The Big Bang and Other Explosions in Nuclear and Particle Astrophysics

The following sections are included:

• CONTENTS

• INTRODUCTION

The relativistic hot Big Bang model for the expanding Universe has yielded a set of interpretations and successful predictions that substantially outnumber the elements used in devising the theory, with no well-established empirical contradictions. It is reasonable to conclude that this standard cosmology has developed into a mature and believable physical model.

At a particular Instant roughly 15 billion years ago, all the matter and energy we can observe, concentrated in a region smaller than a dime, began to expand and cool at an incredibly rapid rate. By the time the temperature had dropped to 100 million times that of the sun's core, the forces of nature assumed their present properties, and the elementary particles known as quarks roamed freely in a sea of energy. When the universe had expanded an additional 1,000 times, all the matter we can measure filled a region the size of the solar system…

A variety of arguments strongly suggest that the density of the universe is no more than a tenth of the value required for closure. Loopholes in this reasoning may exist, but if so, they are primordial and invisible, or perhaps just black.

The following sections are included:

• Threefold achievement

• Exotic objects

Big bang nucleosynthesis provides (with the microwave background radiation) one of the two quantitative experimental tests of the big bang cosmological model. This paper reviews the standard homogeneous-isotropic calculation and shows how it fits the light element abundances ranging from ⁴He at 24% by mass through ²H and ³He at parts in 10⁵ down to ⁷Li at parts in 10¹⁰. Furthermore, the recent LEP (and SLC) results on the number of neutrinos are discussed as a positive laboratory test of the standard scenario. Discussion is presented on the improved observational data as well as the improved neutron lifetime data. Alternate scenarios of decaying matter or of quark-hadron induced inhomogeneities are discussed. It is shown that when these scenarios are made to fit the observed abundances accurately, the resulting conclusions on the baryonic density relative to the critical density Ω_b, remain approximately the same as in the standard homogeneous case, thus, adding to the robustness of the conclusion that Ω_b ≃ 0.06. This latter point is the driving force behind the need for non-baryonic dark matter (assuming Ω_Total = 1) and the need for dark baryonic matter, since Ω_nuble < Ω_b.

The latest nuclear reaction cross sections (including the most recent determinations of the neutron lifetime) are used to recalculate the abundances of deuterium, ³He, ⁴He, and ⁷Li within the framework of primordial nucleosynthesis in the standard (homogeneous and isotropic) hot, big bang model. The observational data leading to estimates of (or bounds to) the primordial abundances of the light elements is reviewed with an emphasis on ⁷Li and ⁴He. A comparison between theory and observation reveals the consistency of the predictions of the standard model and leads to bounds to the nucleon-to-photon ratio, 2.8 ≤ η₁₀ ≤ 4.0 (η_1O ≡ 10¹⁰n_g/n_y), which constrains the baryon density parameter, Ω_Bh²₅₀ = 0.05 ± 0.01 (the Hubble parameter is H_o = 50h₅₀ km s⁻¹ Mpc⁻¹). These bounds imply that the bulk of the baryons in the universe are dark if Ω_TOT = 1 and would require that the universe be dominated by nonbaryonic matter. An upper bound to the primordial mass fraction of ⁴He, Y_p ≤ 0.240, constrains the number of light (equivalent) neutrinos to N_v≤ 3.3, in excellent agreement with the LEP and SLC collider results. Alternatively, for N_v = 3, we bound the predicted primordial abundance of ⁴He: 0.236 ≤ Y_p ≤ 0.243 (for 882 ≤ τ_n 896 s).

A summary is given of the current beliefs regarding the origin and history of the light elements ²D, ³He, ⁴He, ⁶Li, ⁷Li, ⁹Be, ¹⁰B, and ¹¹B in the universe. A description of the various sites of nucleosynthesis for these elements is given, and the results compared with observations. It is found that the galactic cosmic rays (GCR) can spallogenically produce ⁶Li, ⁹Be, and ¹⁰B as well as the bulk of the ¹¹B (perhaps all) and ∼10 percent of the ⁷Li. The deuterium can only be produced pregalactically either in the big bang or in some pregalactic event. The big bang or pregalactic events will also produce ³He and ⁴He and some ⁷Li. Additional ⁷Li (and possibly even some ¹¹B) can be synthesized during the helium-flash stage in red giants. Stellar synthesis might also add ³He to the galactic gas.

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.

The following sections are included:

• Observations of deuterium abundance

• Deuterium production and destruction

• Possible astrophysical production sites

• D from supernova shock waves?

• D versus the light elements

• Deuterium from the primeval fireball?

• An open universe?

• References

It is shown that not only does Big Bang nucleosynthesis provide an upper limit on the baryon density of the Universe, but if one takes into account arguments concerning the production of ³He in stars, one can show that the ³He plus deuterium abundance can also provide a lower limit on the baryon density of the Universe. The derived constraints are that the baryon: photon ratio, η must be between 1.5 × lO⁻¹⁰ and 7 × 10⁻⁹ with a best fit between 3 and 6 × 10⁻¹⁰. This small range for η has implications for our limits on numbers of neutrino types, for Big Bang baryosynthesis, and for arguments about the nature of the dark matter in clusters of galaxies. With reference to the dark matter, the derived baryon density for Big Bang nucleosynthesis corresponds very closely with the implied density of matter in binaries and small groups of galaxies. This implies that non-baryonic matter is not dominant by a large factor on scales as large as binaries and small groups of galaxies. It is also shown that the constraints on the lower limit on the baryon density constrain the lower limit on the primordial ⁴He abundance. Consistency seems to be possible only if the primordial ⁴He is between 23 and 25% by mass if there are three or four species of neutrinos.

