3 ms·

Possible! There certainly is a lot that feels inelegant in the dominant model, and nobody has yet proposed a reason that so much of the universe would be so odd

by Steuard 11y ago

Possible! There certainly is a lot that feels inelegant in the dominant model, and nobody has yet proposed a reason that so much of the universe would be so oddly invisible to us. But (as I just noted in some detail in a different comment here) the "Lambda-CDM" model of a universe evolving in the presence of a cosmological constant and cold dark matter has proven to be quite remarkably consistent under a wide variety of very different observational tests. Everything from supernova data to the cosmic microwave background to large-scale cosmological structure/cluster formation is not just in agreement with the general outline of that model but even gives consistent (and increasingly precise) values for the parameters of the model. (They all lead to pretty much the same value for the percentage of dark matter in the universe, for example. Here's a typical graph from 2003 that I found from a Google image search: https://ned.ipac.caltech.edu/level5/Sept09/Einasto/Figures/figure17.jpg https://ned.ipac.caltech.edu/level5/Sept09/Einasto/Figures/f... The different-colored ovals near the middle-left represent entirely different observational data.) So I would love to learn someday that all these weird phenomena are manifestations of some deeper, more elegant truth. (I've done some limited research vaguely connected to the area myself, though that was long ago and my current focus is much less directly relevant to this topic.) But whatever that deeper truth turns out to be, we can have a whole lot of confidence that it will manifest in a way that behaves an awful lot like the dark matter and dark energy models that are being studied today.

chriswarbo 11y ago
> nobody has yet proposed a reason that so much of the universe would be so oddly invisible to us. Whilst I don't claim to have an answer to this question, I still wouldn't call it "odd". We know of four fundamental forces (strong, weak, electromagnetic and gravitational) and we know that some particles are unaffected by some of these; e.g. leptons don't feel the strong force and neutrinos don't feel the electromagnetic force. I would say the odd part is our assumption that matter should be visible (AKA electromagnetically interacting). (Note: I'm only talking about dark matter here; dark energy is odd ;) )
blincoln 11y ago
> nobody has yet proposed a reason that so much of the universe would be so oddly invisible to us. IANAP, but it seems pretty consistent with the brane element of M-theory to me. If you have a bunch of 3-dimensional branes sitting next to each other in a hyperdimensional bulk, and gravity is the only force which can pass between them, then of course you're going to end up with what appears to be invisible matter. It's the aggregate mass that has clumped together in the respective 3D branes, as close as is possible to the adjacent branes. E.g. dropping one dimension, stack some glass sheets and magnets, with the two interleaved. The magnets on one level will be affected by the fields on other levels, but any 2D flatlanders on any given level will be unable to actually see those other magnets. It will be as if some ghostly "dark magnets" are passing right through their own.
- Steuard 11y ago
  Poor phrasing on my part, I think. Brane models (whether in M-theory or elsewhere) are certainly one way of producing this sort of phenomenon theoretically. (I've done research on branes in both string and M-theory.) But ultimately, what you've described is just as arbitrary as declaring "this quantum field has no electric or color charge": there's no reason known that we'd happen to have those two brane stacks in the right positions to mimic ordinary matter interacting with dark matter. (Why exactly the right branes and intersections to reproduce the standard model? Why also this separate brane or branes at a carefully chosen distance to have meaningful gravitational interactions but not gauge interactions? What stabilizes the separation of the branes over cosmological timescales? etc.)

by Steuard 11y ago

chriswarbo 11y ago

> nobody has yet proposed a reason that so much of the universe would be so oddly invisible to us. Whilst I don't claim to have an answer to this question, I still wouldn't call it "odd". We know of four fundamental forces (strong, weak, electromagnetic and gravitational) and we know that some particles are unaffected by some of these; e.g. leptons don't feel the strong force and neutrinos don't feel the electromagnetic force. I would say the odd part is our assumption that matter should be visible (AKA electromagnetically interacting). (Note: I'm only talking about dark matter here; dark energy is odd ;) )

blincoln 11y ago

> nobody has yet proposed a reason that so much of the universe would be so oddly invisible to us. IANAP, but it seems pretty consistent with the brane element of M-theory to me. If you have a bunch of 3-dimensional branes sitting next to each other in a hyperdimensional bulk, and gravity is the only force which can pass between them, then of course you're going to end up with what appears to be invisible matter. It's the aggregate mass that has clumped together in the respective 3D branes, as close as is possible to the adjacent branes. E.g. dropping one dimension, stack some glass sheets and magnets, with the two interleaved. The magnets on one level will be affected by the fields on other levels, but any 2D flatlanders on any given level will be unable to actually see those other magnets. It will be as if some ghostly "dark magnets" are passing right through their own.

Steuard 11y ago
Poor phrasing on my part, I think. Brane models (whether in M-theory or elsewhere) are certainly one way of producing this sort of phenomenon theoretically. (I've done research on branes in both string and M-theory.) But ultimately, what you've described is just as arbitrary as declaring "this quantum field has no electric or color charge": there's no reason known that we'd happen to have those two brane stacks in the right positions to mimic ordinary matter interacting with dark matter. (Why exactly the right branes and intersections to reproduce the standard model? Why also this separate brane or branes at a carefully chosen distance to have meaningful gravitational interactions but not gauge interactions? What stabilizes the separation of the branes over cosmological timescales? etc.)