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We suggest a dynamical mechanism which explains why in the supersymmetric IKKT matrix model the $SO(9)$ symmetry of the Lagrangian is spontaneously broken to $SO(3) \times SO(6)$, allowing only three large classical spatial dimensions to emerge. The argument relies on the identification of D1-strings as stable excitations of the matrices about the cosmological background, and the absence of forces between parallel D1-string excitations. A spatial dimension can only become large if the D1-strings winding that direction can annihilate. In the absence of forces, this requires the string world sheets to intersect, a process which is highly suppressed if more than three dimensions of space are large. Implications for a recently proposed matrix cosmology scenario are discussed., Comment: 11 pages, 3 figures

We study the effects of long wavelength entropy fluctuations on cosmological probes such as galaxy clustering, luminosity distance, and CMB temperature anisotropies. Specifically, we consider fluctuations of a massless spectator scalar field set up in the early universe, which later acquires mass during the radiation-dominated era. We find that there are non-vanishing effects on observables, and the amplitude of these effects peaks for observables set up at the time of equal matter and radiation, and decreases as $\eta^{-2}$ where $\eta$ is the conformal time. Hence, the back-reaction effects are important for CMB anisotropies, but their impact on late-time observables is suppressed. In particular, the back-reaction effects are unable to explain the Hubble tension while they might alleviate the cosmic dipole tension. In contrast to a lot of the previous work on back-reaction, we work in position rather than momentum space., Comment: 13 pages

We study the effects of superfluid dark matter on the structure of a cosmic string wake, considering both the effects of regular and quantum pressure terms. We consider the total fluid to consist of a combination of baryons and dark matter. Hence, we are also able to study the effects of superfluid dark matter on the distribution of baryons inside the wake. We focus on parameter values for the superfluid dark matter which allow a MONDian explanation of galaxy rotation curves., Comment: 10 pages, 6 figures

We study the interplay of the trans-Planckian censorship conjecture (TCC) and the swampland distance conjecture (SDC) in the context of multifield dark energy in a curved field space. In this scenario, the phase of accelerated expansion is realized as non-geodesic motion in a highly-curved field space, reminiscent of models developed in the context of inflation. The model features a stable attractor solution with near constant equation of state $w\simeq -1$, and predicts that the current era of accelerated expansion is eternal. The latter implies an eventual conflict with the TCC, which holds that the duration of any epoch of cosmic acceleration is bounded by the requirement that the large-scale observable universe is blind to Planck-scale early universe physics. This tension can be resolved by an interplay with the distance conjecture: for suitable parameter values, the apparent violation of the TCC occurs well after the fields have traversed a Planckian distance. The SDC then predicts a breakdown of the effective field theory (EFT) before the TCC can be violated. We derive the constraints on the model arising from the SDC+TCC and the de Sitter conjecture. We demonstrate that the model can be consistent with both swampland conjectures and observational data from Planck 2018 and the Dark Energy Spectroscopic Instrument., Comment: 12 pages, 3 figures

Embedded walls are domain wall solutions which are unstable in the vacuum but stabilized in a plasma of the early Universe. We show how embedded walls in which the electroweak symmetry is restored can lead to an efficient scenario of electroweak baryogenesis. We construct an extension of the Standard Model of particle physics in which embedded walls exist and are stabilized in an electromagnetic plasma., Comment: 15 pages, 1 figure; v2: updated references, discussion of local EWBG and figure 1 added, matches version published in Phys. Rev. D

By means of N-body simulations, we study early structure formation in the presence of a scaling distribution of cosmic string loops. Cosmic string loops dominate the high redshift halo mass function while the fluctuations seeded by the standard structure formation scenario dominate structure at low redshifts. In our study, the effects of the cosmic string loops are taken into account by displacing the dark matter particles and their velocities at the initial time of the simulation by amounts determined by the analytical analysis which makes use of the Zeldovich approximation. We find that the resulting halo mass function is to a good approximation given by the sum of the analytically determined cosmic string halo mass function and the halo mass function obtained from the standard $\Lambda$CDM model., Comment: 8 pages, 10 figures

