Physics and Astronomy
Student Seminar (PASS)
Student Seminar (PASS)
2022-2023
(archived, discontinued)
(archived, discontinued)
| Home | Registration | Program | Directions |
| January 20 | 04:00 pm | Physics 3027 | Impact of Dark Matter annihilation to UV photons on Cosmic Recombination and First Stars |
| Yash Aggarwal (University of California, Riverside) | |
| WIMP-like Dark Matter particles are still the most promising candidates for the invisible matter. In a segment of these models, Dark Matter annihilates to a large multiplicity of soft Standard Model particles, which may have vastly different cosmological implications than the commonly studied s-wave annihilation. In this talk, we consider WIMP-like Dark Matter that annihilates to UV photons between 10-15 eV, the energy range to which most atoms and molecules are sensitive. We study the implications of this energy injection on Cosmic Recombination and the first star-forming Halos. We find that perturbations to Cosmic Recombination are sensitive to the annihilation spectrum, and they can be measured by Planck. We also find that Dark Matter annihilation can alter the dynamics of first star-forming halos, enough to lead to a runaway collapse of gases into a Black Hole, a scenario called Direct Collapse Black Holes. |
| February 1 | 04:00 pm | Physics 3027 | Exoplanets, new targets for Dark Matter search |
| Mehrdad Phoroutan Mehr (University of California, Riverside) | |
| Exoplanets can be new targets to discover Dark Matter. Exoplanets can capture Dark Matter, and the captured Dark Matter particles can annihilate and deposit the annihilation energy within them. The annihilation energy can lead to a change in the Exoplanets' temperature. |
| October 14 | 04:00 pm | Virtual | MF-Box: Multi-fidelity emulation for simulations from different box sizes using a graphical Gaussian process |
| Ming-Feng Ho (University of California, Riverside) | |
| Bayesian inference is necessary for any scientific discipline, especially for explaining cosmological and astrophysical observations. However, Bayesian inference also introduces the computational burden of \(>10^6\) likelihood evaluations on the theoretical model. Emulation is the current only way to perform Bayesian inference using accurate numerical simulations. By approximating the input/output relationship of numerical simulations using a machine learning model, emulators work as cheap surrogates, allowing fast likelihood evaluations suitable for inference using Markov Chain Monte Carlo (MCMC). Nevertheless, as the experiments become highly precise in the future, the training data for an accurate emulator will also become impossible to generate, making the emulator approach impractical. In this talk, I will present MF-Box, a multi-fidelity emulation framework that allows us to combine cosmological simulations from different resolutions and different box sizes. Multi-fidelity emulation augments expensive high-resolution simulations with many cheap low-resolution simulations, minimizing the computational budget. MF-Box extends our previous multi-fidelity emulator, allowing the machine learning model to use simulations from different box sizes to accelerate the training via transfer learning. I will also present a theoretical approach to analyzing the emulator's performance and determining the optimal budget allocation. |
| October 28 | 04:00 pm | Virtual | Detecting CIV Absorption Lines in SDSS by Gaussian Process |
| Reza Monadi (University of California, Riverside) | |
| Metal absorption lines in quasar spectra give us information about the history of the Universe. The spatial distribution of CIV (triply ionized carbon) absorbers in the Intergalactic Medium (IGM) tells us about the effect of feedback and star formation. IGM clouds with CIV may be the main ingredient of future galaxies. CIV is a doublet outside of Lyman-alpha forest which is good for automatic detection. Here we introduce the method and preliminary results of our CIV catalog, which will be the largest one so far. |
| November 4 | 04:00 pm | Virtual | Layer dependence of magneto-transport properties of 2D vdW ferromagnets Fe\(_5\)Ge\(_2\)Te\(_2\) and Fe\(_3\)GeTe\(_2\) |
| Wei-Cheng Liao (University of California, Riverside) | |
| Two-dimensional (2D) metallic ferromagnets, including Fe\(_5\)Ge\(_2\)Te\(_2\) (FG2T) and Fe\(_3\)GeTe\(_2\) (FGT), exhibit a variety of fascinating physical properties that have attracted a lot of attention in the spintronics community. We have fabricated FG2T and FGT nanodevices with a range of thicknesses down to monolayers by exfoliating flakes from crystals grown by solid-state reaction. By performing transport measurements at different temperatures with the external field oriented out-of-plane, we obtained the longitudinal resistivity, anomalous Hall resistivity, Curie temperature, and Kondo temperature for all devices. Discussions of the transport data will be presented. |
| June 22 | 04:00 pm | Phase transition, and universality, and the approach of RG flow at critical point |
| Pak Kau Lim (University of California, Riverside) | |
| I'd like to review a series of background materials. These are broader topics that are either related to my research or materials that I found interesting but have yet to encounter. I will first focus on phase transition, universality, and the approach of RG flow at critical point. The topic will be divided into two talks. |
| June 29 | 04:00 pm | Applications of 3D Ly-\(\alpha\) tomography |
| Mahdi Qezlou (University of California, Riverside) | |
