Cosmologists have long speculated that only around 20 percent of the universe's matter is composed of what they term 'baryonic' matter, encompassing the familiar entities of stars, galaxies, atoms, and life as we know it. The remaining 80 percent has remained an enigma, believed to be composed of dark matter, a substance whose elusive nature continues to elude scientific understanding and may consist of undiscovered subatomic particles.
Now a team of international scientists, including researchers from Chiba University, has unveiled the long-awaited answer to one of cosmology's most perplexing questions: How much matter truly exists in the vast expanse of the universe? Their findings, published in The Astrophysical Journal, reveal that matter constitutes a significant 31 percent of the universe's total matter and energy, shedding light on the mysterious cosmic balance.
This monumental achievement in the realm of cosmology brings us closer to unravelling the universe's mysteries. With a resounding answer to the question of how much matter permeates the cosmos, scientists have taken a significant step toward understanding the fundamental building blocks of our universe, all while uncovering new avenues for cosmological exploration.
To accomplish this feat of cosmic measurement, the research team employed a well-established technique. They compared the observed number and mass of galaxy clusters per unit volume with predictions derived from numerical simulations. The number of these clusters, referred to as 'cluster abundance,' proves to be remarkably sensitive to cosmological conditions and, critically, the total amount of matter in the universe.
However, gauging the mass of galaxy clusters posed a formidable challenge. The majority of matter within these clusters remains hidden in the shroud of darkness, eluding direct observation through telescopes. To circumvent this obstacle, the team ingeniously turned to an indirect method, relying on a correlation known as the 'mass richness relation' (MRR). It stands to reason that more massive clusters should house a greater number of galaxies compared to their less massive counterparts. Given that galaxies are composed of luminous stars, their count within each cluster served as an indirect means of approximating the cluster's total mass.
Using data from the Sloan Digital Sky Survey, the team meticulously tallied the number of galaxies within their sample clusters. This allowed them to estimate the total mass of each cluster. When these estimates were compared against predictions from numerical simulations, an astonishing alignment was discovered—a universe in which matter accounts for precisely 31% of the total matter and energy. Remarkably, this value closely paralleled measurements obtained through a completely independent technique involving cosmic microwave background (CMB) observations from the Planck satellite.
Tomoaki Ishiyama from Chiba University highlighted the significance of their work, emphasising that "this work further demonstrates that cluster abundance is a competitive technique for constraining cosmological parameters and complementary to non-cluster techniques such as CMB anisotropies, baryon acoustic oscillations, Type Ia supernovae, or gravitational lensing."
A key factor in the success of this research lies in their pioneering use of spectroscopy—a technique that dissects radiation into a spectrum of distinct bands or colors. This precision allowed the team to determine not only the distance to each cluster but also to identify the galaxies genuinely bound to the cluster's gravitational influence. This sophisticated approach represented a significant departure from earlier studies, which had to rely on less accurate imaging techniques, such as multi-wavelength sky images, to gauge distances and membership of galaxies within clusters.
