[talanum1 (OP)]

Dark Energy has negative pressure, so does it have negative temperature?

[evan_au]

Dark Energy has negative pressure

That really sucks!

[Bored chemist]

No.
https://www.newscientist.com/article/dn8685-tepid-temperature-of-dark-matter-revealed/

[Eternal Student]

Hi.

Minor concern:    The title of this thread mentions Dark Energy but the Original Post mentions dark matter..

  Bored_Chemist answered for dark matter.
  Evan_au  answered for Dark energy.

    Also, what is so special about negative temperature?  Are you (talanum) measuring in  degrees C ?     A whole lot of space has temperatures that are  below  0 C.

     We think most dark matter would be located at the edge of galaxies and the space between galaxies.  It is quite cold there.   However, we don't know what interactions between dark matter and visible matter are possible other than gravitational interaction.  So maybe it remains isolated from normal matter (not in thermal contact with it) and remains hot.  We also don't know if dark matter is made of particles that can display internal kinetic energy (heat) or support any other known form of heat within it.  So the question is almost impossible to answer.   Dark Matter could have a "temperature" all of it's own that is only relevant if dark matter comes into contact with something it can interact with (perhaps more dark matter).
     Meanwhile  Dark Energy is something we know even less about and it's even harder to assign a temperature to it.  Assuming it is energy contained within the vaccum, then it is in all regions of space in equal density.  If you define temperature as a measure of the energy in a region then vaccum energy adds a constant to the temperature of every point in space.  Since all points are affected equally, it has no effect on heat flow which is determined only by the differences in temperatures.   However, temperature can also be defined by reference to the kinetic energy of particles in an ideal structure (e.g. in an ideal gas or by considering the entropy within a perfect crystal) and vaccum energy would seem to have no effect on this. 

[Eternal Student]

Hi Bored_chemist.

   Thanks for the link to the new scientist article.   I've read it now.
   That was a 2006 article and at that time they were fairly sure Dark Matter was made of particles and WIMPS were the main candidate mentioned in that article.
    It seems they just assumed the particles of dark matter had average kinetic energy proportional to temperature (but the details are not shown).   I'm guessing they modelled dark matter as if it was a gas of particles (i.e. much like an ideal gas).
    I'm really not sure if this still holds well in current times.  WIMPS have certainly lost popularity.  We have not identified a dark matter particle to the best of my knowledge and therefore assigning a kinetic energy to a particle of dark matter (and thus a temperature) is speculative.