F. A. Teppa Pannia, S. E. Perez Bergliaffa

As a first step in the analysis of the influence of inhomogeneities in the evolution of an inflating region, we study the propagation of bubbles of new vacuum in a radially inhomogeneous background filled with dust or radiation, and including a cosmological constant. For comparison, we also analyse the cases with homogeneous dust and radiation backgrounds. We show that the evolution of the bubble in the radiation environments is always slower than in the dust cases, both for homogeneous and inhomogeneous ambients, and leads to appreciable differences in the evolution of the proper radius of the bubble.

Borivoje Dakić, Milan Radonjić

We study the question of whether a macroscopic superposition can naturally exist as a ground state of some gapped many-body Hamiltonian. We derive an upper bound on the energy gap of an arbitrary physical Hamiltonian provided that its ground state is a superposition of two macroscopic "semi-classical" states. For a large class of such macroscopic superposition states we show that the gap vanishes in the macroscopic limit. Our main result shows an interesting quantitative relation between the order of interaction and separation probability between the two constituents of the superposition. This in turn shows that preparation of a "typical" Schr\"odinger cat-like state by simple cooling to the ground state would require the order of interaction to increase with the size of system. We also discuss our results in the context of quantum spin-lattice models and trapped Bose-Einstein condensates and we find that certain classes of states (e.g. W states), although not usually considered to exhibit macroscopic quantum properties, cannot be ground states of gapped local Hamiltonians.

Alan Coley, Daniele Gregoris, Woei Chet Lim

Using the Geroch transformation we obtain the first example of a stiff fluid solution to the Einstein field equations in a closed form exhibiting a spacelike $G_1$ group of symmetries (i.e., with a single isometry). This new solution can be interpreted as an exact example of a close-to-Friedmann-Lemaitre (perturbative) solution. The exact solution is the first (non-null) $G_1$ solution found, and exhibits a spike crossing which persists to the past, which allows us to better understand spike crossings in the context of structure formation.

Jonathan Chardin, Ewald Puchwein, Martin G. Haehnelt

Lyman-alpha forest data probing the post-reionization Universe shows surprisingly large opacity fluctuations over rather large ($\ge$50 comoving Mpc/h) spatial scales. We model these fluctuations using a hybrid approach utilizing the large volume Millennium simulation to predict the spatial distribution of QSOs combined with smaller scale full hydrodynamical simulation performed with RAMSES and post-processed with the radiative transfer code ATON. We produce realictic mock absorption spectra that account for the contribution of galaxies and QSOs to the ionising UV background. This improved models confirm our earlier findings that a significant ($\ge$50%) contribution of ionising photons from QSOs can explain the large reported opacity fluctuations on large scales. The inferred QSO luminosity function is thereby consistent with recent estimates of the space density of QSOs at this redshift. Our simulations still somewhat struggle, however, to reproduce the very long (110 comoving Mpc/h) high opacity absorption through observed in ULAS J0148+0600, perhaps suggesting an even later end of reionization than assumed in our previously favoured model. Medium-deep/medium area QSO surveys as well as targeted searches for the predicted strong transverse QSO proximity effect whould illuminate the origin of the observed large scale opacity fluctuations. They would allow to substantiate whether UV fluctuations due to QSO are indeed primarily responsible, or whether significant contributions from other recently proposed mechansims such as large scale fluctuations in temperature and mean free path (even in the absence of rare bright sources) are required.

Bernhard Brehm

Bianchi models are posited by the BKL picture to be essential building blocks towards an understanding of generic cosmological singularities. We study the behaviour of spatially homogeneous anisotropic vacuum spacetimes of Bianchi type VIII and IX, as they approach the big bang singularity. It is known since 2001 that generic Bianchi IX spacetimes converge towards the so-called Mixmaster attractor as time goes towards the singularity. We extend this result to the case of Bianchi VIII vacuum. The BKL picture suggests that particle horizons should form, i.e. spatially separate regions should causally decouple. We prove that this decoupling indeed occurs, for Lebesgue almost every Bianchi VIII and IX vacuum spacetime.

Ujjal Kumar Dey, Subhendra Mohanty, Gaurav Tomar

The recent data from ATLAS and CMS hint at a new resonance at around 750 GeV in the diphoton invariant mass distribution. The explanation of the significantly large cross section for this diphoton resonance requires coloured particles in its loop-induced production via gluon fusion and subsequent decay to diphoton. A natural candidate for the coloured particle is the colour-triplet leptoquark, lying in the mass range 375-1000 GeV, which can account for such large cross section. The leptoquarks in this mass range can also be produced resonantly from neutrino and quark interactions at IceCube and provide an explanation to the PeV-energy neutrino events. In this work, we show that the scalar leptoquark with quantum number $(3,2,7/6)$ can uniquely provide a unified explanation to both the PeV IceCube events and the 750 GeV diphoton resonance.

Zhe Chang, Chao-Guang Huang, Zhi-Chao Zhao

The response of a detector to gravitational wave is a function of frequency. When the time a photon moving around in the Fabry-Perot cavities is the same order of the period of a gravitational wave, the phase-difference due to the gravitational wave should be an integral along the path. We present a formula description for detector response to gravitational wave with varied frequencies. The LIGO data for GW150914 and GW 151226 are reexamined in this framework. For GW150924, the traveling time of a photon in the LIGO detector is just a bit larger than a half period of the highest frequency of gravitational wave and the similar result is obtained with LIGO and Virgo collaborations. However, we are not always so luck. In the case of GW151226, the time of a photon traveling in the detector is larger than the period of the highest frequency of gravitational wave and the announced signal cannot match well the template with the initial black hole masses 14.2M$_\odot$ and 7.5M$_\odot$.