Cosmology and Gravitation

Quasars are among the most powerful light sources in the universe and, as such, can be seen at cosmological distances. Is some rare occasions (although not that rare), a massive galaxy on their line of sight can act as a gravitational lens and produce multiple images of distant quasars. These can be used both for cosmology and astrophysics by measuring the so-called time delays between the lensed images from photometric monitoring, a quantity directly related to the Hubble-Lemaître parameter H0. The same data, affected by microlensing by stars in the lensing galaxy, can be used to measure the size and energy profile of quasar accretion disks on spatial scales of the order of the light-day, i.e. inaccessible to even the largest interferometers, including the SKA. The results in this talk are on behalf of the COSMOGRAIL and H0LiCOW collaborations. 

Quantum decay of false vacuum states via the nucleation of bubbles may 
have played an important role in the early history of our Universe.  For 
example, in multiverse models that utilize false vacuum eternal 
inflation, the Big Bang of our observable Universe corresponds to one of 
these bubble nucleation events.  Further, our observable Universe may 
have undergone a series of symmetry-breaking first-order phase 
transitions as it cooled, which may have produced a remnant background 
of gravitational waves.

I will present results from a new real-time picture of false vacuum 
decay which, in contrast to existing semiclassical techniques, does not 
rely on classically forbidden tunneling paths.  Lattice simulations are 
used to evolve initial realizations of fluctuations around the false 
vacuum forward in time via the classical equations of motion.  In these 
simulations, we observe the false vacuum decay via the formation and 
subsequent expansion and coalescence of true vacuum bubbles.  By 
sampling initial field realizations, we build up ensembles of these 
decay histories and empirically determine the bubble nucleation rate.  
The rates agree well with standard Euclidean techniques, which cannot 
provide a time-dependent description of the decay.  Some novel 
applications of our new approach include investigation of bubble-bubble 
correlation functions, decay of time-evolving metastable states, decay 
of non-vacuum initial states, and the regime of rapid decays.

Cosmic Microwave Background (CMB) is a powerful probe to the Universe which carries signatures of cosmic secrets over a vast range of redshifts. Along with spatial fluctuations, spectral distortions of CMB blackbody are also a rich source of cosmological information. In my talk, I will introduce a new kind of spectral distortion of CMB which can arise due to the conversion of CMB photons into Axion-Like Particles (ALPs) in the presence of an external magnetic field. This effect leads to both polarized and unpolarized spatially varying spectral distortion signal with a unique spectral shape when CMB photons undergo resonant and non-resonant conversion into ALPs in the presence of the magnetic field of Milky Way, galaxy clusters and voids. I will discuss the spatial structure of this distortion which can arise from Milky Way and galaxy clusters and will show its uniqueness from other known cosmological and astrophysical signals. I will present the first all-sky map of this distortion which is obtained using the data of Planck satellite and will discuss the capability of this new cosmological window to probe ALPs from the upcoming ground-based and space-based CMB experiments such as CMB-S4, LiteBIRD, Simons Observatory, etc.

Infrared sensitivity of the de Sitter decay rate due to particle creation requires that gravitational backreaction be taken into account on the horizon scale. At lowest order, backreaction can be studied by Linear Response of the geometry to quantum matter perturbations around the Bunch-Davies state. In Linear Response the scalar degree of freedom derived from the conformal anomaly gives rise to scalar gravitational waves that grow without bound on the de Sitter horizon scale,  which implies substantial non-linear quantum backreaction effects in cosmology. Fluctuations in the conformalon scalar are potentially responsible for the primordial density fluctuations observed in the CMB anisotropy without an inflaton, and can also lead to dynamical, spacetime dependent dark energy. Possible observational tests of the conformal origin of primordial density fluctuations is suggested by the prediction of equality of scalar and tensor weight spectral indices, and the bispectral shape function of non-Gaussian correlations in the CMB predicted by conformal invariance.

Cosmological vacuum energy does not remain constant for the same reason that a uniform electric field cannot persist indefinitely in the presence of quantum fluctuations. The decay rate of the Bunch-Davies state of QFT in de Sitter space due to particle creation is calculated in real time by the same method as that for an electric field, giving Schwinger’s result.  In both the electric field and de Sitter cases the particles created are verified to be real, in that they persist in the final asymptotic region if the background field is switched off. The energy density of the particles decreases the vacuum energy and the Hubble expansion rate.

