New BBN limits on physics beyond the standard model from 4He
Introduction
Big bang nucleosynthesis (BBN) is one of the most sensitive available probes of physics beyond the standard model. The concordance between the observation-based determinations of the light element abundances of D, 3He, 4He, and 7Li [1], and their theoretically predicted abundances reflects the overall success of the standard big bang cosmology. Many departures from the standard model are likely to upset this agreement, and are tightly constrained [2].
The 4He abundance, in particular, has often been used as a sensitive probe of new physics. This is due to the fact that nearly all available neutrons at the time of BBN end up in 4He and the neutron-to-proton ratio is very sensitive to the competition between the weak interaction rate and the expansion rate. For example, a bound on the number g∗ of relativistic degrees of freedom (at the time of BBN), commonly known as the limit on neutrino flavors, Nν, is derived through its effect on the expansion rate, [3]. However, because the calculated 4He abundance increases monotonically with baryon density (parameterized by the baryon-to-photon ratio, η ≡ nb/nγ), a meaningful limit on Nν requires both a lower bound to η and an upper bound to the primordial 4He mass fraction, Yp[4]. Indeed, for a fixed upper limit to Yp, the upper limit to Nν can be a sensitive function of the lower limit to η, particularly if η is small [4], [5].
The recent all-sky, high-precision measurement of microwave background anisotropies by WMAP [6] has opened the possibility for new precision analyses of BBN. Among the cosmological parameters determined by WMAP, the baryon density has been derived with unprecedented precision. The WMAP best fit assuming a varying spectral index is ΩBh2 = 0.0224 ± 0.0009 which is equivalent to η10,CMB = 6.14 ± 0.25. This result is sensitive mostly to WMAP alone but does include CMB data on smaller angular scales [7], Lyman α forest data, and 2dF redshift survey data [8] on large angular scales. This result is very similar to the corresponding value obtained from combining WMAP with SDSS data and other CMB measurements, which gives [9] and corresponds to . Using the WMAP data to fix the baryon density, one can make quite accurate predictions for the light element abundances [10], [11], [12], [13]. At the WMAP value for η, the 4He abundance is predicted to be [13]On the other hand, accurate 4He abundances have been and continue to be difficult to obtain. It is recognized that there are many potential sources of systematic uncertainties in the derived 4He abundance [14], [15]. As a result there exists a wide range of derived primordial 4He abundances which have typically been relatively low compared with (1). Recently, a reanalysis [16] of the 4He data [17], [18] has led to a significant enlargement in the statistical uncertainty as well as a potential shift in the mean value. A representative result of that analysis isConservatively, it would be difficult to exclude any value of Yp inside the range 0.232–0.258.
Because much of the previous work was based on relatively low values of Yp, tension between the value of η inferred from either D/H or WMAP, and 4He gave rise to very tight constraints on Nν and on other particle properties. In light of the newly suggested range for Yp[16], it is important to reexamine the constraints on physics beyond the standard model. Potential limits from D/H have been discussed recently [10], [12], [13], and we will just quote those results below in comparison with the results derived here. At present, it is not possible to use 7Li to obtain constraints. This is due to (1) the large uncertainty in the BBN prediction of the 7Li abundance, and (2) to the current discrepancy between the BBN prediction and the observational determination of the 7Li abundance (see e.g. [10], [11], [12], [13], [19]).
Section snippets
The 4He abundance
The 4He abundance has had a somewhat checkered history over the last decade. Of the modern determinations, the work of Pagel et al. [20] established the analysis techniques that others were soon to follow [21]. Their value of Yp = 0.228 ± 0.005 was significantly lower than that of a sample of 45 low metallicity HII regions, observed and analyzed in a uniform manner [17], with a derived value of Yp = 0.244 ± 0.002. An analysis based on the combined available data as well as unpublished data yielded an
Standard BBN
Key to BBN analysis is an accurate determination of BBN theory uncertainties, which are dominated by the errors in nuclear cross section data. To this end, several groups have determined reaction rate representations and uncertainties. Smith, Kawano and Malaney [25] presented the first detailed error budget for BBN, generally assuming constant relative errors. In more recent work [26], uncertainties were propagated based on available nuclear data into the light element predictions. The NACRE
Beyond the standard model
For several cases of interest, it will be useful to define a dimensionless cosmic “speed-up” factor ξ = Hnew/Hstd, where is the Hubble expansion rate; ξ = 1 then represents the unperturbed case. The expansion rate itself is given by the Friedmann equation, which for a flat Universe is H2 = (8π/3)GNρ, where ρ is the total mass-energy density. Thus the speed-up factor evolves as . For the case of a radiation-dominated Universe, we have ρ ∝ g∗T4, where g∗ = 2 + 7/2 + 7Nν/4 counts the
Summary
A new and detailed assessment [16] of the observed primordial 4He abundance, and its uncertainties, has important implications for cosmology and BBN generally, and for early Universe and particle physics in particular. The observed 4He abundance is now found to be consistent with the η value given by D, leaving 7Li alone in discordance. Moreover, both D and now 4He are consistent with the precision η range determined by recent observations of CMB anisotropies. The newfound 4He agreement arises
Acknowledgements
R.H.C. would like to thank C. Angulo, P. Descouvemont and P. Serpico for helpful discussions on our respective BBN rate compilations [11], [12], [13]. The work of K.A.O. was partially supported by DOE grant DE-FG02-94ER-40823. The work of B.D.F. was supported by the National Science Foundation under grant AST-0092939. The work of R.H.C. was supported by the Natural Sciences and Engineering Research Council of Canada.
References (54)
- et al.
Phys. Rep.
(1993)Rep. Prog. Phys.
(1996) - et al.
Astrophys. J.
(1981) Astrophys. J. Suppl.
(2003)Astrophys. J. Suppl.
(2003)- et al.
Ap. J.
(1997)et al.Ap. J.
(2000)et al.Ap. J.
(2002)et al.A.A.
(2002) - et al.
MNRAS
(1992) - et al.
New Astron. Rev.
(2003) Nature
(1976)et al.Nucl. Phys. B
(1996)Nucl. Phys. B
(2000)Phys. Rev. D
(2002)- et al.
Phys. Rev. D
(2003) - et al.
Ap. J.
(1991)et al.Phys. Rep.
(2000)et al.Phys. Rev. D
(2002)
Phys. Lett. B
Astrophys. J.
Astrophys. J.
Astrophys. J.
Monthly Not. Royal Astr. Soc.
Phys. Rev. D
Phys. Lett.
Ap. J.
Phys. Rev. D
New Ast.
Ap. J.
Ap. J.
Ap. J.
Ap. J.
Phys. Rev. D
Ap. J.
Ap. J.
Astrophys. J. Suppl.
Astrophys. J.
Ap. J.
Ap. J.
Ap. J.
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