Physics

How the universe's expansion may have stopped dark matter taking over

How the universe's expansion m...
A composite image showing galaxies in optical light, X-ray emissions in pink and invisible mass – or dark matter – in blue
A composite image showing galaxies in optical light, X-ray emissions in pink and invisible mass – or dark matter – in blue
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A composite image showing galaxies in optical light, X-ray emissions in pink and invisible mass – or dark matter – in blue
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A composite image showing galaxies in optical light, X-ray emissions in pink and invisible mass – or dark matter – in blue

Dark matter is a core component of the universe, but how did we end up with exactly the amount needed to hold the universe together? Physicists have proposed a new mechanism where dark matter particles in the early universe converted regular matter into dark matter exponentially, before being slowed down by the expansion of the universe.

Physicists estimate that dark matter particles outnumber regular matter by a ratio of five to one, and that dark matter plays a key role in the large-scale structure of the cosmos. Not only did its gravitational influence seed stars and galaxies to form in the first place, but it still holds galaxies and clusters together today. Without this specific density of dark matter, the universe would have evolved along very different lines.

So how did we get to this dark matter density? For the new study, a team of researchers proposed a new mechanism that they say is relatively simple and can be tested with future observations.

Many models suggest that dark matter was born from a “thermal bath,” the primordial plasma of regular matter particles in the early universe. From there, the team’s new hypothesis follows what’s known as a freeze-in model – essentially, the idea is that there wasn’t much dark matter to begin with, but the thermal bath of regular particles created dark matter particles over time, until it reached the density we see today.

But the team’s model adds a new wrinkle to the story: dark matter particles can convert regular particles into more dark matter. This new dark matter can then turn more regular matter to the dark side, leading to exponential growth of dark matter.

Of course, that model would eventually lead to a universe dominated by dark matter, but there’s an intriguing self-limiting mechanism already built into our understanding of the cosmos: expansion. Dark matter levels can grow very quickly in the early universe because regular matter was extremely dense at the time, but as the universe expanded and matter spread out, there was less fuel for the process and it slowed down.

The researchers showed that the model works to explain the current dark matter density, and can work with a range of dark matter particle masses. It also helps patch up holes in other models, that otherwise work well to explain observations.

Importantly, the idea can be tested. The team says that this mechanism would leave a specific fingerprint on the cosmic microwave background radiation, which could be spotted (or ruled out) by future observations.

The research was published in the journal Physical Review Letters.

Source: Physics

5 comments
5 comments
Altronix
I have always believed that dark matter is simply folds in the fabric of spacetime, created during the expansion phase. Now, these are slowly straightening out so it will be shown that the amount of dark matter in the universe is measurably smaller over time.
paul314
It's always nice when physicists propose a model that's testable with current or near-future technology.
Paparollie Ira
Altronix, I like your proposal, but if this were to be, wouldn't dark matter still be converting matter in these folds of space time? The process wouldn't slow, but just be and unmeasurable at the folds until it straightened out and then would be noticed as larger parts of dark matter over time. Correct?
Paparollie Ira
Unless as you say it was folding and not converting, that is. I think we would end up with greater amount of matter as space time progressed.
FB36
IMHO, Dark Matter is just a side-effect caused by Dark Energy (which is the real mystery)!
As an analogy, consider growing/inflating pockets of gas in rising dough!
(Imagine spacetime as a bubbling/boiling (because of incoming DE!) superfluid!)
As for evidence:
Isn't it true that geometric structure of Cosmic Web looks really like bubbling (instead of collapsing!)? (So it has positive curvature instead of negative!?)
Isn't it true that some galaxies appear to have too much DM & so others too little (contrary to all expectations!)?
(& not to mention, no experiment/observation was ever able to find any new particle which could be what DM is made of!)