## Probing of dark energy properties in the Universe using astrophysical observations

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##### Date

30/11/2017##### Author

Smer Barreto, Vanessa Stephanie Emilia

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##### Abstract

The astrophysical data of the last two decades have allowed cosmologists to
conclude that the present Universe is accelerating. The research carried out to
find the origin of this phenomenon has led to the creation of a vast number of
dark energy and modified gravity theories, of which the simplest is the ˄CDM
model. The latter is, however, plagued with very difficult problems awaiting a
solution. The work here presented seeks to contribute to the discussion of the
possible explanation for the Cosmos' acceleration and other important questions
in modern cosmology using the newest astrophysical observations available.
This thesis starts by exploring a dark energy model dubbed thawing quintessence
which is characterised by allowing a non constant ratio of pressure to density
for dark energy that is however still close to -1 for most of the cosmological
evolution, shifting away from this value when the domination of the radiation and
matter components fades away. The findings are the most up-to-date constraints
for which this model gives a viable theory for dark energy, including a bound
on the equation of state at present of w < -0:88. This exact approach was
contrasted with the use of an approximate equation-of-state parametrisation for
thawing theories. The analysis also includes different parametrisation choices,
and comments on the accuracy of the constraints imposed by CMB anisotropies
alone.
Next, the cosmology of hybrid metric-Palatini gravity is presented. This is a type
of Modified Gravity theory in which the Lagrangian density for the gravitational
action is a function of the Ricci scalars of both the connection and the metric.
The background evolution of two models of this kind is examined explicitly
showing the recovery of standard General Relativity at late times. The maximum
deviation from the gravitational constant G at early times is constrained using a
combination of geometrical data, finding it to be around 1%.
A designer scenario, also introduced under the hybrid metric-Palatini formulation,
is then used to explore to what extent early modifications of gravity, which
become significant after recombination but then decay towards the present, can
be constrained by current and future cosmological observations. This model
is embedded in the effective field theory description of Horndeski scalar-tensor
gravity with an early-time decoupling of the gravitational modification.
Applying cosmological data, the constraints on the early-time deviations from
General Relativity are obtained. These are dependent on the redshift at which
the oscillations in the slip between the gravitational potentials are turned on. For
zon = 1000, the deviation from Einstein's theory is ≤ 10-2 with 95% confidence.
An explanation of the effect that these divergences have on the CMB power
spectrum are discussed, as well as the effect that future 21 cm survey data will
have on this study.
The last part of this work is a move towards inflation, the early epoch of
accelerated expansion undergone by the Universe. Here a parametrisation of
the acceleration trajectory is investigated with the aim of measuring the rolling
of the inflaton corresponding to the value of the tensor-to-scalar ratio r to be
compared with future observations.
Considering five ln ε amplitudes and 14 e-foldings, it was found that the posterior
distribution of (r,∆Φ) is in very good agreement with Lyth's bound. The analysis
included a histogram depiction of the latter result, from which later a minimum
constraint on ∆ϕ for each of the bins was found. These outcomes constitute the
intermediate step of this project which will be made more accurate by extending it
to ~ 50 e-folds, a larger set of cosmological parameters and observational bounds
that are restrictive on small scales.