Wednesday, October 10, 2012

dark energy equation of state

why can the equation of state $w$ of dark energy in a scalar field model never be larger than $+1$ and never be more negative than $-1$? are there physical arguments why this would be weird? what would happen to distance measures and to structure growth if $w$ were outside this range?

bonus question: does the Higgs-mechanism in quantum field theory give rise to mass or to inertia?

second bonus question: why is dark matter called dark matter if it does not interact with light? if that was the reason, it should rather be called transparent matter, don't you agree?


  1. in a scalar field model the dark energy eos $w$ depends on the balance between kinetic energy $\dot{\varphi}^2/2$ and potential energy $V(\varphi)$ of the quintessence field $\varphi$:
    w = \frac{\dot{\varphi}^2/2 - V(\varphi)}{\dot{\varphi}^2/2 + V(\varphi)}
    consequently, if there's only kinetic energy and no potential energy, $w$ would be equal to $+1$, and if there's only potential energy and no kinetic energy, $w$ would be $-1$, which corresponds to the slow roll conditions needed for accelerated expansion.

  2. i think the answer to the bonus question is not so obvious as it might seem. of course the two are identical, but wouldn't you say that mass refers to a quantity (the quanity of matter) whereas inertia is a property of an object resisting forces by showing weaker acceleration? of course it would make sense that a more massive object (in the sense of an object being composed of a larger amount of matter) resists more strongly to forces.

  3. perhaps i try to refer in the future to CTM-models (cold transparent matter) of cosmic structure formation! :)

  4. of course, (adiabatic) cosmic fluids with eos parameters more negative than -1 show increasing density if space expands, and the Hubble function would get larger with increasing scale factor. distances can shrink.