Beyond Feedback: Disentangling Baryonic Effects with tSZ$\times$FRB Cross-Correlations

Imagine the universe as a giant, invisible ocean. Most of the "stuff" that makes up this ocean isn't the stars and galaxies we can see with our telescopes; it's a hot, diffuse gas that fills the space between them. For a long time, astronomers knew this gas was there, but they couldn't quite figure out what it was doing. It's like trying to understand the weather in a room you can't see, only by feeling the breeze on your skin. Two main forces are at play here: "feedback," which is like a cosmic fan blowing gas out of clusters of galaxies, and "non-thermal pressure," which is like invisible, bouncy springs holding the gas up from the inside. The problem is that when we look at the gas using our best tools, these two forces look exactly the same. They are "degenerate," a fancy word meaning they are so tangled together that we can't tell which one is which. If we can't untangle them, we can't accurately weigh the universe or understand how galaxies grow.

Enter a new pair of detectives: the thermal Sunyaev-Zel'dovich (tSZ) effect and Fast Radio Bursts (FRBs). Think of the tSZ effect as a camera that takes a picture of the gas's pressure (how hard it's pushing). It sees both the heat from the "fan" and the bounce from the "springs." Now, imagine FRBs as cosmic lighthouses sending out rapid radio flashes. As these flashes travel through the universe, they get slowed down by the gas they pass through. This slowing down, called the "dispersion measure," acts like a ruler that only counts the amount of gas (density), completely ignoring how hot or bouncy it is. For years, scientists thought these two tools were just two different ways of looking at the same thing. But a new paper suggests that by comparing the "pressure picture" with the "density ruler," we might finally be able to separate the fan from the springs.

In this study, Isabel Medlock and Daisuke Nagai from Yale University propose a clever new way to use these two cosmic tools together. They didn't just look at the data; they built a sophisticated digital model called "Baryon Pasting" to simulate how gas behaves in galaxy clusters. In their simulation, they could turn the "feedback fan" up or down and adjust the "non-thermal springs" independently. They then asked a simple question: If we look at the universe with both our pressure camera and our density ruler at the same time, can we tell the difference between a gas cloud being pushed by a fan versus one being held up by springs?

The answer is a resounding yes, but with a catch. The authors found that looking at just one of these signals leaves the two forces hopelessly mixed up, like trying to taste salt and sugar in a drink when they are perfectly blended. However, when they combined the tSZ data with the FRB data, the mix unraveled. The magic happens in the "inner neighborhoods" of galaxy clusters (specifically at scales smaller than 3000 units on the sky map). In these regions, the two tools react differently: changing the feedback fan changes both the pressure and the density, but changing the springs only changes the pressure. By watching how the signals shift together, the model can isolate the two effects.

When the authors applied this method to real data that has already been collected, they found that the universe seems to have a stronger "fan" (feedback) than some previous, weaker models suggested. They ruled out the idea that feedback is very weak, setting a lower limit on its strength. However, they also discovered that with current data, they still can't measure the "springs" (non-thermal pressure) very precisely; the data is just a bit too noisy to pin down exactly how bouncy the gas is.

Looking ahead, the paper forecasts what will happen when we get better tools. They predict that in the coming decade, with the arrival of massive new radio telescopes like DSA-2000 and ultra-sensitive space missions like CMB-HD, we will be able to measure the "springiness" of the gas with incredible precision—down to about 2.3%. While we aren't there yet, this new method of cross-checking pressure against density offers a clear path forward. It promises to turn our blurry, confusing view of the cosmic ocean into a sharp, detailed map, helping us finally understand how the invisible gas shapes the visible universe.