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We consider the dynamics of a charged particle (e.g., an electron) oscillating in a laser field in flat spacetime and describe it in terms of the variable mass metric. By applying Einstein's equivalence principle, we show that, after representing the electron motion in a time-dependent manner, the variable mass metric takes the form of the Friedmann–Lemaitre–Robertson–Walker metric. We quantize a pseudo-scalar field in this spacetime and derive the production rate of electrically neutral, spinless particles. We show that this approach can provide an alternative experimental method to axion searches.

We find an explicit solution of the source free Maxwell equations in a generalized Kerr-NUT-(A)dS spacetime in all dimensions. This solution is obtained as a linear combination of the closed conformal Killing-Yano tensor $h_{ab}$, which is present in such a spacetime, and a derivative of the primary Killing vector, associated with $h_{ab}$. For the vanishing cosmological constant the obtained solution reduces to the Wald's electromagnetic field generated from the primary Killing vector., Comment: 4 pages, no figures v2: added references

Taking a spacetime average as a measure of the strength of singularities we show that big-rips (type I) are stronger than big-bangs. The former have infinite spacetime averages while the latter have them equal to zero. The sudden future singularities (type II) and $w-$singularities (type V) have finite spacetime averages. The finite scale factor (type III) singularities for some values of the parameters may have an infinite average and in that sense they may be considered stronger than big-bangs., 5 pages, no figures, REVTEX4-1, minor improvements

In this Letter, we propose a new scenario emerging from the conjectured presence of a minimal length $\ell$ in the spacetime fabric, on the one side, and the existence of a new scale invariant, continuous mass spectrum, of un-particles on the other side. We introduce the concept of \textit{un-spectral dimension} $\mathbb{D}_U$ of a $d$-dimensional, euclidean (quantum) spacetime, as the spectral dimension measured by an "un-particle" probe. We find a general expression for the un-spectral dimension $\mathbb{D}_U$ labelling different spacetime phases: a semi-classical phase, where ordinary spectral dimension gets contribution from the scaling dimension $d_U$ of the un-particle probe ; a critical "Planckian phase", where four-dimensional spacetime can be effectively considered two-dimensional when $d_U=1$; a "Trans-Planckian phase", which is accessible to un-particle probes only, where spacetime as we currently understand it looses its physical meaning., 5 pages, 1 figure, version matching that published by Physics Letters B

The differential structure on the kappa-Minkowski spacetime from Jordanian twist of Weyl algebra is constructed, and it is shown to be closed in 4-dimensions in contrast to the conventional formulation. Based on this differential structure, we have formulated a scalar field theory in this kappa-Minkowski spacetime., Comment: 7 pages, no figure;typos corrected

We assume that our universe originated from highly excited and interacting strings with coupling constant g_s = {\cal O} (1). Fluctuations of spacetime geometry are large in such strings and the physics dictating the emergence of a final spacetime configuration is not known. We propose that, nevertheless, it is determined by an entropic principle that the final spacetime configuration must have maximum entropy for a given amount of energy. This principle implies, under some assumptions, that the spacetime configuration that emerges finally is a (3 + 1) -- dimensional FRW universe filled with w = 1 perfect fluid and with 6 -- dimensional compact space of size l_s; in particular, the number of large spacetime dimensions is d = 3 + 1. Such an universe may evolve subsequently into our universe, perhaps as in Banks -- Fischler scenario., 11 pages. Latex file. Version 2: References added

We present classical solutions of Dp-branes $(p \ge 5)$ in plane wave spacetime with nonconstant R-R 3-form flux. We also show the existence of a system of D3-branes in this background. We further analyze the supersymmetric properties of these branes by solving type II Killing spinor equations explicitly., Comment: 13 pages, references added, to appear in Phys. Letts. B

We investigate the supersymmetric extension of the hot NUT–Kerr–Newman–Kasuya spacetime. Along with four standard supersymmetries, this type of spacetime admits fermionic symmetries generated by the square root of bosonic constants of motion except the Hamiltonian. Such new supersymmetries correspond to the Killing–Yano tensors, which play an important role in solving the Dirac equation in curved spacetime.

