[Paper Review] A case for nucleosynthesis in slowly evolving models

Geetanjali Sethi, Pranav Kumar+2 more|arXiv (Cornell University)|Feb 18, 2005

Cosmology and Gravitation Theories3 citations

TL;DR

This paper proposes that cosmological nucleosynthesis in a slowly evolving Friedmann-Robertson-Walker (FRW) universe with a linearly increasing scale factor (a(t) ∼ t) can naturally produce observed primordial helium-4 and low-metallicity environments consistent with Pop II stars. By enabling deuterium production via spallation in low-metallicity, high-energy particle beams, the model circumvents the 'no-go' constraint of Epstein et al., offering a viable alternative to standard big-bang nucleosynthesis with consistent helium and metallicity yields.

ABSTRACT

We present a case for Cosmological Nucleosynthesis in an FRW universe in which the scale factor expands linearly with time: $a(t) \sim t$. It is demonstrated that adequate amount of $^4He$ requires a baryon density that saturates mass bounds from galactic clusters. There is a collataral metallicity production that is quite close to the lowest metallicity observed in metal poor Pop II stars and clouds. On the other hand, sites for incipient low metallicity (Pop II) starformation can support environments conducive to Deuterium production up to levels observed in the universe. A profile of a revised ``Standard Cosmology'' is outlined.

Motivation & Objective

• To challenge the assumption that only standard big-bang nucleosynthesis (SBBN) can produce primordial light elements.
• To address inconsistencies in SBBN, particularly the low observed ⁴He abundances in some metal-poor systems like quasars and HII galaxies.
• To explore whether nucleosynthesis in slowly evolving cosmological models can naturally account for observed deuterium and lithium abundances.
• To propose a mechanism for deuterium production via spallation in low-metallicity, high-energy particle environments, circumventing the 'no-go' constraint from Epstein et al.
• To demonstrate that a linear coasting cosmology (a(t) ∼ t) can reproduce observed ⁴He and metallicity levels without requiring Pop III stars.

Proposed method

• Adopts a Friedmann-Robertson-Walker (FRW) model with a linearly evolving scale factor, a(t) ∼ t, implying a coasting expansion rate.
• Calculates nucleosynthesis yields for ⁴He and metals under this model, using a baryon density parameter η ≈ 7.8 × 10⁻⁹.
• Models particle acceleration in magnetic fields in early star-forming regions, producing beams deficient in ⁴He but rich in protons and light ions.
• Applies spallation reactions to produce deuterium and lithium in low-metallicity environments, using energy spectra from 1 MeV to 500 MeV per nucleon.
• Compares metallicity yields in the linear model to observed values in Pop II stars and metal-poor HII galaxies.
• Evaluates the viability of the model by testing consistency with observed ⁴He mass fraction (Yₚ ≈ 0.21–0.23), deuterium, and lithium abundances.

Experimental results

Research questions

• RQ1 Can a slowly evolving cosmological model with a(t) ∼ t reproduce the observed primordial ⁴He abundance without requiring high baryon density?
• RQ2 Can deuterium be naturally produced in such models through spallation processes in low-metallicity, high-energy environments?
• RQ3 Does the model circumvent the 'no-go' constraint from Epstein et al. that rules out significant deuterium production in ⁴He-rich environments?
• RQ4 Can the observed low-metallicity levels in Pop II stars and HII galaxies be naturally explained without invoking Pop III stars?
• RQ5 Is the linear coasting model consistent with observational constraints on ⁴He, deuterium, and lithium abundances?

Key findings

• The linear coasting model (a(t) ∼ t) with η ≈ 7.8 × 10⁻⁹ produces a ⁴He mass fraction of approximately 23%, consistent with observations.
• Metallicity yields in the model are approximately 10⁻⁵ times solar, matching the lowest observed metallicities in Pop II stars and metal-poor HII galaxies.
• Deuterium can be naturally produced via spallation in high-energy particle beams that are deficient in ⁴He, circumventing the 'no-go' constraint from Epstein et al.
• The model avoids the need for Pop III stars by enabling deuterium and lithium production in the incipient environments of Pop II stars through spallation.
• The model is consistent with Hubble data and the age of the universe, supporting its viability as an alternative to standard big-bang nucleosynthesis.
• Low ⁴He abundances observed in quasars and μ Cassiopeiae A are naturally accommodated without requiring special or contrived explanations.