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Sensing the Unknown

ICTP scientist leads research on dark energy theories

Sensing the Unknown
Sensing the Unknown

10/07/2020 - Trieste

Dark energy is considered one of the Universe’s biggest mysteries. We know that it exists because of its effects on gravity, causing the expansion of the universe at an accelerating rate. Or, in other words, we could say that the only explanation scientists currently have for the acceleration of the universe’s expansion is the existence of an ‘entity’, an unknown form of energy that we call dark energy.

Knowing more about dark energy is one of the main goals of contemporary cosmological research, given its relevance in the overall composition of the universe. It is in fact estimated that about 68% of the total energy in the observable universe is made up of dark energy. In comparison, only 5% of this total is the energetic contribution of the ordinary matter that we encounter in our everyday life. The remaining portion is dark matter, another cosmological enigma.

The first direct observation of gravitational waves by the LIGO and Virgo Scientific Collaboration in 2015 offered scientists a new tool to investigate the Universe and its many undiscovered secrets, including that of dark energy. Gravitational waves are ripples in the fabric of spacetime caused by violent cosmic events such as the collision of two black holes. These waves propagate in all directions and travel at the speed of light, some of them reaching the Earth and eventually being detected by very sensible instruments such as the LIGO and Virgo detectors.

Gravitational waves are the “instrument” that ICTP scientist Paolo Creminelli and his colleagues are using to try and understand more about dark energy, its characteristics and its behaviour. Some of their findings were published in JCAP (Journal of Cosmology and Astroparticle Physics) last October 2019. The research group includes Creminelli, former ICTP postdoc Filippo Vernizzi, now a researcher at Institut de physique théorique, Université Paris Saclay in France, and two PhD students at SISSA, including Vicharit Yingcharoenrat, a former ICTP Diploma Programme student from Thailand.

This work, “Resonant decay of gravitational waves into dark energy”, follows a previous research from the group, in which they started to study the phenomenon of gravitational waves decaying into dark energy. These works are based on the same idea, that is, the analogy between the decay of gravitational waves into dark energy and the absorption of light in a general medium. Just as light gets partially absorbed when it propagates in a medium, so gravitational waves are absorbed by dark energy and the wave’s energy is dissipated as a fluctuation of the medium.

“Following the analogy with the propagation of light in a medium,” explains Creminelli, “we could say that we use a gravitational wave to inspect the medium, that is dark energy, and see how it interacts with these waves, whether they are absorbed, transmitted or reflected.”

In the general theoretical scenario described in the earlier paper, analysing the decay of gravitational waves into dark energy helped the scientists eliminating some models from the list of possible ones.

“What we are trying to do is to put a limit on our fantasy,” says Creminelli. “We are still not in a position of choosing between a couple of possible theories, everything is under consideration right now. We are now in a position where every additional piece of information reduces the possibilities of the models that are under consideration.”

Experimental data show that there is not a significant absorption - or a significant decay - of gravitational waves in dark energy: the experiments confirm the simplest cosmological model available, i.e. the current standard model of cosmology, in which a so-called cosmological constant is used to explain dark energy. The cosmological constant is also called “vacuum energy”, and just as light is not absorbed when it propagates in a vacuum, so there is not absorption of gravitational waves in dark energy in this model.

“This means that our role as theoreticians is to find theories, or models, in which this effect of absorption would be relevant,” says Creminelli, “and thus, to find theories that do not agree with the observations and need to be excluded.”

In this new work, Creminelli and his colleagues consider a more particular case, in which the phenomenon of resonance takes place. In the presence of resonance, the absorption of gravitational waves would be much bigger, and this poses an even stronger limit to the possibilities under consideration.

“What is interesting is that all of this information is new,” says Creminelli. “It is the first time that gravitational waves are observed, so this is a totally new phenomenon, and a new way of looking at the problem.”

The study of possible scenarios continues: in a more recent paper, published on JCAP last May, the group considered the case in which the phenomenon of absorption is so large that the whole system becomes destabilized. The same phenomenon of gravitational wave absorption needs a different mathematical description according to the regime in which it is studied. In some cases, it is described as a simple decay, in some cases it is a decay that only happens in limited resonant areas, or in other cases it could be something that makes the whole medium unstable.  “I think there will still be other work to do,” says Creminelli. “In the future we are trying to thoroughly explore all the possible scenarios. We are very likely going to work on more papers together because the possibilities to explore are many, and I think we will continue on this path for a while.”

The fact that these analyses have the goal to put limits instead of adding new knowledge could maybe look slightly negative. But this is a characteristic feature of this field of investigation. After the introduction of gravitational waves in these experiments, many of the cosmological models traditionally studied have been excluded. Therefore, there is no doubt that some progress is being made, even if it is a negative one.

“This is a good point to be in research right now. With the discovery of gravitational waves, scientists could observe the universe for the first time with waves different from the usual electromagnetic waves,” says Creminelli. “It is like having a new instrument. Of course, it could happen that even if we have a new instrument, we won’t discover anything new, but in the meantime, there is a lot to observe. Observing the Universe with a new instrument means having infinite analysis possibilities and infinite new questions to answer.”


---- Marina Menga