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Cosmological Applications of Lensing and Intrinsic Correlations of Ellipticities

Final Report Summary - CALICE (Cosmological Applications of Lensing and Intrinsic Correlations of Ellipticities)

The expansion of the Universe is known since the 1920s through the observation of the motion of galaxies. Since those early times, various generations of researchers have identified other astrophysical observables that allow one to probe the geometry and dynamics of the Universe. In the late 1990s two independent groups noticed that the light from distant Supernovae (exploding stars emitting a very luminous burst of radiation that can outshine an entire galaxy for a brief period of time) were systematically dimmer than expected. This led to the hypothesis that those stars and their host galaxies were more distant than predicted by the theory of gravity (the general relativity), which in turn might indicate that the expansion of the Universe was accelerating. From these and other observations, such as the discovery of anisotropies in the cosmic microwave background (CMB, the background radiation that permeates the whole Universe and was emitted very early on when the age of the Universe was only 0.003% of its current age), a coherent picture of the Universe started to emerge during the last decade. In this picture, which has become the standard model of cosmology, matter only makes up 25% of all that exists in the Universe. The remaining 75% is a form of energy with puzzling repulsive properties capable of accelerating the Universe. This form of energy was dubbed dark energy. More puzzling yet, the smallness of the CMB anisotropies shows that the attractive properties of matter are not strong enough to form bound structures such as galaxies or stars. This led to the hypothesis that matter made from known atoms and molecules only accounts for 5% of the contents of the Universe (and it is found in the form of galaxies, stars, gas, and all other radiation-emitting astrophysical objects) while the remaining 20% are in the form of a non-luminous unknown substance with strong attractive properties. This form of matter was named dark matter.

CALICE (Cosmological Application of Lensing and Intrinsic Correlations of Ellipticities) is a research project aimed at addressing some of the open questions of Cosmology focussing on a particular cosmological probe: Cosmological Gravitational Lensing. Gravitational lensing describes how light is deflected as it passes through the gravitational field generated by matter. It is well-known from relativity theory and from Einstein’s famous mass-energy equivalence that matter interacts gravitationally with any form of mass or energy. In particular, the mysterious dark matter interacts gravitationally with other forms of matter and energy, including light. This implies that dark matter structures lying between the telescope and a background source of light, such as a distant galaxy, or the CMB photons, will alter the trajectory of that emitted light. This will produce a distortion on the observed image: the dark matter works effectively as a lens. The measurement of coherent distortions across the sky is thus an indirect detection of the presence of invisible matter. Observing the coherent distortions on background galaxies at various distances (and consequently at various periods of the Universe history) will show us the dark matter distribution at different stages of the expansion of the Universe. This “tomographic” weak lensing contains thus information on the properties of the expansion and thus on dark energy and on the properties of gravitation on cosmological scales.

The methodology followed in CALICE was based on the development of a parameter estimation pipeline, made of a set of programming codes. The pipeline includes first the theoretical modelling of a dark energy or gravitation field. Then from this assumed cosmological model the pipeline is able to predict the corresponding gravitational lensing signal. On the other hand, the pipeline is fed with observed or simulated data. Theoretical modelling and data analysis are finally merged in a statistical Bayesian analysis to evaluate the likelihood of the theoretical models given the data, assessing the viability of the model and constraining its parameter values.

With this approach CALICE was able to reach some conclusions. First it studied the viability of modifications of the gravitation theory. For this, it parameterized modifications in the equations of general relativity. The impact of this theoretical modelling on tomographic weak lensing was computed and compared to recent measurements of a gravitational lensing signal on small scales, observed in the COSMOS survey field with the Hubble Space Telescope. It concluded that the observed signal could be well explained with the standard theory of gravitation, and no modifications were apparent at this precision level. This was a contribution to place limits on the allowed range of such modifications and helping in defining viable models of modified gravity. Modified gravity is an active area of research since it is a possible explanation for the late-time acceleration of the Universe. CALICE also addressed the competitive explanation for the acceleration; the presence of a dark energy component. In this case, it considered dark energy models containing late-time oscillations in the expansion rate of the Universe. These models were confronted with forecasted future data and the amplitude of viable oscillations was constrained.

CALICE also produced its own measurements. It attempted to detect the gravitational lensing effect that should be present in cosmic microwave background data. The data available at the time (WMAP) did not have in principle sufficient signal-to-noise ratio to allow for such a detection. We developed a variation of the standard estimation and studied its possible systematic effects. Then on a second publication we applied it to the WMAP data. The results of the measurements were not conclusive. In the mean time higher resolution CMB data, produced by the Planck satellite, were released with a well defined lensing detection.

These results are a small contribution to the large international collaborative effort that is being done in the framework of the European Space Agency’s Euclid Mission. The Euclid space telescope will be launched in 2020 and will stay in operation for 6 years. During that 6-year period it will map the extragalactic sky with a step and stare strategy with a large exposure time such that the density of observed distant galaxies will be greater than 30 galaxies per square arc-minute, which means a total of around 2 billion galaxies over 15000 square degrees of the extragalactic sky. This will allow to map with high-resolution the gravitational lensing signal across the sky, promising to revolutionize our knowledge of the Universe and determining in a decisive way the need for dark energy or modifications of general relativity. Given the strong links of CALICE with Euclid and being the only project of its kind in the host country (Portugal), the fellow was able to make a successful application that resulted in making Portugal a full member of the Euclid Consortium. The fellow became the national scientific coordinator of Euclid and national contact point, which boosted his recognition and integration in the national community.

This national membership gives the opportunity to any researcher from any research centre in Portugal to apply for individual membership in the Euclid consortium and join the Euclid Science Working Groups (SWG) more appropriate to his/her expertise. Nowadays, the scientific preparation of this mission is a main driver of the forefront research in cosmology, and it would be damaging for a national strategy in cosmology to be left out of this mission. The national coordination also brings the responsibility of delivering a technical mandatory contribution to the implementation of the mission. This introduced a new goal and tasks to CALICE at half-time of the project duration. The main goal is to develop algorithms that, in an automated and optimal way, are able to produce a schedule of a full coverage of the survey (including a 6-year wide survey, calibration targets, and additional deeps surveys) while verifying all technical and scientific constraints, such as the range of the rotation angles of the telescope, the amount of propeller, minimum area coverage, desired image quality or required targets. The reference survey thus produced is analysed by the Mission Operations Centre of ESA for operational validation. This is a central product of the implementation phase of Euclid of great interest for all Euclid group coordinators and as such puts the group in a position of great visibility. The active participation in this task greatly opened up the possibility for international collaborations with other research centres and put the group in close contact with ESA departments and with industry. The first results from CALICE activity in this task are already published in a proceeding and in a technical report.