We study the implications of the stringy space-time uncertainty relation (STUR) for inflationary cosmology. By demanding that no fluctuation modes that exit the Hubble radius are affected by the nonlocality resulting from the STUR, we find an upper bound on the number of e-foldings of inflation. The bound is a factor of 2 weaker than what results from the Trans-Planckian Censorship Criterion (TCC). By demanding that the inflationary phase is simultaneously consistent with STUR and sufficiently long for inflation to provide a causal explanation of structure on the scale of the current Hubble radius, we find an upper bound on the energy scale of inflation. The bound is less restrictive than what follows from the TCC, but it remains in conflict with canonical single-field inflation models., Comment: 7 pages, 1 figure

Cosmic string loops are non-linear density fluctuations which form in the early universe and could play an important role in explaining many phenomena which are in tension with the standard $\Lambda$CDM model. Hence, the details of the accretion process onto cosmic string loops should be studied in detail. Most previous works view loops as point masses and ignore the impact of a finite loop size. In this work, we utilize the Zel'dovich approximation to calculate the non-linear mass sourced by a static extended loop with a time-averaged density profile derived from the trajectory of the loop oscillation, and compare the result with what is obtained for a point-mass source. We find that the finite size of a loop mainly affects the evolution of turnaround shells during the early stages of accretion, converging to the point mass result after a critical redshift, $z^{(II)/(III)}_{c}$. For $z>z^{(II)/(III)}_{c}$, the total accreted mass surrounding a loop is suppressed relative to the point mass case and has a growth rate proportional to $(1+z)^{-3/2}$. As an immediate extension, we also qualitatively analyse the accretion onto moving point masses and onto moving extended loops. In addition to the reduction in the nonlinear mass, the loop finite size also changes the shape of the turnaround surface at early stages of accretion., Comment: 19 pages, 7 figures

We consider a cosmic string moving through a gas of superfluid dark matter (SFDM) particles and analyze how it affects the dark matter distribution. We look at two different cases: first, a cosmic string passing through an already condensed region, and second, through a region that is not yet condensed. In the former, the string induces a weak shock in the superfluid, and the Bose-Einstein condensate (BEC) survives. In the latter, a wake of larger density is formed behind the string, and we study under which conditions a BEC can be formed in the virialized region of the wake. By requiring the thermalization of the DM particles and the overlap of their de Broglie wavelengths inside the wake, we obtain an upper bound on the mass of the dark matter particles on the order of 10 eV, which is compatible with typical SFDM models., Comment: 20 pages, 2 figures; added references and footnotes

In this review, a number of approaches to superstring cosmology which make use of key features which distinguish string theory from point particle theories are discussed, with particular emphasis on emergent scenarios. One motivation for the discussion is the realization that, in order to describe the evolution of the very early universe, it is necessary to go beyond a conventional effective field theory (EFT) analysis. Some of the conceptual problems of an EFT analysis will be discussed. The review begins with a summary of the criteria for a successful early universe scenario, emphasizing that cosmic inflation is not the only scenario of early universe cosmology which is consistent with current cosmological observations. Bouncing and emergent scenarios as interesting alternatives are introduced. Some realizations of these scenarios from superstring theory are reviewed, e.g. String Gas Cosmology, the Pre-Big-Bang scenario, the Ekpyrotic model, Double Field Theory cosmology and matrix model cosmology. In light of the difficulties in obtaining cosmic inflation from string theory (at the level of EFT), and realizing that there are promising examples of alternative early universe scenarios which are derived from basic principles of superstring theory, one must entertain the possibility that the cosmology emerging from string theory will not involve an extended period of accelerated expansion., Comment: 25 pages, 8 figures, obvious typos in Eqs. (9) and (10) corrected

An oscillating inflaton field induces small amplitude oscillations of the Hubble parameter at the end of inflation. These Hubble parameter induced oscillations, in turn, trigger parametric particle production of all light fields, even if they are not directly coupled to the inflaton. We here study the induced particle production for a light scalar field (e.g. the Standard Model Higgs field) after inflation as a consequence of this effect. Our analysis yields a model-independent lower bound on the efficiency of energy transfer from the inflaton condensate to particle excitations., Comment: 9 pages, 7 figures

