Chapter 1: A Historical Perspective

In the year 1912, the notion of the American Dream inspired hope among citizens and immigrants in the United States. As farmers moved into the vibrant city, diverse accents filled the streets of New York. Italian workers labored in coal mines, while Russian and Polish couples operated bustling pushcart markets. Stories of figures like Andrew Carnegie and John D. Rockefeller fueled aspirations for prosperity, even amidst the harsh realities of low wages and perilous working conditions that sparked the formation of the first labor unions. Meanwhile, the RMS Titanic tragically sank on the night of April 14th.

At the same time, scientists embarked on experiments utilizing electroscopes—devices designed to detect electric charge, akin to early radiation detectors. Their experiments uncovered a persistent background radiation that seemed to emanate from nowhere. Surprisingly, this radiation persisted even in underground tests, leading researchers to hypothesize it originated from the Earth itself. However, when these electroscopes were launched in hot air balloons, the signals increased dramatically after an initial decline, caused by radiation interacting with air molecules. This indicated that the radiation was not terrestrial; rather, it was cosmic in nature, with higher energy levels suggesting an extragalactic origin.

Chapter 2: Understanding Cosmic Rays

Cosmic rays, as they are aptly named, have puzzled physicists for decades. While some low-energy cosmic rays stem from solar processes, the majority are believed to originate from supernova explosions within our galaxy. These colossal explosions generate shock waves that entrap particles in magnetic fields, accelerating them until they escape into space. The particles vary from positively charged protons to heavier ionized nuclei, such as oxygen and iron, although heavier nuclei constitute only about 1% of cosmic rays. Secondary cosmic rays, including muons and electrons, permeate our daily lives, while the higher-energy particles that astonished scientists upon discovery are rare and elusive, leaving their origins a profound mystery.

While supernovae can explain low-energy cosmic rays, they cannot solely account for the acceleration of particles to ultra-high energies. In October 1991, the detection of the "Oh-My-God" particle in Utah marked the highest energy particle ever recorded, containing millions of times more energy than those created at the Large Hadron Collider. Whereas man-made particles reach a maximum of approximately 10¹³eV (electronvolts), the OMG particle attained energies of 3 x 10²⁰ eV, comparable to a baseball traveling at over 50 mph compressed into a single proton.

Particles exceeding 10¹⁹ eV are classified as ultra-high energy cosmic rays (UHECR) and are thought to originate from outside our galaxy. Their immense energy levels suggest they arise from catastrophic cosmic events. Possible sources include active galactic nuclei, gamma-ray bursts, pulsars, and magnetars, though none have been definitively identified.

The first video explores the enigma of cosmic rays, featuring Kathryn Plant discussing their mysterious origins and implications.

Chapter 3: Potential Sources of UHECR

Recent research proposes a connection between UHECR and starburst galaxies, regions of the universe where star formation occurs at an accelerated rate. A series of supernovae in these galaxies can generate a significant outflow of gas, leading to shockwaves that propel particles to nearly light speed. Magnetic fields ensnare these particles, allowing them to gain energy as they traverse shock fronts until they reach sufficient energy to escape into space. However, according to the Pierre Auger Observatory, this research still leaves 90% of observations unexplained, indicating that further investigation is necessary.

The second video delves into the exploration of the universe using ultra-high energy cosmic rays, shedding light on their potential sources.

Chapter 4: Challenges in Tracing Cosmic Rays

One significant challenge in associating events like gamma-ray bursts or pulsars with cosmic ray origins is the limited distance from which these particles could have traveled. If they originated over 1–200 million light-years away, they would lose energy through interactions with cosmic background radiation. Yet, no clear sources have been identified within that range. Interestingly, a concentration of cosmic rays appears to emanate from the Ursa Major cluster, suggesting a possible "hot spot."

Tracing these particles back to their origins is complex because, unlike neutrinos that travel in straight lines, cosmic rays are deflected by magnetic fields, including those within the Milky Way. Heavier particles can also transform as they lose energy, complicating their identification.

Upon reaching Earth, detectors such as CREAM (Cosmic-Ray Energetics and Mass investigation) stationed in the stratosphere above Antarctica gather data on the energy, type, and trajectory of incoming particles. Although the ISS-CREAM detector, which was positioned to study UHECR, was decommissioned in February of this year, ground-based arrays continue to collect valuable information. When cosmic rays collide with the atmosphere, they trigger a cascade of events—air showers—that excite nitrogen molecules, leading to the emission of detectable fluorescence.

Chapter 5: The Ongoing Mystery of Cosmic Rays

Theoretical models suggest that ultra-heavy dark matter could decay into UHECR particles, although current observations indicate that most of these particles are not photons or neutrinos. Hypothetical remnants from the Big Bang, such as domain walls or cosmic strings, might produce cosmic rays upon their collapse, yet no concrete evidence for these structures has been discovered.

Cosmic rays also impact human health in space, causing bright visual disturbances for astronauts and potentially leading to vision impairment and cognitive effects. These visual phenomena can appear as streaks or clouds, and prolonged exposure may result in cellular mutations, making cosmic rays not only a captivating scientific puzzle but also a significant hurdle for human space exploration. As we continue to study these enigmatic particles, we find ourselves still grappling with questions that date back to their initial discovery in the early 1900s.