Cosmic Growth Signatures of Modified Gravitational Strength

Cosmic growth of large scale structure probes the entire history of cosmic expansion and gravitational coupling. To get a clear picture of the effects of modification of gravity we consider a deviation in the coupling strength (effective Newton's constant) at different redshifts, with different durations and amplitudes. We derive, analytically and numerically, the impact on the growth rate and growth amplitude. Galaxy redshift surveys can measure a product of these through redshift space distortions and we connect the modified gravity to the observable in a way that may provide a useful parametrization of the ability of future surveys to test gravity. In particular, modifications during the matter dominated era can be treated by a single parameter, the "area" of the modification, to an accuracy of $\sim0.3\%$ in the observables. We project constraints on both early and late time gravity for the Dark Energy Spectroscopic Instrument and discuss what is needed for tightening tests of gravity to better than 5% uncertainty.Comment: 12 pages, 14 figure

Evidence for a new SU(4)SU(4) symmetry with J=2J=2 mesons

Recently, a new symmetry of mesons has been found upon truncation of the quasi-zero modes of the Overlap Dirac operator in lattice simulations. Namely, the $\rho,\rho',\omega,\omega',a_1, b_1,h_1$ and possibly $f_1$ $J=1$ mesons get degenerate after removal of the quasi-zero modes. This emergent symmetry has been established to be $SU(4)\supset SU(2)_L \times SU(2)_R \times U(1)_A$. It is higher than the symmetry of the QCD Lagrangian and provides not only a mixing of quarks of given chirality in the isospin space, but also the mixing of left-handed and right-handed components. Here we study, with the Overlap Dirac operator, the isovector $J=2$ mesons upon the quasi-zero mode reduction and observe a similar degeneracy. This result further supports the $SU(4)$ symmetry in mesons of given spin $J \geq 1$.Comment: 6 figure

Isoscalar mesons upon unbreaking of chiral symmetry

In a dynamical lattice simulation with the overlap Dirac operator and $N_f=2$ mass degenerate quarks we study all possible $J=0$ and $J=1$ correlators upon exclusion of the low lying "quasi-zero" modes from the valence quark propagators. After subtraction of a small amount of such Dirac eigenmodes all disconnected contributions vanish and all possible point-to-point $J=0$ correlators with different quantum numbers become identical, signaling a restoration of the $SU(2)_L \times SU(2)_R \times U(1)_A$. The original ground state of the $\pi$ meson does not survive this truncation, however. In contrast, in the $I=0$ and $I=1$ channels for the $J=1$ correlators the ground states have a very clean exponential decay. All possible chiral multiplets for the $J=1$ mesons become degenerate, indicating a restoration of an $SU(4)$ symmetry of the dynamical QCD-like string.Comment: 10 pages, 13 figure

More effects of Dirac low-mode removal

In previous studies we have shown that hadrons, except for a pion, survive the removal of the lowest lying Dirac eigenmodes from the valence quark propagators. The low-modes are tied to the dynamical breaking of chiral symmetry and we found chiral symmetry to be restored by means of matching masses of chiral partners, like, e.g., the vector and axial vector currents. Here we investigate the influence of removing the lowest part of the Dirac spectrum on the locality of the Dirac operator. Moreover, we analyze the influence of low-mode truncation on the quark momenta and thereupon on the hadron spectrum and, finally, introduce a reweighting scheme to extend the truncation to the sea quark sector.Comment: 7pages, 4 figures. Proceedings of the 31st International Symposium on Lattice Field Theory, July 29 - August 3, 2013, Mainz, German

Gravity's Islands: Parametrizing Horndeski Stability

Cosmic acceleration may be due to modified gravity, with effective field theory or property functions describing the theory. Connection to cosmological observations through practical parametrization of these functions is difficult and also faces the issue that not all assumed time dependence or parts of parameter space give a stable theory. We investigate the relation between parametrization and stability in Horndeski gravity, showing that the results are highly dependent on the function parametrization. This can cause misinterpretations of cosmological observations, hiding and even ruling out key theoretical signatures. We discuss approaches and constraints that can be placed on the property functions and scalar sound speed to preserve some observational properties, but find that parametrizations closest to the observations, e.g. in terms of the gravitational strengths, offer more robust physical interpretations. In addition we present an example of how future observations of the B-mode polarization of the cosmic microwave background from primordial gravitational waves can probe different aspects of gravity.Comment: 11 pages, 11 figure

Connecting Primordial Gravitational Waves and Dark Energy

Cosmic acceleration manifested in the early universe as inflation, generating primordial gravitational waves detectable in the cosmic microwave background (CMB) radiation. Cosmic acceleration is occurring again at present as dark energy, detectable in cosmic distance and structure surveys. We explore the intriguing idea of connecting the two occurrences through quintessential inflation by an $\alpha$-attractor potential without a cosmological constant. For this model we demonstrate robustness of the connection $1+w_0\approx 4/(3N^2r)$ between the present day dark energy equation of state parameter $w_0$ and the primordial tensor to scalar ratio $r$ for a wide range of initial conditions. Analytics and numerical solutions produce current thawing behavior, resulting in a tight relation $w_a\approx-1.53(1+w_0)\approx -0.2\,(4\times 10^{-3}/r)$. Upcoming CMB and galaxy redshift surveys can test this consistency condition. Within this model, lack of detection of a dark energy deviation from $\Lambda$ predicts a higher $r$, and lack of detection of $r$ predicts greater dark energy dynamics.Comment: 8 pages, 6 figures; v2 minor changes to match published versio