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Katherine Freese

Publications and source records attributed to Katherine Freese.

At least 163 records · Page 9Linked to original sources
arXiv2008

Slow nucleation rates in Chain Inflation with QCD Axions or Monodromy

The previous proposal (by two of us) of chain inflation with the QCD axion is shown to fail. The proposal involved a series of fast tunneling events, yet here it is shown that tunneling is too slow. We calculate the bubble nucleation rates for phase transitions in the thick wall limit, approximating the barrier by a triangle. A similar problem arises in realization of chain inflation in the string landscape that uses series of minima along the monodromy staircase around the conifold point. The basic problem is that the minima of the potential are too far apart to allow rapid enough tunneling in these two models. We entertain the possibility of overcoming this problem by modifying the gravity sector to a Brans-Dicke theory. However, one would need extremely small values for the Brans-Dicke parameter. Many successful alternatives exist, including other "axions" (with mass scales not set by QCD) or potentials with comparable heights and widths that do not suffer from the problem of slow tunneling and provide successful candidates for chain inflation.

hep-ph↗
arXiv2008

Stellar Structure of Dark Stars: a first phase of Stellar Evolution due to Dark Matter Annihilation

Dark Stars are the very first phase of stellar evolution in the history of the universe: the first stars to form (typically at redshifts $z \sim 10-50$) are powered by heating from dark matter (DM) annihilation instead of fusion (if the DM is made of particles which are their own antiparticles). We find equilibrium polytropic configurations for these stars; we start from the time DM heating becomes important ($M \sim 1-10 M_\odot$) and build up the star via accretion up to 1000 M$_\odot$. The dark stars, with an assumed particle mass of 100 GeV, are found to have luminosities of a few times $10^6$ L$_\odot$, surface temperatures of 4000--10,000 K, radii $\sim 10^{14}$ cm, lifetimes of at least $ 0.5$ Myr, and are predicted to show lines of atomic and molecular hydrogen. Dark stars look quite different from standard metal-free stars without DM heating: they are far more massive (e.g. $\sim 800 M_\odot$ for 100 GeV WIMPs), cooler, and larger, and can be distinguished in future observations, possibly even by JWST or TMT.

astro-ph↗
arXiv2008

Dark Matter Capture in the First Stars: a Power Source and Limit on Stellar Mass

The annihilation of weakly interacting massive particles can provide an important heat source for the first (Pop. III) stars, potentially leading to a new phase of stellar evolution known as a "Dark Star". When dark matter (DM) capture via scattering off of baryons is included, the luminosity from DM annihilation may dominate over the luminosity due to fusion, depending on the DM density and scattering cross-section. The influx of DM due to capture may thus prolong the lifetime of the Dark Stars. Comparison of DM luminosity with the Eddington luminosity for the star may constrain the stellar mass of zero metallicity stars; in this case DM will uniquely determine the mass of the first stars. Alternatively, if sufficiently massive Pop. III stars are found, they might be used to bound dark matter properties.

astro-ph↗
arXiv2008

Dark Stars: Dark Matter in the First Stars leads to a New Phase of Stellar Evolution

The first phase of stellar evolution in the history of the universe may be Dark Stars, powered by dark matter heating rather than by fusion. Weakly interacting massive particles, which are their own antiparticles, can annihilate and provide an important heat source for the first stars in the the universe. This talk presents the story of these Dark Stars. We make predictions that the first stars are very massive ($\sim 800 M_\odot$), cool (6000 K), bright ($\sim 10^6 L_\odot$), long-lived ($\sim 10^6$ years), and probable precursors to (otherwise unexplained) supermassive black holes. Later, once the initial DM fuel runs out and fusion sets in, DM annihilation can predominate again if the scattering cross section is strong enough, so that a Dark Star is born again.

astro-ph↗
arXiv2008

Dangerous implications of a minimum length in quantum gravity

The existence of a minimum length and a generalization of the Heisenberg uncertainty principle seem to be two fundamental ingredients required in any consistent theory of quantum gravity. In this letter we show that they would predict dangerous processes which are phenomenologically unacceptable. For example, long--lived virtual super--Planck mass black holes may lead to rapid proton decay. Possible solutions of this puzzle are briefly discussed.

hep-th↗
arXiv2008

Dark Matter Densities during the Formation of the First Stars and in Dark Stars

The first stars in the universe form inside $\sim 10^6 M_\odot$ dark matter (DM) haloes whose initial density profiles are laid down by gravitational collapse in hierarchical structure formation scenarios. During the formation of the first stars in the universe, the baryonic infall compresses the dark matter further. The resultant dark matter density is presented here, using an algorithm originally developed by Young to calculate changes to the profile as the result of adiabatic infall in a spherical halo model; the Young prescription takes into account the non-circular motions of halo particles. The density profiles obtained in this way are found to be within a factor of two of those obtained using the simple adiabatic contraction prescription of Blumenthal et al. Our results hold regardless of the nature of the dark matter or its interactions and rely merely on gravity. If the dark matter consists of weakly interacting massive particles, which are their own antiparticles, their densities are high enough that their annihilation in the first protostars can indeed provide an important heat source and prevent the collapse all the way to fusion. In short, a ``Dark Star'' phase of stellar evolution, powered by DM annihilation, may indeed describe the first stars in the universe.

