1812.01031
A $\gamma$-ray determination of the Universe's star-formation history
The Fermi-LAT Collaboration, et al
The light emitted by all galaxies over the history of the Universe produces the extragalactic background light (EBL) at UV, optical, and IR wavelengths. The EBL is a source of opacity for gamma rays via photon-photon interactions, leaving an imprint in the spectra of distant gamma-ray sources. Measure this attenuation using 739 active galaxies and one gamma-ray burst detected by Fermi LAT. This allows reconstruction of the evolution of the EBL and determine the SFH of the Universe over 90% of cosmic time. The SFH is consistent with independent measurements from galaxy surveys, peaking at z~2. Upper limits of the EBL at the epoch of re-ionization suggest a turnover in the abundance of faint galaxies at z~6.
1812.07962
A unifying theory of dark energy and dark matter: negative masses and matter creation within a modified $\Lambda$ CDM framework
Farnes
DE and DM constitute 95% of the observable Universe. Yet the physical nature of these 2 phenomena remains a mystery. Einstein suggested a long-forgotten solution: gravitationally repulsive negative masses, which drive cosmic expansion and cannot coalesce into light-emitting structures. However, contemporary cosmological results are derived upon the reasonable assumption that the Universe only contains positive masses. By reconsidering this assumption, construct a toy model which suggests that both dark phenomena can be unified into a single negative mass fluid. The model is a modified LCDM cosmology, and indicates that continuously created negative masses can resemble the cosmological constant and can flatten the rotation curves of galaxies. The model leads to a cyclic universe with a time-variable Hubble parameter, potentially providing compatibility with the current tension that is emerging in cosmological measurements. In the first 3-d N-body sims of negative mass matter in the scientific literature, this exotic material naturally forms haloes around galaxies that extend to several galactic radii. These haloes are not cusp. The proposed cosmological model is therefore able to predict the observed distribution of DM in galaxies from first principles. The model makes several testable predictions and seems to have the potential to be consistent with observational evidence from distant SNe, the CMB, and galaxy clusters. These findings may imply that negative masses are a real and physical aspect of our Universe, or alternatively may imply the existence of a superseding theory that in some limit can be modeled by effective negative masses. Both cases lead to the surprising conclusion that the compelling puzzle of the dark Universe may have been due to a simple sign error.
1812. 1812.01679
Cluster cosmology constraints from the 2500 deg$^2$ SPT-SZ Survey: inclusion of weak gravitational lensing data from Magellan and the Hubble Space Telescope
Bocquet, Dietrich, Schrabback, et al
Derive cosmo constraints using a galaxy cluster sample selected from the 2500 deg2 SPT-SZ survey. The sample spans 0.25<z<1.75 and consists of 377 cluster candidates with SZ detection significance xi>5. The sample is supplemented with optical weak gravitational lensing measurements of 32 clusters in the range 0.29<z<1.13 (using Magelland and HST) and X_ray measurements of 89 cluster in the range 0.25<z<1.75 (Chandra). Rely on minimal modeling assumptions: i) WL provides and accurate means of measuring halo masses ii) the mean SZ and X-ray observables are related to the true halo mass through power-law relations in mass and dimensionless Hubble parameter E(z) with a-priori unknown parameters, iii) there is (correlated, lognormal) intrinsic scatter and scatter due to measurement uncertainties relating these observables to their mean relations. Assuming a flat nuLCDM model, in which the sum of neutrino masses is a free parameter, measure Omega_m=0.276±0.047, sigma8=0.781±0.037, and the parameter combination sigma8(Omega_m/0.3)^0.2=0.766±0.025. The redshift evolution of the X-ray Y_X--mass and Y_X-- mass relation shows a 2.3 sigma deviation from self-similarity. The mass-slope of the M_gas--mass relation is steeper than self-similarity at the 2.5 sigma level. In a nu wCDM cosmology, measure the DE equation of state parameter w=-1.55±0.41 from the cluster data. Perform a measurement of the growth of structure since z~1.7 and find no evidence for tension with the prediction from GR.
