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We use a semianalytical approach, and the standard σ 8 = 1 cold dark matter (SCDM) cosmological model to study the gravitational collapse and virialization, the structure, and the global and statistical properties of isolated dark matter (DM) galactic halos which emerge from primordial Gaussian fluctuations. Firstly, from the statistical properties of the primordial density fluctuation field the possible mass aggregation histories (MAHs) are generated. Secondly, these histories are used as the initial conditions of the gravitational collapse. To calculate the structure of the virialized systems we have generalized the secondary infall model to allow arbitrary MAHs and internal thermal motions. The average halo density profiles we obtained agree with the profile derived as a fitting formula to results of N-body cosmological simulations by Navarro et al. (1996Navarro et al. ( , 1997. The comparison of the density profiles with the observational data is discussed, and some possible solutions to the disagreement found in the inner regions are proposed.The results of our approach, after considering the gravitational dragging of the baryon matter that forms a central disk in centrifugal equilibrium, show that the empirical Tully-Fisher (TF) relation and its scatter can be explained through the initial cosmological conditions, at least for the isolated systems. The σ 8 = 1 SCDM model produces galaxies with high velocities when compared to observations, but when the SCDM power spectrum is normalized to σ 8 = 0.57 an excellent agreement with the

We compare the spectral properties of 79 short and 79 long Gamma-Ray Bursts (GRBs) detected by BATSE and selected with the same limiting peak flux. Short GRBs have a low-energy spectral component harder and a peak energy slightly higher than long GRBs, but no difference is found when comparing short GRB spectra with those of the first 1-2 s emission of long GRBs. These results confirm earlier findings for brighter GRBs. The bolometric peak flux of short GRBs correlates with their peak energy in a similar way to long bursts. Short and long GRBs populate different regions of the bolometric fluence-peak energy plane, short bursts being less energetic by a factor similar to the ratio of their durations. If short and long GRBs had similar redshift distributions, they would have similar luminosities yet different energies, which correlate with the peak energy E peak for the population of long GRBs. We also test whether short GRBs are consistent with the E peak −E iso and E peak −L iso correlations for the available sample of short (6 events) and long (92 events) GRBs with measured redshifts and E obs peak : while short GRBs are inconsistent with the E peak −E iso correlation of long GRBs, they could follow the E peak −L iso correlation of long bursts. All the above indications point to short GRBs being similar to the first phases of long bursts. This suggests that a similar central engine (except for its duration) operates in GRBs of different durations.

The best measure of the universe should be done using a standard "ruler" at any redshift. Type Ia supernovae (SN Ia) probe the universe up to , while the cosmic microwave background (CMB) primary anisotropies z ∼ 1.5 concern basically . Apparently, gamma-ray bursts (GRBs) are all but standard candles. However, their z ∼ 1000 emission is collimated, and the collimation-corrected energy correlates tightly with the frequency at which most of the radiation of the prompt is emitted, as found by Ghirlanda et al. Through this correlation we can infer the burst energy accurately enough to probe the intermediate-redshift ( ) universe. Using the best known 15 z ! 10 GRBs we find very encouraging results that emphasize the cosmological GRB role. A combined fit with SN Ia yields and . Assuming in addition a flat universe, the parameters are con-Q p 0.37 ‫ע‬ 0.10strained to be and . GRBs accomplish the role of "missing link" between Q p 0.29 ‫ע‬ 0.04SN Ia and CMB primary anisotropies. They can provide a new insight on the cosmic effects of dark energy, complementary to the one supplied by CMB secondary anisotropies through the integrated Sachs-Wolfe effect. The unexpected standard candle cosmological role of GRBs motivates us with the most optimistic hopes for what can be obtained when the GRB-dedicated satellite, Swift, is launched.

We report the discovery of a correlation among three prompt emission properties of gamma‐ray bursts (GRBs). These are the isotropic peak luminosity Liso, the peak energy (in νLν) of the time‐integrated prompt emission spectrum Epk, and the ‘high signal’ time‐scale T0.45, previously used to characterize the variability behaviour of bursts. In the rest frame of the source, the discovered correlation reads Liso∝E1.62pkT−0.490.45. We find other strong correlations, but at the cost of increasing the number of variables, involving the variability and the isotropic energy of the prompt emission. With respect to the other tight correlations found in GRBs (i.e. between the collimation corrected energy Eγ and Epk, the so‐called Ghirlanda correlation, and the phenomenological correlation among the isotropic emitted energy Eiso, Epk and the jet break time tbreak), the newly found correlation does not require any information from the afterglow phase of the bursts, nor any model‐dependent assumption. In the popular scenario in which we are receiving beamed radiation originating in a fireball pointing at us, the discovered correlation preserves its form in the comoving frame. This helps to explain the small scatter of the correlation and underlines the role of the local brightness (i.e. the brightness of the visible fraction of the fireball surface). This correlation has been found with a relatively small number of objects and it is hard to establish if any selection bias affects it. Its connection with the prompt local brightness is promising, but a solid physical understanding is still to be found. Despite all that, we find that some properties of the correlation, which we discuss, support its true existence, and this has important implications for the GRB physics. Furthermore, it is possible to use such correlation as an accurate redshift estimator, and, more importantly, its tightness will allow us to use it as a tool to constrain the cosmological parameters.

