arXiv subjects

David N. Schramm

Publications and source records attributed to David N. Schramm.

At least 19 recordsLinked to original sources

arXiv1997

Big-bang Nucleosynthesis Enters the Precision Era

The last parameter of big-bang nucleosynthesis, the baryon density, is being pinned down by measurements of the deuterium abundance in high-redshift hydrogen clouds. When it is determined, it will fix the primeval light-element abundances. D, ^3He and ^7Li will become ``tracers'' for the study of Galactic and stellar chemical evolution, and big-bang nucleosynthesis will become an even sharper probe of particle physics, e.g., the bound to the number of light neutrino species will be tightened significantly. Two key tests of the consistency of the standard theory are on the horizon: an independent, high-precision determination of the baryon density from anisotropy of the cosmic background radiation and a precision determination of the primeval $^4$He abundance.

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arXiv1997

Ultra-high energy cosmic ray sources and large scale magnetic fields

Protons of energies up to 10**20 eV can be subject to significant deflection and energy dependent time delay in lage scale extragalactic or halo magnetic fields of strengths comparable to current upper limits. By performing 3-dimensional Monte Carlo simulations of nucleon propagation, we show how observations of arrival direction and time distributions can be used to measure the structure and strength of large-scale magnetic fields, and constrain the nature of the source of ultra-high energy cosmic rays.

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arXiv1997

Deuteronomy and Numbers

Four light isotopes - D, ^3He, ^4He and ^7Li - were produced by nuclear reactions a few seconds after the big bang. New measurements of ^3He in the ISM by Gloeckler and Geiss and of deuterium in high redshift hydrogen clouds by Tytler and his collaborators provide further confirmation of big-bang nucleosynthesis and new insight about the density of ordinary matter (baryons).

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arXiv1996

A Fresh Look at Axions and SN 1987A

We re-examine the very stringent limits on the axion mass based on the strength and duration of the neutrino signal from SN 1987A, in the light of new measurements of the axial-vector coupling strength of nucleons, possible suppression of axion emission due to many-body effects, and additional emission processes involving pions. The suppression of axion emission due to nucleon spin fluctuations induced by many-body effects degrades previous limits by a factor of about 2. Emission processes involving thermal pions can strengthen the limits by a factor of 3-4 within a perturbative treatment that neglects saturation of nucleon spin fluctuations. Inclusion of saturation effects, however, tends to make the limits less dependent on pion abundances. The resulting axion mass limit also depends on the precise couplings of the axion and ranges from 0.5x10**(-3) eV to 6x10**(-3) eV.

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arXiv1996

Nuclear Reaction Rates and Primordial $^6$Li

We examine the possibility that Big Bang Nucleosynthesis (BBN) may produce non-trivial amounts of $^6$Li. If a primordial component of this isotope could be observed, it would provide a new fundamental test of Big-Bang cosmology, as well as new constraints on the baryon density of the universe. At present, however, theoretical predictions of the primordial $^6$Li abundance are extremely uncertain due to difficulties in both theoretical estimates and experimental determinations of the deuterium-plus-alpha radiative capture reaction cross-section. We also argue that present observational capabilities do not yet allow the detection of primeval $^6$Li in very metal-poor stars of the galactic halo. However, if the critical cross section is towards the upper end of its plausible range, then improvements in $^6$Li detection capabilities may allow the establishment of $^6$Li as another product of BBN. It is also noted that a primordial $^6$Li detection could help resolve current concerns about the extragalactic D/H determination.

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arXiv1996

Cosmological Neutrino Signatures for Grand Unification Scale Physics

Physics beyond the standard model might imply the cosmological production of particles with grand unification scale energies. Nucleons and gamma-rays from such processes are candidates for the cosmic rays observed beyond 100 EeV [10**(20) eV]. Using a new particle propagation code, we calculate the neutrino fluxes predicted by such scenarios if consistency with the observed cosmic ray flux and the universal gamma-ray background at 1-10 GeV is required. Flux levels detectable by proposed km**3 scale neutrino observatories are allowed by these constraints. Bounds on or detection of a neutrino flux above about 1 EeV would allow neutrino astronomy to probe grand unification scale physics.

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arXiv1996

Halo White Dwarfs and the Hot Intergalactic Medium

We present a schematic model for the formation of baryonic galactic halos and hot gas in the Local Group and the intergalactic medium. We follow the dynamics, chemical evolution, heat flow and gas flows of a hierarchy of scales, including: protogalactic clouds, galactic halos, and the Local Group itself. Within this hierarchy, the Galaxy is built via mergers of protogalactic fragments. We find that early bursts of star formation lead to a large population of remnants (mostly white dwarfs), which would reside presently in the halo and contribute to the dark component observed in the microlensing experiments. The hot, metal-rich gas from early starbursts and merging evaporates from the clouds and is eventually incorporated into the intergalactic medium. The model thus suggests that most microlensing objects could be white dwarfs ($m \sim 0.5 \msol$), which comprise a significant fraction of the halo mass. Furthermore, the Local Group could have a component of metal-rich hot gas similar to, although less than, that observed in larger clusters. We discuss the known constraints on such a scenario and show that all local observations can be satisfied with present data in this model. The best-fit model has a halo that is 40% baryonic, with an upper limit of 77%.

