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Katherine Freese

Publications and source records attributed to Katherine Freese.

At least 217 records · Page 12Linked to original sources
arXiv1999

Cosmological Challenges in Theories with Extra Dimensions and Remarks on the Horizon Problem

We consider the cosmology that results if our observable universe is a 3-brane in a higher dimensional universe. In particular, we focus on the case where our 3-brane is located at the $Z_2$ symmetry fixed plane of a $Z_2$ symmetric five-dimensional spacetime, as in the Hořava-Witten model compactified on a Calabi-Yau manifold. As our first result, we find that there can be substantial modifications to the standard Friedmann-Robertson-Walker (FRW) cosmology; as a consequence, a large class of such models is observationally inconsistent. In particular, any relationship between the Hubble constant and the energy density on our brane is possible, including (but not only) FRW. Generically, due to the existence of the bulk and the boundary conditions on the orbifold fixed plane, the relationship is not FRW, and hence cosmological constraints coming from big bang nucleosynthesis, structure formation, and the age of the universe difficult to satisfy. We do wish to point out, however, that some specific choices for the bulk stress-energy tensor components do reproduce normal FRW cosmology on our brane, and we have constructed an explicit example. As our second result, for a broad class of models, we find a somewhat surprising fact: the stabilization of the radius of the extra dimension and hence the four dimensional Planck mass requires unrealistic fine-tuning of the equation of state on our 3-brane. In the last third of the paper, we make remarks about causality and the horizon problem that apply to {\it any} theory in which the volume of the extra dimension determines the four-dimensional gravitational coupling. We point out that some of the assumptions that lead to the usual inflationary requirements are modified.

hep-ph↗
arXiv1999

CP-Violating Effects in Neutralino Scattering and Annihilation

We show that in some regions of supersymmetric parameter space, CP violating effects that mix the CP-even and CP-odd Higgs bosons can enhance the neutralino annihilation rate, and hence the indirect detection rate of neutralino dark matter, by factors of 10^6. The same CP violating effects can reduce the neutralino scattering rate off nucleons, and hence the direct detection rate of neutralino dark matter, by factors of 10^{-7}. We study the dependence of these effects on the phase of the trilinear coupling A, and find cases in the region being probed by dark matter searches which are experimentally excluded when CP is conserved but are allowed when CP is violated.

hep-ph↗
arXiv1999

Limits on Stellar Objects as the Dark Matter of Our Halo: Nonbaryonic Dark Matter Seems to be Required

The nature of the dark matter in the Halo of our Galaxy remains a mystery. Arguments are presented that the dark matter does not consist of ordinary stellar or substellar objects, i.e., the dark matter is not made of faint stars, brown dwarfs, white dwarfs, or neutron stars. In fact, faint stars and brown dwarfs constitute no more than a few percent of the mass of our Galaxy, and stellar remnants must satisfy $Ω_{WD} \leq 3 \times 10^{-3} h^{-1}$, where $h$ is the Hubble constant in units of 100 km/s Mpc^{-1}. On theoretical grounds one is then pushed to more exotic explanations. Indeed a nonbaryonic component in the Halo seems to be required.

astro-ph↗
arXiv1999

Chemical Abundance Constraints on White Dwarfs as Halo Dark Matter

We examine the chemical abundance constraints on a population of white dwarfs in the Halo of our Galaxy. We are motivated by microlensing evidence for massive compact halo objects (Machos) in the Galactic Halo, but our work constrains white dwarfs in the Halo regardless of what the Machos are. We focus on the composition of the material that would be ejected as the white dwarfs are formed; abundance patterns in the ejecta strongly constrain white dwarf production scenarios. Using both analytical and numerical chemical evolution models, we confirm that very strong constraints come from Galactic Pop II and extragalactic carbon abundances. We also point out that depending on the stellar model, significant nitrogen is produced rather than carbon. The combined constraints from C and N give $Ω_{WD} h < 2 \times 10^{-4}$ from comparison with the low C and N abundances in the Ly$α$ forest. We note, however, that these results are subject to uncertainties regarding the nucleosynthesis of low-metallicity stars. We thus investigate additional constraints from D and $^4$He, finding that these light elements can be kept within observational limits only for $Ω_{WD} \la 0.003$ and for a white dwarf progenitor initial mass function sharply peaked at low mass (2$M_\odot$). Finally, we consider a Galactic wind, which is required to remove the ejecta accompanying white dwarf production from the galaxy. We show that such a wind can be driven by Type Ia supernovae arising from the white dwarfs themselves, but these supernovae also lead to unacceptably large abundances of iron. We conclude that abundance constraints exclude white dwarfs as Machos. (abridged)

