PROJECTS
Main Research Projects
Relativistic Cosmology & Large-Scale Structure
Tilted cosmology & Bulk peculiar flows
Observations have repeatedly confirmed the presence of large-scale peculiar motions in our universe. In practice, this means that no real observer in our cosmos follows the smooth universal expansion (the so-called Hubble flow). Instead, large sections of the observable universe seem to be moving coherently towards specific directions in the sky. Our galaxy and the Local Group of galaxies, for example, drift relative to the mean universal expansion at around 600 km/sec. The typical sizes of the reported large-scale "bulk flows" are of the order of few hundred Mpc and their velocities are around few hundred km/sec. There have been also reports of considerably larger and much faster peculiar motions, covering scales close to 1000 Mpc and moving at speeds of up to 1000 km/sec. These are the controversial "dark flows''. The aim of this research project is to investigate the origin, the evolution and the implications of the aforementioned large-scale peculiar motions. We would like, in particular, to know whether these bulk/dark flows are a rather recent (post-recombination) addition to the kinematics of our universe, or whether they might have an earlier (perhaps primordial) origin. Another goal is to investigate the evolution and the effects of such peculiar motions and more specifically their growth-rates during the various epochs in the lifetime of our universe and their role during the structure-formation process. Since nearly all the available theoretical studies are Newtonian, we pursue a fully relativistic treatment of the issue, employing the so-called "tilted" cosmological models that allow for two (at least) families of relatively moving observers. Last, but not least, we want to study the implications of the relative-motion effects on the way we interpret the cosmological data. The key question is the extent our motion with respect to the smooth Hubble flow can ``contaminate'' our measurements and our interpretation of the cosmological data, and more specifically of the sign of the measured deceleration parameter. If it so happens, the recent accelerated expansion of the universe can be a mere illusion, triggered by our galaxy's motion relative to the rest-frame of the cosmos. Astonishing though this may sound, it would not be the first time in the history of Astronomy that relative-motion effects have lead us to a gross misinterpretation of reality.
The project, entitled "Tilted Cosmology", is funded by the Hellenic Foundation for Research and Innovation, with a budget of 200,000 Euros for four years. Currently, the research group comprises the Principal Investigator (PI), one post-doctoral fellow and two PhD and one MSc student. There are also external collaborators from the University of Ioannina (Greece) and from Oxford University (UK).
Currently, researchers participating in the Legacy Survey of Space and Time (LSST) have undertaken the task of comparing the theoretical predictions of the Tilted Cosmology scenario with the observations. The title of the research project in question is ``Testing Tilted Cosmology'' and it is part of the Dark Energy Science Collaboration (DESC -- see https://lsstdesc.org) of the LSST.
Representative papers: # # # # # # # # # # # # # # News features: New Scientist, Inference, Cosmo of '69, NBC
Workshops/Conferences
General Relativity & Electromagnetism
Electromagnetic fields in curved spacetimes
Magnetic and electromagnetic fields are everywhere in the universe. From the Earth, the Sun and the nearby stars, all the way to the distant galaxies and the far away galaxy-clusters, the existence of magnetic fields has been repeatedly verified. At the same time, electromagnetic radiation fills up the entire universe. In addition to its ubiquitous presence, electromagnetism is a rather unique source as well. The Maxwell field, in particular, is the only known energy source of vector nature. This means that, within the geometrical interpretation of gravity that general relativity advocates, electromagnetism couples directly to the gravitational field in two different and mutually complementary ways. The first is through the Einstein field equations, like any other energy source. The second is a purely geometrical coupling, which stems from the vector nature of the Maxwell field and propagates via the Ricci identities. No other known matter source share this direct dual interaction with the gravitational field. As a result, electromagnetic radiation interacts with gravitational waves and electromagnetic signals can travel inside the observer's lightcone. Nevertheless, despite its long known unconventional effects, the coupling between the Maxwell and the Einstein-Weyl fields has remained in the margin of what one may call mainstream research. Perhaps the conceptual and technical challenges of the problem have overshadowed its potentially far-reaching implications. One of the most intriguing theoretical claims made back in the early 1960s, was that magnetic fields showed a generic tendency to resist their gravitational self-collapse. Almost forty years later, around the turn of the millennium, new independent studies seemed to support these earlier claims We have now reasons to believe that magnetism has indeed an inherit tendency to resist gravitational collapse and that the cause is another unique magnetic property, namely its tension. The latter reflects the elasticity of the magnetic forcelines and their generic preference to remain "straight". The main aim of this research project is to take a deeper look into the behaviour of magnetism in strong-curvature environments, establish whether the aforementioned tendency of the Maxwell field to resist gravitational collapse is a viable possibility and, if so, investigate its potential implications. Alongside, we would also like to take a closer look at the wider spectrum of the gravito-electromagnetic interaction and its applications in both astrophysics and cosmology.
The project, entitled "On the Einstein-Maxwell-Weyl coupling", is currently funded by the Hellenic Foundation for Research and Innovation, with a budget of 30,000 Euros for two years. The research group comprises the PI, one PhD and one MSc student.
Representative papers: # # # # # # # # News features: Nature, Science, Der Spiegel
Classical Extensions of General Relativity
Cosmology with torsion & non-metricity
General relativity advocates a geometrical interpretation of gravity, which is no longer a force, but the manifestation of spacetime curvature. The latter is Riemannian in nature, which implies a symmetric connection and a covariantly invariant metric. Relaxing the symmetry of the connection allows for spacetime torsion and thus takes us to the realm of the so-called Einstein-Cartan gravity. Relaxing the covariance of the metric introduces non-metricity to the host spacetime. Geometrically speaking, the presence of torsion implies that the parallel transport of two vectors, along each other's direction, does not close the ensuing parallelogram. Non-metricity, on the other hand, means that lengths and angles are no longer preserved during parallel transport. These are drastic departures from the standard picture of general relativity and the resulting phenomenology is broad and quite often very counter-intuitive. The aims of the ongoing research project are to develop the mathematical framework that will consistently incorporate torsion and non-metricity into the standard cosmological studies and also to investigate their potential phenomenological implications for the universe we live in.