6 ms·

Big Trouble in a Deep Void

158 points by waffle_ss 6y ago

lainga 6y ago
As much as I am fond of MOND (hu hu), is this going to get any traction? It seems like the MOND and lCDM worlds have stopped trying to interact with each other, covering their ears and going "bla bla bla" while building parallel and non-interacting lines of research.
drran 6y ago
Same idea about large mass, which strips smaller mass, leaving void behind: Shapley attractor and Dipole Repeller: https://www.youtube.com/watch?v=1mQr6mzmzbU https://www.youtube.com/watch?v=1mQr6mzmzbU
abcc8 6y ago
I always very much enjoy these posts with many upvotes and few comments.
- 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.
NKosmatos 6y ago
Our very own Milky way is inside the KBC void [0] discussed in the article. It never ceases to amaze me that our local group [1], part of the local Laniakea supercluster [2], despite the hundred of thousand of galaxies it contains is almost empty space and part of the void! [0] https://en.wikipedia.org/wiki/KBC_Void https://en.wikipedia.org/wiki/KBC_Void [1] https://en.wikipedia.org/wiki/Local_Group https://en.wikipedia.org/wiki/Local_Group [2] https://en.wikipedia.org/wiki/Laniakea_Supercluster https://en.wikipedia.org/wiki/Laniakea_Supercluster
- seibelj 6y ago
  Could this be why we exist? Aliens don't like to travel across space desert, it isn't worth their time and risk, so we are in an island of safety.
  - NKosmatos 6y ago
    Nice one!!! Never thought of it that way. On the other hand I now feel more isolated, knowing our civilization is just a flower in the desert that most probably no one is going to notice.
  - wffurr 6y ago
    Grimdark fiction[0] aside, I doubt anything is traveling across intergalactic voids of any size. [0]: https://warhammer40k.fandom.com/wiki/Tyranids https://warhammer40k.fandom.com/wiki/Tyranids
    - koheripbal 6y ago
      I saw an MIT PhD thesis defense video of a paper that described the calculations of how far/fast probes could spread between galaxies. I really wish I could find it again... but my searches have failed. The conclusion was that self-replicating probes could indeed populate (I forget how many - on the order of hundreds of thousands/millions) nearby galaxies within the last couple billion years. It was pretty compelling. The discussion at the end did see some debate of "well then where are they?". A valid question. Why haven't self-replicating probes infected the entire visible universe by now?
tylerjwilk00 6y ago
Is there a galactic equivalent of a solar system's heliosphere?
- drran 6y ago
  Yep, but we cannot send probe to measure it boundaries yet.
unphased 6y ago
I feel like (as someone not too well versed in cosmology) that bigger and better telescopes and interferometry can help to validate stuff like MOND. in particular as it relates to better measurements of galactic structures.

158 points by waffle_ss 6y ago

As much as I am fond of MOND (hu hu), is this going to get any traction? It seems like the MOND and lCDM worlds have stopped trying to interact with each other, covering their ears and going "bla bla bla" while building parallel and non-interacting lines of research.

drran 6y ago

Same idea about large mass, which strips smaller mass, leaving void behind: Shapley attractor and Dipole Repeller: https://www.youtube.com/watch?v=1mQr6mzmzbU https://www.youtube.com/watch?v=1mQr6mzmzbU

abcc8 6y ago

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

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.

NKosmatos 6y ago

Our very own Milky way is inside the KBC void [0] discussed in the article. It never ceases to amaze me that our local group [1], part of the local Laniakea supercluster [2], despite the hundred of thousand of galaxies it contains is almost empty space and part of the void! [0] https://en.wikipedia.org/wiki/KBC_Void https://en.wikipedia.org/wiki/KBC_Void [1] https://en.wikipedia.org/wiki/Local_Group https://en.wikipedia.org/wiki/Local_Group [2] https://en.wikipedia.org/wiki/Laniakea_Supercluster https://en.wikipedia.org/wiki/Laniakea_Supercluster

seibelj 6y ago
Could this be why we exist? Aliens don't like to travel across space desert, it isn't worth their time and risk, so we are in an island of safety.
- NKosmatos 6y ago
  Nice one!!! Never thought of it that way. On the other hand I now feel more isolated, knowing our civilization is just a flower in the desert that most probably no one is going to notice.
- wffurr 6y ago
  Grimdark fiction[0] aside, I doubt anything is traveling across intergalactic voids of any size. [0]: https://warhammer40k.fandom.com/wiki/Tyranids https://warhammer40k.fandom.com/wiki/Tyranids
  - koheripbal 6y ago
    I saw an MIT PhD thesis defense video of a paper that described the calculations of how far/fast probes could spread between galaxies. I really wish I could find it again... but my searches have failed. The conclusion was that self-replicating probes could indeed populate (I forget how many - on the order of hundreds of thousands/millions) nearby galaxies within the last couple billion years. It was pretty compelling. The discussion at the end did see some debate of "well then where are they?". A valid question. Why haven't self-replicating probes infected the entire visible universe by now?

tylerjwilk00 6y ago

Is there a galactic equivalent of a solar system's heliosphere?

drran 6y ago
Yep, but we cannot send probe to measure it boundaries yet.

unphased 6y ago

I feel like (as someone not too well versed in cosmology) that bigger and better telescopes and interferometry can help to validate stuff like MOND. in particular as it relates to better measurements of galactic structures.