# Our Research Fields

### Observational Constraints on Dark Energy

The accelerating expansion of the universe confirmed to a precision level during the past two decades is consistent with the existence of a cosmological constant whose value is fine tuned to be close to the one indicated by the scale of the present day Hubble parameter. The expansion rate predicted in the context of this cosmological constant is of the form $H^2 (z) = H_{0}^{2}[\Omega_{0m}(1+z)^{3} + \Omega_{DE}(z)]$ and is the cornerstone of the standard $\Lambda$CDM cosmological model. One of the main goals of our research is the use of a wide range of cosmological data to test the validity of the $\Lambda$CDM model searching for specific types of deviations of this expansion rate as a function of redshift. This search started in 2004 with the well cited paper entitled  “A comparison of cosmological models using recent supernova data”  (one of the pioneering works in this subject with more than 200 citations) and has continued with a series of well cited papers including (but not limited to) the following:

More recent relevant publications on this subject include:

### Theoretical Models of Dark Energy – Modified Gravity and their Cosmological Predictions

Even though the expansion rate predicted by a cosmological constant appears to be consistent with the majority of cosmological observations, the predicted parameter values of $\Lambda$CDM are not always consistent among different observational probes. These are the well known tensions of $\Lambda$CDM which include the $H_{0}$ tension and the growth tension. This fact, along with the fine-tuning required for fixing the value of the cosmological constant, has lead to a wide range of alternative models and expansion rate parametrizations aiming at providing improved quality of fit to the cosmological data and resolution of the $\Lambda$CDM tensions. One of the main goals of our research is the investigation of the predictions of such theoretical models, the identification of the quality of fit of these predictions to the cosmological data, and the comparison of this quality of fit with the corresponding quality of fit of $\Lambda$CDM. In this context, we have completed a wide range of research projects that include the following representative papers:

More recent relevant publications on this subject include:

### Search for Violations of the Cosmological Principle in the Cosmological Data and related Predictions of Theoretical Models

The cosmological princliple (the assumption of isotropy and homogeneity of the Universe on Hubble scales) is the cornerstone of the standard cosmological model. There are however, recent indications from various large scale cosmological data that this principle may be violated. One of the goals of our research work is to investigate observational data for possible hints of violation of the cosmological principle, the identification of the statistical level of significance of such hints and the construction of theoretical models that may lead to such violation of the cosmological principle. In this context we have completed a wide range of research projects that include the following representative papers:

### Short-range Gravity Experiments and Modified Gravity Theories

Modifications of General Relativity in the form of Modified Gravity Theories may lead to detectable signals in short range gravity experiments on sub-mm scales. An interesting aspect of our research activities involves the identification of the form of these predicted signals for specific modified gravity theories and the search for these signals in the data of short range gravity experiments. In this context we have completed a wide range of research projects that include the following representative papers:

### Gravitational Physics and Field Theory of Cosmic Strings and other Topological Defects

The evolution of scalar fields in a cosmological or a laboratory setup may lead to a wide range of interesting observable effects including the formation and evolution of topological defects. These are topologically trapped energy concentrations which may have spharical symmetry (monopoles), cylindrical symmetry (strings), planar (surface) form (domain walls) or collapsing evolution (textures). The investigation of the field theoretical properties, the gravitational effects and the observational signatures of these objects in both a cosmological and a laboratory setup is also one of the main aspects of our research. In this context we have completed a wide range of research projects that include the following representative papers: