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Journal Club


"Cosmology intertwined: A review of the particle physics, astrophysics, and cosmology associated with the cosmological tensions and anomalies"

Chandra Shekhar Saraf (NCAC, Warsaw)

The standard Λ Cold Dark Matter (ΛCDM) cosmological model provides a good description of a wide range of astrophysical and cosmological data. However, there are a few big open questions that make the standard model look like an approximation to a more realistic scenario yet to be found. In this paper, we list a few important goals that need to be addressed in the next decade, taking into account the current discordances between the different cosmological probes, such as the disagreement in the value of the Hubble constant H₀, the σ₈-S₈ tension, and other less statistically significant anomalies. While these discordances can still be in part the result of systematic errors, their persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the necessity for new physics or generalisations beyond the standard model. In this paper, we focus on the 5.0 σ tension between the Planck CMB estimate of the Hubble constant H₀ and the SH0ES collaboration measurements. After showing the H₀ evaluations made from different teams using different methods and geometric calibrations, we list a few interesting new physics models that could alleviate this tension and discuss how the next decade's experiments will be crucial. Moreover, we focus on the tension of the Planck CMB data with weak lensing measurements and redshift surveys, about the value of the matter energy density Ωₘ, and the amplitude or rate of the growth of structure (σ₈ , fσ₈). We list a few interesting models proposed for alleviating this tension, and we discuss the importance of trying to fit a full array of data with a single model and not just one parameter at a time. Additionally, we present a wide range of other less discussed anomalies at a statistical significance level lower than the H₀-S₈ tensions which may also constitute hints towards new physics, and we discuss possible generic theoretical approaches that can collectively explain the non-standard nature of these signals. Finally, we give an overview of upgraded experiments and next-generation space missions and facilities on Earth that will be of crucial importance to address all these open questions.

Abdalla, E. et al., Journal of High Energy Astrophysics (2022)


"Spin-down driven neutron star to quark star conversion"

Prasad R (IISER Bhopal)

The existence of quark stars is an open problem in astrophysics, and their formation is possible in several astrophysical scenarios via the quark-hadron phase transition. We addressed the spin-down induced phase transition scenario, wherein magnetic braking drives neutron stars from their birth (Keplerian rotation) to later stages of life (slow spin). The central density is found to rise during the slowing down stages, and on reaching a critical phase transition density, the neutron star transits to a hybrid star branch, and a quark core is seeded. The further slowing down results in the growth of the quark core. We computed the mass and size of the quark core during different stages of evolutionary history. The phase transition onset leads to an anomalous change in the magnetic braking index. Also, it can excite the star's f-mode oscillations, leading to burst-type gravitational wave signals in the range of present detectors. The other emissions could be neutrino bursts and GRBs. Detection of these signals and their sky localization may help in finding the quark/hybrid stars formed via phase transition events. This talk is based on:


"Evidence of structural discontinuities in the inner core of red-giant stars"

Christian Eze (NCAC, Warsaw)

Red giants are stars in the late stages of stellar evolution. Because they have exhausted the supply of hydrogen in their core, they burn the hydrogen in the surrounding shell . Once the helium in the core starts fusing, the star enters the clump phase, which is identified as a striking feature in the color-magnitude diagram. Since clump stars share similar observational properties, they are heavily used in astrophysical studies, as probes of distance, extinction through the galaxy, galaxy density, and stellar chemical evolution. In this work, we perform the detailed observational characterization of the deepest layers of clump stars using asteroseismic data from Kepler. We find evidence for large core structural discontinuities in about 6.7% of the stars in our sample, implying that the region of mixing beyond the convective core boundary has a radiative thermal stratification. These stars are otherwise similar to the remaining stars in our sample, which may indicate that the building of the discontinuities is an intermittent phenomenon.

Vrard, Mathieu et al. "Evidence of structural discontinuities in the inner core of red-giant stars". Nature Communications 13. (2022)


"Multiple stellar population in globular clusters"

Gergely Hajdu (NCAC, Warsaw)

We use images collected with the near-infrared camera (NIRCam) on board the James Webb Space Telescope and with the Hubble Space Telescope (HST) to investigate multiple populations at the bottom of the main sequence (MS) of 47 Tucanae. The F115W vs. F115W-F322W2 CMD from NIRCam shows that, below the knee, the MS stars span a wide color range, where the majority of M-dwarfs exhibit blue colors, and a tail of stars are distributed toward the red. A similar pattern is observed from the F160W vs. F110W-F160W CMD from HST, and multiple populations of M-dwarfs are also visible in the optical F606W vs. F606W-F814W CMD. The NIRCam CMD shows a poorly-populated sequence of faint MS stars that we tentatively associate with a population of very low-mass stars. We introduce a chromosome map of M-dwarfs that reveals an extended first population and three main groups of second-population stars. By combining isochrones and synthetic spectra with appropriate chemical composition, we simulate colors and magnitudes of different stellar populations in the NIRCam filters (at metallicities [Fe/H]=-1.5 and [Fe/H]=-0.75) and identify the photometric bands that provide the most efficient diagrams to investigate the multiple populations in globular clusters. Models are compared with the observed CMDs of 47 Tucanae to constrain M-dwarfs' chemical composition. Our analysis suggests that the oxygen range needed to reproduce the colors of first- and second-population M-dwarfs is similar to that inferred from spectroscopy of red giants, challenging the proposal that the chemical variations are due to mass transfer phenomena in proto-clusters.