of 2021) were on a 92mg object. And while in theory the largest mass to be put in a quantum superposition to date was 16mg, the degree of separation in the vibrational superposition is still well below an order of magnitude gap from a difference detectable with current gravitational measurement. For a field where behavior changes in relationship to measurement and the persistence of information, there's not currently any measurable discrepancy between GR and QM. It's literally only in gedankenexperiments there's a conflict. Maybe the difficulty in preserving coherence for larger objects relates to their inadvertently leaving measurable information in spacetime? Perhaps we'll find there's a fundamental limit on gravitational measurement that happens to be right around the limit at which we can maintain coherence. It just seems like the kind of quest that's premature when we can't measure a conflict existing outside of our imaginations, and where the mystery will likely resolve almost immediately after we can actually measure the conflict in the first place (should we ever be able to do so). So yes, if we end up with an entire planet in a quantum superposition near two different spaceships shooting at each other, things might get weird. But at least currently that 'if' seems to be doing a lot of heavy lifting.

143 points by kergonath 3y ago

Related article from The Guardian: https://www.theguardian.com/science/2024/mar/09/controversial-new-theory-of-gravity-rules-out-need-for-dark-matter https://www.theguardian.com/science/2024/mar/09/controversia...

dbacar 3y ago
Thank you, this is much more accessible for me.
boxed 3y ago
That sounds very unconvincing. Afaik there are several found galaxies with more than normal amounts of dark matter, and less. How would these fit into ANY modified gravity theory? This type of theory seems to only handle the common case, but the universe is full of edge cases...
- scotty79 3y ago
  If the base of this theory is some form of chaos you can always fit the distribution of the chaos to explain outliers with the probability they actually occur at.
  - BlueTemplar 3y ago
    Can you ? How is this different from, say, anything involving pressure and temperature, which under current theories, are statistical phenomena arising from quantum behaviour, fundamentally based on our lack of information about what is going on (aka entropy) ?
    - scotty79 3y ago
      I think technically you can. Although it's just moving the question to "why is the chaos distributed like that?". Current explanation for existence of galaxies is those are quantum fluctuations that grew large. So apparently fluctuations can explain everything if you do enough of hand waving in between. I don't think any particular quality of galaxies as we see them today can be traced to specific quantum property. But that doesn't stop us from believing in that explanation.
      - data_maan 3y ago
        Isn't that already enough to know that it is some form chaos that underlies many of the current phenomena? For any explanation X there will always be the question, "now what explains X"?
    - Vecr 3y ago
      I don't think entropy can be defined as your lack of knowledge, at least straight out. You have to be careful as under strict Bayesianism knowledge can't ever decrease, implying that entropy is monotonically non-increasing, and that's clearly wrong.
      - BlueTemplar 3y ago
        Then I'll side with Bayesianism being wrong rather than statistical physics being wrong : http://www.av8n.com/physics/thermo/entropy.html#sec-s-not-knowing http://www.av8n.com/physics/thermo/entropy.html#sec-s-not-kn... http://www.av8n.com/physics/thermo/entropy-more.html#sec-s-context http://www.av8n.com/physics/thermo/entropy-more.html#sec-s-c... http://www.av8n.com/physics/thermo/s-relevance.html#ch-s-relevance http://www.av8n.com/physics/thermo/s-relevance.html#ch-s-rel...
        Vecr 3y ago
        Bayesianism is not wrong (it more or less can't be), the main problem is exactly what you mean by "knowing" and "entropy". Obviously, an embedded agent knowing from within physics is limited, but that's not a problem with theoretical Bayesianism, and it's not a problem if you're dealing with cards.
        BlueTemplar 3y ago
        Wrong when applied in the general context of fundamental physics ?? (See also how entropy can often be approximated as an objective and property of a system, its density intensive, but in the general case that's wrong.) Speaking of, I'm somewhat puzzled, you said that "under strict Bayesianism knowledge can't ever decrease", but how does it then deal with something as simple as card shuffling ??
        Vecr 3y ago
        Depending on how you want to do it, the hypotheses that get probabilities can be represented as computer programs that predict the input the agent receives. If done correctly (perfectly if you want it to be absolute) the probability-weighted sum of the calibration metrics won't ever go down.
  - boxed 3y ago
    That makes no sense. You can't form entire stable galaxies and have them rotate cleanly in a disk for millions of years with "chaos".
- floxy 3y ago
  If a new theory fits 199,999,999,985 galaxies, you deal with the remaining 15 edge cases one at a time.
  - boxed 3y ago
    I hope that was sarcasm...
layer8 3y ago
The Guardian headline stands in contradiction to this statement from the paper’s abstract: “We caution that a greater understanding of this effect is needed before conclusions can be drawn”.
- bloopernova 3y ago
  The byline specifically says: "Exclusive: Paper by UCL professor says ‘wobbly’ space-time could instead explain expansion of universe and galactic rotation" "Could instead explain" is not a definitive statement. Their headline is "Controversial new theory of gravity rules out need for dark matter" "Controversial new" tells me that this isn't settled science, nor is the Guardian trying to say otherwise. "Rules out" by itself, out of context, could imply proven science, but not within the context of the headline and byline.

