arXiv subjects

Axel Krause

Publications and source records attributed to Axel Krause.

27 records · Page 2Linked to original sources

arXiv2004

Enlarging the Parameter Space of Heterotic M-Theory Flux Compactifications to Phenomenological Viability

Heterotic M-Theory is a promising candidate for that corner of M-theory which makes contact with the real world. However, while the theory requires one of its expansion parameters, $ε$, to be perturbatively small, a successful phenomenology requires $ε= {\cal O}(1)$. We show that the constraint to have small $ε$ is actually unnecessary: instead of the original flux compactification background valid to linear order in $ε$ one has to use its appropriate non-linear extension, the exact background solution. The exact background is determined by supersymmetry and consequently one expects the tree-level cosmological constant to vanish which we demonstrate in detail, thereby verifying once more the consistency of this background. Furthermore we show that the exact background represents precisely the 11d origin of the 5d domain wall solution which is an exact solution of the effective 5d heterotic M-theory. We also comment on singularities and the issue of chirality changing transitions in the exact background. The exact background is then applied to determine Newton's Constant for vacua with an M5 brane on the basis of a recent stabilization mechanism for the orbifold length. For vacua without M5 brane we obtain a correction to the lower bound on Newton's Constant which brings it in perfect agreement with the measured value.

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arXiv2004

On De Sitter Vacua in Strongly Coupled Heterotic String Theory

We describe how 4d de Sitter vacua might emerge from 11d heterotic M-theory. Non-perturbative effects and $G$-fluxes play a crucial role leading to vacua with F-term supersymmetry breaking and a positive energy density. Charged scalar matter fields are no longer massless in these vacua thus solving one of the problems of the heterotic string. Moreover, interesting dark matter candidates appear in a natural way.

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arXiv2003

On the Flattening of Negative Curvature via T-Duality with a Non-Constant B-Field

In an earlier paper, Alvarez, Alvarez-Gaume, Barbon and Lozano pointed out, that the only way to "flatten" negative curvature by means of a T-duality is by introducing an appropriate, non-constant NS-NS B-field. In this paper, we are investigating this further and ask, whether it is possible to T-dualize AdS_d space to flat space with some suitably chosen B. To answer this question, we derive a relationship between the original curvature tensor and the one of the T-dualized metric involving the B-field. It turns out that there is one particular component, which is independent of B. By inspection of this component, we then show, that it is not possible to dualize AdS_d to flat space irrespective of the choice of B. Finally, we examine the extension of AdS to an AdS_5 x S^5 geometry and propose a chain of S- and T-dualities together with an SL(2,Z) coordinate transformation, leading to a dual D9-brane geometry.

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arXiv2003

A Small Cosmological Constant and Backreaction of Non-Finetuned Parameters

We include the backreaction on the warped geometry induced by non-finetuned parameters in a two domain-wall set-up to obtain an exponentially small Cosmological Constant $Λ_4$. The mechanism to suppress the Cosmological Constant involves one classical fine-tuning as compared to an infinity of finetunings at the quantum level in standard D=4 field theory.

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arXiv2002

Dual Branes, Discrete Chain States and the Entropy of the Schwarzschild Black Hole

We review a recent proposal towards a microscopic understanding of the entropy of non-supersymmetric spacetimes -- with emphasis on the Schwarzschild black hole. The approach is based at an intermediate step on the description of the non-supersymmetric spacetime in terms of dual Euclidean brane pairs of type-II string-theory or M-theory. By counting specific chain structures on the brane-complex, it is shown that one can reproduce the exact Schwarzschild black hole entropy plus its logarithmic correction.

