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Cosmology with voids and superclusters: combining theory, simulation and observation

Periodic Reporting for period 1 - COSMOVOID (Cosmology with voids and superclusters: combining theory, simulation and observation)

Reporting period: 2015-10-01 to 2017-09-30

This cosmology project developed novel methods of studying the contents, history and evolution of the Universe by analysing the distribution of matter on the largest scales, using voids and supercluster structures identified in galaxy surveys. The aims of the project were to propose new observables and new techniques of data analysis based on these structures in order to test theories of gravitation, dark energy and the initial conditions of the early Universe.

Since the discovery in 1998 that the expansion of the Universe is accelerating, understanding the properties of the dark energy that causes it has been one of the main problems in cosmology. The possibility that the accelerated expansion could be due instead to modifications to the theory of gravity on the largest scales also requires investigation. Data on the large-scale structure of the Universe obtained from galaxy redshift surveys provide one of the primary means to distinguish between these alternatives. In particular, the use of special rare structures in the galaxy distribution, known as voids and superclusters, provide novel probes of many of these theories. At the start of the project, voids in particular were already known to be useful for testing theories of gravity, but there were important shortcomings in theoretical understanding of their properties and precise predictions for their observable effects that could be measured in the data.

The three main objectives of the project were therefore identified as:
1. To use simulations to understand the properties and dynamics of voids and superclusters identified in galaxy data
2. To obtain predictions for observables to enable comparison of theory and data
3. To produce catalogues of voids and superclusters identified in the latest galaxy data from the Sloan Digital Sky Survey and the Dark Energy Survey, to enable observational tests of the results obtained from theory and simulation
The research work performed in this project report period focussed on understanding the properties of voids and superclusters, and their use in testing the current standard cosmological model. This work resulted in the publication of six scientific papers in major international journals. These papers have received a total of 157 citations to date according to the NASA Astronomy Database ADS.

The main results achieved are as follows:

1. Algorithms for identifying voids in galaxy survey data were improved, and a new method specifically designed for application to photometric datasets was developed. These methods were applied to state of the art data from the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES) to create public catalogues of void and supercluster structures for use in a variety of cosmological tests.
2. The properties of voids were studied using large cosmological simulations. New results were obtained identifying scaling relations between void properties and those of the underlying gravitational potential, and calibration of the matter and potential profiles around void centres, leading to improved predictions for their gravitational effects.
3. New statistical methods for measuring the gravitational imprints of voids through gravitational lensing and the integrated Sachs-Wolfe effect were developed.
4. Application of these methods to data catalogues created from SDSS and DES data resulted in the first detection of the void lensing effect from photometric data, and the highest precision measurement of the integrated Sachs-Wolfe imprint of voids and superclusters in the cosmic microwave background to date.

The list of publications produced is:

P1. P. Bull, Y. Akrami, et al. (including S. Nadathur), “Beyond ΛCDM: problems, solutions and the road ahead”, Phys. Dark Univ. 12, 56 (2016),
P2. S. Nadathur, “Testing cosmology with a catalogue of voids in the BOSS galaxy surveys”, Monthly Notices of the Royal Astronomical Society 461, 358 (2016)
P3. S. Nadathur and R. Crittenden, “A detection of the ISW imprint of cosmic superstructures using a matched-filter approach”, Astrophys. J. Lett. 830, L19 (2016)
P4. C. Sanchez, J. Clampitt, A. Kovács, B. Jain, J. Garcia-Bellido, S. Nadathur et al., “Cosmic Voids and Void Lensing in the Dark Energy Survey Science Verification Data”, Monthly Notices of the Royal Astronomical Society 465, 746 (2017)
P5. A. Kovács, C. Sanchez, J. Garcia-Bellido, S. Nadathur et al., “Imprint of DES superstructures on the Cosmic Microwave Background”, Monthly Notices of the Royal Astronomical Society 465, 4166 (2017)
P6. S. Nadathur, S. Hotchkiss and R. Crittenden, “Tracing the gravitational potential using cosmic voids”, Monthly Notices of the Royal Astronomical Society 467, 4067 (2017)

Results from this project were presented at 4 international meetings or conferences, and at 9 invited academic seminars at different university departments.
New superstructure data catalogues of voids and superclusters were created using state of the art data from the SDSS Data Release 12 and the DES Science Verification data. A novel and powerful matched-filter analysis technique for detection of the stacked integrated Sachs-Wolfe imprint of superstructures was proposed and was demonstrated to work by application to the SDSS DR12 catalogue and data from the Planck satellite, thus obtaining the highest precision measurement of the amplitude of the effect from stacking analyses to date. The first ISW stacking measurement using DES data was performed, and the first high-significance detection of the lensing effect of voids in photometric galaxy data was performed. Important insights into the relationship between voids in the galaxy distribution and fluctuations of the gravitational potential were obtained from the analysis of cosmological simulations, including the first discovery of a universal scaling relation between void properties and the potential, which enabled the development of the new stacked ISW analysis technique described above.

Societal impact: The scientific paper on the measurement of the ISW imprint of voids and superclusters was reported in several local and national media sources, including in the online section of a national newspaper in Britain. Following requests, I presented public lectures and talks about these results as well as others from the DES collaboration at the Winchester Science Festival and Stargazing Live public events, to combined audiences of about 150 people. I was also asked to write a public article about the paper for a general audience for The Conversation research news website.
Integrated Sachs-Wolfe imprints of voids (left) and superclusters (right) in the CMB.