Narrow Results

In this paper, we examine the dynamical evolution of flat FRW cosmological model in $f(R,L_m)$ gravity theory. We consider the general form of $f(R,L_m)$ defined as $f(R,L_m)=\mathrm{\Lambda }+\frac{\alpha }{2}R+\beta L_m^n$, where $\mathrm{\Lambda }$, $\alpha$, $\beta$, $n$ are model parameters, with the matter Lagrangian given by $L_m=-p$. We investigate the model through phase plane analysis, actively studying the evolution of cosmological solutions using dynamical systems techniques. To analyze the evolution equations, we introduce suitable transformations of variables and discuss the corresponding solutions by phase-plane analysis. The nature of critical points is analyzed and stable attractors are examined for $f(R,L_m)$ gravity cosmological model. We examine the linear and classical stabilities of the model and discuss it in detail. Further, we investigate the transition stage of the Universe, i.e. from the early decelerating stage to the present accelerating phase of the Universe by evolution of the effective equation of state, $\{r,s\}$ parameters and statefinder diagnostics for the central values of parameters $\alpha ,\beta$ and $n$ constrained using MCMC technique with cosmic chronometer data.

Using the deepest and most complete observations of distant galaxies, we investigate the progenitors of present-day large spirals. Observations include spatially-resolved kinematics, detailed morphologies and photometry from UV to mid-IR. Six billion years ago, half of the present-day spirals were starbursts experiencing major mergers, evidenced by their anomalous kinematics and morphologies. They are consequently modeled using hydrodynamic models of mergers and it perfectly matches with merger rate predictions by state-of-the-art-ΛCDM semi-empirical models. Furthermore imprints in the halo of local galaxies such as M31 or NGC5907 are likely caused by major merger relics. This suggests that the hierarchical scenario has played a major role in shaping the massive galaxies of the Hubble sequence. Linking galaxy properties at different epochs is the best way to fully understand galaxy formation processes and we have tested such a link through generated series of simulations of gas-rich mergers. Mergers have expelled material in galactic haloes and beyond, possibly explaining 60% of the missing baryons in Milky-Way (MW) mass galaxies. A past major merger in M31 might affect drastically our understanding of Local Group galaxies, including MW dwarves. We also propose future directions to observationally constrain the necessary ingredients in galaxy simulations.

Dark energy is the largest fraction of the energy density of our universe — yet it remains one of the enduring enigmas of our times. Here we show that dark energy can be used to solve 2 tantalizing mysteries of the observable universe. We build on existing models of dark energy linked to neutrino masses. In these models, dark energy can undergo phase transitions and form black holes. Here we look at the implications of the family structure of neutrinos for the phase transitions in dark energy and associated peaks in black hole formation. It has been previously shown that one of these peaks in black hole formation is associated with the observed peak in quasar formation at redshifts $z\sim 2.5$. Here, we predict that there will also be an earlier peak in the dark energy black holes at high redshifts $z\sim 18$. These dark energy black holes formed at high redshifts are Intermediate Mass Black Holes (IMBHs). These dark energy black holes at large redshift can help explain both the EDGES observations and the observations of large Supermassive Black Holes (SMBHs) at redshifts of 7 or larger. This work directs us to actively look for these dark energy black holes at these high redshifts as predicted here through targeted searches for these black holes at the redshifts $z$ near 18. There is a slight dependence of the location of the peak on the lightest neutrino mass. This may enable a measurement of the lightest neutrino mass — something which has eluded us so far. Finding these dark energy black holes of Intermediate Mass should be within the reach of upcoming observations — particularly with the James Webb Space Telescope — but perhaps also through the use of other innovative techniques focusing specifically on the redshifts $z$ around 18.

The main target of this study is a search for the origin of two ground level enhancements (GLEs), observed on December of 2003 at sea level by using the TUPI muon telescope. The results show that one of them has a strong correlation with solar flare, while the other has an unknown origin, because there is not satellite report of solar flare, nor the prompt X-ray emission and neither the excess of nuclei during the raster scan where the GLE was observed. Even so, two possibilities are analyzed: the solar flare hypothesis and the gamma ray burst (GRB) hypothesis.

The observational data for the Galactic source Cygnus X-3 collected with the SHALON mirror Cherenkov telescope are presented. The Cygnus X-3 binary have been regularly observed since 1995 with the average γ-ray flux F (E > 0.8 TeV) = (6.8 ± 0.7) × 10^-13cm^-2s^-1. The flux in year 2003 was (1.79 ± 0.33) × 10^-12cm^-2s^-1. Earlier, in 1997, an increase of the flux was also observed.

