Cosmology and Large Scale Structure of the Universe


General Overview

The problem of the formation of the galaxies and the large scale structure of the Universe requires a non barionic component (the dark matter) of the matter in the Universe. We are carrying out a research program where we use the correlation functions (spatial and angular) of galaxies and clusters of galaxies, the study of the cosmological nucleosynthesis and of the cosmic background anisotropies, and the time evolution models of structures, in order to identify the microscopical parameters (mass, spin, chemical potential) of the dark matter.

Gravitational Collapse in the Expanding Universe

In the last developments of our project in Cosmology and LSS we have found, in the linear approximation, that a distribution of fermions in the expanding Universe decreases its minimum collapsing mass as the chemical potential of them increases.
The Hot Dark Matter models based upon massive neutrinos have found great difficulties for explaining the large scale structure of the Universe. Classically only large masses of neutrinos (their greatest Jeans masses) can survive to the free streaming damping process. At late stages of the history of the Universe when they become selfgravitating they have not enough time for collapsing before present time. Therefore the creation of galactic structures in a 'top-down' scenario of large scale structure formation it should not allowed.
How those models are again interesting after the first experimental evidences of neutrino mass and because of the discovery of the 'instability shell' below the Jeans length, in which the fermion density perturbations smaller than Jeans length survive to the collisionless damping.
We are now starting to study this phenomenon integrating numerically the exact equations.

The Role of the Chemical Potential in LSS formation

Participants: Massimiliano Lattanzi, Remo Ruffini, Gregory Vereshchagin

We study the evolution of a gas of massive light neutrinos (m_n <10 eV) in an expanding Friedmann Universe and, using the properties of self-similarity of the distribution function, obtain an expression for the neutrino gas energy density at the present time as a function of masses and chemical potentials of the electronic, m and t neutrinos, for a total of six independent parameters. Then, using the constraints imposed by particle physics and cosmological nucleosyntehsis, we express this energy density as a function of three independent parameters only, namely a common mass and the e and m neutrino chemical potentials. Finally we take for the neutrino mass a value of 2 eV and show how, by varying the e and m neutrino chemical potential beetween the values allowed by cosmological nucleosynthesis, we can obtain for the neutrino +antineutrino density the critical value W=1.
Using WMAP data we find joint constraints on the mass and the chemical potential of light neutrinos. We find that WMAP data alone cannot firmly rule out scenarios with a large lepton number; moreover, a small preference for this kind of scenarios is found.

Dark Energy

Participants: Vahe Gurzadyan, Harutyun Khachatryan, Gregory Vereshchagin, Gegham Yegoryan, She-Sheng Xue

We study cosmological implications of the formula for Dark Energy, derived by Gurzadyan and Xue, which fits the observed value of dark energy density parameter. This formula implies possible variation of physical constants such as speed of light and gravitational constant. Some symmetry and underlying invariance within these models is revealed.

Measurement of the Solar Diameter with SDS Telescope and Total Eclipses

On a secular basis the variation of the solar diameter of few part over 10000 can be direct responsible of the climate changes on the Earth. Its measurement with a relative accuracy of one part in a million is the goal of SDS, Solar Disk Sextant balloon borne Italian-American mission (NASA, Yale, Tor Vergata, Osservatorio Astronomico di Roma and ICRA). We already have 4 eight-hours flights in fall '92, '94 [1], '95 and '96 not completely reduced, and data from total solar eclipses of '84, '94 [2], '95, '98 and '99 observed from the edges [3] which partially overlap the SDS sampling and can provide an absolute calibration for SDS with an accuracy of one part over 10000.
The analysis program Baily Beads [4] for reducing the eclipse data will be calibrated with the previous best ephemeris program [2] and it will be applied to the eclipse data from 1567 up to now. The eclipses of 2001 December and 2002 June will be also observed. In parallel all SDS data will be reduced. The first results both on secular time scales and on the last decade are expected in 2002.

[1] Sofia, S.; Heaps, W.; Twigg, L.; Lydon, T. J.; Sofia, U. J., Results of the September 26, 1994 Balloon Flight of the Solar Disk Sextant: Implications for the Variability of the Solar Diameter and Oblateness,
American Astronomical Society Meeting, 185, #44.07 (1994).
[2] Fiala, Alan D.; Dunham, David W.; Sofia, Sabatino,
Variation of the solar diameter from solar eclipse observations, 1715-1991,
Solar Physics, vol. 152, no. 1, p. 97-104 (1994).
[3] Dunham, David W.; Bixby Dunham, Joan, Observing Total Solar Eclipses from Near the Edge of the Predicted Path,
Lunar Dynamics and Observational Coordinate Systems, Proceedings of IAU Colloq. 24, held in Houston, TX, 15-17 January, 1973. Moon 8, 1973. Revised abstracts published as Lunar Sci. Inst. Contrib. 135. Edited by M. Moutsoulas, p.546 (1973).
[4] Herald, D., Correcting predictions of solar eclipse contact times for the effects of lunar limb irregularities, Journal of the British Astronomical Association, vol.93, no.6, p.241-246 (1983).
Baily Beads Program:
Among the studies on secular solar variability the case of the total-annular solar eclipse of 1567 is the most intriguing and interesting. The current scientific debate on this case still requires historical analysis: a few, undemonstrated, hypotheses on the methods of observation have been used to discriminate the nature of that eclipse (annular or total) [5].
[5] "The solar eclipse observed by Clavius in A.D. 1567", Astronomy and Astrophysics, 322, p.347-351 (1997).
If the eclipse of 1567 was really annular, as Clavius stated, it imply that the Sun in 1567 was larger than nowadays. In addition to a better knowledge of the methods of observation before the invention of the telescope see e.g. [6], this study has some important implications in our understanding of solar astrophysics and, consequently, in terrestrial meteorology.
[6] "Measurements of the Solar Diameter in Kepler's Time", Costantino Sigismondi and Federico Fraschetti; The Observatory Vol. 121 (2001) 380-385.    