1812.02125
The behavior of galactic cosmic ray intensity during solar activity cycle 24
Ross, Chaplin
Studied long-term variations of galactic cosmic ray (GCR) intensity in relation to the sunspot number (SSN) during the most recent solar cycles. This study analyses the time-lag between the GCR intensity and SSN, and hysteresis plots of the GCR count range against SSN for solar activity cycles 20-23 to validate a methodology against previous results in the literature, before applying the method to provide a timely update on the behavior of cycle 24. Cross-plots of SSN vs GCR show a clear difference between the odd-numbered and even-numbered cycles. Linear and elliptical models have been fit to the data with the linear fit and elliptical model proving the more suitable model for even-numbered and odd-numbered solar activity cycles respectively, in agreement with previous literature. Through the application of these methods for the 24th solar activity cycle, it has been show that cycle 24 experienced a lag of 2-4 months and follows the trend of the preceding activity cycles albeit with a slightly longer lag than previous even-numbered cycles. It has been shown through the hysteresis analysis that the linear fit is a better representative model for cycle 24, as the ellipse model doesn't show a significant improvement, which is also in agreement with previous even-numbered cycles.
1812.02148
Dipole distortions in the intergalactic medium
Inman, Pen, Villaescusa-Navarro
Baryonic feedback can significantly modify the spatial distribution of matter on small scales and create a bulk relative velocity between the dominant CDM and the hot gas. Study the consequences of such bulk motions using two high resolution hydro sims, one with no feedback and one with very strong feedback. Find that relative velocities of order 100 km/s are produced in the strong feedback sims whereas it is much smaller when there is no feedback. Such relative motions induce dipole distortions to the gas, which is quantified by computing the dipole correlation function. Find halo coordinates and velocities are systematically changed in the direction of the relative velocity. Finally, discuss potential to observe the relative velocity via large scale structure, SZ and line emission measurements. Given the nonlinear nature of this effect, it should next be studied in simulations with different feedback implementations/strengths to determine the available model space.
1812.02285
How may hydrated NEOs are there?
Rivkin, DeMeo
Hydrated minerals are tracers of early solar system history, and have been proposed as a possible focus for economic activity in space. NEOs are important to both of these, especially the most accessible members of that community. Because there are very few identified hydrated NEOs, use the Ch spectral class of asteroids as a proxy for hydrated asteroids, and use published work about NEO delivery, main-belt taxonomic distributions, NEO taxonomic distributions, and observed orbital distributions to estimate the number of hydrated asteroids with different threshold sizes and at different levels of accessibility. Expect 53±27 Ch asteroids to be present in the known population of NEOs >1 km diameter, and using two different approaches to estimate accessibility, expect 17±9 of them to be more accessible on a round trip than the surface of the Moon. If there is no need to define minimum size, expect 700±350 hydrated objects that meet that accessibility criterion. While there are few unknown NEOs larger than 1 km, the population of smaller NEOs yet to be discovered could also be expected to contain proportionally-many hydrated objects. Finally, estimate that hydrated NEOs are unlikely to bring enough water to account for the ice found at the lunar poles, though it is possible that asteroid-delivered hydrated minerals could be found near their impact sites across the lunar surface.
1812.02333
Null test for interactions in the dark sector
von Martins, et al
Since there is no known symmetry in Nature that prevents a non-animal coupling between the DE and CDM components, such a possibility constitutes an alternative to standard cosmology, with its theoretical and observational consequences being of great interest. In this paper, propose a new null test on the standard evolution of the dark sector based on the time dependence of the ratio between the CDM and DE energy densities which, in the standard LCDM scenario, scales necessarily as a^{-3}. Use the latest measurements of SN Ia, cosmic chronometers and angular BAOs to reconstruct the expansion history using model-independent Machine Learning techniques, namely, the Linear Model formalism and Gaussian Processes. Find that while the standard evolution is consistent with the data at 3 sigma level, some deviations from the LCDM model are found at low redshifts, which may be associated with the current tension between local and global determinations of H0.
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