arXiv subjects

Gary Steigman

Publications and source records attributed to Gary Steigman.

At least 55 records · Page 3Linked to original sources

arXiv2003

Big Bang Nucleosynthesis: Probing the First 20 Minutes

Within the first 20 minutes of the evolution of the hot, dense, early Universe, astrophysically interesting abundances of deuterium, helium-3, helium-4, and lithium-7 were synthesized by the cosmic nuclear reactor. The primordial abundances of these light nuclides produced during Big Bang Nucleosynthesis (BBN) are sensitive to the universal density of baryons and to the early-Universe expansion rate which at early epochs is governed by the energy density in relativistic particles (``radiation'') such as photons and neutrinos. Some 380 kyr later, when the cosmic background radiation (CBR) radiation was freed from the embrace of the ionized plasma of protons and electrons, the spectrum of temperature fluctuations imprinted on the CBR also depended on the baryon and radiation densities. The comparison between the constraints imposed by BBN and those from the CBR reveals a remarkably consistent picture of the Universe at two widely separated epochs in its evolution. Combining these two probes leads to new and tighter constraints on the baryon density at present, on possible new physics beyond the standard model of particle physics, as well as identifying some challenges to astronomy and astrophysics. In this review the current status of BBN will be presented along with the associated estimates of the baryon density and of the energy density in radiation.

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arXiv2003

Hiding relativistic degrees of freedom in the early universe

We quantify the extent to which extra relativistic energy density can be concealed by a neutrino asymmetry without conflicting with the baryon asymmetry measured by the Wilkinson Microwave Anisotropy Probe (WMAP). In the presence of a large electron neutrino asymmetry, slightly more than seven effective neutrinos are allowed by Big Bang Nucleosynthesis (BBN) and WMAP at 2σ. The same electron neutrino degeneracy that reconciles the BBN prediction for the primordial helium abundance with the observationally inferred value also reconciles the LSND neutrino with BBN by suppressing its thermalization prior to BBN.

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arXiv2003

Effective number of neutrinos and baryon asymmetry from BBN and WMAP

We place constraints on the number of relativistic degrees of freedom and on the baryon asymmetry at the epoch of Big Bang Nucleosynthesis (BBN) and at recombination, using cosmic background radiation (CBR) data from the Wilkinson Microwave Anisotropy Probe (WMAP), complemented by the Hubble Space Telescope (HST) Key Project measurement of the Hubble constant, along with the latest compilation of deuterium abundances and measurements of the primordial helium abundance. The agreement between the derived values of these key cosmological and particle physics parameters at these widely separated (in time or redshift) epochs is remarkable. From the combination of CBR and BBN data, we find the 2σranges for the effective number of neutrinos and for the baryon asymmetry (baryon to photon number ratio η) to be 1.7-3.0 and 5.53-6.76 \times 10^{-10}, respectively.

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arXiv2003

BBN And CMB Constraints On Dark Energy

Current observational data favor cosmological models which differ from the standard model due to the presence of some form of dark energy and, perhaps, by additional contributions to the more familiar dark matter. Primordial nucleosynthesis provides a window on the very early evolution of the universe and constraints from Big Bang Nucleosynthesis (BBN) can bound the parameters of models for dark matter/energy at redshifts of order ten billion. The spectrum of temperature fluctuations imprinted on the Cosmic Microwave Background (CMB) radiation opens a completely different window on the universe at epochs from redshifts of order ten thousand to nearly the present. The CMB anisotropy spectrum provides constraints on new physics which are independent of, and complementary to those from BBN. Here we consider three classes of models for the dark matter/energy: extra particles which were relativistic during the early evolution of the universe (X); Quintessence models involving a minimally-coupled scalar field (Q); models with a non-minimally coupled scalar field which modify the strength of gravity during the early evolution of the universe (G). We constrain the parameters of these models using data from BBN and the CMB and identify the allowed regions in their parameter spaces consistent with the more demanding joint BBN and CMB constraints. For X and Q such consistency is relatively easy to find; it is more difficult for the G models with an inverse power law potential for the scalar field.