Cosmic strings, if they exist, source nonlinear and non-Gaussian perturbations all the way back to the time of equal matter and radiation (and earlier). Here, we compute the mass function of halos seeded by a scaling distribution of cosmic string loops, and we compare the results with the predictions of the standard Gaussian $\Lambda$CDM model. Assuming a simple linear relation between stellar mass and halo mass, we also compute the stellar mass function. The contribution of cosmic strings dominates at sufficiently high redshifts $z > z_c$ where $z_c$ depends on the mass of the halo and on the mass per unit length $\mu$ of the strings and is of the order $z_c \sim 12$ for $G\mu = 10^{-8}$. We find that strings with this value of $G\mu$ can explain the preliminary JWST data on the high redshift stellar mass density. Based on an extreme value statistic, we find that the mass of the heaviest expected string-seeded galaxy for the current JWST sky coverage is compatible with the heaviest detected galaxy. Given the uncertainties in the interpretation of the JWST data, we discuss predictions for higher redshift observations., Comment: 13 pages, 8 figures

Thermal inflation was proposed as a mechanism to dilute the density of cosmological moduli. Thermal inflation is driven by a complex scalar field possessing a large vacuum expectation value and a very flat potential, called a `flaton'. Such a model admits cosmic string solutions, and a network of such strings will inevitably form in the symmetry breaking phase transition at the end of the period of thermal inflation. We discuss the differences of these strings compared to the strings which form in the Abelian Higgs model. Specifically, we find that the upper bound on the symmetry breaking scale is parametrically lower than in the case of Abelian Higgs strings, and that the lower cutoff on the string loop distribution is determined by cusp annihilation rather than by gravitational radiation (for the value of the transition temperature proposed in the original work on thermal inflation)., Comment: 6 pages

Oscillating scalar field condensates induce small amplitude oscillations of the Hubble parameter which can induce a decay of the condensate due to a parametric resonance instability [1]. We show that this instability can lead to the decay of the coherence of the condensate of axion-like particle (ALP) fields during the radiation phase of standard cosmology for rather generic ALP parameter values, with possible implications for certain experiments aiming to search for ALP candidates. As an example, we study the application of this instability to the QCD axion. We also study the magnitude of the induced entropy fluctuations., Comment: 9 pages, 2 figures

We argue that, in the same way that in a black hole space-time VECROs will form in order to cancel the gravitational effects of a collapsing mass shell and prevent the formation of a singularity, in a contracting universe a gas of VECROs will form to hold up the contraction, prevent a Big Crunch singularity, and lead to a nonsingular cosmological bounce., Comment: 5 pages

We consider the back-reaction of cosmological fluctuations on the local expansion rate averaged over a space-like hypersurface of constant value of a clock field. We show that in the infrared limit, the fluctuations lead to a decrease in the average expansion rate, measured at a fixed value of the clock field, compared to what would be obtained in a homogeneous universe. We work in the context of Einstein gravity coupled to perfect fluid matter., Comment: 6 pages

Recently, Pei-Ming Ho and Hikaru Kawai have argued that treating particles as wave packets can lead to a shutdown of Hawking radiation after a scrambling time in the case of Schwarzschild black holes. This shutdown arises from viewing quantum field theory near the black hole horizon as an effective field theory, and imposing an appropriate UV cutoff. We show that this effect is also present in the static patch of de Sitter space, leading to a shutdown of Gibbons-Hawking radiation at late times. Assuming this effect is due to the breakdown of effective field theory, we obtain a bound $t \lesssim H^{-1} \ln (H^{-1} M_P)$ on the time scale of validity of effective field theory in de Sitter space, which matches with the predictions of the Trans-Planckian Censorship Conjecture.

