Dark Matter and Its Particles

Physicists revised the minimum mass of dark matter particles from 10^-31 proton masses to 2.3 × 10^-30 proton masses. This change, based on new simulations of a dwarf galaxy, provides deeper insights into the behavior and distribution of dark matter in the universe.

What is Dark Matter?

• Dark Matter is an invisible substance that accounts for about five-sixths of the total matter in the universe.
• Though it cannot be directly observed, its presence is inferred from its gravitational effects on visible matter, such as stars and galaxies.
• Unlike ordinary matter, dark matter does not emit, absorb, or reflect light, making it extremely difficult to detect.
• Minimum mass: For decades, scientists have been trying to understand the properties of dark matter, particularly the mass of its particles. These particles are believed to be non-zero mass to enable the formation of large-scale structures in the universe, such as galaxies and clusters of galaxies.

The Distribution of Dark Matter

• Dark matter is thought to be uniformly spread across the universe, but its distribution on smaller scales remains uncertain.
• Early estimates, like those made by Jacobus Kapteyn in 1922, suggested that dark matter exists at a low density of about 0003 solar masses per cubic light-year. This would mean that even in your house, there could be dark matter, with a mass equivalent to a trillion protons.
• However, this only applies on large scales (millions of light years), and doesn't necessarily hold at smaller scales (e.g., inside a room or your body), where dark matter may be unevenly distributed.
• Dark matter might be spread uniformly or could exist in clumps (lumps). If it is spread uniformly like flour, it would be found all around us, though in very low densities.
• However, if dark matter is clumpy, there could be large regions without dark matter, with the spacing between clumps potentially spanning light years.

The Size and Nature of Dark Matter Particles

The mass of dark matter particles plays a critical role in how they interact and how they are distributed. Here's a breakdown of the behavior based on different particle masses:

• Heavy Particles (100 proton masses or more): If dark matter particles had a mass of around 100 times that of a proton, the separation between them would be about 7 cm. At this scale, dark matter particles would be present not only in your house but in your body as well.
  • The density of dark matter would be high, with particles frequently interacting with each other.
• Moderate Mass Particles (around 10^19 proton masses): If dark matter particles had a mass of about 10^19 times that of a proton, the separation between them would be around 30 km, making dark matter particles rare in everyday life. They would occasionally visit your house, moving at speeds around 300 km/s.
• Light Particles (10^-11 to 10^-31 proton masses): Very light dark matter particles (like those with 10^-11 proton masses) would have very large wavelengths and be spread out more like a fluid rather than discrete particles. If these particles were lighter than this, their wavelengths would be incredibly large (for example, 200 light years for particles with masses of 10^-31 proton masses).
  • As the particles become lighter, the concept of individual particles becomes less meaningful, and they behave more like a collective wave of matter. This explains why smaller masses are associated with the large-scale distribution of dark matter in galaxies and clusters.

New Insights from Computational Physics

• The most recent advances in understanding dark matter come from numerical simulations.
• Theoretical physicists used data from the Leo II dwarf galaxy (a galaxy orbiting the Milky Way) to estimate the density of dark matter within it.
• By solving the modified Schrödinger equation for dark matter particles, they found that the inner regions of galaxies, like Leo II, require heavier dark matter particles than previously thought.
• Particles with masses around 10^-31 proton masses could not account for the amount of invisible mass observed in these inner regions.
• This finding suggests that the mass of dark matter particles is likely heavier than the earlier theoretical lower bound.
• The increased mass helps explain the crowding of dark matter in the inner parts of galaxies, where densities are higher.