1,656 research outputs found

    Limits on the dark matter particle mass from black hole growth in galaxies

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    I review the properties of degenerate fermion balls and investigate the dark matter distribution at galactic centers using NFW, Moore and isothermal density profiles. I show that dark matter becomes degenerate for particles masses of a few keV at distances less than a few parsec from the center of our galaxy. To explain the galactic center black hole of mass of 3.5×106M\sim 3.5 \times 10^{6}M_{\odot} and a supermassive black hole of 3×109M\sim 3 \times 10^{9}M_{\odot} at a redshift of 6.41 in SDSS quasars, the mass of the fermion ball is assumed to be between 3×103M3 \times 10^{3} M_{\odot} and 3.5×106M3.5 \times 10^{6}M_{\odot}. This constrains the mass of the dark matter particle between 0.6keV0.6 {\rm keV} and 82keV82 {\rm keV}. The lower limit on the dark matter mass is improved to about {\rm 6 keV} if exact solutions of Poisson's equation are used in the isothermal power law case. The constrained dark matter particle could be interpreted as a sterile neutrino.Comment: 3 pages, To be published in Proceedings of the 11th Marcel Grossmann meeting on general relativity, 23-29 July 2006, Berlin, German

    Vertical shift and simultaneous Diophantine approximation on polynomial curves

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    The Hausdorff dimension of the set of simultaneously tau well approximable points lying on a curve defined by a polynomial P(X)+alpha, where P(X) is a polynomial with integer coefficients and alpha is in R, is studied when tau is larger than the degree of P(X). This provides the first results related to the computation of the Hausdorff dimension of the set of well approximable points lying on a curve which is not defined by a polynomial with integer coefficients. The proofs of the results also include the study of problems in Diophantine approximation in the case where the numerators and the denominators of the rational approximations are related by some congruential constraint.Comment: 22

    How far can you see in a forest?

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    We address a visibility problem posed by Solomon & Weiss. More precisely, in any dimension n:=d+12n := d + 1 \ge 2, we construct a forest \F with finite density satisfying the following condition : if \e > 0 denotes the radius common to all the trees in \F, then the visibility \V therein satisfies the estimate \V(\e) = O(\e^{-2d-\eta}) for any η>0\eta > 0, no matter where we stand and what direction we look in. The proof involves Fourier analysis and sharp estimates of exponential sums.Comment: This is an extended version of a paper to appear. Minor typos have been correcte

    A note on the Hausdorff dimension of some liminf sets appearing in simultaneous Diophantine approximation

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    Let Q be an infinite set of positive integers. Denote by W_{\tau, n}(Q) (resp. W_{\tau, n}) the set of points in dimension n simultaneously \tau--approximable by infinitely many rationals with denominators in Q (resp. in N*). A non--trivial lower bound for the Hausdorff dimension of the liminf set W_{\tau, n}\W_{\tau, n}(Q) is established when n>1 and \tau >1+1/(n-1) in the case where the set Q satisfies some divisibility properties. The computation of the actual value of this Hausdorff dimension as well as the one--dimensional analogue of the problem are also discussed

    On the Minimum of a Positive Definite Quadratic Form over Non--Zero Lattice points. Theory and Applications

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    Let Σd++\Sigma_d^{++} be the set of positive definite matrices with determinant 1 in dimension d2d\ge 2. Identifying any two SLd(Z)SL_d(\mathbb{Z})-congruent elements in Σd++\Sigma_d^{++} gives rise to the space of reduced quadratic forms of determinant one, which in turn can be identified with the locally symmetric space Xd:=SLd(Z)\SLd(R)/SOd(R)X_d:=SL_d(\mathbb{Z})\backslash SL_d(\mathbb{R})/SO_d(\mathbb{R}). Equip the latter space with its natural probability measure coming from a Haar measure on SLd(R)SL_d(\mathbb{R}). In 1998, Kleinbock and Margulis established sharp estimates for the probability that an element of XdX_d takes a value less than a given real number δ>0\delta>0 over the non--zero lattice points Zd\{0}\mathbb{Z}^d\backslash\{ 0 \}. In this article, these estimates are extended to a large class of probability measures arising either from the spectral or the Cholesky decomposition of an element of Σd++\Sigma_d^{++}. The sharpness of the bounds thus obtained are also established (up to multiplicative constants) for a subclass of these measures. Although of an independent interest, this theory is partly developed here with a view towards application to Information Theory. More precisely, after providing a concise introduction to this topic fitted to our needs, we lay the theoretical foundations of the study of some manifolds frequently appearing in the theory of Signal Processing. This is then applied to the recently introduced Integer-Forcing Receiver Architecture channel whose importance stems from its expected high performance. Here, we give sharp estimates for the probabilistic distribution of the so-called \emph{Effective Signal--to--Noise Ratio}, which is an essential quantity in the evaluation of the performance of this model
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