A Fifth Fundamental Force in The Universe

This article is about a recently floated proposal that there could be a fifth force of nature, manifested as a scalar field, which could explain some shortfalls in the Standard Model of cosmology.

Explaining the dark side

If there really is a fifth fundamental force, then that could go a long way to explaining the strange orbits of some dwarf galaxies.

And more, it could illuminate, metaphorically, the 85% of mass in the universe that we cannot directly observe: Dark Matter.

It would also reset the Standard Model of the universe which relies on three of the four fundamental forces.

This idea of a fifth force is only theoretical, available in a preprint of the authors’ paper available on arXiv.

The Standard Model

The Standard Model of cosmology describes the universe in terms of three components: dark energy, dark matter and normal matter. The model postulates that large galaxies like the Milky Way were formed in a rapid series of mergers of smaller galaxies that occurred inside an immense halo of dark matter.

Although the Standard Model is, arguably, self-consistent and has demonstrated huge successes in providing experimental predictions, it leaves some phenomena unexplained and does not include the gravitational force in its formulation.

The odd behaviour of some dwarf galaxies

Some of these aforementioned dwarf galaxies have persisted until now, and they surround the Milky Way and other big galaxies. These old dwarfs, on the other hand, appear to orbit the Milky Way’s center in a plane rather than being scattered randomly throughout the dark-matter halo.

Several comparable planes of dwarf galaxies can be found around other major galaxies, such as Andromeda. The duration of the existing planes and whether the Standard Model can account for their formation have been the subject of heated debate among theoretical physicists.

The fifth force: A new scalar field?

Astrophysicist Aneesh Naik and particle physicist Clare Burrage at the UK’s University of Nottingham have proposed that a new scalar field and associated fifth force could be the reason that these satellite dwarf galaxies agglomerate in planes.

A scalar field refers to an energy field where every point in space can have a ‘unique’ value. An example of a scalar field might be air pressure at the earth’s surface, where every place has a pressure, with pressure having no associated direction.

Here's the air pressure today over the Indian Ocean (I am writing in Indonesia):

This is different to gravity, for example, which has a value and direction at every point on the Earth’s surface and is therefore a vector field.

The authors propose that below some (currently undefined) critical density threshold, the scalar field changes and adopts two possible minimum energy solutions.

They tag these solutions as positive and negative, with their precise values tied to the parameters of the scalar-field model, which is work yet to be done.

Symmetry-breaking

In an interview with Physics World Aneesh Naik said that “In different regions of the universe the scalar field will adopt the positive solution and in other regions it will adopt the negative solution.” This is symmetry breaking, with the two regions separated by s boundary they dubbed a ‘domain wall’.

Why would it be it a fundamental force?

Naik explained to Physics World, “It’s a new fundamental force because it’s a force that’s mediated by a new scalar particle, in the same way that the electromagnetic force is mediated by the photon”.

The argument continues

Other physicists have described the proposition as interesting, but cautioned about the fact that it is highly speculative.

Another recent preprint on xArchiv has argued that the dwarf galaxy plane in the Milky Way can be explained by the Standard Model.

For me — and I’m not a theoretical physicist, just a sad grad— these are exciting ideas.

What I’m waiting to find out is how Naik and Burrage will propose to put their ideas to test — what observable/measurable effects can they predict?

Proof of a fifth fundamental force would be earth-shattering.

Metaphorically, of course.

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Canonical: this story was first published in Medium on 29 May 2022