PROJECTS
Main Research Projects
Relativistic Cosmology & Large-Scale Structure
Tilted cosmology & Bulk peculiar flows
Observations have repeatedly confirmed that large sections of the universe, between few and several hundred Mpc, move coherently towards a certain celestial direction, with speeds ranging from few hundred to several hundred km/sec. These are the so-called bulk flows, an increasing number of which have sizes and velocities well in excess of the ΛCDM limits. The aim of this research project is to investigate (i) the origin, (ii) the evolution and (iii) the implications of these peculiar motions. In so doing, we have developed a relativistic theoretical model, namely the tilted universe paradigm, specifically designed to the study of cosmological peculiar motions.
(i) With respect to the bulk-flow origin, our work clearly points towards a late, most likely post-recombination, generation, with the responsible agent being the ongoing process of structure formation. Put another way, the ever increasing inhomogeneity of the universe.
(ii) Looking into the evolution, we found that general relativity supports bulk peculiar flows considerably faster and deeper than those anticipated by the Newtonian studies, which have been exclusively employed by all the ΛCDM-based scenarios. This means that relativity can readily explain the fast bulk flows reported by several recent surveys and thus suceed where the Newtonian theory has failed. The reason has been traced back to the fundamentaly different way the two theories treat the gravitational field and its sources.
(iii) When thinking of their potential implications one should keep in mind that bulk flows are large-sale motions relative to the rest-frame of the universe. One should also remember that relative motions have often lead us to gross misinterpretations, some of which lasted for centuries, if not millennia. Our work shows that observers residing in a slightly contracting bulk flow, are likely to mistake their slower local expansion rate as acceleration of the surrounding universe. In the same way that passengers on a train that slows down, may be tricked to believe that it is the train next to theirs which has accelerated away. Interestingly, there have been a number of recent surveys claiming that our galaxy may actually reside within such a contracting bulk flow (e.g. Pasten, et al 2024; Sorrenti, et al 2024). Then, if the celebrated universal acceleration is a mere relative-motion illusion, the data should contain the trademark signature of relative motions, namely an apparent (Doppler-like) dipole in the sky-distribution of the deceleration parameter. In other words, to us the universe should seem to accelerated faster along a certain direction on the celestial sphere and equally slower in the opposite. Moreover the magnitude of this dipole should apppear to decrease with increasing redshift. Intriguingly, a number of recent studies have reported the presence of such a local dipolar anisotropy in the data (Colin, et al 2019; Clocchiatti, et al 2024; Sah, et al 2024).
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.
Representative papers: # # # # # News Features: New Scientist