Articles

1995-02-09T10:55:35-05:00

Mixed dark matter with neutrino chemical potentials

Mixed dark matter models with one low-mass (e.g. 2.4eV) neutrino flavor are shown to be in good agreement with observations if the neutrinos have non-zero chemical potentials. This agreement holds (except for the problem with a low-age Universe) even for high values of the Hubble-parameter. Massless neutrinos with non-zero chemical potentials may reconcile cold dark matter with observations. Some fine-tuning is required to avoid problems with Big Bang nucleosynthesis.

Georg B. Larsen

Jes Madsen

1993-05-24T20:37:08-04:00

"Sample Variance" in Small-Scale CMB Anisotropy Experiments

We discuss the effects of finite sky coverage and the uncertainty in extracting information about the power spectrum from experiments on small angular scales. In general the cosmic variance is enhanced by a factor of $4\pi/A$, where $A$ is the solid angle sampled by the experiment. As a rough guide, an experiment with sensitivity peaking at the $\ell$th multipole has to cover $\simgt\ell$ independent patches to have a lower ``sample variance'' than for a whole-sky measurement of the quadrupole. Our approach gives a relatively simple way of attaching an error bar to the theoretical prediction for a particular experiment, and thereby comparing theories with experimental results, without the need for computationally-intensive Maximum Likelihood or Monte Carlo calculations.

Douglas Scott

Mark Srednicki

Martin White

1996-04-22T13:40:33-04:00

The Tunneling, the Second Order Relativistic Phase Transitions and Problem of the Macroscopic Universe Origin

We propose that the Universe was created from "Nothing" with a relatively small particles number and it very quick relaxed to quasiequilibrium state at the Planck parameters. The classic cosmological solution for this Universe, with the calculation of its ability to be undergo to the second order relativistic phase transition (RPT), has two branches divided by gap. On one from these branches near to "Nothing" state the second order RPT isn't possible at the GUT scale. Other branch is thermodynamically instable. The quantum process of tunneling between the cosmological solution branches and kinetics of the second order RPT after tunneling are investigated by numerical methods. Other quantum geometrodynamics process (the bounce from singularity) is taken into consideration also. It is shown that discussed phenomenon with the calculation of all RPT from the GUT scale to the Salam-Weinberg scale gives the new cosmological scenarios of the macroscopic Universe origin with observable particles number.

1998-09-08T17:42:55-04:00

The Evolutionary Design of Collective Computation in Cellular Automata

We investigate the ability of a genetic algorithm to design cellular automata that perform computations. The computational strategies of the resulting cellular automata can be understood using a framework in which ``particles'' embedded in space-time configurations carry information and interactions between particles effect information processing. This structural analysis can also be used to explain the evolutionary process by which the strategies were designed by the genetic algorithm. More generally, our goals are to understand how machine-learning processes can design complex decentralized systems with sophisticated collective computational abilities and to develop rigorous frameworks for understanding how the resulting dynamical systems perform computation.

James P. Crutchfield

Melanie Mitchell

Rajarshi Das

1993-12-04T09:57:19-05:00

Halo Cold Dark Matter and Microlensing

There is good evidence that most of the baryons in the Universe are dark and some evidence that most of the matter in the Universe is nonbaryonic with cold dark matter (cdm) being a promising possibility. We discuss expectations for the abundance of baryons and cdm in the halo of our galaxy and locally. We show that in plausible cdm models the local density of cdm is at least $10^{-25}\gcmm3$. We also discuss what one can learn about the the local cdm density from microlensing of stars in the LMC by dark stars in the halo and, based upon a suite of reasonable two-component halo models, conclude that microlensing is not a sensitive probe of the local cdm density.

Evalyn Gates

Michael S. Turner

1995-07-14T05:55:01-04:00

The Stability of Primordial Magnetic Fields Produced by Phase Transitions

Primordial magnetic fields seem to be a generic relic of phase transitions in the early universe. We consider a primordial electromagnetic field formed as a result of a second-order phase transition, and show that it is stable to thermal fluctuations in the period immediately following. We also show how such a field arises in first order phase transitions. In both cases there is a transitive electric field produced during the transition.

A. P. Martin

A. C. Davis

1996-01-09T15:25:31-05:00

Does Chaotic Mixing Facilitate Omega<1 Inflation?

