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
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 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.
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.