# name
Implications of Dynamical Dark Energy in the expansion of the Universe and the Structure Formation

# metadata.dc.contributor.other[].value
Universitat de Barcelona. Departament de Física Quàntica i Astrofísica

# metadata.dc.description.abstract[].value
[cat] En aquesta  tesi s’estudien diferents  models cosmològics, tots ells caracteritzats per considerar, de manera efectiva, una lleu evolució temporal de l’Energia Fosca, en contrast amb l’actual model estàndard de la cosmologia.  El terme  Energia  Fosca s’utilitza  per fer referència a una  misteriosa forma  d’energia que sembla  impregnar  tots  els racons  del Univers  i que provoca  que les galàxies s’allunyin les unes de les altres.  El ritme predit  d’expansió del Univers varia d’un model a un altre així com la quantitat d’estructura observada  i la distribució d’aquesta.
Degut al bon moment de la cosmologia observacional  tenim a la nostre disposició una gran quantitat de dades  que ens permeten  posar  a prova  els diferents  models existents.   Un exemple  d’aquests models, seria el Running  Vacuum  Model (RVM),  que ha estat  estudiat en detall  en aquesta  tesi i que considera una expressió per la densitat d’energia del buit motivada en el context  de les Teories Quàntiques de Camp.   Un altre  exemple  de models cosmològics  serien els anomenats models de camp escalar que suposen  que l’equació d’estat  de l’Energia Fosca, en el moment present,  és lleugerament diferent del valor predit  pel model estàndard.
No noées s’han considerat  models acomodats  dins del marc de la teoria  de la Relativitat General, sinó que també s’han estudiat les prediccions  teòriques del model presentat per Brans i Dicke al 1961 i que resulta  ser el primer  intent d’extensió  de la teoria  d’Einstein.    El model  de Brans  i Dicke està caracteritzat pel fet que la interacció gravitatòria està no només mediada  per un camp tensorial,  sinó també per un camp escalar.
Les prediccions teòriques dels diferents  models estudiats, tant a nivell de background com a nivell de pertorbacions, han sigut contrastades amb les mes recents dades cosmològiques revelant que l'anteriorment esmentada evolució temporal  de l’Energia Fosca ajuda a rebaixar,  de manera considerable, algunes de les tensions  que afecten al model estàndard.  La comparació teoria-observacions s’ha dut  a terme  mitjançant una  rigorosa  metodologia  que involucra  diferents  eines estadístiques. Per  tant les conclusions obtingudes  al llarg d’aquesta  tesi  es basen  en un  procés  robust  i en un estudi  detallat dels diferents  models cosmològics considerants.

# metadata.dc.description.abstract[].value
[eng] The high quality  observations performed  during  the last two decades,  have allowed to demonstrate, with  high confidence range,  that the  Universe  is in expansion  and  to  be more precise in accelerated  expansion.  In order to explain the accelerated  evolution  the name of dark  energy was coined.  It  refers to a some mysterious  form of diffuse energy presumably permeating all corners of the Universe as a whole.  We may say that the canonical  picture  of our Universe defined in the framework of General Relativity, whose field equation  were found by Einstein  in 1917, is built upon the assumption that the observed acceleration  is caused, in fact, by a rigid cosmological constant term  denoted  by Λ. Thanks  to the aforementioned cosmological measurements, we have been able to  pin  down its  value  to  an  impressive  level.  Dark  energy  is not  the  only element,  beyond  the conventional  baryons  and photons,  required  by the observations  since we also need large amounts of what is commonly call as dark matter.  We call such an overall picture  of the Universe the “concordance  (or standard) cosmological model” or simply ΛCDM.

Therefore,  we attribute the  observed  accelerated  expansion  of the  Universe  to  the  existence  of a repulsive  force, exerted  by the  Λ term,  which works against  the  attractive gravitational force and tends to push the clusters of galaxies apart at a speed continuously  increasing with the cosmic expansion.

Throughout this  thesis  a wide variety  of models, beyond  the  standard model have been studied. The corresponding  analyses have been carried out by studying  in detail the theoretical predictions at the background  and perturbation level, with the purpose of testing  them with the large amount of cosmological data  which we currently we have access to.  The ultimate goal is to see if we can detect  signals of new physics  that help to alleviate  some of the  tensions  that affect the  ΛCDM. The  concordance  model,  has  remained  robust  and  unbeaten for a long time  since it  is roughly consistent  with  a large body  of cosmological data.   Because  of this  fact,  it  is not  reasonable  to look for models with a very different behaviour  than  the ΛCDM, but  to study  models that exhibit small departures with respect  to the standard model in key aspects.

We have studied  the Running  Vacuum  Models (RVM) in depth.  They are characterized by having a time-evolving  vacuum  energy  density,  whose functional  expression  is motivated in the  context of Quantum Field Theory  in curved  space-time.  It is fundamental that its expression  contains  a constant term,  which mimics the standard behaviour  in order to first generate  the transition from a decelerated  to an accelerated  Universe and to ensure that the fit of the structure formation  data is not ruined.

We have  also studied  the  Peebles  & Ratra model,  which  is a particularly successful scalar  field model φCDM for which the potential takes the form V (φ) ∼ φ−α . The dimensionless parameter α encodes the extra  degree of freedom that this model has with respect  to the standard model.  It is found to be small and positive,  therefore  V (φ) can mimic and approximate cosmological constant that is decreasing  slowly with  time.   In the  late  Universe  the  contribution of the  scalar  field, φ, surfaces over the matter density,  thus  becoming the dominant component.

Not  all  the  models  studied  are  motivated within  a  theoretical framework,  since  we have  also considered  some interesting phenomenological  approaches.  Last  but  not  least,  at  the  end of the thesis the Brans & Dicke (BD) gravity  model was studied  in detail.  The main feature of this model is that the Newtonian  constant coupling GN  is replaced by a dynamical  scalar field G(t) = 1/ψ(t), coupled to the curvature. As a consequence the gravitational interaction is not only mediated  by the metric  field, as in the General  Relativity case but  also for the aforementioned scalar field ψ.

The  obtained  results  clearly point  out  to an interesting conclusion,  those  models which consider an  effective  time-evolving  dark  energy  are  able  to  alleviate  some  of the  tensions  affecting  the
ΛCDM. Among the different tensions  there  are two that stand  out, namely the σ8 -tension and the H0-tension.

# metadata.dc.source[].value
Tesis Doctorals - Departament - Física Quàntica i Astrofísica

# metadata.dc.title[].value
Implications of Dynamical Dark Energy in the expansion of the Universe and the Structure Formation