Unusual occurrences observed right at the centre of the Milky Way might serve as compelling proof for a newly proposed form of dark matter. Should this indeed be true, researchers could have overlooked the discreet influence of such matter.
dark matter
, the cosmos' most enigmatic 'material,' within the realm of cosmic chemistry.
This newly proposed
dark matter candidate
not only would this new form of dark matter be lighter than current theoretical candidates, but it would also annihilate itself. Meaning that upon collision with another particle of dark matter, they would mutually destruct and produce a negative charge.
electron
And its positively charged counterpart, a
positron.
This process and the flood of electrons and positrons would provide the energy needed to strip electrons away from neutral atoms, a process called ionization, in dense gas in the center of the Milky Way. That could explain why there is so much ionized gas in the central region called the
Central Molecular Zone
(CMZ).
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Even if the
annihilation of dark matter
is rare, it stands to reason it would occur more frequently at the heart of galaxies where it is thought to conglomerate.
"We propose that dark matter lighter than a
proton
[the particles found in the nuclei of atoms] could be responsible for an unusual effect seen in the center of
the Milky Way
Team Leader and Postdoctoral Research Fellow at King's College London, Shyam Balaji stated, 'In contrast to many typical dark matter candidates that are usually investigated via their gravitational impacts, this type of dark matter could potentially be identified through its ability to ionize gas, effectively removing electrons from atoms within the Central Molecular Zone.'
This could occur if dark matter particles destroy each other.
electron-positron pairs
, which subsequently interacts with the surrounding gas."
Dark matter chemistry
It is believed that dark matter makes up about 85% of the material in the universe; however, even though it is widespread, scientists cannot observe it like much of regular matter. This is due to dark matter’s lack of interaction with light—or if it does interact minimally—it happens far too infrequently to detect directly.
This indicates that scientists have determined that dark matter cannot consist of baryonic particles such as electrons, protons, and neutrons, which make up the atoms forming ordinary matter.
stars,
planets, moons, along with all the things we encounter regularly.
The sole rationale for scientists hypothesizing the existence of dark matter is due to its interaction with light and matter.
gravity,
And this influence affects both light and "normal" matter.
This has led scientists to explore further than what is known as the "
standard model of particle physics
to look for particles that might explain dark matter.
These particles differ in their mass, measured in electronvolts (eV), as well as in properties. There is speculation that certain ones, akin to this newly identified candidate, might annihilate themselves.
The current "leading suspects" for dark matter are
axions
and axion-like particles, which come in a wide range of masses. However, Balaji and colleagues have mostly ruled out axions and axion-like particles as their dark matter culprits linked to the ionization of gas in the CMZ.
"Most axion models do not predict significant annihilation into electron-positron pairs in the way our proposed dark matter does," Balaji said. "Our proposed dark matter subject is sub-GeV (one billion eV) in mass and self-annihilates into electrons and positrons."
"This sets it apart because it directly affects the
interstellar medium
, generating a distinctive mark through additional ionization, which is not usually anticipated from axions."
Dark matter: Its own greatest adversary
Within the tightly clustered CMZ, generated positrons cannot move far or exit before interacting with surrounding hydrogen molecules, causing them to lose their electrons. This renders the process highly effective in this core area.
"The primary issue this model addresses is an overabundance of ionization in the CMZ," Balaji stated.
Cosmic rays
, the typical agents responsible for ionizing gas appear insufficient to account for the elevated levels of ionization we are observing."
Cosmic rays are charged particles that zip around close to the speed of light.
speed of light
However, based on the findings of this research group, the ionization signal originating from the CMZ appears to suggest a less massive source that is moving at a slower pace compared to numerous alternative dark matter possibilities.
Additionally, if cosmic rays were causing ionization of the gas within the Central Molecular Zone (CMZ), one would expect accompanying emissions.
gamma rays
These are extremely energetic photons. Nonetheless, such emissions are absent from research focusing on the Central Molecular Zone (CMZ).
“If dark matter is causing the ionization of the Central Molecular Zone, it would imply that we’re identifying dark matter through its faint chemical influence on the gases within our galaxy rather than directly observing it,” explained Balaji.
Nevertheless, there is an enigmatic weak gamma-ray emission emanating from the center of our galaxy which could potentially be associated with positrons and ionization.
If we establish a clear link between ionization and this gamma-ray emission, it might bolster the argument for dark matter," Balaji stated. "While there appears to be an association between these two signals, we require additional data at present to make a firmer claim.
Furthermore, this annihilation theory of dark matter might also account for an observable light emission from the Central Molecular Zone caused by collisions between positively charged positrons and negatively charged electrons merging into a state known as
positronium
, which subsequently breaks down quickly into
X-rays,
Light with marginally less energy than gamma rays.
The figures align far more closely than anticipated. Typically, dark matter theories encounter problems as they anticipate signals that would likely have been detected by telescopes already," Balaji stated. "However, with this scenario, the ionization rate generated by sub-GeV dark matter sits comfortably inside recognized limits, without conflicting with current gamma-ray observations.
cosmic microwave background (CMB)
observations."
The researcher noted that the initial convergence with the X-ray emission is equally fascinating.
"That's a rare and exciting situation in dark matter research," Balaji added.
The early stages of this potential dark matter candidate are still underway.
Certainly, this fresh contender for dark matter is still in the early stages of its theoretical existence; it hasn’t been bestowed with a catchy moniker such as WIMP (Weakly Interacting Massive Particle) or MACHO (MAssive Compact Halo Object)!
For comparison, axions have existed since their initial conception by theoretical physicists.
Frank Wilczek
and
Steven Weinberg
in 1978.
This indicates that extensive theoretical work needs to be carried out before this candidate can join others like axions, WIMPs, MACHOs, primordial black holes, and the like in the realm.
dark matter suspect line-up.
We require more accurate measurements of ionization within the Central Molecular Zone; achieving this precision would allow us to determine if the ionization aligns with the anticipated distribution of dark matter," stated Balaji. "By eliminating alternative explanations for the observed ionization, the case for the dark matter theory strengthens considerably.
Additional proof linking annihilating dark matter to unusual signals from the CMZ might come from NASA’s forthcoming mission.
COSI
(The Compton Spectrometer and Imager) gamma-ray observatory, scheduled for launch in 2027.
COSI ought to offer improved data concerning astrophysical phenomena at the MeV (1 million electron volts) level, which might assist in validating or dismissing this particular theory about dark matter.
In any event, this study has yielded a
a fresh perspective on the impact of dark matter
.
Dark matter continues to be one of the greatest enigmas in physics, and this research suggests that we might have missed its gentle chemical influence on the universe," Balaji stated. "If this hypothesis stands true, it could introduce a completely novel approach to investigating dark matter—one that goes beyond its gravitational pull and delves into how it moulds the structure of our Milky Way.
The group's findings were released on Monday (March 10) in the journal Physical Review Letters.
