Dirac-Born-Infeld Research Articles

Flux vacua in DBI type Einstein–Maxwell theory

We study compactification of extra dimensions in a theory of Dirac–Born–Infeld type gravity. We investigate the solution for Minkowski spacetime with an S 2 extra space as well as that for de Sitter spacetime (S 4) with an S 2 extra space. They are derived by the effective potential method in the presence of the magnetic flux on the extra sphere. We also consider the higher-dimensional generalization of the solutions. We find that, in a certain model, the radius of the extra space has a minimum value independent of the higher-dimensional Newton constant.

Inflationary predictions in scalar-tensor DBI inflation

The scalar-tensor Dirac-Born-Infeld (DBI) inflation scenario provides a simple mechanism to reduce the large values of the boost factor associated with single field models with DBI action, whilst still being able to drive 60 efolds of inflation. Using a slow-roll approach, we obtain an analytical expression for the spectral index of the perturbations and, moreover, determine numerically the regions of the parameter space of the model capable of giving rise to a power spectrum with amplitude and spectral index within the observed bounds. We find that regions that exhibit significant DBI effects throughout the inflationary period can be discarded by virtue of a blue-tilted spectral index, however, there are a number of viable cases — associated with a more red-tilted spectral index — for which the boost factor is initially suppressed by the effect of the coupling between the fields, but increases later to moderate values.

On DBI Textures with generalized Hopf fibration

In this Letter we show numerical existence of O(4) Dirac–Born–Infeld (DBI) Textures living in (N+1) dimensional spacetime. These defects are characterized by SN→S3 mapping, generalizing the well-known Hopf fibration into πN(S3), for all N>3. The nonlinear nature of DBI kinetic term provides stability against size perturbation and thus renders the defects having natural scale.

ENHANCED DUALITY SYMMETRIES FOR A PAIR OF DIRAC–BORN–INFELD ACTIONS

We consider a total action composed of two Dirac–Born–Infeld (DBI) actions: one for a vector field Aμ and another for an axial vector field Bμ. We impose a duality condition [Formula: see text], where [Formula: see text] is the Hodge dual of Gμν, and g is a DBI interaction constant. Interestingly, there are two different global duality rotation symmetries in the presence of DBI interactions: (i) [Formula: see text], [Formula: see text], and (ii) δζAμ = - ζBμ, δζBμ = + ζAμ. Both of these symmetry are on-shell symmetries, including nonlinear higher-order terms. The remarkable aspect is that these symmetries are valid even in the presence of DBI interactions. The coupling of this system to N = 1 supergravity is also discussed.

Fermionic DBI and Chaplygin gas unified models of dark energy and dark matter from f-essence

In the present work, we study the cosmological model with fermionic field and with the non-canonical kinetic term (f-essence). We present some important reductions of the model as well as some of its generalizations. We also find the exact solution of the model and examine the influence of such gravity-fermion interaction on the observed accelerated expansion of our universe. Further, we discuss the cosmological implications of the exact models. We show that our f-essence model corresponds to ΛCDM, but not a standard cold dark matter model containing no radiation. Also, our particular f-essence model has no Einstein's static universe. Further we discuss several fermionic Dirac-Born-Infeld (DBI) models as some particular reductions of f-essence, respectively and their Chaplygin gas counterparts are found.

WMAP constraints onk-inflation

We study the K-Inflation models where the inflaton field has non-canonical kinetic term. In particular, we consider the Dirac-Born-Infeld (DBI) form for the kinetic energy of the inflaton field. We consider quadratic and quartic potentials as well as the potential for the natural inflation. We use a modified version of the MODECODE (proposed by Mortonson et al.) to calculate the power spectrum of the primordial perturbations generated by the inflaton field and subsequently use the WMAP7 results to constrain the models. Interestingly with DBI type kinetic term, lesser gravity waves are produced as one approaches more towards scale invariance. This is true for all the potentials considered. Unlike the canonical case, this feature, in particular, helps the quartic ($\lambda\phi^4$) potential with DBI type kinetic term to be consistent with WMAP data.

