## All Dissertations

#### Title

Unveiling the Cosmic History of Light

8-2019

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Physics and Astronomy

#### Committee Member

Marco Ajello, Committee Chair

#### Committee Member

Gerald A Lehmacher

#### Committee Member

Dieter H Hartmann

#### Committee Member

The Universe was created with a Big Bang $\sim$13.7 billion years ago while the first stars and galaxies came after $\sim$400 million years. All the light that was ever emitted in the Universe at ultraviolet, optical, and infrared wavelengths from the period of those first stars till present date makes up the Extragalactic Background Light (EBL). This diffuse background interacts with photons emitted by distant high energy sources, in the GeV and TeV regime, via photon-photon interaction annihilating the high energy photon and producing an electron-positron pair. This gives researchers a powerful and highly effective technique to study the EBL by analyzing the imprint it leaves on the spectra of distant gamma-ray sources. For my PhD thesis project, I made use of this method to study the attenuated spectra of two major high energy sources - gamma ray bursts and active galactic nuclei - observed using the {\it Fermi}-Large Area Telescope and Cherenkov Telescopes. While similar studies have been performed in the past, most of the derived measurements came from just scaling the optical depth due to the EBL according to the observed spectra, making the estimated EBL spectral intensity uncertain. To tackle this, we have recently developed a dedicated technique which deconvolves the EBL into smaller energy and redshift bins. Using this technique along with an extensive GeV+TeV source sample, we were able to obtain the first homogeneous set of measurements of the EBL spectral intensity covering the UV-IR wavelengths. Additionally, we used this result to investigate several, still debated, astrophysical topics like measurements of the star formation history of the Universe, Hubble constant ($H_0$) and matter density ($\Omega_m$).