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We present a model of dark matter as a superconducting fluid of Cooper pairs of right handed neutrinos or of vector-like quarks. The superconducting dark matter is induced by attractive channels in the Standard Model Higgs and color sectors of the Standard Model, respectively. We show that, for each case, the solution to the gap equation provides viable dark matter candidates for suitable chemical potential values. The mechanism yields an ultra-light neutrino condensate with a mass of $m_{\rm DM} \sim 10^{-19} \text{eV}$ or a vector-like quark condensate with wide range of possible masses. Both cosmological and particle physics constraints on the model lead to a connection between the number of effective relativistic species $N_{\rm eff}$, and the chemical potential and CMB temperature at the time of fermion creation. We also find a relation between the superconducting fermion and baryon densities, with implications for the coincidence between the dark matter and baryon densities in standard cosmology. Given the natural $\text{eV}$ scale of neutrinos, this mechanism may have implications for the Hubble tension., Comment: 5 pages, 6 pages of appendices

Given the growing interest in gravitational-wave and cosmological parity-violating effects in dynamical Chern-Simons (dCS) gravity, it is crucial to investigate whether the scalar-gravitational Pontryagin term in dCS persists when formulated in the context of the $\text{U(1)}_{\text{B}-\text{L}}$ anomaly in the Standard Model (SM). In particular, it has been argued that dCS gravity can be reduced to Einstein gravity after ''rotating away'' the gravitational-Pontryagin coupling into the phase of the Weinberg operator $\unicode{x2013}$ analogous to the rotation of the axion zero-mode into the quark mass matrix. We find that dCS is nontrivial if the scalar field $\phi$ has significant space-time dependence from dynamics. We provide a comprehensive consideration of the dCS classical and quantum symmetries relevant for embedding a dCS sector in the SM. We find that, because of the B-L chiral gravitational anomaly, the scalar-Pontryagin term cannot be absorbed by a field redefinition. Assuming a minimal extension of the SM, we also find that a coupling of the dCS scalar with right-handed neutrinos induces both the scalar-Pontryagin coupling and an axion-like phase in the dimension-five Weinberg operator. We comment on the issue of gauging the $\text{U(1)}_{\text{B}-\text{L}}$, the observational effects with these two operators present for upcoming experiments, and the origin of dCS gravity in string theory., Comment: 13 pages, one appendix

When a brane is moving in a compact space, bulk-probing signals originating at the brane can arrive back at the brane outside the lightcone of the emitting event. In this letter, we study how adiabatic perturbations in the brane fluid, coupled to a bulk fluid, propagate in the moving brane. In the non-dissipative regime, we find an effective sound speed for such perturbations, depending on the brane and bulk fluid energy densities, equations of state, and brane speed. In the tight-coupling approximation, the effective sound speed might be superluminal for brane and bulk fluids that satisfy the strong energy condition. This has immediate consequences for brane-world cosmology models., Comment: 15 pages, 3 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

We study the non-linear stability of fixed-point solutions to the $\alpha'$-exact equations from O$(d,d)$ invariant cosmology, with and without matter perturbations. Previous non-linear analysis in the literature is revisited, and its compatibility with known linear perturbation results is shown. Some formal aspects of cosmological perturbations in duality invariant cosmology are discussed, and we show the existence of time-reparameterization invariant variables for perturbations., Comment: 15 pages

The cosmological constant and its phenomenology remain among the greatest puzzles in theoretical physics. We review how modifications of Einstein's general relativity could alleviate the different problems associated with it that result from the interplay of classical gravity and quantum field theory. We introduce a modern and concise language to describe the problems associated with its phenomenology, and inspect no-go theorems and their loopholes to motivate the approaches discussed here. Constrained gravity approaches exploit minimal departures from general relativity; massive gravity introduces mass to the graviton; Horndeski theories lead to the breaking of translational invariance of the vacuum; and models with extra dimensions change the symmetries of the vacuum. We also review screening mechanisms that have to be present in some of these theories if they aim to recover the success of general relativity on small scales as well. Finally, we summarise the statuses of these models in their attempt to solve the different cosmological constant problems while being able to account for current astrophysical and cosmological observations., Comment: 54 pages, 2 figures, 1 table. Invited review for Universe, Special Issue "Cosmological Constant" (https://www.mdpi.com/journal/universe/special_issues/cosmological_constant_problem)

