4 ms·

I too love the idea of there being filaments of matter connecting distant galaxies. I can only imagine the artistic renderings will make the universe look like

I too love the idea of there being filaments of matter connecting distant galaxies. I can only imagine the artistic renderings will make the universe look like a neural network. Can anyone clarify how these filaments remain “hot”?

posterboy 9y ago
>things in motion tend to stay in motion comes to mind
- subroutine 9y ago
  Motion is relative though. And just because something is moving relative to another thing does not necessarily mean it is hot.
  - posterboy 9y ago
    temperature, in my simple understanding, is brownian motion. What ever that means in the end, we know that vacuum is a pretty good thermal isolator, so the counter question should be, why should it cool down? Sure, heat can be given off as radiated photons which is messured coming from the filaments, but you probably don't even begin to understand why, ie. why the T is hot, I sure don't, so the question seems like stabbing in the dark (puns not intended).
  - raattgift 9y ago
    Whee, the best thing to say about "... just because something is moving relative to another thing does not necessarily mean it is hot ..." is that this is unsettled. ;) Ott 1963 shows T' = T / {\sqrt{1 - v^2 / c^2}}, or for short T' = \gamma T, where \gamma is the Lorentz factor, T' is the temperature calculated by an observer in a set of coordinates Lorentz boosted with respect to the coordinates in which the system is at rest with a temperature T. Asymptotically one would expect that as \gamma -> 1 \RightArrow T = T' and \gamma -> \infty \RightArrow T = \infty assuming one is measuring temperature using photocalorimetry (consider a blackbody radiator...). However, there are different proposals that depend on how and if one makes the first or second laws of thermodynamics Lorentz covariant, and how one goes about measuring a distant object's temperature. All of this is also in the context of Special Relativity; a general relativist could say that the whole matter comes down to a choice of gauge anyway, and temperatures are only directly comparable for two objects at the same point in spacetime (and even there you get a can of worms because they would then be "in contact" at the point p on the manifold, and even then there is a choice of gauge and some interesting cut-offs in which they could equilibriate (T_obj1 = T_obj2), dissipate excess stress-energy (T_{p} > T_obj1 + T_obj2), or collapse gravitationally). Unfortunately the matter is not wholly settled and there is an absence of firm experimental evidence, and moreover the debate is confined to flat spacetime. However, one has to do quite a bit of twisting to assume that T = T' (Landsberg did this in 1966 & 1967). Here is a nice recent set of slides on temperature in GR: http://www.dpg-physik.de/dpg/pbh/aktuelles/pdf/Andersson.pdf http://www.dpg-physik.de/dpg/pbh/aktuelles/pdf/Andersson.pdf Q. Is a moving body hot/cold? A. Yes/no/maybe. The discussion is "pointless".
privong 9y ago
> I too love the idea of there being filaments of matter connecting distant galaxies. I can only imagine the artistic renderings will make the universe look like a neural network. There are lots of "cosmic web" renderings from large-scale cosmological simulations of dark mater and the formation of structure in the universe. A major one, now over a decade old is the "Millennium Simulation": http://wwwmpa.mpa-garching.mpg.de/galform/millennium/ http://wwwmpa.mpa-garching.mpg.de/galform/millennium/

posterboy 9y ago

>things in motion tend to stay in motion comes to mind

subroutine 9y ago
Motion is relative though. And just because something is moving relative to another thing does not necessarily mean it is hot.
- posterboy 9y ago
  temperature, in my simple understanding, is brownian motion. What ever that means in the end, we know that vacuum is a pretty good thermal isolator, so the counter question should be, why should it cool down? Sure, heat can be given off as radiated photons which is messured coming from the filaments, but you probably don't even begin to understand why, ie. why the T is hot, I sure don't, so the question seems like stabbing in the dark (puns not intended).
- raattgift 9y ago
  Whee, the best thing to say about "... just because something is moving relative to another thing does not necessarily mean it is hot ..." is that this is unsettled. ;) Ott 1963 shows T' = T / {\sqrt{1 - v^2 / c^2}}, or for short T' = \gamma T, where \gamma is the Lorentz factor, T' is the temperature calculated by an observer in a set of coordinates Lorentz boosted with respect to the coordinates in which the system is at rest with a temperature T. Asymptotically one would expect that as \gamma -> 1 \RightArrow T = T' and \gamma -> \infty \RightArrow T = \infty assuming one is measuring temperature using photocalorimetry (consider a blackbody radiator...). However, there are different proposals that depend on how and if one makes the first or second laws of thermodynamics Lorentz covariant, and how one goes about measuring a distant object's temperature. All of this is also in the context of Special Relativity; a general relativist could say that the whole matter comes down to a choice of gauge anyway, and temperatures are only directly comparable for two objects at the same point in spacetime (and even there you get a can of worms because they would then be "in contact" at the point p on the manifold, and even then there is a choice of gauge and some interesting cut-offs in which they could equilibriate (T_obj1 = T_obj2), dissipate excess stress-energy (T_{p} > T_obj1 + T_obj2), or collapse gravitationally). Unfortunately the matter is not wholly settled and there is an absence of firm experimental evidence, and moreover the debate is confined to flat spacetime. However, one has to do quite a bit of twisting to assume that T = T' (Landsberg did this in 1966 & 1967). Here is a nice recent set of slides on temperature in GR: http://www.dpg-physik.de/dpg/pbh/aktuelles/pdf/Andersson.pdf http://www.dpg-physik.de/dpg/pbh/aktuelles/pdf/Andersson.pdf Q. Is a moving body hot/cold? A. Yes/no/maybe. The discussion is "pointless".

privong 9y ago

> I too love the idea of there being filaments of matter connecting distant galaxies. I can only imagine the artistic renderings will make the universe look like a neural network. There are lots of "cosmic web" renderings from large-scale cosmological simulations of dark mater and the formation of structure in the universe. A major one, now over a decade old is the "Millennium Simulation": http://wwwmpa.mpa-garching.mpg.de/galform/millennium/ http://wwwmpa.mpa-garching.mpg.de/galform/millennium/