arXiv subjects

Gary Steigman

Publications and source records attributed to Gary Steigman.

64 records · Page 4Linked to original sources

arXiv1996

The Crisis Confronting Standard Big Bang Nucleosynthesis

Primordial nucleosynthesis is inevitable during the early evolution of an expanding universe filled with radiation and matter (baryons). However, the precise abundance yields depend sensitively on the baryon density, the radiation content and the early expansion rate. For ``standard" big bang nucleosynthesis (SBBN) only D, $^3$He, $^4$He and $^7$Li are produced in astrophysically interesting abundances which depend on only one ``free" parameter, $η= n_{\rm B}/n_γ$, the ratio of baryons (nucleons) to photons. With four primordial abundances and one adjustable parameter, SBBN is an eminently testable theory. In the confrontation of theory with data current observations suggest a crisis traceable to the tension between deuterium (favoring a ``high" value of $η$) and helium-4 (preferring a ``low" $η$ value). In this contribution, the written version of the material I discussed at the Princeton University ``Critical Dialogues in Cosmology" (June 1996), I will present the case for a conflict and show how recent observations of deuterium in nearly primordial QSO absorbers exacerbate the problem. I conclude with a brief description of an alternate, ``dynamical" approach to bounding the nucleon density which may be of some value given the current deuterium dichotomy.

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arXiv1996

Big Bang Nucleosynthesis

In the first thousand seconds of its evolution the Universe was a primordial nuclear reactor synthesizing the nuclides D, $^3$He, $^4$He and $^7$Li. These messengers from the Big Bang provide a unique window on the early, hot, dense Universe, offering the opportunity for tests of the standard models of particle physics and of cosmology. A new, statistically coherent comparison of the primordial abundances of these nuclides inferred from increasingly accurate observational data with those predicted by the standard model hints at a possible crisis. The case for this emerging crisis is presented and several paths towards ameliorating it are explored.

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arXiv1996

Testing Big Bang Nucleosynthesis

Big Bang Nucleosynthesis (BBN), along with the cosmic background radiation and the Hubble expansion, is one of the pillars ofthe standard, hot, big bang cosmology since the primordial synthesis of the light nuclides (D, $^3$He, $^4$He, $^7$Li) must have occurred during the early evolution of a universe described by this model. The overall consistency between the predicted and observed abundances of the light nuclides, each of which spans a range of some nine orders of magnitude, provides impressive support for the standard models of cosmology and particle physics. Here, the results of recent, statistically consistent tests of BBN are described. This new confrontation between theory and data challenges the standard model. The crises confronting BBN are identified and several possible resolutions are outlined.

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arXiv1996

Galactic Evolution Of D And 3He Including Stellar Production Of 3He

New stellar models which track the production and destruction of $^3$He (and D) have been evolved for a range of stellar masses $(0.65\leq M/M_{\odot}\leq 100)$, metallicities $(0.01 \leq Z/Z_{\odot} \leq 1)$ and initial (main sequence) $^3$He mass fractions $(10^{-5} \leq X_{3,MS} \leq 10^{-3})$. Armed with the $^3$He yields from these stellar models we have followed the evolution of D and $^3$He using a variety of chemical evolution models with and without infall of primordial or processed material. Production of new $^3$He by the lower mass stars overwhelms any reasonable primordial contributions and leads to predicted abundances in the presolar nebula and/or the present interstellar medium in excess of the observationally inferred values. This result, which obtains even for zero primordial D and $^3$He, and was anticipated by Rood, Steigman \& Tinsley (1976), is insensitive to the choice of chemical evolution model; it is driven by the large $^3$He yields from low mass stars. In an attempt to ameliorate this problem we have considered a number of non-standard models in which the yields from low mass stars have been modified. Although several of these non-standard models may be consistent with the $^3$He data, they may be inconsistent with observations of $^{12}$C/$^{13}$C, $^{18}$O and, most seriously, the super-$^3$He rich planetary nebulae (Rood, Bania \& Wilson 1992). Even using the most extreme of these non-standard models (Hogan 1995), we obtain a generous upper bound to pre-galactic $^3$He: X$_{3P} \leq 3.2 \times10^{-5}$ which, nonetheless, leads to a stringent lower bound to the universal density of nucleons.

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arXiv1995

A New Look At Neutrino Limits From Big Bang Nucleosynthesis

We take a fresh look at the limits on the number of neutrino flavors derived from big bang nucleosynthesis. In particular, recent measurements of the \he4 abundance enable one to estimate the primordial \he4 mass fraction at $Y_p = 0.232 \pm .003(stat) \pm .005(syst)$. For a baryon to photon ratio, $η$, consistent with the other light elements, this leads to a best fit for the number of neutrino flavors $N_ν< 3$ (the precise number depends on $η$) indicating a very strong upper limit to $N_ν$. Here, we derive new upper limits on $N_ν$, paying special attention to the fact that the best estimate may lie in an unphysical region ($N_ν< 3$ if all three neutrino flavors are light or massless; the lower bound to $N_ν$ may even be as low as 2, if the small window for a $ν_τ$ mass is exploited.) Our resulting upper limits therefore depend on whether $N_ν\ge 2$ or 3 is assumed. We also explore the sensitivity of our results to the adopted value of $η$ and the assumed systematic errors in $Y_p$.

