arXiv subjects

Edvard Mortsell

Publications and source records attributed to Edvard Mortsell.

At least 37 records · Page 2Linked to original sources

arXiv2011

Cosmological Solutions in Bimetric Gravity and their Observational Tests

We obtain the general cosmological evolution equations for a classically consistent theory of bimetric gravity. Their analytic solutions are demonstrated to generically allow for a cosmic evolution starting out from a matter dominated FLRW universe while relaxing towards a de Sitter (anti-de Sitter) phase at late cosmic time. In particular, we examine a subclass of models which contain solutions that are able to reproduce the expansion history of the cosmic concordance model inspite of the nonlinear couplings of the two metrics. This is demonstrated explicitly by fitting these models to observational data from Type Ia supernovae, Cosmic Microwave Background and Baryon Acoustic Oscillations.

gr-qc↗

arXiv2011

Spherically Symmetric Solutions in Massive Gravity and Constraints from Galaxies

In this paper, analytical solutions describing static and spherically symmetric sources in the decoupling limit of massive gravity are derived. We analyze the model parameter range and specify when a Vainshtein mechanism is possible. Furthermore, we use gravitational lensing and velocity dispersion data from galaxies to put constraints on the mass scale of the graviton. The result for the inverse graviton mass scale lambda_g = h/(2pi)/(c m_g), in units of the Hubble radius r_H=c/H_0, is of the order lambda_g/r_H > 0.01-0.02 at 95% confidence level.

gr-qc↗

arXiv2011

A model independent measure of the large scale curvature of the Universe

Cosmological distances as a function of redshift depend on the effective curvature density via the effect on the geometrical path of photons from large scale spatial curvature and its effect on the expansion history, H(z). Cosmological time, however, depends on the expansion history only. Therefore, by combining distance and lookback time observations (or other estimates of the expansion history), it is possible to isolate the geometrical curvature contribution and measure the curvature in a model independent way, i.e., free from assumptions about the energy content of the universe. We investigate two different approaches to accomplish this task; the differential and the integral approach. The differential approach requires, in addition to distances, derivatives of distance with respect to redshift as well as knowledge of the expansion history. The integral approach is based on measuring the integral of the inverse of the expansion history via measurements of cosmic time as derived, e.g., from galaxy ages. In this paper, we attempt to constrain the large scale curvature of the Universe using distances obtained from observations of Type Ia supernovae together with inferred ages of passively evolving galaxies and Hubble parameter estimates from the large scale clustering of galaxies. Current data are consistent with zero spatial curvature, although the uncertainty on the curvature density is of order unity. Future data sets with on the order of thousands of Type Ia supernovae distances and galaxy ages will allow us to constrain the curvature density with an uncertainty of less than 0.1 at the 95% confidence level.

astro-ph.CO↗

arXiv2010

Constraining dark energy fluctuations with supernova correlations

We investigate constraints on dark energy fluctuations using type Ia supernovae. If dark energy is not in the form of a cosmological constant, that is if the equation of state is not equal to -1, we expect not only temporal, but also spatial variations in the energy density. Such fluctuations would cause local variations in the universal expansion rate and directional dependences in the redshift-distance relation. We present a scheme for relating a power spectrum of dark energy fluctuations to an angular covariance function of standard candle magnitude fluctuations. The predictions for a phenomenological model of dark energy fluctuations are compared to observational data in the form of the measured angular covariance of Hubble diagram magnitude residuals for type Ia supernovae in the Union2 compilation. The observational result is consistent with zero dark energy fluctuations. However, due to the limitations in statistics, current data still allow for quite general dark energy fluctuations as long as they are in the linear regime.

astro-ph.CO↗

arXiv2010

Supernovae as seen by off-center observers in a local void

Inhomogeneous universe models have been proposed as an alternative explanation for the apparent acceleration of the cosmic expansion that does not require dark energy. In the simplest class of inhomogeneous models, we live within a large, spherically symmetric void. Several studies have shown that such a model can be made consistent with many observations, in particular the redshift--luminosity distance relation for type Ia supernovae, provided that the void is of Gpc size and that we live close to the center. Such a scenario challenges the Copernican principle that we do not occupy a special place in the universe. We use the first-year Sloan Digital Sky Survey-II supernova search data set as well as the Constitution supernova data set to put constraints on the observer position in void models, using the fact that off-center observers will observe an anisotropic universe. We first show that a spherically symmetric void can give good fits to the supernova data for an on-center observer, but that the two data sets prefer very different voids. We then continue to show that the observer can be displaced at least fifteen percent of the void scale radius from the center and still give an acceptable fit to the supernova data. When combined with the observed dipole anisotropy of the cosmic microwave background however, we find that the data compells the observer to be located within about one percent of the void scale radius. Based on these results, we conclude that considerable fine-tuning of our position within the void is needed to fit the supernova data, strongly disfavouring the model from a Copernican principle point of view.

