Lorenzo Iorio

Motivated by recent accurate measurements of disk/jet coprecessions around some galactic supermassive black holes, the accelerations experienced by an uncharged, spinless object in the Kerr metric, written in harmonic coordinates, are analytically calculated up to the formal second post-Newtonian order. To such a level, some new accelerations make their appearance. They are proportional to even and odd powers of the hole’s angular momentum. Their counterparts are not known where the primary is a material body. After expressing them in a coordinate-independent, vector form valid for any orientations of the hole’s spin axis in space, their orbital effects are perturbatively worked out in terms of the particle’s Keplerian orbital elements. The resulting expressions, averaged over one orbital revolution, are valid for generic shapes and orientations of the orbit. The orbital plane’s precession proportional to the first power of the hole’s angular momentum and to the reciprocal of the fourth power of the speed of light amounts to about twenty per cent of the corresponding Lense–Thirring effect. The latter is believed to be the cause of the accurately measured disk/jet precessional phenomenology, currently measured to a few per cent accuracy. Although at a lesser extent, also the precession proportional to the second power of the hole’s spin and to the reciprocal of the fourth power of the speed of light is measurable. Allowed domains in the parameter space of the jet precession around M87 ∗ are displayed.

The 3rd International Online Conference on Universe, organized by the journal Universe, was held from 4 to 6 March 2026. It collected distinguished researchers from all over the world who showcased their latest researches on topics like Gravitation and Cosmology, Field Theory, Quantum Gravity, High Energy, Nuclei and Particle Physics, Foundation of Quantum Mechanics, Astrophysics and Compact Objects, Solar and Stellar Physics, and Astroparticle Physics. Both posters and talks were at the forefront of the current research, and the discussions were often vibrant and penetrating with several talks that attracted many stimulating inquiries. This report, collecting the extended abstracts of the participants, gives an idea of the tone of the conference.

Recently, the coprecession of both the accretion disk and the jet formed following the tidal disruption event associated with the optical transient AT2020afhd, driven by a supermassive black hole of almost ten million solar masses, were independently measured in both the X and radio bands, respectively, showing a periodicity of nearly 20 days over about 300 days. An analytical model of the general relativistic gravitomagnetic Lense-Thirring precession of the effective orbit of a fictitious test particle revolving about a spinning primary can explain the observed precessional features. It yields allowed regions in the system’s parameter space which, as far as the hole’s dimensionless spin parameter is concerned, are essentially in agreement with those obtained in the literature with general relativistic magnetohydrodynamic simulations. The present analytical approach can be extended to include the precession due to the hole’s quadrupole mass moment as well. It breaks the degeneracy in the allowed regions occurring for negative and positive values of the spin parameter when only the Lense-Thirring effect is considered. The best estimate for the hole’s mass yields the range 0.185–0.215 for the dimensionless spin parameter. Using the same strategy with the gravitomagnetic frequency for an extended disk of finite size with a parameterized power-law mass density yields to distinct, generally non-overlapping allowed regions for each value of the power-law index adopted. Some of the assumptions on which this work is based are critically examined.

The Planet Nine hypothesis encompasses a body of about 5-8 Earth's masses whose orbital plane would be inclined to the ecliptic by one or two tens of degrees and whose perihelion distance would be as large as about 240-385 astronomical units. Recently, a couple of his epigones have appeared: Planet X and Planet Y. The former is a sort of minor version of Planet Nine in that all its physical and orbital parameters would be smaller. Instead, the latter would have a mass ranging from that of Mercury to the Earth's one and semimajor axis within 100-200 astronomical units. By using realistic upper bounds for the orbital precessions of Saturn, one can get insights on their position which, for Planet Nine, appears approximately confined around its aphelion. Planet Y can be just a Mercury-sized object at no less than about 125 astronomical units, while Planet X appears to be ruled out. Dedicated data reductions by modeling such perturber(s) are required to check the present conclusions, to be intended as hints of what might be detectable should planetary ephemerides include them. A probe on the same route of Voyager 1 would be perturbed by Planet Nine by about 20-40 km after some decades.

The Planet Nine hypothesis encompasses a body of about 5–8 Earth’s masses whose orbital plane would be inclined to the ecliptic by one or two tens of degrees and whose perihelion distance would be as large as about 240–385 astronomical units. Recently, a couple of his epigones have appeared: Planet X and Planet Y. The former is similar to a minor version of Planet Nine in that all its physical and orbital parameters would be smaller. Instead, the latter would have a mass ranging from that of Mercury to Earth’s and a semimajor axis within 100–200 astronomical units. By using realistic upper bounds for the orbital precessions of Saturn, one can obtain insights on their position which, for Planet Nine, appears approximately confined around its aphelion. Planet Y can only be a Mercury-sized object at no less than about 125 astronomical units, while Planet X appears to be ruled out. Dedicated data reductions by modeling such perturber(s) are required to check the present conclusions, to be intended as hints of what might be detectable should planetary ephemerides include them. A probe on the same route of Voyager 1 would be perturbed by Planet Nine by about 20–40 km after some decades.

The long-term rates of change of all the Keplerian orbital elements of a two-body system acted upon by a remote massive point-like object are analytically worked out, to the Newtonian quadrupolar level, without any restriction either on the orbital eccentricity and inclination of the disturbed pair or the position of the distant perturber. The latter is considered fixed during the average over one orbital revolution of the inner binary by means of which its orbital perturbations are calculated. The results are presented in a compact form that facilitates a straightforward application to the case of the supermassive black hole – or megapyknon – M87$^\ast$. In principle, the presence of another distant intermediate-mass black hole may concur to cause the observed jet precession, assumed tightly coupled with the accretion disc. Such a possibility is ruled out by the exclusion plots in the parameter space obtained with the approach presented here.

The adventure of the journal Universe began about ten years ago, when I received an invitation from Dr

The need to choose appropriate and meaningful names for the objects of scientific inquiry, in the spirit of Michael Faraday and, on a different level, of the ancient Chinese doctrine of rectification of names (正名, Zhèngmíng), is illustrated here in the case of the so-called Planet Nine. Since before the discovery of Neptune, the fascinating hypothesis of the possible existence of a new, distant planet in the solar system, yet to be discovered, has regularly surfaced in the pages of astronomy journals in various guises. Its most recent incarnations have been tentatively given names such as Planet X, Planet Y, and, most famously, Planet Nine. Such labels are unsatisfactory because they reveal no significant physical or orbital properties of the object which they are attributed to. I propose here the name Telisto, from the ancient Greek word τήλɩστoς for ‘farthest, most remote’ which captures a feature common to all versions of this scenario that seems destined to remain at the forefront of astronomical research for a long time to come: its supposedly great heliocentric distance, estimated at several hundred astronomical units. By exploring the history of astronomy, I also respond to some criticisms that might be leveled at this proposal. Among other things, I also draw a comparison with the naming of the so-called axions, which are hypothetical elementary particles proposed almost fifty years ago and which continue to be an active object of research.

‘Black hole’ is the denomination of the most extreme prediction of the General Theory of Relativity made popular by J. A. Wheeler in the late sixties of the twentieth century, having now entered widely into the collective imagination. Nonetheless, the term is somewhat misleading since there is nothing that tears apart in black holes, which, furthermore, are not even black. Thus, the new name pyknons, from the ancient Greek word for ‘compact; constricted; close-packed’, is proposed for them since it captures a key distinctive feature of theirs. In deference to the objects thus renamed, it also has the merit of introducing a greater compactness in the terms denoting them.