The evolution and the fate of a bubble-perturbed Bartnik-McKinnon (BK) spacetime are investigated. The infinitesimal neighborhood of the origin in a BK spacetime is shown to be equivalent to a part of a Yang-Mills cosmology (YMC) spacetime. If an arbitrarily chosen small region near the origin in the BK spacetime is replaced by a YMC bubble with the same curvature, the YMC-BK system describes a small perturbed BK spacetime. The bubble can expand as a part of the YMC spacetime in the BK spacetime. There is a thin shell between the YMC bubble and the BK spacetime whose trajectory is non-spacelike. The instability of the BK spacetime is described by the evolution of the bubble.

It is well known that the adiabatic regularization method is applicable to evaluate numerically the renormalized stress-energy tensor for matter in homogeneous spacetime. In this paper we will extend this method to the more general spacetimes with a small inhomogeneity which is invariant under spatial reflection. The difficulty of mode-mixing behavior arising from the inhomogeneity of spacetime is overcome and a possible way of obtaining the WKB solution of the time-dependent mode function under the early-time approximation is presented. Our prescriptions are useful to study the back reaction of a quantized field in the early universe with a small inhomogeneity.

Recently Coleman argued that wormholes have the effect of making the cosmological constant vanish. In this paper we discuss wormholes in multidimensional spacetime in order to investigate whether they can lead to the derivation of the dimensionality of spacetime. A criterion for spontaneous compactification is considered according to which a smaller euclidean action corresponds to the more probable vacuum solution. In the model considered, we find that the probability for the universe to undergo a spontaneous compactification down to a four-dimensional spacetime is greater than to remain in the original homogeneous multidimensional state.

Spontaneous breakdown of continuous symmetry is investigated in field theories in maximally symmetric curved spacetime. It is shown that Coleman's theorem precluding the existence of Nambu-Goldstone bosons in two-dimensional flat spacetime gets modified in curved spacetime, depending in different manners on the sign of spacetime curvature.

It is shown that the chaotic oscillatory behaviour, absent in diagonal homogeneous cosmological models in spacetime dimensions between 5 and 10, can be reestablished when off-diagonal terms are included.

The Born-Infeld theory of the gravitational field formulated in Weitzenbock spacetime is studied in detail. The action, constructed quadratically upon the torsion two-form, reduces to Einstein gravity in the low field limit where the Born-Infeld constant goes to infinity, and it is described by second order field equations for the vielbein field in $D$ spacetime dimensions. The equations of motion are derived, and a number of properties coming from them are discussed. In particular, we show that under fairly general circumstances, the equations of motion are those of Einstein's General Relativity plus an energy-momentum tensor of purely geometrical character. This tensor is obtained solely from the parallelization defining the spacetime structure, which is encoded in a set of D smooth, everywhere non-null, globally defined 1-forms. Spherical symmetry is studied as an example, and we comment on the emergence of the Schwarzschild geometry within this framework. Potential (regular) extensions of it are envisioned., 8 pages, no figures. Additional comments at the end of Sec. II. Two more references are included. Accepted in PLB

A theory of quantum many-particle systems in static and conformally static curved spacetime is discussed. As a simple application, the photon field in Friedman-Robertson-Walker universes is considered.

Building upon the methods used recently, we establish the inexistence of self-gravitating solitonic solutions for both static and strictly stationary asymptotically flat spacetimes in generalised axion electrodynamics. This is an Einstein-Maxwell-axion model, where the axion field $\theta$ is non-minimally coupled to the electromagnetic field. Considering the standard QCD axion coupling, we first present an argument for the absence of static axionic solitons, $i.e.$ localised energy axionic-electromagnetic configurations, yielding an everywhere regular, horizonless, asymptotically flat, static spacetime. Then, for generic couplings $f(\theta)$ and $g(\theta)$ (subject to mild assumptions) between the axion field and the electromagnetic field invariants, we show there are still no solitonic solutions, even when dropping the staticity assumption and merely requiring a strictly stationary spacetime, regardless of the spatial isometries., Comment: 10 pages, no figures