Abstract
The nature of Dark Energy is one of the most open and fascinating questions in all of science. To add to the intrigue, there are questions about whether the cosmological standard model, ΛCDM, is truly the concordance model that it previously appeared to be, as tensions between low and high redshift data persist, particularly with H0 and σ8. This, in addition to the multiple theoretical issues (some old, like the cosmological constant problem and some new like the swampland conjecture) suggest, that a new cosmological model may be needed.One interesting family of models are interacting Dark Energy models. These models allow an interaction between Dark Energy and Dark Matter, letting energy and momentum be exchanged. We will focus on a particular subset of these models; where the interaction is only in the form of momentum exchange. These models come with a rich phenomenology which we explore within this thesis. We initially concentrate on how the coupling effects the sound speed and how this in turn effects the evolution of the Dark Energy perturbations.
To further understand these models, we compare them to observational data from both high and low redshift observations. We find that, when only CMB data is used, interacting models are comparable with ΛCDM. When we include low redshift data, particularly the Planck cluster counts from the Thermal Sunyaev Zel’dovich effect, we see that the data now prefers a lower value of σ8 than ΛCDM can accommodate. This is seen explicitly in the improvement in X2 for the coupled models when compared to ΛCDM. Additionally, one of our chosen interacting models does a good job of fitting the CMB data and the data from cluster counts simultaneously.
| Date of Award | Apr 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Alkistis Pourtsidou (Supervisor), Robert Crittenden (Supervisor), Kazuya Koyama (Supervisor) & Roy Maartens (Supervisor) |