arXiv subjects
Publications and source records attributed to L. Bergstrom.
We investigate the possibility to determine the fraction of compact objects in the Universe by studying gravitational lensing effects on Type Ia supernova observations. Using simulated data sets from one year of operation of the proposed dedicated supernova detection satellite SNAP, we find that it should be possible to determine the fraction of compact objects to an accuracy of < 5 %.
The observational evidence for the existence of a non-zero cosmological constant is getting stronger. It is therefore timely to address the question of its eventual effect on the dynamics of galaxies, clusters and larger structures in the Universe. We find, contrary to a recent claim, that the influence of the cosmological constant has to be negligible for, e.g., the rotation curves of galaxies. On larger scales, starting with large galaxy clusters, there are potentially measurable effects from the repulsive addition to the Newtonian gravitational force caused by the cosmological constant.
The evidence for the existence of dark matter in the universe is reviewed. A general picture emerges, where both baryonic and non-baryonic dark matter is needed to explain current observations. In particular, a wealth of observational information points to the existence of a non-baryonic component, contributing between around 20 and 40 percent of the critical mass density needed to make the universe geometrically flat on large scales. In addition, an even larger contribution from vacuum energy (or cosmological constant) is indicated by recent observations. To the theoretically favoured particle candidates for non-baryonic dark matter belong axions, supersymmetric particles, and of less importance, massive neutrinos. The theoretical foundation and experimental situation for each of these is reviewed. Direct and indirect methods for detection of supersymmetric dark matter are described in some detail. Present experiments are just reaching the required sensitivity to discover or rule out some of these candidates, and major improvements are planned over the coming years.
Dark matter detection experiments are getting ever closer to the sensitivity needed to detect the primary particle physics candidates for nonbaryonic dark matter. Indirect detection methods include searching for antimatter and gamma rays, in particular gamma ray lines, in cosmic rays and high-energy neutrinos from the centre of the Earth or Sun caused by accretion and annihilation of dark matter particles. A review is given of recent progress, both on the theoretical and experimental sides.
We put bounds on the variation of the value of the fine structure constant $α$, at the time of Big Bang nucleosynthesis. We study carefully all light elements up to $^7$Li. We correct a previous upper limit on $|Δα/ α|$ estimated from $^4$He primordial abundance and we find interesting new potential limits (depending on the value of the baryon-to-photon ratio) from $^7$Li, whose production is governed to a large extent by Coulomb barriers. The presently unclear observational situation concerning the primordial abundances preclude a better limit than $|Δα/α| \lsim 2\cdot 10^{-2}$, two orders of magnitude less restrictive than previous bounds. In fact, each of the (mutually exclusive) scenarios of standard Big Bang nucleosynthesis proposed, one based on a high value of the measured deuterium primordial abundance and one based on a low value, may describe some aspects of data better if a change in $α$ of this magnitude is assumed.
The flux of cosmic ray antiprotons from neutralino annihilations in the galactic halo is computed for a large sample of models in the MSSM (the Minimal Supersymmetric extension of the Standard Model). We also revisit the problem of estimating the background of low-energy cosmic ray induced secondary antiprotons, taking into account their subsequent interactions (and energy loss) and the presence of nuclei in the interstellar matter. We consider a two-zone diffusion model, with and without a galactic wind. We find that, given the uncertainties in the background predictions, there is no need for a primary (exotic) component to explain present data. However, allowing for a signal by playing with the uncertainties in the background estimate, we discuss the characteristic features of the supersymmetric models which give a satisfactory description of the data. We point out that in some cases the optimal kinetic energy to search for a signal from supersymmetric dark matter is above several GeV, rather than the traditional sub-GeV region. The large astrophysical uncertainties involved do not, one the other hand, allow the exclusion of any of the MSSM models we consider, on the basis of data. We present besides numerical results also convenient parameterizations of the antiproton yields of all `basic' two-body final states. We also give examples of the yield and differential energy spectrum for a set of supersymmetric models with high rates. We also remark that it is difficult to put a limit on the antiproton lifetime from present measurements, since the injection of antiprotons from neutralino annihilation can compensate the loss from decay.
