Cosmology and Gravitation

The standard cosmological model determined from the accurate cosmic microwave background measurements made by the Planck satellite implies a value of the Hubble constant H0 that is 4.2 standard deviations lower than the one determined from Type Ia supernovae. The Planck best fit model also predicts lower values of the matter density fraction Om and clustering amplitude S8 compared to those obtained from the Dark Energy Survey Year 1 data. We show that accounting for the enhanced recombination rate due to additional inhomogeneities in the baryon density can solve both the H0 and the S8-Om tensions. The additional baryon inhomogeneities can be induced by primordial magnetic fields present in the plasma prior to recombination. The required field strength to solve the Hubble tension is just what is needed to explain the existence of galactic, cluster, and extragalactic magnetic fields without relying on dynamo amplification. Our results show clear evidence for this effect and motivate further detailed studies of primordial magnetic fields, setting several well-defined targets for future observations.

Zoom Link:  https://pitp.zoom.us/j/93581608531?pwd=d3NRQXRGNTNISkhuWmxLYkJMZllTUT09

Based on recent work arXiv:1902.08207 and arXiv:1911.02018 with E. Verlinde.

Over the last decade, the Effective Field Theory of Large Scale Structure (EFTofLSS) has emerged as a frontrunner in the effort to produce accurate models of cosmological statistics. Quantities such as power spectra can be fit with sub-percent precision, and there is a wealth of literature applying the formalism to more complex statistics. It is interesting to ask what lies ahead for the theory. Can it be used for cosmological parameter inference? And is it just for statistics based on the 3D density field?

In this talk, I will present a pedagogical introduction to the EFTofLSS, discussing its motivation and basic formalism, as well as a few of its theoretical challenges. To demonstrate the utility of the theory beyond the blackboard, I will discuss the analysis of galaxy power spectra. Using this model, competitive constraints can be placed on cosmology utilizing all the large-scale spatial information, not just the position of the Baryon Acoustic Oscillations. In particular, this yields the strongest CMB-independent measurement of H0. 

In answering the second question, I will introduce two less conventional applications of the EFTofLSS. Firstly, the marked density field. This is simply the matter field weighted by its local overdensity, and recent works have shown its power spectrum to be capable of placing strong constraints on the neutrino mass. I will discussing its perturbative modeling, highlighting its unusual features, and how the model allows us to shed light on the surprising constraining of the statistic. An additional statistic of interest is weak lensing. Creating an analytic model for this has its own complications, since it is sensitive to a large range of scales, requiring extensions to the usual EFTofLSS modeling. Crucial to this effort is the development of a matter power spectrum model which is accurate on all scales; I will present the results of recent modeling efforts within the ‘Effective Halo Model’, and discuss future applications to integrated statistics such as weak lensing.

Zoom Link: https://pitp.zoom.us/j/91697113596?pwd=OTh3ZjZ5SHd5Q09sTFdReUMyb0hpUT09

COVID-19 is a mysterious disease associated with a large number of unanswered questions. 

In this talk we review what is currently known, what is still a mystery and highlight some of our recent work on the role of climate, blood type and vaccinations on the transmission of the disease and on the extent of "dark infections", the asymptomatic and untested proportion of infections. We end with a list of open research questions that may be amenable to techniques from physics and data science.

The discovery of the Higgs boson has revealed that the quartic Higgs self-coupling becomes small at very high energy scales. Guided by this observation, I introduce Higgs Parity, which is a spontaneously broken symmetry exchanging the standard model Higgs with its parity partner. In addition to explaining the small Higgs quartic coupling, Higgs Parity can provide a dark matter candidate, solve the strong CP problem, and arise from an SO(10) grand unified gauge symmetry. I will show that the Higgs Parity symmetry breaking scale is determined by standard model parameters and predicts experimental signals such as the proton decay rate and the warmness of dark matter. As a result, Higgs Parity provides a tight correlation between future precision measurements of standard model parameters and these experimental signals.

Through their observable properties, the first and smallest dark matter halos represent a rare probe of subkiloparsec-scale variations in the density of the early Universe. These density variations could hold clues to the nature of inflation, the postinflationary cosmic history, and the identity of dark matter. However, the dynamical complexity of these microhalos hinders their usage as cosmological probes. A theoretical understanding of the microhalo-cosmology connection demands numerical simulation, but microhalos are too small and dense to simulate up to the present day in full cosmological context. My research meets this challenge by using controlled numerical simulations to develop (semi)analytic models of dark matter structure. I will discuss these models, which describe the formation of the first halos and their subsequent evolution as they accrete onto larger systems. I will also explore two applications: breaking a degeneracy between the properties of thermal-relic dark matter and the postinflationary history, and probing inflation's late stages via the small-scale primordial power spectrum.

