The cold dark matter of the universe is approximately five times more abundant than ordinary matter. Although today we do not know what it is composed of, it has, like all mass, a gravitational attraction effect that affects the way stars are distributed in galaxies and makes celestial objects rotate (ie move) much more. faster than they would if it didn’t exist.
What would happen if it disappeared? Without dark matter all large galaxies would lose stars, specifically those on the “outskirts”.
Lose stars
If we removed the dark matter from the Milky Way, everything that is now on the periphery of the galaxy, the stars furthest from the center, would separate. And they would do it because they are rotating. It is 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 pull of the Sun, they would continue to move in a straight line, depending on what angular momentum their orbits were in, but in any case they would be thrown off.
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, since it didn’t have dark matter, it couldn’t attract material from outside, that is, the stars and gas on the periphery that are usually present in other galaxies. That is why it is a unique object in the universe, as there is no other like it.
Would it affect the Solar System if dark matter disappeared?
At the center of any massive galaxy, ordinary matter is more concentrated than dark matter and its gravitational effect is predominant. That is why the stars and all objects in the galactic center would not be seriously affected if dark matter disappeared.
Most stars do not escape from the center of the galaxy, they stay there. Every time they get closer, they move faster. But they don’t collapse, because they move. Collapsing, in fact, is very difficult.
The solar system and Earth are in the middle of the Milky Way, so it probably wouldn’t affect us too much if dark matter disappeared. We would have to do numbers to be sure, we would probably notice something, but the Sun is close enough to the center of the galaxy so that the dominant force of gravity is not that of the dark matter but that of the rest of the matter.
Illustration of the Milky Way showing the spiral arms, the central galactic bulge, the location of the Sun, and various X-ray sources. NASA/CXC/M.Weiss, CC BY
We would lose dwarf galaxies
The disappearance of dark matter in low-mass galaxies, the so-called dwarf galaxies, would have a catastrophic effect.
Dwarf galaxies are the most numerous. These galaxies have a higher proportion of dark matter with respect to their total mass than the most massive galaxies. In them, the mass of dark matter can be up to tens of thousands of times greater than light matter. Therefore, in this case, the consequences of losing dark matter would be much greater than those of losing the stars outside: the dwarf galaxies would completely disintegrate.
Nothing changes without Sagittarius A*
We do not know how the black hole at the center of our galaxy, Sagittarius A*, formed. Although it is called a black hole, its origin does not necessarily have to be due to a collapse of dark matter.
The black hole at the center of our galaxy acts very locally. It does not dictate the Sun’s orbit in any way and has no considerable effect. On the scale of the galaxy as a whole, the attraction of the black hole is irrelevant. If we removed it from the system, probably nothing significant would happen.
The union of the Milky Way and Andromeda
Counting its 200 billion stars, supermassive black hole, and dark matter halo, the Milky Way has a mass of 1.15 trillion suns. That is why we say that we are facing a massive galaxy. Andromeda, our most important neighboring galaxy, is also important, with a mass about 1.5 trillion times that of the Sun.
The gravitational attraction they exert on each other means they are getting closer (counteracting the effect of the expansion of the universe) and, at some point, they will merge. What’s more, they are so massive that even if we removed the high dark matter content they would probably end up merging. Of course, they would do it in a much longer period of time than is currently expected given the availability of dark matter.
At the beginning of the universe
Let’s imagine for a moment that dark matter had never existed. That scenario would change things a lot, to begin with because dwarf galaxies might never have formed. Removing dark matter from the equation at the beginning of the universe implies that mainly very massive galaxies would be generated.
The Milky Way would probably have formed, but it would almost certainly have many fewer stars. And we would not see the so-called stellar halos that result from the cannibalization of dwarf galaxies by more massive galaxies.
Without dark matter, would the universe disintegrate?
There is dark matter everywhere. Even so, all the dark matter that exists is incapable of slowing down the acceleration of the expansion of the universe. Dark matter only plays the role of attracting them more to what is relatively close to it.
On the other hand, dark matter is capable of counteracting what we call Hubble flow (Hubble Flow). There are some scales where, thanks to the presence of dark matter, objects decouple from the 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 periphery of massive galaxies, the outermost galaxies of the galaxy clusters would be thrown out. Some of these fleeing galaxies would be incorporated into the Hubble flow and, therefore, into the global expansion of the universe.
And what about dark energy? With what we know about it today, even in the hypothetical scenario that dark matter disappeared, dark energy would not produce such a radical effect as to force the total disintegration of the universe.
This article was originally published on The Conversation.
