arXiv subjects

Edvard Mortsell

Publications and source records attributed to Edvard Mortsell.

46 records · Page 3Linked to original sources

arXiv2005

Supernova Cosmology and the ESSENCE project

The proper usage of Type Ia supernovae (SNe Ia) as distance indicators has revolutionized cosmology, and added a new dominant component to the energy density of the Universe, dark energy. Following the discovery and confirmation era, the currently ongoing SNe Ia surveys aim to determine the properties of the dark energy. ESSENCE is a five year ground-based supernova survey aimed at finding and characterizing 200 SNe Ia in the redshift domain z=[0.2-0.8]. The goal of the project is to put constraints on the equation of state parameter, w, of the dark energy with an accuracy of <10%. This paper presents these ongoing efforts in the context of the current developments in observational cosmology.

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arXiv2005

Limiting the dimming of distant type Ia supernovae

Distant supernovae have been observed to be fainter than what is expected in a matter dominated universe. The most likely explanation is that the universe is dominated by an energy component with negative pressure -- dark energy. However, there are several astrophysical processes that could, in principle, affect the measurements and in order to be able to take advantage of the growing supernova statistics, the control of systematic effects is crucial. We discuss two of these; extinction due to intergalactic grey dust and dimming due to photon-axion oscillations and show how their effect on supernova observations can be constrained using observed quasar colours and spectra. For a wide range of intergalactic dust models, we are able to rule out any dimming larger than 0.2 magnitudes for a type Ia supernova at z=1. The corresponding limit for intergalactic Milky Way type dust is 0.03 mag. For the more speculative model of photons mixing with axions, we find that the effect is independent of photon energy for certain combinations of parameter values and a dimming as large as 0.6 magnitudes cannot be ruled out. These effects can have profound implications for the possibility of constraining dark energy properties using supernova observations.

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arXiv2004

Probing galaxy density profiles with future supernova surveys

In this paper we discuss the possibility to measure the Hubble parameter and the slope of galaxy density profiles using future supernova data. With future supernova surveys such as SNAP, large numbers of core collapse supernovae will be discovered, a small fraction of which will be multiply imaged. Measurements of the image separation, flux-ratio, time-delay and lensing foreground galaxy for these systems will provide tight constraints on the slope of galactic halos as well as providing complementary and independent information to other cosmological tests with respect to the Hubble parameter.

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arXiv2004

Cosmological constraints on the dark energy equation of state and its evolution

We have calculated constraints on the evolution of the equation of state of the dark energy, w(z), from a joint analysis of data from the cosmic microwave background, large scale structure and type-Ia supernovae. In order to probe the time-evolution of w we propose a new, simple parametrization of w, which has the advantage of being transparent and simple to extend to more parameters as better data becomes available. Furthermore it is well behaved in all asymptotic limits. Based on this parametrization we find that w(z=0)=-1.43^{+0.16}_{-0.38} and dw/dz(z=0) = 1.0^{+1.0}_{-0.8}. For a constant w we find that -1.34 < w < -0.79 at 95% C.L. Thus, allowing for a time-varying w shifts the best fit present day value of w down. However, even though models with time variation in w yield a lower chi^2 than pure LambdaCDM models, they do not have a better goodness-of-fit. Rank correlation tests on SNI-a data also do not show any need for a time-varying w.

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arXiv2003

Constraints on intergalactic dust from quasar colours

Colour measurements of quasars are used to constrain the abundance and properties of intergalactic dust and the related extinction effects on high-z sources. For Type Ia supernovae at z = 1, we derive an upper limit on the possible dimming from intergalactic dust of dm=0.2, ruling out the ``grey'' dust scenario as being solely responsible for the observed faintness of high-z SNIa.

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arXiv2003

Constraining photon-axion oscillations using quasar spectra

Using quasar spectra from the SDSS survey, we constrain the possibility of photon-axion oscillations as a source of dimming of high redshift objects. Such a process has been suggested as an explanation of the apparent faintness of distant Type Ia supernovae. For most combinations of magnetic field strengths and plasma densities along the line of sight, large beam attenuations in broad band filters would also lead to significant differential attenuation, not observed in the quasar sample. However, this conservative study does not exclude the possibility of 0.1 mag dimming of Type Ia supernovae for average plasma densities n_e = 10^(-8) cm^(-3). NIR and MIR spectroscopic studies of high-z sources may be used put further constrains or provide indirect evidence for the existence of a very light axion.

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arXiv2002

Probing the dark side: Constraints on the dark energy equation of state from CMB, large scale structure and Type Ia supernovae

We have reanalysed constraints on the equation of state parameter, w_Q = P/rho, of the dark energy, using several cosmological data sets and relaxing the usual constraint w_Q > -1. We find that combining Cosmic Microwave Background, large scale structure and Type Ia supernova data yields a non-trivial lower bound on w_Q. At 95.4% confidence we find, assuming a flat geometry of the universe, a bound of -2.68 < w_Q < -0.78 if w_Q is taken to be a completely free parameter. Reassuringly we also find that the constraint w_Q > -1 does not significantly bias the overall allowed region for w_Q. When this constraint is imposed the 95.4% confidence bound is -1 < w_Q < -0.71. Also, a pure cosmological constant (w = -1) is an excellent fit to all available data. Based on simulations of future data from the Planck CMB experiment and the SNAP and SNfactory supernova experiments we estimate that it will be possible to constrain w_Q at the 5% level in the future.

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arXiv2002

Photon-axion oscillations and Type Ia supernovae

We compute the probability of photon-axion oscillations in the presence of both intergalactic magnetic fields and an electron plasma and investigate the effect on Type Ia supernovae observations. The conversion probability is calculated using a density matrix formalism by following light-paths through simulated universes in a Monte-Carlo fashion. We find, that even though the effect is highly frequency dependent, one needs to analyze relatively narrow spectral features of high redshift objects in order to discern between the dimming effect from oscillations and a cosmological constant, in contrast to earlier claims that broad-band photometry is sufficient.

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arXiv2002

Measuring the properties of extragalactic dust and implications for the Hubble diagram

Scattering and absorption of light by a homogeneous distribution of intergalactic large dust grains has been proposed as an alternative, non-cosmological explanation for the faintness of Type Ia supernovae at $z\s im 0.5$. We investigate the differential extinction for high-redshift sources caused by extragalactic dust along the line of sight. Future observations of Type Ia supernovae up to $z\sim 2$, e.g. by the proposed SNAP satellite, will allow the measurement of the properties of dust over cosmological distances. We show that 1% {\em relative} spectrophotometric accuracy (or broadband photometry) in the wavelength interval 0.7--1.5 $μ$m is required to measure the extinction caused by ``grey'' dust down to $δm=0.02$ magnitudes. We also argue that the presence of grey dust is not necessarily inconsistent with the recent measurement of the brightness of a supernova at $z=1.7$ (SN 1997ff), in the absence of accurate spectrophotometric information of the supernova.

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arXiv2000

Determining gravitational lensing effects on supernovae observations

In this paper we use a generalized version of a method originally proposed by Holz and Wald to investigate the effects from gravitational lensing on Type Ia supernovae measurements. We find that results for different mass distributions in smooth dark matter halos are very similar, making lensing effects predictable for a broad range of density profiles. Also, a sample of 100 supernovae at z = 1, should be sufficient to discriminate between the case of all dark matter in smooth halos and the extreme case of all dark matter in point-like objects.

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