Far out cosmology: Milky Way not hitting Andromeda (Introduction)

by David Turell , Saturday, June 07, 2025, 18:47 (4 days ago) @ David Turell

New calculations:

https://bigthink.com/starts-with-a-bang/milky-way-andromeda-merger-uncertain/?utm_sourc...

"Instead of moving away, Andromeda is headed toward us at an impressive 109 km/s: about twice as fast as Halley’s comet moves at its maximum speed. Because of this speed, its distance of about 2.5 million light-years from us, and the mass-based gravitational attraction of the Milky Way and Andromeda, it was simple and straightforward to conclude that in about four billion years, the Milky Way and Andromeda would collide and merge.

"Only now, with new data from Hubble and Gaia about galaxies from all across the Local Group, it looks like that picture is too simple and naive, and may not be right at all. Here’s what the newest data teaches us.

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"When we take the best position and velocity data that we have from Hubble and Gaia of these objects, and when we include what we know about their masses from a variety of lines of evidence, we can then look at the probability and timescale of a collision-and-merger occurring between the two most massive galaxies in the entire Local Group: Andromeda and the Milky Way.

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"The authors correctly conclude that this implies that the conventional story — of an inevitable merger between the Milky Way and Andromeda in ~4 billion years — is too naive to be true, and in fact their analysis supports a low likelihood for this scenario. (Something that can only be known when we include the gravitational effects of the LMC, which was not known until well into the Gaia era.) By adding in new data, as well as using both simulation and semi-analytical techniques, they confirm that while including the Triangulum galaxy makes a near-term merger more likely, adding in the LMC both pushes out the merger timescale and makes a collision-and-merger less likely overall within the next ~10 billion years.

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"In practically all of the models and simulations — even the most dramatic ones that include the LMC and that experience high-speed near-misses between the Milky Way and Andromeda — the Milky Way and Andromeda remain gravitationally bound to one another and to the greater Local Group, implying that a collision-and-merger will still occur.

"Our entire Local Group eventually should turn into a single galaxy, Milkdromeda, but the timescale for this fate is still largely unknown. It may still be ~4 billion years from now; it’s more likely that it will be ~8 billion years from now; it’s plausible that it will be ~10+ billion years from now, and maybe even tens of billions of years into the future. However, it’s extremely unlikely that either of these galaxies will be ejected from the Local Group, and thus an eventual merger really is inevitable. The question of when this will occur, however, just got a whole lot more interesting."

Comment: We have always been told the Milky Way and Andromeda will join a few million years from now. Now we might not merge at all or do it in many million more years. dhw will comment why did God make such a big universe if His only interest is us on Earth? My answer is God knows what He is doing and for some reason unknown to us the universe's size is required.