Ask Ethan: What’s The Real Story Behind This Dark Matter-Free Galaxy?
“I read a study that said the mystery of a galaxy with no dark matter has been solved. But I thought that this anomalous galaxy was previously touted as evidence FOR dark matter? What’s really going on here, Ethan?”
Imagine you looked at the Universe, and saw a galaxy unlike any other. Whereas every other galaxy we’ve ever looked at exhibited a large discrepancy between the amount of matter that’s present in stars and the total amount of gravitational mass we’d infer, this new galaxy appears to have no dark matter at all. What would you do? If you’re being a responsible scientist, you’d try to knock down this galaxy by any scrupulous means possible. You’d wonder if you had mis-estimated one of its properties. You’d try to re-confirm the measurements with different instruments and techniques. And you’d wonder if there weren’t an alternative explanation for what we were seeing.
Well, if you read that the galaxy has dark matter after all, and the mystery has been resolved, you should definitely read this instead. The story is far from over, and even if the new team’s results hold up, there’s still a mystery at play here.
Cold Dark Matter Is Heated Up By Stars, Even Though It Cannot ‘Feel’ Them
“This effect is what’s known as “dark matter heating.” It isn’t that any of the radiation from the stars or any of the heat from the normal matter is getting transferred to the dark matter itself; it doesn’t involve temperature or energy transfer directly.
Instead, what’s happening is that the additional energy imparted to the normal matter is expelling it from where it was previously the most concentrated: in the galactic center. Once that normal matter is removed from the galactic center, there’s less mass there to hold the dark matter in place, and it, too, has to move to a higher, less-tightly-bound orbit. Because the dark matter gets pushed out and bumped to a higher, more energetic orbit, it has the same effects as though the dark matter was given an extra burst of energy. It’s not actually hotter than it was previously, but the effects are identical.”
Dark matter isn’t supposed to interact with anything. Not with normal matter, not with itself, not with radiation. So how is it, then, that cold dark matter can be heated up by the formation of new stars? Why should that be the solution to the odd distribution of mass in dwarf galaxies?
It actually makes sense, if you reason your way through the physics. Come take that journey, and learn how it actually happens!
How Does Our Earliest Picture Of The Universe Show Us Dark Matter?
“So all you need to do, to know whether your Universe has dark matter or not, is to measure these temperature fluctuations that appear in the CMB! The relative heights, locations, and numbers of the peaks that you see are caused by the relative abundances of dark matter, normal matter, and dark energy, as well as the expansion rate of the Universe. Quite importantly, if there is no dark matter, you only see half as many total peaks! When we compare the theoretical models with the observations, there’s an extremely compelling match to a Universe with dark matter, effectively ruling out a Universe without it.”
If your young Universe is full of matter and radiation, what happens? Gravitation works to pull matter into the overdense regions, but that means that the radiation pressure must rise in those regions, too, and that pushes back against the matter. On small scales, this pushback washes out the gravitational growth, but on large-enough scales, the finite speed that light can travel means that no wash-out can happen. Dark matter, however, doesn’t collide with radiation or normal matter, while normal matter collides with both radiation and itself. If we can calculate exactly how these three species interplay, we can calculate what types of patterns we expect to see in the Big Bang’s leftover glow, and then compare it with what we observe with satellites like WMAP and Planck. And what have we seen, exactly, when we’ve done that?
We see that the Universe must contain dark matter to explain the observations. No alternative theory can match it.
5 Ways To Make A Galaxy With No Dark Matter
“2.) Ejected from galactic mergers. When two galaxies smash together, they usually merge entirely, but sometimes there is ejected material. Sufficient amounts could create a baryons-only galaxy.”
Last week, astronomers announced the discovery of the ultra-diffuse galaxy NGC 1052-DF2 (or DF2), which appears to be completely free of dark matter. Other similar galaxies have been seen before, but all contain more, not less, dark matter than you’d have expected on average. This first galaxy ever seen without the gravitation-altering effects of dark matter was touted to defy theory, but it does no such thing. In fact, there are many explanations that lead directly to galaxies such as DF2 as an inevitable consequence, including one that was put forth in a predictive fashion as much as 20 years ago.