I present some new perspectives on Dark Matter, Dark Energy and the origin of structure in the Universe. First, I argue that in order to understand the two latter issues, one needs to go beyond a standard point particle effective field theory analysis. Next, I review recent work attempting to construct a unified dark sector model from Heterotic superstring theory. I finish by discussing a new research effort to obtain early Universe cosmology directly from a non-perturbative definition of superstring theory., Comment: Invited concluding talk, 33rd Rencontres de Blois

We review a proposal to obtain an emergent metric space-time and an emergent early universe cosmology from the BFSS matrix model. Some challenges and directions for future research are outlined., Comment: 7 pages, 1 figure

We apply the Gaussian expansion method to the BFSS matrix model in the high temperature limit. When the (Euclidean) BFSS action is expanded about a Gaussian ansatz, it is shown that the SO(9) symmetry is spontaneously broken, analogous to what happens in the IKKT model. The analysis of the free energy, using the set of gap equations which determines the width of the Gaussian terms, is sufficient to show that this symmetry breaking happens only when the fermionic terms are included and is absent in the bosonic case., Comment: 23 pages, comments welcome

The IKKT matrix model yields an emergent space-time. We further develop these ideas and give a proposal for an emergent metric. Based on previous numerical studies of this model, we provide evidence that the emergent space-time is continuous and infinite in extent, both in space and in time, and that the metric is spatially flat. The time evolution describes the transition from a string-theoretic emergent phase to a phase in which the $SO(9)$ symmetry of the model is spontaneously broken to $SO(6) \times SO(3)$, with three dimensions of space expanding, becoming classical and at later times evolving like in a radiation-dominated universe, and the remaining six dimensions of space stabilized at the string scale. We speculate on how this analysis can be extended to yield an early universe cosmology which, in addition to the above-mentioned properties, also leads to a roughly scale-invariant spectrum of cosmological fluctuations and gravitational waves., Comment: 9 pages, 1 figure

We study the parametric resonance excitation of the electromagnetic field by a gravitational wave. We show that there is narrow band resonance. For an electromagnetic field in the vacuum the resonance occurs only in the second band, and its strength is thus suppressed by two powers of amplitude of the gravitational wave. On the other hand, in the case of an electromagnetic field in a medium with the speed of light smaller than 1 (in natural units), there is a band of Fourier modes which undergo resonance in the first band., Comment: 7 pages, 2 figures; v2 remarks added; v3 version accepted by Physics of the Dark Universe

The BFSS matrix model is a suggested non-perturbative definition of string theory. Starting from a thermal state of this matrix model, we show how space and time can emerge dynamically. Results from the IKKT matrix model indicate that the $SO(9)$ symmetry of space is spontaneously broken to $SO(3) \times SO(6)$, with the three-dimensional subspace becoming large. Given this initial state for the universe, we show that cosmological perturbations and gravitational waves with scale-invariant spectra are generated, without the need of postulating an early phase of cosmological inflation. The Big Bang singularity is automatically resolved., Comment: Essay written for the Gravity Research Foundation 2022 Awards for Essays on Gravitation, honorary mention, 10 pages, 1 figure

We study the signal of cosmic string wakes present before the time of reionization in the cross-correlation signal of 21-cm redshift and B-mode CMB polarization maps. The specific non-Gaussian signal of strings in the position space cross-correlation maps can be extracted by means of a matched filtering analysis. Signals of strings with tension somewhat lower than those corresponding to the current upper bound can be identified when embedded in a background of Gaussian fluctuations from a Planck best-fit LCDM model., Comment: 11 pages, 7 figures

In the context of Effective Field Theory, the Hilbert space of states increases in an expanding universe. Hence, the time evolution cannot be unitary. The formation of structure is usually studied using effective field theory techniques. We study the constraints on effective field theory analyses of early universe models which come from demanding that the factor of the space of states corresponding to length scales where the primordial fluctuations are manifest does not suffer from the unitarity problem. For bouncing and emergent cosmologies, no constraints arise provided that the energy scale of the bounce or emergent phases is smaller than the ultraviolet (UV) cutoff scale. On the other hand, in the case of the inflationary scenario, non-trivial upper bounds on the energy scale of inflation arise., Comment: 8 pages

The observation of quasars at high redshifts presents a mystery in the theory of black hole formation. In order to source such objects, one often relies on the presence of heavy seeds ($M \approx 10^{4-6} \, M_{\odot}$) in place at early times. Unfortunately, the formation of these heavy seeds are difficult to realize within the standard astrophysical context. Here, we investigate whether superconducting cosmic string loops can source sufficiently strong overdensities in the early universe to address this mystery. We review a set of direct collapse conditions under which a primordial gas cloud will undergo monolithic collapse into a massive black hole (forming with a mass of $M_{BH} \approx 10^5 \, M_{\odot}$ at $z \approx 300$ in our scenario), and systematically show how superconducting cosmic string loops can satisfy such conditions in regions of the $G\mu-I$ parameter space., Comment: 12 pages, 1 appendix, comments welcome!

