As Tim Radford, former science editor of The Guardian newspaper, noted in his 2011 book The Address Book, inquisitive schoolchildren the world over have a certain ritual. Handed a new exercise book and asked to add their name and address, they do so on a Universal scale. House number, street, town and country, postcode even, are followed by their designated continent and the name of our planet. Up the cosmic scale they go, noting the Solar System then the Milky Way, before offering the final identifier: the Universe.
It sounds precise; it is anything but. Most difficult for a deep-space postal service would be the first jump, from the infinite stretch of the known Universe down to our local Galaxy, the Milky Way. Well, things just got a little easier. (Although perhaps not for Radford, who may need to update his book.) This week, scientists add a new line to our planetary coordinates: the Laniakea galaxy supercluster.
Do not bother googling the name. It really is brand new, coined by an international group of astronomers on page 71 of this issue. Our place in the Universe, for so long one of the core mysteries of human existence that scientists and this journal are dedicated to unravelling, just got a little clearer. Laniakea, the scientists write, is our home supercluster, the one in which the Milky Way resides.
What kind of home is it? It is big — some 160 million parsecs across. Although not as big as some superclusters, it is the largest in our local neighbourhood, which is surprisingly crowded given the vast emptiness of most of the cosmological void. Laniakea has several supercluster neighbours, including Coma, Perseus–Pisces and Shapley. (Together they make what? A super-supercluster? A hypercluster?)
It is a home that has been hiding in plain sight, colossal and all around us, yet unnoticed by previous astronomical surveys. As Elmo Tempel discusses in a News & Views article on page 41, this is probably because the boundaries of superclusters are tricky to pin down, even for astronomers.
“It is a home that has been hiding in plain sight, colossal and all around us, yet unnoticed by previous astronomical surveys.”
Laniakea was finally spotted with the help of what Tempel calls a “nifty algorithm” that helped the astronomers to turn incomplete measurements of the motion of galaxies into a map of the distribution and dynamics of cosmic matter. Their map shows galaxy superclusters as hotspots — basins of attractions in fields of velocity flow — that can be hived off from their surroundings.
It is a local map. The nifty algorithm is limited because it depends on direct measurements from Earth of how rapidly galaxies recede owing to cosmic expansion. The rest of the Universe — those galaxies far, far away — remains uncharted territory, for now.
Still, the Laniakea survey is more than cartographical and geographical information. It reveals details of the large-scale structures that surround the Milky Way (best viewed in a video available here: go.nature.com/hpjzwh), which should help astronomers to close in their determinations of cosmological parameters such as the density of dark energy, the hidden power believed to push the Universe away from us.
The name Laniakea has Hawaiian roots, and roughly translated means spacious heaven. It is a beautiful address to have. And one that comes just in time for the new school year and a new curious generation.
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