arXiv subjects

G. Steigman

Publications and source records attributed to G. Steigman.

At least 19 recordsLinked to original sources

arXiv2008

An Accelerating Cosmology Without Dark Energy

The negative pressure accompanying gravitationally-induced particle creation can lead to a cold dark matter (CDM) dominated, accelerating Universe (Lima et al. 1996) without requiring the presence of dark energy or a cosmological constant. In a recent study Lima et al. (2008, LSS) demonstrated that particle creation driven cosmological models are capable of accounting for the SNIa observations of the recent transition from a decelerating to an accelerating Universe. Here we test the evolution of such models at high redshift using the constraint on z_eq, the redshift of the epoch of matter radiation equality, provided by the WMAP constraints on the early Integrated Sachs-Wolfe effect. Since the contribution of baryons and radiation was ignored in the work of LSS, we include them in our study of this class of models. The parameters of these more realistic models with continuous creation of CDM is tested and constrained at widely-separated epochs (z = z_eq and z = 0) in the evolution of the Universe. This comparison reveals a tension between the high redshift CMB constraint on z_eq and that which follows from the low redshift SNIa data, challenging the viability of this class of models.

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arXiv2001

Stellar Mixing and the Primordial Lithium Abundance

We compare the properties of recent samples of the lithium abundances in halo stars to one another and to the predictions of theoretical models including rotational mixing, and we examine the data for trends with metal abundance. We find from a KS test that in the absence of any correction for chemical evolution, the Ryan, Norris, & Beers (1999} sample is fully consistent with mild rotational mixing induced depletion and, therefore, with an initial lithium abundance higher than the observed value. Tests for outliers depend sensitively on the threshold for defining their presence, but we find a 10$--$45% probability that the RNB sample is drawn from the rotationally mixed models with a 0.2 dex median depletion (with lower probabilities corresponding to higher depletion factors). When chemical evolution trends (Li/H versus Fe/H) are treated in the linear plane we find that the dispersion in the RNB sample is not explained by chemical evolution; the inferred bounds on lithium depletion from rotational mixing are similar to those derived from models without chemical evolution. We find that differences in the equivalent width measurements are primarily responsible for different observational conclusions concerning the lithium dispersion in halo stars. The standard Big Bang Nucleosynthesis predicted lithium abundance which corresponds to the deuterium abundance inferred from observations of high-redshift, low-metallicity QSO absorbers requires halo star lithium depletion in an amount consistent with that from our models of rotational mixing, but inconsistent with no depletion.

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arXiv2001

CMB (And Other) Challenges To BBN

Primordial nucleosynthesis provides a probe of the universal abundance of baryons when the universe was only a few minutes old. Recent observations of anisotropy in the cosmic microwave background (CMB) probe the baryon abundance when the universe was several hundred thousand years old. Observations of type Ia supernovae and clusters of galaxies in the very recent past, when the universe is several billion years old and older, provide a complementary measure of the baryon density in excellent agreement with the early universe values. The general agreement among the three measurements represents an impressive confirmation of the standard model of cosmology. However, there is a hint that the CMB observations may not be in perfect agreement with those from big bang nucleosynthesis (BBN). If this "tension" between BBN and the CMB persists, the standard model of cosmology may need to be modified. Here, in a contribution dedicated to Silvia Torres-Peimbert and Manuel Peimbert, we describe how an asymmetry between neutrinos and antineutrinos ("neutrino degeneracy") has the potential for resolving this possible conflict between BBN and the CMB.

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arXiv2000

Global Constraints On Key Cosmological Parameters

Data from Type Ia supernovae, along with X-ray cluster estimates of the universal baryon fraction and Big Bang Nucleosynthesis (BBN) determinations of the baryon-to-photon ratio, are used to provide estimates of several global cosmological parameters at epochs near zero redshift. We show that our estimate of the present baryon density is in remarkably good agreement with that inferred from BBN at high redshift, provided the primordial abundance of deuterium is relatively low and the Universe is flat. We also compare these estimates to the baryon density at z = 1100 as inferred from the CMB angular power spectrum.

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arXiv2000

Nucleosynthesis in Power-Law Cosmologies

We have recently considered cosmologies in which the Universal scale factor varies as a power of the age of the Universe and concluded that they cannot satisfy the observational constraints on the present age, the magnitude-redshift relation for SN Ia, and the primordial element (D, He3, He4, and Li7) abundances. This claim has been challenged in a proposal that suggested a high baryon density model (Omega_B*h*h = 0.3) with an expansion factor varing linearly with time could be consistent with the observed abundance of primoridal helium-4, while satisfying the age and magnitude-redshift constraints. In this paper we further explore primordial nucleosynthesis in generic power-law cosmologies, including the linear case, concluding that models selected to satisfy the other observational constraints are incapable of accounting for all the light element abundances.

