Archive for weak gravitational lensing

The Most Ancient Heavens

Posted in Art, Biographical, Poetry, The Universe and Stuff with tags , , , , , , , , on March 21, 2019 by telescoper

So here I am, in that London, getting ready for the start of a two-day conference at the Royal Astronomical Society on cosmology, large-scale structure, and weak gravitational lensing, to celebrate the work of Professor Alan Heavens, on (or near) the occasion of his 60th birthday. Yes, it is a great name for an astronomer.

I was honoured to be invited to give a talk at this meeting, though my immediate reaction when I was told about was `But he can’t be sixty! He’s only a few years older than me…oh.’ I gather I’m supposed to say something funny after the conference dinner tomorrow night too.

Courtesy of alphabetical order it looks like I’m top of the bill!

Anyway, I’ve known Alan since I was a research student, i.e. over thirty years, and we’re co-authors on 13 papers (all of them since 2011). I’m looking forward to the HeavensFest not only for the scientific programme (which looks excellent) but also for the purpose of celebrating an old friend and colleague.

Just to clear up a couple of artistic points.

First, the title of the meeting, The Most Ancient Heavens, is taken from Ode to Duty by William Wordsworth.

Second, the image on the conference programme shown above is a pastiche of The Creation of Alan Adam which is part of the ceiling of the Sistine Chapel painted by Michelangelo di Lodovico Buonarroti Simoni, known to his friends as Michelangelo. Apparently he worked flat out painting this enormous fresco. It was agony but the ecstasy kept him going. I’ve often wondered (a) who did the floor of the Sistine Chapel and (b) how could Michelangelo create such great art when it was so clearly extremely cold? Anyway, I think that is a picture of Alan at high redshift on the far right, next to the man with beard who at least had the good sense to wear a nightie to spare his embarrassment.

Anyway, that’s all for now. I must be going. Time for a stroll down to Piccadilly.

Update: you can find a bunch of pictures of this conference here.

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Subaru and Cosmic Shear

Posted in The Universe and Stuff with tags , , , , , , on February 15, 2019 by telescoper

Up with the lark this morning I suddenly remembered I was going to do a post about a paper which actually appeared on the arXiv some time ago. Apart from the fact that it’s a very nice piece of work, the first author is Chiaki Hikage who worked with me as a postdoc about a decade ago. This paper is extremely careful and thorough, which is typical of Chiaki’s work. Its abstract is here:

The work described uses the Hyper-Suprime-Cam Subaru Telescope to probe how the large-scale structure of the Universe has evolved by looking at the statistical effect of gravitational lensing – specifically cosmic shear – as a function of redshift (which relates to look-back time). The use of redshift binning as demonstrated in this paper is often called tomography. Gravitational lensing is sensitive to all the gravitating material along the line of sight to the observer so probes dark, as well as luminous, matter.

Here’s a related graphic:

The article that reminded me of this paper is entitled New Map of Dark Matter Spanning 10 Million Galaxies Hints at a Flaw in Our Physics. Well, no it doesn’t really. Read the abstract, where you will find a clear statement that these results `do not show significant evidence for discordance’. Just a glance at the figures in the paper will convince you that is the case. Of course, that’s not to say that the full survey (which will be very much bigger; the current paper is based on just 11% of the full data set) may not reveal such discrepancies, just that analysis does not. Sadly this is yet another example of misleadingly exaggerated science reporting. There’s a lot of it about.

Incidentally, the parameter S8 is a (slightly) rescaled version of the more familiar parameter σ8  – which quantifies the matter-density fluctuations on a scale of 8 h-1 Mpc – as defined in the abstract; cosmic shear is particularly sensitive to this parameter.

Anyway, if this is what can be done with just 11%, the full survey should be a doozy!

Cosmological Results from the Dark Energy Survey

Posted in The Universe and Stuff with tags , , , , , on August 4, 2017 by telescoper

At last the Dark Energy Survey has produced its first cosmological results. The actual papers have not yet hit the arXiv but they have been announced at a meeting in the USA and are linked to from this page.

I’ll jump straight to this one, which shows the joint constraints on S8 which is related to σ8 (a measure of the level of fluctuations in the cosmological mass distribution) via S8= σ8m/0.3)0.5 against the cosmological density parameter, Ωm.

These constraints, derived using DES Y1 measurements of galaxy clustering, galaxy-galaxy lensing, and weak lensing cosmic shear are compared with those obtained from the cosmic microwave background using Planck data, and also combined with them to produce a joint constraint. Following usual practice, the contours are 68% and 95%  posterior probability regions.

The central values of DES and Planck values are different, but the discrepancy is only marginal. Compare this with a an equivalent diagram from a paper I discussed last year.

The KIDS analysis used to produce this plot uses only weak lensing tomography, so you can see that using additional measures reduces the viable region in this parameter space.

It’s great to see new data coming in, but at first sight it seems it is tending to confirm the predictions of the standard cosmological model, rather than providing evidence of departures from it.

Incidentally, this little video shows the extent to which the Dark Energy Survey is a global project, including some of my former colleagues at the University of Sussex!

