487 research outputs found
Observer and Particle Transformations and Newton's Laws
A frequently confused point in studies of symmetry violation is the
distinction between observer and particle transformations. In this work, we
consider a model in which a coefficient in the Standard-Model Extension leads
to violations of rotation invariance in Newton's second law. The model
highlights the distinction between observer and particle transformations.Comment: Presented at the Sixth Meeting on CPT and Lorentz Symmetry,
Bloomington, Indiana, June 17-21, 201
Antimatter, Lorentz Symmetry, and Gravity
A brief introduction to the Standard-Model Extension (SME) approach to
testing CPT and Lorentz symmetry is provided. Recent proposals for tests with
antimatter are summarized, including gravitational and spectroscopic tests.Comment: Presented at the 12th International Conference on Low Energy
Antiproton Physics, Kanazawa Japan, March 6-11, 2016, Accepted for
publication in JPS Conference Proceeding
Reheating of the Universe and Population III
We note that current observational evidence strongly favors a conventional
recombination of ionized matter subsequent to redshift z=1200, followed by
reionization prior to redshift z=5 and compute how this would have occurred in
a standard scenario for the growth of structure. Extending prior semi-analytic
work, we show by direct, high-resolution numerical simulations (of a COBE
normalized CDM+Lambda model) that reheating, will occur in the interval 15>z>7,
followed by reionization and accompanied by a significant increase in the Jeans
mass. However, the evolution of the Jeans mass does not significantly affect
star formation in dense, self-shielded clumps of gas, which are detached from
the thermal evolution of the rest of the universe. On average, the growth of
the Jeans mass tracks the growth of the nonlinear mass scale, a result we
suspect is due to nonlinear feedback effects. Cooling on molecular hydrogen
leads to a burst of star formation prior to reheating which produces Population
III stars with Omega_* reaching 10^{-5.5} and Z/Z_sun reaching 10^{-3.7} by
z=14. Star formation subsequently slows down as molecular hydrogen is depleted
by photo-destruction and the rise of the temperature. At later times, z<10,
when the characteristic virial temperature of gas clumps reach 10,000 degrees,
star formation increases again as hydrogen line cooling become efficient.
Objects containing Pop III stars accrete mass with time and, as soon as they
reach 10,000 K virial temperature, they engage in renewed star formation and
turn into normal Pop II objects having an old Pop III metal poor component.Comment: six postscript figures included, submitted to ApJ
Dependence of the Inner DM Profile on the Halo Mass
I compare the density profile of dark matter (DM) halos in cold dark matter
(CDM) N-body simulations with 1 Mpc, 32 Mpc, 256 Mpc and 1024 Mpc box sizes. In
dimensionless units the simulations differ only for the initial power spectrum
of density perturbations. I compare the profiles when the most massive halos
are composed of about 10^5 DM particles. The DM density profiles of the halos
in the 1 Mpc box show systematically shallower cores with respect to the
corresponding halos in the 32 Mpc simulation that have masses, M_{dm}, typical
of the Milky Way and are fitted by a NFW profile. The DM density profiles of
the halos in the 256 Mpc box are consistent with having steeper cores than the
corresponding halos in the 32 Mpc simulation, but higher mass resolution
simulations are needed to strengthen this result. Combined, these results
indicate that the density profile of DM halos is not universal, presenting
shallower cores in dwarf galaxies and steeper cores in clusters. Physically the
result sustains the hypothesis that the mass function of the accreting
satellites determines the inner slope of the DM profile. In comoving
coordinates, r, the profile \rho_{dm} \propto 1/(X^\alpha(1+X)^{3-\alpha}),
with X=c_\Delta r/r_\Delta, r_\Delta is the virial radius and \alpha
=\alpha(M_{dm}), provides a good fit to all the DM halos from dwarf galaxies to
clusters at any redshift with the same concentration parameter c_\Delta ~ 7.
