Spanning across a distance of 180 thousand light-years, the Milky Way is home to an estimated 100 to 400 billion stars, some of which are as old as the universe itself, and probably at least 100 billion planets. It is surrounded by many other galaxies, all of which are moving away from each other just like everything in the universe according to Big Bang cosmology. The Milky Way itself is moving at a speed of 600 kilometers per second with respect to extragalactic frames of reference. It is also moving towards something elusive known as the Great Attractor, a gravitational anomaly far away, the nature of which we don’t yet know.
Over four decades ago, astronomers began to notice that the Milky Way is moving much faster through space than it should be. When they looked at the sky for a possible explanation, they found none.
The anomaly was first observed in 1973 and then again in 1978 when astronomers were making detailed maps of the cosmic microwave background, the radiation left from the early days of the universe. According to Hubble’s law, all the astronomical objects in the universe are in motion because of the expansion of the universe, an observation that supports the Big Bang theory. This motion is known as Hubble’s flow.
One conceptual model of the universe’s expansion is to consider a rubber sheet or a balloon with dots on its surface. As the sheet is stretched or the balloon is filled with air, all the dots on the surface are being moved away from each other at the same rate. This means that the net motion of the dots around any specific dot should be zero. This, however, does not seem to be the case of the galaxies around the Milky Way. The only explanation is that there is an additional gravitational force nearby that astronomers call the Great Attractor pulling us towards it.
The Great Attractor is located at the center of the Laniakea Supercluster between 150 and 250 million light years away from us in what is known as the Zone of Avoidance, a part of the universe that is notoriously difficult to observe at visible wavelengths.
The Laniakea Supercluster is 520 million light-years long and contains over 100,000 galaxies. The Milky Way is part of a group of 54 galaxies called the Local Group. The Local Group is part of the Virgo Supercluster, one of the four subparts of Laniakea. Though astronomers were able to detect many galaxies and stars in Laniakea, they cannot see the area of the sky that is obscured by the disk of the Milky Way because the light from beyond it is blocked by all the stars, gas, planets, and interstellar dust of our own galaxy. This area is known as the Zone of Avoidance. It roughly covers 10 degrees on either side of the galactic plane obstructing the view of around 20 percent of the sky outside our galaxy at visible wavelengths.
Since it’s impossible to observe at visible wavelengths, radio and X-ray surveys are used to observe what lies beyond the Milky Way.
The location of the Great Attractor was determined in 1986 and was found to be in the direction of the constellations Triangulu Australe (The Southern Triangle) and Norma (The Carpenter’s Square). Through X-ray surveys, astronomers have also found that the Zone of Avoidance is dominated by the Norma cluster, a massive cluster of very old galaxies many of which are colliding with each other and giving off radio waves. The area beyond the center of the Milky Way, however, where the dust is thickest, is still impossible to observe.
According to Lister Staveley-Smith of the University of Western Australia, Australian astronomers are in prime position to observe the nearby universe and possibly the Great Attractor.
Radio surveys such as CAASTRO‘s 2MTF survey which uses the Parkes telescope and the WALLABY survey which uses the ASKAP telescope are believed to help explore the radio universe in detail. Another new survey known as TAIPAN and conducted by the Australian Astronomical Observatory and the Australian National University targets elliptical galaxies at optical wavelengths. Some theorists even believe that the space-time metric we use to describe the universe may become obsolete and general relativity itself might need to be modified at large scales.
While the Great Attractor still remains a mystery, in 2005, astronomers conducting X-ray surveys found that the Milky Way and Laniakea are moving towards the Shapley Attractor, which is ten times more massive. The Shapley Attractor is a massive cluster of galaxies near the Shapley Supercluster, a very close neighbor of the Local Group of galaxies, including the Milky Way.