astro-ph.CO↗

arXiv2009

On the use of black hole binaries as probes of local dark energy properties

Accretion of dark energy onto black holes will take place when dark energy is not a cosmological constant. It has been proposed that the time evolution of the mass of the black holes in binary systems due to dark energy accretion could be detectable by gravitational radiation. This would make it possible to use observations of black hole binaries to measure local dark energy properties, e.g., to determine the sign of 1+w where w is the dark energy equation of state. In this Letter we show that such measurements are unfeasible due to the low accretion rates.

astro-ph.CO↗

arXiv2009

Weighing dark matter haloes with gravitationally lensed supernovae

High redshift Type Ia supernovae (SNe Ia) are likely to be gravitationally lensed by dark matter haloes of galaxies in the foreground. Since SNe Ia have very small dispersion after light curve shape and colour corrections, their brightness can be used to measure properties of the dark matter haloes via gravitational magnification. We use observations of galaxies and SNe Ia within the Great Observatories Origins Deep Survey (GOODS) to measure the relation between galaxy luminosity and dark matter halo mass. The relation we investigate is a scaling law between velocity dispersion and galaxy luminosity in the B-band. The best-fitting values to this relation (velocity dispersion normalisation and exponent) are 136 km/s and 0.27. We find the velocity dispersion normalisation to be less than 190 km/s at the 95 per cent confidence level. This method provides an independent cross-check of measurements of dark matter halo properties from galaxy-galaxy lensing studies. Our results agree with the galaxy-galaxy lensing results, but have much larger uncertainties. The GOODS sample of SNe Ia is relatively small (we include 24 SNe) and the results therefore depend on individual SNe Ia. We have investigated a number of potential systematic effects. Light curve fitting, which affects the inferred brightness of the SNe Ia, appears to be the most important one. Results obtained using different light curve fitting procedures differ at the 68.3 per cent confidence level.

astro-ph.CO↗

arXiv2009

Model independent constraints on the cosmological expansion rate

We investigate what current cosmological data tells us about the cosmological expansion rate in a model independent way. Specifically, we study if the expansion was decelerating at high redshifts and is accelerating now, without referring to any model for the energy content of the universe, nor to any specific theory of gravity. This differs from most studies of the expansion rate which, e.g., assumes some underlying parameterised model for the dark energy component of the universe. To accomplish this, we have devised a new method to probe the expansion rate without relying on such assumptions. Using only supernova data, we conclude that there is little doubt that the universe has been accelerating at late times. However, contrary to some previous claims, we can not determine if the universe was previously decelerating. For a variety of methods used for constraining the expansion history of the universe, acceleration is detected from supernovae alone at >5 sigma, regardless of the curvature of the universe. Specifically, using a Taylor expansion of the scale factor, acceleration today is detected at >12 sigma. If we also include the ratio of the scale of the baryon acoustic oscillations as imprinted in the cosmic microwave background and in the large scale distribution of galaxies, it is evident from the data that the expansion decelerated at high redshifts, but only with the assumption of a flat or negatively curved universe.

astro-ph↗

arXiv2008

Reducing the gravitational lensing scatter of Type Ia supernovae without introducing any extra bias

Magnification and de-magnification due to gravitational lensing will contribute to the brightness scatter of Type Ia supernovae (SNe Ia). The purpose of this paper is to investigate the possibility to decrease this scatter by correcting individual SNe Ia using observations of galaxies in the foreground, without introducing any extra bias. We simulate a large number of SN Ia lines of sight populated by galaxies. For each line of sight the true magnification factor and an estimate thereof are calculated. The estimated magnification factor corresponds to what an observer would infer from a survey like SNLS. Using the simulated data we investigate the possibility to estimate the magnification of individual supernovae with enough precision to be able to correct their brightness for gravitational lensing with negligible bias. Our simulations show that the bias arising from gravitational lensing corrections of individual SNe Ia is negligible for current and next generation surveys and that the scatter from lensing can be reduced by approximately a factor 2. The total scatter in the SN Ia magnitudes could be reduced by 4% for an intrinsic dispersion of 0.13 mag. For an intrinsic dispersion of 0.09 mag, which might not be to unrealistic for future surveys, the total scatter could be reduced by 6%. This will reduce the errors on cosmological parameters derived from supernova data by 4-8%. The prospect of correcting for lensing is thus very good.

