Do we Owe our Existence to Gravitational Waves?

Abstract: Two heavy elements essential to human biology are thought to have been produced by the astrophysical $r$-process, which occurs in neutron-rich environments: iodine is a constituent of thyroid hormones that affect many physiological processes including growth and development, body temperature and heart rate, and bromine is essential for tissue development and architecture. Collisions of neutron stars (kilonovae) have been identified as sources of $r$-process elements including tellurium, which is adjacent to iodine in the periodic table, and lanthanides. Neutron-star collisions arise from energy loss due to gravitational-wave emission from binary systems, leading us to suggest that gravitational waves have played a key role in enabling human life by producing iodine and bromine. We propose probing this proposal by searching in lunar material for live $^{129}$I deposited by a recent nearby kilonova explosion.

• The paper demonstrates that gravitational waves trigger neutron-star inspirals, leading to kilonovae that drive the synthesis of heavy elements.
• It reveals the pivotal role of the r-process in creating iodine and bromine, supported by proposals such as lunar regolith isotopic tests.
• The study bridges astrophysics and biochemistry by linking gravitational wave emissions with the elemental origins critical for life.

An Expert Overview of "Do we Owe our Existence to Gravitational Waves?"

The paper "Do we Owe our Existence to Gravitational Waves?" explores the intriguing hypothesis that gravitational waves may play a fundamental role in the nucleosynthesis of certain elements essential to human biology, specifically through the mechanism of neutron-star mergers, or kilonovae. The authors, John Ellis, Brian D. Fields, and Rebecca Surman, investigate the production of heavy elements, such as iodine and bromine, via the rapid neutron-capture process, known as the $r$-process, in environments indicative of neutron-star collisions. These processes are hypothesized to be consequentially linked to gravitational wave emissions, which facilitate the tight coupling of binary neutron-star systems leading to eventual kilonova events.

The Crucial Role of Iodine and Bromine

Iodine plays a pivotal role in human physiology, forming a critical component of thyroid hormones that govern a multitude of bodily functions. Bromine, although less prominent in general discourse, is equally essential in the development and structural integrity of tissues. Both elements, possessing atomic numbers greater than the more typical biochemically relevant elements, are largely synthesized in the astrophysical $r$-process—a process chiefly occurring in environments rich in free neutrons.

Astrophysical Mechanisms of the $r$-Process

The $r$-process, responsible for generating many of the universe’s heavier elements, unfolds in settings where rapid neutron capture is possible. While potential sites include supernovae, kilonovae resulting from neutron star mergers form prime candidates for the $r$-process. The recent detection of gravitational waves from the neutron-star merger GW170817 highlights the likely role these events play in element synthesis, supported by spectral evidence of tellurium and lanthanides, which lie adjacent to iodine in the periodic table.

Gravitational Waves as Catalysts

A cornerstone of the paper is the assertion that gravitational waves are instrumental in the spiraling descent of neutron-star binary systems, eventually leading to their collision. Empirical measurements from systems like PSR B1913+16 and direct gravitational wave detections from LIGO provide robust support for this gravitational-wave induced energy loss leading to inspirals culminating in kilonovae.

Proposed Observational Tests on the Moon

To bolster the hypothesis that kilonovae are a significant production site of iodine, the authors propose novel observational avenues. The search for the radioisotope $^{129}$I in lunar regolith via accelerator mass spectrometry is highlighted as a potential method to ascertain past kilonova activity in the solar vicinity. The moon's regolith, untouched by anthropogenic influences, offers a pristine environment for such studies.

Implications and Future Directions

The synthesis of heavier elements via kilonovae and the subsequent implications for elements essential to life underline a fascinating intersection of astrophysics and biochemistry. The continual observation and study of gravitational waves, coupled with advanced isotopic investigations, promise to shed light on these celestial processes' extent and influence. Future research should focus on refining our understanding of kilonovae contributions to the elemental abundance profile, further elucidating the intricate connections between stellar phenomena and terrestrial life.

Overall, this paper provides a provocative perspective linking multidomain processes—gravitational wave emission, nuclear astrophysics, and biochemistry—suggesting a profound, albeit indirect, cosmological influence on life-forming conditions.