Thursday, January 21, 2016

Double “inflation” and dark matter

A new theory developed by the Brookhaven National Laboratory suggests that, after the Big Bang, there was a secondary inflation, able to explain the quantity of dark matter present in the Universe

A dense, boiling hot primordial nucleus, extremely rapid expansion and a progressive and increasingly slow cooling period. These are the main stages that, according to standard cosmological theory, mark our Universe's very first “action” after the Big Bang: inflation.

The inflation theory is in fact the theory stating that the Universe inflated exponentially a few moments after its birth (to be precise, over an interval of time equal to 10 to the power of minus 35 seconds) and is still expanding today. Most scientists agree with this theory, because it can explain a great many known physical phenomena. Nevertheless, it does not explain all of them: for example, it does not explain one of the greatest mysteries of modern cosmology - that of the quantity of dark matter present in the Universe.

For this reason, a group of physicists from the American Brookhaven National Laboratory has speculated that there could be a passage missing from standard cosmological theory. An intermediate stage, something that happened after the first, great expansion of the Universe: a smaller and shorter expansion that researchers have called “secondary inflation”.

This theory, which will be published on 18 January in the Physical Review Letters magazine, could offer a more precise explanation of what Hooman Davoudiasl, the first author of the study, calls “dilution of dark matter”.

In fact, this phenomenon cannot be explained by the many theories branching from the standard cosmological model, which envisage more dark matter than can be demonstrated by empirical observations.

This is where the idea of “adding” another cosmic inflation came from - an inflation that took place between the great expansion and the beginning of the Universe's cooling period. It is believed that this second inflation took place when temperatures were still very high: this is a fundamental feature, because it is precisely the heat that would have made it possible for the particles of dark matter to collide with each other and annihilate one another, thus creating particles like electrons and quarks. Above all, this makes sense with regard to the quantity of dark matter calculated today.

The secondary inflation suggested by the Brookhaven National Laboratory was much gentler than the primary one, but crucial in taking the primordial particles to conditions of temperature, volume and density consistent with what we can observe almost 14 billion years later.

Therefore, this is a potentially simple and coherent theory. Nevertheless, we are still a long way from moving from theory to practice, as Hooman Davoudiasl, the father of the inflation theory himself, asserts: the next step will be to try to identify particle interactions according to the new theory through experiments within the scope of the LHC. Thus, empirical proof of this double inflation at the origin of our Universe can be found.

“The best explanations in physics are the simple ones – explains the president of the Italian Space Agency (ASI), Roberto Battiston - but in the Universe's case the simplest one may not be the best! In order to make sense of the quantity of dark matter observed in our universe, which is approximately 6 times more abundant than normal matter, according to this new theory, the hyper-fast phase of expansion that accompanied the big-bang (called inflation, editor’s note) had a sort of hiccup and repeated itself a second time, in quick succession to the first. There was much less intensity in this phase, but it was sufficient to dilute the dark matter produced in the first phase and reach a figure compatible with that measured.”

Battiston points out that “This is a theory and, as such, requires verification. For this reason - he concludes - a new generation of instruments will be needed, able to observe the effect of what happened during the initial moments, such as, for example, the impression made in the radiation of the Big Bang, due to the effect of the gravitational waves produced during the initial phases of the great explosion”.