One of the fundamental challenges in string theory is to derive realistic four-dimensional cosmological backgrounds from it despite strict consistency conditions that constrain its possible low-energy backgrounds. In this work, we focus on energy conditions as covariant and background-independent consistency requirements in order to classify possible backgrounds coming from low-energy string theory in two steps. Firstly, we show how supergravity actions obey many relevant energy conditions under some reasonable assumptions. Remarkably, we find that the energy conditions are satisfied even in the presence of objects which individually violate them due to the tadpole cancellation condition. Thereafter, we list a set of conditions for a higher-dimensional energy condition to imply the corresponding lower-dimensional one, thereby categorizing the allowed low-energy solutions. As for any no-go theorem, our aim is to highlight the assumptions that must be circumvented for deriving four-dimensional spacetimes that necessarily violate these energy conditions, with emphasis on cosmological backgrounds., Comment: 22 pages; typos corrected; updated discussion of DEC for p-form fields added; extra typos corrected

We present a kinetically mixed dark sector (KMIX) model to address the Hubble and $S_8$ tensions. Inspired from string theory, our model includes two fields: an axion, which plays a role similar to the scalar field in early dark energy models, and a dilaton. This theory differs from other axio-dilaton models aimed at the Hubble tension in that there is necessarily kinetic mixing between the two fields which allows for efficient energy transfer from the axion into the dilaton which has $w\approx1$. As a direct consequence of these dynamics, we find the model does not need to resort to a fine-tuned potential to solve the Hubble tension and naturally accommodates a standard axion potential. Furthermore, the axion will necessarily makeup a small (fuzzy) fraction of $\Omega_{\rm cdm}$ once it begins to oscillate at the bottom of its potential and will suppress the growth of perturbations on scales sensitive to $S_8$. Interestingly, the scale of the potential for the dilaton has to be small, $\lesssim \mathcal{O}(10~{\rm meV})^4$, suggesting the possibility for a connection to dark energy. Implementing the dynamics for the background and perturbations in a modified Boltzmann code we calculate the CMB and matter power spectra for our theory. Exploring the parameter space of our model, we find regions which can accommodate a $\sim 10\%$ increase in $H_0$ from the Planck inferred value and $S_8$ values that are consistent with large-scale structure constraints., Comment: 22 pages, 5 figures, replaced with version accepted for publication in JCAP

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

We provide further evidence to support the fact that a four-dimensional effective field theory description with de Sitter isometries in IIB string theory, overcoming the no-go and the swampland criteria, can only exist if de Sitter space is realized as a Glauber-Sudarshan state. We show here that this result is independent of the choice of de Sitter slicings. The Glauber-Sudarshan state, constructed by shifting the interacting vacuum in the M-theory uplift of the scenario, differs from a standard coherent state in QFT in the sense that the shape and size of the state changes with respect to time, implying changes in the graviton and the flux quanta. Despite this, the expectation values of the graviton and flux operators in such a state reproduce the exact de Sitter background, as long as we are within the temporal bound set by the onset of the strong coupling in the dual type IIA side, and satisfy the corresponding Schwinger-Dyson's equations in the presence of hierarchically controlled perturbative and non-perturbative quantum corrections. Additionally, we provide a detailed study of the fluxes supporting the Glauber-Sudarshan state in the M-theory uplift of the IIB scenario. We show explicitly how the Bianchi identities, anomaly cancellation and flux quantization conditions, along-with the constraints from the Schwinger-Dyson's equations, conspire together to provide the necessary temporal dependences to support such a state in full M-theory. We also discuss how our analysis points towards a surprising connection to the four-dimensional null energy condition, for a Friedman-Lemaitre-Robertson-Walker state in the IIB side, as a consistency condition for the existence of an effective field theory description in M-theory., Comment: 188 pages, 33 tables, LaTex; comments welcome. v2: Typos corrected and reference updated; Matches version to appear in "Fortschritte der Physik"