The slope, \gamma, of the outer parts of the halo appears to depend on the
acceleration of the universe: when the scale parameter is a=(1+z)^{-1} < 1, the
slope is \gamma ~ 3 as in the NFW profile, but \gamma ~ 4 at a > 1 when
\Omega_\Lambda ~ 1 and the universe is inflating.[abridged]Comment: Accepted for publication in MNRAS. 13 pages, including 11 figures and
2 tables. The revised version has an additional discussion section and work
on the velocity dispersion anisotrop
The Omega Dependence of the Evolution of xi(r)
The evolution of the two-point correlation function, xi(r,z), and the
pairwise velocity dispersion, sigma(r,z), for both the matter and halo
population, in three different cosmological models:
(Omega_M,Omega_Lambda)=(1,0), (0.2,0) and (0.2,0.8) are described. If the
evolution of xi is parameterized by xi(r,z)=(1+z)^{-(3+eps)}xi(r,0), where
xi(r,0)=(r/r_0)^{-gamma}, then eps(mass) ranges from 1.04 +/- 0.09 for (1,0) to
0.18 +/- 0.12 for (0.2,0), as measured by the evolution of at 1 Mpc (from z ~ 5
to the present epoch). For halos, eps depends on their mean overdensity. Halos
with a mean overdensity of about 2000 were used to compute the halo two-point
correlation function tested with two different group finding algorithms: the
friends of friends and the spherical overdensity algorithm. It is certainly
believed that the rate of growth of this xihh will give a good estimate of the
evolution of the galaxy two-point correlation function, at least from z ~ 1 to
the present epoch. The values we get for eps(halos) range from 1.54 for (1,0)
to -0.36 for (0.2,0), as measured by the evolution of xi(halos) from z ~ 1.0 to
the present epoch. These values could be used to constrain the cosmological
scenario. The evolution of the pairwise velocity dispersion for the mass and
halo distribution is measured and compared with the evolution predicted by the
Cosmic Virial Theorem (CVT). According to the CVT, sigma(r,z)^2 ~ G Q rho(z)
r^2 xi(r,z) or sigma proportional to (1+z)^{-eps/2}. The values of eps measured
from our simulated velocities differ from those given by the evolution of xi
and the CVT, keeping gamma and Q constant: eps(CVT) = 1.78 +/- 0.13 for (1,0)
or 1.40 +/- 0.28 for (0.2,0).Comment: Accepted for publication in the ApJ. Also available at
http://manaslu.astro.utoronto.ca/~carlberg/cnoc/xiev/xi_evo.ps.g
POTENT Reconstruction from Mark III Velocities
We present an improved POTENT method for reconstructing the velocity and mass
density fields from radial peculiar velocities, test it with mock catalogs, and
apply it to the Mark III Catalog. Method improvments: (a) inhomogeneous
Malmquist bias is reduced by grouping and corrected in forward or inverse
analyses of inferred distances, (b) the smoothing into a radial velocity field
is optimized to reduce window and sampling biases, (c) the density is derived
from the velocity using an improved nonlinear approximation, and (d) the
computational errors are made negligible. The method is tested and optimized
using mock catalogs based on an N-body simulation that mimics our cosmological
neighborhood, and the remaining errors are evaluated quantitatively. The Mark
III catalog, with ~3300 grouped galaxies, allows a reliable reconstruction with
fixed Gaussian smoothing of 10-12 Mpc/h out to ~60 Mpc/h. We present maps of
the 3D velocity and mass-density fields and the corresponding errors. The
typical systematic and random errors in the density fluctuations inside 40
Mpc/h are \pm 0.13 and \pm 0.18. The recovered mass distribution resembles in
its gross features the galaxy distribution in redshift surveys and the mass
distribution in a similar POTENT analysis of a complementary velocity catalog
(SFI), including the Great Attractor, Perseus-Pisces, and the void in between.
The reconstruction inside ~40 Mpc/h is not affected much by a revised
calibration of the distance indicators (VM2, tailored to match the velocities
from the IRAS 1.2Jy redshift survey). The bulk velocity within the sphere of
radius 50 Mpc/h about the Local Group is V_50=370 \pm 110 km/s (including
systematic errors), and is shown to be mostly generated by external mass
fluctuations. With the VM2 calibration, V_50 is reduced to 305 \pm 110 km/s.Comment: 60 pages, LaTeX, 3 tables and 27 figures incorporated (may print the
most crucial figures only, by commenting out one line in the LaTex source
Newtonian versus relativistic nonlinear cosmology
Both for the background world model and its linear perturbations Newtonian
cosmology coincides with the zero-pressure limits of relativistic cosmology.
However, such successes in Newtonian cosmology are not purely based on Newton's
gravity, but are rather guided ones by previously known results in Einstein's
theory. The action-at-a-distance nature of Newton's gravity requires further
verification from Einstein's theory for its use in the large-scale nonlinear
regimes. We study the domain of validity of the Newtonian cosmology by
investigating weakly nonlinear regimes in relativistic cosmology assuming a
zero-pressure and irrotational fluid. We show that, first, if we ignore the
coupling with gravitational waves the Newtonian cosmology is exactly valid even
to the second order in perturbation. Second, the pure relativistic correction
terms start appearing from the third order. Third, the correction terms are
independent of the horizon scale and are quite small in the large-scale near
the horizon. These conclusions are based on our special (and proper) choice of
variables and gauge conditions. In a complementary situation where the system
is weakly relativistic but fully nonlinear (thus, far inside the horizon) we
can employ the post-Newtonian approximation. We also show that in the
large-scale structures the post-Newtonian effects are quite small. As a
consequence, now we can rely on the Newtonian gravity in analyzing the
evolution of nonlinear large-scale structures even near the horizon volume.Comment: 8 pages, no figur
Evidence for universal structure in galactic halos
The late infall of dark matter onto a galaxy produces structure (such as
caustics) in the distribution of dark matter in the halo. We argue that such
structure is likely to occur generically on length scales proportional to , where is the age of the universe and is the
rotation velocity of the galaxy. A set of 32 extended galactic rotation curves
is analyzed. For each curve, the radial coordinate is rescaled according to
, where we choose . A
linear fit to each rescaled rotation curve is subtracted, and the residuals are
binned and averaged. The sample shows significant features near and . This is consistent with the predictions of the
self-similar caustic ring model of galactic halos.Comment: 4 pages, LaTeX, 1 epsf figur
- …