Richard H. Cyburt

PRIMORDIAL NUCLEOSYNTHESIS IN THE NEW AGE OF COSMOLOGY: DETERMINING UNCERTAINTIES, EXAMINING CONCORDANCE, AND PROBING NEW PHYSICS

Chapter 1

Overview -- An Introduction to Cosmology -- Primordial Nucleosynthesis -- Cosmic Microwave Background -- Bibliography

Cosmic Microwave Background

The cosmic microwave background is the relic photon background that decoupled from ordinary matter shortly after neutral atoms formed, about 380,000 years after the big

bang. With the first observations of the CMB by Penzias and Wilson [10], not only was the prediction of Alpher and Herman [9] vindicated, but also provided direct evidence in support of the hot big bang model. Subsequent observations went on to show the remarkable uniformity in the background, further in support of a homogeneous and isotropic universe. With the first observations of anisotropies in the CMB by the COBE satellite [13], a new era in cosmology was born. The wealth of information stored in the power spectrum of these anisotropies is what is driving this new age of cosmology. For reviews of CMB theory see Van der Veen (1998) [30] and Tegmark (1995) [31] and of CMB observations see Wang et al (2002) [32] and the individual group papers from MAXIMA, BOOMERANG, DASI, CBI, ACBAR and WMAP [14, 15, 16, 17, 18, 19].

In a perfectly homogeneous and isotropic universe, the hot and dense gas cools adiabatically, with the ordinary matter coupled to photons through their respective interactions with electrons. The point at which the radiation (photons & neutrinos) mass-energy density becomes equal to that of the matter (baryons & dark) density is called matter-radiation equality. Now the matter dominates the universe and thus governs its expansion. When the universe reaches a temperature below 13.6 eV, electrons combine with protons to form neutral hydrogen. However, since there are many more photons than baryons, neutral hydrogen is rapidly dissociated. Akin to the BBN deuterium-bottleneck, this atomic-hydrogen bottleneck stalls recombination until the high-energy photons are not abundant enough to dissociate neutral hydrogen. The photons then scatter for the last time and decouple from the baryons. Each then can adiabatically cool, following the time-temperature relation we discussed in section 1.1. Today, a 2.725 K blackbody spectrum for the photons is observed [13].

As COBE observed, the universe is not quite this perfect, with inhomogeneities at the level of 1 : 105 [13]. These fluctuations in the CMB carry information about the seeds of structure formation and the content of the universe. Inflation is the favored source of these fluctuations. In a radiation dominated universe, small scale perturbations (< c=H) do not grow because the the density and pressure is high enough that the baryon-photon plasma remains smooth, thus preventing the dark matter from collapsing under the influence of its own gravity. It is not until matter-radiation equality that perturbations can grow, at which point the baryons fall into the now growing, dark matter potential wells. However, the baryons are still highly coupled to the photons, thus once they get dense enough from collapse, the photon pressure causes the collapse to rebound. A series of acoustic oscillations are set up in the matter dominated plasma. It is not until recombination and decoupling, that these oscillations cease.

The temperature fluctuations in the CMB are thus a record of the state these acoustic oscillations were in at the time of last scattering. Over-dense clumps are hotter, while under-dense clumps are cooler than the average CMB temperature. Clumps which are moving towards or away from an observer, makes the clumps appear hotter or cooler, respectively. Photons traveling from the last scattering surface to an observer can appear cooler or hotter by having to climb out of or into gravitational wells just after last scattering. These are known as the primary anisotropies in the CMB angular power spectrum. The physics described above sets the scales we should observe these fluctuations and the content of the universe determines its amplitude. By comparing these scales and amplitudes in the CMB power spectrum, one can determine many cosmological parameters. Shown in figure 1.2 is the angular power spectrum, as measured by WMAP, and its best fit theory [19].

Of major importance in this work is the CMB determination of the cosmic baryon density, Bh2 / . This independent measurement of the baryon content examines the general concordance of the BBN light element abundance theory predictions and their observed values, and tests the basic framework of the hot big bang model. Key to this test, is an understanding of the dominant uncertainties in the light element predictions. These uncertainties stem from the systematic errors in nuclear cross sections. We present a new procedure for determining cross section representations and their uncertainties and describe

Figure 1.2: The data and best fit of the CMB angular power spectrum, with the top panel showing the temperature-temperature cross-correlation and the bottom panel showing the temperature-polarization (E-type) cross-correlation. The grey band is the cosmic variance about the best fit. This figure is from the WMAP team [19].

how they propagate into thermal rates and the light element predictions. With this updated nuclear network, we then quantify the concordance between the light element abundance observations and their predictions, and the CMB. With this level of concordance set for the standard cosmological model, we can test and constrain non-standard models. We use primordial nucleosynthesis and the cosmic microwave background together to probe early universe physics spanning times from 1 sec to 400,000 yrs after the big bang and beyond.