Neutrino telescopes of kilometer size are currently being planned. They will be two or three orders of magnitude bigger than presently operating detectors, but they will have a much higher muon energy threshold. We discuss the trade-off between area and energy threshold for indirect detection of neutralino dark matter captured in the Sun and in the Earth and annihilating into high energy neutrinos. We also study the effect of a higher threshold on the complementarity of different searches for supersymmetric dark matter.
We investigate the possibility to detect neutralino dark matter in a scenario in which the galactic dark halo is clumpy. We find that under customary assumptions on various astrophysical parameters, the antiproton and continuum gamma-ray signals from neutralino annihilation in the halo put the strongest limits on the clumpiness of a neutralino halo. We argue that indirect detection through neutrinos from the Earth and the Sun should not be much affected by clumpiness. We identify situations in parameter space where the gamma-ray line, positron and diffuse neutrino signals from annihilations in the halo may provide interesting signals in upcoming detectors.
We investigate if the gamma ray halo, for which recent evidence has been found in EGRET data, can be explained by neutralino annihilations in a clumpy halo. We find that the measured excess gamma ray flux can be explained through a moderate amount of clumping in the halo. Moreover, the required amount of clumping implies also a measureable excess of antiprotons at low energies, for which there is support from recent measurements by the BESS collaboration. The predicted antiproton fluxes resulting from neutralino annihilations in a clumpy halo are high enough to give an excess over cosmic-ray produced antiprotons also at moderately high energies (above a few GeV). This prediction, as well as that of one or two sharp gamma lines coming from annihilations into 2 gammas or Z gamma can be tested in upcoming space-borne experiments like AMS and GLAST.
Recent advances in N-body simulations of cold dark matter halos point to a substantial density enhancement near the center. This means that, e.g., the $γ$ ray signals from neutralino dark matter annihilations would be significantly enhanced compared to old estimates based on an isothermal sphere model with large core radius. Another important development concerns new detectors, both space- and ground-based, which will cover the window between 50 and 300 GeV where presently no cosmic $γ$-ray data are available. Thirdly, new calculations of the $γ$-ray line signal (a sharp spike of $10^{-3}$ relative width) from neutralino annihilations have revealed a hitherto neglected contribution which, for heavy higgsino-like neutralinos, gives an annihilation rate an order of magnitude larger than previously predicted. We make a detailed phenomenological study of the possible detection rates given these three pieces of new information. We show that the proposed upgrade of the Whipple telescope will make it sensitive to a region of parameter space, with substantial improvements possible with the planned new generation of Air Cherenkov Telescope Arrays. We also comment on the potential of the GLAST satellite detector. An evaluation of the continuum $γ$-rays produced in neutralino annihilations into the main modes is also done. It is shown that a combination of high-rate models and very peaked halo models are already severely constrained by existing data.
The need for dark matter is briefly reviewed. A wealth of observational information points to the existence of a non-baryonic component. To the theoretically favoured candidates today belong axions, supersymmetric particles, and to some extent massive neutrinos. The theoretical foundation and experimental situation for each of these is reviewed. In particular, indirect detection methods of supersymmetric dark matter are described. Present experiments are just reaching the required sensitivity to discover or rule out some of these candidates, and major improvements are planned over the next few years.
A full one-loop calculation of neutralino S-wave annihilation into the $Zγ$ final state is performed in the minimal supersymmetric extension of the Standard Model. This process, like the similar one with two photons in the final state, may be of importance for the indirect detection of supersymmetric dark matter through the very narrow $γ$ ray line that would result from neutralino annihilations in the galactic halo. We give the complete analytical formulas for this loop-induced process and treat the case of a pure Higgsino as a first application of our expressions. Predictions for the gamma line flux are given for the halo model which is of the form suggested by Kravtsov et al. and for the profile proposed by Navarro, Frenk and White. For heavy neutralinos, the lines from $2γ$ and $Zγ$ would have indistiguishable energy in a realistic detector, making the fluxes add and facilitating discovery. For lighter neutralinos, the positions and relative strengths of the two lines would give valuable information on the nature of the supersymmetric dark matter particles.
