Cosmology and Gravitation

The astrophysical stochastic gravitational wave background (SGWB) originates from numerous faint sub-threshold  gravitational wave (GW) signals arising from the coalescing binary compact objects. This background is expected to be discovered from the current (or next-generation) network of GW detectors by cross-correlating the signal between multiple pairs of GW detectors. However, detecting this signal is challenging and the correlation is only detectable at low frequencies due to the arrival time delay between different detectors. In this work, we propose a novel technique, Spectrogram Correlated Stacking (or SpeCS), which goes beyond the usual cross-correlation (and to higher frequencies)  by exploiting the higher-order statistics in the time-frequency domain which accounts for the chirping nature of the individual events that comprise SGWB.

We show that SpeCS improves the signal-to-noise for the detection of SGWB by up to an order of magnitude, compared to standard optimal cross-correlation methods which are tuned to measure only the power spectrum of the SGWB signal. SpeCS can probe beyond the power spectrum and its application to the GW data available from the current and next-generation GW detectors would speed up the SGWB discovery. 

based on work with Ramit Dey, Luis Longo, and Suvodip Mukherjee

Zoom link:   https://pitp.zoom.us/j/97091817158?pwd=MHNkdjFQT0plVzJJY2lsOHRxdDdwdz09

The fundamental nature of dark matter (DM) so far eludes direct detection experiments, but it has left its imprint in the large-scale structure (LSS) of the Universe. I will present a search using cosmic microwave background (CMB) and galaxy surveys for ultra-light DM particle candidates called axions that are well motivated from high energy theory. In combining these datasets, I will discuss how the presence of axions can improve consistency between these probes and, in particular, help alleviate the S_8 cosmological parameter tension (the discrepancy in the amplitude of density fluctuations as inferred from CMB and galaxy data). I will then present complementary searches for ultra-light and light (sub-GeV) DM using a LSS probe called the Lyman-alpha forest. By combining complementary large- and small-scale structure probes, I will demonstrate how current and forthcoming cosmological data will systematically test the nature of DM. In order to model novel DM physics accurately and efficiently in CMB and LSS probes, I will present new machine learning approaches using neural network "emulators" to accelerate DM parameter inference from days to seconds and active learning to reduce massively the computational expense.

Zoom Link: TBD

The angular resolution of a stellar interferometer, as for a single telescope, becomes better at smaller wavelengths and larger baselines. The goal for ground detectors would then be optical interferometers with baselines as long as the Earth’s diameter. The latter goal has been achieved in radio, but it becomes prohibitive in the optical, as the electromagnetic field oscillates too rapidly to record and analyze directly over km-long baselines. Intensity interferometry relying on second-order correlations can make this possible: rather than the amplitude and phase of incoming light, we need only count photons. This technique has a long history and to date the best measurements of nearby stellar radii, dating back to the 1950s. Its main limitations are the need for very bright sources and its narrow field of view, restricting kilometer-long baselines to sources only a few μas away. In this talk, I will propose an optical-path modification of astronomical intensity interferometers, which introduces an effective time delay in the two-photon interference amplitude, splitting the main intensity correlation fringe into others at finite opening angles, allowing for differential astrometry of multiple compact sources such as stars or quasar images. Together with the exponential progress in the field of single photon detection, such a modification will immensely increase the scope of intensity interferometry beyond measurements of the optical emission region morphology. I will lay out the theory and technical requirements of time-delay intensity interferometry and, time permitting, I will talk about some promising applications, which include astrometric microlensing of stars and quasar images, binary-orbit characterization, exoplanet detection, Galactic acceleration measurements and calibration of the cosmic distance ladder, all at unprecedented relative astrometric precision.