CMB lensing tomography has the potential to map the amplitude and growth of structure over cosmic time, provide some of the most stringent tests of gravity, and break important degeneracies between cosmological parameters. I use the unWISE photometric galaxy catalog to create three samples at median redshifts z~0.6, 1.1, and 1.5, and cross-correlate them with the most recent Planck CMB lensing maps. The resulting significance of ~80 at 100 < ell < 1000 is the highest significance detection to date of CMB lensing cross-correlation.  The redshift distribution of the two-band unWISE galaxies is a major source of systematic error. I primarily use cross-correlations with BOSS galaxies and quasars and eBOSS quasars to measure the redshift distribution, supplemented with cross-matching to deep COSMOS photometric redshifts.  I demonstrate how to propagate the uncertainty in the redshift distribution to the modeling of the signal, and perform a number of null tests. Finally, I discuss the cosmological implications of this measurement and lessons learned for CMB lensing cross-correlations with future photometric surveys.

We propose a model for combining the Standard Model (SM) with gravity. It relies on a non-minimal coupling of the Higgs field to the Ricci scalar and on the Palatini formulation of gravity. Without introducing any new degrees of freedom in addition to those of the SM and the graviton, this scenario achieves two goals. First, it generates the electroweak symmetry breaking by a non-perturbative gravitational effect. In this way, it does not only address the hierarchy problem but opens up the possibility to calculate the Higgs mass. Second, the model incorporates inflation at energies below the onset of strong-coupling of the theory. Provided that corrections due to new physics above the scale of inflation are not unnaturally large, we can relate inflationary parameters to data from collider experiments.
 

Cosmologists wish to explain how our universe, in all its complexity, could ever have come about. This is the problem of initial conditions and the first step towards its solution is the assessment of the universe’s entropy today. It is widely agreed upon that the entropy of vacuum energy, given by the Bekenstein bound, makes up the bulk of the current entropy budget, dominating over that of gravity and over thermal motions of the cosmic radiation background.

Have we counted all significant contributions to the entropy inventory? There is one number which we have not considered: the number of degrees of freedom in our vibrant, complex biosphere. What is the entropy of life? Is it sizeable enough to need to be accounted for at the Big Bang, or negligible compared to vacuum entropy?

In this talk, I will outline the forward model approach to reconstruct cosmological fields in a Bayesian framework. I will focus on two examples - galaxy clustering and neutral hydrogen intensity mapping.

In galaxy clustering example, I will use the observations of galaxy surveys to reconstruct the initial Lagrangian field. Here, we develop a novel framework with neural networks to forward model halo masses and positions and demonstrate that our method outperforms standard reconstruction in both real and redshift space. This reconstructed initial field has enhanced signal for baryon acoustic oscillations and can enhance science returns for surveys like DESI.

For neutral hydrogen surveys, we lose over 50% of the modes at high redshifts due to foregrounds and it severely hampers their feasibility for cosmological analysis. With a novel bias framework for the forward model, I will show that we are able to reconstruct over 90% of these modes and this recovers cross-correlations with photo-z surveys like LSST and tracers like CMB lensing.

Lastly, I will briefly touch upon assumptions made in this reconstruction framework regarding noise models and likelihood. I will discuss preliminary ways to improve upon them using deep learning.

Primordial SU(2) gauge fields and axions can contribute to the physics of
inflation. In this class of models, both the gauge field and axion acquire
a VEV, which is P and CP breaking and enriches the phenomenology of
particles with spin. Their multifaceted phenomenology and unique
observational signatures, e.g., chiral primordial gravitational waves and
gravitational leptogenesis, turned this class of models to a hot topic of
research in the past nine years. In this talk, first, I will briefly
review the different realizations of this setup. Next, I will tell you
about the three different types of particles produced by the SU(2) gauge field,
i.e., scalar, fermion, and spin-2 particles and their observable
signatures. In particular, the new spin-2 field produces
chiral gravitational waves, which can be detected with LiteBIRD, CMB-S4,
adv LISA, and BBO. Last but not least, I will tell you about the fermion
generation during inflation, dark and visible, as yet another generous
opportunity offered by the primordial SU(2)!

Thus far, the non-linear regime of structure formation is only accessible through expensive numerical N-body simulations since the conventional ana-
lytic treatment of cosmic density fluctuations via the hydrodynamical equations runs into severe problems even in a mildly non-linear regime. I will give an overview of Kinetic Field Theory, a novel analytic approach to cosmic large-scale structure formation, which provides a density fluctuation power spectrum that agrees well with state of the art N-body simulations up to k = 10h Mpc−1. I will point out how Kinetic Field Theory avoids the difficulties of standard perturbation theory by construction and allows to proceed deeply into the non-linear regime of density fluctuations. I will give a summary of the most important results and recent developments for cosmic structure formation and briefly show applications of Kinetic Field Theory outside of cosmology, stressing the opportunities this method provides for the study of the physics behind structure formation. I will then focus my discussion on how the form of the interaction potential influences the formation of cosmic structures in the highly non-linear regime and the implications it bears on the NFW density profile of dark matter halos.