For the first time, a full one-loop calculation of the process $χχ\to 2γ$ is performed, where $χ$ is the lightest neutralino in the minimal supersymmetric extension of the Standard Model. This process is of interest for dark matter detection, since it would give a sharp $γ$ ray line with $E_γ=m_χ$. We improve upon and correct published formulas, and give cross sections for supersymmetric models with $χ$ masses between 30 GeV and several TeV. We find a new contribution, previously neglected, which enhances the $2γ$ rate for TeV higgsinos by up to an order of magnitude. As a byproduct, we obtain a new expression for the related process $χχ\to 2 gluons$, which on the other hand is generally smaller than previously calculated. There has been a recent claim that evidence for a 3.5 TeV higgsino annihilating into a $γ$ line may already exist from balloon emulsion and Air Cherenkov Telescope data. We comment on attractive features and problems with this interpretation.
An overview is given of high-energy gamma-ray and neutrino astronomy, emphasizing the links between the two fields. With several new large detectors just becoming operational, the TeV gamma-ray and neutrino sky will soon be surveyed with unprecedented sensitivity.
We investigate the improvement in sensitivity to astrophysical point sources of energetic ($\gtrsim1$ GeV) neutrinos which can be achieved with angular and/or energy resolution of the neutrino-induced muon. As a specific example we consider WIMP annihilation in the Sun and in the Earth as a neutrino source. The sensitivity is improved by using the angular and energy distribution to reduce the atmospheric-neutrino background. Although the specific improvements depend on the backgrounds and assumed sources, the sensitivity to a WIMP signal may be improved, with equal exposure, by up to roughly a factor of two with good angular resolution, and by up to roughly a factor of three with good energy resolution. In case of a positive detection, energy resolution would also improve the measurement of the neutrino energy spectrum and therefore provide information on the WIMP mass and composition.
We discuss recent measurements of the wavelength-dependent absorption coefficients in deep South Pole ice. The method uses transit time distributions of pulses from a variable-frequency laser sent between emitters and receivers embedded in the ice. At depths of 800 to 1000 m scattering is dominated by residual air bubbles, whereas absorption occurs both in ice itself and in insoluble impurities. The absorption coefficient increases approximately exponentially with wavelength in the measured interval 410 to 610 nm. At the shortest wavelength our value is about a factor 20 below previous values obtained for laboratory ice and lake ice; with increasing wavelength the discrepancy with previous measurements decreases. At around 415 to 500 nm the experimental uncertainties are small enough for us to resolve an extrinsic contribution to absorption in ice: submicron dust particles contribute by an amount that increases with depth and corresponds well with the expected increase seen near the Last Glacial Maximum in Vostok and Dome C ice cores. The laser pulse method allows remote mapping of gross structure in dust concentration as a function of depth in glacial ice.
At the AMANDA South Pole site, four new holes were drilled to depths 2050 m to 2180 m and instrumented with 86 photomultipliers (PMTs) at depths 1520-2000 m. Of these PMTs 79 are working, with 4-ns timing resolution and noise rates 300 to 600 Hz. Various diagnostic devices were deployed and are working. An observed factor 60 increase in scattering length and a sharpening of the distribution of arrival times of laser pulses relative to measurements at 800-1000 m showed that air bubbles are absent below 1500 m. Absorption lengths are 100 to 150 m at wavelengths in the blue and UV to 337 nm. Muon coincidences are seen between the SPASE air shower array and the AMANDA PMTs at 800-1000 m and 1500-1900 m. The muon track rate is 30 Hz for 8-fold triggers and 10 Hz for 10-fold triggers. The present array is the nucleus for a future expanded array. The potential of AMANDA for SUSY dark matter search through the detection of high-energy neutrinos from the centre of the Sun or Earth is discussed.
Neutralinos annihilating in the center of the Sun or the Earth may give rise to a detectable signal of neutrinos. We derive the indirect detection rates for neutrino telescopes in the minimal supersymmetric extension of the standard model. We show that even after imposing all phenomenological and experimental constraints that make the theories viable, regions of parameter space exist which can already be probed by existing neutrino telescopes. We compare with the discovery potential of supersymmetry at LEP2 as well as direct detections and point out the complementarity of the methods.