The cosmic abundance of ⁴He provides a fundamental test of the standard hot big bang model. In this Comment we discuss the importance of proper corrections for contaminating helium produced by stars when inferring the primordial ⁴He abundance from observations of gas in galaxies. These corrections have traditionally relied on oxygen as a tracer of the degree of stellar processing, which ignores the possibility that excess helium is produced by intermediate mass stars that do not make oxygen. A modest extension of the simple galactic chemical evolution model is proposed in which carbon or nitrogen abundances are combined with oxygen abundances to give a more robust measure of stellar helium contamination levels in metal-poor galaxies.

The observed abundance of ³He can be used, in conjunction with big bang nucleosynthesis, to set a lower bound to the density of nucleons in the universe. Critical to this approach is an estimate of the destruction of ³He in stars. Detailed stellar evolution calculations which explicitly examine ³He destruction are presented. The effect of stellar mass, composition, and mass-loss rate on ³He destruction is studied. Limits on ³He destruction appropriate for big bang nucleosynthesis and for understanding present interstellar ³He abundances are discussed.

The parameter space for baryon inhomogeneous big bang models is explored with the goal of determining the minimum helium abundance obtainable in such models while still satisfying the other light-element constraints. We find that the constraint of (D + ³He)/H < 10⁻⁴ restricts the primordial helium mass fraction from baryon-inhomogeneous big bang models to be ≥ 0.231 even for a scenario which optimizes the effects of the inhomogeneities and destroys the excess lithium production. Thus, this modification to the standard big bang as well as the standard homogeneous big bang model itself would be falsifiable by observation if the primordial ⁴He abundance were observed to be less than 0.231. Furthermore, a present upper limit to the observed helium mass fraction of y^obs_p ≤ 0.24 implies that the maximum baryon-to-photon ratio allowable in the inhomogeneous models corresponds to η ≤ 2.3 × 10⁻⁹ (Ω_bh² ≤ 0.088) even if all conditions are optimized.

Two DECADES ago lithium was considered an obscure light element, bypassed by mainline nucleosynthetic processes in stars, and thought to be synthesized with the other light nuclei in some mysterious proto-solar process (the ‘x’-process)¹ — now it has become one of the confirming cornerstones of the big bang itself and one of the best probes of stellar evolution…

We discuss the importance of Population II ⁶Li as a diagnostic for models of primordial nucleosynthesis, cosmic-ray nucleosyntheses in the early Galaxy, and the structure and evolution of metal-poor solar-type stars. The observation of ⁶Li in the subdwarf HD 84937 is shown to be consistent with the existing Population II LiBeB data within the context of a simple three-component model: (1) standard big bang nucleosynthesis, (2) Population II cosmic-ray nucleosynthesis, (3) standard (nonrotating) stellar LiBeB depletion. If this interpretation is correct, we predict a potentially detectable boron abundance for this star: B/H ∼ 2 × 10⁻¹². Subsequent Population II LiBeB observations, and in particular further observations of Population II ⁶Li, are shown to be crucial to our understanding of the primordial and early galactic creation and destruction mechanisms for light elements.

The significant dip in observed lithium abundances for Population I stars near M ∼ 1.3 M_⊙ is discussed. It is noted that this dip occurs near where the instability strip crosses the main sequence on the lower edge of the δ Scuti stars and that stellar pulsations are expected to give rise to mass loss. A total mass loss of 0.05 M_⊙ over the main-sequence lifetime of these stars would be sufficient to explain the observations of lithium depletion. The absence of a dip in the Pleiades and of significant depletion of beryllium in the Hyades places tight constraints on the rate of mass loss. These constraints make unlikely the high main-sequence mass-loss rates which would significantly affect globular cluster ages.

A current problem in stellar evolution is to understand the lithium depletion in low-mass main-sequence stars. Standard stellar models do not produce temperatures in the outer convective zone high enough to allow lithium burning to occur. Convective overshoot could extend the mixing region deep enough to allow lithium burning. However, the strong temperature dependence of the relevant reaction rates initially seems to imply either total destruction or no destruction. Nevertheless, observations of main-sequence stars indicate a smooth variation of lithium abundance with stellar mass between 0.8 and 1.1 M_⊙ as well as a dependence on stellar age. A gradual dependence of the degree of convective overshoot with stellar mass can be obtained if one uses a dynamical approach for describing the convective process. Thus both the mass and age dependence of lithium depletion might be understood. Specific examples of the Hyades and Pleiades clusters as well as the Sun are discussed.

It is shown that the recent observation of a subplateau lithium abundance for a high surface temperature (T ∼ 6300 K) Population II star relative to the Population II lithium plateau can be explained by mainsequence mass loss. This explanation is identical to our previously proposed explanation for the Population I lithium dip and predicts a similar dip for Population II. It is assumed that the main-sequence mass loss in both cases is associated with the instability strip intersecting the main sequence. This mass-loss process can also decrease globular cluster ages by ∼1 Gyr.

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 ⁴He. It is shown that observational limits to the primordial abundance of ⁴He lead to the constraint that the total number of types of heavy lepton must be less than or equal to 5.

The standard Big Bang nucleosynthesis arguments are reviewed. The primordial He abundance is inferred from He-C and He-N and He-O correlations. The strengthened Li constraint as well as ²D plus ³He are used to limit the baryon density. The allowed number of neutrino families, N_v is delineated using the new value of τ_n = 890 ± 4 s (τ_½ = 10.3 min) reported at this meeting. The formal statistical result is N_v = 2.6 ± 0.3 (lσ), providing a reasonable fit (1.3σ) to three families but making a fourth light (m_v ≲ 10 MeV) neutrino family exceedingly unlikely (≳ 4.7σ) (barring significant systematic errors either in D + ³He, and Li and/or ⁴He and/or τ_n). It is also shown that uncertainties induced by postulating a first-order quark-hadron phase transition do not seriously affect the conclusions.