astro-ph↗

arXiv2008

Probing Dark Energy Inhomogeneities with Supernovae

We discuss the possibility to identify anisotropic and/or inhomogeneous cosmological models using type Ia supernova data. A search for correlations in current type Ia peak magnitudes over a large range of angular scales yields a null result. However, the same analysis limited to supernovae at low redshift, shows a feeble anticorrelation at the two sigma level at angular scales of about 40 degrees. Upcoming data from, e.g., the SNLS (Supernova Legacy Survey) and the SDSS-II (SDSS: Sloan Digital Sky Survey) supernova searches will improve our limits on the size of - or possibly detect - possible correlations also at high redshift at the per cent level in the near future. With data from the proposed SNAP (SuperNova Acceleration Probe) satellite, we will be able to detect the induced correlations from gravitational lensing on type Ia peak magnitudes on scales less than a degree.

astro-ph↗

arXiv2008

Prospects and pitfalls of gravitational lensing in large supernova surveys

To investigate the effect of gravitational lensing of supernovae in large ongoing surveys, we simulate the effect of gravitational lensing magnification on individual supernovae using observational data input from two large supernova surveys. To estimate the magnification due to matter in the foreground, we simulate galaxy catalogs and compute the magnification along individual lines of sight using the multiple lens plane algorithm. The dark matter haloes of the galaxies are modelled as gravitational lenses using singular isothermal sphere or Navarro-Frenk-White profiles. Scaling laws between luminosity and mass, provided by Faber-Jackson and Tully-Fisher relations, are used to estimate the masses of the haloes. While our simulations show that the SDSSII supernova survey is marginally affected by gravitational lensing, we find that the effect will be measurable in the SNLS survey that probes higher redshifts. Our simulations show that the probability to measure a significant (3 sigma) correlation between the Hubble diagram residuals and the calculated lensing magnification is ~95% in the SNLS data. Moreover, with this data it should be possible to constrain the normalisation of the masses of the lensing galaxy haloes at the 1 sigma and 2 sigma confidence level with ~30% and ~60% accuracy, respectively.

astro-ph↗

arXiv2008

Extinction properties of lensing galaxies

Context. Observations of quasars shining through foreground galaxies, offer a way to probe the dust extinction curves of distant galaxies. Interesting objects for this study are found in strong gravitational lensing systems, where the foreground galaxies generate multiple images. Aims. The reddening law of lensing galaxies is investigated by studying the colours of gravitationally-lensed quasars, and a handful of other quasars where a foreground galaxy is detected. Methods. We compare the observed colours of quasars reported in the literature, with spectral templates reddened by different extinction laws and dust properties. The data consists of 21 quasar-galaxy systems, with a total of 48 images. The galaxies, which are both early- and late-type, have redshifts in the interval z=0.04-1.51. Results. We measure a difference in rest-frame B-V between the quasar images we study, and quasars without resolved foreground galaxies. This difference in colour is indicative of significant dust extinction in the intervening galaxy. Good fits to standard extinction laws were found for 22 of the images, corresponding to 13 different galaxies. Our fits imply a wide range of possible values for the total-to-selective extinction ratio, Rv. The distribution was found to be broad with a weighted mode of Rv=2.4 and a FWHM of 2.7 (sigma = 1.1). Thus the bulk of the galaxies for which good reddening fits could be derived, have dust properties compatible with the Milky Way value (Rv=3.1).

astro-ph↗

arXiv2006

Tentative detection of the gravitational magnification of type Ia supernovae

The flux from distant type Ia supernovae (SN) is likely to be amplified or de-amplified by gravitational lensing due to matter distributions along the line-of-sight. A gravitationally lensed SN would appear brighter or fainter than the average SN at a particular redshift. We estimate the magnification of 26 SNe in the GOODS fields and search for a correlation with the residual magnitudes of the SNe. The residual magnitude, i.e. the difference between observed and average magnitude predicted by the "concordance model" of the Universe, indicates the deviation in flux from the average SN. The linear correlation coefficient for this sample is r=0.29. For a similar, but uncorrelated sample, the probability of obtaining a correlation coefficient equal to or higher than this value is ~10%, i.e. a tentative detection of lensing at ~90% confidence level. Although the evidence for a correlation is weak, our result is in accordance with what could be expected given the small size of the sample.

