Light-front (LF) form of quantum field theory has a few serious conceptual advantages, but also some puzzling features. In my talk, I shall suggest potential solutions to some of the LF "failures". First, presence of the vacuum "bubbles" in the Hamiltonian LF perturbation theory will be clarified. The resulting vacuum amplitudes are non-vanishing despite the notorious "triviality" of the LF vacuum and agree with the results of Feynman perturbation theory evaluated in terms of the LF variables. Next, I will show that some apparent consistency problems of the LF theory can be solved if mathematically adequate treatment is adopted. Contradictions related to the LF restriction of the two-point function are removed if the scalar field contains regularization terms in the plane-wave factors. As a consequence, the correct equal-LF time commutators are reproduced from the Pauli-Jordan function, and the sign function present in them is naturally replaced by a function suppressed for large x^- - values. The value of the two-point function at coinciding points is also correctly obtained in the Hamiltonian (”on shell”) formulation. Then a novel look at the LF zero modes will be described, based on a consistent quantization of two-dimensional massless LF fields. Finally, the operator solution of the Thirring-Wess model in the light-front formulation will be presented.

Since direct detection experiments over the last few decades have been unsuccessful in finding dark matter consistent with a WIMP undergoing thermal freeze-out, we should re-examine whether this is indeed the right way to think about dark matter. One can make two possible departures from this paradigm — either by changing the mechanism that determines the DM density or by changing the nature of DM as an elementary particle. Often, when we move away from the WIMP paradigm, the direct detection cross section is too small to be observable and possibly production at colliders becomes our first line of attack. We will examine the generic signatures of both these possibilities at the LHC.

It is well understood the light element lithium in stars generally gets destroyed. Thus, stars are known as lithium sinks. Contrary to this general understanding a few stars show large amount of lithium. In some rare cases this amount exceeds the present interstellar value of A(Li)= 3.2 dex by about a factor of 10. This has been a nagging problem in stellar astrophysics for more than 4 decades since its first discovery in early 1980s. In this discuss the problem and observational efforts leading to evidence of large lithoum origin in red giants.

In 2015 the LIGO collaboration announced the first direct detection of gravitational waves from the coalesce of two intermediate mass black holes, ushering in a new era in direct observations of extreme gravitational environments. Since then, laser interferometers have detected 100s more binary systems. The next gravitational frontier is to use observations of pulsars to form what is effectively a galactic sized gravitational wave detector. These pulsar timing arrays (PTAs) have been in operation for nearly two decades in North America, Europe, and Australia. I will discuss the basic physics behind PTAs and some of the key details of their data analysis. If there is time, I will discuss ways in which we can use PTAs to detect physics beyond the standard model.

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.

Large self interactions between dark matter particles alter the predicted properties of dark matter halos and may help address small-scale structure issues, while maintaining the successes of standard cold dark matter at large scales. Self interactions allow for efficient heat transfer within a halo, which can be modeled using a gravothermal fluid approximation. In this talk, I will discuss how the choice of the particle physics model for self interactions, as well as the environment of the halo, impacts the phases of halo evolution.

Upcoming surveys of cosmic structures will probe scales ranging from the nonlinear regime to scales close to the cosmological horizon. This opens up the possibility of probing the LCDM model, as well as early universe scenarios with non-Gaussianity. Modeling the galaxy bispectrum is challenging, as it involves general relativity, radiation, and large nonlinearities. In this talk, I will present a numerical modeling of the matter bispectrum on the light cone including relativistic and radiation effects. This is a crucial step towards modeling the observable bispectra, i.e. the weak lensing bispectrum and the galaxy bispectrum.

While the standard $\Lambda$CDM model provides an astonishing fit to cosmological data, there is a 4-6 $\sigma$ tension in the Hubble constant, $H_0$, between the value inferred from early-Universe observables, Planck CMB anisotropy spectra, and the value determined by local measurements from SH0ES collaboration using Type 1a SN calibrated with Cepheid variables. As an effort to solve this tension, we develop a method of searching for data-driven solutions to the Hubble tension given cosmological dataset, based on the standard Fisher bias formalism. As an example, we apply our method to time-varying electron mass modifying cosmological recombination history. With Planck CMB data, we find a solution entirely resolving the Hubble tension. However, once BOSS BAO and PantheonPlus uncalibrated SN data are included, it is not possible to fully solve the tension. The method we develop in this work can be a useful tool to search for any other possible extensions to the $\Lambda$CDM model, and is expected to inspire a model-building effort from the cosmology and particle physics community.