Two neighboring star systems the Milky Way and Andromeda Galaxy collided once before, some 10 billion years ago,
according to a European team of astronomers led by Hongsheng Zhao of the University of St Andrews.
The commonly accepted theory postulates that our Galaxy, the Milky Way, is set to crash into its larger neighbor,
the Andromeda Galaxy, in about 3 billion years' time and that this will be the first time such a collision has taken place.
The new study also suggests 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.
Milky Way/Andromeda Collision (artist's impression) Credits: James Gitlin
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.
Based on the conventional model of gravity devised by Newton and modified by Einstein a century ago, the dark matter
in both Andromeda and the Milky Way 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.
Dr Zhao and his team say that this model to explain some properties of the galaxies we see around us.
Therefore, 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, the astronomers say, 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.
A schematic diagram showing how the Andromeda Galaxy (at bottom right) collided with the Milky Way (at the intersection of the axes) 10 billion years ago, moved out to a maximum distance of more than 3 million light years and is now approaching
our Galaxy once again. The yellow line shows the track of Andromeda with respect to the Milky Way.
Credit: Fabian Lueghausen / University of Bonn.
Dr Zhao's team has for the first time used MOND to calculate the motion of Local Group galaxies, and the results
indicate that the Milky Way and Andromeda galaxies already had a close encounter about 10 billion years ago.
"Dark matter would work like honey: in a close encounter, the Milky Way and Andromeda would get stuck together,
figuratively speaking", says team member Prof. Pavel Kroupa from Bonn University.
"But if Milgrom's theory is right", says his colleague Dr Benoit Famaey (Observatoire Astronomique de Strasbourg),
"then there are no dark particles and the two large galaxies could have simply passed each other thereby drawing
matter from each other into long thin tidal arms."
Now, the team's next step is to model the encounter using Prof . Milgromian's dynamics and are developing a
computer code at Bonn University for this purpose.
In the new model, the Milky Way and Andromeda are still going to crash into each other again in the next few
billion years, but it will feel like 'deja vu'.
"If we are right, the history of the cosmos will have to be rewritten from scratch," Pavel Kroupa concludes.
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It looks like something taken straight from a horror movie. An enormous hole leading to hell, some would say. But this is not a movie.
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