Meaning of the inertial energy density

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Arman777
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I am reading an article, which talks about graduated dark energy (gDE) model. In this model, it's assumed
that the inertial mass density exhibits power-law dependence to its energy density

$$\rho_{inert} = \gamma\rho_0(\frac{\rho}{\rho_0})^{\lambda}$$

Where ##\gamma## and ##\lambda## are real constant. Authors define an EoS parameter in the form of

$$w = \frac{p}{\rho} = -1 + \frac{\rho_{inert}}{\rho}=-1 + \gamma(\frac{\rho}{\rho_0})^{\lambda-1}$$

At this point, I confused about the "inertial mass density" term. I never heard the use of it in cosmology. What does inertial mass density is proportional to energy density means in this context?

I understand that we are trying to make DE as a time-dependent function but I did not understand why we need a term like "inertial mass density"?

I hope my questions make sense. You can find the article from here. I am talking about the second Equation.

https://ui.adsabs.harvard.edu/abs/2020PhRvD.101f3528A/abstract

Thanks
 
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