5 ms·

I always very much enjoy these posts with many upvotes and few comments.

by abcc8 6y ago

detritus 6y ago
I upvoted here 50% because I found it interesting, 50% because I wanted to hear criticism from people knowledgable on the matter and 50% because I'm actually too stupid to fully comprehend it and hope for an ELI5 (as you may tell, I'm not hot on math).
- mellosouls 6y ago
  WRT ELI5 (and bear in mind I'm a layman who's only briefly skimmed the article, but this may do till an expert chips in, corrections welcome): There are a couple of main ways of calculating the speed of the expansion of the universe; they are very different and act at different scales, but should obviously tally. They don't, and the mismatch is often referred to as a crisis in cosmology in popular articles on the subject. It's a serious issue because each of the two different methods is foundational in our understanding of the structure of the universe in their domain; we can't just prefer one over the other. Our standard way of modelling the universe relies on Einstein's general relativity in a larger framework that caters for dark matter; a (controversial) "Newtonian" alternative is called MOND and this article proposes ways that elements of the two (but favouring the latter) might be combined to counter the discrepancies in the measurements, taking into account other difficulties in the process like the void in the title. Further reading; Crisis: https://www.quantamagazine.org/cosmologists-debate-how-fast-the-universe-is-expanding-20190808/ https://www.quantamagazine.org/cosmologists-debate-how-fast-... MOND https://www.wikipedia.org/wiki/Modified_Newtonian_dynamics https://www.wikipedia.org/wiki/Modified_Newtonian_dynamics Standard Cosmological Model https://www.wikipedia.org/wiki/Lambda-CDM_model https://www.wikipedia.org/wiki/Lambda-CDM_model
- orclev 6y ago
  Another ELI5 by someone not at all qualified to understand this stuff (I.E. a layman). We know from measurements of the leftover energy (CMB) approximately how long ago the big bang happened, and how fast it appears to be expanding. Our most favored model of the universes physics, General Relativity makes certain predictions that mostly match up with reality up until you get to galaxy scales at which point they start to diverge. In order for our measurements to work under General Relativity our galaxies need to be more massive than they appear to be based on all the stuff we can actually see in them. This needed excess mass is called dark matter, but even with dark matter the universe appears to be expanding faster than it should. The theory proposed in the paper is that the universe as whole isn't actually expanding faster, but due to a quirk of where we are in the universe it only looks like it is when we look at nearby galaxies. Unfortunately for that to be true we would need to be in a void in the structure of the universe which General Relativity predicts shouldn't be possible. An alternative theory of universal physics exists called MOND. MOND is similar to General Relativity, but rather than solving the problem at the galaxy scale through theoretical dark matter, it instead just assumes that gravity works differently once you reach a certain cutoff point. This aligns with observations of actual galaxies (not entirely unsurprisingly because the cutoff point was chosen in order to align with those observations) without needing dark matter to exist. From the perspective of the paper there's another nice property of MOND which is that simulations based on it allow for the kind of void to form that the paper predicts would be necessary to explain the locally observed expansion. Basically, General Relativity can't explain how fast galaxies spin without Dark Matter, nor how fast the universe appears to be expanding. MOND combined with our galaxy being in the middle of a big void can explain both. Both theories, General Relativity and MOND require a certain amount of hand waving in order to align with reality. MOND requires a bit less but is highly suspect because it's solution is basically "gravity just acts different sometimes" which is suspiciously close to "it's that way because it is". As for the actual math involved in all of this, beats me, we'll need to wait for someone who's actually in this field to look it over and explain what if anything is wrong with it all.
  - SCHiM 6y ago
    Isn't there also observational data that disfavors MOND? I remember it had something to do with galaxies colliding and the dark matter staying behind or something along those lines.
    - dnautics 6y ago
      The bullet cluster is an n of one and the dark matter estimates come from redshifting/lensing, which could have come from other sources in such an unusual scenario. Some of the other crazy observations like ultradiffuse galaxies are, on recalculation, not as extreme as initially guessed, and predicated on an indirect empirical estimate of mass (number of globular clusters) with no mechanistic confirmation.
    - orclev 6y ago
      Not sure honestly. That might have something to do with the vHDM theory mentioned in the paper. I didn't really follow a lot of it, but I think (major grain of salt here) it's saying that that theory predicts some very light neutrinos exist and tend to collect inside of galactic clusters, but not galaxies themselves and that it's those neutrinos that make up the missing mass at universal scales that general relativity explains using dark matter. Either case seems pretty hand wavy honestly. When it comes to galaxy and universe level physics it all seems pretty weak compared to the sort of particle physics and classical physics that we can actually measure and test on Earth. It's all just a bunch of theoretical math with relatively few actual measurements to pin it all down. I don't think we're anywhere near having a solid theory of the universe so it's mostly an exercise in trying to prove which theory is the least wrong at this point, rather than which one is correct.
  - marcus_holmes 6y ago
    So General Relativity is a theoretical framework that has been proven to match observation[0], while MOND's details have been chosen to match observation, without any theoretical basis? Is that right? Or is there some mechanism proposed as to why gravity's cutoff point is where it is? [0]up to a certain point, and if you include Dark Matter, which we still don't understand (the original assumption was that it was WIMPs, but as we still haven't found a WIMP that would do this, then we don't know what it is).