ramshanker 3y ago

[flagged]

deleted 3y ago
[deleted]

quantum_mcts 3y ago

Good. Now explain the Bullet Cluster.

naasking 3y ago
No one can explain the Bullet Cluster.
- quantum_mcts 3y ago
  Cold dark matter made of weakly interacting massive particles can.
  - naasking 3y ago
    No, LCDM can't explain the observed velocities.
    - quantum_mcts 3y ago
      Yes it can. https://arxiv.org/abs/1410.7438 https://arxiv.org/abs/1410.7438
raattgift 3y ago
> Good. Is it, or isn't it, and why? You don't say. The authors asked themselves how their quantum gravity theory differs from General Relativity, and whether the successes of General Relativity in astrophysical settings would be fatal if their theory has strong differences, and that's the basis for this paper. The tl;dr is that their theory predicts different trajectories outside of large central masses, but that might not conflict with evidence from galactic-dynamics astronomy. This is the second paper released in the past few days by the University College London Oppenheim group. It's a preliminary investigation of the longer length scale features of their classical stochastic theory. The central question is how its version of Schwarzschild-de Sitter (SdS) differs from standard General Relativity. The first paper, and I think the more interesting one, is about the short length scale aspects of their asymptotically free theory, in which the gravitational interaction weakens as distances between interacting sources decreases. The asymptotic freedom means the theory is amenable to renormalization, unlike perturbative quantum gravity and a number of other approaches. That paper is at <https://arxiv.org/abs/2402.17844 https://arxiv.org/abs/2402.17844>. Note that they do not know how to make the gravitational part quantum mechanical without introducing problems (i.e., it is haunted by "bad" ghosts in the sense of <https://en.wikipedia.org/wiki/Ghost_(physics) https://en.wikipedia.org/wiki/Ghost_(physics)>); their classical and stochastic gravitational sector is ghost-free (a point also made at the end of Appendix A in the large-scale paper), and it is reasonable for them to believe that could be good enough that it's worth continuing to investigate what the theory predicts and how its parameters are set. The second paper was motivated by the first: "The theory was not developed to explain dark matter, but rather, to reconcile quantum theory with gravity. However, it was [noted] that diffusion in the metric could result in stronger gravitational fields when one might otherwise expect none to be present, and that this raised the possibility that gravitational diffusion may explain galactic rotation curves". That MOND-like effects might arise in their approach to the problem of small-scale quantum gravity is at least interesting. It was not the starting point. Moreover, they did not start with the idea of modifying General Relativity to get rid of the need for (some or all) cold dark matter. As they say: "While this study demonstrates that galactic rotation curves can undergo modification due to stochastic fluctuations, a phenomenon attributed to dark matter, it is important to acknowledge the existence of separate, independent evidence supporting ΛCDM. In particular, in the CMB power spectrum, in gravitational lensing, in the necessity of dark matter for structure formation, and in a varied collection of other methods used to estimate the mass in galaxies." > Now explain the Bullet Cluster This paper does not seek to do so. "To make it tractable analytically, we have restricted ourselves to spherically symmetric and static spacetimes, with metrics of the form of Eqs. (17)." Eqn 17 describes out an adapted Schwarzschild-de Sitter spacetime and leans on an argument that Birkhoff's theorem applies (in particular that their model spacetime is stable against certain perturbations, notably those concentric upon the source mass). There is further detail in Appendix B. Studying this restricted model, the de Sitter expansion of the spacetime and MOND-like anomalous Kepler orbits at some remove from the Schwarzschild central mass are in their theory driven by entropic forces generated by the fluctuations in the gravitational field of the central mass (and they do a good job