Editor’s note: Ignacio Trujillo Cabrera is a Research Professor at the Canary Islands Institute of Astrophysics.
Dark matter is about five times more abundant than ordinary matter in the Universe. Although we currently don’t know what it is composed of, it has, like all mass, a gravitational pull effect that affects the way stars are distributed in galaxies and causes celestial objects to rotate around their center of gravity (or that is, make a movement of revolution, or translation) much faster than they would do if it did not exist.
But what would happen if dark matter disappeared? Without dark matter, all large galaxies would lose stars, especially those on their “periphery”.
Loss of stars
If we removed the dark matter in the Milky Wayall that is now on the edge of the galaxy, like the stars furthest from their center, would “fly” out of the galaxy. And this would happen because all of this is in translation. It’s as if we removed the Sun from the Solar System: the planets that normally revolve around it, attracted by its mass, would escape. Without the Sun’s pull, they would still move in a straight line depending on the angular momentum they had in their orbits, but they would still fly away.
NGC 1277 is a good example that a massive galaxy can exist without dark matter. Although we don’t really know what happened to this galaxy, the interesting thing is that because it lacks dark matter, it was not able to attract material from outside, that is, the stars and gas on the outskirts that are normally present in other galaxies. This makes it a unique object in the Universe, as we don’t know another like it.
Would the Solar System be affected?
At the center of any massive galaxy, ordinary matter is more concentrated than dark matter, and its gravitational effect is predominant. This is why stars and all objects in the galactic center would not be seriously affected if dark matter disappeared.
Most stars don’t escape the center of the galaxy, they stay there. Every time they get closer, they move faster and faster. But they do not “fall” in their center, because they move. The fall, in fact, is very difficult.
O Solar system and Earth are close to the “middle” of Milky Way, so we probably wouldn’t be affected much if dark matter disappeared. We would need to crunch the numbers to be sure, we would probably notice something, but the Sun is close enough to the center of the galaxy that the dominant gravitational pull is not on dark matter, but on the rest of the matter.
We would lose dwarf galaxies
The disappearance of dark matter in low-mass galaxies, so-called dwarf galaxies, however, would have a catastrophic effect.
Dwarf galaxies are the most numerous in the Universe. These galaxies have a greater proportion of dark matter to total mass than more massive galaxies. In these galaxies, the mass of dark matter can be up to tens of thousands of times greater than that of visible matter. Therefore, in this case, the consequences of the loss of dark matter would be much greater than the loss of outer stars: the dwarf galaxies would completely fragment.
Nothing changes without Sagittarius A*
We don’t know how black hole at the center of our galaxy, Sagittarius A* graduated. Although it is called a black hole, its origin is not necessarily due to a collapse of dark matter.
The black hole at the center of our galaxy acts very locally. In any case, it does not dictate the Sun’s orbit and has no appreciable effect. On the scale of the galaxy as a whole, the pull of the black hole is irrelevant. If we removed it from the system, nothing significant would likely happen.
The merger of the Milky Way and Andromeda
Counting its 200 billion stars, its supermassive black hole and its dark matter halo, the Milky Way has a mass of about 1.15 trillion suns. That’s why we call it a massive galaxy. Andromeda, our most important neighboring galaxy, is also massive, with a mass about 1.5 trillion times that of the Sun.
The gravitational attraction that they exert on each other means that they are getting closer (neutralizing the effect of the expansion of the Universe) and, at some point, they will merge. In fact, they are so large that even if we removed the high dark matter content, they would probably end up merging. However, they would do so over a much longer period of time than currently expected because of dark matter.
At the beginning of the Universe
But let’s imagine for a moment that dark matter had never existed. This would make a big difference, because dwarf galaxies might never have formed in the first place. Removing dark matter from the equation in the early Universe meant that mainly very massive galaxies would be generated.
The Milky Way would probably have formed, but it would almost certainly have far fewer stars. And we also wouldn’t see the so-called stellar halos that result from the cannibalization of dwarf galaxies by massive galaxies.
Would the Universe disintegrate?
There is dark matter everywhere. However, all the dark matter that exists is incapable of stopping the acceleration of the expansion of the Universe. Dark matter only plays the role of attracting more dark matter to what is relatively close to it.
On the other hand, dark matter is capable of neutralizing what we call the Hubble flow (Hubble Flow). There are some scales at which, thanks to the presence of dark matter, objects decouple from expansion and collapse (the case of Andromeda and the Milky Way is a good example).
If we removed all the dark matter at once, as would happen with the stars on the outskirts of massive galaxies, the outermost galaxies in galaxy clusters would come rushing out. Some of these runaway galaxies would be incorporated into the Hubble stream and thus into the global expansion of the Universe.
What about dark energy? With what we know today about dark energy, even in the hypothetical scenario of dark matter disappearing, it would not have such a strong effect that it would cause the Universe to completely fragment.
This article is republished from The Conversation. Read the original article.
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