One of the most fascinating predictions that astronomy has made about the fate of our galaxy, the Milky Way, is that of a future cosmic collision with the Andromeda Galaxy in about 4.5 billion years. The prediction comes from a 2008 study published in the journal Monthly Notices of the Royal Astronomical Society based on the fact that the two galaxies are approaching each other. This collision event would lead to the formation of an elliptical galaxy that astronomers have dubbed “Milkomeda”. A new study published on the portal arXiv it seems though to deny this prediction: new simulations raise the probability of an actual collision between the Milky Way and the Andromeda galaxy to “only” 50%.
Using the latest data from the space telescope Hubble and from the ESA satellite Gaiaa team of European astronomers has simulated the orbit of the galaxies that are part of the Local group, the group of galaxies that both the Milky Way and Andromeda are part of.
Taking into account the gravitational influences of other objects, such as the Triangulum galaxy M33 and the Large Magellanic Cloudmuch to their surprise, astronomers discovered that the chance that Andromeda and the Milky Way will collide in 10 billion years is only 50%!
As we know Andromeda and the Milky Way are on a collision course
That the Milky Way and the Andromeda Galaxy are on a collision course is a well known fact in astronomy since 1912. In that year, the astronomer I wear Slipher he noticed how the light coming from Andromeda was shifted towards the blue (blueshift in jargon), that is, squashed towards shorter wavelengths by theDoppler effect (the same one that makes us perceive the sound of ambulances as more acute as they get closer), and not shifted towards the red (redshift in jargon), thus implying a approach motion between the two objects (net of the expansion of the Universe which tends to push all objects away from each other).
It was only with the advent of space telescopes, however, that astronomers were able to reconstruct the complex three-dimensional speed of Andromeda, finding that it is moving toward Earth at the astonishing speed of 110 kilometers per second (396,000 km/h).
A 2008 study published in Monthly Notices of the Royal Astronomical Society He also provided an estimate of when this cosmic collision would occur, which is about 4.5 billion yearsto then be completed in approximately 10 billion years with the formation of a new elliptical galaxy, which astronomers have renamed “Milkomeda”.
The complexity of the measures
However, a study by a team of European astronomers (not yet peer-reviewed), led by Till Sawala of the University of Helsinki, seems to challenge this consolidated belief.
Accurately predicting the collision between two complex cosmic objects such as galaxies is in fact potentially subject to a large number of uncertaintiesfor example in calculating the mass of the objects involved, in their velocity vector, in the presence of intergalactic gas which can cause friction with the gas inside galaxies and change their orbits.
What astronomers can do then is update from time to time predictions based on the new data available. This is exactly what has been done in this new study, where the most recent data from the Hubble Space Telescope and the ESA satellite Gaia have been used to determine as accurately as possible the speed and mass of the objects in the game.
To give an idea of the complexity of the measurements, just think that determining the exact speed of Andromeda, which is approximately 2.5 million light-years from the Earth, is equivalent to measuring the growth rate of a human hair at the distance of the Moon.
Details from the new study: What the simulations predict
Till Sawala and colleagues took the latest and best estimates of the speed and mass of the four galaxies more massive than the Local Group, they have inserted them into the most advanced numerical simulations available and have begun to evaluate under what conditions Andromeda and the Milky Way collide to form a new elliptical galaxy.
In the first case, astronomers have considered only the Milky Way and Andromeda and ran several simulations under this scenario using slightly different initial values that accounted for observational uncertainties in the mass and velocity. In this first scenario, the galaxies trace orbits that lead them to merge in just under half of the cases.
By including in the simulations the third most massive galaxy of the local group, the Triangulum galaxy M33, collision probability rises and Andromeda and the Milky Way merge in 75% of cases.
However, by also introducing the fourth most massive member of the local group, the Large Magellanic Clouda satellite of the Milky Way, the probability of collision decreases again, dropping to 50%. Furthermore, in cases where a collision occurs, it does not occur before 8 billion yearscompared to the 4.5 previously predicted.
What would happen if the collision between the two galaxies were to actually happen?
Although this new study questions the merger between Andromeda and the Milky Way in cosmically short timescales, it must be taken into account that the galaxies of the Local Group are all gravitationally bound among themselves. This implies that as the tens of billions of yearseventually all the galaxies in the Local Group will tend to merge into a single one immense elliptical galaxy.
But what will happen to Earth in this scenario? First of all, the timescales involved are such that, even in the best case scenario, the collision between Andromeda and the Milky Way will not occur before 4.5 billion yearsat which point the Sun will begin to transform into a red giant, swallowing the inner planets, including Earth.
As for the Sun, however, considering that galaxies are mostly empty spacea collision with another star is highly unlikely and at most the gravitational ballet will lead to the complete ejection of the Sun from the new galaxy that is forming.
