Galaxies collides probably due to these two scenarios. The left one depicts the two galaxies flying away in an almost parallel path. The gravity of the two galaxies slowly pull the two galaxies together. They meet and pass through each other.
The right one shows a light galaxy, indicated as the red path is moving initially ahead of another massive galaxy, indicated as the blue path. The light galaxy is being pulled and decelerated by the heavy galaxy. Until the light galaxy is being taken over by the heavy galaxy. It looks like the two galaxies go into a head on collision. Then the heavy galaxy pass over the light galaxy. The heavy galaxy afterwards pull the light galaxy in the same direction as the expansion of the universe.
Yes, 13 billion years is merely an estimation of the age of universe. It is very likely to be the lower bound of the age of universe. Yet, the real age of the universe should not be too far away from 13 billion years. For example, the age of the universe is extremely unlikely to exceed 20 billion years. It is because the measurement of background radiation gives scientists a relatively precise measurement.
Unlike what is described by the video, detection of background radiation differs from detection of stars and galaxies greatly. Stars and galaxies far away from earth may be too far to be observed. The light scattered so thin that the Hubble Telescope simply omitted their existence. However, background radiation presents everywhere. It can be detected with earth-based radiotelescope pointing to anywhere in space. Simply say, on the one hand, stars and constellations are birds and gulls in the sky. They are small dots which are hard to be observed distant away. On the other hand, background radiation is the azure sky, which can be seen immediately if we look up. Even if the blue sky is ten times higher than the flying animals, the sky is still infinitely more visible than other airborne objects.
Therefore, measuring time by background radiation is pretty accurate. Although more power telescopes can help scientists discover more distant galaxies; the boundary of the universe, defined by background radiation, will not be pushed further away.
Yet, some of the points in the video is correct. For instance, the nature of dark matter and dark energy is largely unknown by current physics. I won't be surprised if a few years later, the age and size of the universe turn out to be much older and larger. Or that big bang theory is invalid. After all, science is evolving every minute. Yet, the big bang theory is pretty much the only handful of scientific theories we have currently that describes our universe the most precisely.
yes there are
