Introduction to NANOGrav's Discovery

As the holiday weekend approached, anticipation surged across social media when the NANOGrav collaboration announced a significant discovery set to be revealed on Thursday, June 29. This network of observatories across North America had been observing the universe through radio wavelengths for over 15 years, and they were ready to share their monumental findings.

Those closely following NANOGrav's research were filled with hope. The team had been diligently tracking the rhythmic signals of radio pulsars, aiming to detect subtle changes in the structure of space-time caused by gravitational waves. The big news was expected to provide insights into how our universe might be warping in observable ways.

Insights into the Universe

“We can gain unique insights about our Universe through observations with the multiple messengers of gravitational waves and electromagnetic waves at radio to gamma-ray frequencies observed with telescopes on Earth and in space.” Credit: NANOGrav.org

Established in 2015 with funding from the National Science Foundation, NANOGrav has utilized various radio telescopes, including the Arecibo and Greenbank dishes, along with CHIME and the Very Large Array, to monitor a growing number of pulsars.

These rapidly spinning neutron stars emit radio waves like a hyperactive lighthouse, rotating at speeds of up to 1000 times per second. Neutron stars are the dense remnants of massive stars that have exhausted their nuclear fuel, resulting in colossal balls of neutrons. Due to the close proximity of these particles, neutron stars can condense substantial mass into a volume comparable to Manhattan Island, with masses ranging from 1.4 to about 2.2 times that of our Sun.

The pulsars' intense rotation means they serve as incredibly precise timekeepers. When pulsars exist in binary systems, their orbits can be accurately measured through slight shifts in their pulse timings. As a pulsar moves toward Earth, the distance traveled by each pulse decreases, causing the ticking to appear faster. Conversely, as it moves away, each pulse has to travel a greater distance, leading to a perceived slowdown in the ticking.

This remarkable timing accuracy was exemplified in 1992 when the first exoplanet was discovered orbiting a pulsar, detectable through its minuscule movements.

Pulsars have been featured in numerous science fiction narratives as navigational references. For instance, the Voyager missions included a gold plate with a map indicating the location of our solar system in relation to various pulsars.

Stretching Space-Time

If the universe consists of elastic space-time as per Einstein's theory, these pulsars should exhibit movement in tandem with the fluctuations of space itself, manifesting as minute timing variations in their pulse emissions.

According to Einstein, significant mass interactions can generate gravitational waves that ripple through the cosmos. If a solitary pulsar appears to oscillate slightly, it may indicate the contraction and expansion of space in response to a passing gravitational wave while the pulsar remains stationary within the space-time fabric.

However, measuring these minute changes in pulsar timing is complex. If researchers only observe a single pulsar showing a variation, they cannot definitively ascertain whether the timing is incorrect or if space is indeed stretching. This is why the team aimed to monitor as many pulsars as possible.

By observing multiple pulsars in various locations, researchers can detect the timing discrepancies produced by a passing gravitational wave, as pulsars aligned with the wave will experience changes differently compared to those positioned off to the side.

The Evolution of NANOGrav

When NANOGrav first received funding, existing gravitational wave detectors had yet to yield any results, leading some, including myself, to doubt the feasibility of detecting minute variations in space-time. While facilities like LIGO and Virgo are sensitive to various disturbances, pulsars offer the potential to observe massive gravitational waves resulting from the merging of supermassive black holes and other colossal systems.

Thus, in 2007, NANOGrav commenced its mission. In 2015, with NSF funding, the team established a Physics Frontiers Center dedicated to their research. The following year, LIGO and Virgo announced their inaugural detection of a merger between two stellar mass black holes, while NANOGrav continued its observations.

On June 23, NANOGrav unveiled its findings through several papers, proclaiming, “The signal is here… we can observe the fabric of space exhibiting wobbly variations through the minuscule timing changes in pulsar emissions.”

The core finding, featured in a special issue of the Astrophysical Journal Letters, revealed correlations in timing variations among 68 pulsars with three or more years of data. By analyzing 2,211 distinct event pairs, they successfully replicated the anticipated signals indicative of supermassive black hole mergers, which ripple through the universe as gravitational waves.

The signal is evident, yet the data remains noisy, preventing researchers from definitively linking timing events to specific cosmic occurrences. We are not yet at the stage where we can detect a gravitational wave signal and immediately direct telescopes to observe its source.

Reflecting on the Journey

Over the past four centuries, our exploration of the universe through light has allowed us to construct a comprehensive understanding of cosmic expansion, star formation, and planetary evolution. We recognize that we are made of stardust, yet there remains much to discover, as light only reveals a fraction of the universe's mysteries. Events from the universe's inception up to 400,000 years post-creation remain obscured by the cosmic microwave background.

In time, future generations will look back at 2015—the year NANOGrav began, 2016—the year LIGO and Virgo made their groundbreaking detection, and 2023—the year of these pivotal findings, acknowledging it as the dawn of a new era in astronomical discovery. This progress is not attributed to a solitary individual with a rudimentary device but rather to the collective efforts of scientists striving to push technological boundaries for one fundamental purpose: to unveil the secrets of our universe.

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