Come find out the five ways to make a galaxy without dark matter, and learn why this is such an important test of dark matter theory itself!
Only Dark Matter (And Not Modified Gravity) Can Explain The Universe
“Modified gravity cannot successfully predict the large-scale structure of the Universe the way that a Universe full of dark matter can. Period. And until it can, it’s not worth paying any mind to as a serious competitor. You cannot ignore physical cosmology in your attempts to decipher the cosmos, and the predictions of large-scale structure, the microwave background, the light elements, and the bending of starlight are some of the most basic and important predictions that come out of physical cosmology. MOND does have a big victory over dark matter: it explains the rotation curves of galaxies better than dark matter ever has, including all the way up to the present day. But it is not yet a physical theory, and it is not consistent with the full suite of observations we have at our disposal. Until that day comes, dark matter will deservedly be the leading theory of what makes up the mass in our Universe.”
You’ve heard of the big controversy: between dark matter explaining the missing mass of the Universe on one hand, and on the possibility of modifying gravity on the other. If you’re not a physical cosmologist yourself, how do you know which camp is right? Should you just go with whatever answer sounds better to you, or fits your gut instinct better? Of course not! Instead, you need to look at the full suite of data, and you need to look in the regime where the predictions are the most robust and the easiest to discern from one another. Where is that? On the largest scales, at the earliest times, and in general in the linear regime of structure formation. There are four observations that I highlight here, and remarkably, dark matter can explain all four with ease. Modified gravity? It can’t get you even two of them with the same modification, not unless you also include dark matter.
This is the one in-depth article you should read if you want to know why cosmologists strongly and almost universally prefer dark matter to modified gravity!
Satellite Galaxies Live In The Same Plane As Their Hosts, Defying Dark Matter Predictions
“So who is correct? As simulations become better at adding in additional dynamics such as dark matter/radiation/normal matter interactions, star formation feedback, local peculiar velocity effects and more, they match better with the observations. Alternatives to dark matter still suffer the same failures when attempting to reproduce the cosmic web, the cosmic microwave background, or the dynamics of colliding galaxy clusters. However, it’s important to keep an open mind so long as the smoking-gun evidence for CDM is missing, and also remember that this is a puzzle that may say more about galaxy evolution and mergers than it does about dark matter.”
When we run our most advanced simulations of dark matter, we find that they create large, massive halos, which correspond to the existence of galaxies. However, these halos also obtain large clumps around them: sub-halos, which should house orbiting, dwarf satellite galaxies. They ought to be distributed randomly, in all directions, similarly to how we find globular clusters. Instead, however, observations of three different large galaxies now – Andromeda, the Milky Way, and now Centaurus A – show strong evidence for a plane of dwarf galaxies. Moreover, that plane may be co-rotating along with the disk of the galaxy it’s orbiting with. Is it possible that these dwarf galaxies have nothing to do with dark matter at all, and instead formed via a completely different mechanism?
The possibility is intriguing, but the article’s conclusion that “this challenges cold dark matter cosmology” is not robust at all. Take a detailed look to see why.
The Bullet Cluster Proves Dark Matter Exists, But Not For The Reason Most Physicists Think
“When your cluster is undisturbed, the gravitational effects are located where the matter is distributed. It’s only after a collision or interaction has taken place that we see what appears to be a non-local effect. This indicates that something happens during the collision process to separate normal matter from where we see the gravitational effects. Adding dark matter makes this work, but non-local gravity would make differing before-and-after predictions that can’t both match up, simultaneously, with what we observe.
Interestingly, this argument has been made for over a decade, now, with no satisfactory counterargument coming from detractors of dark matter. It isn’t the displacement of gravitation from normal matter that “proves” dark matter exists, but rather the fact that the displacement only occurs in environments where dark matter and normal matter would be separated by astrophysical processes. This is a fundamental issue that must be addressed, if alternatives to dark matter are to be taken seriously as complete theories, rather than ideas in their infancy. That time is not yet at hand.”
