Lecture Notes: The Missing Baryon Problem

Introduction

• Sponsored by KiwiCo (more information at the end).
• Discussion on the missing baryon problem, which refers to the normal ordinary matter in the universe that remains undetected.

Understanding Baryonic Matter

• The universe is composed of:
  • 27% dark matter
  • 68% dark energy
  • 5% baryonic matter (ordinary matter)
• Current detection shows only 2.5% baryonic matter present.
• Baryonic matter includes protons and neutrons, which make up stars, planets, and everything visible.

Origin of Baryonic Matter

• Expectation of 5% baryonic matter based on elemental abundances in the universe.
• Key ratios: deuterium to hydrogen to helium observed from the early universe.
• After the Big Bang, fusion of protons and neutrons formed helium-4, with deuterium as a precursor.

The Big Bang and Element Formation

• Universe was radiation-dominated and hot initially, allowing for fusion.
• By 10 seconds after the Big Bang, conditions allowed deuterium formation.
• By 20 minutes post-Big Bang, fusion rates decreased, locking in elemental abundances: 75% hydrogen and 25% helium.
• Deuterium is stable and originates from the Big Bang, with minimal production post-Big Bang.

Observations and Measurements

• Cosmic Microwave Background Radiation provides insights into early universe conditions.
• Density of radiation helps calculate expected baryonic matter.
• Late 1990s: scientists conducted a census of observable matter (stars, galaxies, etc.) and found only 20% of expected baryonic matter.

Finding the Missing Baryons

• Ordinary matter not easily visible; much is in darkness.
• Quasars can serve as backlights to detect neutral hydrogen along the line of sight (Lyman-alpha forest).
• Lyman-alpha forest shows where neutral hydrogen exists and contributes to baryon budget, revealing almost 50% of baryonic matter.

Discovery of the Warm-Hot Intergalactic Medium (WHIM)

• WHIM consists of ionized baryons spread thinly between galaxies.
• Possesses a temperature of 100,000 to 10 million Kelvin, making detection difficult.
• WHIM primarily emits/absorbs in high energy UV or low energy X-rays.

Fast Radio Bursts (FRBs)

• In 2007, first FRBs detected from distant galaxies.
• Powerful pulses of radio waves that can help estimate the presence of ionized baryons in the WHIM.
• Dispersion of FRBs correlates with the amount of ionized baryons encountered, allowing for calculations of total baryonic matter.

Conclusion

• Recent findings confirm that about 50% of missing baryons exist in the WHIM, consistent with earlier simulations.
• Highlights the inefficiency of ordinary matter's formation into visible structures (10-20% in stars and galaxies).
• Distinction between scientists and non-scientists: scientists seek unexpected results for new discoveries.

Sponsorship by KiwiCo

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