Inflation assisted by heterotic axions

Abstract

We explore the possibility of obtaining inflation in weakly coupled heterotic string theory, where the model dependent axions are responsible for driving inflation. This model can be considered as a certain extrapolation of m 2 φ 2 -inflation, and is an attempt to explicitly realize the so called N-flation proposal in string theory. The instanton generated potential for the axions essentially has two parameters; a natural mass scale M and the string coupling g s . For isotropic compactifications leading to of order O(10 4 ) axions in the four dimensional spectrum we find that with (M, g s ) ≃ (M GU T , 0.5) the observed temperature fluctuations in the CMB are correctly reproduced. We assume an initially random distribution for the vevs of the axions. The spectral index, n s , is generically more red than for m 2 φ 2 -inflation. The greater the vevs, the more red the spectral index becomes. Allowing for a wide range of vevs 55 e-foldings from the end of inflation, we find 0.946 n s 0.962. The tensor-to-scalar ratio, r, is more sensitive to the vevs, but typically smaller than in m 2 φ 2 -inflation. Furthermore, in the regime where the leading order theory is valid, r is bounded by r < 0.10. The spectral index and the tensor-to-scalar ratio are correlated. For example, n s ≃ 0.951 corresponds to r ≃ 0.036.

Large gravitational waves and the Lyth bound in multi-brane inflation

“…Similar conclusions hold for all multi-brane inflation models [23] or the string-theory realization [58] of N-flation [59].…”

A review of axion inflation in the era of Planck

“…This implies that even producing observable tensor modes and satisfying the Lyth bound, this two field model is unaffected by the theoretical difficulties associated with large field excursion. Let us now discuss the model with a large number of axions, called axion N-flation, which was proposed in [31] and further discussed in [32,33,34,35,36,37]. Ignoring higher instanton terms, the potential can be written as…”

“…Several attempts were made to embed this model in string theory (e.g. [33,34,35,38]) but different obstacles such as backreaction make it very hard to control the large N limit. One point worth stressing is that loop corrections induced by the N fields renormalize the Planck scale [31] in such a way to prevent inflation for very large N .…”

Multifield coupled quintessence

“…It was studied for quadratic and quartic potentials in the context of higher dimensional reduction in Ref. [7][8][9], for Bianchi models [10,11], for an ensemble of tachyon fields [12][13][14][15], in braneworld models [16,17], in particular string theory realisations [18][19][20] and taking into account Loop Quantum Cosmology corrections [21]. A multi-field dynamics was also studied in the context of Logamediate inflation [22] and k-inflation [23].…”