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arXiv1996

Constraints on the Strength of Primordial Magnetic Fields from Big Bang Nucleosynthesis Revisited

In this paper, we revisit in detail the effects of primordial magnetic fields on big bang nucleosynthesis (BBN) including a discussion of the magnetic field geometry and the anomalous magnetic moment. The presence of magnetic fields affects BBN by (1) increasing the weak reaction rates; (2) increasing the electron density due to changes to the electron phase space; and (3) by increasing the expansion rate of the universe, due both to the magnetic field energy density and to the modified electron energy density. Of the effects considered, the increase in the expansion rate due to the magnetic field energy is the most significant for the interests of BBN. The allowed magnetic field intensity at the end of nucleosynthesis (0.01 MeV) is about $2 \times 10^{9}$G and corresponds to an upper limit on the magnetic field energy density of about 28\% of the neutrino energy density ($ρ_B \le 0.28 ρ_ν$).

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arXiv1996

Implications of a Primordial Origin for the Dispersion in D/H in Quasar Absorption Systems

We consider possible implications if the recent and disconcordant measurements of D/H in quasar absorption systems are real and indicate a dispersion in D/H in these primitive systems. In particular we examine the option that the D/H abundances in these systems, which are separated on cosmological scales, are primordial, implying a large scale inhomogeneity in the baryon content of the Universe. We show that such large scale isocurvature perturbations are excluded by current cosmic microwave background observations. We also discuss the implications of a smaller (in amplitude) inhomogeneity on the problem of the baryon density in clusters.

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arXiv1996

The Big-Bang Nucleosynthesis Limit to the Number of Neutrino Species

Concern about systematic uncertainty in the $^4$He abundance as well as the chemical evolution of $^3$He leads us to re-examine this important limit. It is shown that with conservative assumptions no more than the equivalent of 4 massless neutrino species are allowed. Even with the most extreme estimates of the astrophysical uncertainties a meaningful limit still exists, less than 5 massless neutrino species, and illustrates the robustness of this argument. A definitive measurement of the deuterium abundance in high-redshift hydrogen clouds should soon sharpen the limit.

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arXiv1996

Implications of a Possible Clustering of Highest Energy Cosmic Rays

Very recently, a possible clustering of a subset of observed ultrahigh energy cosmic rays above about 40EeV (4x10^19eV) in pairs near the supergalactic plane was reported. We show that a confirmation of this effect would provide information on origin and nature of these events and, in case of charged primaries, imply interesting constraints on the extragalactic magnetic field. The observed time correlation would most likely rule out an association of these events with cosmological gamma ray bursts. If no prominent astrophysical source candidates such as powerful radiogalaxies can be found, the existence of a mechanism involving new fundamental physics would be favored.

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arXiv1996

Geological Isotope Anomalies as Signatures of Nearby Supernovae

Nearby supernova explosions may cause geological isotope anomalies via the direct deposition of debris or by cosmic-ray spallation in the earth's atmosphere. We estimate the mass of material deposited terrestrially by these two mechanisms, showing the dependence on the supernova distance. A number of radioactive isotopes are identified as possible diagnostic tools, such as Be-10, Al-26, Cl-36, Mn-53, Fe-60, and Ni-59, as well as the longer-lived I-129, Sm-146, and Pu-244. We discuss whether the 35 and 60 kyr-old Be-10 anomalies observed in the Vostok antarctic ice cores could be due to supernova explosions. Combining our estimates for matter deposition with results of recent nucleosynthesis yields, we calculate the expected signal from nearby supernovae using ice cores back to $\sim 300$ kyr ago, and we discuss using deep ocean sediments back to several hundred Myr. In particular, we examine the prospects for identifying isotope anomalies due to the Geminga supernova explosion, and signatures of the possibility that supernovae might have caused one or more biological mass extinctions.

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arXiv1996

Cosmological Implications of the First Measurement of the Local ISM Abundance of $^3$He

Deuterium plays a crucial role in testing big-bang nucleosynthesis. Its chemical evolution, while simple (it is burned to $^3$He), is intertwined with the more complicated evolution of $^3$He. Gloeckler \& Geiss' new measurement of the $^3$He abundance and the HST measurement of D, both in the local ISM today, can be compared to the pre-solar nebula abundances of D and $^3$He. Within the uncertainties, the sum of D + $^3$He relative to hydrogen is unchanged. This provides some validation of the cosmological utility of D + $^3$He, first suggested by Yang et al (1984), and further, indicates that over the past 4.5 Gyr there has been at most modest stellar production of $^3$He, in contradiction with stellar modeling, or modest stellar destruction of $^3$He, in contradiction with some ``solar spoons.'' While the earlier Galactic evolution of D + $^3$He cannot be constrained directly, it is expected to be dominated by massive stars, which deplete their $^3$He and produce metals. Based on the Galactic metallicity and the constancy of D + $^3$He over the past 4.5 Gyr, we derive a more empirically based lower bound to the cosmological baryon density; while not dramatically different from the original bound of Yang et al (1984) based on D + $^3$He, it alleviates some of the cosmic tension between the big-bang $^4$He abundance and those of D and $^3$He.