The relationship between big bang nucleosynthesis constraints on the number of types of neutrinos and the width of the Z⁰ is discussed. The standard model with m_v < m_z has a well known relationship between the width of the Z⁰. Гz, and the number of neutrinos, N_v. However, for a heavy neutrino, with 1 MeV ≲ m_v < mz/2 there will be a contribution to the width of the Z⁰, which will not be counted by primordial nucleosynthesis. On the other hand, right-handed neutrinos or light supersymmetric “inos” may not couple directly to the Z⁰ and thus not contribute to Гz but could make a contribution to N_v. No model is cosmologically acceptable with more than 4 effective neutrinos (or 1 more than v_e, v_μ and v_T). If ГZ ≳3.4 GeV then other new massive particles must exist.

Over the past decade two subfields of science, cosmology and elementary-particle physics, have become married in a symbiotic relationship that has produced a number of exciting offspring. These offspring are beginning to yield insights on the creation of spacetime and matter at epochs as early as l0⁻⁴³ to 10⁻³⁵ second after the birth of the universe in the primordial explosion known as the big bang. Important clues to the nature of the big bang itself may even come from a theory currently under development, known as the ultimate theory of everything (T.O.E.). A T.O.E. would describe all the interactions among the fundamental particles in a single bold stroke…

Previous work has used the primordial abundance of ⁴He to infer limits on the number of neutrinos with full-strength neutral-current weak interactions. By accounting for the quark-gluon constituents of hadrons, we extend the analysis to earlier times and higher temperatures and densities and, therefore, to considerably weaker interactions. The maximum number of new, superweakly interacting, light (≲ MeV) particles is between ∼1 and ∼20.

It is shown that, since the bulk of the approximately 10⁵³ ergs of neutron star binding energy emerges from a supernova as neutrinos the presence of a radiative decay channel for any type of neutrino with mass less than about 10 MeV can be severely constrained. Lifetimes longer than a thousand seconds but less than the age of the universe are eliminated by the observed X- and γ-ray background limits, as discussed previously by Cowsik [1]. Lifetimes between about 10⁻³s and 10³ s are restricted by a combination of X- and γ-ray background limits, and by the total energetics of supernova events themselves, since such decays would occur within the presupernova star. These limits would also apply to axions.

We recently determined a lower bound on the axion decay constant (upper bound on the axion mass) by demanding that the axion emission rate in SN1987A be small enough to be consistent with the observed neutrino emission. We estimate here the magnitude of corrections to our previous results due to high density effects, ρ exchange and the restoration of chiral symmetry. Assuming a non-degenerate core, we now find that for naive quark model couplings f_a≳0.2× 10¹² GeV (m_a≲ 3.6 × 10⁻⁴ eV) which is a factor of 4 different from our previous limits assuming a degenerate core. Limits using couplings derived from EMC measurements are a factor of 2–4 weaker. We show that there is no window in f_a for non-freely-streaming axions. These limits still imply that the remaining window for axions is the one which is the most interesting for cosmology.

The following sections are included:

• Cosmological mass and decay limits

• Unstable v's

• Limits to the number of families

• Supernova 1987A and neutrino counting

• Other constraints from SN 1987A

• Magnetic moment

• Neutrino charge

• Neutrino mass

• Neutrino mixing

• Secret interactions

• Radiative decays

• References

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/c². 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 arguments favoring non-baryonic dark matter are summarized. In particular, if the cosmological density parameter Ω ≥ 0.15, the universe must be dominated by non-baryonic matter. General cosmological constraints, independent of detailed galaxy formation scenarios, are presented on the masses of stable neutrinos and other “inos.” where ino represents any candidate particle for the dark matter in the universe: (i) The requirement that the total mass density not exceed Ω ≤ 4 restricts neutrinos to two mass ranges, ∑m_v ≤ 400 eV and m_v ≥ 1 GeV. (ii) From age of the universe arguments, tighter constraints on the lower of the two mass ranges become ∑m_v ≤ 25 eV (∑m_ino ≤ 400 eV) or ∑m_v ≤ 100 eV (∑m_ino ≤ 2 keV) depending on age technique used. An actual determination of a neutrino mass puts an upper limit on the age of the universe, and in a neutrino-dominated universe Ω = 1 is only possible for ∑m_v ≥ 25 eV. (iii) From phase-space density arguments, a necessary (but not sufficient) condition for the clustering of neutrinos on large scales is that m_v ≥ 3 eV. (iv) For the formation of large-scale structure, the maximum neutrino Jeans mass should not exceed supercluster scales and therefore m_v ≥ 10 eV. Three neutrinos of equal mass can be excluded if one uses globular cluster determinations of the age in (ii) above, (v) If the formation of large-scale structure requires damping of small scales and hence a minimum value of the maximum neutrino Jeans mass. m_ino≤200 eV for dominant particles. In a universe with Ω = 1 and photon temperature T_yo = 2.7 K, this also leads to the constraint that decoupling temperature T_D of the dominant ino is T_D ≤ 100 MeV. (vi) Big-bang nucleosynthesis restricts the number of neutrino species to at most four, probably only three.

These arguments are then synthesized to show that all of the independent constraints can only be simultaneously met in a “best-fit” model with 10 eV≤ m_v≤25 eV for the most massive neutrino eigenstate. Independently of galaxy formation arguments and with only the extremely conservative age limit, it can still be said that 3 eV ≤ m_v ≤ 100 eV. Note also that if constraint (v) is valid then the dominant ino acts in every way like a massive neutrino and thus if (vi) also holds, it probably is a massive neutrino. Differences in adiabatic and isothermal fluctuation models are discussed; in particular the GUTs preferred adiabatic mode is only consistent with limits on 3 K anisotropies if non-baryonic matter dominates. Problems on small-scales with galaxy correlation studies and equilibrium time scales in a neutrino-dominated universe with adiabatic fluctuations are discussed. For Ω - 0.2-0.6 cold matter such as axions or GeV mass inos could be the dominant matter, but in an Ω = 1 universe these as well as keV mass “inos” are not optimal for the dominant matter and the best fit is a neutrino of mass m_v = 25 eV.

