Astrophysicists from the Institute of Cosmology and Gravitation at the University of Portsmouth, UK, have produced the largest ever map of cosmic superstructures (voids and superclusters), which helps solve a long-standing cosmological mystery.
The effect of voids and superclusters seen in patches of the cosmic microwave background (CMB). Photons of the CMB that have traveled through void regions on average appear slightly colder than average, and those coming from supercluster regions appear slightly hotter. The color scale shows the temperature differences, with blue being coldest and red hottest. Image credit: Seshadri Nadathur / Robert Crittenden.
The map of the positions of cosmic voids (vast, empty spaces which contain relatively few or no galaxies) and superclusters (large conglomerations of clusters and groups of galaxies) can be used to measure the effect of dark energy ‘stretching’ the Universe.
“We used a new technique to make a very precise measurement of the effect that these structures have on photons from the cosmic microwave background (CMB) – light left over from shortly after the Big Bang – passing through them,” said Dr. Seshadri Nadathur, lead author of the paper reporting the results in the Astrophysical Journal Letters (arXiv.org preprint).
“Light from the CMB travels through such voids and superclusters on its way to us. According to Einstein’s General Theory of Relativity, the stretching effect of dark energy causes a tiny change in the temperature of CMB light depending on where it came from.”
“Photons of light traveling through voids should appear slightly colder than normal and those arriving from superclusters should appear slightly hotter. This is known as the integrated Sachs-Wolfe (ISW) effect.”
“When this effect was studied by B.R. Granett et al in 2008 using an older catalogue of voids and superclusters, the effect seemed to be five times bigger than predicted. This has been puzzling scientists for a long time, so we looked at it again with new data.”
To create the map of cosmic superstructures, Dr. Seshadri Nadathur and his colleague, Prof. Robert Crittenden, used more than three-quarters of a million galaxies identified by the Sloan Digital Sky Survey. This gave them a catalogue of structures more than 300 times bigger than the one previously used.
The astrophysicists then used computer simulations of the Universe to predict the size of the ISW effect.
Because the effect is so small, they had to develop a new statistical technique to be able to measure the CMB data.
They applied this technique to CMB data from ESA’s Planck space telescope, and were able to make a very precise measurement of the ISW effect of the voids and superclusters.
Unlike in the previous work, they found that the new result agreed extremely well with predictions using Einstein’s gravity.
“Our results resolve one long-standing cosmological puzzle, but doing so has deepened the mystery of a very unusual ‘Cold Spot’ in the CMB,” Dr. Nadathur said.
“It has been suggested that the Cold Spot could be due to the ISW effect of a gigantic ‘supervoid’ which has been seen in that region of the sky. But if Einstein’s gravity is correct, the supervoid isn’t big enough to explain the Cold Spot.”
“It was thought that there was some exotic gravitational effect contradicting Einstein which would simultaneously explain both the Cold Spot and the unusual ISW results from B.R. Granett et al. But this possibility has been set aside by our new measurement – and so the Cold Spot mystery remains unexplained.”
_____
Seshadri Nadathur & Robert Crittenden. 2016. A detection of the integrated Sachs-Wolfe imprint of cosmic superstructures using a matched-filter approach. ApJL 830, L19; doi: 10.3847/2041-8205/830/1/L19
