Dark matter or cooling primordial soup could create gravitational waves


Now a pair of German astrophysicists has investigated two intriguing possible sources, the universe cooling down after the Big Bang, and a field of particles that could be dark matter. In January the NANOGrav collaboration reported the first detection of these waves, with a very low frequency of about 10-8 Hz. These were picked up by observing a series of bright pulsars across the galaxy for over a decade, watching how their light is stretched or squeezed at different intervals as it follows the ebb and flow of background gravitational waves.

In a new study, scientists at Johannes Gutenberg University Mainz (JGU) investigated what may have produced those ripples. They found that the signal is consistent with two scenarios that lie outside the Standard Model of physics, a phase transition in the early universe and a field of axion-like particles (ALPs) that may constitute dark matter.

Wolfram Ratzinger, co-author of the study said, even though so far the data only provides us with a first hint of the existence of low-frequency gravitational waves, it is still very exciting for us to work with them. This is because such waves could be produced by various processes that occurred in the early universe. We can now use the data we already have to decide, which of these come into consideration and which do not fit the data at all.

The first scenario takes us back almost to the dawn of the universe. After the Big Bang, the universe was filled with quark-gluon plasma, a dense primordial soup of elementary particles that slowly cooled into the kinds of matter we see today. This phase transition is thought to have given off massive turbulences, which could be detectable as low-frequency gravitational waves.

These signals could give scientists a glimpse behind the previously inaccessible curtain of the early universe. The cosmic microwave background (CMB) is a field of radiation that pervades the universe, often described as the afterglow of the Big Bang. However, this field obscures the earliest days of the universe from view by telescopes, meaning the furthest back we can see is about 300,000 years after the Big Bang. These gravitational waves could carry information from before then.

The second scenario is that the low-frequency gravitational waves were produced by a dark matter field. Axions are hypothetical elementary particles that, if they exist are believed to float around the universe in waves rarely interacting with normal matter. As such, they’ve been proposed as a dark matter candidate.

The team on the new study suggests that a field of axion like particles could be responsible for the gravitational wave signals. These ALPs would transfer energy to another hypothetical particle called a dark photon, which would be a force carrier similar to the familiar electromagnetic photon. This energy transfer would amplify quantum fluctuations in the dark photon field, the team says, which could grow into the kind of gravitational wave background that was detected.

The researchers say that of the two scenarios, phase transition is the more likely candidate. They also point out, however, that the original team's conclusion, that the ripples represent background noise of collisions between supermassive black holes is still the neatest explanation. But either way, further investigation could potentially help unlock secrets from beyond the Standard Model.

The researchers said, our work is a first, but important development it gives us a lot of confidence that with more precise data we can draw reliable conclusions about the message gravitational waves are sending us from the early universe.

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