High-frequency gravitational waves from a first-order phase transitions

First-order phase transitions in the early Universe are a well-motivated source of gravitational waves (GWs). In this paper, we identify a previously overlooked GW production mechanism: , arising from graviton emission by particles whose mass changes as they pass through expanding bubble walls. Unlike conventional sources, such as bubble collisions or sound waves, this mechanism operates at the microscopic scale set by the Lorentz-contracted wall thickness, leading to GW emission at significantly higher frequencies. The resulting spectrum features a distinctive shape with a peak frequency redshifting to <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:msub> <a:mi>f</a:mi> <a:mrow> <a:mi>peak</a:mi> </a:mrow> </a:msub> <a:mo>∼</a:mo> <a:msub> <a:mi>T</a:mi> <a:mn>0</a:mn> </a:msub> <a:mo>∼</a:mo> <a:msup> <a:mn>10</a:mn> <a:mn>10</a:mn> </a:msup> <a:mtext> </a:mtext> <a:mtext> </a:mtext> <a:mi>Hz</a:mi> </a:math> , where <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:msub> <c:mi>T</c:mi> <c:mn>0</c:mn> </c:msub> </c:math> is the current temperature of the Universe. This mechanism is generic and is expected to operate similarly for domain walls and other relativistic interfaces.