Comparing Infrared Dirac-Born-Infeld Brane Inflation to Observations

TL;DR

The paper tests the Infrared Dirac-Born-Infeld (IR DBI) brane inflation model against cosmological data using Bayesian MCMC techniques, exploring how string-theoretic microphysics maps to observables. It finds current data cannot decisively distinguish IR DBI from \LambdaCDM but constrains microscopic parameters such as the fundamental string scale \(m_s\), warp factors, and throat charges, while predicting distinctive signatures. A central result is the regional, stringy-phase-induced running of the spectral index near a phase transition at scale \(k_c\), together with sizable equilateral non-Gaussianity ${f_{NL}^{eq}}$, offering a concrete route to probe string theory with cosmological observations. The work demonstrates how observations can constrain and potentially reveal string-theoretic early-universe physics, guiding future measurements like Planck and large-scale structure surveys.

Abstract

We compare the Infrared Dirac-Born-Infeld (IR DBI) brane inflation model to observations using a Bayesian analysis. The current data cannot distinguish it from the \LambdaCDM model, but is able to give interesting constraints on various microscopic parameters including the mass of the brane moduli potential, the fundamental string scale, the charge or warp factor of throats, and the number of the mobile branes. We quantify some distinctive testable predictions with stringy signatures, such as the large non-Gaussianity, and the large, but regional, running of the spectral index. These results illustrate how we may be able to probe aspects of string theory using cosmological observations.

Figures (13)

• Figure 1: The inflation phase diagram for UV models. The shaded regions correspond to parameter space that can give rise to inflation. The darker the region is, the larger $e$-folds it can provide. "S.R." stands for slow-roll inflation; "DBI" stands for DBI inflation. The arrows indicate the starting point and rolling direction of the inflaton. In brane inflation, inflatons have to stay below the horizontal solid line (at $\phi_A = R_A \sqrt{n_A T_3}$); the two vertical lines (at $m=\sqrt{V_0}/M_{\rm Pl}$ and $m=M_{\rm Pl}/\sqrt{\lambda_A}$) are widely separated. The curve stretching from $m=\sqrt{V_0}/M_{\rm Pl}$ to $\phi_A=\sqrt{2}M_{\rm Pl}$ corresponds to $\eta_V=\beta/3=1$. See text for discussion.
• Figure 2: Multi-throat brane inflation scenario. In the first figure, antibranes are settled down in throats. In the second figure, in some throats antibranes annihilate fluxes and generate branes. For a throat with tachyonic brane moduli, branes fall out and settle down somewhere else, triggering either IR or UV models of brane inflation.
• Figure 3: The inflation phase diagram for IR models. The notation used here is the same as in Fig. \ref{['Fig:UVmodels']}. The vertical line at $m=\sqrt{V_0}/M_{\rm Pl}$ corresponds to $|\eta_V| = \beta/3 =1$. The unshaded region may support a certain amount of non-relativistic fast-roll inflation.
• Figure 4: Attractor solutions and numerical results. The dashed lines are the analytical attractor solutions. The solid lines are numerical solutions with different initial velocities. The upper-left panel shows the evolution of the ratio of the inflaton-velocity $\dot r$ to the warped-speed-of-light $h^2$. The two dashed lines are DBI and non-relativistic rolling, respectively. The upper-right panel shows the evolution of the Lorentz factor $\gamma$. The lower panels are the blow-ups of the upper panels. The parameters are $\beta=2$, $N_B=10^{9}$, $n_B=10^{5}$, $m_s g_s^{-1/4}= 10^{-6} M_{\rm Pl}$, $n_A h_A^4 = 1$. In the simulation, branes are started at $h_B=2.9 \times 10^{-7}$.
• Figure 5: Solid lines show the marginalized 2D-joint 68% and 95% probability contours (off-diagonal panels) and 1D marginalized probability distribution (diagonal panels) for the microphysical IR DBI parameters. The color coding in the off-diagonal panels shows the marginalized probability density in these 2D parameter spaces, ranging from red for the highest density to blue for the lowest.