Faculty of Physics UW: Dark inflation opens the gravitational window to the first moments after the Big Bang

What was the course of evolution of the Universe just after the Big Bang? Despite decades of research, current cosmological models still do not allow to clarify the chronology of events.

But researchers at the Faculty of Physics, University of Warsaw have developed a new cosmological model of the evolution of the Universe, in which rapid expansion of dark matter and dark energy plays a key role. The dark inflation model not only puts the thermal history of the Universe in order. A spectacular prediction of the model is the demonstration of the possibility of detection of gravitational waves created only fractions of seconds after the formation of space-time, the Faculty of Physics UW researchers inform in the release sent to PAP.

According to the release, the earliest structure of the Universe available for observation today is cosmic microwave background (CMB) radiation. "This electromagnetic relic comes from around 380,000 years after the Big Bang and shows amazing homogeneity, even between areas so far apart that the light has not yet been able to travel the distance separating them" - researchers from the Faculty of Physics UW inform.

In 1979, Alan Guth proposed a simple explanation of this homogeneity: the distances between homogeneous areas were so large, because inflation had occurred - an extremely violent expansion of space-time (even one billion billion billion times over a fraction of a second). A new physical field, called the inflatonic field, with which a specific associated particle inflaton, would be responsible for the inflation.

"The fundamental problem with inflation is that we don`t really know when exactly it occurred and at what energy levels. The range of energies at which inflation could have occurred is vast, stretching over 70 orders of magnitude" - says Prof. Zygmunt Lalak, quoted in the release, and adds: "Inflation is described as a period of supercooled expansion. However, for cosmological models to be consistent, following inflation the Universe should have undergone reheating to a very high temperature, and we have no idea how or when this might have occurred. Just like with inflation itself, we are dealing with energies across a range of 70 orders of magnitude. As a result, the thermal history of the Universe is yet to be described".

Measurements of CMB radiation made with the Planck satellite have been used to estimate the composition of the contemporary Universe. It turns out that dark energy comprises as much as 69% of all extant energy/matter, with dark matter comprising 26% and ordinary matter just 5%, researchers from the Faculty of Physics UW remind. Dark matter and ordinary matter don`t interact at all, or their interactions are so weak we are only just starting to notice dark matter`s gravitational impact on the movement of stars in galaxies and galaxies in clusters. Dark energy should be a factor responsible for the accelerated expansion of the Universe.

"Our inflation model is significantly different from those proposed in the past. We started with the assumption that since today dark matter and dark energy comprise up to 95% of the Universe`s structure, then both factors must have also been extremely important immediately after the Big Bang. This is why we describe the dark sector of the Universe as responsible for the inflation process" - explains Dr. Michal Artymowski (Faculty of Physics UW), main author of the paper published in the Journal of Cosmology and Astroparticle Physics.

In the model proposed by the theoretical physicists from the University of Warsaw, inflation is driven by a scalar field. The properties of the field mean that inflation is not permanent and it must come to an end: at some point the rate of expansion of the Universe will start slowing down instead of accelerating. At the point of this shift, new relativistic particles are formed, behaving in the same way as radiation. Some of these particles are described by the Standard Model, while others may correspond to particles predicted by theories beyond the Standard Model, such as supersymmetry - the release reads.

"In our models, the new particles are the result of gravitation, which is a very weak force. The process of formation of particles is ineffective, and at the end of inflation inflatons continue to dominate the Universe" - says Olga Czerwińska, PhD student at the Faculty of Physics UW.

In order to recreate the observed dominance of radiation in the Universe, inflatons should lose energy rapidly. The researchers from Warsaw propose two physical mechanisms that could be responsible for this process. They reveal that the new model predicts the course of events of the Universe`s thermal history with a far greater accuracy than the previous models.

"The model’s predictions concerning primordial gravitational waves are especially interesting. Gravitational waves are vibrations of spacetime itself, and they have already been detected several times. In each case their source has been a merger of a pair of black holes or neutron stars. Current cosmological models predict that gravitational waves should also appear as a result of inflation. However, all the evidence suggested that vibrations of spacetime caused by inflation would be so weak by now that no existing or future detectors would have been able to register them. These predications were revised when physicists from the University of Warsaw took into account the effects of the dark sector of the Universe" - reads the release.

"Gravitational waves lose energy like radiation does. However, inflatons must lose it significantly faster" - says Dr. Artymowski. "If the inflation involved the dark sector, the input of gravitational waves increased proportionally. This means that traces of the primordial gravitational waves are not as weak as we originally thought!".

The estimates of Warsaw physicist are optimistic. Data suggest that primordial gravitational waves could be detected by observatories currently at the design stage or under construction, such as the Deci-Hertz Interferometer Gravitational Wave Observatory (DECIGO), Laser Interferometer Space Antenna (LISA), European Pulsar Timing Array (EPTA) and Square Kilometre Array (SKA).

"The first events could be detected in the coming decade. For cosmologists this would be an unprecedented discovery, paving the way for research into gravitational events which took place immediately after the Big Bang, a period hitherto impossible to study" - researchers from the Faculty of Physics UW note.

But according to the researchers, the dark inflation model has another fascinating aspect: it is highly dependent on gravitational theory. "By comparing the model`s predictions with data collected by gravitational observatories, cosmologists should be able to provide new verifications of Einstein’s general theory of relativity. What happens if they find discrepancies? It would mean that observational data provides the first information on the properties of real gravity" - the researchers conclude in the release.

The research of Warsaw theoreticians was funded by grants from the Polish Ministry of Science and Higher Education and the National Science Centre.

PAP - Science in Poland

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