I always very much enjoy these posts with many upvotes and few comments.

by abcc8 6y ago

I always very much enjoy these posts with many upvotes and few comments.

I upvoted here 50% because I found it interesting, 50% because I wanted to hear criticism from people knowledgable on the matter and 50% because I'm actually too stupid to fully comprehend it and hope for an ELI5 (as you may tell, I'm not hot on math).

mellosouls 6y ago
WRT ELI5 (and bear in mind I'm a layman who's only briefly skimmed the article, but this may do till an expert chips in, corrections welcome): There are a couple of main ways of calculating the speed of the expansion of the universe; they are very different and act at different scales, but should obviously tally. They don't, and the mismatch is often referred to as a crisis in cosmology in popular articles on the subject. It's a serious issue because each of the two different methods is foundational in our understanding of the structure of the universe in their domain; we can't just prefer one over the other. Our standard way of modelling the universe relies on Einstein's general relativity in a larger framework that caters for dark matter; a (controversial) "Newtonian" alternative is called MOND and this article proposes ways that elements of the two (but favouring the latter) might be combined to counter the discrepancies in the measurements, taking into account other difficulties in the process like the void in the title. Further reading; Crisis: https://www.quantamagazine.org/cosmologists-debate-how-fast-the-universe-is-expanding-20190808/ https://www.quantamagazine.org/cosmologists-debate-how-fast-... MOND https://www.wikipedia.org/wiki/Modified_Newtonian_dynamics https://www.wikipedia.org/wiki/Modified_Newtonian_dynamics Standard Cosmological Model https://www.wikipedia.org/wiki/Lambda-CDM_model https://www.wikipedia.org/wiki/Lambda-CDM_model
orclev 6y ago
Another ELI5 by someone not at all qualified to understand this stuff (I.E. a layman). We know from measurements of the leftover energy (CMB) approximately how long ago the big bang happened, and how fast it appears to be expanding. Our most favored model of the universes physics, General Relativity makes certain predictions that mostly match up with reality up until you get to galaxy scales at which point they start to diverge. In order for our measurements to work under General Relativity our galaxies need to be more massive than they appear to be based on all the stuff we can actually see in them. This needed excess mass is called dark matter, but even with dark matter the universe appears to be expanding faster than it should. The theory proposed in the paper is that the universe as whole isn't actually expanding faster, but due to a quirk of where we are in the universe it only looks like it is when we look at nearby galaxies. Unfortunately for that to be true we would need to be in a void in the structure of the universe which General Relativity predicts shouldn't be possible. An alternative theory of universal physics exists called MOND. MOND is similar to General Relativity, but rather than solving the problem at the galaxy scale through theoretical dark matter, it instead just assumes that gravity works differently once you reach a certain cutoff point. This aligns with observations of actual galaxies (not entirely unsurprisingly because the cutoff point was chosen in order to align with those observations) without needing dark matter to exist. From the perspective of the paper there's another nice property of MOND which is that simulations based on it allow for the kind of void to form that the paper predicts would be necessary to explain the locally observed expansion. Basically, General Relativity can't explain how fast galaxies spin without Dark Matter, nor how fast the universe appears to be expanding. MOND combined with our galaxy being in the middle of a big void can explain both. Both theories, General Relativity and MOND require a certain amount of hand waving in order to align with reality. MOND requires a bit less but is highly suspect because it's solution is basically "gravity just acts different sometimes" which is suspiciously close to "it's that way because it is". As for the actual math involved in all of this, beats me, we'll need to wait for someone who's actually in this field to look it over and explain what if anything is wrong with it all.
- SCHiM 6y ago
  Isn't there also observational data that disfavors MOND? I remember it had something to do with galaxies colliding and the dark matter staying behind or something along those lines.
  - dnautics 6y ago
    The bullet cluster is an n of one and the dark matter estimates come from redshifting/lensing, which could have come from other sources in such an unusual scenario. Some of the other crazy observations like ultradiffuse galaxies are, on recalculation, not as extreme as initially guessed, and predicated on an indirect empirical estimate of mass (number of globular clusters) with no mechanistic confirmation.
  - orclev 6y ago
    Not sure honestly. That might have something to do with the vHDM theory mentioned in the paper. I didn't really follow a lot of it, but I think (major grain of salt here) it's saying that that theory predicts some very light neutrinos exist and tend to collect inside of galactic clusters, but not galaxies themselves and that it's those neutrinos that make up the missing mass at universal scales that general relativity explains using dark matter. Either case seems pretty hand wavy honestly. When it comes to galaxy and universe level physics it all seems pretty weak compared to the sort of particle physics and classical physics that we can actually measure and test on Earth. It's all just a bunch of theoretical math with relatively few actual measurements to pin it all down. I don't think we're anywhere near having a solid theory of the universe so it's mostly an exercise in trying to prove which theory is the least wrong at this point, rather than which one is correct.
- marcus_holmes 6y ago
  So General Relativity is a theoretical framework that has been proven to match observation[0], while MOND's details have been chosen to match observation, without any theoretical basis? Is that right? Or is there some mechanism proposed as to why gravity's cutoff point is where it is? [0]up to a certain point, and if you include Dark Matter, which we still don't understand (the original assumption was that it was WIMPs, but as we still haven't found a WIMP that would do this, then we don't know what it is).