Was our Universe boiling?

October 25, 2023 by francesco costa

Their story starts in 1960 when the largest radio waves antenna was built in Holmdel, USA as a part of a satellite communication system. Radio waves are a type of light that carries way way less energy than the one we can see, which we call visible light.

Figure 1. Horn reflector antenna at Bell Telephone Laboratories in Holmdel, New Jersey 1959. Credit: NASA on The Commons.

We cannot see radio waves but we have antennas that can capture them. The idea of radio waves for communication is that you can make them travel long distance and bounce them off satellites to receive them on the other side of the globe. Penzian and Wilson at that time were studying the radio wave light coming from the Milky Way and did not care too much about radio communication, they were dying to use the antenna for research. Unfortunately they had to wait until 1962 when it was finally free from commercial use. After all the wait the two scientists were finally able to begin their work. However, they soon encountered a problem: they kept hearing background noise, similar to static on a radio. They thought that this could have been radio waves coming from some source they didn't take into account. So they tried to understand where it was coming from, maybe from New York? Or maybe from the pigeons nesting in the antenna? However, all their efforts were in vain. Finally, they came up with a solution: they decided to point their antenna towards a spot in the sky where no galaxies were visible. If they still detected

Figure 2. Light at different energies. What we can see is the visible light. It is only a small part of whole range of "light-types". Radio waves have less energy than visible light. Credit: GRAPPA.

the noise, the problem must have been with their instruments. The two scientists worked hard to understand what went wrong with their equipment but they could not find any problem. Still, they kept seeing the same radio wave "disturbance" coming from all the directions in the sky. The only explanation at this point was that there were actual radio waves coming from everywhere in the sky! We now know that this phenomenon is known as the Cosmic Microwave Background (CMB). Physicists named it so because it originates from the Universe or cosmos (C), and it consists of microwaves (M), which are a type of radio waves. The signal is always present in the background (B), which is why it is also referred to as the CMB.

This light comes from a very early time in the Universe and today it is the best way we have to know what was going on at that time, we can sort of see what the Universe looked like back then! But unfortunately this cosmic microwave background was formed thousands of years after the Big Bang. Therefore using this light we cannot observe anything that happened before this time. This is unfortunate because exploring the physics of earlier times is crucial as it may hold valuable insights into the Universe that are still unknown to us.

However physicists are not easily discouraged in their attempts to understand the Universe! In fact, there is an even cooler background signal or disturbance that was recently discovered, a background of gravitational waves! Similar to the one made of lightwaves, this is made of waves of gravity and it comes from every direction in the sky. Every gravitational wave antenna that is good enough will see it. In the same way Penzian and Wilson kept seeing that radiowave disturbance. So, what are these waves of gravity? The idea comes directly from the way we understand gravity - thanks Albert. In fact the best way we can describe gravity is as the warping of the spacetime that massive objects cause, and how the shape and warping of the spacetime determines how objects move in it. Spacetime is what we are immersed in everyday: it is just all the space and the time surrounding us. You can imagine it as a swimming pool. Imagine it like a cosmic swimming pool, and mass behaves like a drain in the middle of the pool's floor, which pulls all the water, and anything in it, towards it. Similarly, Earth's gravity pulls objects towards itself, like a drain in the pool of spacetime. Imagine now that there is an explosion underwater. This will create waves moving from the point of the explosion outwards everywhere in the pool. This is how we can picture gravitational waves, which are ripples in spacetime. These are not conjecture anymore: we know they are real, since we observed them for the first time in 2016!

Figure 3. An artist's impression of gravitational waves generated by two stars orbiting around each other. Credit: R. Hurt/Caltech-JPL.

Gravitational waves can come from very different sources. The ones that were observed in 2016 were caused by two black holes colliding which led to a big explosion sending out these waves. This happened at a specific time and place: we can calculate where and when it occurred given the time we received them and the direction they came from. However, the waves generated by a single event are not considered as background waves. They arrive at a specific time and then stop. But what if we had many explosions constantly occurring in the pool in different places; some closer, some further away; some stronger, some weaker. We would keep seeing waves coming towards us at all times. These ongoing waves would form a background: What we would observe on our gravitational wave antennas is the sum of all waves from various events. This is what we could see if the Universe was indeed boiling at some time in the past. But let us first understand what we mean by boiling.

Let's go back to something we know very well: boiling water. What we see when we boil water is a what we call a transition of phase: the water goes from being a liquid to being a gas as it becomes steam. If you watch water boil, you see bubbles forming in thewater. At this point, two phases exist at the same time: gas inside the bubbles and liquid outside them. This happens at a particular temperature, and if you keep the flame under your pot, all the water becomes steam and you end up with only one phase: gas.

Figure 4. Boiling water. Credit: Karen Bailey.

The transition from water to steam is over. A similar process of phase transition could have happened in the early Universe. In this case, the temperature was decreasing instead of increasing, but like with the water, when it reached the special temperature for the transition to start happening, bubbles began to form, with two different phases, one inside and one outside the bubbles. However, this time the difference between the phases is the mass. Outside the bubbles, the particles making up the Universe had no mass and inside instead they did (it is crazy I know). These bubbles then expanded, bringing the whole Universe into the final phase (in which we are today, with all particles having mass), like when all water in the pot is completely evaporated and all the water became steam. But most importantly in this process, different bubbles collided with each other and these blasts produced gravitational waves.

The initial discovery of gravitational waves dates back to 2016. However, it wasn't until 2023 that NANOGrav, an international collaboration, succeeded in detecting background gravitational waves. At the moment they do not have that much data and the observations are not yet very precise, but it looks like this background could be explained by a transition of phase in the early Universe: bubbles formed and then collided producing these gravity waves. However, we need to be cautious because we are still not sure what this background actually is and what produced it - a phase transition of the Universe is still only a hypothesis. And it raises a significant question mark: a phase transition like this does not actually make sense with the physics we have today. Therefore, if this theory is proven to be true and the origin of the background gravitational waves, it would indicate new and thrilling advancements in the field of physics as we know it. The journey of discovery continues, and who knows what is next to discover in the magnificent Universe?