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arXiv1995

Concordance of X-Ray Cluster Data With BBN In Mixed Dark Matter Models

If the hot, X-ray emitting gas in rich clusters forms a fair sample of the universe (as in Cold Dark Matter (CDM) models), and the universe is at the critical density, $Ω_T = 1$, then the data appears to imply a baryon fraction, $Ω_{b,x}$ ($Ω_{b,x}\equiv Ω_b$ derived from X-ray cluster data), larger than that predicted by Big Bang Nucleosynthesis (BBN). While various other systematic effects such as clumping can lower $Ω_{b,x}$, in this paper we use an elementary analysis to show that a simple admixture of Hot Dark Matter (HDM, low mass neutrinos) with CDM to yield mixed dark matter shifts $Ω_{b,x}$ down so that significant overlap with $Ω_b$ from BBN can occur for $H_0 \lsim 75\;{\rm km/sec/Mpc}$, even without invoking the possible aforementioned effects. The overlap interval is slightly larger for lower mass neutrinos since fewer cluster on the scale of the hot X-ray gas. We illustrate this result quantitatively in terms of a simple isothermal model. More realistic velocity dispersion profiles, with less centrally-peaked density profiles, imply that fewer neutrinos are trapped and, thus, further increase the interval of overlap. However, we also note that if future observations of light element abundances find that $Ω_b h^2 \lsim 0.018$, the range of concordance in this simple mixed dark matter model vanishes.

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arXiv1995

Big-Bang Nucleosynthesis and Galactic Chemical Evolution

Deuterium is the best indicator of the baryon density; however, only its present abundance is known (and only locally) and its chemical evolution is intertwined with that of $^3$He. Because galactic abundances are spatially heterogeneous, mean chemical-evolution models are not well suited for extrapolating the pre-solar D and $^3$He abundances to their primeval values. We introduce a new approach which explicitly addresses heterogeneity, and show that the decade-old big-bang nucleosynthesis concordance interval $η\approx (2 -8)\times 10^{-10}$ based on D and $^3$He is robust.

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arXiv1995

Assessing Big-Bang Nucleosynthesis

Systematic uncertainties in the light-element abundances and their evolution make a rigorous statistical assessment difficult. However, using Bayesian methods we show that the following statement is robust: the predicted and measured abundances are consistent with 95\% credibility only if the baryon-to-photon ratio is between $2\times 10^{-10}$ and $6.5\times 10^{-10}$ and the number of light neutrino species is less than 3.9. Our analysis suggests that the $^4$He abundance may have been systematically underestimated.

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arXiv1995

The Nuclear Impact on Cosmology: The $H_0\hbox{-}Ω$ Diagram

The \HOmega\ diagram is resurrected to dramatically illustrate the nature of the key problems in physical cosmology today and the role that nuclear physics plays in many of them. In particular it is noted that the constraints on \OmegaB\ from big bang nucleosynthesis do not overlap with the constraints on \OmegaVis\ nor have significant overlap with the lower bound on $Ω$ from cluster studies. The former implies that the bulk of the baryons are dark and the later is the principle argument for non-baryonic dark matter. A comparison with hot x-ray emitting gas in clusters is also made. The lower bound on the age of the universe from globular cluster ages (hydrogen burning in low mass stars) and from nucleocosmochronology also illustrates the Hubble constant requirement $H_0 \le 66 \Hunits$ for $Ω_0 = 1$\@. It is also noted that high values of $H_0$ ($\sim 80\Hunits$) even more strongly require the presence of non-baryonic dark matter. The lower limit on $H_0$ ($\ge 38\Hunits$) from carbon detonation driven type~Ia supernova constrains long ages and only marginally allows \OmegaB\ to overlap with \OmegaClus. Diagrams of \HOmega\ for $Λ_0=0$ and $Λ_0\neq 0$ are presented to show that the need for non-baryonic dark matter is independent of $Λ$.

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arXiv1994

What's The Problem With ^3He?

We consider the galactic evolutionary history of \he3 in models which deplete deuterium by as much as a factor of 2 to $\sim$ 15 from its primordial value to its present day observed value in the ISM. We show that when \he3 production in low mass stars (1 -- 3 $M_\odot$) is included over the history of the galaxy, \he3 is greatly over-produced and exceeds the inferred solar values and the abundances determined in galactic \hii regions. Furthermore, the ISM abundances show a disturbing dispersion which is difficult to understand from the point of view of standard chemical evolution models. In principle, resolution of the problem may lie in either 1) the calculated \he3 production in low mass stars; 2) the observations of the \he3 abundance; or 3) an observational bias towards regions of depleted \he3. Since \he3 observations in planetary nebula support the calculated \he3 production in low mass stars, option (1) is unlikely. We will argue for option (3) since the \he3 interstellar observations are indeed made in regions dominated by massive stars in which \he3 is destroyed. In conclusion, we note that the problem with \he3 seems to be galactic and not cosmological.

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