arXiv subjects

Katherine Freese

Publications and source records attributed to Katherine Freese.

At least 199 records · Page 11Linked to original sources

arXiv2003

Future Type Ia Supernova Data as Tests of Dark Energy from Modified Friedmann Equations

In the Cardassian model, dark energy density arises from modifications to the Friedmann equation, which becomes $H^2 = g(\rhom)$, where $g(\rhom)$ is a new function of the energy density. The universe is flat, matter dominated, and accelerating. The distance redshift relation predictions of generalized Cardassian models can be very different from generic quintessence models, and can be differentiated with data from upcoming pencil beam surveys of Type Ia Supernovae such as SNAP. We have found the interesting result that, once $Ω_m$ is known to 10% accuracy, SNAP will be able to determine the sign of the time dependence of the dark energy density. Knowledge of this sign (which is related to the weak energy condition) will provide a first discrimination between various cosmological models that fit the current observational data (cosmological constant, quintessence, Cardassian expansion). Further, we have performed Monte Carlo simulations to illustrate how well one can reproduce the form of the dark energy density with SNAP. To be concrete we study a class of two parameter ($n$,$q$) generalized Cardassian models that includes the original Cardassian model (parametrized by $n$ only) as a special case. Examples are given of MP Cardassian models that fit current supernovae and CMB data, and prospects for differentiating between MP Cardassian and other models in future data are discussed. We also note that some Cardassian models can satisfy the weak energy condition $w>-1$ even with a dark energy component that has an effective equation of state $w_X < -1$.

astro-ph↗

arXiv2003

Hagedorn Inflation: Open Strings on Branes Can Drive Inflation

We demonstrate an inflationary solution to the cosmological horizon problem during the Hagedorn regime in the early universe. Here the observable universe is confined to three spatial dimensions (a three-brane) embedded in higher dimensions. The only ingredients required are open strings on D-branes at temperatures close to the string scale. No potential is required. Winding modes of the strings provide a negative pressure that can drive inflation of our observable universe. Hence the mere existence of open strings on branes in the early hot phase of the universe drives Hagedorn inflation, which can be either power law or exponential. We note the amusing fact that, in the case of stationary extra dimensions, inflationary expansion takes place only for branes of three or less dimensions.

hep-th↗

arXiv2002

An Accelerating Universe from Dark Matter Interactions with Negative Pressure

As an explanation for the acceleration of the universe, we propose dark matter with self-interactions characterized by a negative pressure; there is no vacuum energy whatsoever in this Cardassian model. These self-interactions may arise due to a long-range "fifth force" which grows with the distance between particles. We use the ordinary Friedmann equation, $H^2 = 8 πG ρ/3$, and take the energy density to be the sum of two terms: $ρ= ρ_M + ρ_K$. Here $ρ_M \equiv m n_M$ is the ordinary mass density and $ρ_K \equiv ρ_K(ρ_M)$ is a new interaction term which depends only on the matter density. For example, in the original version of the Cardassian model, $ρ_K = b ρ_M^n$ with $n<2/3$; other examples are studied as well. We use the ordinary four-dimensional Einstein's equations $G_{μν} = 8 πG T_{μν} $ and assume a perfect fluid form for the energy momentum tensor. Given this ansatz, we can compute the accompanying pressure, and use the conservation of the energy-momentum tensor to obtain modified forms of the Euler's equation, Poisson equation, and continuity equation. With this fully relativistic description, one will then be able to compute growth of density perturbations, effects on the Cosmic Background Radiation, and other effects with an eye to observational tests of the model.