On the 1-loop effective action for the IKKT model and non-commutative branes

We study the one-loop effective action of the IKKT or IIB model on a 4-dimensional non-commutative brane background. The trace-U(1) sector is governed by non-commutativity, and leads - assuming no SUSY breaking - to a higher-derivative effective action. In contrast, the non-Abelian sector at low energies reduces to SU(n) N=4 Super-Yang-Mills on the brane, with a global SO(9,1) symmetry broken spontaneously by the background. In the Coulomb branch, we recover the leading contribution to the Dirac-Born-Infeld (DBI) action, exhibiting a S^5 \times AdS^5 bulk geometry around a stack of branes. SUSY may be broken by compact extra dimensions M^4 x K, leading to an induced gravitational action on M^4 due to the trace-U(1) sector. The one-loop effective action is UV finite on such backgrounds, and the UV/IR mixing is non-pathological.

Stochastic Inflation in Compact Extra Dimensions

While moving down the potential on its classical slow roll trajectory, the inflaton field is subject to quantum jumps, which take it up or down the potential at random. In "stochastic inflation", the impact of these quantum jumps is modeled by smoothing out the field over (at least) Hubble-patch sized domains and treating fluctuations on smaller scales as noise. The inflaton thus becomes a stochastic process whose values at a given time are calculated using its probability distribution. We generalize this approach for non-canonic kinetic terms of Dirac Born Infeld (DBI) type and investigate the resulting modifications of the field's trajectory. Since models of DBI inflation arise from string-inspired scenarios in which the scalar field has a geometric interpretation, we insist that field value restrictions imposed by the model's string origin must be respected at the quantum level.

Strong coupling BCS superconductivity and holography

We attempt to give a holographic description of the microscopic theory of a BCS superconductor. Exploiting the analogy with chiral symmetry breaking in QCD we use the Sakai–Sugimoto model of two D8 branes in a D4 brane background with finite baryon number. In this case there is a new tachyonic instability which is plausibly the bulk analog of the Cooper pairing instability. We analyze the Yang–Mills approximation to the non-Abelian Dirac–Born–Infeld action. We give some exact solutions of the non-linear Yang–Mills equations in flat space and also give a stability analysis, showing that the instability disappears in the presence of an electric field. The holographic picture also suggests a dependence of T c on the number density which is different from the usual (weak coupling) BCS. The flat space solutions are then generalized to curved space numerically and also, in an approximate way, analytically. This configuration should then correspond to the ground state of the boundary superconducting (superfluid) ground state. We also give some preliminary results on Green functions computations in the Sakai–Sugimoto model without any chemical potential.

Heating up the BIon

We propose a new method to consider D-brane probes in thermal backgrounds. The method builds on the recently developed blackfold approach to higher-dimensional black holes. While D-brane probes in zero-temperature backgrounds are well-described by the Dirac-Born-Infeld (DBI) action, this method addresses how to probe thermal backgrounds. A particularly important feature is that the probe is in thermal equilibrium with the background. We apply our new method to study the thermal generalization of the BIon solution of the DBI action. The BIon solution is a configuration in flat space of a D-brane and a parallel anti-D-brane connected by a wormhole with F-string charge. In our thermal generalization, we put this configuration in hot flat space. We find that the finite temperature system behaves qualitatively different than its zero-temperature counterpart. In particular, for a given separation between the D-brane and anti-D-brane, while at zero temperature there are two phases, at finite temperature there are either one or three phases available.

Warm Dirac-Born-Infeld inflation

We propose a warm inflationary model in the context of relativistic D-brane inflation in a warped throat, which has Dirac-Born-Infeld (DBI) kinetic term and is coupled to radiation through a dissipation term. The perturbation freezes at the sound horizon and the power spectrum is determined by a combination of the dissipative parameter and the sound speed parameter. The thermal dissipation ameliorates the eta problem and softens theoretical constraints from the extra-dimensional volume and from observational bounds on the tensor-to-scalar ratio. The warm DBI model can lead to appreciable non-Gaussianity of the equilateral type. As a phenomenological model, ignoring compactification constraints, we show that large-field warm inflation models do not necessarily yield a large tensor-to-scalar ratio.

