Whenever anyone talks about the cosmological constant, they usually cite the difference of $\approx 120$ orders of magnitude between the observed and calculated values for the vacuum energy ($\rho_\text{obs} \approx 10^{-8}\text{erg}/\text{cm}^3$ and $\rho_\text{calc} \approx 10^{112}\text{erg}/\text{cm}^3$) and then claim that this leads to a serious fine-tuning problem. I've been trying to look into this, and, as best I can tell, the root of it appears to be that people claim that there is a bare cosmological constant: $$\Lambda_{\text{eff}} = \Lambda_{\text{bare}} + \Lambda_{\text{vac}}.$$ Where it appears that $\Lambda_\text{eff}$ is the "actual" cosmological constant that gets introduced into the Einstein Field Equations: $$R_{\mu\nu} - \frac{1}{2}Rg_{\mu\nu} + \Lambda_\text{eff}g_{\mu\nu} = 8\pi G T_{\mu\nu},$$ $\Lambda_\text{vac}$ corresponds to the roughly calculated value of the vacuum energy from all of the zero-point energies, and $\Lambda_\text{bare}$ is the the bare cosmological constant. The claim is that in order to get the extremely small observed value, the bare constant must cancel the vacuum constant out to $120$ orders of magnitude, which is a pretty serious fine-tuning problem.
My question, then, is this: What is the nature of the bare cosmological constant? My understanding of the "actual" cosmological constant that enters the Einstein Field Equations is that it arises from the vacuum energy (I guess, in my mind, I've always thought $\Lambda_\text{eff} = \Lambda_\text{vac}$), which leaves me confused about where the bare constant comes from. I've seen several papers make mention of the bare constant, but no one ever actually explains why it's present or how it is justified, so a reference would be greatly appreciated. Thanks!
