What Happens When Matter and Antimatter Galaxies Collide?
Usually nothing: 1 star in about 10 years in the visible Universe dies in a galactic collision.
Relates to galaxies formation in No More Antimatter Enigma. There is a popular opinion that antimatter galaxies will annihilate/explode/disappear in dramatic collision with matter galaxies. Here is why it is not happening:
Let’s run some numbers. The closest-to-our-Sun star is at the distance of ~4×10¹³ km (the symbol ~ stands for “approximately”). Our Sun is an average-sized star, with the diameter of 1.4×10⁶ km. Roughly, in the empty space around the Sun we can pack ~(4×10¹³ / 1.4×10⁶)³ ~ 10²² stars. In other words, stars’ density around us is ~10⁻²². But to calculate the probability of stars collision, when two similar galaxies collide, we have to calculate flat, 2-dimensional density of stars instead of 3-dimensional density. That is because we need to estimate galaxies’ flat projections collision (flat projections passing through each other momentarily, instead of billion-year-long collision in real time). So, flat density (meaning, using math power of 2 instead of math power of 3) is ~ 1/(4×10¹³ / 1.4×10⁶) ² ~ 0.35×10⁻¹⁴. That is the probability for a single star, the Sun for example, to collide with any star from another similar-to-the-Milky-Way galaxy (in a single galactic collision). And if we need the probability of any star in the Milky Way suffering in this collision, not just for the Sun, we have to multiply that by 100 billion stars of Milky Way: ~ 0.35×10⁻¹⁴×10¹¹ ~ 0.00035. So, in about 3,000 galactic collisions with our Milky Way, 1 of our stars will suffer/collide. The earliest collision of our Milky Way with another galaxy is expected in about 4.5 billion years (that is about the Earth age) — with the Andromeda galaxy. Multiply 4.5 billion years by 3,000 collisions, we get 13.5 trillion years. Thus, in about 13.5 trillion years one of our stars will not survive a galactic collision. Now, with 1–2 trillion galaxies estimated / observed in the whole Universe, we have (13.5 trillion years) / (1–2 trillion galaxies) ≈ 10. Therefore, 1 star in about 10 years in the whole Universe perishes in a galactic collision.
This is my rough estimate / back of envelope calculation, which explains why matter and antimatter galaxies coexist without spectacular annihilation fireworks all over the Universe. That is the probability of head-to-head collision. But what if stars pass near each other at a distance of, let’s say, 1,000 Suns? Probability of such an event is 1000² (not 1000³, because we estimate probability using flat projections) = million times higher than the probability of head-to-head collision. That translates into 100,000 events a year ( = 1,000,000 / 1 event in 10 years ). With trillion galaxies around, that translates into 1 event per 10 million galaxies a year. Still, it is rare. Even for 1,000,000-Sun distance between stars passing by, the probability of such an event is 1 in 10 years within a galaxy. Near-passing stars may form binary stars (which are ubiquitous/normal). What happens in a binary system of a matter star and an antimatter star, where solar/star winds annihilate matter-antimatter: they burn brighter with a light of extremely high frequency, anything else (gamma-ray bursts, supernovae)? Apart from galactic collisions, every galaxy, including our Milky Way, has a halo where matter-antimatter annihilates/burns, so we get some of such high frequency light from halo and from galactic centers.
