The standard model of cosmology tells us that only 4.9% of the universe consists of ordinary matter (of what we can see), while the rest is 26.8% dark matter and 68.3% dark energy. As the name of these concepts suggests, we cannot see them, so their existence must follow from theoretical models, observations of the large-scale structure of the Universe, and the obvious gravitational effects that appear on visible matter.
Ever since this was first talked about, there has certainly been no shortage of speculation about what dark matter particles look like. Not so long ago, many scientists began to think that dark matter consists of weakly interacting massive particles (WIMPs, WIMPs), which are about 100 times the mass of a proton, but interact like neutrinos. Nevertheless, all attempts to find WIMPs using particle accelerator experiments have led to nothing. Therefore, scientists began to sort out possible alternatives to the composition of dark matter.
Modern cosmological models tend to assume that the mass of dark matter lies within 100 GeV (gigaelectronvolt), which corresponds to the mass limits of many other particles that interact with the help of a weak nuclear force. The existence of such a particle would correspond to a supersymmetric extension of the Standard Model of particle physics. In addition, it is believed that such particles should have been born in a hot, dense, early Universe, with the mass-density of matter, which has remained unchanged to this day.
However, ongoing experiments to identify WIMPs have found no concrete evidence for the existence of such particles. These included searches for WIMP annihilation products (gamma rays, neutrinos, and cosmic rays) in nearby galaxies and clusters, as well as direct particle detection experiments using supercolliders like the LHC.
By supersymmetry, wimps annihilate among themselves, creating a cascade of particles and radiation, including medium-energy gamma rays
Finding nothing, many scientists decided to move away from the WIMP paradigm and look for dark matter elsewhere. One such group of cosmologists CERN and CP3-Origins in Denmark recently published a study showing that dark matter may be much heavier and weaker to interact than previously thought.
One of the members of the CP-3 Origins research team, Dr. McCullen Sandora, spoke about his team's efforts:
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“We cannot yet rule out the WIMP scenario, but every year we suspect more and more than we have seen nothing. In addition, the usual weak scale of physics suffers from a hierarchy problem. It is unclear why all the particles we know are so light, especially if you look at the natural scale of gravity, the Planck scale, which is about 1019 GeV. So if dark matter were closer to the Planck scale, it wouldn't be affected by the hierarchy problem, and that would also explain why we haven't seen signatures associated with WIMPs.”
Using a new model they call Planck's Interacting Dark Matter (PIDM), scientists are investigating an upper limit on the mass of dark matter. While the WIMPs place the mass of dark matter at the upper end of the electroweak scale, the Danish research team of Martias Garney, McCullen Sandora and Martin Slot proposed a particle with a mass that is on a completely different natural scale - the Planck scale.
On the Planck scale, one mass unit is equivalent to 2.17645 x 10-8 kilograms - about a microgram, or 1019 times the mass of a proton. At this mass, each PIDM is essentially as heavy as a particle can be before becoming a miniature black hole. The group also suggested that these PIDM particles interact with ordinary matter only gravitationally and that a lot of them were formed in the very early Universe during the era of strong heating - a period that began at the end of the inflationary era, somewhere from 10-36 to 10- 33 or 10-32 seconds after the Big Bang.
This era is so called because during inflation, space temperatures are believed to have dropped 100,000 times. When inflation ended, temperatures returned to their pre-inflationary level (about 1027 Kelvin). By this time, most of the potential energy of the inflationary field has decayed into particles of the Standard Model, which filled the Universe, and among them - dark matter.
Naturally, the new theory comes with its share of consequences for cosmologists. For example, for this model to work, the temperature of the heating epoch must have been higher than currently believed. Moreover, a hotter heating period would also create more primary gravitational waves that would be reflected in the cosmic microwave background (CMB).
“This high temperature tells us two interesting things about inflation,” says Sandora. - If dark matter is PIDM: first, inflation proceeded at very high energies, which would produce not only fluctuations in the temperature of the early Universe, but also in space-time itself, in the form of gravitational waves. Second, it tells us that the energy of inflation should have decayed into matter extremely quickly, because if it took a long time, the Universe could cool down to the point after which it would no longer be able to produce PIDM at all.
The existence of these gravitational waves may be confirmed or excluded in future studies of the cosmic microwave background. This is extremely exciting news, as the recent discovery of gravitational waves is expected to lead to renewed efforts to detect primordial waves that are rooted in the very creation of the universe.
As Sandora explained, all of this represents a clear win-win scenario for scientists, as the newest candidate for dark matter will either be discovered or disproved in the near future.
“Our scenario makes an ironclad prediction: we will see gravitational waves in the next generation of experiments with the cosmic microwave background. That is, this is a win-win: if we see them, it’s great, and if we don’t see them, then we will know that dark matter is not a PIDM, from which it follows that some of its interaction with ordinary matter should be expected. If all this happens in the next ten years, we can only wait impatiently."
Ever since Jacobus Kaptein first suggested the existence of dark matter in 1922, scientists have looked for direct evidence of its existence. One by one, particle candidates - from gravitinos to axions - were proposed, screened out, and left in the realm of perpetual quest. Well, if this last candidate is unequivocally denied or confirmed, that option is already not bad.
After all, if it is confirmed, we will solve one of the biggest cosmological mysteries of all time. Let's get one step closer to understanding the universe and how its mysterious forces interact with each other.
