In a recently proposed theory, the cosmological constant (CC) does not curve spacetime in our universe, but instead gets absorbed into another universe endowed with its own dynamical metric, nonlocally coupled to ours. Thus, one achieves a long standing goal of removing entirely any cosmological constant from our universe. Dark energy then cannot be due to a cosmological constant, but must be obtained via other mechanisms. Here we focus on the scenario in which dark energy is due to massive gravity and its extensions. We show how the metric of the other universe, that absorbs our CC, also gives rise to the fiducial metric known to be necessary for the diffeomorphism invariant formulation of massive gravity. This is achieved in a framework where the other universe is described by 5D AdS gravity, while our universe lives on its boundary and is endowed with dynamical massive gravity. A non-dynamical pullback of the bulk AdS metric acts as the fiducial metric for massive gravity on the boundary. This framework also removes a difficulty caused by the quantum strongly coupled behavior of massive gravity at the Λ 3 scale: in the present approach, the massive gravity action does not receive any loop-induced counterterms, despite being strongly coupled. An Unconventional PathThe importance of the cosmological constant (CC) problem is well-known, so are the difficulties in solving it (see [1,2], and references therein). It is clear that unconventional approaches are needed. Along these lines, Tseytlin [3] had made an interesting proposal building on an earlier idea of [4], and inspirations from T-duality of string theory. He suggested to apply the least action principle to the "volume normalized action,"S:instead of that principle being applied to S. Here, V g = d 4 x |g| is the invariant space-time volume in the 16πG N = 1 units (it is assumed that V g is regularized, as in Section 5). The above modified action should be thought as a certain low energy effective action, hopefully emerging from more conventional high energy physics [3]. Since the CC problem is a low energy problem, it is reasonable to expect that its solution will not be influenced by an exact form of the high energy completion of (1.1). Furthermore, L SM in (1.1) denotes a Lagrangian of all the fields of nature but gravity, coupled to gravity. As argued in [5], for consistency with the empirical data the Lagrangian L SM needs to be regarded as a quantum effective Lagrangian coupled to classical gravity. This classical gravity will be subsequently quantized using (1.1). Such an unconventional procedure of quantization will be reviewed in Section 6, where earlier works that made this procedure more precise will be referenced. Till then we will not use any specific form of L SM , but discuss only its constant part.It is straightforward to see that a cosmological constant is an unphysical parameter in (1.1): a shift of L SM by any constant changesS by an additive constant, and the latter does not affect the equations of motion obtained by varyingS.Wh...

We present a classical analysis on the issue of vector superluminality in the decoupling limit ghost-free massive gravity with a Minkowski reference metric. We show explicitly in the Lorenz gauge that the theory is free of superluminal vector excitations around a nontrivial solution at the cubic order in the fields. In the same gauge, we demonstrate that superluminal vector modes arise at the quartic order and compute some superluminal propagating solutions. We then generalize our findings to all orders in a gauge-independent way. We check the physical consistency of the vector superluminalities, arguing that they are not physically detectable in the perturbation theory but could be trusted classically in the strong coupling region. Nevertheless, these superluminalities involve only low frequency group and phase velocities and are unable to determine the acausality of the theory.

In the theories described by a volume normalized action functional, an arbitrary cosmological constant is eliminated from the physical picture of our Universe, and dynamical alternatives must be responsible for today's accelerated expansion as well as the conjectured early-time inflation in the Universe. A few wellknown such scenarios realized by a single homogeneous scalar field are examined in this new context, and their diverse fates are elucidated. Typical inflationary models are not affected at the level of the background evolution, and also give rise to the scalar perturbations equivalent to those obtained in the standard general relativity; however, the primordial quantum tensor fluctuations are absent in the new framework, irrespective of the inflationary model. As a consequence, our proposal would be ruled out should the primordial tensor modes, or their indirect consequences, be observed experimentally.

Recently, de Visser and Blaauboer [Phys. Rev. Lett. 96, 246801 (2006)] proposed the most efficient deterministic teleportation protocol T for electron spins in a semiconductor nanostructure consisting of a single and a double quantum dot. However, it is as yet unknown if T can be completed before decoherence sets in. In this paper we analyze the detrimental effect of nuclear spin baths, the main source of decoherence, on T . We show that nonclassical teleportation fidelity can be achieved with T provided certain conditions are met. Our study indicates that realization of quantum computation with quantum dots is indeed promising.

A certain class of nonlocal theories eliminates an arbitrary cosmological constant (CC) from a universe that can be perceived as our world. Dark energy then cannot be explained by a CC; it could however be due to massive gravity. We calculate the new corrections, which originate from the nonlocal terms that eliminate the CC, to the decoupling limit Lagrangian of massive gravity. The new nonlocal terms also have internal field space Galilean symmetry and are referred here as "nonlocal Galileons." We then study a self-accelerated solution and show that the new nonlocal terms change the perturbative stability analysis. In particular, small fluctuations are now stable and non-superluminal for some simple parameter choices, whereas for the same choices the pure massive gravity fluctuations are unstable. We also study stable spherically symmetric solutions on this background.