On the nature of dark energy: the lattice Universe

There is something unknown in the cosmos. Something big. Which causes the acceleration of the Universe expansion, that is perhaps the most surprising and unexpected discovery of the last decades, and thus represents one of the most pressing mysteries of the Universe. The current standard $\Lambda$CDM model uses two unknown entities to make everything fit: dark energy and dark matter, which together would constitute more than 95% of the energy density of the Universe. A bit like saying that we have understood almost nothing, but without openly admitting it. Here we start from the recent theoretical results that come from the extension of general relativity to antimatter, through CPT symmetry. This theory predicts a mutual gravitational repulsion between matter and antimatter. Our basic assumption is that the Universe contains equal amounts of matter and antimatter, with antimatter possibly located in cosmic voids, as discussed in previous works. From this scenario we develop a simple cosmological model, from whose equations we derive the first results. While the existence of the elusive dark energy is completely replaced by gravitational repulsion, the presence of dark matter is not excluded, but not strictly required, as most of the related phenomena can also be ascribed to repulsive-gravity effects. With a matter energy density ranging from $\sim5$ (baryonic matter alone, and as much antimatter) to $\sim25$ of the so-called critical density, the present age of the Universe varies between about 13 and $15\rm\,Gyr$. The SN Ia test is successfully passed, with residuals comparable with those of the $\Lambda$CDM model in the observed redshift range, but with a clear prediction for fainter SNe at higher $z$. Moreover, this model has neither horizon nor coincidence problems, and no initial singularity is requested. In conclusion, we have replaced all the tough problems of the currentComment: 10 pages, 3 figures, accepted as a Letter to the Editor by Astrophysics and Space Scienc

The long-term optical spectral variability of BL Lacertae

We present the results from a study of the long-term optical spectral variations of BL Lacertae, using the long and well-sampled B and R-band light curves of the Whole Earth Blazar Telescope (WEBT) collaboration, binned on time intervals of 1 day. The relation between spectral slope and flux (the spectrum gets bluer as the source flux increases) is well described by a power-law model, although there is significant scatter around the best-fitting model line. To some extent, this is due to the spectral evolution of the source (along well-defined loop-like structures) during low-amplitude events, which are superimposed on the major optical flares, and evolve on time scales of a few days. The "bluer-when-brighter" mild chromatism of the long-term variations of the source can be explained if the flux increases/decreases faster in the B than in the R band. The B and R-band variations are well correlated, with no significant, measurable delays larger than a few days. On the other hand, we find that the spectral variations lead those in the flux light curves by ~ 4 days. Our results can be explained in terms of Doppler factor variations due to changes in the viewing angle of a curved and inhomogeneous emitting jet.Comment: 7 pages, 5 figures, accepted for publication in A&

Reply to "Comment to a paper of M. Villata on antigravity"

In this short paper we reply to the Comment [arXiv:1108.4543] by M. J. T. F. Cabbolet on Villata's theory of antigravity [arXiv:1103.4937]. The criticisms of methodological and ontological kind presented by that author come from a misinterpretation of some concepts, perhaps due to some lack of clarity or omission of details in Villata's original article. In order to clarify these points, here we provide additional explanations regarding the assumptions and results of the theory.Comment: 3 pages, accepted as a Letter to the Editor by Astrophysics and Space Scienc

Optical variability of the BL Lacertae object GC 0109+224. Multiband behaviour and time scales from a 7-years monitoring campaign

