The search for dark matter (DM) is one of the most active and challenging areas of current research. Possible DM candidates are ultralight fields such as axions and weak interacting massive particles (WIMPs). Axions piled up in the center of stars are supposed to generate matter/DM configurations with oscillating geometries at a very rapid frequency, which is a multiple of the axion mass $m_B$ [1,2]. Borra and Trottier recently found peculiar ultrafast periodic spectral modulations in $236$ main sequence stars in the sample of $2.5$ million spectra of galactic halo stars of the Sloan Digital Sky Survey that were interpreted as optical signals from extraterrestrial civilizations, possible candidates for the search for extraterrestrial intelligence (SETI) program [3]. We argue, instead, that this could be the first indirect evidence of bosonic axion-like DM fields inside main sequence stars, with a stable radiative nucleus, where a stable DM core can be hosted. These oscillations were not observed in earlier stellar spectral classes probably because of the impossibility of starting a stable oscillatory regime due to the presence of chaotic motions in their convective nuclei. The axion mass values, $(50 < m_B < 2.4 times 10^{3})~ mathrm{mu eV}$, obtained from the frequency range observed by Borra and Trottier, $(0.6077< f <0.6070$) THz, agree with the recent theoretical results from high-temperature lattice quantum chromodynamics [4,5].
Context: Strings and other alternative theories describing the quantum properties of space-time suggest that space-time could present a foamy structure and also that, in certain cases, quantum gravity (QG) may manifest at energies much below the Planck scale. One of the observable effects could be the degradation of the diffraction images of distant sources. Aims: We searched for this degradation effect, caused by QG fluctuations, in the light of the farthest quasars (QSOs) observed by the Hubble Space Telescope with the aim of setting new limits on the fluctuations of the space-time foam and QG models. Methods: We developed a software that estimates and compares the phase variation in the interference patterns of the high-redshift QSOs, taken from the snapshot survey of HST-SDSS, with those of stars that are expected to not be affected by QG effects. We used a two-parameter function to determine, for each test star and QSO, the maximum of the diffraction pattern and to calculate the Strehl ratio. Results: Our results go far beyond those already present in the literature. By adopting the most conservative approach where the correction terms, that describe the possibility for space-time fluctuations cumulating across long distances and partially compensate for the effects of the phase variations, are taken into account. We exclude the random walk model and most of the holographic models of the space-time foam. Without considering these correction terms, all the main QG scenarios are excluded. Finally, our results show the absence of any directional dependence of QG effects and the validity of the cosmological principle with an independent method; that is, viewed on a large scale, the properties of the Universe are the same for all observers, including the effects of space-time fluctuations.
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Fabrizio Tamburini (Department of Astronomy University of Padova
, n Vicolo dellOsservatorio 3
, Padova
2009
We propose a thought experiment to detect low-energy Quantum Gravity phenomena using Quantum Optical Information Technologies. Gravitational field perturbations, such as gravitational waves and quantum gravity fluctuations, decohere the entangled photon pairs, revealing the presence of gravitational field fluctuations including those more speculative sources such as compact extra dimensions and the sub-millimetric hypothetical low-energy quantum gravity phenomena and then set a limit for the decoherence of photon bunches and entangled pairs in space detectable with the current astronomical space technology.
We propose the use of heralded photons to detect Gravitational Waves (GWs). Heralded photons are those photons that, produced during a parametric downconversion process, are labelled by the detection and counting of coincidences of their correlated or entangled twins and therefore can be discriminated from the background noise, independently of the type of correlation/entanglement used in the setup. Without losing any generality, we illustrate our proposal with a gedankenexperiment, in which the presence of a gravitational wave causes a relative rotation of the reference frames associated to the double-slit and the test polarizer, respectively, of a Walborns quantum eraser cite{wal02}. In this thought experiment, the GW is revealed by the detection of heralded photons in the dark fringes of the recovered interference pattern by the quantum eraser. Other types of entanglement, such as momentum-space or energy-time, could be used to obtain heralded photons to be used in the future with high-frequency GW interferometric detectors when enough bright sources of correlated photons will be available.
We propose a thought technique for detecting Gravitational Waves using Einstein-Podolski-Rosen photon Entangled States. GWs decohere the entangled photon pairs, introduce a relative rotation and de-synchronize Alice and Bobs reference frames thus reducing the measured non-locality of correlated quanta described by Bells inequalities. Gravitational Waves, distorting quantum encryption key statistics away from a pure white noise, act then as shadow eavesdroppers. The deviation from the intrinsic white-noise randomness of a Quantum Key Distribution process can reveal the presence of a gravitational wave by analyzing the emerging color distortions in the key. Photon entangled states provide the key advantage of revealing the polarization rotation introduced by GWs without the need of previously fixed reference frames
Recently it has become clear that the resonant amplification of quantum field fluctuations at preheating must be accompanied by resonant amplification of scalar metric perturbations, since the two are united by Einsteins equations. Furthermore, this metric preheating enhances particle production and leads to gravitational rescattering effects even at linear order. In multi-field models with strong preheating (q gg 1), metric perturbations are driven nonlinear, with the strongest amplification typically on super-Hubble scales (k to 0). This amplification is causal, being due to the super- Hubble coherence of the inflaton condensate, and is accompanied by resonant growth of entropy perturbations. The amplification invalidates the use of the linearized Einstein field equations, irrespective of the amount of fine-tuning of the initial conditions. This has serious implications at all scales - from the large-angle cosmic microwave background (CMB) anisotropies to primordial black holes. We investigate the (q,k) parameter space in a two-field model, and introduce the time to nonlinearity, t_{nl}, as the timescale for the breakdown of the linearized Einstein equations. Backreaction effects are expected to shut down the linear resonances, but cannot remove the existing amplification, which threatens the viability of strong preheating when confronted with the CMB. We discuss ways to escape the above conclusions, including secondary phases of inflation and preheating solely to fermions. Finally we rank known classes of inflation from strongest (chaotic and strongly coupled hybrid inflation) to weakest (hidden sector, warm inflation) in terms of the distortion of the primordial spectrum due to these resonances in preheating.
Grand unified theories may display multiply interacting fields with strong coupling dynamics. This poses two new problems: (1) What is the nature of chaotic reheating after inflation, and (2) How is reheating sensitive to the mass spectrum of these theories ? We answer these questions in two interesting limiting cases and demonstrate an increased efficiency of reheating which strongly enhances non-thermal topological defect formation, including monopoles and domain walls. Nevertheless, the large fluctuations may resolve this monopole problem via a modified Dvali-Liu-Vachaspati mechanism in which non-thermal destabilsation of discrete symmetries occurs at reheating.