in Appendix D explaining this). In GR's Schwarzschild-de Sitter the free-fall trajectories of test particles around the central mass are totally determined by the mass; the gravitational field doesn't fluctuate. The (Boltzmann) gravitational entropy of the region outside the central mass is everywhere very high. In GR-SdS we can consider adaptations where with M=const. we turn the pointlike central mass into a spherically symmetric shell, or a concentric set of such shells, or even a ball of fluid, or a ball of dust, or a ball of stars and other galactic matter. None of these symmetry-preserving adaptations changes the free-fall trajectories of test particles outside the outer surface, or the gravitational entropy at any outside point. In the author's theory, the spacetime is stochastic. It fluctuates. Close to the central mass fluctuations are unnoticeably small; the gravitational entropy is very low. Far from the central mass the gravitational entropy is very high, and gravitational fluctuations are noticeable. A sort of thermodynamics leads to a diffusive flow outwards from the central mass, from the low entropy near there to the high entropy at increasing radial distance. This diffusion is carefully constructed so that the outwards flow is only really appreciable at large-scale distances. The effect is that large-radius orbits are statistically pulled inwards by something describable as stronger gravity at larger radiuses (see around Eqn (21)). This is an "entropic force", very roughly analogous to squashing a sponge ball in your hand then releasing the pressure and watching the sponge ball expand, where the material of the sponge represents the gravitational field. Their stochastic fluctuations are still generated by the spherically-symmetric central mass. These fluctuations break the spherical symmetry of the outside metric. Consequently they have to do some work to make the outside metric look appropriately Schwarzschild-like in their "diffusion regime", and to keep that stable against the stochastic perturbations. The authors contend that with reasonable choices of parameters, and restricted to static spherical symmetry of the central mass (and no additional dynamics), this effect comes close to duplicating MOND's low-acceleration regime. They don't go into anything like a backreaction upon the Schwarzschild metric by large fluctuations. (They do have an idea about how to get the de Sitter trajectories though, but that doesn't fit very naturally into this comment, which is already long.) > Bullet cluster The authors know full well that the metric for a gravitationally bound cluster of galaxies isn't well-represented by their choice of SdS-like metric. A galaxy cluster is too lumpy for the Schwarzschild part. Two gravitationally bound galaxy clusters having passed through each other (trailing collided gas and dust, and tidally stripped stars and other matter) is even less like Schwarzschild. This is because SdS solutions of the Einstein Field Equations do not linearly superpose. So their metric is a poor description of any sort of "close call" interaction between galaxies or galaxy clusters, even if the individual components are "close enough" to Schwarzschild from the perspective of an observer sufficiently large (as in cosmologically large) distances. They do not (and within this initial paper should not really be expected to) offer a more suitable metric. I'm sure they'd love to look into things like that though. The non-linear superposeability of useful solutions of General Relativity is a problem for asking how astrophysics differ in most theories that preserve the equivalence principle (this one does, it's a metric theory of gravitation). As the replacement for the Einstein Field Equations lose symmetries (sphericity, staticity) they tend to become analytically intractable and non-numerical approximations become unreliable. The authors -- imho in a strikingly principled way -- call attention to various difficulties in using this work to describe astrophysical systems, particulary from the middle of the fifth page of the PDF. They are not obviously worse off than the Verlinde programme of emergent-entropic gravity, where the gravitational field is generated by entropic forces rather than vice-versa.