We investigate the physical measurability of the infrared instability of a de Sitter phase in the formalism recently proposed by Kitamoto et al.. We find that the logarithmic decay of the effective cosmological constant is only measurable if an additional clock field is introduced., Comment: 9 pages

An embedding of a unified dark sector model into string theory with the following features is proposed: The model-independent axion descending from the Kalb-Ramond 2-form field is identified with the dark-matter field, and the real part of a K\"ahler modulus field -- the ``radius'' of one of the extra spatial dimensions -- accounts for dark energy. The expectation value of the dilaton field is stabilized by a gaugino condensation mechanism. A dark-energy potential corresponding to a realistic low energy scale results from some gentle tuning of the stabilized expectation value of the dilaton. The resulting potential reproduces the one in a previous dark-sector model proposed by two of us., Comment: 21 pages; construction slightly refined, references added

Most current cosmological models of the very early universe are based on local point particle effective field theories coupled to gravity. I will discuss some conceptual limitations of this approach and argue that an improved description of the early universe needs to go beyond this framework. I will outline a couple of ideas based on superstring theory., Comment: Invited talk at the workshop "Quantum Gravity, Higher Derivatives and Nonlocality", March 9 - 13 2021; 12 pages

Assuming that superstring theory is the fundamental theory which unifies all forces of Nature at the quantum level, I argue that there are key limitations on the applicability of effective field theory techniques in describing early universe cosmology., Comment: 16 pages, 4 figures; invited review for the special issue "The future of mathematical cosmology" of Philosophical Transactions A

Matrix theory is a proposed non-perturbative definition of superstring theory in which space is emergent. We begin a study of cosmology in the context of matrix theory. Specifically, we show that matrix theory can lead to an emergent non-singular cosmology which, at late times, can be described by an expanding phase of Standard Big Bang cosmology. The horizon problem of Standard Big Bang cosmology is automatically solved. We show that thermal fluctuations in the emergent phase source an approximately scale-invariant spectrum of cosmological perturbations and a scale-invariant spectrum of gravitational waves. Hence, it appears that matrix theory can lead to a successful scenario for the origin of perturbations responsible for the currently observed structure in the universe while providing a consistent UV-complete description., Comment: 12 pages; comments welcome

A cosmic string wake produces a distinct non-Gaussian signal in 21-cm intensity maps at redshifts above that of reionization. While the string signal is (locally) larger in amplitude than the signal of the Gaussian fluctuations of the $\Lambda$CDM model, they are overwhelmed (even locally in position space) by astrophysical and instrumental foregrounds. Here, we study to what extent the signal can be extracted from noisy interferometric data. The narrowness of the string-induced feature in redshift direction allows for a subtraction of astrophysical and instrumental foregrounds. Based on the specific geometry of the string signal we identify a particular three-point statistic which is promising in order to extract the signal, and we find that, having in mind a telescope of specifications similar to that of the MWA instrument, the string signal can be successfully extracted for a value of the string tension of $G\mu = 3 \times 10^{-7}$. Prospects for further improvements of the analysis are discussed., Comment: 35 pages, 21 figures

We identify potential sources of decoherence for $U(1)$ gauge bosons from a cosmological standpoint. Besides interactions with different species in the cosmological medium, we also consider effects due to the expansion of the Universe, which can produce particles (especially scalars) than can potentially interact with the photon in a quantum state. We look in particular at the case of axion-like particles and their predicted decay channels in our analysis. These interactions are shown to have a negligible effect as far as decoherence goes. Interaction rates with CMB radiation or through Thomson scattering are small, so that the interstellar medium remains the biggest decoherence factor. Thus, quantum teleportation experiments with photon energies in the range $1$-$10$ keV should be feasible at cosmological distances up to the galaxy formation epoch or beyond ($z \sim 100$)., Comment: 11 pages