astro-ph↗
arXiv2008

Dark Matter in the MSSM Golden Region

Dark matter is examined within the ``golden region'' of the Minimal Supersymmetric Standard Model. This region satisfies experimental constraints, including a lower bound on the Higgs mass of 114 GeV, and minimizes fine-tuning of the Z boson mass. Here we impose additional constraints (particularly due to experimental bounds on $b\to s γ$). Then we find the properties of the Dark Matter in this region. Neutralinos with a relic density that provides the amount of dark matter required by cosmological data are shown to consist of a predominant gaugino (rather than higgsino) fraction. In addition, the U(1)$_Y$ gaugino mass parameter must satisfy $M_1 < 300$ GeV.

hep-ph↗
arXiv2008

The Phantom Bounce: A New Proposal for an Oscillating Cosmology

An oscillating universe cycles through a series of expansions and contractions. We propose a model in which ``phantom'' energy with a supernegative pressure ($p < - ρ$) grows rapidly and dominates the late-time expanding phase. The universe's energy density is then so large that the effects of quantum gravity are important at both the beginning and the end of each expansion (or contraction). The bounce can be caused by high energy modifications to the Friedmann equation governing the expansion of the universe, which make the cosmology nonsingular. The classic black hole overproduction of oscillating universes is resolved due to their destruction by the phantom energy.

astro-ph↗
arXiv2008

The Phantom Bounce: A New Oscillating Cosmology

An oscillating universe cycles through a series of expansions and contractions. We propose a model in which ``phantom'' energy with $p < -ρ$ grows rapidly and dominates the late-time expanding phase. The universe's energy density is so large that the effects of quantum gravity are important at both the beginning and the end of each expansion (or contraction). The bounce can be caused by high energy modifications to the Friedmann equation, which make the cosmology nonsingular. The classic black hole overproduction of oscillating universes is resolved due to their destruction by the phantom energy.

astro-ph↗
arXiv2008

Natural Inflation: the status after WMAP 3-year data

The model of Natural Inflation is examined in light of recent 3-year data from the Wilkinson Microwave Anisotropy Probe and shown to provide a good fit. The inflaton potential is naturally flat due to shift symmetries, and in the simplest version takes the form $V(ϕ) = Λ^4 [1 \pm \cos(Nϕ/f)]$. The model agrees with WMAP3 measurements as long as $f > 0.7 m_{Pl}$ (where $m_{Pl} = 1.22 \times 10^{19}$GeV) and $Λ\sim m_{GUT}$. The running of the scalar spectral index is shown to be small -- an order of magnitude below the sensitivity of WMAP3. The location of the field in the potential when perturbations on observable scales are produced is examined; for $f > 5 m_{Pl}$, the relevant part of the potential is indistinguishable from a quadratic, yet has the advantage that the required flatness is well-motivated. Depending on the value of $f$, the model falls into the large field ($f \ge 1.5 m_{Pl}$) or small field ($f < 1.5 m_{Pl}$) classification scheme that has been applied to inflation models. Natural inflation provides a good fit to WMAP3 data.

hep-ph↗
arXiv2007

Dark matter and the first stars: a new phase of stellar evolution

A mechanism is identified whereby dark matter (DM) in protostellar halos dramatically alters the current theoretical framework for the formation of the first stars. Heat from neutralino DM annihilation is shown to overwhelm any cooling mechanism, consequently impeding the star formation process and possibly leading to a new stellar phase. A "dark star'' may result: a giant ($\gtrsim 1$ AU) hydrogen-helium star powered by DM annihilation instead of nuclear fusion. Observational consequences are discussed.

astro-ph↗
arXiv2007

The Effect of Dark Matter on the First Stars: A New Phase of Stellar Evolution

Dark matter (DM) in protostellar halos can dramatically alter the current theoretical framework for the formation of the first stars. Heat from supersymmetric DM annihilation can overwhelm any cooling mechanism, consequently impeding the star formation process and possibly leading to a new stellar phase. The first stars to form in the universe may be ``dark stars''; i.e., giant (larger than 1 AU) hydrogen-helium stars powered by DM annihilation instead of nuclear fusion. Possibilities for detecting dark stars are discussed.

astro-ph↗
arXiv2006

Chain Inflation via Rapid Tunneling in the Landscape

Chain inflation takes place in the string theory landscape as the universe tunnels rapidly through a series of ever lower energy vacua such as may be characterized by quantized changes in four form fluxes. The string landscape may be well suited to an early period of rapid tunneling, as required by chain inflation, followed by a later period of slow tunneling, such as may be required to explain today's dark energy and small cosmological constant. Each tunneling event (which can alternatively be thought of as a nucleation of branes) provides a fraction of an e-folding of inflation, so that hundreds of tunneling events provide the requisite amount of inflation. A specific example from M-theory compactification on manifolds with non-trivial three-cycles is presented.

