48,395 research outputs found
The Effect of Spatial Curvature on the Classical and Quantum Strings
We study the effects of the spatial curvature on the classical and quantum
string dynamics. We find the general solution of the circular string motion in
static Robertson-Walker spacetimes with closed or open sections. This is given
closely and completely in terms of elliptic functions. The physical properties,
string length, energy and pressure are computed and analyzed. We find the {\it
back-reaction} effect of these strings on the spacetime: the self-consistent
solution to the Einstein equations is a spatially closed spacetime with
a selected value of the curvature index (the scale f* is normalized to
unity). No self-consistent solutions with exist. We semi-classically
quantize the circular strings and find the mass in each case. For
the very massive strings, oscillating on the full hypersphere, have {\it independent} of and the level spacing {\it
grows} with while the strings oscillating on one hemisphere (without
crossing the equator) have and a {\it finite} number of
states For there are infinitely many string states
with masses that is, the level spacing grows {\it slower} than
The stationary string solutions as well as the generic string fluctuations
around the center of mass are also found and analyzed in closed form.Comment: 30 pages Latex + three tables and five figures (not included
Hawking Radiation in String Theory and the String Phase of Black Holes
The quantum string emission by Black Holes is computed in the framework of
the `string analogue model' (or thermodynamical approach), which is well suited
to combine QFT and string theory in curved backgrounds (particulary here, as
black holes and strings posses intrinsic thermal features and temperatures).
The QFT-Hawking temperature T_H is upper bounded by the string temperature T_S
in the black hole background. The black hole emission spectrum is an incomplete
gamma function of (T_H - T_S). For T_H << T_S, it yields the QFT-Hawking
emission. For T_H \to T_S, it shows highly massive string states dominate the
emission and undergo a typical string phase transition to a microscopic
`minimal' black hole of mass M_{\min} or radius r_{\min} (inversely
proportional to T_S) and string temperature T_S. The semiclassical QFT black
hole (of mass M and temperature T_H) and the string black hole (of mass M_{min}
and temperature T_S) are mapped one into another by a `Dual' transform which
links classical/QFT and quantum string regimes. The string back reaction effect
(selfconsistent black hole solution of the semiclassical Einstein equations
with mass M_+ (radius r_+) and temperature T_+) is computed. Both, the QFT and
string black hole regimes are well defined and bounded: r_{min} leq r_+ \leq
r_S, M_{min} \leq M_+ \leq M, T_H \leq T_+ \leq T_S. The string `minimal' black
hole has a life time tau_{min} \simeq \frac{k_B c}{G \hbar} T^{-3}_S.Comment: LaTex, 31 pages, no figure
Constraining the Warm Dark Matter Particle Mass through Ultra-Deep UV Luminosity Functions at z=2
We compute the mass function of galactic dark matter halos for different
values of the Warm Dark Matter (WDM) particle mass m_X and compare it with the
abundance of ultra-faint galaxies derived from the deepest UV luminosity
function available so far at redshift z~2. The magnitude limit M_UV=-13 reached
by such observations allows us to probe the WDM mass functions down to scales
close to or smaller than the half-mass mode mass scale ~10^9 M_sun. This
allowed for an efficient discrimination among predictions for different m_X
which turn out to be independent of the star formation efficiency adopted to
associate the observed UV luminosities of galaxies to the corresponding dark
matter masses. Adopting a conservative approach to take into account the
existing theoretical uncertainties in the galaxy halo mass function, we derive
a robust limit m_X>1.8 keV for the mass of thermal relic WDM particles when
comparing with the measured abundance of the faintest galaxies, while m_X>1.5
keV is obtained when we compare with the Schechter fit to the observed
luminosity function. The corresponding lower limit for sterile neutrinos
depends on the modeling of the production mechanism; for instance m_sterile > 4
keV holds for the Shi-Fuller mechanism. We discuss the impact of observational
uncertainties on the above bound on m_X. As a baseline for comparison with
forthcoming observations from the HST Frontier Field, we provide predictions
for the abundance of faint galaxies with M_UV=-13 for different values of m_X
and of the star formation efficiency, valid up to z~4.Comment: 14 pages, 3 figures. Accepted for publication in The Astrophysical
Journa
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