We show that an S-Brane which arises in the inside of the black hole horizon when the Weyl curvature reaches the string scale induces a continuous transition between the inside of the black hole and the beginning of a new universe. This provides a simultaneous resolution of both the black hole and Big Bang singularities. In this context, the black hole information loss problem is also naturally resolved., Comment: 9 pages, 3 figures; essay written for the Gravity Research Foundation 2021 Awards for Essays on Gravitation, awarded Honorable Mention

Superconducting cosmic strings emit electromagnetic waves between the times of recombination and reionization. Hence, they have an effect on the global 21cm signal. We compute the resulting absorption features, focusing on strings with critical current, study their dependence on the string tension $\mu$, and compare with observational results. For string tensions in the range of $G \mu = 10^{-10}$, where $G$ is Newton's gravitational constant, there is an interesting amplification of the two characteristic absorption features, one during the cosmic dawn, $z \lesssim 30$, and the other during the cosmic dark age, $z \sim 80$, the former being comparable in amplitude to what was observed by the EDGES experiment., Comment: 8 pages, 3 figures

We consider the cosmology obtained using scalar fields with a negative potential energy, such as employed to obtain an Ekpyrotic phase of contraction. Applying the covariant entropy bound to the tower of states dictated by the distance conjecture, we find that the relative slope of the potential $|V^{\prime}| / |V|$ is bounded from below by a constant of the order one in Planck units. This is consistent with the requirement to obtain slow Ekpyrotic contraction. We also derive a refined condition on the potential which holds near local minima of a negative potential., Comment: 8 pages, 2 figures; v2: 14 pages, 2 figures; references and comments on conformal diagrams added; matches published version

We demonstrate that cosmic string loops may provide a joint resolution of two mysteries surrounding recently observed black holes. For a string tension in an appropriate range, large radius string loops have the potential to provide the nonlinearities in the early universe which seed supermassive black holes. The more numerous smaller radius string loops can then seed intermediate mass black holes, including those with a mass in the region between 65 and 135 solar masses in which standard black hole formation scenarios predict no black holes are able to form, but which have recently been detected by the LIGO/VIRGO collaboration. We find that there could be as many as $10^6$ of intermediate mass black holes per galaxy, providing a tantalizing target for gravitational wave observatories to look for., Comment: 8 pages

In theories beyond the Standard Model, in particular in theories with extra spatial dimensions such as superstring theory, there are a large number of scalar fields which appear in the low energy effective action. These moduli fields must be stabilized. Often, moduli stabilization involves a stage during which the moduli fields oscillatte coherently about their ground state value. Here, we study moduli and graviton production during the period during which the background modulus field is oscillating, assuming that this period takes place in the radiation phase. We find a resonant production of moduli fluctuations, and a tachyonic instability to the generation of long wavelength gravitational waves. As a consequence, the period of moduli stabilization is short on a Hubble time scale., Comment: 8 pages, 2 figures, minor typos corrected

We propose a construction with which to resolve the black hole singularity and enable an anisotropic cosmology to emerge from the inside of the hole. The model relies on the addition of an S-brane to the effective action which describes the geometry of space-time. This space-like defect is located inside of the horizon on a surface where the Weyl curvature reaches a limiting value. We study how metric fluctuations evolve from the outside of the black hole to the beginning of the cosmological phase to the future of the S-brane. Our setup addresses i) the black hole singularity problem, ii) the cosmological singularity problem and iii) the information loss paradox since the outgoing Hawking radiation is entangled with the state inside the black hole which becomes the new universe., Comment: 14 pages, 3 figues, comments welcome

I review the "Trans-Planckian Censorship Conjecture" (TCC) and its implications for cosmology, in particular for the inflationary universe scenario. Whereas the inflationary scenario is tightly constrained by the TCC, alternative early universe scenarios are not restricted., Comment: Invited short review for LHEP Special Issue: Swampland and String Theory Landscape. 9 pages, 3 figures. References added

We study the effects of particle production on the evolution of the inflaton field in an axion monodromy model with the goal of discovering in which situations the resulting dynamics will be consistent with the {\it swampland constraints}. In the presence of a modulated potential the evolving background field (solution of the inflaton homogeneous in space) induces the production of long wavelength inflaton fluctuation modes. However, this either has a negligible effect on the inflaton dynamics (if the field spacing between local minima of the modulated potential is large), or else it traps the inflaton in a local minimum and leads to a graceful exit problem. On the other hand, the production of moduli fields at enhanced symmetry points can lead to a realization of {\it trapped inflation} consistent with the swampland constraints, as long as the coupling between the inflaton and the moduli fields is sufficiently large., Comment: 11 pages, 5 figures

