Solution of the size and horizon problems from classical string geometry
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Springer
Received: August 18, 2020 Accepted: September 21, 2020 Published: October 23, 2020
Heliudson Bernardo,a Robert Brandenbergera and Guilherme Franzmannb a
Department of Physics, McGill University, Montr´eal, QC, H3A 2T8, Canada b Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, SE-106 91 Stockholm, Sweden
E-mail: [email protected], [email protected], [email protected] Abstract: In a recent paper we developed a string cosmology background from classical string geometry. Here, we show that this background yields a solution to the size and horizon problems of Standard Big Bang cosmology while remaining compatible with the Transplanckian Censorship Conjecture. We also take a first look at the evolution of cosmological perturbations in this model. Keywords: Strings and branes phenomenology ArXiv ePrint: 2007.14096
c The Authors. Open Access, Article funded by SCOAP3 .
https://doi.org/10.1007/JHEP10(2020)155
JHEP10(2020)155
Solution of the size and horizon problems from classical string geometry
Contents 1
2 Review of α0 -cosmology and summary of the model
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3 Solving the size and horizon problems of standard cosmology
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4 Connection with the Trans-Planckian Censorship conjecture
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5 A first look at the evolution of cosmological perturbations
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6 Summary and conclusions
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Introduction
If superstring theory is the correct framework which unifies all forces of nature at the quantum level, then strings must have played a crucial role at the high densities of the early universe. A theory based on one-dimensional strings as the fundamental objects has new symmetries and new degrees of freedom which theories based on point particles do not have. In particular, strings do not only have momentum modes, but also oscillatory and winding modes. Associated with the existence of winding modes is a new symmetry, namely the T-duality symmetry [1–5] which implies that, for a compact background (e.g. a toroidal background), physics on a space of radius R is equivalent to physics on a space of radius 1/R (in string units). From the partition function of a gas of closed strings [6] it then follows that the temperature T (R) obeys this symmetry which implies that the temperature of a gas of closed strings is finite [7] for any radius and always below the Hagedorn temperature [8]. String Gas Cosmology (SGC) [7] (see also [9]) is a scenario based on the above results in which an emergent phase dominated by a gas of strings with temperature close to TH precedes the Standard Big Bang expansion. To attain thermal equilibrium, a quasi-static phase in Einstein frame is postulated, with a nearly constant scale factor. Regarding cosmological perturbations, it was shown in [10] that thermal fluctuations yield a nearly scale-invariant spectrum of scalar and tensor perturbations, with a red tilt for the former and blue tilt for the latter [11, 12], and non-Gaussianities which are Poisson-suppressed on large scales [13]. Hence, String Gas Cosmolo
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