Most astronomers say it is likely to happen. Is this true? Are there any scientists that reject this?
Relatively high-resolution measurements were made with the Hubble Space Telescope over a 8 year period. From these measurements, astronomers determined that the collision was a certainty. It is predicted to occur in about 4 billion years. I suppose there may be some astronomers who disagree with the measurements or calculations but I can find no mention of that. The results were reported in an article in "Nature" (http://www.nature.com/news/andromeda-on-collision-course-with-the-milky-way-1.10765#/b1) and published in "Astrophysical Journal" Both are well-respected publications. The "Astrophysical Journal" reference is below.
Sangmo Tony Sohn; Jay Anderson; Roeland van der Marel (Jul 1, 2012). "The M31 velocity vector. I. Hubble Space Telescope proper-motion measurements". The Astrophysical Journal. doi:10.1088/0004-637X/753/1/7.