The familiar nucleosynthesis constraint on the number of neutrino species, N_V ≤ 3.4, applies to massless neutrino species. An MeV-mass neutrino can have even greater impact, and we show that primordial nucleosynthesis excludes a τ-neutrino mass from 0.3 to 25 MeV (Dirac) and 0.5 to 25 MeV (Majorana) provided that its lifetime τ_v≳ 1 sec, and from 0.3 to 30 MeV (Dirac) and 0.5 to 32 MeV (Majorana) for τ_v≳ 10³ sec. A modest improvement in the laboratory mass limit — from 35 to 25 MeV — would imply that the τ-neutrino mass must be less than 0.5 MeV (provided τ_v≳ 1 sec).

The spontaneous breaking of a global symmetry leads to the existence of Nambu-Goldstone bosons, which through their coupling to electrons and/or photons, can transport energy from the cores of stars and affect significantly the course of stellar evolution. We find by following in detail the evolution of stars that if the couplings to electrons and/or photons is too strong, helium never ignites—in contradiction with the observational evidence. Our limits restrict the axion mass to less than 0.01 eV, the familon breaking scale to < 7 × 10⁹ GeV, and the triplet Majoron vacuum expectation value to < 9 keV.

THERE are several clues to the history of the early Universe. The 3-K microwave background indicates that the Universe was highly isotropic when it was about a half million years old, and the abundance of ⁴He (also D, ³He and ⁷Li) indicates that nucleosynthesis was taking place when the Universe was about three minutes old. These two facts are strong evidence that the Universe began from a hot big bang¹. In addition, Hawking, Ellis and Penrose have proved (within general relativity) that the existence of the 3-K radiation implies the Universe must have been singular in its past^2,3. The existence and clustering of galaxies indicate there were some deviations from homogeneity in the early Universe. Perhaps the most curious fact about the Universe is that it is composed almost entirely of matter (the Universe contains negligible amounts of antimatter) and that the number of baryons per photon (∼baryon/entropy ratio) is between 10⁻¹⁰ and 10⁻⁸ (ref. 4). If baryon number is absolutely conserved, then it follows that the baryon number of the Universe must be viewed merely as an initial condition. Recent ideas in particle physics when applied to the early Universe may explain how an initially baryon-symmetrical (zero net baryon number) Universe could have evolved into one with net baryon number. Here, we will discuss these ideas and other interesting astrophysical implications…

The following sections are included:

• Recent developments

• Constraints from nucleosynthesis

• Other astrophysical constraints

• The origin of baryons

• References

We review the cosmology/particle physics interface. Focusing on two of its most active areas, inflation and dark matter.

It is shown that the amazing coincidence of recombination and radiation to matter dominance in the early universe may be due to an underlying connection between leptonic and hadronic mass scales.

A long-standing problem in cosmology has been to understand the origins of density inhomogeneities in the Universe. Calculations involving inhomogeneities have for the most part required the assumption of some initial spectrum of perturbations, which is inserted ‘by hand’. We present here a mechanism for the spontaneous creation of density perturbations during the transition in the early Universe from unconfined quark matter to hadronic matter. The subsequent evolution of these perturbations remains a problem for further study.

A review is made of the relation of the quark-hadron and chiral symmetry transitions and the big bang universe. Possible signatures at de-confinement are mentioned and related to cosmology. It is noted that to produce surviving perturbations at this epoch requires black hole production which in turn would require a first-order phase transition. The effect of a quark matter phase in neutron stars is also discussed and related to monopole flux limits through its role in cooling.

The possible implications of the quark-hadron transition for cosmology are explored. Possible surviving signatures are discussed. In particular, the possibility of generating a dark matter candidate such as strange nuggets or planetary mass black holes is noted. Much discussion is devoted to the possible role of the transition for cosmological nucleosynthesis. It is emphasized that even an optimized first order phase transition will not significantly alter the nucleosynthesis constraints on the cosmological baryon density nor on neutrino counting. It is also noted that, contrary to some initial hopes, Be and B observations in old stars are not a signature of a cosmologically significant quark-hadron transition. The spallagenic origin of Be and B in these old stars has now been firmly spported by a recent ⁶Li observation. Furthermore, is is shown that first order quark-hadron transitions are unable to produce measurable amounts of Be and B when they are constrained to fit the other observed light element abundances.

An examination and a brief review are made of the effects of quark-hadron transition induced fluctuations on big bang nucleosynthesis. It is shown that cosmologically critical densities in baryons are difficult to reconcile with observation, but the traditional baryon density constraints from homogenous calculations might be loosened by as much as 50%, to 0.3 of critical density, and the limit on the number of neutrino flavors remains about N_V≲ 4. To achieve baryon densities ≳0.3 of critical density would require initial density contrasts R ≫ 103, whereas the simplest models for the transition seem to restrict R to ≲, 10₂.

We investigate the possibility that inhomogeneous nucleosynthesis may eventually be used to explain the abundances of ⁶Li, ⁹Be, and B in Population II stars. The present work differs from previous studies in that we have used a more extensive reaction network. It is demonstrated that in the simplest scenario the abundances of the light elements with A ≤ 7 constrain the separation of inhomogeneities to sufficiently small scales that the model is indistinguishable from homogeneous nucleosynthesis and that the abundances of ⁶Li, ⁹Be, and B are then below observations by several orders of magnitude. This conclusion does not depend on the ⁷Li constraint. We also examine alternative scenarios which involve a post-nucleosynthesis reprocessing of the light elements to reproduce the observed abundances of Li and B, while allowing for a somewhat higher baryon density (still well below the cosmological critical density). Future B/H measurements may be able to exclude even this exotic scenario and further restrict primordial nucleosynthesis to approach the homogeneous model conclusions.

This review focuses on the impact of several new observations on cosmology. In particular, it is shown that the basic picture of big bang nucleosynthesis continues to strengthen and that there are important clues to the dark matter and large-scale structure problems. It is shown that the interface of cosmology with nuclear and particle physics is one of the most active areas of physics.

