Exacerbating the Cosmological Constant Problem with Interacting Dark Energy Models

Future cosmological surveys will probe the expansion history of the Universe and constrain phenomenological models of dark energy. Such models do not address the fine-tuning problem of the vacuum energy, i.e., the cosmological constant problem (CCP), but can make it spectacularly worse. We show that this is the case for “interacting dark energy” models in which the masses of the dark matter states depend on the dark energy sector. If realized in nature, these models have far-reaching implications for proposed solutions to the CCP that require the number of vacua to exceed the fine-tuning of the vacuum energy density. We show that current estimates of the number of flux vacua in string theory, $N_{\mathrm{vac}}\sim \mathcal{O}(10^{272000})$, are far too small to realize certain simple models of interacting dark energy and solve the cosmological constant problem anthropically. These models admit distinctive observational signatures that can be targeted by future gamma-ray observatories, hence making it possible to observationally rule out the anthropic solution to the cosmological constant problem in theories with a finite number of vacua.

Figure 1

Contributions to the vacuum energy.

Figure 2

From bottom and up: black lines correspond to $p=1–4$, 6, 8; grey horizontal lines to ${\log }_{10}(f_{\mathrm{tot}})=506$ and 272 000.