Beyond the atmosphere, past the stars we see, farther than the Milky Way, and continuing past Andromeda, we reach the real cosmic ocean. So called because like an ocean on Earth, it is vast, homogeneous, and impossible to navigate by common sense alone. In the cosmic ocean, an impossibly huge amount of space separates island galaxies, whose strong gravity binds them across incredible distances, dictating their course, and forming the largest and most massive structures in the universe: galaxy clusters.
Because these immense structures are so vast and so distant, it requires the work of several telescopes to map out their locations. But the reward is the understanding of these great structures that are defined by, and directly relatable to, the evolution of the universe.
The XXL survey is a collaboration of over 100 Astronomers and multiple ground-based and space-based telescopes. Their goal is to discover, characterize, and study galaxy clusters that cover nearly half the age of the universe. The added bonus is the measure of the third dimension, their distance to us. This is possible because the telescopes cover a wide wavelength range on the electromagnetic spectrum, allowing them to precisely measure what we call spectroscopic redshift. The XMM-Newton X-ray Space Telescope imaged two patches of sky, each 100 times the size of the full moon, in order to discover these new galaxy clusters, while ground based telescopes gave us the spectroscopic redshift, the 3D element.
When it comes to the most distant objects in the universe, like galaxy clusters and superclusters, we can measure their distance by how quickly they are moving away from us. It was Edwin Hubble that discovered the expansion of the universe, and the Hubble constant tells us how quickly space is expanding. The true power of the Hubble constant is the relationship between redshift and distance, meaning a precise measure of a galaxy’s redshift correlates with how far away it is, allowing us to build a 3D map of the entire universe.
The understanding of clusters and their distribution helps us to better characterize the enigmatic Dark Matter and Dark Energy, one a gravitationally dominant matter that pulls clusters of Galaxies together, the other a strange vacuum energy that is accelerating the expansion of the universe. Every day we come one step closer to finding out what is really going on in the universe.