astro-ph↗
arXiv1999

Constraining the cosmic abundance of stellar remnants with multi-TeV gamma-rays

If galactic halos contain stellar remnants, the infra-red flux from the remnant progenitors would contribute to the opacity of multi-TeV gamma-rays. The multi-TeV gamma-ray horizon is established to be at a redshift z>0.034 by the observation of the blazar Mkn501 . By requiring that the optical depth due to γγ---> e+ e- be less than one for a source at z=0.034 we limit the cosmological density of stellar remnants, Omega_rm \le (2-4) x 10^-3 h_70^-1 (h_70 is the Hubble constant in units of 70 km sec-1 Mpc-1) and thus strongly constrain stellar remnants as a cosmologically significant source of dark matter.

astro-ph↗
arXiv1999

What are Machos? Limits on Stellar Objects as the Dark Matter of our Halo

The nature of the Massive Compact Halo objects seen in microlensing experiments and interpreted as dark matter in the Halo of our Galaxy remains a mystery. Arguments are presented that these events are probably not ordinary stellar or substellar objects, i.e., they are probably not faint stars, brown dwarfs, white dwarfs, or neutron stars. On theoretical grounds one is then pushed to either exotic explanations or a "no-Macho" Halo (in which the Machos reside elsewhere than in the Halo). Indeed a nonbaryonic component in the Halo seems to be required.

astro-ph↗
arXiv1998

Massive Compact Halo Objects Viewed from a Cosmological Perspective: Contribution to the Baryonic Mass Density of the Universe

[Abridged] We estimate the contribution of Massive Compact Halo Objects (Machos) and their stellar progenitors to the mass density of the Universe. If the Machos that have been detected reside in the Halo of our Galaxy, then a simple extrapolation of the Galactic population (out to 50 kpc) of Machos to cosmic scales gives a cosmic density $ρ_{Macho} = (1-5) \times 10^9 h \msun \Mpc^{-3}$, which in terms of the critical density corresponds to $Ω_{Macho}=(0.0036-0.017) h^{-1}$. Such a mass density is comparable to the baryon density implied by Big Bang Nucleosynthesis. If we take the central values of the estimates, then Machos dominate the baryonic content of the Universe today, with $Ω_{Macho}/Ω_{Baryon} \sim 0.7 h$. However, the cumulative uncertainties in the density determinations only require that $Ω_{Macho}/Ω_{Baryon} \geq 1/6 h f_{gal}$, where the fraction of galaxies that contain Machos $f_{gal} > 0.17$, and $h$ is the Hubble constant in units of 100 km s$^{-1}$ Mpc$^{-1}$. Our best estimate for $Ω_{Macho}$ is hard to reconcile with the current best estimates of the baryonic content of the intergalactic medium indicated by measurements of the Lyman-$α$ forest. We explore the addition constraints that arise if the Machos are white dwarfs as suggested by the present microlensing data. We discuss the challenges this scenario presents at both the local and cosmic scales, emphasizing in particular the constraints on the required mass budget and nucleosynthesis products (particularly carbon).

astro-ph↗
arXiv1998

A Bound on the Flux of Magnetic Monopoles from Catalysis of Nucleon Decay in White Dwarfs

Catalysis of nucleon decay in white dwarfs is used to constrain the abundance of magnetic monopoles arising from Grand Unified Theories. Recent discoveries of the dimmest white dwarf ever observed, WD 1136-286 with $L = 10^{-4.94} L_{\odot}$, place limits on the monopole flux. An abundance of monopoles greater than the new bound would heat this star to a luminosity higher than what is observed. The new bound is $(F/$cm $^{-2}$ s$^{-1}$ sr$^{-1}$) $(σ\upsilon/10^{-28} cm^2) < 1.3 \times 10^{-20} (\upsilon/10^{-3}c)^2$, where $\upsilon $ is the monopole velocity. The limit is improved by including the monopoles captured by the main-sequence progenitor of the white dwarf: $(F/$cm $^{-2}$ s$^{-1}$ sr$^{-1}$) $(σ\upsilon /10^{-28} cm^2) < 3.5(26) \times 10^{-21}$ for $10^{17}$ ($10^{16}$) GeV monopoles. We also note that the dependence on monopole mass of flux bounds due to catalysis in neutron stars with main sequence accretion has previously been calculated incorrectly (previously the bound has been stated as $F (σ\upsilon/10^{-28} cm^2) < 10^{-28} $ cm $^{-2}$ s$^{-1}$ sr$^{-1}$). We show that the correct bounds are somewhat weaker for monopole mass other than $10^{17}$ GeV.

