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One of the biggest challenges in cosmology is figuring out what happened to the universe in its first moments to expand so fast

The direct observation of the first gravitational waves that humans have been able to identify a little over six years ago is undoubtedly the best thing that has happened to recent cosmology. Are gravitational disturbances generated by massive objects that are subjected to a certain acceleration propagate through the space-time continuum at the speed of light in the form of waves, which, under certain conditions, scientists are able to detect.

Over the past six years, gravitational waves, as they are also known, have shown us that they are a very valuable tool that can help us better understand the history of the universe. And they are because carry information about the cosmic event that originated them. The sensitivity of the interferometers that we currently use to identify them requires that these disturbances have been caused by events of great magnitude, such as the collision of two black holes.

Scientists are gradually expanding our knowledge of the stages through which the universe has passed, but there is a particularly unaffordable phase: the moment that happened to the Big Bang

In fact, at the end of last June the research groups that are in charge of analyzing the data collected by the interferometers LIGO, in the United States, and Virgo, in Italy, claimed to have very solid reasons to suspect that their experiments had identified gravitational waves produced by the merger of two binary systems made up of a black hole and a neutron star. This is the kind of cosmic cataclysm that we can currently identify through the disturbances they introduce into the fabric of space-time.

Fortunately, gravitational waves are not the only tool that cosmologists have to gradually unravel the past of the universe; observation and analysis of cosmic microwave background it is also very helpful. Microwave background radiation, as it is also known, is a type of electromagnetic radiation that permeates the entire universe and that began its journey through the cosmos no less than approximately 13.77 billion years ago. This is, in fact, how old the universe is according to the most accurate estimates that astrophysicists have ever made.

Using these and other tools, scientists are gradually expanding our understanding of the stages through which the universe has passed, but there is a particularly unaffordable phase. In reality, that period is a very small period of time; a seemingly insignificant moment that covers only a tiny fraction of a second, but which is vitally important because it was the moment in which an event occurred that we still do not understand: the spark that unleashed the accelerated expansion of the universe on which cosmologists have built the hypothesis of cosmic inflation.

Primordial gravitational waves may have the answer we seek

Cosmic inflation proposes an explanation for the exponential expansion of the primeval cosmos. During a minimum instant that took place after the Big Bang, the universe expanded with a very high speed, and from that moment its growth continued, although with a significantly more moderate expansion rate. Observations and the most advanced models currently used by cosmologists suggest this behavior, but the big problem is that they do not know the mechanism that can reliably explain that very short period of cosmic inflation.

However, all is not lost. Scientists conducting research in this area believe that the answer they are looking for may lie in the primordial gravitational waves, which, precisely, were originated during the final phase of this period of inflation of the cosmos. These gravitational disturbances should have left a trace on the cosmic microwave background, but finding it is extraordinarily difficult because, broadly speaking, it is such a light trace that it is apparently imperceptible. And, furthermore, it is extraordinarily easy to confuse it with the signal originated by galactic dust, which also emits radiation and has a structure similar to what in theory primordial gravitational waves should have.

One of the great challenges faced by the scientists participating in this experiment is to increase the signal-to-noise ratio of their measurements.

Still, cosmologists don’t give up easily. In recent years, several research groups have developed radio telescopes located in geographical enclaves where atmospheric conditions allow us to observe the cosmos with fewer disturbances, and, therefore, in a more precise way, such as the South Pole. One of the most relevant collaborative scientific projects is BICEP / Keck, and fortunately over the last few years it has been delivering very promising results. In fact, the researchers involved in the BICEP3 experiment They just published their first results, and they confirm that we are one step closer to detecting primordial gravitational waves.

One of the great challenges faced by the scientists participating in this experiment is to increase signal-to-noise ratio of their measurements with one purpose: to avoid that the signal originated by the galactic dust masks the information that really interests them and that could lead them to identify the primordial gravitational waves that could underpin, perhaps in a definitive way, the model of cosmic inflation. The good news is that the BICEP / Keck collaboration is slowly refining its experiments, so that the measurements it is collecting are becoming more and more precise. And the not so good is that there is still a lot of work to do.

If you are curious and are not intimidated by relatively advanced physics, I suggest you take a look at the great article in which Francisco R. Villatoro breaks down the results obtained by the BICEP3 experiment. This is a long-distance race in which every little step is importantEspecially if we bear in mind the difficulty involved in the careful exploration of the cosmic microwave background in search of the long-awaited indications of the presence of primordial gravitational waves. But there is no doubt that the effort is worth it even if we are forced to accept that perhaps the result we are looking for will still take time to arrive.

Cover Image | POT

More information | Physical Review Letters | arXiv: 2110.00483