Scientists may have found a new way to unlock the vast secrets of the Big Bang—the cosmic event thought to have kicked off the expansion of the universe billions of years ago. The revelation came in 2023, when scientists found nearly imperceptible ripples within the very fabric of space and time as we know it.

The ripples appear to be associated directly with rapidly spinning neutrons that we call pulsar timing arrays. Researchers believe that studying gravitational waves—more specifically, the low-frequency background hum they emit—may allow us to learn more about the Big Bang and the universe’s very beginning.

For a long time, researchers have believed that the low-frequency background hum of gravitational waves in our universe was part of a “phase transition” that occurred just after the Big Bang. However, a new bit of research could further unlock the secrets of the Big Bang and suggests that this might not be the case at all.

Several pulsar timing array collaborations recently reported evidence of a stochastic gravitational wave background (SGWB) at nHz frequencies. While the SGWB could originate from the merger of supermassive black holes, it could be a signature of new physics near the 100 MeV scale. Supercooled first-order phase transitions (FOPTs) that end at the 100 MeV scale are intriguing explanations, because they could connect the nHz signal to new physics at the electroweak scale or beyond. Here, however, we provide a clear demonstration that it is not simple to create a nHz signal from a supercooled phase transition, due to two crucial issues that could rule out many proposed supercooled explanations and should be checked. As an example, we use a model based on nonlinearly realized electroweak symmetry that has been cited as evidence for a supercooled explanation.

In this episode of Cosmology 101, we dive into the concept of an expanding universe. From the first moments of the Big Bang, our cosmos has been stretching in every direction. We explore what this expansion means for us, how we know it’s happening, and the fascinating implications of living in an ever-growing universe.

Join Katie Mack, Perimeter Institute’s Hawking Chair in Cosmology and Science Communication, on an incredible journey through the cosmos in our new series, Cosmology 101.

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