I understand that the question is about the speed of the galaxies in relation to the rest of the universe and not, say, the rotation of the galaxy on its own axis. The first thing I wanted to clarify is that a single velocity cannot be quantified for all galaxies. It can be said that each galaxy moves in its own way. And the same thing happens with the other objects that make up the universe, everything moves in it.
The speed of galaxies depends on several factors: their age, location and environment. The overall speed depends on how it is swept up by the rate of expansion of space in the universe (Hubble flux), because we already know that the cosmos has been expanding since the Big Bang, causing the space between objects to grow over time. This is the dominant factor in the most remote universe, the most primitive. This most distant universe is much more chaotic than the current one, and the galaxies in this universe are moving the fastest, much faster than those in the current universe.
As I said, not all galaxies move at the same speed, but to give you an idea, those around our Milky Way move more or less in the range of 100 to 2,000 kilometers per second; those that are slightly more distant, at about 5,000 kilometers per second, and the most distant ones can move at more than 10,000 kilometers per second.
For example, the galaxy where a supermassive black hole called Messier 87 was first discovered is moving at about 1,000 miles per second, and ours, the Milky Way, is moving at 370 miles per second. These are very high numbers because we are talking about kilometers per second. In the case of the Milky Way, its speed is more or less like traveling from Madrid to Cadiz in the blink of an eye.
To this velocity of drag due to the expansion of space we must add the velocities deviating from the distance caused by the cosmic expansion known as peculiar motion (peculiar velocity in English). In this case we are talking about the local motion/velocity of the galaxies around them. It depends, for example, on whether the galaxy is in a group that is groups of about fifty galaxies or in a cluster that is groups of hundreds of galaxies together.
The Milky Way is moving at this speed of 600 kilometers per second towards a large concentration of matter called the Great Attractor, located between Virgo and Leo. And that’s because of the gravity that this Great Attractor is exerting on it.
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If, in addition to being in the universe and being affected by its expansion, and in addition to being in a large or small neighborhood, it has a neighbor, it also creates a kind of dance between them, approaching and receding , which also affects their overall speed. This happens, for example, with the Milky Way with Andromeda (or more generally with galaxies in the process of interaction-fusion). And that can also cause the speed to change. Although these changes are usually relatively small if we compare them with the rate of expansion of space in the universe and with the peculiar movement due to the environment.
Something that may surprise you is that the speed of the Milky Way is about 600 kilometers per hour, that of Andromeda is the same but negative. And that’s because you also have to consider the direction of movement. A very important thing is the reference system that tells us if the speed is positive or negative. In everyday life it’s easier to have a common fixed reference point and that’s why we don’t use negative velocities. The sign of velocity tells us where it’s going (whether it’s moving toward or away from something), since we can’t go back in time or move in negative space that doesn’t exist. But when we measure the speeds of galaxies, the sign is important because it is due to the reference system we use.
For a long time it was difficult to measure the speed of objects in the universe because it was difficult to find common reference systems when everything is moving. But in the 1960’s an all-pervasive fossil radiation from the Big Bang was discovered, the microwave background radiation, which appeared very, very shortly after the Big Bang, has spread with the universe and we find it everywhere. No matter where we point the telescope, there is background microwave radiation. And that radiation that fills everything can be considered the most fixed point we can find, it’s also moving, but it’s the most constant thing in the universe, and all the velocities I’ve mentioned in my answer are related to this Radiation cosmic background.
Sarah Cazzoli is PhD in Astrophysics, researcher at the Institute of Astrophysics of Andalusia (CSIC)
Question submitted by Cesar Mesa
Coordination and writing: Victoria Bull
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