We present the most continuous data base of optical $BVR_{c}I_{c}$ observations ever published on the BL Lacertae object GC 0109+224, collected mainly by the robotic telescope of the Perugia University Observatory in the period November 1994-February 2002. These observations have been complemented by data from the Torino Observatory, collected in the period July 1995-January 1999, and Mt. Maidanak Observatory (December 2000). GC 0109+224 showed rapid optical variations and six major outbursts were observed at the beginning and end of 1996, in fall 1998, at the beginning and at the end of 2000, and at the beginning of 2002. Fast and large-amplitude drops characterized its flux behaviour. The $R_c$ magnitude ranged from 13.3 (16.16 mJy) to 16.46 (0.8 mJy), with a mean value of 14.9 (3.38 mJy). In the periods where we collected multi-filter observations, we analyzed colour and spectral indexes, and the variability patterns during some flares. The long-term behaviour seems approximatively achromatic, but during some isolated outbursts we found evidence of the typical loop-like hysteresis behaviour, suggesting that rapid optical variability is dominated by non-thermal cooling of a single emitting particle population. We performed also a statistical analysis of the data, through the discrete correlation function (DCF), the structure function (SF), and the Lomb-Scargle periodogram, to identify characteristic times scales, from days to months, in the light curves, and to quantify the mode of variability. We also include the reconstruction of the historical light curve and a photometric calibration of comparison stars, to favour further extensive optical monitoring of this interesting blazar.Comment: 13 pages, 11 PS figures, 1 EPS figure, 3 tables, accepted by Astronomy and Astrophysics. Uses A&A documentclass aa.cls, and the package graphicx.st

Optical and radio behaviour of the BL Lacertae object 0716+714

Eight optical and four radio observatories have been intensively monitoring the BL Lac object 0716+714 in the last years: 4854 data points have been collected in the UBVRI bands since 1994, while radio light curves extend back to 1978. Many of these data are presented here for the first time. The long-term trend shown by the optical light curves seems to vary with a characteristic time scale of about 3.3 years, while a longer period of 5.5-6 years seems to characterize the radio long-term variations. In general, optical colour indices are only weakly correlated with brightness. The radio flux behaviour at different frequencies is similar, but the flux variation amplitude decreases with increasing wavelength. The radio spectral index varies with brightness (harder when brighter), but the radio fluxes seem to be the sum of two different-spectrum contributions: a steady base level and a harder-spectrum variable component. Once the base level is removed, the radio variations appear as essentially achromatic, similarly to the optical behaviour. Flux variations at the higher radio frequencies lead the lower-frequency ones with week-month time scales. The behaviour of the optical and radio light curves is quite different, the broad radio outbursts not corresponding in time to the faster optical ones and the cross-correlation analysis indicating only weak correlation with long time lags. However, minor radio flux enhancements simultaneous with the major optical flares can be recognized, which may imply that the mechanism producing the strong flux increases in the optical band also marginally affects the radio one.Comment: 18 pages, 15 Postscript figures, 5 JPEG figures, accepted for publication in A&

Multifrequency monitoring of the blazar 0716+714 during the GASP-WEBT-AGILE campaign of 2007

Since the CGRO operation in 1991-2000, one of the primary unresolved questions about the blazar gamma-ray emission has been its possible correlation with the low-energy (in particular optical) emission. To help answer this problem, the Whole Earth Blazar Telescope (WEBT) consortium has organized the GLAST-AGILE Support Program (GASP) to provide the optical-to-radio monitoring data to be compared with the gamma-ray detections by the AGILE and GLAST satellites. This new WEBT project started in early September 2007, just before a strong gamma-ray detection of 0716+714 by AGILE. We present the GASP-WEBT optical and radio light curves of this blazar obtained in July-November 2007, about various AGILE pointings at the source. We construct NIR-to-UV spectral energy distributions (SEDs), by assembling GASP-WEBT data together with UV data from the Swift ToO observations of late October. We observe a contemporaneous optical-radio outburst, which is a rare and interesting phenomenon in blazars. The shape of the SEDs during the outburst appears peculiarly wavy because of an optical excess and a UV drop-and-rise. The optical light curve is well sampled during the AGILE pointings, showing prominent and sharp flares. A future cross-correlation analysis of the optical and AGILE data will shed light on the expected relationship between these flares and the gamma-ray events.Comment: 5 pages, 5 figures, to be published in A&A (Letters); revised to match the final version (changes in Fig. 5 and related text