Illustration of the early universe in the context of the primordial black hole scenario and induced gravitational waves. Figure made by myself with Goodnotes.
The early universe and gravitational waves
In the last decades we learned a lot about the early universe. We are certain that the universe, since its first seconds, was so dense and hot that all particles blended in a uniform soup. We also know that this primordial soup was slightly denser in some regions than others; we observed density fluctuations. We have strong evidence that such fluctuations emerged much earlier, in the infant universe, from quantum fluctuations during a period called cosmic inflation. Unfortunately, we still know little about this period of the universe. But we definitely know there is more to discover.
So far, we have been observing the universe with electromagnetic waves. However, the early universe was at some point so dense and hot that it was essentially opaque to electromagnetic waves. The first “light” that we observe comes from the moment when the universe expanded and cooled down enough so that photons were able to propagate freely through the universe. The information we obtained from these photons tells us that the universe was filled with relativistic particles since it was one second old. But it does not tell us anything prior to that moment.
The detection of gravitational waves by LIGO just provided the perfect means to explore it. These ripples of spacetime can travel through the primordial soup as if it were transparent; they could reach us even from the infant universe. Gravitational waves, for the first time, provide a window into the unexplored infant universe. Undoubtedly, the discovery potential of gravitational waves, and what we may learn, is fascinating and exciting.
I investigate one of the most appealing possibilities to probe the infant universe: ripples of spacetime created by the oscillations of primordial density fluctuations. We have evidence that such induced gravitational waves should be there, since we observed density fluctuations, and they promise key information about the infant universe. There are ongoing efforts to detect these waves, with which I collaborate and which need a strong theoretical basis in order to progress. Within a decade, the gravitational wave detector LISA is expected to launch into space. And, it could observe such gravitational waves.
Interestingly, if such primordial density fluctuations were large enough, they may have collapsed under their own gravity and created black holes. These primordial black holes could be the explanation of several observations, such as lensing events from planets-mass objects observed by OGLE or some of the black holes seen by LIGO. Induced gravitational waves will have the final word in any primordial black hole model, either by confirming it or ruling it out.
This field is gathering considerable attention in the recent years as there is tantalizing evidence for a gravitational wave background by Pulsar Timing Arrays. Perhaps, induced gravitational waves could explain such signal and, depending on the early universe model, their primordial black hole counterpart is the LIGO black holes or the OGLE objects. In the next decades we will know the answer.
