Sunday, February 9, 2020

MOND Performs Well In Its First Galaxy Formation Simulation

Doing a simulation of galaxy formation in MOND gravity is quite difficult. But, the first attempt, with admittedly simplified assumptions, has performed well.
In cooperation with Dr. Benoit Famaey in Strasbourg, we have now simulated for the first time whether galaxies would form in a MOND universe and if so, which ones," says Kroupa's doctoral student Nils Wittenburg. To do this he used a computer program for complex gravitational calculations which was developed in Kroupa's group. Because with MOND, the attraction of a body depends not only on its own mass, but also on whether other objects are in its vicinity.
The scientists then used this software to simulate the formation of stars and galaxies, starting from a gas cloud several hundred thousand years after the Big Bang. "In many aspects, our results are remarkably close to what we actually observe with telescopes," explains Kroupa. For instance, the distribution and velocity of the stars in the computer-generated galaxies follow the same pattern that can be seen in the night sky. "Furthermore, our simulation resulted mostly in the formation of rotating disk galaxies like the Milky Way and almost all other large galaxies we know," says the scientist. "Dark matter simulations, on the other hand, predominantly create galaxies without distinct matter disks - a discrepancy to the observations that is difficult to explain." 
Calculations based on the existence of dark matter are also very sensitive to changes in certain parameters, such as the frequency of supernovae and their effect on the distribution of matter in galaxies. In the MOND simulation, however, these factors hardly played a role.
From here. The paper is:
The formation and evolution of galaxies is highly dependent on the dynamics of stars and gas, which is governed by the underlying law of gravity. To investigate how the formation and evolution of galaxies takes place in Milgromian gravity (MOND), we present full hydrodynamical simulations with the Phantom of Ramses (POR) code. These are the first-ever galaxy formation simulations done in MOND with detailed hydrodynamics, including star formation, stellar feedback, radiative transfer and supernovae. These models start from simplified initial conditions, in the form of isolated, rotating gas spheres in the early Universe. These collapse and form late-type galaxies obeying several scaling relations, which was not a priori expected. The formed galaxies have a compact bulge and a disk with exponentially decreasing surface mass density profiles and scale lengths consistent with observed galaxies, and vertical stellar mass distributions with distinct exponential profiles (thin and thick disk). This work thus shows for the first time that disk galaxies with exponential profiles in both gas and stars are a generic outcome of collapsing gas clouds in MOND. These models have a slight lack of stellar angular momentum because of their somewhat compact stellar bulge, which is connected to the simple initial conditions and the negligible later gas accretion. We also analyse how the addition of more complex baryonic physics changes the main resulting properties of the models and find this to be negligibly so in the Milgromian framework.
Nils Wittenburg, Pavel Kroupa and Benoit Famaey "The formation of exponential disk galaxies in MOND." (February 5, 2020) Astrophysical Journal,


neo said...

is this the first time MOND simulation?

i'd like to see how MOND does in large scale structures and the variety of galaxies.

is every single dark matter free galaxy observed thus far supposed to be the result of MOND external field effect, or are there any that contradicts this, thus falsifying MOND?

andrew said...

"is this the first time MOND simulation?"

Pretty much. I've seen some very, very crude efforts mentioned by Stacy McGaugh (I probably cite them in an old blog post somewhere at this blog tagged "dark matter"). In general, it appears that gravity modification leads to earlier galaxy formation than the CDM prediction.

"i'd like to see how MOND does in large scale structures and the variety of galaxies"

Me too.

"is every single dark matter free galaxy observed thus far supposed to be the result of MOND external field effect, or are there any that contradicts this, thus falsifying MOND?"

There are only a handful and one of the difficulties in the case of some of them is that the distance from the nearest large galaxy has large error bars. Also, the language of some of the key articles sometimes reads as if it is talking about the distance from the nearest large galaxy when it is really talking about the distance from Earth.

It is also worth recalling that a falsification of the original 1983 era toy model MOND is not a falsification of other gravity based explanations of DM phenomena that produce similar results in most circumstances. It turns out that there are lots of ways to write an equation of gravity to reproduce the most strongly experimentally confirmed features of MOND.

One of the stronger indications that one of these very similar theories to MOND that isn't actually MOND (which of course also needs to be generalized to a relativistic theory to work in many contexts), is that wide binaries which would seem at first glance to be subject to the external field effect, seems to display MOND-like behavior.

andrew said...

Some basic discussion of large scale structure in MOND with bibliography can be found at:

neo said...

ethan siegel blog in the past criticized MOND for failing large scale structure over dark matter but apparently MOND simulations havent been down till now.

"If you add a new ingredient to the Universe, like dark matter, the way you make predictions about it is to simulate the Universe on large scales. When you add a new ingredient, many cosmic observables change in easily quantifiable ways that lead to clean predictions and clean signals. It’s like dropping a sheet of paper or a feather at the surface of the Moon, rather than on Earth; you’ll measure what you intend to measure, rather than the contaminating, messy effects that might get in the way. The best laboratory for that? Examining the large-scale structures present within the Universe."

andrew said...

I have been very unimpressed with Ethan Siegel's writing on the topic. He is less informed about it than he claims to be.

neo said...

in many cases he is summarizing standard research, for example, that dark matter is needed to create the gravity to cause large scale structures.

Gravitational lensing is also evidence of dark matter, esp in the bullet cluster.

andrew said...

The existence of dark matter phenomena is absolutely a real thing and there is no doubt that something beyond the SM + GR core theory is necessary to explain these phenomena.

A dark matter particle hypothesis to explain that phenomena is much less well supported than he claims when compared to the alternative possibility of gravity modification. As numerous posts under the tag "dark matter" at this blog document, the dark matter particle hypothesis has multiple serious discrepancies between this theory and observation (including the Bullet Cluster often used to support it).

The Bullet Cluster and other issues like large scale structure (see ) can be successfully modeled in a modified gravity regime (not necessarily MOND). The claim that these things prove that dark matter particles rather than gravity modification is the cause of dark matter phenomena is simply false.

No dark matter particle theory and no gravity modification theory in existence at this time and widely accepted or studied has been demonstrated to simultaneously resolve all dark matter phenomena.

In particular, pure CDM theory is pretty much completely ruled out by the evidence. There needs to be some force component as well, not just gravity, interacting with it.