We calculate the entanglement entropy of scalar perturbations due to gravitational non-linearities present in any model of canonically-coupled, single-field ekpyrosis. Specifically, we focus on a recent model of improved ekpyrosis which is able to generate a scale-invariant power spectrum of curvature perturbations and gravitational waves as well as have a non-singular bounce due to an S-brane at the end of ekpyrotic contraction. By requiring that the entanglement entropy remians subdominant to the thermal entropy produced during reheating, we get an upper bound on the energy scale of the bounce., Comment: 7 pages

In String Gas Cosmology, the simplest shape modulus fields are naturally stabilized by taking into account the presence of string winding and momentum modes. We determine the resulting effective potential for these fields and show that it obeys the de Sitter conjecture, one of the swampland criteria for effective field theories to be consistent with superstring theory., Comment: 4 pages

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., Comment: 7 pages, 2 figures

In recent work, two of us proposed a nonsingular Ekpyrotic cosmology making use of an S-brane which forms at the end of the phase of Ekpyrotic contraction. This S-Brane mediates a transition between contraction and expansion. Graviitational waves passing through the S-Brane acquire a roughly scale-invariant spectrum, and if the S-Brane has zero shear, then a roughly scale-invariant spectrum of cosmological perturbatiions results. Here, we study the production of gauge field fluctuations driven by the decay of the S-Brane, and we show that the reheating process via gauge field production will be efficient, leading to a radiation-dominated expanding phase., Comment: 7 pages, 1 figure, one reference added

We consider thermal, trapped and chromo-natural inflation in light of the swampland criteria and the Trans-Planckian Censorship Conjecture (TCC). Since thermal inflation occurs at energies low compared to those of Grand Unification, it is consistent with the TCC, and it is also consistent with the refined swampland conditions. Trapped and chromo-natural inflation are candidates for primordial (high energy scale) inflation. Since in both of these scenarios there are effective damping terms in the scalar field equation of motion, the models can easily be consistent with the swampland criteria. The TCC, on the other hand, constrains these scenarios to only take place at low energies., Comment: 8 pages

We compute the energy spectrum of photons and neutrinos produced by the unwinding of a scaling distribution of cosmic textures, and discuss the implications for the spectrum of high energy cosmic rays, and for CMB spectral distortions. Textures lead to a contribution to the photon flux which scales as $E^3 F(E) \sim E^{3/2}$. Hence, the tightest constraints on the texture model come from the highest energies from which primordial photons can reach us without being scattered by the CMB and other foregrounds. Textures lead to both $\mu$ type and $y$ type distortions. While the constraints on the texture model coming from the current COBE bounds are weaker than the bounds from the angular power spectrum of the CMB, future surveys such as PIXIE can lead to stronger bounds. The high energy neutrino flux is constrained by data from the Pierre Auger experiment and yields a bound on the energy scale of textures which is competitive with CMB bounds., Comment: 8 pages, discussion of the neutrino flux added, version accepted for publication in JCAP

We show that the entropy of cosmological perturbations originating as quantum vacuum fluctuations in the very early universe, including the contribution of the leading nonlinear interactions, can be viewed as momentum space entanglement entropy between sub- and super-Hubble modes. The interactions between these modes causes decoherence of the super-Hubble fluctuations which, in turn, leads to a non-vanishing entropy of the reduced density matrix corresponding to the super-Hubble inhomogeneities. In particular, applying this to inflationary cosmology reveals that the entanglement entropy produced by leading order nonlinearities dominates over that coming from the squeezing of the vacuum state unless inflation lasts for a very short period. Furthermore, demanding that this entanglement entropy be smaller than the thermal entropy at the beginning of the radiation phase of standard cosmology leads to an upper bound on the duration of inflation which is similar to what is obtained from the Trans-Planckian Censorship Conjecture., Comment: 18 pages, comments welcome; v2: refs updated; v3: matches published version