Iranian Journal of Astronomy and AstrophysicsIranian Journal of Astronomy and Astrophysics
http://ijaa.du.ac.ir/
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http://ijaa.du.ac.ir/
Feed provided by Iranian Journal of Astronomy and Astrophysics. Click to visit.Spin and Isospin Asymmetry, Equation of State and Neutron Stars
http://ijaa.du.ac.ir/article_60_10.html
In the present work, we have obtained the equation of state for neutron star matter considering the in uence of the ferromagnetic and antiferromagnetic spin state. We have also investigated the structure of neutron stars. According to our results, the spin asymmetry stiens the equation of state and leads to high mass for the neutron star.Wed, 30 Nov 2016 20:30:00 +0100Structure formation and generalized second law of thermodynamics in some viable f(R)-gravity models
http://ijaa.du.ac.ir/article_67_10.html
We investigate the growth of matter density perturbations as well as the generalized second law (GSL) of thermodynamics in the framework of f(R)-gravity. We consider a spatially ﬂat FRW universe ﬁlled with the pressureless matter and radiation which is enclosed by the dynamical apparent horizon with the Hawking temperature. For some viable f(R) models containing the Starobinsky, Hu-Sawicki, Exponential, Tsujikawa and AB models, we ﬁrst explore numerically the evolution of some cosmo-logical parameters like the Hubble parameter, the Ricci scalar, the deceleration param-eter, the density parameters and the equation of state parameters. Then, we examine the validity of GSL and obtain the growth factor of structure formation. We ﬁnd that for the aforementioned models, the GSL is satisﬁed from the early times to the present epoch. But in the farther future, the GSL for all models is violated. Our numerical results also show that for all models, the growth factor for larger structures, like the ΛCDM model, ﬁt the data very well.Wed, 30 Nov 2016 20:30:00 +0100Formation of Large Structures in the Acceleration Universe with a Hybrid Expansion Law
http://ijaa.du.ac.ir/article_68_10.html
In the current paper, we have studied the eﬀect of dark energy on for-mation where dark energy exists in the background. For this purpose, we used both WMAP9 and Planck data to study how the radius changes with redshift in these mod-els. We used diﬀerent data sets to ﬁx the cosmological parameters to obtain a solution for a spherical region under collapse. The mechanism of structure formation for dark and baryonic matter is diﬀerent. When processed by gravitational instability, den-sity perturbations have given rise to collapsed dark matter structures, called halos. These dark matter halos oﬀer the backdrop for the subsequent formation of all col-lapsed baryonic structures, including stars, galaxies, and galaxy clusters. In Planck Data forΛCDM, with the presence of dark energy in the background, the formation of baryonic matter is delayed. Therefore, it is a factor for the largening of the baryonic matter radius. Accompanying dark energy is entailing an increment of dark matter virial radius. For WACDM Data, dark energy alongside time-dependent parameter of state and baryon acoustic oscillations are the reasons for the delay of dark matter formation and the radius reduction. Due to the lack of data without baryonic acoustic waves in the background, we are left unable to delineate its impact on the structures. In WCDM(BAO +H0) and WCDM(H0), the lack of BAO shows a critical role in the delaying of baryonic matter structure formation. Respectively, it causes growing virial radius of dark matter. BAO, without taking dark energy into accounts, is the reason for the increasing and decresing of radius of dark and baryonic matter. It also delays baryonic matter formation. In ΛCDM(BAO +H0) and ΛCDM(H0), We have studied ΛCDM data for standard model under two circumstances: (a) ΛCDM(BAO + H0),(b) ΛCDM(H0) data. Dark energy in this data delays formation and intensiﬁes virial radius of baryonic matter. Our studies show WCDM andΛCDM have the same eﬀect on formation if we do not consider dark energy in BG. Planck data, in comparing with WMAP, has important role in describing standard model.Wed, 30 Nov 2016 20:30:00 +0100