My PhD research focuses on early universe cosmology. Here is my INSPIRE-HEP author page.
We have ample indirect evidence for the existence of dark matter, but actually finding it has proven difficult. We haven’t found any evidence of it in direct detection experiments or colliders. The third possible avenue of detection is indirect: seeing the products of annihilating dark matter.
Because we haven’t seen any dark matter, there is interest in theories in which this dark matter lives in a “hidden sector”, decoupled from the visible sector of Standard Model particles. Although this sector is hidden, it can leave observable gravitational signatures in certain scenarios. One such class of scenarios involves early matter domination.
Standard cosmology tells us that the universe was radiation dominated after inflation until the matter-dominated epoch began. In fact, the history of the universe between inflation and Big Bang Nucleosynthesis (BBN) is poorly understood. The universe could have seen deviations from radiation domination in this phase, including possible epochs in which something like matter dominates the energy density of the universe.
In many hidden sector theories, heavy particles in the hidden sector can come to dominate the early universe before BBN, causing an era of early matter domination. This kind of era causes the rapid growth of matter perturbations, leading to the collapse of dense small-scale structures much earlier than they would form in vanilla cosmologies without early matter domination. These dense structures can massively enhance the dark matter annihilation signal and also be detected via their impact on pulsar timing arrays and the microlensing of stars. My research deals with modeling the properties and distributions of these “microhalos” to connect potential observations to these hidden sector scenarios.
Publications / Preprints
The Matter Power Spectrum Resulting from Early Matter Domination
In my first paper, I considered an early matter-dominated era (EMDE) caused by a massive particle in the hidden sector. I calculated how this particle transitions from being relativistic to nonrelativistic as the hidden sector temperature decreases in time. As a result of the early relativistic pressure of this particle, density perturbations experience inhibited growth, causing a small-scale cut-off on the matter power spectrum. This cut-off determines the formation times, central densities and scales of the earliest forming microhalos, controlling the prospects of detection and the boost to the dark matter annihilation signal. The paper provides transfer functions for the easy calculation of this cut-off and discusses possible allowed regions of the EMDE parameter space in addition to avenues of observation of these microhalos. Published in JCAP.
Simulations of Gravitational Heating
In some EMDE histories, the power spectrum of matter is enhanced enough to cause the creation of microhalos during the EMDE. When the EMDE ends, the hidden sector particle causing it decays away, destroying these microhalos. The dark matter particles residing in these halos are suddenly bereft of gravitational support and shoot out at high speeds in random directions. This halo formation-evaporation “heats up” the dark matter. We simulated this process using a modified version of the N-body simulation code GADGET-2 (thanks to Sten Delos) and furnished the form of the free-streaming cut-off that develops as a result of this heating. Submitted to JCAP.
Cosmology of Hidden Sector Dark Matter (3rd author)
Using work from my first paper, we laid down the time-evolution equations for a general dark matter species in the hidden sector that is initially relativistic and becomes cold later. Submitted to JCAP.
Ongoing Projects
Formation of Prompt Cusps in EMDE Cosmologies
(with Sten Delos)
There is evidence that the first bound structures, or cusps, form directly from the collapse of the peaks in the dark matter field. These cusps have a sharp power-law density profile, different from the Navarro-Frenk-White profiles of bigger, later-forming halos that are seen in N-body simulations. This work focuses on the distribution of cusps resulting from enhanced power spectra in EMDE cosmologies, with an aim to assess the impact of the cusps on the dark matter annihilation signal.
Simulations of Microhalo (Re)Formation
(with Sten Delos)
Although the aforementioned gravitational heating suppresses the matter power spectrum after the EMDE ends, some of the bump in the power spectrum caused by the EMDE still remains intact. This bump causes the (re)formation of microhalos as early as the epoch of matter-radiation equality, or even before that in some cases. In my second paper, we explored the statistics of these (re)forming structures using Press-Schechter theory, but a detailed investigation of their formation requires N-body simulations, which is the aim of this project.
Painting Halos onto Galaxies
(with Jordan Krywonos and Matthew Johnson at PI)
Can we use galaxy properties along with machine learning techniques to infer the dark matter density field?
Himanish Ganjoo 