astro-ph↗
arXiv1997

MACHOs, White Dwarfs, and the Age of the Universe

(Abridged Abstract) A favored interpretation of recent microlensing measurements towards the Large Magellanic Cloud implies that a large fraction (i.e. 10--50%) of the mass of the galactic halo is composed of white dwarfs. We compare model white dwarf luminosity functions to the data from the observational surveys in order to determine a lower bound on the age of any substantial white dwarf halo population (and hence possibly on the age of the Universe). We compare various theoretical white dwarf luminosity functions, in which we vary hese three parameters, with the abovementioned survey results. From this comparison, we conclude that if white dwarfs do indeed constitute more than 10% of the local halo mass density, then the Universe must be at least 10 Gyr old for our most extreme allowed values of the parameters. When we use cooling curves that account for chemical fractionation and more likely values of the IMF and the bolometric correction, we find tighter limits: a white dwarf MACHO fraction of 10% (30%) requires a minimum age of 14 Gyr (15.5 Gyr). Our analysis also indicates that the halo white dwarfs almost certainly have helium-dominated atmospheres.

astro-ph↗
arXiv1997

Constraints on the Intergalactic Transport of Cosmic Rays

Motivated by recent experimental proposals to search for extragalactic cosmic rays (including anti-matter from distant galaxies), we study particle propagation through the intergalactic medium (IGM). We first use estimates of the magnetic field strength between galaxies to constrain the mean free path for diffusion of particles through the IGM. We then develop a simple analytic model to describe the diffusion of cosmic rays. Given the current age of galaxies, our results indicate that, in reasonable models, a completely negligible number of particles can enter our Galaxy from distances greater than $\sim 100$ Mpc for relatively low energies ($E$ $< 10^6$ GeV/n). We also find that particle destruction in galaxies along the diffusion path produces an exponential suppression of the possible flux of extragalactic cosmic rays. Finally, we use gamma ray constraints to argue that the distance to any hypothetical domains of anti-matter must be roughly comparable to the horizon scale.

astro-ph↗
arXiv1996

Baryogenesis During Reheating in Natural Inflation and Comments on Spontaneous Baryogenesis

We calculate the baryon asymmetry created by the decay of a pseudo Nambu-Goldstone boson whose interactions violate baryon number conservation. Our results are in disagreement with previous results in the original spontaneous baryogenesis models for the asymmetry produced by the decay of an oscillating scalar field with B number violating derivative couplings; we find that the net baryon number density is proportional to $þ_i^3$, where $þ_i$ is the amplitude of the PNGB-field in natural inflation at the onset of reheating. We also discuss our disagreement with the interpretation of $\dotθ$ as an effective chemical potential for baryon number in spontaneous baryogenesis models. While our calculation of the asymmetry is carried out in the context of natural inflation our approach is generally valid for baryogenesis models using decaying classical fields. In the Appendices, we include a complete derivation of the number density of particles produced by the decay of a classical scalar field; this number density is proportional to the integral over momenta of the one pair production amplitude.

hep-ph↗
arXiv1996

Moduli Inflation with Large Scale Structure Produced by Topological Defects

It is tempting to inflate along one of the many flat directions that arise in supersymmetric theories. The required flatness of the potential to obtain sufficient inflation and to not overproduce density fluctuations occurs naturally. However, the density perturbations (in the case of a single moduli field) that arise from inflaton quantum fluctuations are too small for structure formation. Here we propose that topological defects (such as cosmic strings), which arise during a phase transition near the end of moduli inflation can provide an alternative source of structure. The strings produced will be `fat', yet have the usual evolution by the time of nucleosynthesis. Possible models are discussed.

hep-ph↗
arXiv1996

Indirect Detection of a Light Higgsino Motivated by Collider Data

Kane and Wells recently argued that collider data point to a Higgsino-like lightest supersymmetric partner which would explain the dark matter in our Galactic halo. They discuss direct detection of such dark-matter particles in laboratory detectors. Here, we argue that such a particle, if it is indeed the dark matter, might alternatively be accessible in experiments which search for energetic neutrinos from dark-matter annihilation in the Sun. We provide accurate analytic estimates for the rates which take into account all relevant physical effects. Currently, the predicted signal falls roughly one to three orders of magnitude below experimental bounds, depending on the mass and coupling of the particle; however, detectors such as MACRO, super-Kamiokande, and AMANDA will continue to take data and should be able to rule out or confirm an interesting portion of the possible mass range for such a dark-matter particle within the next five years.