Yes, if the universe has compact topology. Inflation is currently the most elegant explanation of why the universe is old, large, nearly flat, homogeneous on large scales and structured on small scales. One of the weaknesses of the inflationary paradigm is the problem of initial conditions for inflation: the pre-inflationary universe must be somewhat old, somewhat large and somewhat homogeneous. These initial condition requirements are even more severe in Omega < 1 inflationary models: if the universe does not inflate enough to appear flat, then it does not inflate enough to appear homogeneous. One solution is to have two inflationary epochs. Here, we propose another solution to the problem of pre-inflationary homogeneity: if the universe is compact, then during the pre-inflationary period, there is sufficient time to homogenize the universe as chaotic mixing smoothes out primordial fluctuations. Gradients in the energy-density are reduced as e^(-kappa d), where kappa is the Kolomogorov-Sinai entropy of the flow, and d is the distance the flow travels. We explore this homogenization process, outline why compact negatively-curved universes are the most natural, and conclude with a discussion of the implications of living in such a universe.

Neil J. Cornish

David N. Spergel

Glenn D. Starkman

1993-10-11T23:12:02-04:00

The Meaning of Eros/Macho

Most of the mass density in the Universe---and in the halo of our own galaxy---exists in the form of dark matter. Overall, the contribution of luminous matter (in stars) to the mass density of the Universe is less than 1\%; primordial nucleosynthesis indicates that baryons contribute between 1\% and 10\% of the critical density ($0.01h^{-2}\la \Omega_B\la 0.02h^{-2}$; $h=$ the Hubble constant in units of $100\kms\Mpc^{-1}$); and other evidence indicates that the total mass density is at least 10\% of critical density, and likely much greater. If the universal density is as low as 10\% of the critical density there may be but one kind of dark matter. More likely, the universal density is greater than 10\%, and there are two kinds of dark matter, and thus two dark matter problems: In what form does the baryonic dark matter exist? and In what form does the nonbaryonic dark matter exist? The MACHO and EROS collaborations have presented evidence for the microlensing of stars in the LMC by $10^{-1\pm 1}M_\odot$ dark objects in the halo of our own galaxy and may well have solved {\it one} of the dark matter puzzles by identifying the form of the baryonic dark matter.

Michael S. Turner

1995-09-15T19:09:17-04:00

Mode Identification of the Slowly Pulsating F0V Star V398 Aurigae (9 Aur)

We have investigated the F0V star V398\,Aurigae (= 9 Aur) under the assumption that it is undergoing non-radial gravity mode oscillations and that the two principal periods given by Krisciunas et al. (1995) are correct. We find that the two periods are manifestations of an $\ell=3, |m|=1$ spheroidal mode and its toroidal corrections due to the rotation of the star. As far as we know, this is the first detection of toroidal correction terms in a real star. The two modes probably are the result of rotational splitting. Our analysis provides for the first time a physical explanation of certain characteristics of the observed behavior of the star. The amplitude of the radial part of the pulsation for $f_1 = 0.795$\,d$^{-1}$ is a factor of 4 larger than the one for $f_2 = 0.346$\,d$^{-1}$. Since the photometric variability is determined mostly by temperature variations, which in turn are determined by the radial part of the pulsation, the photometric variability is dominated by the mode with frequency $f_1$. On the other hand, $f_2$ is the more pronounced one in all three spectroscopic moment variations (including the radial velocity), reflecting that the transverse displacement of $f_2$, and not the one of $f_1$, dominates the velocity behavior.

Conny Aerts

Kevin Krisciunas

1995-06-21T14:36:35-04:00

The MACHO Project First Year LMC Results: The Microlensing Rate and the Nature of the Galactic Dark Halo

The MACHO collaboration reports on the analysis of our first year LMC data, 9.5 million light curves with an average of 235 observations each. Automated selection procedures give 3 events consistent with microlensing. We evaluate our experimental detection efficiency using a range of Monte- Carlo simulations. Using a `standard' halo density profile we find that a halo comprised entirely of Machos in the mass range 3 \ten{-4} to 0.06 \msun would predict > 15 detected events in this dataset; thus a standard spherical halo cannot be dominated by objects in this mass range. Assuming all three events are microlensing of halo objects and fitting a naive spherical halo model to our data yields a Macho halo fraction f =0.19+0.16-0.10, a total mass in Machos (inside 50 kpc) of 7.6+6-4 \ten{10} \msun, and a microlensing optical depth 8.8+7-5 \ten{-8} (68\% CL). Exploring a wide range of halo models we find that our constraints on the Macho fraction are quite model-dependent, but constraints on the total mass in Machos within 50 kpc are quite secure.