2023's Biggest Breakthroughs in Physics

A billion years in the past, two black holes collided at an incredibly high speed, causing a violent explosion that distorted the fabric of SpaceTime. These distortions, known as gravitational waves, involve the literal stretching and compressing of SpaceTime. This phenomenon was predicted by Einstein over a century ago, as he understood that mass has the ability to bend SpaceTime. Therefore, if mass experiences any sort of movement or vibration, SpaceTime will also be affected. However, Einstein believed that detecting gravitational waves would be nearly impossible due to their minuscule nature. Nevertheless, in 2015, scientists at the gravitational wave Observatory called LIGO achieved a significant breakthrough by successfully detecting gravitational waves. This accomplishment marked the beginning of a new era in which we can study and understand gravitational waves. Gravitational waves travel at the speed of light and carry valuable information about the most extreme objects in the universe, such as black holes. While ground-based detectors like LIGO are effective in measuring black holes of solar mass or around 100 solar masses, detecting the most massive black holes or different types of gravitational waves requires instruments that operate at much lower wavelengths. For the past 15 years, an international coalition known as Nanograph has been dedicated to detecting these elusive gravitational waves.

Using a technique known as a pulsar timing array, we employ some of the largest radio telescopes in the world to observe a multitude of pulsars and, with any luck, directly detect gravitational waves at low frequencies in the universe. At the end of its life, a massive star undergoes gravitational collapse, forming a neutron star at its core. Remarkably dense, just a teaspoon of neutron star material could weigh as much as a mountain. Certain neutron stars begin rotating rapidly, emitting beams of radiation, which we refer to as pulsars. The pulsars we study rotate at speeds comparable to a kitchen blender, and when their beams align with our line of sight, similar to a lighthouse, we detect radio pulses. Gravitational waves have the ability to subtly affect the timing of these pulses. These waves may originate from beyond our galaxy, causing disturbances in the space between Earth and the numerous pulsars we observe. Essentially, we have transformed our local region of the Milky Way, along with the millisecond pulsars within it, into a large-scale detector capable of capturing the passage of gravitational waves.

In June 2023, Nanograph released its 15-year data, presenting compelling evidence for the existence of low-frequency gravitational waves that permeate our universe. For the very first time, we have strong evidence of what we refer to as "The Smoking Gun" of gravitational waves in this frequency range. Although it may appear as noise or a random signal, when we correlate the signal with multiple pulsars, a distinct correlation pattern emerges, known as the Helens and Downs curve. This pattern serves as the fingerprint of the universe, where gravitational waves from various sources combine to create a hum that we measure. The most probable source of these waves is believed to be the violent collisions of supermassive black holes. However, physicists remain hopeful for more exotic explanations. It may seem paradoxical, but supermassive black holes could be considered the most ordinary explanation for our observed signal. We are aware that most galaxies harbor supermassive black holes, and galaxy mergers occur frequently. Nevertheless, numerous theoretical papers have proposed alternative explanations for this background, involving fundamental physics beyond the standard model, such as strings or new forms of dark matter. These possibilities are incredibly exciting and hold the potential to expand our understanding of the universe.