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arXiv2000

The Supernova Relic Neutrino Background

An upper bound to the supernova relic neutrino background from all past Type II supernovae is obtained using observations of the Universal metal enrichment history. We show that an unambiguous detection of these relic neutrinos by the Super-Kamiokande detector is unlikely. We also analyze the event rate in the Sudbury Neutrino Observatory (where coincident neutrons from anti-nu_e + D --> n + n + e+ might enhance background rejection), and arrive at the same conclusion. If the relic neutrino flux should be observed to exceed our upper bound and if the observations of the metal enrichment history (for z<1) are not in considerable error, then either the Type II supernova rate does not track the metal enrichment history or some mechanism may be responsible for transforming anti-nu_{mu,tau} --> anti-nu_e.

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arXiv2000

Primordial Nucleosynthesis For The New Millennium

The physics of the standard hot big bang cosmology ensures that the early Universe was a primordial nuclear reactor, synthesizing the light nuclides (D, 3He, 4He, and 7Li) in the first 20 minutes of its evolution. After an overview of nucleosynthesis in the standard model (SBBN), the primordial abundance yields will be presented, followed by a status report (intended to stimulate further discussion during this symposium) on the progress along the road from observational data to inferred primordial abundances. Theory will be confronted with observations to assess the consistency of SBBN and to constrain cosmology and particle physics. Some of the issues/problems key to SBBN in the new millenium will be highlighted, along with a wish list to challenge theorists and observers alike.

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arXiv1999

Ionization Corrections For Low-Metallicity H II Regions and the Primordial Helium Abundance

Helium and hydrogen recombination lines observed in low-metallicity, extragalactic H II regions provide the data used to infer the primordial helium mass fraction, Y_P. The ionization corrections for unseen neutral helium (or hydrogen) are usually assumed to be absent; i.e., the ionization correction factor is taken to be unity (icf = 1). In this paper we revisit the question of the icf for H II regions ionized by clusters of young, hot, metal-poor stars. Our key result is that for the H II regions used in the determination of Y_P, there is a ``reverse'' ionization correction: icf < 1. We explore the effect on the icf of more realistic inhomogeneous H II region models and find that for those regions ionized by young stars, with ``hard'' radiation spectra, the icf is reduced further below unity. In Monte Carlos using H II region data from the literature (Izotov and Thuan 1998) we estimate a reduction in the published value of Y_P of order 0.003, which is roughly twice as large as the quoted statistical error in the Y_P determination.

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arXiv1999

Omega_B and Omega_0 From MACHOs and Local Group Dynamics

We obtain restrictions on the universal baryon fraction, f_B = Omega_B/Omega_0, by assuming that the observed microlensing events towards the Large Magellanic Cloud are due to baryonic MACHOs in the halo of the Galaxy and by extracting a bound to the total mass of the Milky Way from the motion of tracer galaxies in the Local Group. We find a lower bound f_B > 0.29^{+0.18}_{-0.15}. Consistency with the predictions of primordial nucleosynthesis leads to the further constraint on the total mass density, Omega_0 < 0.2.

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arXiv1998

Non-Nucleosynthetic Constraints on the Baryon Density and Other Cosmological Parameters

Because the baryon-to-photon ratio eta_{10} is in some doubt, we drop nucleosynthetic constraints on eta_{10} and fit the three cosmological parameters (h, Omega_M, eta_{10}) to four observational constraints: Hubble parameter h_o = 0.70+-0.15, age of the universe t_o = 14+7-2 Gyr, cluster gas fraction f_o \equiv f_G h^{3/2} = 0.060 +- 0.006, and effective shape parameter Gamma_o = 0.255 +- 0.017. Errors quoted are 1 sigma, and we assume Gaussian statistics. We experiment with a fifth constraint Omega_o = 0.2 +- 0.1 from clusters. We set the tilt parameter n = 1 and the gas enhancement factor Upsilon = 0.9. We consider CDM models (open and Omega_M = 1) and flat LambdaCDM models. We test goodness of fit and draw confidence regions by the Delta chi^2 method. CDM models with Omega_M = 1 (SCDM models) are accepted only because the large error on h_o allows h < 0.5. Baryonic matter plays a significant role in Gamma_o when Omega_M \sim 1. Open CDM models are accepted only for Omega_M \gtrsim 0.4. The combination of the four other constraints with Omega_o = 0.2 +- 0.1 is rejected in CDM models with 98% confidence, suggesting that light may not trace mass. LambdaCDM models give similar results. In all of these models, eta_{10} \gtrsim 6 is favored strongly over eta_{10} \lesssim 2. This suggests that reports of low deuterium abundances on QSO lines of sight may be correct and that observational determinations of primordial 4He may have systematic errors. Plausible variations on n and Upsilon in our models do not change the results much. If we drop or change the crucial Gamma_o constraint, lower values of Omega_M and eta_{10} are permitted. The constraint Gamma_o = 0.15 +- 0.04, derived recently from the IRAS redshift survey, favors Omega_M \approx 0.3 and eta_{10} \approx 5 but does not exclude eta_{10} \approx 2.

