Category Archives: Euclid

Compromised Cosmic Shear: Approximations That Bias Cosmology

The distortion of the images of distant galaxies by the gravity of the large-scale structure of the Universe can be a powerful tool to help us understand our Universe. By measuring this distortion, known as cosmic shear, we can constrain cosmological parameters. However, in our analyses, we make certain approximations that may no longer be valid. Anurag Deshpande, a second-year PhD student tells us about two such effects; the reduced shear approximation and magnification bias.

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Non-parametric Cosmology with Cosmic Shear

Cosmology is in pretty bad shape; we don’t know what makes up 95% of the Universe. Galaxies spin too fast and the expansion of the Universe is unexpectedly accelerating. Cosmologists deal with this by inferring the existence of dark matter and dark energy. These form the backbone of the Lambda Cold Dark Matter (LCDM) model. The problem is that we have no physical explanation for the existence of these two components.

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The National Astronomy Meeting 2013

This week Alvina On, Dave Barnes and Idunn Jacobsen all PhD students at MSSL blog about the experience of attending the UK National Astronomy Meeting 2013 in St. Andrews, Scotland.


This year’s National Astronomy Meeting was located in a (partially) warm and sunny St. Andrews, Scotland, between 1st and 5th of July. With a programme incorporating a wide range of research areas, with talks ranging from Solar physics to Cosmology to the future of instrumentation, a reported 600 registered astronomers and space scientists descended on the quiet, coastal town.

NAM 2013 Banner.
NAM 2013 Banner

We arrived on Sunday evening, which we spent getting familiar with the town. It is usually dominated by students attending the university. However, due to the summer holidays, the town centre was very quiet, and our accommodation was a 20 minute walk out of the centre, so it was even quieter.

Between Monday and Friday we enjoyed a packed programme, with parallel sessions, plenaries and public talks. The frequent coffee and lunch breaks allowed attendees to meet and greet fellow researchers, engage in discussions both within their respective fields, as well as across areas, and to view the many posters that were complimenting the talks of the parallel sessions.

The poster room at NAM.
The poster room at NAM.

With a large fraction of sessions this year allocated to UK Solar Physics (UKSP) and Magnetosphere Ionosphere Solar-Terrestrial physics (MIST) meetings, it gave us from the numerical and theoretical corners of large-scale astrophysics a glimpse of astronomy on much smaller scales than we are accustomed to. The opportunity to speak to fellow PhD- students and astronomy researchers from other institutions was great, not only to discuss the science being done, but to have the chance to discuss career paths and progress, and the general news from the science community.

NAM2013 venue

A number of social events allowed everyone to meet in more relaxed settings, such as the Whiskey tasting event. This provided both the history of Scottish Whiskey, by Dr David Wishart from the host university, and, equally importantly, ample choice in whiskies – arranged by taste and complexity analogous to the Hertzsprung-Russell diagram, for the benefit of the audience.

The conference programme commenced Monday afternoon, with registration and a plenary talk to ease us in to the week ahead. Mike Thompson from HAO Boulder gave an insight into the study of the Solar interior, including the methods of helioseismology and modelling as ways of learning more about the parts of the Sun which cannot be reached by traditional means of detection. Tuesday’s plenary talks were on cosmology, with a special focus on the recent results from the Planck satellite, from the theoretical point of view by UCL’s Hiranya Peiris, leading us through the process of obtaining the CMB map to the current 6-parameter cosmological model, and from an observational perspective by Catherine Heymans of ROE Edinburgh, emphasising the puzzle of the dark Universe.

Wednesday’s plenaries were dedicated to astronomy at slightly smaller scales: first was Rob Kennicutt Jr. (of the Kennicutt-Schmidt relation) from IoA Cambridge – who, in the morning after a night of whiskey tasting, gave an insightful talk on star formation in the Universe, and down to galactic scales, promoting upcoming data from ALMA to greatly help fill in the unknowns, especially star formation at high redshifts. In the afternoon, we continued on galactic scales, with a discussion by Andy Lawrence of ROE Edinburgh (aka the eAstronomer) on the science done with mega-surveys, focusing around AGN and the high redshift quasar population, such as the furthest quasar known to date (at a redshift of just over 7). Its massive black hole leaves us wondering how it was able to form so early on in the history of the universe, and welcomes the large quasar surveys of PanSTARRS, WISE, UKIDSS and SDSS (and we got a short reminder of the future with Gaia and Euclid).

St. Andrews from the beach!
St. Andrews from the beach!

The parallel sessions ran on average twice a day, and for each of the 11 parallel sessions, we had about six topics to choose from. The areas of science presented in each were from all corners of research represented at the conference, thus for an astrophysics PhD student, the choice would primarily fall on the most relevant session in the group. A number of people from Astro-group chaired and presented talks and posters, as did we – David and Alvina presented talks in the Heating & Turbulence session, which were received well and accumulated some good questions. In addition, David had a poster on his GCMHD+ code for cosmological simulations, for which he won the UKSP student poster competition!