hep-ph↗

arXiv2002

Observational Tests of Open Strings in Braneworld Scenarios

We consider some consequences of describing the gauge and matter degrees of freedom in our universe by open strings, as suggested by the braneworld scenario. We focus on the geometric effects described by the open string metric and investigate their observational implications. The causal structure of spacetime on the brane is altered; it is described not by the usual metric $g_{μν}$, but instead by the open string metric, that incorporates the electromagnetic background, $G_{μν} = g_{μν} - (2πα^\prime)^2 (F^2)_{μν}$. The speed of light is now slower when propagating along directions transverse to electromagnetic fields or an NS-NS two form, so that Lorentz invariance is explicitly broken. A generalized equivalence principle guarantees that the propagation of {\it all} particles, not just photons, (with the exception of gravitons) is slower in these transverse directions. We describe a variety of astrophysical and laboratory-based experiments designed to detect the predicted variations in the causal structure. We show that current technology cannot probe beyond open string lengths of $10^{-13}$ cm, corresponding to MeV string scales. We also point out that in a braneworld scenario, constraints on large scale electromagnetic fields together with a modest phenomenological bound on the NS-NS two-form naturally lead to a bound on the scale of canonical noncommutativity that is two orders of magnitude below the string length. By invoking theoretical constraints on the NS-NS two-form this bound can be improved to give an extremely strong bound on the noncommutative scale well below the Planck length, $\sqrt{|θ|_{max}} < 10^{-35} {\rm cm} \times ({{\rm TeV} \over {\rm string} {\rm scale}}

hep-ph↗

arXiv2002

The positron excess and supersymmetric dark matter

Using a new instrument, the HEAT collaboration has confirmed the excess of cosmic ray positrons that they first detected in 1994. We explore the possibility that this excess is due to the annihilation of neutralino dark matter in the galactic halo. We confirm that neutralino annihilation can produce enough positrons to make up the measured excess only if there is an additional enhancement to the signal. We quantify the `boost factor' that is required in the signal for various models in the Minimal Supersymmetric Standard Model parameter space, and find that a boost factor >30 provides good fits to the HEAT data. Such an enhancement in the signal could arise if we live in a clumpy halo.

astro-ph↗

arXiv2002

Cardassian Expansion: a Model in which the Universe is Flat, Matter Dominated, and Accelerating

A modification to the Friedmann Robertson Walker equation is proposed in which the universe is flat, matter dominated, and accelerating. An additional term, which contains only matter or radiation (no vacuum contribution), becomes the dominant driver of expansion at a late epoch of the universe. During the epoch when the new term dominates, the universe accelerates; we call this period of acceleration the Cardassian era. The universe can be flat and yet consist of only matter and radiation, and still be compatible with observations. The energy density required to close the universe is much smaller than in a standard cosmology, so that matter can be sufficient to provide a flat geometry. The new term required may arise, e.g., as a consequence of our observable universe living as a 3-dimensional brane in a higher dimensional universe. The Cardassian model survives several observational tests, including the cosmic background radiation, the age of the universe, the cluster baryon fraction, and structure formation.

astro-ph↗

arXiv2002

Hagedorn Inflation: Open Strings on Branes Can Drive Inflation

hep-th↗

arXiv2002

The Ultimate Fate of Life in an Accelerating Universe

The ultimate fate of life in a universe with accelerated expansion is considered. Previous work showed that life cannot go on indefinitely in a universe dominated by a cosmological constant. In this paper we consider instead other models of acceleration (including quintessence and Cardassian expansion). We find that it is possible in these cosmologies for life to persist indefinitely. As an example we study potentials of the form $V \propto ϕ^n$ and find the requirement $n < -2$.

astro-ph↗

arXiv2002

A Wavelet Analysis of Solar Climate Forcing: I) Solar Cycle Timescales

We use the technique of wavelet analysis to quantitatively investigate the role of solar variability in forcing terrestrial climate change on solar cycle timescales (roughly 11 years). We examine the connection between mean annual solar irradiance, as reconstructed from sunspot and isotope records, and the climate, as proxied by two Northern Hemisphere surface air temperature reconstructions. By applying wavelet transforms to these signals, we are able to analyze both the frequency content of each signal, and also the time dependence of that content. After computing wavelet transforms of the data, we perform correlation analyses on the wavelet transforms of irradiance and temperature via two techniques: the Pearson's method and the conditional probability method. We thus track the correlation between individual frequency components of both signals as a function of time. A nonzero correlation between the irradiance and temperature wavelet spectra requires a phase lag between the two data sets (i.e., terrestrial response to solar output is not instantaneous). We search for the optimal phase lag that maximizes the correlation. By choosing an appropriate phase for each year, we find a significant, positive sun-climate correlation for most of the period AD 1720-1950. We find that this phase-optimized correlation varies in time, oscillating between 0.12 and 0.71 throughout the past 400 years. We find that the phase lag varies from 0-10 years. We also present a test to determine whether terrestrial response to solar output is enhanced via stochastic resonance, wherein the weak periodic solar signal is amplified by terrestrial noise.

