Galaxies are islands of stars in the sea of space. But galaxies are much more than the stars we see. According to the Standard Model of Cosmology – which explains many observed properties of our universe and suggests that a Big Bang created all matter, space and time – most galaxies should be surrounded by a halo of dark matter . Dark matter is invisible to our eyes, but detectable via its gravitational pull. Occasionally, however, there have been attempts to replace the standard model with a different model. And now another new study – announced on August 5, 2022 – challenges the view of the universe presented by the Standard Model. The study is based on an analysis of dwarf galaxies in the Fornax cluster, the second closest large cluster of galaxies to our Milky Way. The Standard Model says that these small galaxies should have dark matter halos. Observations show no trace of dark matter halos.
Scientists from the University of Bonn in Germany and the University of Saint Andrews in Scotland conducted the study. These researchers published their peer-reviewed findings in the Royal Astronomical Society Monthly Notices June 25, 2022.
A free version of the document is also available on the arXiv preprint server.
A tidal force acting between galaxies
Thus, the Fornax cluster is the second closest large cluster of galaxies to our Milky Way. And, like all galaxy clusters, it contains many dwarf galaxies, often captured orbiting the larger galaxies in the cluster. Being smaller, these dwarf galaxies are susceptible to the gravitational pull of larger galaxies. And so they are subject to gravitational “tides”, similar to ocean tides on Earth, caused by the pull of the moon and sun.
Consider that the moon pulls most strongly on the side of the Earth facing it. And it pulls less hard on the side of the Earth furthest from the Moon. That’s why we have two tidal bulges on Earth and two high tides (and two low tides) every day across the planet (read more about Earth’s tidal bulges here).
Similarly, a larger galaxy exerts a differential tidal force on a dwarf galaxy. It pulls more strongly on the near side of the dwarf galaxy than on the far side. And these scientists studied these tidal forces in dwarf galaxies in order to draw their conclusions. As the study’s lead author, Elena Asencio from the University of Bonn, said:
We introduce an innovative way to test the Standard Model based on the amount of dwarf galaxies perturbed by gravitational “tides” from nearby larger galaxies.
Dwarf galaxies didn’t look good
So these researchers noticed something strange about the dwarf galaxies in the Fornax cluster. The small galaxies didn’t look like they should, according to the Standard Model. They looked deformed. What was causing this? And could another model of the universe better explain these observations? According to Pavel Kroupa of the University of Bonn and Charles University in Prague:
Such perturbations in Fornax dwarfs are not expected according to the Standard Model. Indeed, according to the standard model, the dark matter halos of these dwarfs should partly protect them from the tides raised by the cluster.
In other words, if dark matter surrounded these clusters, it should partially shield them from the pull of larger nearby galaxies. Yet these dwarf galaxies appeared unprotected in this way. They were visibly distorted.
What was seen
How did these researchers conduct their study? They analyzed the expected amount of disturbance in small galaxies, in line with the Standard Model predictions. They then compared these results with the observed level of disturbance in the images taken by the European Southern Observatory’s VLT Survey Telescope. As Asencio stated:
The comparison showed that, if one wants to explain the observations in the Standard Model, the dwarfs of Fornax should already be destroyed by gravity from the center of the cluster…
So the theory and the observations didn’t match.
An alternative model, MOND
The researchers said the observed disturbances cannot be explained using the Standard Model. So they repeated their analysis using an alternative model, called Modified Newtonian Dynamics (MOND). MOND is a hypothesis proposing a modification of Newton’s law of universal gravitation. It is an alternative theory of the universe. Indranil Banik from the University of St. Andrews commented:
We were unsure whether dwarf galaxies would be able to survive the extreme environment of a galaxy cluster in MOND, due to the lack of protective dark matter halos in this model. But our results show a remarkable agreement between the observations and the MOND expectations for the level of disturbance of the Fornax dwarfs.
Co-authors Aku Venhola from the University of Oulu in Finland and Steffen Mieske from ESO added:
It is exciting to see that the data we have obtained with the VLT survey telescope has allowed such thorough testing of cosmological models.
Where does that leave the standard model?
The study of the dwarf galaxy Fornax is the latest of several recent studies suggesting that the observed dynamics and evolution of certain galaxies can be better explained if no dark matter surrounds them. Yet the Standard Model calls for dark matter around galaxies. Pavel Kroupa from the University of Bonn said:
The number of publications showing incompatibilities between observations and the dark matter paradigm continues to increase every year. It’s time to start investing more resources in more promising theories.
Hongsheng Zhao from the University of St Andrews added:
Our results have major implications for fundamental physics. We expect to find more perturbed dwarfs in other clusters, a prediction that other teams should verify.
Last December, scientists reported finding six more galaxies that appear to be lacking in dark matter. Astronomers also found two other similar galaxies in 2019 as well.
If in the future researchers find more galaxies that lack dark matter halos, astronomers may need to reconsider whether the Standard Model – with its Big Bang and dark matter framework for galaxy evolution – is always the best model describing our universe, and open the possibility of other scenarios.
Conclusion: A new study suggests that dwarf galaxies in the Fornax cluster lack dark matter halos. This discovery is the latest in a series that challenges the standard cosmological model, the dominant theory of our universe.
Source: The distribution and morphologies of Fornax Cluster dwarf galaxies suggest they lack dark matter
Source (preprint): The distribution and morphologies of Fornax Cluster dwarf galaxies suggest they lack dark matter
Via the University of Bonn