Fifth force could explain puzzling orbits of dwarf galaxies


Andromeda galaxy
Andromeda: this image of the galaxy was taken with a small telescope and includes two dwarf galaxies. (Courtesy: David (Deddy) Dayag/CC BY-SA 4.0)

New physics, in the form of a “fifth force”, could be responsible for the odd and unexplained arrangement of dwarf galaxies orbiting the Milky Way and other large galaxies – according to new research done it the UK. The new force could also shed light on the nature of dark matter, a mysterious substance that accounts for about 85% of the matter in the universe.

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

Some of these dwarf galaxies remain distinct to this day and they surround the Milky Way and other large galaxies. But rather than being distributed throughout the dark-matter halo, the dwarfs preferentially orbit the centre of the Milky Way in a plane. Similar planes exist around other nearby large galaxies, such as Andromeda. How long-lived these planes are, and whether the Standard Model can explain their existence, has been a source of great debate among researchers.

New scalar field

Now, astrophysicist Aneesh Naik and particle physicist Clare Burrage at the University of Nottingham propose that a new scalar field and associated fifth force could be the cause of these planes of satellite dwarf galaxies.

Our current understanding of particle physics involves four fundamental forces: electromagnetism, gravity, the strong force and the weak force. However, this model is known to be incomplete – it has no adequate description of dark matter, for example. As a result, researchers are developing “new physics” to attempt to create a better understanding of nature.

In this latest work, Naik and Burrage have delved into the world of scalar fields to develop their new physics. A scalar field refers to an energy field where every point in space can have a unique value. For example, a temperature map of the Earth, where different locations have different temperatures, is a familiar example of a scalar field.

When the universe began, the duo’s scalar field would have adopted the same minimum energy level everywhere. But, as the universe expanded, the density of the matter distribution in space became greatly reduced.  Naik and Burrage’s scalar field is tightly coupled to matter, so below some critical density threshold, the scalar field changes and adopts two possible minimum energy solutions. These solutions are described as positive and negative, although their exact values depend upon the parameters of the scalar-field model, which have yet to be nailed down.

Symmetry breaking

“This is what we call symmetry breaking,” Naik tells Physics World. “In different regions of the universe the scalar field will adopt the positive solution and in other regions it will adopt the negative solution.”

The boundary between these positive and negative regions is referred to as a domain wall, and Naik and Burrage propose that domain walls have sliced through the Milky Way and other galaxies, creating the planes of satellites.

A domain wall would act as a fifth force, attracting satellite galaxies to the plane. Although they admit that their model is simplified at the moment, the duo was able to broadly replicate the Milky Way’s plane of satellites, and even the Andromeda galaxy’s bi-modal plane, where some of the dwarfs orbit within a plane, while others are randomly distributed off the plane.

Geraint Lewis, who is an astrophysicist at Australia’s University of Sydney, describes the work as “interesting,” and he agrees that it provides a “possible mechanism for shaping satellite distributions into planes”. However, he cautions that the idea is only at the proof-of-concept stage. “It is a limited exploration and while it makes something that looks like a plane, it’s hard to assess how generic this result is and whether the resultant planes match those observed in galaxies.”

Short-lived and coincidental

Meanwhile, proponents of the Standard Model of cosmology still believe it can explain dwarf-galaxy planes. Earlier this month, a preprint was posted that argues that the Milky Way’s plane of satellites is consistent with the Standard Model. The authors’ simulations suggest that the plane is short-lived and coincidental, but Lewis is not convinced by it. “This is another ‘post-diction’ and so I am not too swayed, especially if the planes are long-lived structures.”

Naik and Burrage’s scalar-field model and fifth force could also have something to say about the nature of dark matter. Naik explains, “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”. This particle goes by numerous names in the various scalar field models developed by researchers, but one common name is the symmetron.

The models suggest that the symmetron would be a massive particle. “It could – potentially – be dark matter,” says Naik.

Naik and Burrage will now finesse their model and perform more sophisticated N-body simulations.

A preprint of the duo’s paper is available on arXiv.

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