astro-ph↗

arXiv2002

Lensed Density Perturbations in Braneworlds: An Alternative to Perturbations from Inflation

We consider a scenario in which our observable universe is a 3-dimensional surface (3-brane) living in extra dimensions with a warped geometry. We show that ``lensed'' density perturbations from other branes serve as possible seeds for structure formation on our observable brane (without inflation), and, in addition, provide constraints on braneworld scenarios with warped bulk geometry. Due to the warped bulk metric, any perturbation generated on one brane (or in the bulk matter) appears to an observer on a second brane to have a significantly different amplitude. We analyze lensed perturbations in the Randall-Sundrum type scenarios and the ``shortcut metric'' scenarios. For Lorentz violating metrics in the bulk, we find the attractive possibility that large density fluctuations that are causally produced elsewhere can lead to small density fluctuations on our brane on superhorizon (acausal) length scales, as required by structure formation. Our most interesting result is that the ``shortcut metrics'' in which geodesics traverse the extra dimensions provide an alternative to inflation with two important features: a possible solution to the horizon problem and a mechanism to generate perturbations necessary for structure formation.

astro-ph↗

arXiv2001

The cosmic ray positron excess and neutralino dark matter

Using a new instrument, the HEAT collaboration has confirmed the excess of cosmic ray positrons that they first detected in 1994. We explore the possibility that this excess is due to the annihilation of neutralino dark matter in the galactic halo. We confirm that neutralino annihilation can produce enough positrons to make up the measured excess only if there is an additional enhancement to the signal. We quantify the `boost factor' that is required in the signal for various models in the Minimal Supersymmetric Standard Model parameter space, and study the dependence on various parameters. We find models with a boost factor greater than 30. Such an enhancement in the signal could arise if we live in a clumpy halo. We discuss what part of supersymmetric parameter space is favored (in that it gives the largest positron signal), and the consequences for other direct and indirect searches of supersymmetric dark matter.

astro-ph↗

arXiv2001

On the direct detection of extragalactic WIMPs

We consider the direct detection of WIMPs reaching the Earth from outside the Milky Way. If these WIMPs form a distinct population they will, although of much lower flux than typical galactic halo WIMPs, have a number of features which might aid in their ultimate detectability: a high and essentially unique velocity ($\sim 600$ km/s in the galactic rest frame) due to their acceleration in entering the Milky Way, and most likely one or two unique flight directions at the Earth. This high velocity may be experimentally advantageous in direct detection experiments, since it gives a recoil signal at relatively high energy where background is generally much reduced. For a density of extragalactic WIMPs comparable to the critical density of the universe the count rate expected is very roughly the same as that of fast galactic WIMPs. If there is an increased density relative to critical associated with the Local Group of galaxies, say 10-30 times the critical density, there is a corresponding increase in rate and the extragalactic WIMPs would show up as a high energy shoulder in the recoil energy distribution. Evidence of such WIMPs as a separate population with these distinct properties would offer interesting information on the formation and prehistory of the galaxy.

astro-ph↗

arXiv2000

Can Geodesics in Extra Dimensions Solve the Cosmological Horizon Problem?

