The end of the Dark Ages in MOND

We study the evolution of a spherically symmetric density perturbation in the Modified Newtonian Dynamics (MOND) model applied to the net acceleration over Hubble flow. The background cosmological model is a $\Lambda$-dominated, low-$\Omega_b$ Friedmann model with no Cold Dark Matter. We include thermal processes and non-equilibrium chemical evolution of the collapsing gas. We find that under these assumptions the first low-mass objects ($M \le 3\times 10^4 M_{\odot}$) may collapse already for $z\sim 30$, which is in quite good agreement with the recent WMAP results. A lower value of $a_0$ would lead to much slower collapse of such objects.Comment: 6 pages, 11 figures, LaTeX 2e with MN2e, MNRAS submitte

Cosmological fluctuation growth in bimetric MOND

I look at the growth of weak density inhomogeneities of nonrelativistic matter, in bimetric-MOND (BIMOND) cosmology. I concentrate on matter-twin-matter-symmetric versions of BIMOND, and assume that, on average, the universe is symmetrically populated in the two sectors. MOND effects are absent in an exactly symmetric universe, apart from the appearance of a cosmological constant, Lambda~(a0/c)^2. MOND effects-local and cosmological-do enter when density inhomogeneities that differ in the two sectors appear and develop. MOND later takes its standard form in systems that are islands dominated by pure matter. I derive the nonrelativistic equations governing small-scale fluctuation growth. The equations split into two uncoupled systems, one for the sum, the other for the difference, of the fluctuations in the two sectors. The former is governed strictly by Newtonian dynamics. The latter is governed by MOND dynamics, which entails stronger gravity, and nonlinearity even for the smallest of perturbations. These cause the difference to grow faster than the sum, conducing to matter-twin-matter segregation. The nonlinearity also causes interaction between nested perturbations on different scales. Because matter and twin matter (TM) repel each other in the MOND regime, matter inhomogeneities grow not only by their own self gravity, but also through shepherding by flanking TM overdensitie. The relative importance of gravity and pressure in the MOND system depends also on the strength of the perturbation. The development of structure in the universe, in either sector, thus depends crucially on two initial fluctuation spectra: that of matter alone and that of the matter-TM difference. I also discuss the back reaction on cosmology of BIMOND effects that appear as "phantom matter" resulting from inhomogeneity differences between the two sectors.Comment: 14 pages. Some clarifications added. Version published in Phys. Rev.

Confrontation of MOND Predictions with WMAP First Year Data

I present a model devoid of non-baryonic cold dark matter (CDM) which provides an acceptable fit to the WMAP data for the power spectrum of temperature fluctuations in the cosmic background radiation (CBR). An a priori prediction of such no-CDM models was a first-to-second peak amplitude ratio A1:2 = 2.4. WMAP measures A1:2 = 2.34 +/- 0.09. The baryon content is the dominant factor in fixing this ratio; no-CDM models which are consistent with the WMAP data are also consistent with constraints on the baryon density from the primordial abundances of 2H, 4He, and 7Li. However, in order to match the modest width of the acoustic peaks observed by WMAP, a substantial neutrino mass is implied: m(nu) ~ 1 eV. Even with such a heavy neutrino, structure is expected to form rapidly under the influence of MOND. Consequently, the epoch of reionization should occur earlier than is nominally expected in LCDM. This prediction is realized in the polarization signal measured by WMAP. An outstanding test is in the amplitude of the third acoustic peak. Experiments which probe high-L appear to favor a third peak which is larger than predicted by the no-CDM model.Comment: ApJ, in press. 33 pages, 7 figure

The Formation of the First Stars in the Universe

In this review, I survey our current understanding of how the very first stars in the universe formed, with a focus on three main areas of interest: the formation of the first protogalaxies and the cooling of gas within them, the nature and extent of fragmentation within the cool gas, and the physics -- in particular the interplay between protostellar accretion and protostellar feedback -- that serves to determine the final stellar mass. In each of these areas, I have attempted to show how our thinking has developed over recent years, aided in large part by the increasing ease with which we can now perform detailed numerical simulations of primordial star formation. I have also tried to indicate the areas where our understanding remains incomplete, and to identify some of the most important unsolved problems.Comment: 74 pages, 4 figures. Accepted for publication in Space Science Review