The 10-billion-light-year cosmic megastructures helping us understand dark matter and dark energy

Reported by Robert Lea

How truly insignificant our own planetary system is on the grand scale of the universe. Like cosmic nesting dolls, the Solar System H is part of the Milky Way, with the Sun just one of around 100 billion stars, and the Milky Way is just one of over 54 members that comprise the Local Group of galaxies. And this Local Group is part of something even larger – the Virgo Supercluster. This supercluster contains the Milky Way, Andromeda and at least 100 groups and clusters of galaxies spanning a width of around 110 million light years. The mass of the Virgo Supercluster, or the Local Supercluster as it is sometimes called, is estimated to be equivalent to around 1.5 quadrillion suns – that’s 15 followed by 14 zeros. That means our Solar System, estimated to be three light years across, could fit within the width of our home supercluster around 37 million times.

And the Virgo Supercluster is also part of something bigger… In 2014, astronomers discovered that the Virgo Supercluster is an ‘appendage’ of the larger Laniakea Supercluster, with Laniakea being Hawaiian for ‘open skies’ or ‘immense heaven’. This is an apt name for a supercluster that contains 100,000 to 150,000 galaxies and stretches for an estimated 520 million light years, with a mass equivalent to around 100 quadrillion Suns. Perhaps even more shocking than this is the fact that the Laniakea Supercluster is just one of an estimated 10 million superclusters in the observable universe.

“Superclusters represent the largest and most massive aggregations of matter in the universe,” Shishir Sankhyayan, a postdoctoral fellow at the University of Tartu, tells All About Space. “To draw a rough analogy, they are akin to vast mountain ranges amid the otherwise relatively flat expanses of Earth. Unlike smaller structures such as galaxies or our Solar System, superclusters are not necessarily gravitationally bound structures.” As that hints, it’s not easy answering the question of what a supercluster actually is.

As Sankhyayan explains, the term ‘supercluster’ is still a topic of active debate in astronomical circles. Some scientists consider superclusters to be gravitationally bound entities, while other researchers define them based on density. Others focus on the large-scale convergence of galaxies moving at similar speeds. He adds that this is complicated further when the choice of astronomical objects, such as individual galaxies, galaxy clusters or other things, is factored in and used to define supercluster types.