Some relatively model-independent results for structure formation via late time phase transitions (LTPT) are discussed. In particular, generic LTPT power spectra are presented. The implication of the recent COBE detection of the cosmic background radiation (CBR) anisotropy at large angular scales (≳7°) and the tight upper limits from small angular scales (˜1°) to LTPT models are discussed. Special attention is focused on the observational constraints and possible non-Gaussian signatures of CBR temperature anisotropies from LTPT and other non-Gaussian models. It is shown that while LTPT have been seriously constrained by the recent data, viable models do remain which provide more power on the 100–200 Mpc scales than do more traditional primordial Gaussian density fluctuation models. Tests for such models are presented, including possible anisotropies on angular scales less than 8'.

Observations of galaxy-galaxy and cluster-cluster correlations as well as other large-scale structure can be fit with a “limited” fractal with dimension D ≈ 1.2. This is not a “pure” fractal out to the horizon: the distribution shifts from power law to random behavior at some large scale. If the observed patterns and structures are formed through an aggregation growth process, the fractal dimension D can serve as an interesting constraint on the properties of the stochastic motion responsible for limiting the fractal structure. In particular, it is found that the observed fractal should have grown from two-dimensional sheetlike objects such as pancakes, domain walls, or string wakes. This result is generic and does not depend on the details of the growth process.

One of the most powerful tools used in attempts to understand the structure of the Universe is the correlation function ξ(r), the excess probability over random that there are two objects separated by a distance r. In particular, distributions of galaxies and of clusters of galaxies have been investigated using this parameter. Here we show that if the amplitudes of the cluster-cluster correlation function is made dimensionless, systematic changes with cluster richness vanish, implying scale Invariance in the clustering process. The dimensionless galaxy-galaxy correlation seems stronger, implying gravitational enhancement on smaller scales.

Some models have extremely low-mass pseudo-Goldstone bosons that can lead to vacuum phase transitions at late times, after the decoupling of the microwave background. This can generate structure formation at redshifts z ≳ 10 on mass scales as large as M ˜ 10_1MM Such low energy transitions can lead to large but phenomenologically acceptable density inhomogeneities in “soft topological defects” (e.g., domain walls) with minimal variations in the microwave anisotropy, as small as δT/T ≲ 10−6. This mechanism is independent of the existence of hot, cold, or baryonic dark matter. It is a novel alternative to both cosmic string and inflationary quantum fluctuations as the origin of structure in the Universe.

One of the crucial aspects of density perturbations that are produced by the standard inflation scenario is that they are Gaussian where seeds produced by topological defects tend to be non-Gaussian. The three-point correlation function of the temperature anisotropy of the cosmic microwave background radiation (CBR) provides a sensitive test of this aspect of the primordial density field. In this paper, this function is calculated in the general context of various allowed non-Gaussian models. It is shown that the Cosmic Background Explorer and the forthcoming South Pole and balloon CBR anisotropy data may be able to provide a crucial test of the Gaussian nature of the perturbations.

THE idea that superheavy ‘strings’, formed at a phase transition in the very early Universe^1–4, provide an explanation of the origin of galaxies, has been the subject of much discussion (in part in these columns: see Nature 310, 365; 1984). We should like to take the discussion further as it is now becoming apparent that strings can make galaxies in ways quite different from other methods and thus may help solve or remove the dark-matter problems…

Particles which decouple before quark confinement in a hot big bang will have a number density significantly below that of relic neutrinos. Fermions whose mass exceeds ˜500 eV may cluster in dwarf galaxies without violating phase space constraints; however, the free-streaming mass scale for such particles is much larger than a dwarf galaxy mass. Results from a numerical simulation show that this free streaming does not mean such particles cannot provide the missing mass in dwarf galaxies, so long as such dwarf galaxy halos constitute a small fraction of the dark matter in the universe. It is shown that the results of the simulation are not affected by two-body scattering.

Galaxy and structure formation in a neutrino-dominated universe with cosmic strings is investigated. Strings survive neutrino free streaming to seed galaxies and clusters. The effective maximum Jeans mass is about 1.5x 10¹⁴h₅₀⁻⁴M_⊙, lower than in the adiabatic scenario. Hence cluster formation is only marginally different from that in the cold-dark-matter and strings model, but galaxy masses are lower. The mass spectrum of galaxies is flatter than with cold dark matter, and the density profile about an individual loop is less steep, in better agreement with observations.

Relic neutrinos will be abundant today (n_v ≈ n_Y) and could, if they have a small mass (m_v ≳ 1.4 eV), dominate the universal mass density. Ordinary matter (nucleons) appears to be incapable of accounting for the dynamically inferred mass on scales of clusters of galaxies; recent indications suggest that this problem may persist down to the scale of binary galaxies and small groups of galaxies. The difficulty is that, were the mass on these scales in nucleons, too much helium and too little deuterium would have been produced during primordial nucleosynthesis. Light neutrinos with m_v ≲ 4 eV will remain unclustered but could supply a nonnegligible contribution to the total mass density if m_v ≳ 1 eV. Heavy neutrinos with m_v ≳ 20 eV could have collapsed along with galaxies and, unless there were a subsequent segregation of nucleonic matter from neutrinos, would contribute too much invisible mass on such scales. Relic neutrinos with 4 ≲ m_v ≲ 20 eV could supply most of the unseen mass on scales ranging from binaries through small groups to large clusters. Such neutrinos will dominate the mass of the universe and, along with ordinary nucleons, could come close to closing the universe without violating the nucleosynthesis constraints.

The following sections are included:

• TRIUMPHS OF THE BIG BANG

• THE MISSING MASS

• SEEDS OF STRUCTURE

• TYPES OF SEEDS

• BEYOND COBE

• YESTERDAY AND TODAY

Arguments on the Age of the Universe, t_u, are reviewed. The four independent age determination techniques are:

(1) Dynamics (Hubble Age and deceleration);

(2) Oldest stars (globular clusters);

(3) Radioactive dating (nucleocosmochronology);

(4) White dwarf cooling (age of the disk).

