[Paper Review] Multi-feature universe: large parameter space cosmology and the swampland

Deng Wang|arXiv (Cornell University)|Sep 13, 2018

Cosmology and Gravitation Theories7 citations

TL;DR

This paper investigates the swampland conjecture's cosmological implications by analyzing a 12-parameter interacting dark energy model using current cosmological data. Employing a model-by-model comparison approach, it derives the tightest upper bounds to date on the swampland constant $c$, finding $c \lesssim 1.94$ at 3σ for dark energy and $c \lesssim 0.58$ at 3σ for Dirac-Born-Infeld inflation with concave potential, indicating modest tension with the swampland conjecture.

ABSTRACT

Under the belief that the universe should be multi-feature and informative, we employ a model-by-model comparison method to explore the possibly largest upper bound on the swampland constant $c$. Considering the interacting quintessence dark energy as the comparison model, we constrain the large parameter space interacting dark energy model, a 12-parameter extension to the $Λ$CDM cosmology, in light of current observations. We obtain the largest $2σ$ ($3σ$) bound so far, $c\lesssim1.62$ $(1.94)$, which would allow the existences of a number of string theory models of dark energy such as 11-dimensional supergravity with double-exponential potential, $O(16) imes O(16)$ heterotic string and some Type II string compactifications. For inflationary models with concave potential, we find the $2σ$ ($3σ$) bound $c\lesssim0.13$ $(0.14)$, which is still in strong tension with the string-based expectation $c \sim \mathcal{O}(1)$. However, combining Planck primordial non-Gaussianity with inflation constraints, it is interesting that the Dirac-Born-Infeld inflation with concave potential gives the $2σ$ ($3σ$) bound $c\lesssim0.53$ $(0.58)$, which is now in a modest tension with the swampland conjecture. Using the Bayesian evidence as the model selection tool, it is very surprising that our 18-parameter multi-feature cosmology is extremely strongly favored over the $Λ$CDM model.

Motivation & Objective

• To determine the largest possible upper bound on the swampland constant $c$ using current cosmological observations.
• To test the viability of the swampland conjecture in the context of large parameter space cosmological models.
• To explore whether string theory-inspired dark energy models and inflationary scenarios are compatible with observational constraints under the swampland framework.
• To investigate the consistency of multi-feature cosmological models with Planck-2018 data, particularly regarding primordial power spectrum and lensing amplitude.
• To address the 'helium puzzle'—an unexpected degeneracy in constraints on primordial helium abundance $Y_{\text{He}}$—in the context of high-dimensional parameter spaces.

Proposed method

• A model-by-model comparison method is employed, using interacting quintessence dark energy as the comparison model in a 12-parameter extension of $\Lambda$CDM.
• The analysis uses combined datasets TLLBP for dark energy and TLLBK for inflation, including CMB, BAO, SNe Ia, and weak lensing data.
• The swampland conjecture's second criterion, $M_p |V'|/V > c \sim \mathcal{O}(1)$, is applied to constrain $c$ via likelihood analysis and Markov Chain Monte Carlo sampling.
• Constraints on $c$ are derived from the $2\sigma$ and $3\sigma$ credible intervals of the posterior distribution for each model.
• The study incorporates Planck-2018 primordial non-Gaussianity data to refine bounds in inflationary models, particularly for Dirac-Born-Infeld (DBI) inflation with concave potentials.
• The scale invariance of the primordial power spectrum ($n_s=1$) and lensing amplitude $A_L$ are tested for consistency with theoretical predictions in the multi-feature models.

Experimental results

Research questions

• RQ1 What is the largest possible upper bound on the swampland constant $c$ consistent with current cosmological data?
• RQ2 Can string theory-inspired dark energy models, such as 11D supergravity or $O(16)\times O(16)$ heterotic string, be viable under the swampland conjecture?
• RQ3 How does the inclusion of primordial non-Gaussianity data affect the constraint on $c$ in DBI inflation with concave potentials?
• RQ4 To what extent do multi-feature cosmological models with 12 parameters reproduce standard predictions like $n_s=1$ and $A_L=1$?
• RQ5 Why is the primordial helium abundance $Y_{\text{He}}$ unconstrained in the current analysis, and what does this imply about parameter degeneracies?

Key findings

• The 3σ upper bound on the swampland constant $c$ is found to be $c \lesssim 1.94$ for the interacting dark energy model, representing the tightest bound to date.
• This bound allows for the viability of several string theory models of dark energy, including 11-dimensional M-theory with double-exponential potential and $O(16)\times O(16)$ heterotic string compactifications.
• For inflationary models with concave potentials, the 3σ bound is $c \lesssim 0.14$, consistent with previous results and in strong tension with the $c \sim \mathcal{O}(1)$ expectation from string theory.
• When combining Planck primordial non-Gaussianity data with inflation constraints, the DBI inflation model yields a 3σ bound of $c \lesssim 0.58$, indicating only modest tension with the swampland conjecture.
• The scale invariance of the primordial power spectrum ($n_s = 1$) is well satisfied at less than the 2σ confidence level in the large parameter space LPSIDE model.
• The lensing amplitude scaling parameter $A_L$ is consistent with the theoretical prediction $A_L = 1$ at approximately the 1σ confidence level in both LPSIDE and LPSCPL models.