A mysterious cold spot in the cosmic microwave background could be a cosmic defect called a “texture” that was left over from the aftermath of the Big Bang, according to physicists in Spain and the UK. If confirmed, the existence and properties of such a spot would provide physicists with a unique window on the early universe and allow them to test different theories for the unification of the fundamental forces of nature (Science DOI: 10.1126/science.1148694).
The cosmic microwave background (CMB) was born when the universe was about 380,000 years old. Before this time, space was filled with hot plasma that did not allow light to travel very far without being scattered. But as the universe expanded, the plasma cooled enough to allow neutral atoms to form. This “decoupling” of matter and radiation suddenly enabled photons to travel across space largely unimpeded, their wavelengths being stretched over time to produce a faint glow of radiation in the microwave region that we can detect today.
While the CMB is remarkably uniform, it appears to contain a cold spot that is over a billion light years across. The spot was first seen in images taken by NASA’s WMAP satellite in 2004, as it mapped out the tiny fluctuations in the CMB’s temperature. Various possibilities have been suggested, such as instrumental effects, foreground contamination from the Milky Way, the effect of rotating universes and giant voids bereft of galaxies – but none of these are very convincing.
Now, Marcos Cruz, Patricio Vielva and Enrique Martínez-González at the Institute of Physics of Cantabria (IFCA), Spain, as well as Neil Turok and Michael Hobson at the University of Cambridge believe that a better explanation for the cold spot can be found in events that occurred in the very early universe.
The team suggests that as the universe expanded and cooled after the Big Bang, it underwent a series of phase transitions, similar to water freezing into ice. These transitions corresponded to the breaking of fundamental symmetries that occurred as the fundamental particles and forces of nature separated out from the single unified substance that was the very early universe. Importantly, just as misalignments that occur in the crystallization of water can lead to visible defects like cloudy spots in ice, misalignments in this symmetry breaking pattern could form cosmic defects.
“Depending on the nature of the symmetry being broken, different types of defect can form, such as cosmic strings,” explains Hobson. “Our work investigates the exciting possibility that the cold spot is due to the presence of a cosmic texture – a three-dimensional object like a knot of energy anything between a few millimetres and many light-years across.”
These knots are predicted to collapse and unravel, concentrating mass into a rapidly shrinking region and creating a strong gravitational field that attracts nearby matter. The gravitational field could also affect the energy of CMB photons passing through the texture, leading to a cold spot in the CMB.
If confirmed, a cosmic defect could provide invaluable information about how the fundamental forces and particles evolved in the very early universe. Moreover, it could allow cosmologists to study the physical processes that happened at the extremely high energies prevalent in the aftermath of the Big Bang, which are far beyond those accessible to any terrestrial particle-physics experiment.
According to the researchers have performed a statistical analysis ranking that suggest that their explanation is the most probable of those put forth so far, they are careful to say that more evidence is needed to support their conclusion. Further measurements required include determining the pattern of polarization of the CMB radiation from the cold spot, searching for additional textures and looking for gravitational lensing of background objects by the texture. Some of this could be done over the over the next decade.
“The texture hypothesis is the most convincing explanation for the cold spot, although it still remains far from compelling,” says Jason McEwen an astrophysicist at the University of Cambridge, who was not involved in the research. “One of the most promising aspects is that tests may be performed using future observations of CMB polarization. If the cold spot does indeed turn out to be a texture, this will be a revolutionary discovery, probing the highest energies yet studied by physicists and extending our knowledge of the very early Universe.”