Gamma Ray Bursts (GRBs) are among the most powerful sources in the Universe: they emit up to 10 54 erg in the hard X-ray band in few tens of seconds. The cosmological origin of GRBs has been confirmed by several spectroscopic measurements of their redshifts, distributed in the range z ∈ (0.1, 6.3). These two properties make GRBs very appealing to investigate the far Universe. Indeed, they can be used to constrain the geometry of the present day universe and the nature and evolution of Dark Energy by testing the cosmological models in a redshift range hardly achievable by other cosmological probes. Moreover, the use of GRBs as cosmological tools could unveil the ionization history of the universe, the IGM properties and the formation of massive stars in the early universe. The energetics implied by the observed fluences and redshifts span at least four orders of magnitudes. Therefore, at first sight, GRBs are all but standard candles. But there are correlations among some observed quantities which allows us to know the total energy or the peak luminosity emitted by a specific burst with a great accuracy. Through these correlations, GRBs becomes "known" candles, and then they become tools to constrain the cosmological parameters. One of these correlation is between the rest frame peak spectral energy E peak and the total energy emitted in γ-rays E γ , properly corrected for the collimation factor. Another correlation, discovered very recently, relates the total GRB luminosity L iso , its peak spectral energy E peak and a characteristic timescale T 0.45 , related to the variability of the prompt emission. It is based only on prompt emission properties, it is completely phenomenological, model independent and assumption-free. These correlations have been already used to find constraints on Ω M and Ω Λ , which are found to be consistent with the concordance model. The present limited sample of bursts and the lack of low redshift events, necessary to calibrate the correlations used to standardize GRBs energetics, makes the cosmological constraints obtained with GRBs still large compared to those obtained with other cosmological probes (e.g. SNIa or CMB). However, the newly born field of GRB-cosmology is very promising for the future.

We test the spectral-energy correlation including the new bursts detected (mostly) by Swift with firm measurements of their redshifts and peak energy. The problem of identifying the jet breaks in the complex and multibreak/flaring X-ray light curves observed by Swift is discussed in the complex and multibreak/flaring X-ray light curves observed by Swift. We use the optical data as the most reliable source for the identification of the jet break, since the X-ray flux may be produced by a mechanism different from the external shocks between the fireball and the circumburst medium, which are responsible for the optical afterglow. We show that the presence of an underlying SN event in XRF 050416A requires a break to occur in the afterglow optical light curve at around the expected jet break time. The possible presence of a jet break in the optical light curve of GRB 050401 is also discussed. We point out that, for measuring the jet break, it is mandatory that the optical light curve extends after the epoch where the jet break is expected. The interpretation of the early optical breaks in GRB 050922C and GRB 060206 as jet breaks is controversial because they might instead correspond to the flat-to-steep decay transition common in the early X-ray light curves. All the 16 bursts coming from Swift are consistent with the E peak − E γ and E peak − E iso − t jet correlation. No outlier is found to date. Moreover, the small dispersion of the E peak − E γ and E peak − E iso − t jet correlation, confirmed also by the Swift bursts, strengthens the case of using GRBs as standard candles.

A series of high-resolution ΛCDM cosmological N-body simulations are used to study the properties of galaxy-size dark halos as a function of global environment. We analyse halos in three types of environment: "cluster" (cluster halos and their surroundings), "void" (large regions with density contrasts −0.85), and "field" (halos not contained within larger halos). We find that halos in clusters have a median spin parameter ∼ 1.3 times lower, a minor-to-major axial ratio ∼ 1.2 times lower (more spherical), and a less aligned internal angular momentum than halos in voids and the field. For masses 5 × 10 11 h −1 M ⊙ , halos in cluster regions are on average ∼ 30 − 40% more concentrated and have ∼ 2 times higher central densities than halos in voids. While for halos in cluster regions the concentration parameters decrease on average with mass with a slope of ∼ 0.1, for halos in voids these concentrations do not seem to change with mass. When comparing only parent halos from the samples, the differences are less pronounced but they are still significant. We obtain also the maximum circular velocity-and rms velocity-mass relations. These relations are shallower and more scattered for halos in clusters than in voids, and for a given circular velocity or rms velocity, the mass is smaller at z = 1 than at z = 0 for all environments. At z = 1, the differences in the halo properties with environment almost dissapear, suggesting this that the differences were stablished mainly after z ∼ 1. The halos in the cluster regions undergo more dramatic changes than those in the field or the voids. The differences in halo properties with environment are owing to (i) the dependence of halo formation time on global environment, and (ii) local effects as tidal stripping and the tumultuos histories that halos suffer in high-density regions.We calculate seminumerical models of disk galaxy evolution using halos with the concentrations and spin parameters found for the different environments. For a given disk mass, the galaxy disks have higher surface density, larger maximum circular velocity and secular bulge-to-disk ratio, lower gas fraction, and are redder as one goes from cluster to void environments. Although all these trends agree with observations, the latter tend to show more differences, suggesting this that physical ingredients not considered here as missalignment of angular momentum, halo triaxility, merging, ram pressure stripping, harassment, etc. play an important role for galaxy evolution, specially in high-density environments.