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arXiv2002

On the Variation of Deuterium and Oxygen Abundances in the Local Interstellar Medium

The first observations of deuterium and oxygen in the Local Interstellar Medium (LISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) can be used to search for local abundance variations. While the very limited sample of these first data may be consistent with no variations, they do offer a hint of anti-correlated variations between D/H and O/H. If confirmed by more data (which will require independently determined, accurate HI column densities), these hints suggest that observations of interstellar gas within a few kpc of the solar neighborhood will reveal clear signs of the evolution of the abundance of deuterium from there and then (the Big Bang), to here and now (the Local Interstellar Medium of the Galaxy).

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arXiv2002

Primordial Alchemy: From The Big Bang To The Present Universe

Of the light nuclides observed in the universe today, D, 3He, 4He, and 7Li are relics from its early evolution. The primordial abundances of these relics, produced via Big Bang Nucleosynthesis (BBN) during the first half hour of the evolution of the universe provide a unique window on Physics and Cosmology at redshifts of order 10^10. Comparing the BBN-predicted abundances with those inferred from observational data tests the consistency of the standard model of cosmology over ten orders of magnitude in redshift, constrains the baryon and other particle content of the universe, and probes both Cosmology and Physics beyond their current standard models. These lectures are intended to introduce students, both of theory and observation, to those aspects of the evolution of the universe relevant to the production and evolution of the light nuclides from the Big Bang to the present. The current observational data is reviewed and compared with the BBN predictions and the implications for cosmology (e.g., universal baryon density) and particle physics (e.g., relativistic energy density) are discussed. While this comparison reveals the stunning success of the standard model(s), there are currently some challenges which leave open the door for more theoretical and observational work with potential implications for astronomy, cosmology, and particle physics.

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arXiv2002

The Baryon Density Through The (Cosmological) Ages

The light element abundances probe the baryon density of the universe within a few minutes of the Big Bang. Of these relics from the earliest universe, deuterium is the baryometer of choice. By comparing its primordial abundance inferred from observations with that predicted by Big Bang Nucleosynthesis (BBN), the early universe baryon density is derived. This is then compared to independent estimates of the baryon density several hundred thousand years after the Big Bang and at present, more than 10 billion years later. The excellent agreement among these values represents an impressive confirmation of the standard model of cosmology.

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arXiv2001

The Evolution of Helium and Hydrogen Ionization Corrections as HII Regions Age

Helium and hydrogen recombination lines observed in low-metallicity, extragalactic, HII regions provide the data used to infer the primordial helium mass fraction, Y_P. In deriving abundances from observations, the correction for unseen neutral helium or hydrogen is usually assumed to be absent; i.e., the ionization correction factor is taken to be unity (icf = 1). In a previous paper (VGS), we revisited the question of the icf, confirming a "reverse" ionization correction: icf < 1. In VGS the icf was calculated using more nearly realistic models of inhomogeneous HII regions, suggesting that the published values of Y_P needed to be reduced by an amount of order 0.003. As star clusters age, their stellar spectra evolve and so, too, will their icfs. Here the evolution of the icf is studied, along with that of two, alternate, measures of the "hardness" of the radiation spectrum. The differences between the icf for radiation-bounded and matter-bounded models are also explored, along with the effect on the icf of the He/H ratio (since He and H compete for some of the same ionizing photons). Particular attention is paid to the amount of doubly-ionized helium predicted, leading us to suggest that observations of, or bounds to, He++ may help to discriminate among models of HII regions ionized by starbursts of different ages and spectra. We apply our analysis to the Izotov & Thuan (IT) data set utilizing the radiation softness parameter, the [OIII]/[OI] ratio, and the presence or absence of He++ to find 0.95 < icf < 0.99. This suggests that the IT estimate of the primordial helium abundance should be reduced by Delta-Y = 0.006 +- 0.002, from 0.244 +- 0.002 to 0.238 +- 0.003.