indeyets 3y ago

But… WHY is it wobbly? Is there a particle which represents this wobbliness? :)

SideburnsOfDoom 3y ago
Is that the "postquantum" bit? i.e. not that spacetime is quantized as such, but that it's not a uniform continuous medium at all scales, that classical mechanics would assume.
scotty79 3y ago
I can imagine that wobbliness comes from chaotically passing strong gravity waves of very high wavelengths coming from outside of the observable universe and from before times of Big Bang. There's no specific reason that forces us to believe that Big Bang was the actual beginning of everything. It might have been "local" high energetic event happening in much larger structure. For example try to imagine how the collision of two dense clusters of trillions of black holes each traveling at nearly light speed would look like from the point of view of the dust swirling in-between and around them. I can't imagine it would be very different from what we observe in our universe.
- MauranKilom 3y ago
  Don't you think that we would be able to observe some anisotropy in the data if that were the case? (Note: I have no clue, but it seems unlikely...)
  - scotty79 3y ago
    Maybe? I don't know. I imagine we don't have any good ways to observe gravitational waves, even fairly strong ones with periods of millenia or even centuries which are still very short times on a galactic scale. And apart from that, I think that we can only look at CMB and conclude that it's a bit wobbly. Must be quantum fluctuations, sure, why not, but is it only the quantum fluctuations? Or maybe the spacetime between us and CMB source is a bit wobbly too? Another thing is large scale structure of our universe. Visualizations look like foam. Planty of chaos that wobblines can safely compose into. I think the work of those scientists is very important because it might allowed us to pinpoint how strong distortions of the spacetime would need to be to explain what we see and maybe we could narrow down the range of frequencies which might gives us ideas how to look for them. Simplistic and grandiose assumptions make our current best model of the universe a bit restrictive. To the point that we are starting to find direct counterexamples for our theories derived from it. Mature galaxies way too old. Megastructures way too large. CMB fluctuation not exactly fitting best theoretical models. Unreconcilable differences between Hubble constant measured from CMB and that measured from galaxies. I think accepting a bit of chaos beyond what we currently believe is an inevitable way out.

oneshtein 3y ago

Finally, someone is at right path.

A_D_E_P_T 3y ago

There's something distinctly unsatisfying about this sort of paper, because the number of potential solutions to the problem they're trying to solve is in principle vast, and you can arrive at any number of them if you laboriously work backwards and fit your equations to the data. Without very rigorous efforts towards empirical validation, in novel ways if required, this sort of thing is just another wholly speculative theory to add to the large and growing pile. The interesting thing, as far as I'm concerned, is the size and shape of the answer space. How many theoretical solutions can be coaxed to fit the cosmological data? It can't be infinite, but it seems as though it's a rather large number.

szundi 3y ago
This is exactly what Sabine Hossenfelder is talking about for years now.
- naasking 3y ago
  Indeed, and even she thinks this is at least a fairly novel approach in this space which hasn't seen much new thinking recently (20+ years).
hnaccount_rng 3y ago
To be fair, everything that astrophysics comes up with will be massively under constrained. For all intents and purposes we work with a single datapoint and try to infer dynamics.
- Nevermark 3y ago
  Exactly. A few "data points", i.e. unexplained effects, and then Occamm's razor, parsimony, etc. I really like the parsimony of this paper. If a very small detail of space-time, i.e. stochastic behavior at the Planck scale, results in an emergent effect that explains something on the cosmological scale, that is explaining a lot with a little. Stochastic might not even mean random or non-deterministic (as I understand - and as the paper suggests, i.e. non-quantum), it could just be chaotic. Which would be very parsimonious. So many seemingly different effects we observe are the result of very simple things, under stochastic, statistical, entropy, thermodynamic type situations.

