Modified Gravity Could Soon Be Ruled Out, Says New Research On Dwarf Galaxies

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Only approximately 1000 stars are present in the entirety of dwarf galaxies Segue 1 and Segue 3, which has a gravitational mass of 600,000 Suns. The stars making up the dwarf satellite Segue 1 are circled here. If new research is correct, then dark matter will obey a different distribution depending on how star formation, over the galaxy’s history, has heated it.Marla Geha and Keck Observatories

When you look out at the Universe, there are a few things you’d rationally expect. You’d expect that the same things that made up everything we saw — like atoms and light — made up everything there was. You’d expect that the fundamental laws would apply equally well everywhere you looked, from small scales to large scales. And you’d expect that if you had multiple ways of measuring the same physical quantity, they’d give you the same answer.

Which is why the dark matter problem is such a puzzle. There are a huge variety of measurements we can make that indicate that about 5/6ths of the Universe, by mass, isn’t made up of any of the known particles. It doesn’t interact with normal matter or light. And if you measure the mass of a galaxy directly, from its light, it doesn’t match the mass you infer from gravity.

According to models and simulations, all galaxies should be embedded in dark matter halos, whose densities peak at the galactic centers. On long enough timescales, of perhaps a billion years, a single dark matter particle from the outskirts of the halo will complete one orbit. The effects of gas, feedback, star formation, supernovae, and radiation all complicate this environment, making it extremely difficult to extract universal dark matter predictions.NASA, ESA, and T. Brown and J. Tumlinson (STScI)

Traditionally, the way to approach this problem has been to add a single ingredient: dark matter. If you assume that the Universe isn’t simply made up of the matter we can directly detect, but that there’s an additional component, you wouldn’t expect that those two mass measurements would line up. If there’s something besides protons, neutrons, and electrons making up the Universe, their gravitational effects would show themselves without necessarily leaving a visible light signature.

But another option would be to modify the law of gravity. If you simply add in an additional term to Newton’s law of gravity that defines a minimum acceleration scale, you can explain how galaxies rotate to a superior degree to the dark matter idea. The great hope of modified gravity is to reproduce the entire observable Universe without adding in dark matter.

Individual galaxies could, in principle, be explained by either dark matter or a modification to gravity, but they are not the best evidence we have for what the Universe is made of, or how it got to be the way it is today.Stefania.deluca of Wikimedia Commons

While attempts to make a modification to gravity that explain all the cosmic observations have proved elusive thus far, this remains the best option to explain how galaxies (and smaller objects) behave. Without a direct detection of a theoretical particle that could be responsible for dark matter, the door must remained open for alternatives. Despite the overwhelming cosmological evidence pointing to dark matter, other options deserve consideration, too.

Our galaxy is thought to be embedded in an enormous, diffuse dark matter halo, indicating that there must be dark matter flowing through the solar system. But it isn’t very much, density-wise, and that makes it extremely difficult to detect locally.Robert Caldwell & Marc Kamionkowski Nature 458, 587-589 (2009)

In science, the way you decide which ideas are admissible versus which ones are no longer possible is to put them to the test against one another. Dark matter and modified gravity have a hard time going head-to-head on galactic scales because there are a number of confounding elements involved. For galaxies, star formation, feedback between gas, radiation, and dark matter, as well as stellar winds and complicated merger scenarios make universal predictions difficult on these small scales. Modified gravity might give you much cleaner predictions on these small scales, but fail catastrophically when you attempt to extend these modifications to larger ones, where dark matter achieves its greatest successes.

The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. Alternative theories now need to be so contrived that they are considered by many to be quite ridiculous. But dark matter and modified gravity are both contenders for explaining the Universe on small (galactic) scales.X-ray: NASA/CXC/Ecole Polytechnique Federale de Lausanne, Switzerland/D.Harvey NASA/CXC/Durham Univ/R.Massey; Optical/Lensing Map: NASA, ESA, D. Harvey (Ecole Polytechnique Federale de Lausanne, Switzerland) and R. Massey (Durham University, UK)

But there’s a new paper out that has devised a brilliant, head-to-head test for dark matter against modified gravity. If the law of gravity is truly different from Einstein’s General Relativity, then it should apply equally well to all galaxies under all conditions.

If we can find two galaxies with the same mass profiles — where they’re not only the same overall mass, but have the same mass-as-a-function-of-radius as one another — we’d expect them to exhibit the same internal motions as one another. If there’s no dark matter, but just the matter we observe, the force of gravity, even if it’s a modified force of gravity, would have to be the same.