| Ly-\(\alpha\) tomography surveys have begun to produce three-dimensional (3D) maps of the intergalactic medium (IGM) opacity at \(z \sim 2.5\) with Mpc resolution. These surveys provide an exciting new window to discover and characterize high-redshift overdensities, including the progenitors of today's massive groups and clusters of galaxies, known as protogroups and protoclusters. In this talk, I will introduce our novel method for delineating the boundaries of structures detected in 3D Ly-\(\alpha\) flux maps by applying classical image segmentation techniques. Moreover, I will argue how the Ly-\(\alpha\) tomography is complementary to the conventional galaxy surveys by discussing an interesting population of observed protoclusters, in Lyman-\(\alpha\) Tomographic IMACS Survey (LATIS), which contain surprisingly fewer UV-bright galaxies compared with the dark matter halos in mock-observed simulations. Finally, I will discuss other potential applications of these tomography surveys, such as the synergies with line intensity map (LIM) experiments. |
| July 6 | 04:00 pm | === Cancelled === |
| Reza Monadi (University of California, Riverside) | |
| July 13 | 04:00 pm | Back projection methods for the refinement of SARS-Cov-2 sequence data |
| Elizabeth Finney (University of California, Riverside) | |
| As the SARS-CoV-2 virus continues to evolve, viral sequencing data has extensively been used to identify novel genome mutations. Reliable genomic surveillance data is needed in order to investigate viral variants, but important epidemiological features are either absent from the data, or inherently un-observable. In this talk I will explain how we apply a deconvolution method from epidemiology, known as back-projection, to these missing data problems that arise in our SARS-Cov-2 selection model. This approach can improve the quality of data in regions where sampling is rare or infrequent, and aid in the detection of Covid-19 variants of concern. |
| May 16 | 04:00 pm | Multi-Fidelity Emulation for Cosmological Inference in the Roman Era |
| Ming-Feng Ho (University of California, Riverside) | |
| Next-generation telescopes (e.g., Roman space telescope) will probe the small-scale structures of the Universe with high accuracy. Thus, expensive high-resolution simulations are required to extract cosmological information at these non-linear scales, making Bayesian parameter inference impractical for future surveys. One solution is multi-fidelity emulation, which uses simulations of varying qualities (or resolutions) to train an accurate emulator at a much lower cost. We present the first implementation of multi-fidelity emulation in cosmology, where we use machine learning to model summary statistics of varying accuracies using a sequence of Gaussian processes. We show two applications of our emulation framework: the non-linear matter power spectrum and the Lya flux power spectrum. We demonstrate that our multi-fidelity emulation can achieve percent accuracy with only a few high-resolution simulations and outperform single-fidelity emulators. Our proposed multi-fidelity emulator shows a new way to predict non-linear scales, making the emulation development substantially more practical for future cosmological inference problems. |
| May 23 | 04:00 pm | Probing Cosmic Reionization Using Hydrogen Absorption Lines |
| Yongda Zhu (University of California, Riverside) | |
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Reionization is the last major phase transition in the history of the Universe. During the epoch of reionization, which is widely believed to occur between roughly 150 million and one billion years after the Big Bang (redshift 20 ≳ z ≳ 6), neutral hydrogen in the intergalactic medium (IGM) was ionized by ultraviolet (UV) photons emitted by the first generations of stars and galaxies. The timing of reionization is therefore essential for us to understand the formation and evolution of the first galaxies. Large-scale fluctuations in IGM Lyman-alpha opacity at z < 6 suggest that reionization may have ended later than is normally assumed. Alternatively, the opacity fluctuations may be driven by variations in the ionizing UV background that reionization has ended. To resolve this dilemma, we can use hydrogen absorption lines in quasar spectra to probe the IGM during the reionization epoch. In this talk, I will present the novel investigations of the IGM over 5 < z < 6 using dark gaps in the Lyman-ɑ&Lyman-β forest. I will show our latest results and discuss the implications for reionization. |
| May 31 | 04:00 pm | Seeding Supermassive Black Holes with Self-Interacting Dark Matter |
| Wei-Xiang Feng (University of California, Riverside) | |
| Observations show that supermassive black holes (SMBHs) with a mass of \(\sim 10^{9} M_{\odot}\) exist when the universe is just \(6\%\) of its current age. Recently, we have proposed a scenario where a self-interacting dark matter halo experiences gravothermal instability and its central region collapses into a seed black hole. Our results indicate that self-interacting dark matter can provide a unified explanation for diverse dark matter distributions in galaxies today and the origin of SMBHs at redshifts \(z \sim 6–7\). If time permits, I will talk about the other related topics of BHs. |
| June 6 | 04:00 pm | Review on the Renormalization Group Approach |
| Pak Kau Lim (University of California, Riverside) | |
| Before shifting my focus to my works, I'd like to review a series of background materials until mid-summer. These are broader topics that are either related to my research or materials that I found interesting but have yet to encounter. In my upcoming PASS talk on Monday, I will discuss the renormalization group approach in lower dimension condensed matter theories. |