Variable Stars

Also we are investigating, in collaboration with Prof. Dorrit Hoffleit, Yale University and Riccardo Coccioli (Physics Student at "La Sapienza"), the Long-term behavior of Mira-type variable stars. This problem, naturally related to the solar variability, presents some new aspects on the study of the time series of maxima: there exist negative correlations between the luminosity of two consecutive maxima.

[7] "Long-term Behavior of Mira Ceti Maxima", Costantino Sigismondi, Dorrit Hoffleit and Riccardo Coccioli; Presented in the 90th AAVSO meeting in Madison (WI) and published in JAAVSO, The Journal of American Association of Variable Stars Observers, (2001). .
[8] "Mira and the Star of Bethlehem" C. Sigismondi, Online Journal of Christian Theology and Philosophy, vol. 4 n. 1  (2002).

Lunar Impacts

The studies on Lunar Impacts [9-10] and Dynamics of Small Bodies in Solar System [11] is carried out with Giovanni Paolo Imponente (ICRA and Napoli University) and David W. Dunham (Johns Hopkins University, Applied Physics Laboratory). We posed and studied the problem of luminous efficiency of hypervelocity impacts [12] and we are opening now the collaboration with other European and American partners. This problem is related with the nature and the number of the impactors, this is the reason why we are studying also the dynamic of the Leonids, meteoroids originated by the Comet P/Tempel-Tuttle [11].

[9] "The observation of Lunar Meteors", C. Sigismondi and G. Imponente, WGN, the Journal of the IMO, vol. 28 n. 2/3, p. 54-57 (2000).
[10] "The observation of Lunar Meteors II." C. Sigismondi and G. Imponente, WGN, the Journal of the IMO, vol. 28 n. 6, p. 230-232 (2000).
[11] "Ejection Velocity of Meteoroids from Comet Surfaces", Giovanni Imponente and Costantino Sigismondi; WGN, The Journal of the IMO, vol. 29, No. 5 (2001) 176-181. .
[12] "The Luminous Efficiency of Hypervelocity Impacts on the Moon", Costantino Sigismondi and David W. Dunham; New Views of the Moon, Europe Future Lunar Exploration, Science Objectives, and Integration of Datasets, David Heather editor, Berlin, Germany (2002).    .

The Role of the Chemical Potential in LSS formation

  1. R. Ruffini, M. Lattanzi and G. Vereshchagin, "On the possible role of massive neutrinos in cosmological structure formation" in Cosmology and Gravitation: Xth Brazilian School of Cosmology and Gravitation, edited by M. Novello and S.E. Perez Bergliaffa, AIP Conference Proceedings, Vol. 668, Melville, New York, 2003, pp.263-287.
  2. M. Lattanzi, R. Ruffini and G.V. Vereshchagin, "Do WMAP data constraint the lepton asymmetry of the Universe to be zero?" in Albert Einstein Century International Conference, edited by J.-M. Alimi, and A. Füzfa, AIP Conference Proceedings, Vol. 861, Melville, New York, 2006, pp.912-919.
  3. M. Lattanzi, R. Ruffini and G.V. Vereshchagin, "Joint constraints on the lepton asymmetry of the Universe and neutrino mass from the Wilkinson Microwave Anisotropy Probe", Physical Review D, Vol. 72 (2005) 063003.

Dark Energy

  1. V.G. Gurzadyan, S.-S. Xue in: "From Integrable Models to Gauge Theories; volume in honor of Sergei Matinyan", ed. V.G. Gurzadyan, A.G. Sedrakian, p.177, World Scientific, 2002; "On the estimation of the current value of the cosmological constant", Modern Physics Letters A, Vol. 18, No. 8 (2003) pp. 561-568.
  2. G.V. Vereshchagin, G. Yegorian, "Cosmological models with Gurzadyan-Xue dark energy", Classical and Quantum Gravity, Vol. 23, (2006) No 15, pp. 5049-5061.
  3. G.V. Vereshchagin, G. Yegorian, "Hidden invariance in Gurzadyan-Xue cosmological models", Physics Letters B, Vol. 636, (2006) pp. 150-153.
  4. G.V. Vereshchagin, "Physical constants and the Gurzadyan-Xue formula for the dark energy", Modern Physics Letters A, Vol. 21, (2006) No. 9, pp. 729-733.
  5. H.G. Khachatryan, "Invatiants and solutions of Gurzadyan-Xue Dark Energy cosmological models", Modern Physics Letters A, Vol. 22, No. 5 (2007) pp. 333- 338.