We analyze four-dimensional Friedmann-Lemaitre-Robertson-Walker cosmologies in type IIB, arising from a M-theory dual, and find that the null energy condition (NEC) has to be obeyed by them (except for the negatively curved case) in order for the M-theory action to have a Wilsonian effective description. However, this does not imply that the M-theory metric has to obey the 11d NEC. Thus, we propose a new swampland conjecture -- the 4d NEC is a consistency condition for any theory to have a completion within M-theory -- with an explicit derivation of it for cosmological backgrounds from a top-down perspective. We briefly discuss the cosmological consequences of such a condition derived from M-theory., Comment: 14 pages (including an Appendix); comments welcome; v2: double-columned 8 pages (includes Supplementary Materials), matches published version

The cosmology of the fully $\alpha'$-corrected duality-invariant action for the Neveu-Schwarz sector of string theory is revisited, with special emphasis on its coupling to matter sources. The role of the duality covariant pressure and dilatonic charge of the matter sector is explored in various contexts, from the low-curvature regime to non-perturbative solutions in $\alpha'$. We comment on how an infinite tower of $\alpha'$ corrections allows for fixed-dilaton de Sitter solutions, even in vacuum. We further investigate the necessary conditions for accelerated expansion in the Einstein frame, as well as for non-singular bounces that could resolve the big bang singularity. In particular, explicit examples are constructed, which show that the tower of $\alpha'$ corrections may support an Einstein-frame non-singular cosmological bouncing background, even when the matter sector respects the null energy condition., Comment: 37 pages, 5 figures; v2: minor changes, references added; v3: minor changes, matches published version

We study the matter-coupled equations of motion for cosmological NS massless fields including all $\alpha'$ corrections in an O$(d,d)$ duality invariant approach, with emphasis on the Kalb-Ramond two-form field $B_{(2)}$ and its source. Solutions for the vacuum and matter case are found and the corresponding Einstein frame cosmologies are discussed. We also show that the ansatz for $B_{(2)}$ required by the duality invariant framework implies that the two-form is non-isotropic., Comment: 24 pages

In recent times, a considerable effort has been dedicated to identify certain conditions -- the so-called swampland conjectures -- with an eye on identifying effective theories which have no consistent UV-completions in string theory. In this paper, we examine the anti-de Sitter vacua corresponding to solutions which arise from purely non-perturbative contributions to the superpotential and show that these solutions satisfy the (axionic) weak gravity conjecture and the AdS-moduli scale separation conjecture. We also sketch out their advantages over other constructions., Comment: 13 pages, 1 figure

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 construct purely non-perturbative anti-de Sitter vacua in string theory which, on uplifting to a de Sitter (dS) one, have a decay time many orders of magnitude smaller than those of standard constructions, such as the KKLT and LVS scenarios. By virtue of being constructed purely from non-perturbative terms, these vacua avoids certain obstructions plaguing other constructions of dS in string theory. This results in a new class of phenomenological dS vacua in string theory with novel distinctive characteristics such as having two maxima. After examining whether these uplifted dS vacua obey the TCC, we revisit some old problems of realization of dS space as a vacuum. We find that not only is it phenomenologically hard to construct TCC-compatible vacua, but also inherent temporal dependences of the degrees of freedom generically arise in such constructions, amongst other issues. This reinforces the idea that dS, if it exists in string theory, should be a Glauber-Sudarshan state and not a vacuum., Comment: 41 pages, 2 figures; references added

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

We introduce a very early universe model based on the thermodynamics of a gas of closed strings in a background which is non-perturbative in $\alpha'$. Upon considering the fully $\alpha'$-corrected equations extended to include certain anisotropic cosmological backgrounds, we describe the evolution of the system in three different stages parametrized by the gas' equation of state. Using standard string thermodynamical arguments, we start with an isotropic 10-dimensional universe inside the string scale and evolve it towards a universe with four large spacetime dimensions and six stabilized internal dimensions in the Einstein frame., Comment: 28 pages, 1 figure; references and appendix C added

