According to the results of a redshift survey of nearly 7,000 galaxies, a gigantic ‘hole’ in the Universe is unlikely to explain a ‘Cold Spot’ in the Cosmic Microwave Background, leaving room for exotic explanations like a collision between universes.
Map of the CMB; the insets show the environment of the Cold Spot. Image credit: ESA Planck Collaboration / Gergö Kránicz.
The Cosmic Microwave Background (CMB) provides the earliest snapshot of the evolution of the Universe.
At a temperature of 2.73 degrees Kelvin (minus 454.77 degrees Fahrenheit, or minus 270.43 degrees Celsius), the CMB has some anomalies, including the Cold Spot.
The Cold Spot, which is 0.00015 degrees Kelvin colder than its surroundings, was discovered in 2004 by P. Vielva et al while examining a map of the CMB created by NASA’s Wilkinson Microwave Anisotropy Probe (WMAP).
This feature was previously claimed to be caused by an enormous ‘hole,’ or supervoid, a vast region approximately 1.8 billion light-years across in which the density of galaxies is much lower than usual in the known Universe.
In seeking to confirm or dispel a link between a supervoid and the Cold Spot, a research team led by University of Durham scientists conducted a comprehensive redshift survey of 6,879 galaxies.
From their dataset, the researchers see no evidence of a supervoid capable of explaining the Cold Spot within the standard theory.
They instead found that the Cold Spot region is split into smaller voids, surrounded by clusters of galaxies. This ‘soap bubble’ structure is much like the rest of the Universe.
3D galaxy distribution in the foreground of the Cold Spot, where each point is a galaxy. The galaxy distribution in the Cold Spot (black points, at right) is compared to the same in an area with no background Cold Spot (red points, at left). The number and size of low galaxy density regions in both areas are similar, making it hard to explain the existence of the Cold Spot by the presence of ‘voids.’ Image credit: Mackenzie et al.
“The voids we have detected cannot explain the Cold Spot under standard cosmology,” said Ruari Mackenzie, a postgraduate student in the Department of Physics and a member of the Centre for Extragalactic Astronomy at the University of Durham.
“There is the possibility that some non-standard model could be proposed to link the two in the future but our data place powerful constraints on any attempt to do that.”
If there really is no supervoid that can explain the Cold Spot, simulations of the standard model of the Universe give odds of 1 in 50 that the Cold Spot arose by chance.
“This means we can’t entirely rule out that the Cold Spot is caused by an unlikely fluctuation explained by the standard model,” said Tom Shanks, a professor in the Department of Physics and a member of the Centre for Extragalactic Astronomy at the University of Durham.
“But if that isn’t the answer, then there are more exotic explanations.”
“Perhaps the most exciting of these is that the Cold Spot was caused by a collision between our Universe and another bubble Universe,” Prof. Shanks said.
“If further, more detailed, analysis of CMB data proves this to be the case then the Cold Spot might be taken as the first evidence for the Multiverse — and billions of other universes may exist like our own.”
“For the moment, all that can be said is that the lack of a supervoid to explain the Cold Spot has tilted the balance towards these more unusual explanations, ideas that will need to be further tested by more detailed observations of the CMB.”
Details of the research were recently published in the Monthly Notices of the Royal Astronomical Society. The article is also publicly available at arXiv.org.
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Ruari Mackenzie et al. Evidence against a supervoid causing the CMB Cold Spot. Mon Not R Astron Soc, published online April 25, 2017; doi: 10.1093/mnras/stx931