On the 5-hour train home, we’re leaving St. Andrews behind in bright sunshine, and look forward to next years NAM in Portsmouth !

Live from the Euclid Meeting!

In this post we have a live report from Sami Niemi, Euclid Visible Imager Instrument Scientist, from the Euclid Consortium Annual Meeting

This blog post discusses briefly the Euclid Mission and Euclid Consortium Meeting held in Leiden on May 13 – 16, 2013.


Our very best knowledge, based on many astronomical observables, implies that the Universe we live in is made mostly out of two entities we currently know rather little about. Because we know so little about them we have decided to call them simply dark energy and dark matter. Together these two dark components constitute about 95 per cent of the energy density of the Universe. We do know that these two entities interact with light and with more common material called baryons, we are made out of, via gravity. However, because we have not managed to detect any light from either dark energy or matter (hence the name “dark”), the little knowledge we have managed to gather thusfar is based on indirect probes. It is clearly unsatisfactory to not know about 95 per cent of everything that surrounds us, but how can we make progress on something we cannot directly see?

A part of the astronomical community had acknowledged the lack of knowledge in dark matter and energy already some time ago and hence decided to propose a space mission to the European Space Agency (ESA) to study the dark Universe. After competitive process two proposals were joined to form a single space mission to help solve the mysteries of dark energy and dark matter. The Euclid mission was born.

The Euclid mission will use two complementary probes, namely weak gravitational lensing and galaxy clustering, to study the dark Universe. The launch date for the Euclid mission is 2020. But before we can unravel the mysteries of the Universe, a lot of work is required to make the mission reality.

The Euclid Concept.
The Euclid Concept.


To help make the Euclid mission reality a Euclid Consortium (EC) was founded. The Euclid Consortium consists of scientists, engineers, project managers, and technical staff and it is the largest astronomical community in Europe with about 1150 members. In the current Euclid organisation, the EC is responsible for the definitions of the scientific goals, the science requirements and the Euclid survey. It is also in charge of the design, construction, tests, integration and delivery to ESA of the imaging and spectroscopic instruments (VIS and NISP); the design, development tests, integration and operation of the data processing tools, pipelines and data centers; and the scientific analysis and interpretation of the Euclid data.

Members of the EC are working all the way from hardware to building of large cameras through development of shape and clustering measurement algorithms to finally the cosmological parameters describing the dark energy and dark matter. The Consortium therefore consists of experts from many disciplines. To fully exploit and share the expertise a Consortium level meeting is organised yearly. It is also the place to learn about Euclid and all the cool science it will enable.


I, Sami-Matias Niemi (VIS Instrument Scientist), am writing this blog post from the fourth Euclid Consortium Meeting held in Stadsgehoorzaal in a historic city of Leiden. In many ways the yearly EC meeting is not your typical astronomical science meeting. Firstly, about 400 people participate this years meeting, implying a large astronomical meeting. Secondly, the meeting is a mixture of technical and engineering talks and science presentations from theory to simulations and finally observations. Thus, the meeting is very multidisiplinary and provides enourmous amount of information regarding Euclid.

The Stadsgehoorzaal.
The Stadsgehoorzaal.

Now when the first meeting day is behind us, it is safe to say that this years meeting is the largest EC meeting ever. Up to six parallel splinter sessions are running simultaneously, so one must choose carefully to which ones to attend. For the morning part I had decided to catch up on the simulation activities and chose to join the Euclid Simulations splinter session.

Given that it will still be many years before Euclid will see its first light, we currently must rely on lab data and simulations. However, simulations are increasingly important in any astronomical exploitations, not only to predict the performance but also in achieving the scientific accuracy required. For example, to derive the cosmological parameters describing dark matter and energy a suite of simulations are required, so that we can be sure that we have probed the parameter space in an unbiased fashion. Without advancements, both technical and mathematical, the current brute force simulations would require billions of computer hours, so that even with the largest super computers (1 million CPUs) the simulations would still take a decade to run. Clearly advancements are needed to make the problem more manageable.

After the simulation session a few plenary talks were presented. The EC lead Yannick Mellier spoke briefly about the Euclid mission and the milestones since the last year’s meeting. Rene Laureijs (ESA) recapped the Euclid mission history since the selection six years ago, while Guiseppe Racca (ESA) enlightened us about the ESA Euclid management structure. The last plenary session, shared between Jerome Amiaux and Jose Lonrenzo Alvarez, discussed system engineering aspects of this 1 Billion Euro space mission.

In the afternoon more parallel splinter sessions discussing for example photometric redshifts, calibrations, supernova science, and science ready data and catalogues took place. I shall not go into details, but simply say that I was awed by the amount of work that has taken place since the previous meeting. I must however now stop my report, I do have a presentation to give…

Sami Niemi's Euclid VIS Presentation
Sami Niemi’s Euclid VIS Presentation