The sensitivity to infrared physics of this decay rate requires that gravitational backreaction be taken into account on the horizon scale. A Linear Response analysis shows the relevance of the scalar degree of freedom of the conformal anomaly to de Sitter instability, backreaction and screening of vacuum energy. The conformalon scalar gives rise to cosmological horizon modes that can generate the primordial density fluctuations observed in the CMB anisotropy, and lead to dynamical, spacetime dependent dark energy. Possible observational tests of the conformal quantum origin of primordial density fluctuations is suggested by the prediction of equality of scalar and tensor weights spectral indices, and the bispectral shape function of non-Gaussian correlations in the CMB.

Introduction of the torsion tensor on the space-time manifold  leads to a Weyl invariant geometry, in which t e torsion trace acts as a "U(1)" compensating field for the conformal transformations. Such  symmetry can be extended to the whole matter sector included in the  standard model by coupling the Higgs scalar (and all other possible  fundamental scalars) to the torsion. A relevant question is then whether such framework can be used to describe cosmic inflation, as a spontaneous symmetry breaking phenomenon. It turns out that this is indeed possible because the Weyl invariance imposes a distinct field space geometry, which introduces a slow roll plateau in the otherwise  too steep quartic potential dictated by Weyl symmetry. The model can  have observable consequences that distinguish it from other inflationary scenarios, at least under some choices of initial conditions.

In this talk I will discuss several aspects of using the 21cm Intensity Mapping (IM) as a Large-Scale Structure (LSS) probe in order to better constrain the cosmological parameters. I will start with a Baryon Acoustic Oscillations (BAO) reconstruction method intended for 21cm IM observations at low redshifts. I will then present the predictions and gains of performing 21cm IM surveys at redshift range which is currently vastly unobserved (2<z<6). Finally I will show the results of using existing data (ALFALFA & SDSS) to place constraints on the distribution of neutral hydrogen (HI) in dark matter halos as a function of the halo mass. This is then used to constrain and improve the HI halo model needed to make accurate and precise predictions on the 21cm signal and the noise.

In July 2018 the Planck Collaboration released its final set of cosmology results. I will discuss some of the  interesting new science that remains to be done with the CMB, including some not so often discussed topics such as the kinetic SZ effect and 21cm cross-correlations.

Hyper Suprime-Cam (HSC) is an imaging camera mounted at the Prime Focus of the Subaru 8.2-m telescope operated by the National Astronomical Observatory of Japan on the summit of Maunakea in Hawaii. A consortium of astronomers from Japan, Taiwan and Princeton University is carrying out a three-layer, 300-night, multiband survey from 2014-2019 with this instrument. In this talk, I will focus on the HSC survey Wide Layer, which will cover 1400 square degrees in five broad bands (grizy), to a 5 sigma point-source depth of r~26. We have covered 240 square degrees of the Wide Layer in all five bands, and the median seeing in the i band is 0.60 arcseconds. This powerful combination of depth and image quality makes the HSC survey unique compared to other ongoing imaging surveys. In this talk I will describe the HSC survey dataset and the completed and ongoing science analyses with the survey Wide layer, including galaxy studies, strong and weak gravitational lensing, but with an emphasis on weak lensing. I will demonstrate the level of systematics control, the latest cosmology results, and describe some lessons learned that will be of use for other ongoing and future lensing surveys.

In the second lecture, I will extend the previous discussion to gravity, and show that the conformal trace anomaly must play a special role in the effective field theory of low energy gravity. Classical General Relativity receives an infrared relevant modification from the conformal trace anomaly of the energy-momentum tensor of massless, or nearly massless, quantum fields, which implies the existence of a new long range massless scalar degree of freedom, not present in the classical Einstein theory, that contributes to gravitational scattering processes and has long range gravitational effects. Similar to the axial anomaly, the local form of the effective action associated with the conformal anomaly is expressible in terms of a dynamical scalar field that couples to the conformal factor of the spacetime metric, allowing it to propagate over macroscopic distances. Among the significant implications of this effective field theory of gravity are the prediction of scalar gravitational wave solutions—a spin-0 breather mode— in addition to the transversely polarized tensor waves of the classical Einstein theory. Astrophysical sources for scalar gravitational waves are considered, with the excited gluonic condensates in the interiors of neutron stars in merger events with other compact objects likely to provide the strongest burst signals. The conformal anomaly also implies generically large quantum back reaction effects and conformal correlators in the vicinity of black hole horizons which are relevant to the formation of a non-singular interior, as well as an additional scalar degree of freedom in cosmology, providing a theoretical foundation for dynamical dark energy.