ESA and Durham University
A mysterious ‘cold spot’ in the Universe is not caused by a massive void, according to a recent paper.
According to the authors of the paper, the most likely explanation for the spot is our Universe, in its early stages, collided with another ‘bubble’ Universe – so-called because it grows like a bubble out of a vacuum.
The cosmic microwave background (CMB) is a relic of the Big Bang covering the whole sky. By peering into space at great distances, astronomers can look back in time to study the Universe in its early stages.
The CMB is a temperature of 2.73 degrees above absolute zero, -270.43 degrees Celsius, but it has some anomalies. These include the Cold Spot, an area spanning 1.8 billion light years across that was discovered in 2004 and is about 0.00015 degrees colder than its surroundings.
In 2015, a paper claimed the most likely explanation for the spot was a huge void, billions of light years across, containing relatively few galaxies. Now, a team of researchers led by Professor Tom Shanks from Durham University has studied the spot in further detail, and found this is not the case.
“Some authors claimed that such a supervoid had been found in the galaxy distribution,” Shanks told WIRED. “But they were using less accurate “photometric redshifts” to make their survey.”
Galaxies that are further away have their light shifted to longer wavelengths, an effect known as a cosmological redshift, caused by the expansion of the Universe.
By measuring the colours of galaxies, their redshifts, and thus their distances, can be estimated. These measurements have a high degree of uncertainty, and the new paper used a more precise method.
“When we used highly accurate spectroscopic redshift to make our survey we failed to find their supervoid,” Shanks told WIRED. “So this rules out the idea that a foreground supervoid could have caused the Cold Spot.”
Now the supervoid theory has been ruled out, the question as to what causes the spot still remains.
This shows the 3D galaxy distribution in the foreground of the CMB Cold Spot (black) where each point is a galaxy, compared to a scenario with no Cold Spot (red). The number of dots in each are similar, suggesting there are no ‘voids’
“There is still a chance that the Cold Spot could have been caused by a statistical fluctuation in the standard cosmology,” Shanks told WIRED, “but only 1 in 50 simulated universes show a similar feature.”
The most likely explanation other than a statistical fluctuation is our Universe was involved in a huge collision in its early stages.
“Of the more exotic explanation, an early collision with another bubble Universe could be regarded as the most likely, given that standard inflation theory predicts trillions of such universes,” said Shanks.
For this to have been the case, there must be an infinite number of other Universes, a theory known as the ‘multiverse’ theory. This theory, which has been around for years, says each of these Universes has a different version of reality.
Last year, the astronomer Martin Rees, Astronomer Royal, mentioned the multiverse theory when WIRED met Rees in Tenerife, Spain at the Starmus astrophysics conference.
“Our universe could be just one island in a vast cosmic archipelago,” he said. “Far beyond the horizon we could all have avatars.”
There are still other possible explanations for the Cold Spot, Shanks says.
“It could be evidence for a topological defect like a texture created by symmetry breaking in early Universe,” he said, “like when water crystallises to ice- but inflation is liked because it inflates away, ie erases the evidence for, such defects.”
Further study of the CMB will provide the answers. “The most standard explanation is the bubble collision and the multiverse,’ Shanks told WIRED, “and this hypothesis is still testable via CMB polarisation maps from Planck and future CMB satellites.”
The paper was published in Monthly Notices of the Royal Astronomical Society.