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arXiv2002

G-Fluxes and Non-Perturbative Stabilisation of Heterotic M-Theory

We examine heterotic M-theory compactified on a Calabi-Yau manifold with an additional parallel M5 brane. The dominant non-perturbative effect stems from open membrane instantons connecting the M5 with the boundaries. We derive the four-dimensional low-energy supergravity potential for this situation including subleading contributions as it turns out that the leading term vanishes after minimisation. At the minimum of the potential the M5 gets stabilised at the middle of the orbifold interval while the vacuum energy is shown to be manifestly positive. Moreover, induced by the non-trivial running of the Calabi-Yau volume along the orbifold which is driven by the G-fluxes, we find that the orbifold-length and the Calabi-Yau volume modulus are stabilised at values which are related by the G-flux of the visible boundary. Finally we determine the supersymmetry-breaking scale and the gravitino mass for this open membrane vacuum.

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arXiv2002

Schwarzschild Black Holes from Brane-Antibrane Pairs

We show that D=4 Schwarzschild black holes can arise from a doublet of Euclidean D3-antiD3 pairs embedded in D=10 Lorentzian spacetime. By starting from a D=10 type IIB supergravity description for the D3-antiD3 pairs and wrapping one of them over an external 2-sphere, we derive all vacuum solutions compatible with the symmetry of the problem. Analysing under what condition a Euclidean brane configuration embedded in a Lorentzian spacetime can lead to a time-independent spacetime, enables us to single out the embedded D=4 Schwarzschild spacetime as the unique solution generated by the D3-antiD3 pairs. In particular we argue on account of energy-conservation that time-independent solutions arising from isolated Euclidean branes require those branes to sit at event horizons. In combination with previous work this self-dual brane-antibrane origin of the black hole allows for a microscopic counting of its Bekenstein-Hawking entropy. Finally we indicate how Hawking-radiation can be understood from the associated tachyon condensation process.

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arXiv2000

Four-Flux and Warped Heterotic M-Theory Compactifications

In the framework of heterotic M-theory compactified on a Calabi-Yau threefold 'times' an interval, the relation between geometry and four-flux is derived {\it beyond first order}. Besides the case with general flux which cannot be described by a warped geometry one is naturally led to consider two special types of four-flux in detail. One choice shows how the M-theory relation between warped geometry and flux reproduces the analogous one of the weakly coupled heterotic string with torsion. The other one leads to a {\it quadratic} dependence of the Calabi-Yau volume with respect to the orbifold direction which avoids the problem with negative volume of the first order approximation. As in the first order analysis we still find that Newton's Constant is bounded from below at just the phenomenologically relevant value. However, the bound does not require an {\it ad hoc} truncation of the orbifold-size any longer. Finally we demonstrate explicitly that to leading order in $κ^{2/3}$ no Cosmological Constant is induced in the four-dimensional low-energy action. This is in accord with what one can expect from supersymmetry.

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arXiv2000

Testing Stability of M-Theory on an S^1/Z_2 Orbifold

We analyse perturbatively, whether a flat background with vanishing G-flux in Horava-Witten supergravity represents a vacuum state, which is stable with respect to interactions between the ten-dimensional boundaries, mediated through the D=11 supergravity bulk fields. For this, we consider fluctuations in the graviton, gravitino and 3-form around the flat background, which couple to the boundary $E_8$ gauge-supermultiplet. They give rise to exchange amplitudes or forces between both boundary fixed-planes. In leading order of the D=11 gravitational coupling constant $κ$, we find an expected trivial vanishing of all three amplitudes and thereby stability of the flat vacuum in the static limit, in which the centre-of-mass energy $\sqrt{s}$ of the gauge-multiplet fields is zero. For $\sqrt{s}>0$, however, which could be regarded a vacuum state with excitations on the boundary, the amplitudes neither vanish nor cancel each other, thus leading to an attractive force between the fixed-planes in the flat vacuum. A ground state showing stability with regard to boundary excitations, is therefore expected to exhibit a non-trivial metric. Ten-dimensional Lorentz-invariance requires a warped geometry. Finally, we extrapolate the amplitudes to the case of coinciding boundaries and compare them to the ones resulting from the weakly coupled $E_8 \times E_8$ heterotic string theory at low energies.

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