We demonstrate a non-inflationary solution to the cosmological horizon problem in scenarios in which our observable universe is confined to three spatial dimensions (a three-brane) embedded in a higher dimensional space. A signal traveling along an extra-dimensional null geodesic may leave our three-brane, travel into the extra dimensions, and subsequently return to a different place on our three-brane in a shorter time than the time a signal confined to our three-brane would take. Hence, these geodesics may connect distant points which would otherwise be ``outside'' the four dimensional horizon (points not in causal contact with one another).

hep-ph↗

arXiv2000

Death of Stellar Baryonic Dark Matter Candidates

The nature of the dark matter in the Universe is one of the outstanding questions in astrophysics. In this talk, I address possible stellar baryonic contributions to the 50-90% of our Galaxy that is made of unknown dark matter. First I show that faint stars and brown dwarfs constitute only a few percent of the mass of the Galaxy. Next, I show that stellar remnants, including white dwarfs and neutron stars, are also insufficient in abundance to explain all the dark matter of the Galaxy. High energy gamma-rays observed in HEGRA data place the most robust constraints, $Ω_{WD} < 3 \times 10^{-3} h^{-1}$, where $h$ is the Hubble constant in units of 100 km s$^{-1}$ Mpc$^{-1}$. Overproduction of chemical abundances (carbon, nitrogen, and helium) provide the most stringent constraints, $Ω_{WD} < 2 \times 10^{-4} h^{-1}$. Comparison with recent updates of microlensing data are also made. According to the gamma-ray limit, all Massive Compact Halo Objects seen by the experiments (Machos) can be white dwarfs if one takes the extreme numbers; however, from chemical overproduction limits, NOT all Machos can be white dwarfs. Comments on recent observations of the infrared background and of white dwarfs are also made. In conclusion, a nonbaryonic component in the Halo seems to be required.

astro-ph↗

arXiv2000

Hagedorn inflation of D-branes

We examine the cosmological effects of the Hagedorn phase in models where the observable universe is pictured as a D-brane. It is shown that, even in the absence of a cosmological constant, winding modes cause a negative `pressure' that can drive brane inflation of various types including both power law and exponential. We also find regimes in which the cosmology is stable but oscillating (a bouncing universe) with the Hagedorn phase softening the singular behavior associated with the collapse.

hep-th↗

arXiv2000

Death of Stellar Baryonic Dark Matter

The nature of the dark matter in the haloes of galaxies is one of the outstanding questions in astrophysics. All stellar candidates, until recently thought to be likely baryonic contributions to the Halo of our Galaxy, are shown to be ruled out. Faint stars and brown dwarfs are found to constitute only a few percent of the mass of the Galaxy. Stellar remnants, including white dwarfs and neutron stars, are shown to be very constrained as well. High energy gamma-rays observed in HEGRA data place the strongest constraints, $Ω_{WD} < 3 \times 10^{-3} h^{-1}$, where $h$ is the Hubble constant in units of 100 km s$^{-1}$ Mpc$^{-1}$. Hence one is left with several unanswered questions: 1) What are MACHOs seen in microlensing surveys? 2) What is the dark matter in our Galaxy? Indeed a nonbaryonic component in the Halo seems to be required.

astro-ph↗

arXiv2000

Effects of CP-Violation in Neutralino Scattering and Annihilation

We summarize our paper hep-ph/9908390, where we find 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 by factors of 10^6, and reduce the neutralino scattering rate off nucleons by factors of 10^{-7}.

hep-ph↗

arXiv1999

Protogalactic Extension of the Parker Bound

We extend the Parker bound on the galactic flux $\cal F$ of magnetic monopoles. By requiring that a small initial seed field must survive the collapse of the protogalaxy, before any regenerative dynamo effects become significant, we develop a stronger bound. The survival and continued growth of an initial galactic seed field $\leq 10^{-9}$G demand that ${\cal F} \leq 5 \times 10^{-21} (m/10^{17} {GeV}) {cm}^{-2} {sec}^{-1} {sr}^{-1}$. For a given monopole mass, this bound is four and a half orders of magnitude more stringent than the previous `extended Parker bound', but is more speculative as it depends on assumptions about the behavior of magnetic fields during protogalactic collapse. For monopoles which do not overclose the Universe ($Ω_m <1$), the maximum flux allowed is now $8 \times 10^{-19}$ cm^{-2} s^{-1} sr^{-1}, a factor of 150 lower than the maximum flux allowed by the extended Parker bound.

astro-ph↗