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arXiv1998

Observational Constraints On Power-Law Cosmologies

In a class of models designed to solve the cosmological constant problem by coupling scalar or tensor classical fields to the space-time curvature, the universal scale factor grows as a power law in the age, $a \propto t^α$, regardless of the matter content or cosmological epoch. We investigate constraints on such "power-law cosmologies" from the present age of the Universe, the magnitude-redshift relation, and from primordial nucleosynthesis. Constraints from the current age of the Universe and from the high-redshift supernovae data require "large" $α$ ($\approx 1$), while consistency with the inferred primordial abundances of deuterium and helium-4 forces $α$ to lie in a very narrow range around a lower value ($\approx 0.55$). Inconsistency between these independent cosmological constraints suggests that such power-law cosmologies are not viable.

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arXiv1998

Halo Star Lithium Depletion

The depletion of lithium during the pre-main sequence and main sequence phases of stellar evolution plays a crucial role in the comparison of the predictions of big bang nucleosynthesis with the abundances observed in halo stars. In the past a wide range of possible depletion factors, ranging from minimal in standard (non-rotating) stellar models to as much as an order of magnitude in models including rotational mixing have been suggested. Using recent progress in the study of the angular momentum evolution of low mass stars, which now permits the construction of theoretical models that reproduce the angular momentum evolution of low mass open cluster stars, along with the observed distribution of initial angular momenta inferred from stellar rotation data in young open clusters, we study lithium depletion in main sequence halo stars. We predict a well-defined, nearly flat (in T_eff) halo lithium plateau with modest scatter and a small population of outliers. We also examine the relative depletions of 6Li and 7Li finding that the dispersion in the plateau and the 6Li/7Li depletion ratio scale with the absolute 7Li depletion. Using observational data to bound the 7Li depletion in main sequence halo stars, we find a maximum depletion of 0.4 dex is set by the observed dispersion and the 6Li/7Li depletion ratio, and a minimum depletion of 0.2 dex is required by both the presence of highly overdepleted halo stars and consistency with the solar and open cluster 7Li data. The cosmological implications of these bounds on the primordial abundance of 7Li are discussed. (Abridged)

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arXiv1998

Big Bang Nucleosynthesis: Current Status

During its hot, dense, early evolution the Universe was a primordial nuclear reactor, synthesizing the light nuclides D, 3He, 4He and 7Li in the first thousand seconds. The presently observed abundances of these relic nuclides provide a unique window on the early Universe. The implications of current observations for cosmology (the universal density of nucleons) and for particle physics (new particles beyond the standard model) will be reviewed. The present data appear to be in rough agreement with the predictions of the standard, hot big bang model for three species of light neutrinos, and a nucleon-to-photon ratio restricted to a narrow range of 3-4 parts in 10 billion. On closer inspection, however, a tension is revealed between the inferred primordial abundances of deuterium and helium-4. Although observations of deuterium in nearly primordial, high-redshift QSO absorbers may help to relieve this tension, current data appear to exacerbate the crisis. Resolution of this conflict may lie with the data (statistical uncertainties?), with the analysis of the data (systematic uncertainties?), or with the fundamental physics (massive, unstable, and/or degenerate neutrinos?). Independent (non-BBN) evidence from cosmological observations of large-scale dynamics and structure may be useful in deciding among the current options.

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arXiv1998

Non-BBN Constraints On The Key Cosmological Parameters

Since the baryon-to-photon ratio "eta" is in some doubt at present, we ignore the constraints on eta from big bang nucleosynthesis (BBN) and fit the three key cosmological parameters (h, Omega_M, eta) to four other observational constraints: Hubble parameter, age of the universe, cluster gas (baryon) fraction, and effective shape parameter "Gamma". We consider open and flat CDM models and flat "Lambda"-CDM models, testing goodness of fit and drawing confidence regions by the Delta-chi^2 method. CDM models with Omega_M = 1 (SCDM models) are accepted only because we allow a large error on h, permitting h < 0.5. Open CDM models are accepted only for Omega_M \gsim 0.4. Lambda-CDM models give similar results. In all of these models, large eta (\gsim 6) is favored strongly over small eta, supporting reports of low deuterium abundances on some QSO lines of sight, and suggesting that observational determinations of primordial 4He may be contaminated by systematic errors. Only if we drop the crucial Gamma constraint are much lower values of Omega_M and eta permitted.

