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NASA's Hubble, Chandra Find Clues that May Help Identify Dark Matter

Using observations from NASA's Hubble Space Telescope and Chandra X-ray Observatory, astronomers have found that dark matter does not slow down when colliding with itself, meaning it interacts with itself less than previously thought. Researchers say this finding narrows down the options for what this mysterious substance might be.
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Dark Matter is Darker Than Once Thought

Galaxy clusters, the largest objects in the Universe held together by their own gravity, are made up of three main components: stars, clouds of hot gas, and dark matter. When galaxy clusters collide, the clouds of gas enveloping the galaxies crash into each other and slow down or stop. The stars are much less affected by the drag from the gas and, because they occupy much less space, they glide past each other like ships passing in the night.
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Detection of gamma rays from a newly discovered dwarf galaxy may point to dark matter

A newly discovered dwarf galaxy orbiting our own Milky Way has offered up a surprise - it appears to be radiating gamma rays, according to an analysis by physicists at Carnegie Mellon, Brown, and Cambridge universities. The exact source of this high-energy light is uncertain at this point, but it just might be a signal of dark matter lurking at the galaxy's center.
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Title: How Late can the Dark Matter form in our universe?
Author: Abir Sarkar, Subinoy Das, Shiv K. Sethi
Version v2

We put constraints on the epoch of dark matter formation for a class of non-WIMP (Weakly Interacting Massive Particle) dark matter candidates. These models allow a fraction of Cold Dark Matter (CDM) to be formed between the epoch of Big Bang Nucleosynthesis (BBN) and the matter radiation equality. We show that for such models the matter power spectra might get strong suppression even on scales that could be probed by linear perturbation theory at low redshifts. Unlike the case of Warm Dark Matter (WDM), where the mass of the dark matter particle controls the suppression scale, in Late Forming Dark Matter (LFDM) scenario, it is the redshift of the dark matter formation which determines the form of the matter power spectra. We use the Sloan Digital Sky Survey (SDSS) galaxy clustering data and the linear matter power spectrum reconstructed from the Lyman-alpha data to find the latest epoch of the dark matter formation in our universe. If all the observed dark matter is late forming, we find lower bounds on the redshift of dark matter formation z_f>1.08 x 10^5 at 99.73 % C.L from the SDSS data and z_f>9 x 10^5, at the same C.L, from the Lyman-alpha data. If only a fraction of the dark matter is late forming then we find tentative evidence of the presence of LFDM from the Lyman-alpha data. Upcoming data from SDSS-III/BOSS (Baryon Oscillation Spectroscopic Survey) will allow us to explore this issue in more detail.

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Title: Dark matter in galaxies: the dark matter particle mass is about 7 keV
Author: H. J. de Vega, N. G. Sanchez
(Version v3)

Warm dark matter (WDM) means DM particles with mass m in the keV scale. For large scales, (structures beyond ~ 100 kpc) WDM and CDM yield identical results which agree with observations. For intermediate scales, WDM gives the correct abundance of substructures. Inside galaxy cores, below ~ 100 pc, N-body WDM classical physics simulations are incorrect because at such scales quantum WDM effects are important. WDM quantum calculations (Thomas-Fermi approach) provide galaxy cores, galaxy masses, velocity dispersions and density profiles in agreement with the observations. For a dark matter particle decoupling at thermal equilibrium (thermal relic), all evidences point out to a 2 keV particle. Remarkably enough, sterile neutrinos decouple out of thermal equilibrium with a primordial power spectrum similar to a 2 keV thermal relic when the sterile neutrino mass is about 7 keV. Therefore, WDM can be formed by 7 keV sterile neutrinos. Excitingly enough, Bulbul et al. (2014) announced the detection of a cluster X-ray emission line that could correspond to the decay of a 7.1 keV sterile neutrino and to a neutrino decay mixing angle of \sin^2 2 \theta ~ 7 10^{-11} . This is a further argument in favour of sterile neutrino WDM. Baryons, represent 10 % of DM or less in galaxies and are expected to give a correction to pure WDM results. The detection of the DM particle depends upon the particle physics model. Sterile neutrinos with keV scale mass (the main WDM candidate) can be detected in beta decay for Tritium and Renium and in the electron capture in Holmiun. The sterile neutrino decay into X rays can be detected observing DM dominated galaxies and through the distortion of the black-body CMB spectrum. So far, not a single valid objection arose against WDM.