Some galaxies are observed, if we try to fit them with dark matter, to have a ‘core’ in the center where the density is low, while others have a ‘cusp’ where the density is high. If dark matter gets heated based on the galaxy’s star formation history, this mystery could at last be solved.J. I. Read, M. G. Walker, P. Steger; arXiv:1808.06634

So if we look at two galaxies and see that they don’t match, either at least one of the galaxies must be out of equilibrium, meaning it’s in a state of change, or modified gravity can’t explain it.

On the other hand, there is a tremendously powerful explanation that dark matter offers that could explain it all, even if both galaxies are in equilibrium. The reason? Because galaxies could have formed stars at different times or different rates, and star-formation history affects not just the normal matter, but the dark matter as well.

While the web of dark matter (purple) might seem to determine cosmic structure formation on its own, the feedback from normal matter (red) can severely impact galactic scales. Even small galaxies are subject to these effects, and if dark matter heats up from star formation, the effect can be quite severe.Illustris Collaboration / Illustris Simulation

While it’s true that only normal matter interacts (i.e., scatters) with photons, both normal matter and dark matter should respond to radiation pressure. If a galaxy formed stars only a very long time ago, and not for many billions of years, there should be plenty of dark matter that now populated in the inner reaches of a galaxy. But if there has been a lot of recent star formation, that dark matter should have been heated up by that radiation pressure, lowering the inner density of dark matter. (There was a nice review of this back in 2014.)

The best laboratory to test this is with small, dwarf galaxies, where these effects should be the largest.

Dwarf galaxy NGC 5477 is one of many irregular dwarf galaxies. The blue regions are indicative of new star formation, but many such galaxies have formed no new stars in many billions of years. Even with the same light profiles, their mass profiles appear to be different, a challenge for modified theories of gravity.ESA/Hubble and NASA

If the galaxies all demonstrate the same gravitational behavior, it would be a victory for modified gravity. But if we can trace out the star-formation histories of these galaxies — which we can do by examining the stellar populations found inside them — and if these galaxies exhibit different gravitational behaviors because of them, that would be a victory for dark matter, and a blow to the theories of modified gravity that make contrary predictions.

The number of galaxies we’ve found and examined to test this is small, but in a new paper led by Justin Read, they look at 16 such galaxies, and find that the dark matter “heating” explanation appears to work!

The dwarf ‘twins’ Carina and Draco: a challenge for alternative gravity explanations for DM.
The solid and dashed black and purple lines show predictions
for Draco and Carina in MOND, which clearly fare poorly. Despite their similarities in terms of light, stellar kinematics imply that Draco is substantially denser than Carina.Fig 7. from J. I. Read, M. G. Walker, P. Steger; arXiv:1808.06634

They looked at 8 dwarf spheroidal and 8 dwarf irregular galaxies, and found that there were two populations: one where star formation hasn’t occurred for the past 6 billion years, and one where it has. The ones where star formation didn’t occur recently are consistent with lots of dark mass in the center (no recent heating), and the ones where it did occur recently show far less dark matter in their centers (evidence for recent heating). It’s an indication that there is dark matter, it is cold and collisionless, and that it can be heated up by recent star formation.

The Draco dwarf spheroidal galaxy is one of the 16 galaxies examined in the Read et al. paper, and displays extremely different mass profiles from its gravitational effects than the Carina galaxy, which otherwise appears extremely similar except for a different star formation history.Bernhard Hubl / astrophoton.com

In particular, two of the galaxies (Draco and Carina) have almost the same masses and normal mass profiles, but widely different gravitational effects.

The Carina dwarf galaxy, very similar in size, star distribution, and morphology to the Draco dwarf galaxy, exhibits a very different gravitational profile from Draco. This can be cleanly explained with dark matter if it can be heated up by star formation, but not by modified gravity.ESO/G. Bono & CTIO

The authors note:

These two galaxies require different dynamical mass profiles for almost the same radial light profile. This is a challenge not only for MOND, but for any weak-field gravity theory that seeks to fully explain DM.

The fact that these two galaxies exhibit such different gravitational effects tell us that either something is very funny with one of them (something must be out-of-equilibrium), or that dark matter gets heated up by star formation and modified gravity cannot explain this. As always, more data, additional galaxies, and further research will be required to solve this mystery, but at long last, we’re looking at a viable way to prove modified gravity wrong on galaxy scales. Even without directly detecting a particle, dark matter might just achieve a knockout blow over its greatest competing alternative.


Thanks to Justin Read and Rhys Taylor for their explanatory takes on this new work.

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