The new distant metagalactic γ-sources 1739+522 (z = 1.375) and 3c454.3 (z = 0.857) are detected at energies E > 0.8 TeV with the fluxes (0.53 ± 0.10) × 10^-12cm^-2s^-1 and (0.43 ± 0.13) × 10^-12cm^-2s^-1, respectively. The γ-ray spectra and fluxes of known blazars Mkn 421, Mkn 501 and distant flat-spectrum radio quasars 1739+522 and 3c454.3 are presented.

Dark energy and the accelerated expansion of the universe have been the direct predictions of the distant supernovae Ia observations which are also supported, indirectly, by the observations of the CMB anisotropies, gravitational lensing and the studies of galaxy clusters. Today these results are accommodated in what has become the concordance cosmology: a universe with flat spatial sections t = constant with about 70% of its energy in the form of Einstein's cosmological constant Λ and about 25% in the form of dark matter (made of perhaps weakly-interacting massive particles). Though the composition is weird, the theory has shown remarkable successes at many fronts.

However, we find that as more and more supernovae Ia are observed, more accurately and towards higher redshift, the probability that the data are well-explained by the cosmological models decreases alarmingly, finally ruling out the concordance model at more than 95% confidence level. This raises doubts against the "standard candle"-hypothesis of the supernovae Ia and their use in constraining the cosmological models. We need a better understanding of the entire SN Ia phenomenon in order to extract cosmological consequences from them.

We present the analysis of the quadrupole phases of the Internal Linear Combination map, ILC(I) and (III), derived by the WMAP team (one- and three-year data release). This approach allows us to see the global trend of non-Gaussianity of the quadrupoles for the ILC(III) map through phase correlations with the foregrounds. Significant phase correlations are found between the ILC(III) quadrupole and the WMAP foreground phases for the K-W band: the phases of the ILC(III) quadrupole ξ_2,1, ξ_2,2 and those of the foregrounds at K–W bands Φ_2,1, Φ_2,2 display significant symmetry: ξ_2,1 + Φ_2,1 ≃ ξ_2,2 + Φ_2,2, which is a strong indication that the morphology of the ILC(III) quadrupole is a mere reflection of that the foreground quadrupole through coupling. To clarify this issue we exploit the symmetry of the CMB power, which is invariant under permutation of the index m = 1 ⇔ 2. By simple rotation of the ILC(III) phases with the same angle we reach the phases of the foreground quadrupole. We discuss possible sources of phase correlation and come to the conclusion that the phases of the ILC(III) quadrupole reflect most likely systematic effects such as changing of the gain factor for the three-year data release with respect to the one-year, rather than manifestation of the primordial non-Gaussianity.

The evidence is reviewed that the primary form of energy that escapes to infinity from gamma-ray bursts (GRBs) is gamma-rays, and/or Poynting flux, and that the kinetic energy in ultrarelativistic baryons is a secondary component resulting from acceleration of baryons by radiation pressure near or beyond the photosphere. This could account for several observed characteristics of observed GRB spectra and light curves, such as the typical peak photon energy, the correlation of this peak with apparent GRB energy, and the profiles and spectral lagging of GRB subpulses.

In a few dozen seconds, gamma ray bursts (GRBs) emit up to ~10⁵⁴ erg in terms of an equivalent isotropically radiated energy E_iso, so they can be observed up to z ~ 10. Thus, these phenomena appear to be very promising tools to describe the expansion rate history of the universe. Here, we review the use of the E_p,i–E_iso correlation of GRBs to measure the cosmological density parameter Ω_M. We show that the present data set of GRBs, coupled with the assumption that we live in a flat universe, can provide independent evidence, from other probes, that Ω_M ~ 0.3. We show that current (e.g. Swift, Fermi/GBM, Konus-WIND) and forthcoming gamma ray burst (GRB) experiments (e.g. CALET/GBM, SVOM, Lomonosov/UFFO, LOFT/WFM) will allow us to constrain Ω_M with an accuracy comparable to that currently exhibited by Type Ia supernovae (SNe–Ia) and to study the properties of dark energy and their evolution with time.

In a previous paper (part I), the mathematical properties of the cosmic microwave background radiation (CMBR) power spectrum which presents oscillations were discussed. Here, we discuss the physical interpretation: a power spectrum with oscillations is a rather normal characteristic expected from any fluid with clouds of overdensities that emit/absorb radiation or interact gravitationally with the photons, and with a finite range of sizes and distances for those clouds. The standard cosmological interpretation of "acoustic" peaks is just a particular case; peaks in the power spectrum might be generated in scenarios within some alternative cosmological model that have nothing to do with oscillations due to gravitational compression in a fluid.