Zoom Link: TBD

The angular resolution of a stellar interferometer, as for a single telescope, becomes better at smaller wavelengths and larger baselines. The goal for ground detectors would then be optical interferometers with baselines as long as the Earth’s diameter. The latter goal has been achieved in radio, but it becomes prohibitive in the optical, as the electromagnetic field oscillates too rapidly to record and analyze directly over km-long baselines. Intensity interferometry relying on second-order correlations can make this possible: rather than the amplitude and phase of incoming light, we need only count photons. This technique has a long history and to date the best measurements of nearby stellar radii, dating back to the 1950s. Its main limitations are the need for very bright sources and its narrow field of view, restricting kilometer-long baselines to sources only a few μas away. In this talk, I will propose an optical-path modification of astronomical intensity interferometers, which introduces an effective time delay in the two-photon interference amplitude, splitting the main intensity correlation fringe into others at finite opening angles, allowing for differential astrometry of multiple compact sources such as stars or quasar images. Together with the exponential progress in the field of single photon detection, such a modification will immensely increase the scope of intensity interferometry beyond measurements of the optical emission region morphology. I will lay out the theory and technical requirements of time-delay intensity interferometry and, time permitting, I will talk about some promising applications, which include astrometric microlensing of stars and quasar images, binary-orbit characterization, exoplanet detection, Galactic acceleration measurements and calibration of the cosmic distance ladder, all at unprecedented relative astrometric precision.

Zoom link:  https://pitp.zoom.us/j/92041231568?pwd=cWo2c0hwTEdmOTRCc042SHNxRWw5UT09

For more than a decade, enigmatic extragalactic flashes called fast radio bursts (FRBs) have defied a definitive explanation for their origin. In addition, the unique properties of FRBs make them promising probes of both cosmology and the distribution of gas on intergalactic scales. The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is the only radio telescope capable of instantaneously observing hundreds of square degrees with the sensitivity of a 100-meter scale aperture. As a result, its transient search instrument, CHIME/FRB, has detected thousands of FRBs, increasing the known sample by an order of magnitude. I will give an overview of CHIME/FRB's most recent results, where observations of particular sources and statistical analyses of the FRB population are starting to reveal the nature of this mysterious phenomenon. I will then describe an effort to augment CHIME/FRB's capabilities by adding Outrigger telescopes, which will be located across North America and will precisely localize FRB sources using very long baseline interferometry. The resulting large sample of localized FRBs will allow for detailed measurements of the large-scale distribution of baryons in the universe, providing precise constraints on feedback processes in galaxy evolution.

Zoom link:  https://pitp.zoom.us/j/93157775112?pwd=bzZDMVc3VnF1WHZzeVlLOTdDZmtQQT09

Inflationary cosmology is notoriously past geodesically incomplete in many situations. However, it is generally unknown whether the geodesic incompleteness implies the existence of an initial spacetime curvature singularity or whether the spacetime may be extended beyond its null past boundary. In homogeneous and isotropic cosmology with flat spatial sections, we classify which past inflationary histories have a scalar curvature singularity and which might be extendible/non-singular. We derive rigorous extendibility criteria of various regularity classes for quasi-de Sitter spacetimes that evolve from infinite proper time in the past. Beyond homogeneity and isotropy, we show that continuous extensions respecting the Einstein field equations with a perfect fluid must have the equation of state of a de Sitter universe asymptotically. An interpretation of our results is that past-eternal inflationary scenarios are most likely physically singular, except in very special situations.

Zoom link:  https://pitp.zoom.us/j/98334550627?pwd=UnR3eUxZRFZpeEZoOGEwNkJuc0M0UT09

One of the exciting new frontiers in cosmology and structure formation is the Epoch of Reionization (EoR), a period when the radiation from the early stars and galaxies ionized almost all gas in the Universe. This epoch forms an important evolutionary link between the smooth matter distribution at early times and the highly complex structures seen today. Fortunately, a whole slew of instruments that have been specifically designed to study the high-redshift Universe (JWST, ALMA, Roman Space Telescope, HERA, SKA, CCAT-p, SPHEREx), have started providing valuable insights, which will usher  the study of EoR into a new, high-precision era. It is, therefore, imperative that theoretical/numerical models achieve sufficient accuracy and physical fidelity to meaningfully interpret this new data. In this talk, I will introduce the THESAN simulation framework that is designed to efficiently leverage current and upcoming high redshift observations to constrain the physics of reionization. The multi-scale nature of the process is tackled by coupling large volume (~100s Mpc) simulations designed to study the large-scale statistical properties of the intergalactic medium (IGM) that is undergoing reionization, with high-resolution (~ 10 pc) simulations that zoom-in on single galaxies which are ideal for predicting the resolved properties of the sources responsible for it. I will discuss applications from the first set of papers, including predictions for high redshift galaxy properties, the galaxy-IGM connection, Ly-α transmission and back reaction of reionization on galaxy formation. I will finish by highlighting recent improvements to the model and proposed future work.