astro-ph↗

arXiv2006

Tuning Gravitationally Lensed Standard Sirens

Gravitational waves emitted by chirping supermassive black hole binaries could in principle be used to obtain very accurate distance determinations. Provided they have an electromagnetic counterpart from which the redshift can be determined, these standard sirens could be used to build a high redshift Hubble diagram. Errors in the distance measurements will most likely be dominated by gravitational lensing. We show that the (de)magnification due to inhomogeneous foreground matter will increase the scatter in the measured distances by a factor ~10. We propose to use optical and IR data of the foreground galaxies to minimize the degradation from weak lensing. We find that the net effect of correcting the estimated distances for lensing is comparable to increasing the sample size by a factor of three when using the data to constrain cosmological parameters.

astro-ph↗

arXiv2006

The neutrino mass bound from WMAP-3, the baryon acoustic peak, the SNLS supernovae and the Lyman-alpha forest

We have studied bounds on the neutrino mass using new data from the WMAP 3 year data, the Sloan Digital Sky Survey measurement of the baryon acoustic peak, the Type Ia supernovae from SNLS, and the Lyman-alpha forest. We find that even in the most general models with a running spectral index where the number of neutrinos and the dark energy equation of state are allowed to vary, the 95% C.L. bound on the sum of neutrino masses is sum m_nu < 0.62 eV (95% C.L.), a bound which we believe to be robust. In the more often used constrained analysis with N_nu =3, w = -1, and alpha_s = 0, we find a bound of 0.48 eV without using the Lyman-alpha data. If Lyman-alpha data is used, the bound shrinks to \sum m_nu < 0.2-0.4 eV (95% C.L.), depending strongly on the Lyman-alpha analysis used.

astro-ph↗

arXiv2006

Strong lensing, cosmology and lensing halos

With future wide and deep cosmological sky surveys, a large number of gravitationally lensed, multiply imaged systems will be found. In addition to multiply imaged galaxies and quasars, sources will include transient events like supernovae and gamma ray bursts in which case very accurate time delay measurements are possible. Also, large numbers of systems with several lensed sources behind a single lens will be observed. In this paper, we review and compare different possibilities of using future strong lensing data to probe lens matter distributions and to determine the Hubble parameter and the matter density of the universe. Specifically, we investigate the possibility to break the well-known degeneracy between dark matter halo profiles and the Hubble parameter using observed flux ratios. We also investigate how strong lensing can provide useful constraints on the matter density of the universe independently of the flux ratios and other cosmological probes.

astro-ph↗

arXiv2005

Corrections for gravitational lensing of supernovae: better than average?

We investigate the possibility of correcting for the magnification due to gravitational lensing of standard candle sources, such as Type Ia supernovae. Our method uses the observed properties of the foreground galaxies along the lines-of-sight to each source and the accuracy of the lensing correction depends on the quality and depth of these observations as well as the uncertainties in translating the observed luminosities to the matter distribution in the lensing galaxies. The current work is limited to cases where the matter density is dominated by the individual galaxy halos. However, it is straightforward to generalize the method to include also gravitational lensing from cluster scale halos. We show that the dispersion due to lensing for a standard candle source at z=1.5 can be reduced from about 7% to ~< 3%, i.e. the magnification correction is useful in reducing the scatter in the Type Ia Hubble diagram, especially at high redshifts where the required long exposure times makes it hard to reach large statistics and the dispersion due to lensing becomes comparable to the intrinsic Type Ia scatter.

astro-ph↗

arXiv2005

Lensing magnification of supernovae in the GOODS-fields

Gravitational lensing of high-redshift supernovae is potentially an important source of uncertainty when deriving cosmological parameters from the measured brightness of Type Ia supernovae, especially in deep surveys with scarce statistics. Photometric and spectroscopic measurements of foreground galaxies along the lines-of-sight of 33 supernovae discovered with the Hubble Space Telescope, both core-collapse and Type Ia, are used to model the magnification probability distributions of the sources. Modelling galaxy halos with SIS or NFW-profiles and using M/L scaling laws provided by the Faber-Jackson and Tully-Fisher relations, we find clear evidence for supernovae with lensing (de)magnification. However, the magnification distribution of the Type Ia supernovae used to determine cosmological distances matches very well the expectations for an unbiased sample, i.e.their mean magnification factor is consistent with unity. Our results show that the lensing distortions of the supernova brightness can be well understood for the GOODS sample and that correcting for this effect has a negligible impact on the derived cosmological parameters.

astro-ph↗