We also calculate the angular correlation function of the anisotropies from the Wilkinson Microwave Anisotropy Probe (WMAP)-7 yr and ACT data, in an attempt to derive the minimum number of parameters a polynomial function should have to fit it: a set of polynomial functions with a total of ≈ 6 free parameters, apart from the amplitude, is enough to reproduce the first two peaks. However, the standard model with six tunable free parameters also reproduces higher-order peaks, giving the standard model a higher confidence. At present, while no simple function with six free parameters is found to give a fit as good as the one given by the standard cosmological model, we may consider the predictive power of the standard model beyond an instrumentalist approach (such as the Ptolemaic astronomy model of the orbits of the planets).

In this paper, we have investigated a very natural question regarding the dynamics of the universe, namely, the possibility of its decelerating phase immediately after the present accelerating phase. To begin with, we have focused on the matter creation theory which is considered to be a viable alternative to dark energy and modified gravity models. Moreover, we have introduced the cosmographic approach which allows us to express the free parameters of a cosmological model in terms of the known cosmographic parameters. Assuming a generalized matter creation rate, we have discussed the theoretical bounds on the model parameters allowing the future deceleration of the universe. Moreover, using the observational bounds on the cosmographic parameters obtained from the low redshifts observational probes, we have also examined the chance of a decelerating phase of the universe. Finally, considering a variety of known cosmological models and parametrizations, we have tested the same possibility. Our analysis shows that the chance of a future decelerating expansion of the universe is highly dependent on the choice of the cosmological models and parametrizations and also on the observational data. Even though the future decelerating expansion is allowed in some cosmological frameworks, but we do not see any strong evidence in favor of this. Perhaps, the future cosmological surveys could offer some more information regarding the fate of the universe.

Recent observations from the James Webb Space Telescope have identified a population of massive galaxy sources ($>10^{10}{M}_{\odot }$) at $z>7$–10, formed less than 700Myr after the Big Bang. Such massive galaxies do not have enough time to form within the standard cosmological model, and hence these observations pose a significant challenge. A number of possible solutions to this problem have been put forward. In this essay, we discuss two more theoretical and speculative possibilities.

In this paper, we present a cosmology-independent method to constrain cosmological models from the latest 221 gamma-ray bursts (GRBs) sample, including 49 GRBs from Fermi catalog with the Amati relation (the $E_{\mathrm{\text{p}}}-E_{\mathrm{\text{iso}}}$ correlation), which are calibrated by using a Gaussian process from the Pantheon+ type Ia supernovae (SNe Ia) sample. With 182 GRBs at $0.8\le z\le 8.2$ in the Hubble diagram and the latest observational Hubble data (OHD) by the Markov Chain Monte Carlo (MCMC) method, we obtained ${\mathrm{\Omega }}_{\mathrm{\text{m}}}=0.348_{-0.066}^{+0.048}$ and $h=0.680_{-0.029}^{+0.029}$ for the flat $\mathrm{\Lambda }$CDM model and ${\mathrm{\Omega }}_{\mathrm{\text{m}}}=0.318_{-0.059}^{+0.067}$, $h=0.704_{-0.068}^{+0.055}$, $w=-1.21_{-0.67}^{+0.32}$ for the flat wCDM model. These results are consistent with those in which the coefficients of the Amati relation and the cosmological parameters fitted simultaneously.

We suggest novel statistics for the CMB maps that are sensitive to non-Gaussian features. These statistics are natural generalizations of the geometrical and topological methods that have been already used in cosmology such as the cumulative distribution function and genus. We compute the distribution functions of the Partial Minkowski Functionals for the excursion set above or bellow a constant temperature threshold. Minkowski Functionals are additive and are translationally and rotationally invariant. Thus, they can be used for patchy and/or incomplete coverage. The technique is highly efficient computationally (it requires only O(N) operations, where N is the number of pixels per one threshold level). Further, the procedure makes it possible to split large data sets into smaller subsets. The full advantage of these statistics can be obtained only on very large data sets. We apply it to the 4-year DMR COBE data corrected for the Galaxy contamination as an illustration of the technique.