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Dark matter half what we thought

A new measurement of dark matter in the Milky Way has revealed there is half as much of the mysterious substance as previously thought.
Australian astronomers used a method developed almost 100 years ago to discover that the weight of dark matter in our own galaxy is 800 000 000 000 (or 8 x 1011) times the mass of the Sun.

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Large number of Dark Matter peaks found using Gravitational Lensing.

A number of studies have shown that Dark Matter is the principal mass component of the Universe making up about 80% of the mass budget. The most direct technique to reveal the Dark Matter distribution is by using the gravitational lensing technique. Indeed, following Einstein's theory of Gravitation, we know that a mass concentration will deform locally the Space-Time and the observed shapes of distant galaxies seen through the such concentration will be deflected and distorted. By measuring the exact shapes of millions of these distant galaxies we can then map accurately the mass distribution in the Universe, and identify the mass peaks tracing mass concentration along their line of sight. Importantly, the number of mass peaks as a function of the mass peak significance encodes important information on the cosmological world model. In particular this distribution is sensitive to the nature of Gravitational force at large scales as well as the geometry of the Universe. Measuring mass peaks is thus one of the most attractive way to probe the relative importance and nature of Dark Matter and Dark Energy, measure the evolution the Universe and predict its fate.
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Title: Is the effect of the Sun's gravitational potential on dark matter particles observable?
Author: Nassim Bozorgnia, Thomas Schwetz

We consider the effect of the Sun's gravitational potential on the local phase space distribution of dark matter particles, focusing on its implication for the annual modulation signal in direct detection experiments. We perform a fit to the modulation signal observed in DAMA/LIBRA and show that the allowed region shrinks if Solar gravitational focusing (GF) is included compared to the one without GF. Furthermore, we consider a possible signal in a generic future direct detection experiment, irrespective of the DAMA/LIBRA signal. Even for scattering cross sections close to the current bound and a large exposure of a xenon target with 270 ton yr it will be hard to establish the presence of GF from data. In the region of dark matter masses below 40 GeV an annual modulation signal can be established for our assumed experimental setup, however GF is negligible for low masses. In the high mass region, where GF is more important, the significance of annual modulation itself is very low. We obtain similar results for lighter targets such as Ge and Ar. We comment also on inelastic scattering, noting that GF becomes somewhat more important for exothermic scattering compared to the elastic case.

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Fermi Telescope data tantalize with new clues to dark matter

A new study of gamma-ray light from the center of the galaxy makes the strongest case to date that some of this emission may arise from dark matter, an unknown substance making up most of the material universe.
Using publicly available data from NASAs Fermi Gamma-ray Space Telescope, independent scientists at the Fermi National Accelerator Laboratory, the Harvard-Smithsonian Center for Astrophysics (CfA), the Massachusetts Institute of Technology and the University of Chicago have developed new maps showing that the galactic center produces more high-energy gamma rays than can be explained by known sources and that this excess emission is consistent with some forms of dark matter.

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Title: Relic Gravity Waves and 7 keV Dark Matter from a GeV scale inflaton
Author: F. Bezrukov, D. Gorbunov

We study the mechanism of generation of 7 keV sterile neutrino Dark Matter (DM) in the model with light inflaton chi, which serves as a messenger of scale invariance breaking. In this model the inflaton, in addition to providing reheating to the Standard Model (SM) particles, decays directly into sterile neutrinos. The latter are responsible for the active neutrino oscillations via seesaw type I like formula. While the two sterile neutrinos may also produce the lepton asymmetry in the primordial plasma and hence explain the baryon asymmetry of the Universe, the third one being the lightest may be of 7 keV and serve as DM. For this mechanism to work, the mass of the inflaton is bound to be light (0.1-1 GeV) and uniquely determines its properties, which allows to test the model. For particle physics experiments these are: inflaton lifetime (10^-5-10^-12 s), partial decay width of B-meson to kaon and inflaton (10^-6-10^-4) and inflaton branching ratios into light SM particles like it would be for the SM Higgs boson of the same mass. For cosmological experiments these are: spectral index of scalar perturbations (ns=0.957-0.967), and amount of tensor perturbations produced at inflation (tensor-to-scalar ratio r=0.15-0.005).

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