hep-ph↗
arXiv1996

The Mass-Function of Low Mass Halo Stars: Limits on Baryonic Halo Dark Matter

We derive mass functions (MF) for halo red dwarfs (the faintest hydrogen burning stars) and then extrapolate to place limits on the total mass of halo brown dwarfs (stars not quite massive enough to burn hydrogen). The mass functions are obtained from the luminosity function of a sample of 114 local halo stars in the USNO parallax survey (Dahn \etal 1995). We use stellar models of Alexander \etal (1996) and make varying assumptions about metallicity and about possible unresolved binaries in the sample. We find that the MF for halo red dwarfs cannot rise more quickly than $1/m^2$ as one approaches the hydrogen burning limit. Using recent results from star formation theory, we extrapolate the MF into the brown-dwarf regime. We see that likely extrapolations imply that the total mass of brown dwarfs in the halo is less than $\sim 3\%$ of the local mass density of the halo ($\sim 0.3\%$ for the more realistic models we consider). Our limits apply to brown dwarfs in the halo that come from the same stellar population as the red dwarfs.

astro-ph↗
arXiv1995

Analysis of a Hubble Space Telescope Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo

We re-examine a deep {\it Hubble Space Telescope} pencil-beam search for red dwarfs, stars just massive enough to burn Hydrogen. The authors of this search (Bahcall, Flynn, Gould \& Kirhakos 1994) found that red dwarfs make up less than 6\% of the galactic halo. First, we extrapolate this result to include brown dwarfs, stars not quite massive enough to burn hydrogen; we assume a $1/{\cal M}$ mass function. Then the total mass of red dwarfs and brown dwarfs is $\leq$18\% of the halo. This result is consistent with microlensing results assuming a popular halo model. However, using new stellar models and parallax observations of low mass, low metallicity stars, we obtain much tighter bounds on low mass stars. We find the halo red dwarf density to be $<1\%$ of the halo, while our best estimate of this value is 0.14-0.37\%. Thus our estimate of the halo mass density of red dwarfs drops to 16-40 times less than the reported result of Bahcall et al (1994). For a $1/{\cal M}$ mass function, this suggests a total density of red dwarfs and brown dwarfs of $\sim$0.25-0.67\% of the halo, \ie , $(0.9-2.5)\times 10^9\msun$ out to 50 kpc. Such a low result would conflict with microlensing estimates by the \macho\ group (Alcock \etal 1995a,b).

astro-ph↗
arXiv1994

Calculation of Particle Production by Nambu Goldstone Bosons with Application to Inflation Reheating and Baryogenesis

A semiclassical calculation of particle production by a scalar field in a potential is performed. We focus on the particular case of production of fermions by a Nambu-Goldstone boson $θ$. We have derived a (non)local equation of motion for the $θ$-field with the backreaction of the produced particles taken into account. The equation is solved in some special cases, namely for purely Nambu-Goldstone bosons and for the tilted potential $ U(θ) \propto m^2 θ^2 $. Enhanced production of bosons due to parametric resonance is investigated; we argue that the resonance probably disappears when the expansion of the universe is included. Application of our work on particle production to reheating and an idea for baryogenesis in inflation are mentioned.

hep-ph↗
arXiv1994

The Scalar Field Potential in Inflationary Models: Reconstruction and Further Constraints

In this paper, we present quantitative constraints on the scalar field potential for a general class of inflationary models. (1) We first consider the reconstruction of the inflationary potential for given primordial density fluctuation spectra. Our work differs from previous work on reconstruction in that we find a semi-analytic solution for the potential for the case of density fluctuations with power-law spectra. In addition, for the case of more general spectra, we show how constraints on the density fluctuation spectra imply corresponding constraints on the potential. We present a series of figures which show how the shape of the potential depends on the shape of the perturbation spectrum and on the relative contribution of tensor modes. (2) We show that the average ratio $\rave$ of the amplitude of tensor perturbations (gravity wave perturbations) to scalar density perturbations is bounded from above: $\rave \le$ 1.6. We also show that the ratio $\rave$ is proportional to the change $Δϕ$ in the field: $\rave \approx 0.42 Δϕ/\mp$. Thus, if tensor perturbations are important for the formation of structure, then the width $Δϕ$ must be comparable to the Planck mass. (3) We constrain the change $ΔV$ of the potential and the change $Δϕ$ of the inflation field during the portion of inflation when cosmological structure is produced. We find both upper and lower bounds for $Δϕ$ and for $ΔV$. In addition, these constraints are then used to derive a bound on the scale $Λ$, which is the scale of the height of the potential

astro-ph↗