Zoom link:  https://pitp.zoom.us/j/94595023029?pwd=b0c0MVZHZ01rQnprS1ZCSzVIZktqUT09

Abstract: TBD

Zoom Link: https://pitp.zoom.us/j/91979071868?pwd=Z0VWSFhJTEJFWkZRbzBtcWdtV0Ezdz09

A dark energy-like component in the early universe, known as early dark energy (EDE), is a proposed solution to the Hubble tension. In this talk, I will describe how a frequentist profile likelihood yields important complementary information compared to a Bayesian MCMC analysis. While in an MCMC analysis, the EDE model is clearly disfavoured by Cosmic Microwave Background and large-scale structure data, a profile likelihood analysis prefers consistently larger amounts of EDE and with that a Hubble constant consistent with the SH0ES measurement for the same data sets. The difference between MCMC and profile likelihood can be explained by prior volume effects in the MCMC analysis. I will discuss how frequentist and Bayesian methods can give important complementary information in the context of beyond-LCDM models.

Zoom link:  https://pitp.zoom.us/j/97257428766?pwd=ckx4ajRsQVRQUnpXaGEvZEtEWW9ldz09

Optimal extraction of the non-Gaussian information encoded in the Large-Scale Structure (LSS) of the universe lies at the forefront of modern precision cosmology. In this talk, I will discuss recent efforts to achieve this task using the Wavelet Scattering Transform (WST), which subjects an input field to a layer of non-linear transformations that are sensitive to non-Gaussianity in spatial density distributions through a generated set of WST coefficients. In order to assess its applicability in the context of LSS surveys, I will present the first WST application to actual galaxy observations, through a WST re-analysis of the BOSS DR12 CMASS dataset. After laying out the procedure on how to capture all necessary layers of realism for an application on data obtained from a spectroscopic survey, I will show results for the marginalized posterior probability distributions of 5 cosmological parameters obtained from a WST likelihood analysis of the CMASS data. The WST is found to deliver a substantial improvement in the values of the predicted 1σ errors compared to the regular galaxy power spectrum, both in the case of flat and uninformative priors and also when a Big Bang Nucleosynthesis prior is applied to the value of ω_b. Finally, I will discuss ongoing follow-up work towards applying this estimator to the next generation of spectroscopic observations to be obtained by the DESI and Euclid surveys.

Zoom link:  https://pitp.zoom.us/j/96291506998?pwd=TVVFYnNIQ1F0cktna000cUp3SU1kQT09

The dark universe may host physics as rich and complex as the visible sector, but the only guaranteed window to the dark sector(s) is through gravity. If the dark matter has a dissipative self-interaction, dark gas can cool and collapse to form compact object whose mergers may be accessible to LIGO. The mass spectrum of the merging compact objects encodes fundamental physical information--a purely gravitational probe of dark matter microphysics.
In this talk, I will present our work to forward-model the gas collapse process in the "atomic dark matter" model, beginning with a retelling of the standard cosmological history including this new ingredient and culminating in a description of the fragmentation scale of the dark gas.

Zoom link:  https://pitp.zoom.us/j/99141938599?pwd=T0I1d1A5R0JBNWFlSHlCREl5dElTUT09

Scientific programs involving joint analyses of different tracers of large-scale structure and CMB are increasingly gaining attention as they often increase the prospects to detect and characterise new signals by reducing systematics, cancelling cosmic variance and breaking degeneracies. In this talk, I will demonstrate how these programs will provide the most precise tests of fundamental physics by measuring galaxy peculiar velocity throughout cosmic time, opening new and unique windows into unexplored epochs of structure formation such as the epoch helium reionization, making pioneering first detections of multiple CMB signals and reducing the confusion effects from scattering and lensing on the CMB, while not requiring new experiments other than those being built or proposed.

Zoom link:  https://pitp.zoom.us/j/98508740176?pwd=a3BUc1lpZi82c0R0SkJyd1FPRFRUZz09