hep-ph↗

arXiv1995

Generic Evolution Of Deuterium And Helium-3

The primordial abundances of deuterium and of helium-3, produced during big bang nucleosynthesis, depend sensitively on the baryon density. Thus, the observed abundances of D and \he may provide useful ``baryometers'' provided the evolution from primordial to present (or, presolar nebula) abundances is understood. Inevitably, the derivation of primordial from observed abundances requires the intervention of a model for galactic evolution and, so, the inferred primordial abundances are, necessarily, model dependent. Here, an analytic framework for the evolution of D and \he is presented which is ``generic'' in the sense that it should describe the results of any specific galactic evolution model. The ``effective \he survival fraction'', $Γ_3$, is the one free parameter which is model specific. Solar system and interstellar data are used to infer upper and lower bounds to the primordial deuterium mass fraction $(X_{2P})$ as a function of $Γ_3$ and, these bounds are used to constrain the present baryon-to-photon ratio $(η)$ and baryon density $(Ω_B)$. For $Γ_3 \geq$ 1/4 it is found that (from D and \he alone): $3.1 \leq η_{10} \leq 9.0$; $0.045 \leq Ω_B h^2_{50} \leq 0.133$ (where $H_0 = 50h_{50}$ km$\,$s$^{-1}$ Mpc$^{-1}$).

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arXiv1995

THE X-RAY CLUSTER BARYON CRISIS

Nucleosynthesis in the standard hot big bang cosmology offers a successful account of the production of the light nuclides during the early evolution of the Universe. Consistency among the predicted and observed abundances of D, $^3$He, \hel4 and \li7 leads to restrictive lower and upper bounds to the present density of nucleons. In particular, the upper bound ensures that nucleons cannot account for more than a small fraction $(<0.06 h{^{-2} _{50}})$ of the mass in a critical density (Einstein-de Sitter) Universe. In contrast, x-ray observations of rich clusters of galaxies suggest strongly that baryons (in hot gas) contribute a significant fraction of the total cluster mass $(\ge 0.2 h{^{-3/2} _{50}})$. If, indeed, clusters do provide a ``fair" sample of the mass in the Universe, this ``crisis" forces us to consider other ways of mitigating it, including the politically incorrect possibility that $Ω< 1$. The options, including magnetic or turbulent pressure, clumping, and non-zero space curvature and/or cosmological constant, are discussed.

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arXiv1994

Big Bang Nucleosynthesis: Consistency Or Crisis ?

The early, hot, dense, expanding Universe was a primordial reactor in which the light nuclides D, 3He, 4He and 7Li were synthesized in astrophysically interesting abundances. The challenge to the standard hot big bang model (Big Bang Nucleosynthesis = BBN) is the comparison between the observed and predicted abundances, the latter which depend only on the universal abundance of nucleons. The current status of observations is reviewed and the inferred primordial abundances are used to confront BBN. This comparison suggests consistency for BBN for a narrow range in the nucleon abundance but, looming on the horizon are some potential crises which will be outlined.

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arXiv1994

Cosmic Deuterium Or A Hydrogen Interloper ?

Two groups have independently reported the possible detection of extragalactic deuterium in the absorption spectrum of the same high redshift, low metalicity QSO. Although the high value for the inferred deuterium abundance poses no problems for cosmology (i.e., big bang nucleosynthesis), it is in apparent conflict with solar system observations of deuterium and helium-3. This latter inconsistency is explained and made quantitative and it is shown that, unless the inferred D/H ratio is too high by a factor of three, these data challange our understanding of the stellar and galactic evolution of helium-3. This conflict is resolved if the observed absorption feature is, in fact, due to a hydrogen interloper rather than to high z, low Z deuterium.

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arXiv1994

On the Abundance of Primordial Helium

We have used recent observations of helium-4, nitrogen and oxygen from some four dozen, low metallicity, extra-galactic HII regions to define mean $N$ versus $O$, $^4He$ versus $N$ and $^4He$ versus $O$ relations which are extrapolated to zero metallicity to determine the primordial $^4He$ mass fraction $Y_P$. The data and various subsets of the data, selected on the basis of nitrogen and oxygen, are all consistent with $Y_P = 0.232 \pm 0.003$. For the 2$σ$ (statistical) upper bound we find $Y_P^{2σ} \le 0.238$. Estimating a 2\% systematic uncertainty $(σ_{syst} = \pm 0.005)$ leads to a maximum upper bound to the primordial helium mass fraction: $Y_P^{MAX} = Y_P^{2σ} + σ_{syst} \le 0.243$. We compare these upper bounds to $Y_P$ with recent calculations of the predicted yield from big bang nucleosynthesis to derive upper bounds to the nucleon-to-photon ratio $η$ ($η_{10} \equiv 10^{10}η$) and the number of equivalent light (\lsim 10 MeV) neutrino species. For $Y_P \le 0.238$ ($0.243$), we find $η_{10} \le 2.5 (3.9)$ and $N_ν\leq 2.7 (3.1)$. If indeed $Y_P \le 0.238$, then BBN predicts enhanced production of deuterium and helium-3 which may be in conflict with the primordial abundances inferred from model dependent (chemical evolution) extrapolations of solar system and interstellar observations. Better chemical evolution models and more data - especially $D$-absorption in the QSO Ly-$α$ clouds - will be crucial to resolve this potential crisis for BBN. The larger upper bound, $Y_P \leq 0.243$ is completely

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