An old puzzle solved: astronomers discover the world is flat
A pan-European group of astronomers has used newly developed computer technology to solve a 100 year-old puzzle. While most galaxies in our neighborhood move away from us almost as expected for an unperturbed cosmic expansion, our nearest giant neighbour is approaching at high speed. Systematic numerical experimentation demonstrates this rapid approach is due to massive dark matter haloes surrounding both Andromeda and our own Milky Way, but this mass does not slow down somewhat more distant galaxies because its effects are counteracted by more distant dark matter which lies in a vast flattened sheet out to distances well beyond the neighboring galaxies considered.
Why is the Andromeda Nebula heading straight for us, while other nearby galaxies are receding?
It is nearly a century since the American astronomer Edwin Hubble discovered the expansion of the Universe. Distant galaxies similar to our own Milky Way move away from us at speeds that increase in proportion to their distance, reflecting the origin of the Universe in a Big Bang, an enormous explosion 14 billion years ago. Hubble already knew, however, that this is not true for our nearest giant neighbour, the Andromeda Nebula, which is 2.5 million light-years away and coming towards us at 100 kilometers per second. In 1959, two European astronomers, Franz Kahn and Lodewijk Woltjer, calculated that in order for the gravity of the two galaxies to have reversed the initial expansion, their total mass must be more than 1000 billion times the mass of the Sun – much more than the mass of all their stars put together. This was the first detection of unseen Dark Matter around our Milky Way and its neighbour.
In the 1970s and 1980s, accurate distances began to be measured for somewhat more distant galaxies. It became clear that not only are they are mostly moving away from us but that their speeds are close to those predicted by the overall cosmic expansion – starting in a “Big Bang” 14 billion years ago. Studies of galaxies at distances from 1.5 to 4 times the Milky Way-Andromeda separation found the deviations to be actually quite small – the total amount of matter required to account for these deviations out to the most distant galaxy cannot be larger than that already needed to explain the approach speed of the Milky Way and Andromeda. However, there are several other large galaxies in this region, which should contribute additional mass. Why then does the cosmic expansion around us appear so weakly perturbed?
A pan-European group of astronomers has recently used newly developed computer technology to find the solution to this puzzle. They set the machine the following task: Find representative regions of the early Universe with small deviations from uniformity that are statistically similar to the Cosmic Microwave Background, but that evolve to produce galaxies similar to the Milky Way and Andromeda, with the appropriate positions and velocities. At the same time, motions at the positions of other nearby galaxies should match those observed.
Apparently, the puzzle was not hard for the computer: it was able to find hundreds of examples satisfying all the given conditions. The average mass distribution for a large number of these is shown in the figure. In the region containing the local galaxies, motions relative to a uniform expansion are indeed small – the Hubble flow is almost unperturbed – while at larger distances material is actually moving away from the Milky Way faster than the Hubble flow.
How the computer solved the puzzle can be seen in the right image of the figure, which shows a view of the same box rotated by 90 degrees. The mass is concentrated to a flattened sheet extending well beyond the region occupied by the local galaxies considered. All the galaxies are inside the sheet and even at larger distances most known galaxies are still found in a flattened distribution known as the Local Supercluster. The computer has inferred this larger structure even though it was not told about its existence. The large low-density regions above and below the sheet are also seen in the galaxy distribution and are known as the Local Voids. However, the large velocities predicted there are not observable, because in the real universe there are no galaxies there to be measured.
Thus, there are two reasons why the local Hubble flow seems so weakly perturbed despite the large combined mass of the Milky Way and Andromeda. Mass at larger distances is counteracting the gravity of the central galaxies by pulling material outwards. In addition, there are no galaxies where the predicted infall effects are large, so inflow onto the Local Sheet is hidden.
The solution to the puzzle is that the total mass distribution in our environment is at least as sheet-like as the distribution of galaxies. The world around our Local Group of galaxies is indeed flat out to distances of tens of millions of light-years.
