Abstract
The ΛCDM model has celebrated great success as the standard cosmological model over the past decades. However, with the increasing precision of cosmological measurements, various observational challenges have arisen within the context of ΛCDM. The most pressing issue is the 5σ discrepancy in the inferred value of the Hubble parameter between local distance measurements and the cosmic microwave background (CMB). If the mismatch between these probes does not arise from systematic errors within the datasets, the so-called Hubble tension could indicate the need for a reviewed standard model description incorporating new physics. In this thesis, I explore Early Dark Energy (EDE) as a potential solution to this tension.I begin by providing a brief overview of the fundamental pillars of the concordance model before addressing the observational challenges that ΛCDM faces and how these challenges can inform the search for extended models. EDE allows for higher H0 values than ΛCDM when combining CMB and Supernovae data by introducing an additional scalar degree of freedom around the time of recombination. Galaxy clustering data has the potential to constrain EDE effectively and might be crucial in determining whether EDE remains a valid solution to the Hubble tension. However, there is currently no consensus in the literature on whether EDE can adequately fit cosmological data from both the CMB and large-scale structure (LSS) at the same time. This is mainly due to possible projection effects within the analysis, which complicates cosmological parameter interpretation.
I present a comprehensive full shape analysis of EDE, incorporating constraints from BOSS and eBOSS data. My analysis evaluates the effectiveness of different priors in mitigating projection effects and explores the interplay between EDE parameters and the data. My findings indicate that while EDE can alleviate the Hubble tension, significant projection effects persist, making the inclusion of additional external data necessary. Stage-IV surveys like DESI and Euclid will provide measurements an order of magnitude beyond current capabilities, potentially playing a decisive role in determining the viability of EDE as a solution to the Hubble tension.
To this end, I discuss constraints on the Hubble parameter from the first data release of DESI. DESI’s first-year data, with over six million spectroscopic redshifts, provides the most precise mea- surement of the Hubble constant independent of CMB anisotropies. I assess the internal consistency of DESI Baryon Acoustic Oscillation (BAO) measurements and their agreement with previous SDSS results, highlighting their implications for the Hubble tension. My results show that LSS data can play a pivotal role in the discussion of the Hubble tension. Stage-IV datasets will provide the un- precedented precision needed to confirm or reject EDE as a viable model, ultimately guiding our understanding of the Universe’s expansion history.
| Date of Award | 25 Nov 2024 |
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| Original language | English |
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| Supervisor | David Bacon (Supervisor), Kazuya Koyama (Supervisor) & Robert Crittenden (Supervisor) |