David N. Schramm

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

Cosmological Limits to the Number of Massive Leptons

Abstract If massive leptons exist, their associated neutrinos would have been copiously produced in the early stages of the hot, big bang cosmology. These neutrinos would have contributed to the total energy density and would have had the effect of speeding up the expansion of the universe. The effect of the speed-up on primordial nucleosynthesis is to produce a higher abundance of 4 He. It is shown that observational limits to the primordial abundance of 4 He lead to the constraint that the total number of types of heavy lepton must be less than or equal to 5.

Some Astrophysical Consequences of the Existence of a Heavy Stable Neutral Lepton

Recently, high-energy particle theorists have constructed new extended gauge theories which may fit experiment somewhat better than previous already very successful theories. One of the predictions which is often discussed is the possible existence of a stable neutral lepton, probably with a mass of a few GeV/c/sup 2/. Following this motivation we here investigate some cosmological consequences of the existence of any stable, massive, neutral lepton, and show that it could well dominate the present mass density in the universe. The contribution to the mass density depends on the mass of the lepton, which should eventually be determined with high-energy accelerators. It is interesting that the more massive the lepton, the smaller its contribution to the present mass density. It is unlikely that these leptons affect big bang nucleosynthesis or condense into stellar size objects. However, such a lepton is an excellent candidate for the material in galactic halos and for the mass required to bind the great clusters of galaxies. Annihilation radiation from these structures should be detectable. At the end of the paper a brief mention is made of the astrophysical constraints on the mass-lifetime relationship if the neutral lepton is unstable.

The origin of deuterium

General nuclear constraints are used to show that deuterium is most likely of pregalactic origin. Big-bang nucleosynthesis is the most plausible source for significant amounts of this isotope, but other, more speculative, sources are not ruled out.

Constraints on Cosmology and Neutrino Physics from Big Bang Nucleosynthesis

New calculations of primordial nucleosynthesis are presented and compared with the abundances of D, /sup 4/He, and /sup 7/Li to derive constraints to the number of two-component neutrino types (N/sub L/), the time-variation of the gravitational constant (Gapprox.t/sup -x/), and the present nucleon density (rho/sub N/=2 x 10/sup -29/..cap omega../sub N/h/sub 0//sup 2/). The observational and theoretical considerations related to the helium abundance are reviewed, and a primordial mass fraction Y< or approx. =0.25 is proposed. It follows from this limit to the /sup 4/He mass fraction that N/sub L/< or approx. =3,x<0.005,..cap omega../sub N/h/sub 0//sup 2/< or approx. =0.1. Independent considerations from the abundances of D, /sup 4/He, and /sup 7/Li all lead to the conclusion that the universe cannot be closed by nucleons.

The isotopic abundance of26Mg and limits on26Al in the early solar system

Abstract The isotopic composition of Mg is investigated in meteoritic, lunar and terrestrial feldspar samples and standard reagents using a mass spectrometer with on-line data processing. Isotopically enriched standards were run demonstrating a resolution of better than five parts in 10 4 for the 26 Mg/ 24 Mg ratio. No 26 Mg/ 24 Mg anomalies are found in any samples analyzed to within experimental errors. (These analyses include some samples run by Clarke, de Laeter, Schwarcz and Shane [1] and found by them to have 26 Mg/ 24 Mg anomalies of 4–6 parts in 10 3 relative to terrestrial values.) There is at the present time no positive evidence indicating the existence of 26 Al in the early solar system. From these results, limits are placed on the amount of 26 Al(τ ½ = 7.4 × 10 5 yr) and the resulting heating effects in the solar system at the time of crystallization of the meteorites. It is clear that when the meteorites solidified, 26 Al was not an important heat source; however, this would not rule out the possibility that a few million years prior to solidification 26 Al was a major heat source.

Big-bang nucleosynthesis revisited

Abstract We compute the homogeneous big-bang nucleosynthesis yields of D, 3 He, 4 He, and 7 Li, taking into account recent measurements of the neutron mean-life as well as updates of several nuclear reactions rates which primarily affect the production of 7 Li. We discuss the extraction of primordial abundances from observation and the likelihood that the primordial mass fraction of 4 He, Y p , is less than or equal to 0.24. Using the primordial abundances of D+ 3 He and 7 Li we limit the baryon-to-photon ratio ( η in units of 10 −10 ): 2.6 ⩽ η 10 ⩽ 4.3 (or, in terms of the present mass density in baryons, 1.8×10 −31 ⩽ ϱ B ⩽ 3.0×10 −31 g/cm 3 , for a microwave background temperature of 2.75°) which we use to argue that baryons contribute between 0.02 and 0.11 to the critical energy density of the universe. An upper limit to Y p of 0.24 constrains the number of light neutrinos to N ν ⩽ 3.4, in excellent agreement with the LEP and SLC collider results. We turn this argument around to show that the collider limit of 3 neutrino species can be used to bound the primordial abundance of 4 He: 0.235 ⩽ Y p ⩽ 0.245.

Dust in the Universe

The possible existence of intergalactic dust with properties similar to interstellar dust is examined in some detail. If such dust does exist, it could result in a correction to the observed deceleration parameter. (As little as 2 x 10/sup -4/ mag Mpc/sup -1/ absorption would cause a measurement error of (..delta..q/sub 0/)/sub Dust/ approx.1.) The relation of the dust to possible intergalactic gas is examined, including survival times in a hot gas. The possible origins of intergalactic dust are also discussed. It is shown that uniform filling of the unverse with dust is exceedingly unlikely whereas dust halos around galaxies and clusters of galaxies may be quite. The dust optical depth across a cluster halo is estimated to be ..pi..approx. =0.1.

Big Bang Nucleosynthesis as a Probe of Cosmology and Particle Physics

The mass fraction of 4He synthesized in the big bang, YP, depends upon the neutron half-life T1/2, the ratio of baryons to photons eta, and the number of two-component neutrino species NV. New observational and experimental data have led us to reexamine the constraints on cosmology and particle physics which follow from primordial nucleosynthesis. We find that eta must lie in the range 10-9.9±1, implying that baryons alone cannot close the universe; the related ratio of the baryon number to the specific entropy must lie in the range l0-10.8±1. If baryons provide most of the mass which binds binary and small groups of galaxies, then Nbeta must be V. If the universe is dominated by nonbaryonic matter, than there is no contradiction between the predictions of primordial nucleosynthesis and the observations of 4He, provided that YP 0.15.

Neutrino Opacities at High Temperatures and Densities

A detailed calculation is made of the major cross sections contributing to neutrino opacities at high temperatures and densities such as those encountered in gravitational collapse. These calculations include the effects of neutral currents, where applicable, and electron degeneracy. The processes considered are electron-neutrino scattering (including both electron and muon neutrinos and antineutrinos), neutrino-nucleon absorption and scattering, and coherent neutrino scattering. Results for these interactions are also given for the average energy transferred by the neutrino as well as the mean scattering angle (thus yielding momentum transfer). (AIP)