hep-th↗
arXiv2006

Natural Inflation: status after WMAP 3-year data

The model of Natural Inflation is examined in light of recent 3-year data from the Wilkinson Microwave Anisotropy Probe and shown to provide a good fit. The inflaton potential is naturally flat due to shift symmetries, and in the simplest version takes the form $V(ϕ) = Λ^4 [1 \pm \cos(Nϕ/f)]$. The model agrees with WMAP3 measurements as long as $f > 0.7 m_{Pl}$ (where $m_{Pl} = 1.22 \times 10^{19}$GeV) and $Λ\sim m_{GUT}$. The running of the scalar spectral index is shown to be small -- an order of magnitude below the sensitivity of WMAP3. The location of the field in the potential when perturbations on observable scales are produced is examined; for $f > 5 m_{Pl}$, the relevant part of the potential is indistinguishable from a quadratic, yet has the advantage that the required flatness is well-motivated. Depending on the value of $f$, the model falls into the large field ($f \ge 1.5 m_{Pl}$) or small field ($f < 1.5 m_{Pl}$) classification scheme that has been applied to inflation models. Natural inflation provides a good fit to WMAP3 data.

hep-ph↗
arXiv2006

Annual Modulation of Dark Matter in the Presence of Streams

In addition to a smooth component of WIMP dark matter in galaxies, there may be streams of material; the effects of WIMP streams on direct detection experiments is examined in this paper. The contribution to the count rate due to the stream cuts off at some characteristic energy. Near this cutoff energy, the stream contribution to the annual modulation of recoils in the detector is comparable to that of the thermalized halo, even if the stream represents only a small portion (~5% or less) of the local halo density. Consequently the total modulation may be quite different than would be expected for the standard halo model alone: it may not be cosine-like and can peak at a different date than expected. The effects of speed, direction, density, and velocity dispersion of a stream on the modulation are examined. We describe how the observation of a modulation can be used to determine these stream parameters. Alternatively, the presence of a dropoff in the recoil spectrum can be used to determine the WIMP mass if the stream speed is known. The annual modulation of the cutoff energy together with the annual modulation of the overall signal provide a "smoking gun" for WIMP detection.

astro-ph↗
arXiv2006

New Models for a Triaxial Milky Way Spheroid and Effect on the Microlensing Optical Depth to the Large Magellanic Cloud

We obtain models for a triaxial Milky Way spheroid based on data by Newberg and Yanny. The best fits to the data occur for a spheroid center that is shifted by 3kpc from the Galactic Center. We investigate effects of the triaxiality on the microlensing optical depth to the Large Magellanic Cloud (LMC). The optical depth can be used to ascertain the number of Massive Compact Halo Objects (MACHOs); a larger spheroid contribution would imply fewer Halo MACHOs. On the one hand, the triaxiality gives rise to more spheroid mass along the line of sight between us and the LMC and thus a larger optical depth. However, shifting the spheroid center leads to an effect that goes in the other direction: the best fit to the spheroid center is_away_ from the line of sight to the LMC. As a consequence, these two effects tend to cancel so that the change in optical depth due to the Newberg/Yanny triaxial halo is at most 50%. After subtracting the spheroid contribution in the four models we consider, the MACHO contribution (central value) to the mass of the Galactic Halo varies from \~(8-20)% if all excess lensing events observed by the MACHO collaboration are assumed to be due to MACHOs. Here the maximum is due to the original MACHO collaboration results and the minimum is consistent with 0% at the 1 sigma error level in the data.

astro-ph↗
arXiv2005

Devaluation: a dynamical mechanism for a naturally small cosmological constant

We propose a natural solution to the cosmological constant problem consistent with the standard cosmology and successful over a broad range of energies. This solution is based on the existence of a new field, the devaluton, with its potential modeled on a tilted cosine. After inflation, the universe reheats and populates the devaluton's many minima. As the universe cools, domain walls form between different regions. The domain wall network then evolves and sweeps away regions of higher vacuum energy in favor of lower energy ones. Gravitation itself provides a cutoff at a minimum vacuum energy, thus leaving the universe with a small cosmological constant comparable in magnitude to the present day dark energy density.

hep-ph↗
arXiv2005

Age of the Universe in the Cardassian Model

The age of the universe is obtained in a subset of Cardassian models by using WMAP data. Cardassian expansion is a modification to the Friedmann equation that allows the universe to be flat, matter dominated, and accelerating, without a vacuum component. Since this model changes the evolution of the universe, we should not a priori expect the Cardassian age to be the same as the WMAP Friedmann derived result of 13.7 +/- 0.2 Gyrs. However, in the subset of Cardassian models we consider, we discover that the age of the universe varies from 13.4 - 13.8 Gyr over the range of parameter space we explore, a result close to that of the standard Lambda Cold Dark Matter model. The Hubble constant h, which may also vary in these models, likewise varies little from the Friedmann result.

astro-ph↗