arXiv subjects
Publications and source records attributed to Marieke Postma.
We study the spectrum of fermionic modes on cosmic string loops. We find no fermionic zero modes nor massive bound states - this implies that vortons stabilized by fermionic currents do not exist. We have also studied kink-(anti)kink and vortex-(anti)vortex systems and find that all systems that have vanishing net topological charge do not support fermionic bound modes.
We show that the parameter space for F-term inflation which predict the formation of cosmic strings is larger than previously estimated. Firstly, because realistic embeddings in GUT theories alter the standard scenerio, making the inflationary potential less steep. Secondly, the strings which form at the end of inflation are not necessarily topologically stable down to low scales. In shifted and smooth inflation strings do not form at all. We also discuss D-term inflation; here the possibilities are much more limited to enlargen paramer space.
Cosmic strings form at the end of standard supersymmetric hybrid inflation, and both inflation and strings contribute to the CMB anisotropies. If the symmetry which is broken at the end of inflation is gauged $B$-$L$, there is a mixed scenario for leptogenesis: Right-handed neutrinos can be produced non-thermally during reheating via inflaton decay as well as via cosmic string decay. We show that the parameter space consistent with CMB data can accommodate either or both scenarios depending on the mass of the right-handed neutrinos.
$F$-term GUT inflation coupled to N=1 Supergravity is confronted with CMB data. Corrections to the string mass-per-unit-length away from the Bogomolny limit are taken into account. We find that a superpotential coupling $10^{-7}/\mcN \lesssim κ\lesssim 10^{-2}/\mcN$, with $\mcN$ the dimension of the Higgs-representation, is still compatible with the data. The parameter space is enlarged in warm inflation, as well as in the curvaton and inhomogeneous reheat scenario. $F$-strings formed at the end of $P$-term inflation are also considered. Because these strings satisfy the Bogomolny bound the bounds are stronger: the gauge coupling is constrained to the range $10^{-7} < g <10^{-4}$.
We derive the zero mode solutions for a Majorana fermion in the background of a cosmic string and contrast it with the zero mode solution for a (neutral) Dirac fermion. A Majorana zero mode carries no vector or axial charge, and it cannot be bosonised. We study the implications for vorton formation and stability. In the massless limit stability of the zero mode is guaranteed by energy-momentum conservation. However, zero modes obtain an effective mass on string loops. It is found that the conditions under which current formation can be effective are exactly those for which zero mode decay is most likely to occur.
We consider F and D-term cosmic strings formed in supersymmetric theories. Supersymmetry is broken inside the string core, but restored outside. In global SUSY, this implies the existence of goldstino zero modes, and the string potentially carries fermionic currents. We show that these zero modes do not survive the coupling to gravity, due to the super Higgs mechanism. Therefore the superconductivity and chirality properties are different in global and local supersymmetry. For example, a string formed at the end of D-term inflation is chiral in supergravity but non-chiral in global SUSY.
In its simplest form the curvaton paradigm requires the Hubble parameter during inflation to be bigger than $10^8 \GeV$, but this bound may be evaded in non-standard settings. In the heavy curvaton scenario the curvaton mass increases significantly after the end of inflation. We reanalyze the bound in this set up, taking into account the upper bound on the curvaton mass from direct decay. We obtain $H_* > 10^8 \GeV$ if the mass increase occurs at the end of inflation, and $H_* > 10^{-14} \GeV$ if it occurs just before nucleosynthesis. We then discuss two implementations of the heavy curvaton. Parameters are constrained in these explicit models, and as a result even obtaining TeV scale inflation is hard to obtain.
We discuss the constraints on the inhomogeneous reheating scenario. In particular, we discuss the prospects for low scale inflation with a Hubble constant of the order of the gravitino mass, and the possibility that an MSSM flat direction is responsible for the density fluctuations. Thermal effects are generically fatal for the scenario, and can only be avoided for small enough mass and couplings of the flat direction field, excluding MSSM flat directions. Prompt decay at the end of inflation bypasses thermal constraints, and is compatible with both low scale inflation and MSSM flat directions. However, the level of non-Gaussianity is acceptably small only for a small window of inflaton mass and couplings. The upshot is that tuning of parameters is needed for the inhomogeneous reheating scenario to work.
We present a gauge invariant formalism to study the evolution of curvature perturbations during decay of the inflaton or some other field that dominates the energy density. We specialize to the case where the total curvature perturbation arises predominantly from a spatially fluctuating decay rate. Gaussian fluctuations in the decay rate are present if the inflaton coupling or mass depends on some light field, which freely fluctuates during inflation. The isocurvature fluctuations of the light field seeds the total curvature perturbations, whose amplitude freezes after inflaton decay. We also analyze the resulting curvature perturbation starting with non-negligible values of the initial inflaton curvature. Lastly, we study the effects of the energy density stored in the light field.