Stability and superluminality of spherical DBI Galileon solutions

The Dirac-Born-Infeld (DBI) Galileons are a generalization of the Galileon terms, which extend the internal Galilean symmetry to an internal relativistic symmetry, and can also be thought of as generalizations of DBI which yield second order field equations. We show that, when considered as local modifications to gravity, such as in the Solar System, there exists a region of parameter space in which spherically symmetric static solutions exist and are stable. However, these solutions always exhibit superluminality, casting doubt on the existence of a standard Lorentz invariant UV completion.

Embedding DBI inflation in scalar-tensor theory

The Dirac-Born-Infeld (DBI) action has been widely studied as an interesting example of amodel of k-inflation in which the sound speed of the cosmological perturbations differs from unity.In this article we consider a scalar-tensor theory in which the matter component is a field with a DBI action.Transforming to the Einstein frame, we explore the effect of the resulting coupling on thebackground dynamics of the fields and the first-order perturbations. We find thatthe coupling forces the scalar field into the minimum of its effective potential. While the additional scalar field contributes significantly to the energy density during inflation, the dynamics are determined by the DBI field, which has the interesting effect of increasing the number of efolds of inflation and decreasing the boost factor of the DBI field.Focusing on this case, we show, with the benefit of numerical examples,that the power spectrum of the primordial perturbations is determined by the behaviour of the perturbations of the modified DBI field.

NON-GAUSSIANITY FROM THE BISPECTRUM IN GENERAL MULTIPLE FIELD INFLATION

We study the non-gaussianity from the bispectrum in multi-field inflation models with a general kinetic term. The models include the multi-field K-inflation and the multi-field Dirac-Born-Infeld (DBI) inflation as special cases. We derive the three-point function for the curvature perturbation which depends on both adiabatic and entropy perturbations, in the small sound speed limit and at leading order in the slow-roll expansion. The contribution from the entropy perturbations has a different momentum dependence if the sound speed for the entropy perturbations is different from the adiabatic one, which provides a possibility to distinguish the multi-field models from single field models.

Gauge field back-reaction in Born–Infeld cosmologies

In this paper, we investigate the back-reaction of U(1) gauge fields into a specific class of inflationary settings. To be more precise, we employ a Bianchi-I geometry (taken as an anisotropic perturbation of a flat FRW model) within two types of Born–Infeld (BI) theories. First, we consider pure Born–Infeld electromagnetism. For either a constant or a b(ϕ) coupling, inflationary trajectories are modified but anisotropies increase. In particular, for the former coupling we find that a quadratic inflaton potential, within a constant ratio for the scalar and gauge energy densities, does not induce sufficient inflation, while in the latter, the back-reaction in the cosmology determines (from the tensor–scalar ratio) a narrow range where inflation can occur. A Dirac–Born–Infeld framework is analysed afterwards in both non-relativistic and relativistic regimes. In the former, for different cases of the coupling (richer with respect to mere BI setups) between scalar and gauge sectors, we find that inflationary trajectories are modified, with anisotropy increasing or decreasing. In particular, a tachyonic solution is studied, allowing for a nonstandard ratio between scalar and gauge matter densities, enhancing sufficient inflation, but with the anisotropy increasing. For the relativistic limit, inflationary trajectories are also modified and anisotropies increase faster than in the non-relativistic limit. Finally we discuss how magnetic seed fields could evolve in these settings.

Geometrically consistent approach to stochastic DBI inflation

Stochastic effects during inflation can be addressed by averaging the quantum inflaton field over Hubble-patch--sized domains. The averaged field then obeys a Langevin-type equation into which short-scale fluctuations enter as a noise term. We solve the Langevin equation for an inflaton field with a Dirac-Born-Infeld (DBI) kinetic term perturbatively in the noise and use the result to determine the field value's probability density function (PDF). In this calculation, both the shape of the potential and the warp factor are arbitrary functions, and the PDF is obtained with and without volume effects due to the finite size of the averaging domain. DBI kinetic terms typically arise in string-inspired inflationary scenarios in which the scalar field is associated with some distance within the (compact) extra dimensions. The inflaton's accessible range of field values therefore is limited because of the extra dimensions' finite size. We argue that in a consistent stochastic approach the inflaton's PDF must vanish for geometrically forbidden field values. We propose to implement these extra-dimensional spatial restrictions into the PDF by installing absorbing (or reflecting) walls at the respective boundaries in field space. As a toy model, we consider a DBI inflaton between two absorbing walls and use the method of images to determine its most general PDF. The resulting PDF is studied in detail for the example of a quartic warp factor and a chaotic inflaton potential. The presence of the walls is shown to affect the inflaton trajectory for a given set of parameters.