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arXiv2001

Precision Neutrino Counting

In the framework of the standard, hot big bang cosmological model the dynamics of the early evolution of the universe is controlled by the energy density of relativistic particles, among which neutrinos play an important role. In equilibrium, the energy density contributed by one flavor of relativistic neutrinos is 7/8 of that of the cosmic background radiation (CBR) photons. As the universe expands and cools, neutrinos decouple and their subsequent contribution to the energy density is modified by the relative heating of the CBR photons when electron-positron pairs annihilate. The small corrections to the post-annihilation energy density of the standard model neutrinos due to incomplete decoupling and finite-temperature QED effects are reviewed (correcting an error in the literature) and extended to account for possible additional relativistic degrees of freedom whose presence might modify the predictions of primordial nucleosynthesis and of the predicted CBR anisotropies.

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arXiv2001

How Does CMB + BBN Constrain New Physics?

Recent cosmic microwave background (CMB) results from BOOMERANG, MAXIMA, and DASI provide cosmological constraints on new physics that can be competitive with those derived from Big Bang Nucleosynthesis (BBN). In particular, both CMB and BBN can be used to place limits on models involving neutrino degeneracy and additional relativistic degrees of freedom. However, for the case of the CMB, these constraints are, in general, sensitive to the assumed priors. We examine the CMB and BBN constraints on such models and study the sensitivity of ``new physics" to the assumed priors. If we add a constraint on the age of the universe ($t_0 \ga 11$ Gyr), then for models with a cosmological constant, the range of baryon densities and neutrino degeneracy parameters allowed by the CMB and BBN is fairly robust: $η_{10} = 6.0 \pm 0.6$, \deln $\la 6$, $ξ_e \la 0.3$. In the absence of new physics, models without a cosmological constant are only marginally compatible with recent CMB observations (excluded at the 93% confidence level).

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arXiv2001

Tracking The Baryon Density From The Big Bang To The Present

The primordial abundances of deuterium, helium, and lithium probe the baryon density of the universe only a few minutes after the Big Bang. Of these relics from the early universe, deuterium is the baryometer of choice. After reviewing the current observational status (a moving target!), the BBN baryon density is derived and compared to independent estimates of the baryon density several hundred thousand years after the Big Bang (as inferred from CMB observations) and at present, more than 10 billion years later. The excellent agreement among these values represents an impressive confirmation of the standard model of cosmology, justifying -- indeed, demanding -- more detailed, quantitative scrutiny. To this end, the corresponding BBN-predicted abundances of helium and lithium are compared with observations to further test and constrain the standard, hot, big bang cosmological model.

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arXiv2001

Extended quintessence and the primordial helium abundance

In extended quintessence models, a scalar field which couples to the curvature scalar R provides most of the energy density of the universe. We point out that such models can also lead naturally to a decrease in the primordial abundance of helium-4, relieving the tension which currently exists between the primordial helium-4 abundance inferred from observations and the amount predicted by Standard Big Bang Nucleosynthesis (SBBN) corresponding to the observed deuterium abundance. Using negative power-law potentials for the quintessence field, we determine the range of model parameters which can lead to an interesting reduction in the helium-4 abundance, and we show that it overlaps with the region allowed by other constraints on extended quintessence models.

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arXiv2000

Light Element Nucleosynthesis

An introductory review of the early evolution of the Universe relevant to the primordial synthesis of the light nuclides deuterium, helium-3, helium-4 and lithium-7. The predictions of the element abundances in the "standard", hot, big bang cosmological model (SBBN) are described. After descriptions of the evolution of the primordial abundances from "there and then" to "here and now", the SBBN predictions are compared to current observational data. The implications for the standard model and for physics beyond the standard model are discussed.

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arXiv1999

Primordial Nucleosynthesis: Theory and Observations

We review the Cosmology and Physics underlying Primordial Nucleosynthesis and survey current observational data in order to compare the predictions of Big Bang Nucleosynthesis with the inferred primordial abundances. From this comparison we report on the status of the consistency of the standard hot big bang model, we constrain the universal density of baryons (nucleons), and we set limits to the numbers and/or effective interactions of hypothetical new "light" particles (equivalent massless neutrinos).