josh-sematic 3y ago

Note that this isn’t a “get out of dark matter free” card, as there are clear signatures of dark matter in the cosmic microwave background. https://physicsworld.com/a/dark-energy-spotted-in-the-cosmic-microwave-background/ https://physicsworld.com/a/dark-energy-spotted-in-the-cosmic...

willis936 3y ago
Only if we've constrained ourselves to take inflation as true.
- Vecr 3y ago
  Inflation in what era? There has to have been some inflation at some point, right, otherwise how could the very hot period that's considered to be right after the start of the universe be low entropy? Sure, maybe you could say there's no big bang and time runs the other way on the other side of that low entropy state, but there has to be some sort of low entropy state.
  - tsimionescu 3y ago
    It could be that it is actually not low entropy, and there are some other processes that explain a recution in entropy at some scale. That is, maybe the second law of thermodynamics is not that universal.
    - Vecr 3y ago
      That would be really weird. I'd be more willing to buy galaxies just coming into existence somehow in the gaps as the universe expands, as that at least drives the arrow of time consistently in the right direction. The arrow of time can survive local entropy reduction, but probably not what you're talking about.
  - willis936 3y ago
    >Inflation in what era? Why the inflationary epoch of course. https://en.wikipedia.org/wiki/Inflationary_epoch https://en.wikipedia.org/wiki/Inflationary_epoch I'm saying the very notion of the big bang warrants more scrutiny. All singularities do.

nabla9 3y ago

>"explain galactic rotation curves without needing to evoke dark matter." I wonder if this makes it incompatible with dark matter? If stochastic spacetime explains the rotation curves exactly without black matter, then black matter can't exist at the same time. The evidence for dark matter is find in, galaxy rotation curves, gravitational lensing, cosmic microwave background, structure formation, bullet cluster and other galaxy cluster collisions, baryon acoustic oscillations, Lyman-alpha forest absorption lines, etc. There is plenty of evidence for dark matter and it's existence is widely accepted. Explaining just one piece of evidence can be evidence against the new explanation if it's incompatible.

raattgift 3y ago
> Can dark matter and [the author's theory of gravitation] fit together...? Nobody knows yet. The authors aren't seeking to overthrow the concordance cosmology; this paper is essentially trying to see if their theory is quickly killed by being unable to model a stable spherically symmetric galaxy in an expanding universe. Their theory predicts significant differences from the Schwarzschild-de Sitter (SdS) metric in General Relativity, with SdS serving as a proxy for an isolated galaxy. The paper in part investigates whether those differences can be "hidden" by removing the cold dark matter which otherwise would be the standard source of outer orbit anomalies. They don't go deeper than that, and at this stage just can't: "To make it tractable analytically, we have restricted ourselves to spherically symmetric and static spacetimes". They restrict themselves in other ways too. > The evidence for dark matter From just after Eqn (23) in the preprint: "While this study demonstrates that galactic rotation curves can undergo modification due to stochastic fluctuations, a phenomenon attributed to dark matter, it is important to acknowledge the existence of separate, independent evidence supporting ΛCDM. In particular, in the CMB power spectrum, in gravitational lensing, in the necessity of dark matter for structure formation, and in a varied collection of other methods used to estimate the mass in galaxies"
- nabla9 3y ago
  If stochastic spacetime explains the rotation curves exactly without black matter, then how black matter can't exist at the same time?

