The Standard Model of Cosmology based on Einstein’s theory of general relativity suggests that most of the mass and energy in the universe is made up of invisible substances known as dark matter and dark energy. While over the last couple of decades the model has been successful at explaining different phenomena, we still don’t for sure what makes up dark matter. Thus far, we only know that it exists because of the gravitational pull it has on galaxy clusters and other structures. Although there have been several proposals in the past, scientists don’t know for sure which particles make up dark matter. Interestingly, a new study has hinted that neutrinos are likely to make up some of the dark matter.
The standard model which tries to explain the phenomenon of dark matter claims that the particles responsible for making up dark matter are cold. Most of these particles are relatively heavy with sluggish motions. As a result, it is pretty easy for particles to form objects bound by gravity. As a result of this, the standard model predicts that the universe is filled with small dark matter “haloes” which ultimately merge and form more massive systems. However, it is possible that some of the dark matter is hot and would result in some particles escaping from dense regions. Ultimately, this would slow the accumulation of new matter and lead to a universe where the formation of structure is suppressed.
If you are familiar with neutrinos, then you know that these high velocity particles are good candidate for hot dark matter. Moreover, these particles don’t emit or absorb light which makes them appear dark. While in past scientists assumed that neutrinos don’t have mass, recent experiments have shown that these particles can change from one species to another which require these particles to have mass. As a result, they are legitimate candidate for hot dark matter.
However, in recent times, particle physicists have ruled out the argument the neutrinos make up most of the dark matter. Moreover, the standard model claims that the neutrinos have so little mass that they can be completely ignored.
Despite that, as the quality of cosmological observations has gone up significantly over the last couple of years, astronomers are now being presented with new datasets to test the standard model with more details. Thanks to increasing number of examples thanks to “gravitational lensing observations”, scientists have concluded that the mass distribution in the universe is less lumpy than previously thought if the dark matter is entirely cold.
Now, the group of researchers have created the largest suite of cosmological simulations of normal and dark matter to date. After analyzing the data from the simulation, the researchers have concluded that the discrepancy between the new observational data sets and the standard cold dark matter model is even larger than previously claimed.
The researchers found out that when neutrinos were included in the model, the structure formation in the cosmos was washed out, making the universe less lumpy. Interestingly, the results suggest that the neutrinos make up between 3 to 5 percent of the total dark matter mass. The results from the latest model is enough to reproduce a wide variety of observations – including the new gravitational lensing measurements. If a larger fraction of the dark matter is “hot”, the growth of structure in the universe is suppressed too much.
While it is too early to conclude anything, the research might go a long way to solve the mystery of the mass of a neutrino. In past, particle physicists have claimed that the sum of three neutrino species (there are three different species of neutrino) should be at least 0.06 electron volts. According to the previous calculation, the total neutrino contribution to dark matter would be 0.05 percent. However, since the researchers found that the total neutrino is six to ten times larger, it’s likely that the neutrino mass is about 0.3-0.5 eV.