While discussing all four, this review will concentrate more on nucleocosmochronology due in part to recent possible controversies there. It is shown that all four techniques are in general agreement, which is an independent argument in support of a catastrophic creation event such as the Big Bang. It is shown that the most consistent range of cosmological ages is for 12 ≲ t_u ≲ 17Gyr. It is argued that the upper bound from white dwarf cooling is only ˜ 10Gyr due to the disk of the Gaxaxy probably forming several Gyr after the Big Bang itself. Only values of the Hubble constant, H_o ≲ 60km/sec/Mpc, are consistent with the other age arguments if the universe is at its critical density. An interesting exception to this limit is noted for the case of a domain wall dominated universe where ages as large as 2/H_o are possible.

The equations for a nucleosynthetic chronology are shown to be separable with the equations for extremely long-lived and stable nuclei yielding the mean age of the elements. This result is independent of the time-dependent production model used. This mean age is a lower bound on the age of the elements. The age of the elements is critically model-dependent. The short-lived isotopes are shown to yield the formation interval for the solar system which also is essentially model-independent. The short-lived and intermediate-lived isotopes taken relative to stable isotopes are shown to yield information on the rate of f-process nucleosynthesis with time and thus may provide the distribution of supernovae in time within the Galaxy.

We have found values of the r-process production ratios of galactic chronologic importance, taking into account the uncertainties in the mass law as well as other calculation parameters. The values prior to any reactions during freezing out and ejection are: ²⁸²Th/²³⁸U = 1.96 ± 0.25, ²⁴⁴Pu/²³⁸U = 0.96 ±0.21, ²³⁷Np/²³⁸U = 1.89 + 0.26, and ²³⁵U/²³⁸U = 1.89 + 0.36.

Neutrinos, produced by the collisions of ultra high energy cosmic rays and the 3 K background radiation, require careful treatment of the evolving cosmic ray spectrum. The resulting neutrino differential energy spectrum is flat up to energies of order 10¹⁹eV, thus most events are expected at this energy. The total V_e-flux should be significantly larger than the proton flux at }5 × l0¹⁹eV when the recoil proton is correctly treated in photomeson production and when the Bethe–Heitler process (e⁺e⁻ pair production) is incorporated. Based upon the observed primary proton spectrum we obtain a lower limit on the neutrino flux of }5.2/km²yr sr implying roughly 0.4 detected upward moving events per year in the present Fly's Eye range of sensitivity.

The formation of the solar system inside an OB association is examined with particular attention to the elemental abundances which would have been ejected by the association's first few supernovae. It is found that the solar system material may have been significantly contaminated by these supernovae and thus the average interstellar composition may differ from the solar system composition. In particular, we find that many of the so-called isotopic and elemental abundance anomalies (e.g., Ne, C, O. s-process/r-process, etc.) found in meteoritic inclusions and in cosmic rays may be more representative of the average interstellar abundance. In other words, it may be that the average solar system abundances are what is “anomalous.”

We discuss the potential implications of the recent results of the Fly's Eye detector on the ultrahigh-energy cosmic-ray spectrum. The data suggest that the observed slope flattening immediately prior to the cutoff appears to be a recoil-proton pileup associated with an E^−2.5±0.3 injection spectrum. We discuss the induced neutrino spectrum and its detectability in Fly's Eye and DUMAND detectors.

The recent observations of Be and B in metal-deficient halo dwarfs are used to constrain a “bright phase” of enhanced cosmic-ray flux in the early Galaxy. Assuming that this Be and B arises from cosmic-ray spallation in the early Galaxy, limits are placed on the intensity of the early (Population II) cosmic-ray flux relative to the present (Population I) flux. A simple estimate of bounds on the flux ratio is 1 ≲ Φ_{pop II}/Φ_{pop I} ≲ 40. This upper bound would restrict galaxies like our own from producing neutrino fluxes that would be detectable in any currently proposed detectors. It is found that the relative enhancement of the early flux varies inversely with the relative time of enhancement. It is noted that associated gamma-ray production via pp → π^°pp may be a significant contribution to the gamma-ray background above 100 MeV.

The ultrahigh-energy (UHE) proton and neutrino spectra resulting from collapse or annihilation of topological defects surviving from the grand unification (GUT) era are calculated. Irrespective of the specific process under consideration, the UHE proton spectrum “recovers” at ~1.8 × 10¹¹ GeV after a partial Greisen-Zatsepin-Kuz'min “cutoff” at ~ 5 × 10¹⁰ GeV and continues to a GUT-scale energy with a universal shape determined by the physics of hadronic jet fragmentation. The shape of the UHE neutrino spectrum is, however, sensitive also to the cosmological evolution of the defects involved.

The hypothesis of topological defects (from grand unified and/or Planck scales) as the sources of extremely high-energy (> 10¹⁸ eV) cosmic rays predicts an unusually high content of γ rays at energies E ≳ 10²⁰ eV (γ/p ≦ 1) and E ≲ 10¹⁴ eV (γ/p ≳ 10⁻³). This can be used as a signature for testing the hypothesis in forthcoming experiments.

In this paper we show that the conventional diffusive shock acceleration mechanism for cosmic rays associated with relativistic astrophysical shocks in active galactic nuclei (AGNs) has severe difficulties to explain the highest energy cosmic ray events. We show that protons above around 2 × 10²⁰ eV could have marginally been produced by this mechanism in an AGN or a rich galaxy cluster not further away than around 100 Mpc. However, for the highest energy Fly's Eye and Yakutsk events this is inconsistent with the observed arrival directions. Galactic and intergalactic magnetic fields appear unable to alter the direction of such energetic particles by more than a few degrees. We also discuss some other options for these events associated with relativistic particles including pulsar acceleration of high Z nuclei. At the present stage of knowledge the concept of topological defects left over from the early universe as the source for such events appears to be a promising option. Such sources are discussed and possible tests of this hypothesis are proposed.