_obviously 3y ago

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clysm 3y ago
The fuck... I can't believe what I've just read. That's like saying Newton set us back over a hundred years. There was no regression because of Einstein.
- _obviously 3y ago
  [flagged]
- oneshtein 3y ago
  Mainstream theories are so good that they dwarfed competition. Lack of competition caused lack of progress. It was easy to collect hundreds of downvotes on HN just by talking about alternative theories or about physical representation of abstract theories. Now such comments will collect mere dozens of downvotes. It also should be obvious that a new successful theory will account for more things, so it will be much more complex than previous theories.
AnimalMuppet 3y ago
Yeah, um... if you're going to claim something like that, you need more than the claim. What evidence do you see for your position? What evidence can you give us so that we can evaluate your position?

nukeyoular 3y ago

[flagged]

fallingfrog 3y ago

I wonder how this relates to Erik Verlinde’s entropic gravity? https://arxiv.org/abs/1611.02269 https://arxiv.org/abs/1611.02269 It seems like this theory is proposing some entropic term on top of classical general relativity whereas Verlinde came up with a completely entropic theory of gravity. They both claim to explain dark matter.

kromem 3y ago

I love their terminology of 'postquantum.' There's a certain irony that a hundred years ago Physics was divided between bitterly divided camps declaring light as continuous or as quantized until finally they found it was a duality of both, specifically converting around the inflection point of interactions with persistence (i.e. no quantum eraser). So it's a bit bizarre to watch modern physics repeating the process with spacetime and the commitment to finding a 'solution' to quantum gravity, particularly when including notions of quantized particles like gravitons or dark matter that can't be directly interacted with in persistent ways (in which case, why are they expected to be quantized and not continuous?). I know most physicists proudly state that they only concern themselves with 'how' and not 'why' - but it's refreshing to see work that clearly had the wherewithal to ask themselves 'why' regarding a number of broad assumptions about the fundimentality of quantization as a default mode and I look forward to more 'postquantum' work in the future.

eigenket 3y ago
I think there's a bit of language problem here. In the way physicists use the word in 2024, making gravity "quantum" means something very different to just having discrete "quanta" particles. It means a theory with stuff like superpositions of classical states,interference and entanglement. It also means having a theory which can consistently interact with other quantum stuff. The standard through experiment goes something like this. Take a particle with some mass (maybe am electron, maybe a bowling ball), general relativity tells us what its mass does to space-time around it. Now take that particle and put it in a superposition of being located at point A and point B (you can do this if the particle is quantum). What the heck is it doing to spacetime now? GR doesn't tell us, because GR can't deal with quantum particles. Naively it seems like space-time should also be in a superposition, but we tried to make theories like that and they didn't really work.
- kromem 3y ago
  Yes, I'm familiar with the thought experiments. My favorite to date was Zych, et al Bell’s theorem for temporal order (2019). But it still seems a bit like putting the cart ahead of the horse given the amount of resources dedicated to the search when the smallest gravitational field measurements to date (or at least as of 2021) were on a 92mg object. And while in theory the largest mass to be put in a quantum superposition to date was 16mg, the degree of separation in the vibrational superposition is still well below an order of magnitude gap from a difference detectable with current gravitational measurement. For a field where behavior changes in relationship to measurement and the persistence of information, there's not currently any measurable discrepancy between GR and QM. It's literally only in gedankenexperiments there's a conflict. Maybe the difficulty in preserving coherence for larger objects relates to their inadvertently leaving measurable information in spacetime? Perhaps we'll find there's a fundamental limit on gravitational measurement that happens to be right around the limit at which we can maintain coherence. It just seems like the kind of quest that's premature when we can't measure a conflict existing outside of our imaginations, and where the mystery will likely resolve almost immediately after we can actually measure the conflict in the first place (should we ever be able to do so). So yes, if we end up with an entire planet in a quantum superposition near two different spaceships shooting at each other, things might get weird. But at least currently that 'if' seems to be doing a lot of heavy lifting.