Few things are as mysterious as dark matter, the strange substance that appears to outnumber ordinary matter by a ratio of five to one. Now, a physicist from Johns Hopkins University has outlined a new theory that helps to explain the stuff but at the same time makes it seem even more bizarre. According to the study, dark matter may have originated before the Big Bang.
Today the universe is expanding, with everything rushing away from everything else at an accelerating rate. By that logic, if you track the timeline back everything gets closer together, until eventually you find a point in time when everything was crammed into one tiny space – a singularity of infinite density.
All matter that exists today sprung forth from this singularity in an epic explosion commonly called the Big Bang, about 13.7 billion years ago. Dark matter is thought to have arisen from the Big Bang just like regular matter – and it seems to be responsible for the large-scale structure of the universe as we see it.
As the universe grew, tiny fluctuations in the density of dark matter meant that in regions where there was more mass, expansion slowed down. The gravity of dark matter's extra mass attracted normal matter to the area, and as the density of that matter grew it eventually collapsed and fired up into the first stars. This explains why stars tend to gather in galaxies, and galaxies in turn gather into clusters.
While dark matter makes itself known through its gravitational influence on normal matter, it's completely invisible to the other fundamental forces, making it tricky to study. As such, we don't really know what it's made of, but there's no shortage of candidate particles.
Suggested candidates include dark photons, weakly-interacting massive particles (WIMPs), axions, and even "Macro" particles as big as a dwarf planet and as dense as a neutron star. Unfortunately, no sign of any of these candidates have turned up yet, despite extensive experiments designed to detect them.
The reason for this, according to the new study, is because of the assumption that dark matter – whatever it is – was produced during the Big Bang.
"If dark matter were truly a remnant of the Big Bang, then in many cases researchers should have seen a direct signal of dark matter in different particle physics experiments already," says Tommi Tenkanen, an author of the study.
So what's the alternative? According to Tenkanen, dark matter may predate the Big Bang itself.
Before the Big Bang
The idea is not as crazy as it sounds. In most people's minds, the Big Bang was the very beginning of all space and time, and nothing existed before that point. But this may not be entirely accurate. Some physicists say that the Cosmic Microwave Background – the baseline radiation permeating the universe – gives us clues that there may have been something before the Big Bang.
For one, the universe could never have been hot enough to reach a singularity. On top of that, there are also two observed features of the universe that suggest something more: it seems to be spatially flat, not curved. And the temperature fluctuations average out to be the same in all directions.
These two things can be explained through a theory called cosmological inflation, a period in which the universe exponentially expanded. The theory goes that this period ended 13.7 billion years ago, with a burst of matter that we commonly call the Big Bang, and then the universe went on expanding at a different rate. That means the Big Bang set in motion the universe we now call home – but it wasn't the very beginning of everything. Case in point: dark matter itself.
Early dark matter
The new study suggests that while regular matter was created during the Big Bang, dark matter arose earlier, during the period of cosmic inflation. As the universe expanded rapidly, it produced a type of particle called scalars, which have a spin integer of zero. Only one type of scalar particle has been discovered so far – the famous Higgs boson – but according to Tenkanen, dark matter may fall into this category too.
Tenkanen proposes what he describes as the simplest possible mathematical scenario for the origin of dark matter. In this framework, dark matter is a type of scalar particle that was produced during the cosmic inflation, and importantly it works well to explain how dark and regular matter interact as we've already measured.
"We do not know what dark matter is, but if it has anything to do with any scalar particles, it may be older than the Big Bang," says Tenkanen. "With the proposed mathematical scenario, we don't have to assume new types of interactions between visible and dark matter beyond gravity, which we already know is there."
Conveniently, this form of dark matter fits neatly into the rest of the story that we know. After the Big Bang, the patchy structure of the dark matter that's already there can then go on to influence the way regular matter clumps together, and eventually form the galaxies and clusters that we see today.
That means that the theory can be backed up by studying these cosmic structures, and it might even tell us more about this mysterious pre-Big Bang time.
"The study revealed a new connection between particle physics and astronomy," says Tenkanen. "If dark matter consists of new particles that were born before the Big Bang, they affect the way galaxies are distributed in the sky in a unique way. This connection may be used to reveal their identity and make conclusions about the times before the Big Bang too."
Tenkanen says that these particular particles will be difficult to detect with the kinds of experiments currently searching for other candidates, but upcoming satellites like the European Space Agency's Euclid could help.
"While this type of dark matter is too elusive to be found in particle experiments, it can reveal its presence in astronomical observations," says Tenkanen. We will soon learn more about the origin of dark matter when the Euclid satellite is launched in 2022. It's going to be very exciting to see what it will reveal about dark matter and if its findings can be used to peek into the times before the Big Bang."
It will be interesting to see whether this new theory gathers any momentum. Other ideas suggest that the gravitational effects attributed to dark matter could instead be the influence of a dark fluid with negative mass, or we may simply have to tweak our understanding of gravity itself.
The new research was published in the journal Physical Review Letters.
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