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Register a new userNew Cosmology with Clusters of Galaxies
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Peter Schuecker
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The review summarizes present and future applications of galaxy clusters to cosmology with emphasis on nearby X-ray clusters. The discussion includes the density of dark matter, the normalization of the matter power spectrum, neutrino masses, and especially the equation of state of the dark energy, the interaction between dark energy and ordinary matter, gravitational holography, and the effects of extra-dimensions.
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A modified gravitational theory explains early universe and late time cosmology, galaxy and galaxy cluster dynamics. The modified gravity (MOG) theory extends general relativity (GR) by three extra degrees of freedom: a scalar field $G$, enhancing the strength of the Newtonian gravitational constant $G_N$, a gravitational, spin 1 vector graviton field $phi_mu$, and the effective mass $mu$ of the ultralight spin 1 graviton. For $t < t_{rm rec}$, where $t_{rm rec}$ denotes the time of recombination and re-ionization, the density of the vector graviton $rho_phi > rho_b$, where $rho_b$ is the density of baryons, while for $t > t_{rm rec}$ we have $rho_b > rho_phi$. The matter density is parameterized by $Omega_M=Omega_b+Omega_phi+Omega_r$ where $Omega_r=Omega_gamma+Omega_ u$. For the cosmological parameter values obtained by the Planck Collaboration, the CMB acoustical oscillation power spectrum, polarization and lensing data can be fitted as in the $Lambda$CDM model. When the baryon density $rho_b$ dominates the late time universe, MOG explains galaxy rotation curves, the dynamics of galaxy clusters, galaxy lensing and the galaxy clusters matter power spectrum without dominant dark matter.
Clusters of galaxies contain a hot gas, which emits in X-rays. X-ray telescopes such as XMM-Newton allow to study this plasma to obtain information on physical quantities of these objects. We present here some results on the total mass density distribution of clusters obtained with XMM-Newton based on the hydrostatic approach. These results can be compared to models based on cold dark matter. Generally good agreement is found between observations and models. Furthermore we present a study on physical properties of a distant merging cluster of galaxies, which demonstrates the potential of XMM-Newton studies on this class of objects.
Motivated by recent suggestions that a number of observed galaxy clusters have masses which are too high for their given redshift to occur naturally in a standard model cosmology, we use Extreme Value Statistics to construct confidence regions in the mass-redshift plane for the most extreme objects expected in the universe. We show how such a diagram not only provides a way of potentially ruling out the concordance cosmology, but also allows us to differentiate between alternative models of enhanced structure formation. We compare our theoretical prediction with observations, placing currently observed high and low redshift clusters on a mass-redshift diagram and find -- provided we consider the full sky to avoid a posteriori selection effects -- that none are in significant tension with concordance cosmology.
Power-law cosmologies, in which the cosmological scale factor evolves as a power law in the age, $a propto t^{alpha}$ with $alpha ga 1$, regardless of the matter content or cosmological epoch, is comfortably concordant with a host of cosmological observations.} {In this article, we use recent measurements of the X-ray gas mass fractions in clusters of galaxies to constrain the $alpha$ parameter with curvature $k = pm1, 0$. We find that the best fit happens for an open scenario with the power index $alpha = 1.14 pm 0.05$, though the flat and closed model can not be rule out at very high confidence level.} {Our results are in agreement with other recent analyses and show that the X-ray gas mass fraction measurements in clusters of galaxies provide a complementary test to the power law cosmology.
NIKA2 is a dual-band millimetric camera of thousands of Kinetic Inductance Detectors (KID) installed at the IRAM 30-meter telescope in the Spanish Sierra Nevada. The instrument commissioning was completed in September 2017, and NIKA2 is now open to the scientific community and will operate for the next decade. NIKA2 has well-adapted instrumental design and performance to produce high-resolution maps of the thermal Sunyaev-Zeldovich (SZ) effect toward intermediate and high redshift galaxy clusters. Moreover, it benefits from a guaranteed time large program dedicated to mapping a representative sample of galaxy clusters via SZ and that includes X-ray follow-ups. The main expected outputs of the SZ large program are the constraints on the redshift evolution of the pressure profile and the mass-observable relation. The first SZ mapping of a galaxy cluster with NIKA2 was produced, as part of the SZ large program. We found a sizable impact of the intracluster medium dynamics on the integrated SZ observables. This shows NIKA2 capabilities for the precise characterisation of the mass-observable relation that is required for accurate cosmology with galaxy clusters.
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