Setting limits on phase transitions in the dark energy era

Using the cosmic microwave background (CMB) as a backlight to see the dark universe boil. False vacuum decay in a dark sector occurs in different places at different times. CMB photons from different directions enter the true vacuum region earlier or later, creating patterns (anisotropies) that observatories like Planck can discover or constrain. Researchers at the University of Notre Dame and Fudan University recently showed that fluctuations in the cosmic microwave background (CMB), small variations in the radiation left over from the Big Bang, could be used to probe phase transitions during the dark energy era. Using this theoretical approach, outlined in a paper published in Physical Review Letters, Seth Koren, Yuhsin Tsai and Runqing Wang examined a hypothetical first-order phase transition (FOPT) occurring after dark energy became predominant and placed limits on how much vacuum energy it could have released. A FOPT is a sudden change from one physical state to another that unfolds through the formation of bubbles and releases energy. "As we continue to find null results on interactions of dark sector particles with Standard Model particles—including, importantly, dark matter—we must take more seriously the possibility that these sectors may only interact with us gravitationally," Koren told Phys.org. "This 'nightmare case' is harder to probe, but it doesn't mean the dark sector is invisible. Our best hope is to look for the cosmological footprint of the dynamics of dark sectors, where the gravitational coupling with our sector may result in observable effects on our particles."

• "The researchers considered whether a hidden part of the universe could have undergone a similar phase transition more recently."
• "They showed that using CMB data, they could place tighter limits on the energy involved if the phase transition occurred unevenly."
• "They used the absence of detectable variations matching these predictions in existing CMB observations to place tight limits on the hypothetical phase transition."

In its early history, the universe underwent profound changes, including the electroweak and quantum chromodynamics (QCD) phase transitions. The researchers considered whether a hidden part of the universe could have undergone a similar phase transition more recently. "This is a good place to start testing how well we can see purely gravitational dark sector effects," said Koren. "While science fiction often portrays vacuum decay as an instantaneous, apocalyptic catastrophe, our work highlights that a phase transition in a secluded dark sector would be a silent cosmic event, entirely imperceptible in our daily lives. To observe or constrain such an event, we must carefully look at precise cosmological measurements to check for subtle changes in the spectrum of photons from the CMB."

Using CMB data to probe recent phase transitions. If the hypothetical phase transition considered by the researchers had occurred uniformly throughout the universe, the energy involved could only be constrained to around 10% of the total dark energy using observations of the universe's expansion. Yet if it occurred unevenly, with bubbles of the new state forming and spreading across space, astronomers could place tighter limits on the energy it released. "We show that we can use the cosmic microwave background as a backlight to probe these inhomogeneities by considering how the CMB photons coming to us from various directions would be redshifted anisotropically," said Koren. "This anisotropic effect can be discovered or constrained by experiments that probe the microwave sky, such as the Planck observatory."

Finally, another motivation for our study comes from quantum gravity and ideas about the 'Swampland.' Some physicists who specialize in string theory, the idea that the smallest building blocks in the universe are tiny vibrating strings as opposed to point-like particles, have proposed that the universe cannot expand forever. They hav

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