Dark matter is considered to be the initial ingredient for the first galaxies in the universe. After the Big Bang, dark matter particles would have combined in gravitational “halos”, pulling gas into cores which on condensation resulted in the first galaxies. Scientists, however, know very little about these particles since they have not been detected directly.
Researchers at Princeton, MIT and Cambridge discovered that the appearance of the very first galaxies would differ a lot depending on the type of dark matter. The team has successfully simulated the appearance of early galaxies if the dark matter were “fuzzy” instead of cold or warm. The findings appear in the Physical Review Letters journal.
Dark matter is cold in most scenarios composed of slow-moving particles and does not interact with normal matter besides gravitational forces. The warm version is lighter and faster than cold dark matter. On the other hand, fuzzy dark matter consists of ultralight particles, each with the mass of 1 octillionth of an electron mass, cold dark matter particle is 105 times heavier than an electron.
It was found in simulations that when dark matter is cold, the first galaxies would have been formed in spherical halos. But with the dark matter being warm or fuzzy, the early galaxies would have formed in tail-like filaments. Researchers can look back into the earlier universes with telescopes coming online and deduce from the galaxy formation patterns if the dark matter is fuzzy instead of being warm or cold.
Mark Vogelsberger, a physics professor at Kavli Institute for Astrophysics and Space Research, MIT said the type of dark matter present today may be known from the earlier universes. If the filament pattern is visible, then it is a fuzzy dark matter.
Till now the hypothesis of dark matter being cold has been successful in the description of the grand scale of the observable universe. Hence, galaxy formation is modeled on the assumption that dark matter is cold. However, Vogelsberger pointed out some of the discrepancies between predictions and observations of cold dark matter. This is highlighted in the case of smaller galaxies where the theoretical models do not agree with the actual distribution of dark matter. This is where alternate theories of warm and fuzzy dark matter have been proposed.
Anastasia Fialkov of Cambridge University, co-author of the paper said that the motivation of fuzzy dark matter is from fundamental physics such as string theory. The final validation lies in the cosmic structures.
Fuzzy dark matter consists of particles so light that they exhibit quantum, wave behavior instead of individual particles. Philip Mocz of Princeton University, the lead author of the paper said that the first galaxies would differ from the galaxies in the late universe providing hints about the nature of the dark matter.
A cubic portion of the early universe measuring 3 million light-years was simulated by the researchers to observe a fuzzy and cold early universe. It was tested through different periods of time to understand the formation of galaxies if the dark matter were either cold or warm or fuzzy.
Simulations of how galaxies form in cold, warm and fuzzy (left to right) dark matter scenarios. (Credit: Universities of Princeton, Sussex, Cambridge
The simulation was started based on the dark matter distribution known to researchers based on cosmic microwave radiation, “relic radiation” detected 400,000 years after Big Bang took place. Vogelsberger said that there is no constant density for dark matter. For warm and cold scenarios, existing algorithms were used for simulation. However, for fuzzy dark matter, a new method was used.
The simulation of cold dark matter was modified for solving two extra equations in order to simulate the formation of galaxies under the fuzzy dark matter. Schrodinger’s equation describes the behavior of quantum particle as a wave and Poisson’s equation tells how the wave creates a density field, dark matter distribution leading to gravity which is the pulling force for the formation of galaxies. This was added to the model about gas behavior in the universe and how it condenses to galaxies responding to gravity.
In each of the scenarios, the formation of galaxies took place where the concentration of dark matter collapsed from gravitation was high. The pattern differed on it being cold or warm or fuzzy.
In cold dark matter, the formation of galaxies took place in spherical halos and subhalos. Warm dark matter created galaxies in filaments resembling tails with the absence of subhalos. The fuzzy dark matter led to formation along filaments succeeded by effects of quantum wave as a result of which galaxies took shape of striated filaments. This is due to interference, overlapping of waves which resulted in an alternating pattern of dark matter concentrations, over-dense and under dense.
Vogelsberger said that there would be high gravity in over-dense regions resulting in the formation of galaxies in such areas. This is replicated throughout the early universe. Researchers are developing detailed predictions of the appearances of early galaxies in a universe of fuzzy dark matter. The main target is to give a map to telescopes such as James Webb Space Telescope that can run back in time to find the earliest galaxies. If filamentary galaxies are observed, then it is the indication that dark matter is fuzzy.
Journal Reference: Physical Review Letters