 

KiDS-450: Testing extensions to the standard cosmological model [CEA]

Posted in The Universe and Stuff with tags , , , on October 19, 2016 by telescoper

Since I’ve just attended a seminar in Cardiff by Catherine Heymans on exactly this work, I couldn’t resist reblogging the arXiver entry for this paper which appeared on arXiv a couple of days ago.

The key finding is that the weak lensing analysis of KIDS data (which is mainly to the distribution of matter at low redshift) does seem to be discrepant with the predictions of the standard cosmological model established by Planck (which is sensitive mainly to high-redshift fluctuations).

Could this discrepancy be interpreted as evidence of something going on beyond the standard cosmology? Read the paper to explore some possibilities!

arXiver

http://arxiv.org/abs/1610.04606

We test extensions to the standard cosmological model with weak gravitational lensing tomography using 450 deg$^2$ of imaging data from the Kilo Degree Survey (KiDS). In these extended cosmologies, which include massive neutrinos, nonzero curvature, evolving dark energy, modified gravity, and running of the scalar spectral index, we also examine the discordance between KiDS and cosmic microwave background measurements from Planck. The discordance between the two datasets is largely unaffected by a more conservative treatment of the lensing systematics and the removal of angular scales most sensitive to nonlinear physics. The only extended cosmology that simultaneously alleviates the discordance with Planck and is at least moderately favored by the data includes evolving dark energy with a time-dependent equation of state (in the form of the $w_0-w_a$ parameterization). In this model, the respective $S_8 = sigma_8 sqrt{Omega_{rm m}/0.3}$ constraints agree at the $1sigma$ level, and there is `substantial concordance’ between…

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(Guest Post) The GREAT10 Challenge

Posted in The Universe and Stuff with tags , , on December 8, 2010 by telescoper

I haven’t had any guest posts for a while, so I was happy to respond to an offer from Tom Kitching to do one about the GREAT10 challenge. I’ve been working a bit on weak gravitational lensing myself recently – or rather my excellent and industrious postdoc Dipak Munshi has, and I’ve been struggling to keep up! Anyway, here’s Tom’s contribution…

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This guest post is about the the GREAT10 challenge, which was launched this week, I’ll briefly explain why this is important for cosmology, what the GREAT10 challenge is, and how you can take part. For more information please visit the website, or read the GREAT10 Handbook.

GREAT10 is focussed on weak gravitational lensing. This is an effect that distorts the shape of every galaxy we see, introducing a very small additional ellipticity to galaxy images. Weak lensing is a interesting cosmological probe because it can be used to measure both the rate of growth of structure and the geometry of the Universe. This enables extremely precise determinations of dark energy, dark matter and modified gravity. We can either use it to make maps of the dark matter distribution or to generate statistics, such as correlation functions, that depend sensitively on cosmological parameters.

As shown in the Figure (click it for a higher-resolution version), the weak lensing effect varies as a function of position (left; taken from Massey et al. 2007), which can be used to map dark matter (centre) or the correlation function of the shear can be constructed (right; taken from Fu et al. 2008).

However, the additional ellipticity induced by weak lensing generates only about a 1% change in the surface brightness profile for any galaxy, far too small to been seen by eye, so we need to extract this “shear” signal using software and analyse its effect statistically over many millions of galaxies. To make things more complicated,  images contain noise, and are blurred by a PSF (or convolution kernel) caused by atmospheric turbulence and telescope effects.

So the image of a galaxy is sheared by the large scale structure, then blurred by the PSF of the atmosphere and telescope, and finally distorted further by being represented by pixels in a camera. Star images are not sheared, but are blurred by the PSF. The challenge is to measure the shear effect (which is small) in the presence of all these other complications.

GREAT10 provides an environment in which algorithms and methods for measuring the shear, and dealing with the PSF, can be developed. GREAT10 is a public challenge, and we encourage everyone to take part, in particular we encourage new ideas from different areas of astronomy, computer science and industry. The challenge contains two aspects :

  • The Star Challenge : Is to the reconstruct the Point Spread Function, or convolution kernel, in astronomical images, which occurs because of the slight blurring effects of the telescope and atmosphere. The PSF varies across each image and is only sparsely sampled by stars, which are pixelated and noisy. The challenge is to reconstruct the PSF at non-star positions.
  • The Galaxy Challenge : Is to measure the shapes of galaxies to reconstruct the gravitational lensing signal in the presence of noise and a known Point Spread Function. The signal is a very small change in the galaxies’ ellipticity, an exactly circular galaxy image would be changed into an ellipse; however real galaxies are not circular. The challenge is to measure this effect over 52 million galaxies.

The challenges are run as a competition, and will run for 9 months. The prize for the winner is a trip to the final meeting at JPL, Pasadena, and an iPad or similar (sorry Peter! I know you don’t like Apple), but of course the real prize is the knowledge that you will have helped in creating the tools that will enable us to decipher the puzzle of understanding our Universe.

For more discussion on GREAT10 see MSNBC, WIRED and NASA.

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EDITOR’S NOTE: I assume that second prize is two iPads…


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