Thursday, January 15, 2015

Quick Dark Matter and Dark Energy Hits

* The Xenon 100 direct dark matter detection experiment has put strict new constraints on potential dark matter masses and cross-sections of interactions, including limitations on axion dark matter.  It has not observed any dark matter signal.

* The DarkSide50 direct dark matter detection experiment has been calibrated and is ready to start making high precision observations.

* A new way of quantifying how weak lensing in general relativity would be modified by modifications to gravity laws has been devised.

* Modified gravity theories better explain the fact that lots of galaxies are bulgeless or have only pseudo-bulges naturally, while dark matter models, generically, expect almost all galaxies to have true bulges.

* Astronomy data is consistent with a cosmological constant in the formulas of general relativity, but is arguably not consistent with any form of dark energy fluid, because it would have to violate some very basic axioms applicable due to the laws of thermodynamics to any such fluid. As the conclusion to this paper explains:
We have shown that the thermal and mechanical stability conditions forbid the existence of negative pressure fluids with a constant EoS parameter which excludes the vacuum energy as a candidate to explain the cosmic acceleration. We also show that the observational data are in conflict with the thermodynamic constraints that a general dark energy fluid with a time-dependent EoS parameter must satisfy. This result suggest that adding dark energy to the content of the Universe may not be the answer to the cosmic acceleration problem.

We must noting that, although our analysis excludes the vacuum energy, this does not represent the end of the cosmological constant. A bare geometrical Λ-term remains in the game if interpreted as a constant of the nature whose value must be determined by observations.

However, what happens to the vacuum energy? Is it null? If so, why? We know that the vacuum energy has a significant role in the quantum world but, should it play any significant role in the Universe at large scales? Can we add the quantum vacuum energy so naively to classical general relativity field equations? We know that the fluid description works for relativistic and non relativistic matter but, can we describe the vacuum energy simply as a fluid? Perhaps only a quantum theory of gravity can provide an answer to these issues. 
Beyond the issues raised above a question still remains: is the geometrical cosmological constant the explanation to the accelerated expansion? The Λ-term certainly is the simplest solution but nobody can guarantee that it is the true answer. Thus, finding out deviations of the cosmological term will remain as one of the hottest theoretical investigation lines concerning cosmic acceleration. If the dark energy is out of the game, approaches such as the kinematic method developed in can be a useful tool to search for such deviations.

Obviously, we can sacrifice the thermodynamical stability conditions to keep the dark energy hypothesis alive. For example, if we relax the mechanical stability condition, but keep the thermal stability we have, from (28),that w ≥ −1 saving vacuum energy and quintessence. If additionally we give up the thermal stability, phantom fields (w < −1) are also allowed. However, we do not think that this is a good way to address the problem.
* An effort is made to fit gamma ray emission data from the Milky Way and its nearby dwarf galaxies to a self-interacting dark matter model.

UPDATE January 19, 2015: Resonaances has a nice chart on spin-dependent dark matter exclusions in direct searches by the Ice Cube experiment, and the ATLAS experiment at the LHC has new exclusions on dark matter production there, which a more strict for bosonic matter and less strict for fermionic matter.  The ATLAS exclusion is also a de facto exclusion of heavy fertile neutrinos up to 150 GeV.

1 comment:

andrew said...

Lubos has lots of quick hits from the CMB Conference underway in Minnesota right now.