We review O$(d,d)$ Covariant String Cosmology to all orders in $\alpha'$ in the presence of matter and study its solutions. We show that the perturbative analysis for a constant dilaton in the absence of a dilatonic charge does not lead to a time-independet equation of state. Meanwhile, the non-perturbative equations of motion allow de Sitter solutions in the String frame parametrized by the equation of state and the dilatonic charge. Among this set of solutions, we show that a cosmological constant equation of state implies a de Sitter solution both in String and Einstein frames while a winding equation of state implies a de Sitter solution in the former and a static phase in the latter. We also consider the stability of these solutions under homogeneous linear perturbations and show that they are not unstable, therefore defining viable cosmological scenarios., Comment: 13 pages, 1 figure, references updated, typos fixed

In this letter we discuss the implications of the recently proposed TransPlanckian Censorship Conjecture (TCC) for Holographic Cosmology. For models of holographic inflation that are dual to inverse renormalization group (RG) flows, assuming the TCC to be true, we found a bound on the running of the field theory coupling and the corresponding maximum energy scale above which the RG flow should be modified. We also discuss on the transPlanckian problem in non-geometrical holographic models., Comment: 10 pages, references added

Recently, all duality invariant $\alpha'$-corrections to the massless NS-NS sector of string theory on time-dependent backgrounds were classified and the form of their contribution to the action were calculated. In this paper we introduce matter sources in the resulting equations of motion in an O$(d,d)$ covariant way. We show that either starting with the corrected equations and sourcing them with matter or considering corrections to the matter sourced lowest order equations give the same set of equations that defines string cosmology to all orders in $\alpha'$. We also discuss perturbative and non-perturbative de Sitter solutions including matter., Comment: 27 pages, 1 figure

It is argued that the Unruh effect may be interpreted as a duality of a theory on different backgrounds. This issue is revisited in String Theory in the path integral formalism. By using T-duality and the Unruh effect, the T-dual transformation for acceleration is investigated and a maximum effective physical acceleration for observers in String Theory is found., Comment: 12 pages, references added

In this paper we present cosmological solutions of Double Field Theory in the supergravity frame and in the winding frame which are related via T-duality. In particular, we show that the solutions can be viewed without the need of complexifying the cosmological scale factor., Comment: 7 pages

We propose a way to obtain holographic cosmology models for 3+1 dimensional cosmologies vs. 3 dimensional field theories from a "dimensional reduction" procedure, obtained by integrating over the time direction, of (modifed) standard holographic duals of 3+1 dimensional field theories. The example of a modified ${\cal N}=4$ SYM vs. $AdS_5\times S^5$ is presented, and in perturbation theory doesn't match observations, though at strong coupling it might. But the proposed mechanism is more general, and it could in principle be applied to other top down holographic models., Comment: 15 pages, no figures; clarifications about Wick rotation and susy added, references added

We investigate the possibility that the inflaton, in particular in conformal inflation models, is also a chameleon, i.e. that it couples to the energy density of some heavy non-relativistic matter present during inflation. We find new and interesting attractor behaviours, either prolonging inflation, or changing the observables $n_s,r$, depending on the sign of the chameleon coupling exponent. We also check that the chameleon coupling with the heavy matter field strongly suppress entropy modes during inflation., Comment: 28 pages, 11 figures; reference added; section 4 reviewed

We consider "new inflation" inflationary models at small fields, embedded in minimal ${\cal N}=1$ supergravity with a single chiral superfield. Imposing a period of inflation compatible with experiment severely restricts possible models, classified in perturbation theory. If moreover we impose that the field goes to large values and very small potential at the current time, like would be needed for instance for the inflaton being the volume modulus in large extra dimensional scenarios, the possible models are restricted to very contrived superpotentials., Comment: 25 pages, no figures; references and explanation added