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arXiv1997

Is High Primordial Deuterium Consistent With Galactic Evolution?

Galactic destruction of primordial deuterium is inevitably linked through star formation to the chemical evolution of the Galaxy. The relatively high present gas content and low metallicity suggest only modest D-destruction. In concert with deuterium abundances derived from solar system and/or interstellar observations this suggests a primordial deuterium abundance in possible conflict with data from some high-redshift, low-metallicity QSO absorbers. We have explored a variety of chemical evolution models including infall of processed material and early, supernovae-driven winds with the aim of identifying models with large D-destruction which are consistent with the observations of stellar-produced heavy elements. When such models are confronted with data we reconfirm that only modest destruction of deuterium (less than a factor of 3) is permitted. When combined with solar system and interstellar data these results favor the low deuterium abundances derived for the QSO absorbers observed by Tytler et al (1996) and by Burles & Tytler (1996).

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arXiv1997

Temperature Fluctuations and Abundances in HII Galaxies

There is evidence for temperature fluctuations in Planetary Nebulae and in Galactic HII regions. If such fluctuations occur in the low-metallicity, extragalactic HII regions used to probe the primordial helium abundance, the derived 4He mass fraction, Y_P, could be systematically different from the true primordial value. For cooler, mainly high-metallicity HII regions the derived helium abundance may be nearly unchanged but the oxygen abundance could have been seriously underestimated. For hotter, mainly low-metallicity HII regions the oxygen abundance is likely accurate but the helium abundance could be underestimated. The net effect is to tilt the Y vs. Z relation, making it flatter and resulting in a higher inferred Y_P. Although this effect could be large, there are no data which allow us to estimate the size of the temperature fluctuations for the extragalactic HII regions. Therefore, we have explored this effect via Monte Carlos in which the abundances derived from a fiducial data set are modified by Δ-T chosen from a distribution with 0 < Δ-T < Δ-T_max where Δ-T_max is varied from 500K to 4000K. It is interesting that although this effect shifts the locations of the HII regions in Y vs. O/H plane, it does not introduce any significant additional dispersion.

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arXiv1996

The Primordial Abundance of He4: An Update

We include new data in an updated analysis of helium in low metallicity extragalactic HII regions with the goal of deriving the primordial abundance of He4 (Y_P). We show that the new observations of Izotov et al (ITL) are consistent with previous data. However they should not be taken in isolation to determine (Y_P) due to the lack of sufficiently low metallicity points. We use the extant data in a semi-empirical approach to bounding the size of possible systematic uncertainties in the determination of (Y_P). Our best estimate for the primordial abundance of He4 assuming a linear relation between He4 and O/H is Y_P = 0.230 \pm 0.003 (stat) based on the subset of HII regions with the lowest metallicity; for our full data set we find Y_P = 0.234 \pm 0.002 (stat). Both values are entirely consistent with our previous results. We discuss the implications of these values for standard big bang nucleosynthesis (SBBN), particularly in the context of recent measurements of deuterium in high redshift, low metallicity QSO absorption-line systems.

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arXiv1996

Primordial Deuterium and Big Bang Nucleosynthesis: A Tale of Two Abundances

Recent confrontations of the predictions of standard big bang nucleosynthesis (SBBN) with the primordial abundances of the light nuclides inferred from observational data reveal a conflict. Simply put, compared to theoretical expectations the inferred primordial abundances of either deuterium or helium-4 (or both) are ``too small". Here I outline the ``tension" between D and $^4$He in the context of SBBN. The incipient crisis for SBBN may be resolved by observations of deuterium in nearly pristine environments such as the high-redshift, low-metallicity QSO absorbers. At present the big bang abundances of deuterium inferred from such data fall into two, apparently mutually exclusive, groups. I describe the deuterium dichotomy and its implications for SBBN as well as for cosmology in general.

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