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arXiv1997

Rotational Mixing And The Primordial Lithium Abundance

There has been recent progress in the study of the angular momentum evolution of low mass stars (Krishnamurthi et al 1997a). Theoretical models can now be constructed which reproduce the angular momentum evolution of low mass open cluster stars and the distribution of initial conditions can be inferred from young clusters. In this poster we report on the application of these models to the problem of rotational mixing in halo stars. The distribution of initial conditions inferred from young clusters produces a well-defined halo lithium "plateau" with modest scatter and a small population of outliers. Different choices for the solar calibration produce a range of absolute depletion factors. We show that both the dispersion and the ratio of $^6$Li depletion to $^7$Li depletion increase as the absolute $^7$Li depletion increases. The measured $^6$Li in HD 84937 and the dispersion in the plateau set independent upper bounds on the $^7$Li depletion. Consistency with open clusters and the Sun, along with claims of an intrinsic dispersion in the plateau, set a lower bound. We derive a range of 0.2-0.4 dex $^7$Li depletion in halo field stars. Implications for cosmology are discussed.

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arXiv1996

Cosmological Implications of Two Conflicting Deuterium Abundances

Constraints on big bang nucleosynthesis (BBN) and on cosmological parameters from conflicting deuterium observations in different high red-shift QSO systems are discussed. The high deuterium observations by Carswell et al., Songaila et al., and Rugers & Hogan is consistent with 4He and 7Li observations and Standard BBN (N_nu =3) and allows N_nu \leq 3.6 at 95% C.L., but is inconsistent with local observations of D and 3He in the context of conventional theories of stellar and Galactic evolution. In contrast, the low deuterium observations by Tytler, Fan & Burles and Burles & Tytler are consistent with the constraints from local Galactic observations, but require N_nu = 1.9 +/- 0.3 at 68% C.L., excluding Standard BBN at 99.9% C.L., unless the systematic uncertainties in the 4He observations have been underestimated by a large amount. The high and low primordial deuterium abundances imply, respectively, Omega_B h^2 = 0.005 - 0.01 and Omega_B h^2 = 0.02 - 0.03 at 95% C.L. When combined with the high baryon fraction inferred from x-ray observations of rich clusters, the corresponding total mass densities (for 50 \le H_0 \le 90) are Omega_M = 0.05 - 0.20 and Omega_M = 0.2 - 0.7, respectively (95\% C.L.) The range of Omega_M corresponding to high D is in conflict with dynamical constraints (Omega_M > 0.2 - 0.3) and with the shape parameter constraint (Gamma = Omega_M h = 0.25 +/- 0.05) from large scale structure formation in CDM and Lambda-CDM models.

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arXiv1995

Predicting Big Bang Deuterium

We present new upper and lower bounds to the primordial abundances of deuterium and helium-3 based on observational data from the solar system and the interstellar medium. Independent of any model for the primordial production of the elements we find (at the 95\% C.L.): $1.5 \times 10^{-5} \le (D/H)_P \le 10.0 \times 10^{-5}$ and $(^3He/H)_P \le 2.6\times 10^{-5}$. When combined with the predictions of standard big bang nucleosynthesis, these constraints lead to a 95\% C.L. bound on the primordial abundance of deuterium: $(D/H)_{best} = (3.5^{+2.7}_{-1.8})\times 10^{-5}$. Measurements of deuterium absorption in the spectra of high redshift QSOs will directly test this prediction. The implications of this prediction for the primordial abundances of helium-4 and lithium-7 are discussed, as well as those for the universal density of baryons.

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arXiv1995

Big Bang Nucleosynthesis in Crisis?

A new evaluation of the constraint on the number of light neutrino species (N_nu) from big bang nucleosynthesis suggests a discrepancy between the predicted light element abundances and those inferred from observations, unless the inferred primordial 4He abundance has been underestimated by 0.014 +/- 0.004 (1 sigma) or less than 10% (95%C.L.) of 3He survives stellar processing. With the quoted systematic errors in the observed abundances and a conservative chemical evolution parameterization, the best fit to the combined data is N_nu = 2.1 +/- 0.3 (1 sigma) and the upper limit is N_nu < 2.6 (95% C.L.). The data are inconsistent with the Standard Model (N_nu = 3) at the 98.6% C.L.

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