A huge amount of good quality astrophysical data converges towards the picture of a spatially flat universe undergoing the today observed phase of accelerated expansion. This new observational trend is commonly addressed as Precision Cosmology. Despite of the quality of astrophysical surveys, the nature of dark energy dominating the matter-energy content of the universe is still unknown and a lot of different scenarios are viable candidates to explain cosmic acceleration. Methods to test these cosmological models are based on distance measurements and lookback time toward astronomical objects used as standard candles. I discuss the characterizing parameters and constraints of three different classes of dark energy models pointing out the related degeneracy problem which is the signal that more data at low (z ~ 0 ÷ 1), medium (1 < z < 10) and high (10 < z < 1000) redshift are needed to definitively select realistic models.

The South Pole Telescope (SPT) is a 10-meter telescope designed to survey the millimeter-wave sky, taking advantage of the exceptional observing conditions at the Amundsen-Scott South Pole Station. The telescope and its ground-breaking 960-element bolometric camera finished surveying 2500 square degrees at 95. 150, and 220 GHz in November 2011. We have discovered hundreds of galaxy clusters in the SPT-SZ survey through the Sunyaev-Zel’dovich (SZ) effect. The formation of galaxy clusters the largest bound objects in the universe is highly sensitive to dark energy and the history of structure formation. I will discuss the cosmological constraints from the SPT-SZ galaxy cluster sample as well as future prospects with the soon to-be-installed SPT-3G camera.

We introduce Probing Radio Intensity at high-Z from Marion (PRI^ZM), a new experiment designed to measure the globally averaged sky brightness, including the expected redshifted 21cm neutral hydrogen absorption feature arising from the formation of the first stars. PRI^ZM consists of two dual-polarization antennas operating at central frequencies of 70 and 100MHz, and the experiment is located on Marion Island in the sub-Antarctic. We describe the initial design and configuration of the PRI^ZM instrument that was installed in 2017, and we present preliminary data that demonstrate that Marion Island offers an exceptionally clean observing environment, with essentially no visible contamination within the FM band.

Measurements of redshifted 21cm emission of neutral hydrogen at $\lesssim 30$MHz have the potential to probe the cosmic “dark ages,” a period of the universe’s history that remains unobserved to date. Observations at these frequencies are exceptionally challenging because of bright Galactic foregrounds, ionospheric contamination, and terrestrial radio-frequency interference. Very few sky maps exist at $\lesssim 30$MHz, and most have modest resolution. We introduce the Array of Long Baseline Antennas for Taking Radio Observations from the Sub-Antarctic (ALBATROS), a new experiment that aims to image low-frequency Galactic emission with an order-of-magnitude improvement in resolution over existing data. The ALBATROS array will consist of antenna stations that operate autonomously, each recording baseband data that will be interferometrically combined offline. The array will be installed on Marion Island and will ultimately comprise 10 stations, with an operating frequency range of 1.2–125MHz and maximum baseline lengths of $\sim 20$km. We present the ALBATROS instrument design and discuss pathfinder observations that were taken from Marion Island during 2018–2019.

An increasing number of instances in extreme weather events over the global oceans have deepened the concerns on the impact of climate change. The frequency of extreme weather events is also seen to increase attributes to climate change across the globe. In the Indian context, there has been about 285 reported flooding events over the period from 1950 to 2017 that affected nearly 850 million people with many causalities. As the global oceans become stormier, the effects are seen in rising sea level and infrastructural facilities. Major flooding events are caused by tropical cyclone-induced storm surge and associated breaking waves. These extreme weather events coupled with sea level rise have serious repercussions on the coastal vulnerability. Also recently, there is an upsurge in the intensity and tropical cyclone size that forms over the North Indian Ocean region that brought attention among the scientific community. The worst possible scenario of extreme water level can occur when the time of storm surge occurrence coincides with the astronomical high water. This review paper provides a comprehensive overview on the research developments and efforts made in ocean wave modeling in particular for the Indian seas. As per the Fifth Assessment Report of Intergovernmental Panel on Climate Change (IPCC), the role and influence of ocean surface gravity wave in the climate system are considered to be very important. At present, numerical models are widely used and that can be used to hindcast and forecast the wave characteristics over both regional and global ocean basin scales. A detailed overview on the observational techniques is listed along with the historical perspective and recent developments in wind-wave modeling for the Indian seas. Recent developments in computing technology and advanced numerical techniques have made it possible to solve the complex problems in coastal science and engineering using state-of-the-art numerical models providing realistic estimates, cost effective and having immense potential in operational weather centers. This review also deals with some of the important issues and future directions in wind-wave modeling studies such as improvements required in momentum transfer, bottom dissipation, and rain–wave interaction effects that require detailed understanding and concentrated efforts.