The yellow line shows the track of Andromeda with respect to the Milky Way.
Credit: Fabian Lueghausen / University of Bonn
For many years scientists have believed that our Galaxy, the Milky Way, is set to crash into its larger neighbour, the Andromeda Galaxy, in about 3 billion years' time and that this will be the first time such a collision has taken place.
But now a European team of astronomers led by Hongsheng Zhao of the University of St Andrews propose a very different idea; that the two star systems collided once before, some 10 billion years ago and that our understanding of gravity is fundamentally wrong.
Remarkably, this would neatly explain the observed structure of the two galaxies and their satellites, something that has been difficult to account for until now.
Dr Zhao will present the new work at the RAS National Astronomy Meeting in St Andrews on Thursday 4 July.
The Milky Way, made up of about 200 billion stars, is part of a group of galaxies called the Local Group.
Astrophysicists often theorise that most of the mass of the Local Group is invisible, made of so-called dark matter.
Most cosmologists believe that across the whole universe, this matter outweighs 'normal' matter by a factor of five.
The dark matter in both Andromeda and the Milky Way then makes the gravitational pull between the two galaxies strong enough to overcome the expansion of the cosmos, so that they are now moving towards each other at around 100 km per second, heading for a collision 3 billion years in the future.
But this model is based on the conventional model of gravity devised by Newton and modified by Einstein a century ago, and it struggles to explain some properties of the galaxies we see around us.
Dr Zhao and his team argue that at present the only way to successfully predict the total gravitational pull of any galaxy or small galaxy group, before measuring the motion of stars and gas in it, is to make use of a model first proposed by Prof. Mordehai Milgrom of the Weizmann Institute in Israel in 1983.
This modified gravity theory (Modified Newtonian Dynamics or MOND) describes how gravity behaves differently on the largest scales, diverging from the predictions made by Newton and Einstein.
Dr Zhao (University of St Andrews) and his colleagues have for the first time used this theory to calculate the motion of Local Group galaxies.
Their work suggests that the Milky Way and Andromeda galaxies had a close encounter about 10 billion years ago.
If gravity conforms to the conventional model on the largest scales then taking into account the supposed additional pull of dark matter, the two galaxies would have merged.
More information: A preprint of the the paper, entitled "Local Group timing in Milgromian dynamics. A past Milky Way-Andromeda encounter at z>0.8", is available from arxiv.org/abs/1306.6628
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