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Register a new userThe self-tuning of the cosmological constant and the holographic relaxion
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We propose a brane-world setup based on gauge/gravity duality that permits the simultaneous realisation of self-tuning of the cosmological constant and a stabilisation of the electroweak hierarchy. The Standard Model dynamics including the Higgs sector is confined to a flat 4-dimensional brane, embedded in a 5-dimensional bulk whose dynamics is governed by Einstein-dilaton-axion gravity. The inclusion of a dynamical bulk axion is new compared to previous implementations of the self-tuning mechanism. Because of the presence of the axion, the model generically exhibits a multitude of static solutions, with different values for the equilibrium position for the brane. Under mild assumptions regarding the dependence of brane parameters on bulk fields, a number of these solutions exhibit electroweak symmetry breaking with a hierarchically small Higgs mass as compared to the cutoff-scale of the brane theory. The realisation of self-tuning of the cosmological constant is generic and as efficient as in previous constructions without a bulk axion. Vacua with a hierarchically small Higgs mass can sometimes be found, regardless of whether the brane theory depends explicitly on the bulk axion. Because it is expected on general principles that the brane action will depend on the axion, the generation of solutions with a large hierarchy is a robust feature.
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The cosmology of branes undergoing the self-tuning mechanism of the cosmological constant is considered. The equations and matching conditions are derived in several coordinate systems, and an exploration of possible solution strategies is performed. The ensuing equations are solved analytically in the probe brane limit. We classify the distinct behavior for the brane cosmology and we correlate them with properties of the bulk (static) solutions. Their matching to the actual universe cosmology is addressed.
String theory has no parameter except the string scale $M_S$, so the Planck scale $M_text{Pl}$, the supersymmetry-breaking scale, the EW scale $m_text{EW}$ as well as the vacuum energy density (cosmological constant) $Lambda$ are to be determined dynamically at any local minimum solution in the string theory landscape. Here we consider a model that links the supersymmetric electroweak phenomenology (bottom up) to the string theory motivated flux compactification approach (top down). In this model, supersymmetry is broken by a combination of the racetrack Kahler uplift mechanism, which naturally allows an exponentially small positive $Lambda$ in a local minimum, and the anti-D3-brane in the KKLT scenario. In the absence of the Higgs doublets in the supersymmetric standard model, one has either a small $Lambda$ or a big enough SUSY-breaking scale, but not both. The introduction of the Higgs fields (with their soft terms) allows a small $Lambda$ and a big enough SUSY-breaking scale simultaneously. Since an exponentially small $Lambda$ is statistically preferred (as the properly normalized probability distribution $P(Lambda)$ diverges at $Lambda=0^{+}$), identifying the observed $Lambda_{rm obs}$ to the median value $Lambda_{50%}$ yields $m_{rm EW} sim 100$ GeV. We also find that the warped anti-D3-brane tension has a SUSY-breaking scale of $100m_{rm EW}$ in the landscape while the SUSY-breaking scale that directly correlates with the Higgs fields in the visible sector has a value of $m_{rm EW}$.
Recently, the variation of the Planck mass in the General Relativistic Einstein-Hilbert action was proposed as a self-tuning mechanism of the cosmological constant, preventing Standard Model vacuum energy from freely gravitating and enabling an estimation of the magnitude of its observed value. We explore here new aspects of this proposal. We first develop an equivalent Einstein-frame formalism to the current Jordan-frame formulation of the mechanism and use this to highlight similarities and differences of self-tuning to the sequestering mechanism. We then show how with an extension of the local self-tuning action by a coupled Gauss-Bonnet term and a companion four-form field strength, graviton loops can be prevented from incapacitating the degravitation of the Standard Model vacuum energy. For certain cases, we furthermore find that this extension can be recast as a Horndeski scalar-tensor theory and be embedded in the conventional local self-tuning formalism. We then explore the possibility of a unification of inflation with self-tuning. The resulting equations can alternatively be used to motivate a multiverse interpretation. In this context, we revisit the coincidence problem and provide an estimation for the probability of the emergence of intelligent life in our Universe as a function of cosmic age, inferred from star and terrestrial planet formation processes. We conclude that we live at a very typical epoch, where we should expect the energy densities of the cosmological constant and matter to be of comparable size. For a dimensionless quantity to compare the emergence of life throughout the cosmic history of different universes in an anthropic analysis of the multiverse, we choose the order of magnitude difference of the evolving horizon size of a universe to the size of its proton as the basic building block of atoms, molecules, and eventually life. (abridged)
String theory has no parameter except the string scale, so a dynamically compactified solution to 4 dimensional spacetime should determine both the Planck scale and the cosmological constant $Lambda$. In the racetrack Kahler uplift flux compactification model in Type IIB theory, where the string theory landscape is generated by scanning over discrete values of all the flux parameters, a statistical preference for an exponentially small $Lambda$ is found to be natural (arXiv:1305.0753). Within this framework and matching the median $Lambda$ value to the observed $Lambda$, a mass scale ${bf m}simeq 100$ GeV naturally appears. We explain how the electroweak scale can be identified with this mass scale.
We examine the relaxion mechanism in string theory. An essential feature is that an axion winds over $N gg 1$ fundamental periods. In string theory realizations via axion monodromy, this winding number corresponds to a physical charge carried by branes or fluxes. We show that this monodromy charge backreacts on the compact space, ruining the structure of the relaxion action. In particular, the barriers generated by strong gauge dynamics have height $propto e^{-N}$, so the relaxion does not stop when the Higgs acquires a vev. Backreaction of monodromy charge can therefore spoil the relaxion mechanism. We comment on the limitations of technical naturalness arguments in this context.
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