The video explores the mystery of fast radio bursts (FRBs), their discovery, and the challenges in understanding their origins. Initially thought to be cataclysmic events, the repeated detection of FRBs suggests a different source, possibly young neutron stars. The video also presents a challenge question about the travel distance of cosmic microwave background photons and the mean free path of photons before recombination, explaining the calculations involved. The episode concludes with acknowledgments and rewards for correct answers.

What was the initial belief about the origin of Fast Radio Bursts before the discovery of their true source?

• They were signals from alien civilizations.

• They were from within the Milky Way.

• They were reflections of Earth's radio signals.

• They were caused by solar flares.

What significant discovery about Fast Radio Bursts was made in March 2016?

• They are caused by supernovae.

• They can repeat themselves.

• They originate from black holes.

• They are signals from extraterrestrial life.

What is the distance that cosmic microwave background photons traveled to reach us?

• 13.7 million light years

• 13.7 billion light years

• 137 billion light years

• 1.37 billion light years

What was the main challenge for photons traveling through the universe before recombination?

• Absorption by dark matter

• Deflection by magnetic fields

• Interacting with free electrons

• Colliding with protons

What is the Thompson scattering cross section used to describe?

• The mass of a neutron

• The speed of light

• The interaction region of electrons

• The size of a proton

How many electrons were present per cubic meter at the moment of recombination?

• 20 million

• 2 million

• 200 million

• 2 billion

What is the mean free path of photons in the early universe?

• 75,000 light years

• 7500 light years

• 750,000 light years

• 750 light years