The possibility of doing point source neutrino astronomy is discussed. Probable sources include galactic nuclei, Seyferts, quasars, radio galaxies, pulsars and supernovae.

Explosions of massive stars (8 ≲ M/M_ȯ ≲ 70) are examined as the source of Galactic cosmic rays. Detailed nucleosynthetic and evolutionary calculations suggest that these massive stars produce the heavy elements (Z ≥ 6) in their proper relative abundances. This is particularly significant because lower-mass stars (in particular the 4–8 M_ȯ range) are unable to produce the observed abundances of C and O relative to the iron peak. A small (~ 1.4 M_ȯ) dense remnant star left after the explosion may provide a location for an electromagnetic acceleration mechanism. Those abundance ratios which can now be predicted (He, C, O, Ne, Mg) for the material to be accelerated by the pulsar give a reasonable match to the observed cosmic-ray data. The conditions at the outer edge of the remnant and the inner edge of the ejected material may be appropriate for an r-process to occur; the high-Z cosmic rays seem to show an enrichment of r-process material. It appears that these stars may be the astrophysical source for the Galactic cosmic rays.

It is shown that if more accurate neutrino opacities (including effects of electron degeneracy) are used in a gravitational collapse calculation, then the effects of neutral currents and coherent scattering may be considerably greater than was previously thought. It is also shown that a careful inclusion of the electron-capture neutrinos should increase the importance of the region near densities of ~ 2 × 10¹¹ g/cm³.

The astrophysical site for the production of the r-process nuclei is not presently known. Part of the reason for this lack of knowledge is that the range of conditions which enable the production of these nuclei has not been accurately determined. Dynamic r-process nucleosynthesis calculations provide a means by which limits can be placed on the range of initial conditions that can produce r-process nuclei. Such calculations have several important time scales inherent to them. By requiring the time scales for neutron captures, β-decays, hydrodynamic expansion, and seed nucleus production to be mutually compatible, it has been found that constraints can be placed on the initial temperatures, densities, neutron/proton ratios, and chemical compositions that can produce the r-process nuclei. The allowed ranges of these parameters are presented along with the results of r-process calculations performed for several different sets of initial conditions.

Using simple classical neutron gas models, we show that neutrino radiation may in some cases be more efficient than gravitational radiation for damping out nonradial pulsations during gravitational collapse. This implies that previous estimates of gravitational radiation from neutron-star-black-hole formation following supernovae may have been overly optimistic. Final conclusions for any particular model await detailed hydrodynamic calculations. However, this paper shows that neutrino damping is potentially important in such calculations and must be taken into account.

The observed ²⁶Mg and ¹⁶O anomalies in meteorites can be consistently understood if a supernova occurred within a few million years of the condensation of the Solar System. Grains condensing in the ejecta from this supernova may be an integral aspect of this process.

Detailed calculations are made of the neutrino spectra emitted during gravitational collapse events (Type II supernova?). Those aspects of the neutrino signal which are relatively independent of the collapse model and those aspects which are sensitive to model details are discussed. The easier to detect high-energy tail of the emitted neutrinos has been calculated using the Boltzmann equation. This is compared with the result of the traditional multigroup flux-limited diffusion calculations. The harder to detect electron antineutrino background from historical supernova might be enhanced by matter oscillation of higher energy mu and tau neutrinos to electron antineutrinos.

An overview of the significance for physics of the closest visual supernova in almost 400 years is presented. The supernova occurred in the Large Magellanic Cloud (LMC),˜ 50 kpc away. The supernova star was a massive star of ˜ 15 – 20M_ȯ. Observations now show that it was once a redgiant but lost its outer envelope. The lower than standard luminosity and higher observed velocities are a natural consequence of the pre-supernova star being a blue rather than a red [supergiant]. Of particular importance to physicsts is the detection of neutrinos from the event by detectors in the United States and Japan. Not only did this establish extra-solar system neutrino astronomy, but it also constrained the properties of neutrino. It is shown that the well established Kamioka-IMB neutrino burst experimentally implies an event with about 2 to 4 × 10⁵³ergs emitted in neutrinos and a temperature, T_ve, of between 4 and 4.5 MeV. This event is in excellent agreement with what one would expect from the gravitational core collapse of a massive star. A neutrino detection, such as that reported earlier in Mt. Blanc, would require more than the rest mass energy of a neutron star to be converted to neutrinos, if it were to have its origin in the LMC. Thus it is probably unrelated to the supernova. The anticipated frequency of collapse events in our Galaxy, will also be discussed, with a rate as high as 1/10 year shown to be not unreasonable.

Despite their apparently very different objectives, astrophysics — the study of the largest structures in the Universe — and particle physics — the study of the smallest — have always had common ground. On 23 February 1987 a supernova explosion provided additional impetus to reinforce these links. In this article, David Schramm of the University of Chicago and the NASA/Fermilab Astrophysics Center, explains why.

The stellar explosions called supernovas are rare events: among the 100 billion stars of our galaxy there are probably about three supernovas a century. In spite of their rarity, however, supernovas had a seminal role in the origin of the solar system. Much of the material that came together to form the sun and the planets was dust and gas that had been ejected by supernovas over a period of several billion years. Now there is evidence for a more intimate connection with a particular supernova. It appears that a massive star exploded in the vicinity of the developing solar system at about the time the system condensed…

We construct schematic models for chemical evolution and cosmochronology within the expanding and collapsing protogalactic halo followed by formation of the local disk. Star formation is associated with both the rate of protogalactic mergers and the intrinsic gas density of protogalactic clouds and the disk. This leads naturally to a scenario in which star formation in the disk can be delayed by several billion years relative to the formation of the oldest globular clusters. We analyze various cosmochronometers in the context of this model and show that the apparent differences between the maximum globular-cluster ages, the white-dwarf cooling age, and nuclear chronometric ages can be understood. The merger models which satisfy the age constraints imply a relatively late forming peak in luminosity and therefore may be consistent with the observed peak in galaxy number counts at intermediate redshifts. Versions of the model with and without nonbaryonic dark matters can yield significant dark baryonic halos.