Cosmological dynamics of a Dirac-Born-Infeld field

We analyze the dynamics of a Dirac-Born-Infeld (DBI) field in a cosmological set-up which includes a perfect fluid. Introducing convenient dynamical variables, we show the evolution equations form an autonomous system when the potential and the brane tension of the DBI field are arbitrary power-law or exponential functions of the DBI field. In particular we find scaling solutions can exist when powers of the field in the potential and warp-factor satisfy specific relations. A new class of fixed-point solutions are obtained corresponding to points which initially appear singular in the evolution equations, but on closer inspection are actually well defined. In all cases, we perform a phase-space analysis and obtain the late-time attractor structure of the system. Of particular note when considering cosmological perturbations in DBI inflation is a fixed-point solution where the Lorentz factor is a finite large constant and the equation of state parameter of the DBI field is $w=-1$. Since in this case the speed of sound $c_s$ becomes constant, the solution can be thought to serve as a good background to perturb about.

Consistency relation for the Lorentz invariant single-field inflation

In this paper we compute the sizes ofequilateral and orthogonal shape bispectrum for the generalLorentz invariant single-field inflation. The stability offield theory implies a non-negative square of sound speedwhich leads to a consistency relation between the sizes oforthogonal and equilateral shape bispectrum, namelyfNLorth. ⩽ −0.054fNLequil.. In particular,for the single-field Dirac-Born-Infeld (DBI) inflation, theconsistency relation becomes fNLorth. = 0.070fNLequil. ⩽ 0. These consistency relations arealso valid in the mixed scenario where the quantumfluctuations of some other light scalar fields contributeto a part of total curvature perturbation on thesuper-horizon scale and may generate a local formbispectrum. A distinguishing prediction of the mixedscenario is τNLloc. > ((6/5)fNLloc.)2.Comparing these consistency relations to WMAP 7yr data,there is still a big room for the Lorentz invariantinflation, but DBI inflation has been disfavored at morethan 68% CL.

Nonlinear supersymmetry, brane–bulk interactions and super-Higgs without gravity

We derive the coupling of a hypermultiplet of N = 2 global supersymmetry to the Dirac–Born–Infeld Maxwell theory with linear N = 1 and a second nonlinear supersymmetry. At the level of global supersymmetry, this construction corresponds to the interaction with Maxwell brane fields of bulk hypermultiplets, such as the universal dilaton of type IIB strings compactified on a Calabi–Yau manifold. It displays in particular the active role of a four-form field. Constrained N = 1 and N = 2 superfields and the formulation of the hypermultiplet in its single-tensor version are used to derive the nonlinear realization, allowing a fully off-shell description. Exact results with explicit symmetries and supersymmetries are then obtained. The electric–magnetic dual version of the theory is also derived and the gauge structure of the interaction is exemplified with N = 2 nonlinear QED of a charged hypermultiplet. Its Higgs phase describes a novel super-Higgs mechanism without gravity, where the goldstino is combined with half of the hypermultiplet into an N = 1 massive vector multiplet.

Cosmological constant behavior in DBI theory

Cosmological constant behavior can be realized as solutions of the Dirac–Born–Infeld (DBI) action within Type IIB string theory and the AdS/CFT correspondence. We derive a family of attractor solutions to the cosmological constant that arise purely from the “relativistic” nature of the DBI action without an explicit false vacuum energy. We also find attractor solutions with values of the equation of state near but with w≠−1; the forms for the potential arising from flux interactions are renormalizable and natural, and the D3-brane tension can be given by the standard throat form. We discuss present and future observational constraints on the theory.