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?

4 comments:

  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 $\varphi$:
    \begin{equation}
    w = \frac{\dot{\varphi}^2/2 - V(\varphi)}{\dot{\varphi}^2/2 + V(\varphi)}
    \end{equation}
    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.

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  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.

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

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  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.

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