No Evidence of Massive Black Holes in the Milky Way's Dark Matter Halo

Core Concepts

Gravitational wave detectors have revealed a population of massive black holes that do not resemble those observed in the Milky Way, leading to the hypothesis that they may be primordial black holes. This study finds no evidence of such primordial black holes in the Milky Way's dark matter halo based on a long-term gravitational microlensing search.

Abstract

The content discusses the search for long-timescale gravitational microlensing events that could indicate the presence of massive black holes in the Milky Way's dark matter halo. The authors present the results of a 20-year monitoring of nearly 80 million stars in the Large Magellanic Cloud by the OGLE survey.

Key highlights:

• Gravitational wave detectors have discovered a population of massive black holes that do not resemble those observed in the Milky Way.
• One hypothesis is that these black holes may have formed from density fluctuations in the early Universe (primordial black holes) and could make up a significant fraction of dark matter.
• If such primordial black holes existed in the Milky Way's dark matter halo, they would cause long-timescale gravitational microlensing events lasting years.
• The authors did not find any microlensing events with timescales longer than one year, and all shorter events detected can be explained by known stellar populations.
• The study concludes that compact objects in the mass range from 1.8 × 10^-4 to 6.3 solar masses cannot compose more than 1% of dark matter, and those in the mass range from 1.3 × 10^-5 to 860 solar masses cannot make up more than 10% of dark matter.
• Therefore, primordial black holes in this mass range cannot simultaneously explain a significant fraction of dark matter and the gravitational wave events observed.

Stats

The search was conducted on the light curves of nearly 80 million stars located in the Large Magellanic Cloud, monitored for 20 years by the OGLE survey.
Compact objects in the mass range from 1.8 × 10^-4 to 6.3 solar masses cannot compose more than 1% of dark matter.
Compact objects in the mass range from 1.3 × 10^-5 to 860 solar masses cannot make up more than 10% of dark matter.

Quotes

"We did not find any events with timescales longer than one year, whereas all shorter events detected may be explained by known stellar populations."
"Thus, primordial black holes in this mass range cannot simultaneously explain a significant fraction of dark matter and gravitational wave events."

Key Insights Distilled From

No massive black holes in the Milky Way halo - Nature

by Prze... at www.nature.com 06-24-2024

https://www.nature.com/articles/s41586-024-07704-6

Deeper Inquiries

What other potential explanations or alternative hypotheses could account for the population of massive black holes detected by gravitational wave observatories?

One potential explanation for the population of massive black holes detected by gravitational wave observatories, apart from primordial black holes, could be the direct collapse of massive stars. When a massive star exhausts its nuclear fuel, it can undergo a rapid gravitational collapse, forming a black hole without a supernova explosion. These black holes would be more massive than those formed through stellar evolution and could account for the observed population. Additionally, interactions in dense stellar environments, such as globular clusters, could lead to the formation of massive black holes through stellar collisions and mergers.

How could the sensitivity and duration of the microlensing search be improved to potentially detect the presence of primordial black holes in the Milky Way's dark matter halo?

To improve the sensitivity and duration of the microlensing search for primordial black holes in the Milky Way's dark matter halo, several strategies could be employed. Firstly, increasing the number of monitored stars and extending the observation period would enhance the chances of detecting long-timescale microlensing events. Utilizing advanced telescopes with higher resolution and sensitivity, such as the upcoming James Webb Space Telescope, could also improve the detection capabilities. Implementing machine learning algorithms to analyze the vast amount of data collected from microlensing surveys could help in identifying subtle and extended microlensing events caused by primordial black holes.

What implications would the existence of primordial black holes have for our understanding of the early Universe and the formation of structure in the cosmos?

The existence of primordial black holes would have profound implications for our understanding of the early Universe and the formation of structure in the cosmos. If primordial black holes make up a significant fraction of dark matter, it would suggest that they were formed in the extreme conditions of the early Universe, possibly during phase transitions or inflationary periods. The presence of primordial black holes could provide insights into the nature of dark matter and the mechanisms responsible for seeding the formation of galaxies and large-scale structures in the Universe. Studying the properties and distribution of primordial black holes could offer valuable clues about the fundamental processes that shaped the cosmos in its infancy.