## Thesis etd-11252015-141438 |

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Thesis type

Tesi di dottorato di ricerca

Author

PIZZA, LIBERATO

URN

etd-11252015-141438

Thesis title

The role of Gravity in the comprehension of the early and late time Universe

Academic discipline

FIS/02

Course of study

SCIENZE DI BASE

Supervisors

**tutor**Prof. Strumia, Alessandro

**tutor**Prof. Lambiase, Gaetano

Keywords

- agravity
- cosmografia
- cosmologia
- cosmology
- dark energy
- dark matter
- dynamic generation of the Planck mass
- energia oscura
- f(R)
- fenomenologia
- generazione dinamica della massa di Panck
- gravità
- gravità modificate
- heavy neutrino
- indice spettrale
- inflation
- inflazione
- leptogenesi
- leptogenesis
- materia oscura
- modified gravity
- modified thermal history
- multifield inflation
- neutrino pesante
- PAMELA
- phenomenology
- Planckion
- reconstruction
- reheating
- relic abundance
- scalar-tensor theory
- spectral index
- storia termica modificata

Graduation session start date

26/12/2015

Availability

Full

Summary

In this thesis we study modiﬁed theories of gravity as a solution to longstanding open problems in Cosmology. In particular our main contribution relies in the determination of a viable form of f(R) that allows us to explain the current accelerated phase of the Universe. To set the initial conditions on the f(R) functions, we involve the use of the so called cosmography of the Universe, i.e. the technique of ﬁxing constraints on the observable Universe by comparing expanded observables with current data. This powerful approach is essentially model independent and correspondingly we got a model independent reconstruction of f(z) classes within the interval z ∈ [0,1]. To allow the Hubble rate to evolve around z ≤ 1, we considered three relevant frameworks of eﬀective cosmological dynamics, i.e. the ΛCDM model, the CPL parametrization and a polynomial approach to dark energy. Finally, cumbersome algebra permits us to pass from f(z) to f(R) and the general outcome of our work is the determination of a viable f(R) function, that eﬀectively describes the observed Universe dynamics. Furthermore, we study a form of f(R) that can justify the relic Dark Matter abundance and the asymmetry between matter and antimatter. In particular we study two models of f(R) theories of gravitation that, with the opportune choice of free parameters, introduce a little perturbation to the scale factor of the Universe in the radiation dominated (RD) phase predicted by General Relativity (GR), i.e., a(t) ≈ t^(1/2). This little perturbation generates a Ricci scalar diﬀerent by zero, i.e., R 6= 0 that reproduces the correct magnitude for the asymmetry factor η computed in the frame of the theories of gravitational baryogenesis and gravitational leptogenesis. Besides, the evolution of relics particles (WIMPs) in the f(R) context is analysed in order to explain PAMELA data. Furthermore, theories where the Planck scale is dynamically generated from dimensionless interactions are studied here in the context of the inﬂationary scenario. We ﬁrst study the minimal single-ﬁeld realisation in the low-energy eﬀective ﬁeld theory limit, ﬁnding the predictions ns ≈ 0.96 for the spectral index and r ≈ 0.13 for the tensor-to-scalar ratio, which can be reduced down to ≈ 0.04 in presence of large couplings. Next we consider agravity as a dimensionless quantum gravity theory ﬁnding a multi-ﬁeld inﬂation that converges towards an attractor trajectory that predicts ns ≈ 0.96 and 0.003 < r < 0.13, interpolating between the quadratic and Starobinsky inﬂation.

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