The evolution of s-process abundances in the solar neighborhood is studied, using alternative stellar production sites and Galactic models. Production in either low-mass or medium-mass stars, as suggested in Ulrich's alternative models for FG Sge for example, can account for the solar-system abundances. Either case is consistent with independent limits on subsequent neutron exposure of nuclei produced in explosive oxygen and silicon burning and of r-process material. The cases could be distinguished by observations of the ratios of s-process to primary metal abundances in stars of different ages. The predictions are not strongly dependent on the model used for Galactic evolution.

The tidal breakup of a neutron star near a black hole is examined. A simple model for the interaction is calculated, and the results show that the amount of neutron-star material ejected into the interstellar medium may be significant. Using reasonable stellar statistics, the estimated quantity of ejected material is found to be roughly comparable to the abundance of r-process material.

Magneto-hydrodynamic effects may play a crucial role in supernova dynamics. Here we suggest that such effects could result in a novel type of jet structure associated with pulsar formation. We point out that speckle interferometry data from SN1987a is consistent with this hypothesis as well as some spectroscopic and polarimetry data.

While the dynamics governing supernova explosions are still not completely understood, the exciting observation of a supernova in the LMC has provided us with unprecedented information with which to test various models. The observed neutrino emission suggests that a neutron star was formed in the initial collapse and that a shock wave traversed the remaining stellar matter establishing an ejection trajectory. Because of possible high magnetic fields at the neutron star surface, trapped magnetic flux might also play an important role in supernova dynamics. Here we explore roles that magneto-hydrodynamic effects might play in the dynamics of pulsar formation. Assuming some dipole flux is trapped in a rotating neutron star and in the surrounding expanding envelope, we suggest this might result in one, or perhaps two, oppositely directed magnetic jets penetrating the expanding envelope. We note that several observations of anisotropy associated with SN1987a–the possible observation of a transient secondary light image using speckle interferometry, and the observations of asymmetry of the envelope–are consistent with the model we describe here. Other indications of anisotropy are the optical polarization and some spectroscopic data.

Radioactive decays inside the Earth produce antineutrinos that may be detectable at the surface. Their flux and spectrum contain important geophysical information. New detectors need to be developed, discriminating between sources of antineutrinos, including the cosmic-background. The latter can be related to the frequency of supernovas.

NEUTRON-STAR collisions occur inevitably when binary neutron stars spiral into each other as a result of damping of gravitational radiation. Such collisions will produce a characteristic burst of gravitational radiation, which may be the most promising source of a detectable signal for proposed gravity-wave detectors¹. Such signals are sufficiently unique and robust for them to have been proposed as a means of determining the Hubble constant². However, the rate of these neutron-star collisions is highly uncertain³. Here we note that such events should also synthesize neutron rich heavy elements, thought to be formed by rapid neutron capture (the r-process)⁴. Furthermore, these collisions should produce neutrino bursts⁵ and resultant bursts of γ-rays; the latter should comprise a subclass of observable γ-ray bursts. We argue that observed r-process abundances and γ-ray-burst rates predict rates for these collisions that are both significant and consistent with other estimates.

It is shown that the current solar neutrino situation, now that we have the SAGE and GALLEX result along with the results from the Kamiokande and the Homestake experiments, is unfortunately still quite ambiguous. The differences between observations and the standard solar theory may still be due to either astrophysical inputs or new neutrino physics. In particular, the astrophysical solution, which requires a cooler Sun than the standard solar model of Bahcall et al., may still be viable. The need for new neutrino physics, MSW or vacuum neutrino mixing, is sensitive to the results of the Homestake experiment and SAGE. The use of future experiments, SNO, Borexino, the Super Kamiokande, and the Iodine experiment to resolve this ambiguity are explicitly discussed.

We explore the impact of astrophysical uncertainties on the Mikheyev-Smirnov-Wolfenstein (MSW) solution by calculating the allowed MSW solutions for 1000 different solar models with a Monte Carlo selection of solar model input parameters, assuming a full three-family MSW mixing. Applications are made to the chlorine, gallium, Kamiokande, and Borexino experiments. The initial GALLEX result limits the mixing parameters to the upper diagonal and the vertical regions of the MSW triangle. We also calculate the expected event rates in the Borexino experiment assuming the MSW solutions implied by GALLEX.

A detailed estimate is presented of the possible uncertainty range for the neutrino flux from a standard solar model. Using present estimated errors in the key input parameters, detailed solar models are calculated to give an uncertainty in the theoretical v_e capture rate in both the ongoing ³⁷Cl experiment and the proposed experiment using ⁷¹Ga. The uncertainty in capture rate is investigated by considering individual parameter variations about a mean model, by simultaneously varying several key parameters to yield upper and lower limits, and by a Monte Carlo method. It is found that with the most recent experimental value of Davis—2.2 ± 0.4 SNU—and the best-estimate mean model of the present work—7.0 ± 3.0 SNU—the theoretical to experimental ratio is 3.2 ± 1.5, or 1.5 standard deviations from agreement. The prediction for ⁷¹Ga is 111 ± 13 SNU. While the mean value for the ³⁷Cl capture rate is in good agreement with the recent calculation of Bahcall et al., the estimated uncertainty is larger by a factor of 2. The largest sources of uncertainty in these predictions are due to the range used in the ³He(x, γ)⁷Be cross section and to the estimate of the error range of the calculated opacities. The results are also discussed, in light of recent experimental and theoretical work on neutrino oscillations, to determine if the present situation provides any evidence for such oscillations. It is found that while there is only weak evidence in the ³⁷Cl experiment, a discrepancy in the ⁷¹Ga experiment could be interpreted as strong evidence for neutrino oscillations.