What if Dark Matter Doesn't Really Exist?

Dark Matter makes up 85% of the universe and scientists have been looking for it for decades. 

They have yet to find it, so how do we know that it exists?

It might seem a mystery how we could be unaware of so much of the universe. Eighty-five percent is an awful lot of stuff to be missing.

But there is good reason to believe that it really is there. This is because the existence of Dark Matter would explain why the movement of galaxies appear to flout the laws of physics.

The thing is that motion of the outer parts of galaxies ought to slow down, the further they get from the centre, as the effect of gravity diminishes. The trouble is, they don't.

However, if the outer parts of these galaxies contained large amounts of matter that we are not aware of, then this would increase the gravitational effect and thus explain the anomaly. 

And that is the currently received wisdom: Dark Matter must exist because that explains the movement of galaxies.

The problem is that no one has been able to detect it. 

One theory is that the universe is full of weakly interacting massive particles (WIMPs). These would be, as yet, undetected particles that have a mass but, because they don't interact with other matter, cannot easily be seen.

But there might be a different explanation.

A new theory suggests that the gravitational effect of a large body (a star, for example) changes depending on how far you are away from it. For things that are relatively close to an object the effect is different to those that are further away.

The theory proposes that there is a conceptual bubble around objects whose size is proportional to their mass. Within the bubble, gravity behaves according to well-known Newtonian principles, the gravitational effects that we see in the orbits of planets in our own solar system, but as you get further away gravitational effects become larger.

The reason that we haven't been able to measure this, up to now, is because the distances involved are so big: the gravitational effects within our solar system are well understood and conform to conventional physics. But they are also well within the bubble that would be associated with our Sun. 

We wouldn't know about a change in the effect of gravity without being able to observe objects that are well beyond the solar system.

In order to prove or disprove this theory, we need to be able to detect the effects of a variable gravity that does not involve the motion of galaxies (as this is also explained by the existence of Dark Matter).

One method of doing this would be to examine the gravitational lensing effect of galaxies. Gravitational lensing is the bending of the trajectory of light as it passes a massive object. The more massive the object the greater the effect. If the galaxies truly are more massive than they appear due to the existence of dark matter, then the lensing effect would be different to a galaxy with no dark matter.

This leaves us with the satisfying conclusion that we have a theory that provides an alternative to the existence of dark matter that comes with a method to disprove itself.

The original research paper is in the Journal of Cosmology and Astroparticle Research, here,

Long range effects in gravity theories with Vainshtein screening by Moritz Platschera, Juri Smirnovb, Sven Meyerc and Matthias Bartelmannc

and there is more in this article in The Conversation.

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