We study preheating, i.e., non-perturbative resonant decay, of flat direction fields, concentrating on MSSM flat directions and the right handed sneutrino. The difference between inflaton preheating and flaton preheating, is that the potential is more constraint in the latter case. The effects of a complex driving field, quartic couplings in the potential, and the presence of a thermal bath are important and cannot be neglected. Preheating of MSSM flat directions is typically delayed due to out-of-phase oscillations of the real and imaginary components and may be preceded by perturbative decay or $Q$-ball formation. Particle production due to the violation of adiabaticity is expected to be inefficient due to back reaction effects. For a small initial sneutrino VEV, $ \lesssim m_N/h$ with $m_N$ the mass of the right handed sneutrino and $h$ a yakawa coupling, there are tachyonic instabilities. The $D$-term quartic couplings do not generate an effective mass for the tachyonic modes, making it an efficient decay channel. It is unclear how thermal scattering affects the resonance.
We analyse the curvaton scenario in the context of supersymmetry. Supersymmetric theories contain many scalars, and therefore many curvaton candidates. To obtain a scale invariant perturbation spectrum, the curvaton mass should be small during inflation $m \ll H$. This can be achieved by invoking symmetries, which suppress the soft masses and non-renormalizable terms in the potential. Other model-independent constraints on the curvaton model come from nucleosynthesis, gravitino overproduction, and thermal damping. The curvaton can work for masses $m \gtrsim 10^4 \GeV$, and very small couplings (e.g. $h \lesssim 10^{-6}$ for $m \lesssim 10^8 \GeV$).
We discuss the possibility of generating adiabatic density perturbations from spatial fluctuations in the inflaton decay rate which are due to quantum fluctuations of light moduli fields coupling to the inflaton. We point out that non-renormalizable operators, which lift the flatness of the moduli potential, play an important role for the density perturbations. In particular, the non-renormalizable terms give rise to a considerable damping of the fluctuations and thereby pose an obstruction to the construction of possible models.
We analyze the curvaton scenario in the context of supersymmety. Supersymmetric theories contain many scalars, and therefore many curvaton candidates. To obtain a scale invariant perturbation spectrum, the curvaton mass should be small during inflation $m \ll H$. This can be achieved by invoking symmetries, which suppress the soft masses and non-renormalizable terms in the potential. Other constraints on the curvaton model come from nucleosynthesis, gravitino overproduction, and thermal evaporation. The curvaton coupling to matter should be very small to satisfy these constraints ({\it e.g.} $h \lesssim 10^{-8}$ for typical soft masses $m \sim \TeV$).
We show that neutrinos can be produced through standard electroweak interactions in matter with time-dependent density.
We study the formation of Q-balls in the early universe, concentrating on potentials with a cubic or quartic attractive interaction. Large Q-balls can form via solitosynthesis, a process of gradual charge accretion, provided some primordial charge assymetry and initial ``seed'' Q-balls exist. We find that such seeds are possible in theories in which the attractive interaction is of the form $A H ψ^* ψ$, with a light ``Higgs'' mass. Condensate formation and fragmentation is only possible for masses $m_ψ$ in the sub-eV range; these Q-balls may survive untill present.
We show that neutrinos can be produced through standard electroweak interactions in matter with time-dependent density.
Sources of ultrahigh energy photons operating at high red shift produce a diffuse background of neutrinos. At high red shift, when the cosmic microwave background radiation has a higher temperature, an electromagnetic cascade originated by an energetic photon can generate neutrinos via muon and pion production and decay. We have calculated numerically the neutrino spectrum produced by various photon sources. A distinctive feature of the produced flux is a ``bump'' in the spectrum at neutrino energies $E \sim 10^{17}$eV. The produced flux is largest for $m=3$ sources ({\it e.g} necklaces), with $E^2 J(E) \sim 10 {\rm eV} {\rm cm}^{-2} {\rm s}^{-1} {\rm sr}^{-1}$ at these energies. Detection of such neutrinos can help understand the origin of ultrahigh energy cosmic rays.
Some of the proposed explanations for the origin of ultrahigh-energy cosmic rays invoke new sources of energetic photons (e.g., topological defects, relic particles, etc.). At high red shift, when the cosmic microwave background has a higher temperature but the radio background is low, the ultrahigh-energy photons can generate neutrinos through pair-production of muons and pions. Neutrinos produced at high red shift by slowly evolving sources can be detected. Rapidly evolving sources of photons can be ruled out based on the existing upper limit on the neutrino flux.