Recently, a paper came out challenging alternative theories to dark matter and claiming that many of them were invalid. The basis for that argument? That those theories predict different arrival times for gravitational waves and light waves from a neutron star merger, when we saw them arrive practically simultaneously. One of those theories, MOG, claims to survive, but it’s already been discredited for another reason that’s discussed far less frequently: the Bullet Cluster. When the apparent effects of gravitation are well-separated in space from where we see the matter, you require non-locality to save your theory. MOG is a non-local theory of gravity, so you might think everything is fine. But if gravitational effects aren’t where the matter is located, we’d expect to see these non-local effects in clusters that are in a pre-merger state, and those don’t exist.
Can a theory like MOG survive in this context? I don’t believe so. The Bullet Cluster proves dark matter exists, but not for the reason most physicists think!
Merging Neutron Stars Deliver Deathblow To Dark Matter And Dark Energy Alternatives
“No-dark-matter modified gravity theories like Bekenstein’s TeVeS or Moffatt’s MoG/Scalar-Tensor-Vector ideas have the property that gravitational waves propagate on different geodesics — a.k.a. different spacetime paths — from those followed by photons and neutrinos. In short, gravitational waves should travel along the paths defined by the normal matter alone, while the photons and neutrinos should travel along paths defined by the effective mass: the normal matter plus the effects that emulate dark matter. This would give a difference in arrival times between photons and gravitational waves by approximately 800 days, instead of the 1.7 seconds observed.
With the cross-correlation of gravitational waves and electromagnetic signals, these no-dark-matter scenarios are busted.”
One of the most puzzling facts about the Universe is that 95% of the energy in it, in the forms of dark matter and dark energy, are completely invisible, and have never been directly detected. Perhaps, the story goes, it’s our theory of gravity that’s to blame, rather than needing new components in the Universe. While dark matter and dark energy can explain a whole slew of observations, gravity modifications do a better job of explaining galactic rotation, but require altering Einstein’s theory of gravity. But merging neutron stars provide a unique test: electromagnetic and gravitational waves both originate from an ultra-distant source over 100 million light years away. The first signals arrive separated by mere seconds, allowing us to constrain models where gravity and light are bent (and delayed) differently by the presence of masses. While theories like Bekenstein’s TeVeS and Moffat’s Scalar-Tensor-Vector predict differing delays by years, the observed arrival time difference was just 1.7s.
With these new observations, models that attempt to do away with dark matter and dark energy are largely busted, leaving only contrived, non-local modified gravity theories behind. It’s an incredible victory for Einstein and the dark Universe.
Dark Matter Theory Triumphs In Sweeping New Study
“It’s a revolutionary breakthrough that dark matter can reproduce both the relationships between luminosity and galactic speeds and the stellar mass function in galaxies simultaneously, as this new study accomplishes for the first time. By incorporating advanced techniques and more detailed physical models and the interplay between different components, relations that has only been observed, never explained, are finally seen to emerge. If we can throw our cosmic ingredients into a simulation and get out the Universe exactly as we observe it, that’s as big a success for our theories and models as one can ask for.”
On the largest scales, dark matter has been undoubtedly the most successful theory in modern cosmology for explaining a huge variety of observations. From the motions of galaxies in clusters to the separation of mass and light when they collide, from the correlations between galactic positions to the fluctuations in the CMB, from the bending of starlight to the formation of large-scale structure, it’s clear that the Universe needs dark matter. But individual galaxies have always been the most difficult test for dark matter. In particular, there have been empirical correlations – or relationships between two different observables – that have never had an underlying explanation successfully presented. One of the most difficult has been the Tully-Fisher relation, which relates the luminosity to the rotational speed of spiral galaxies. But a new simulation, at long last, has finally cracked that nut by incorporating not only gravitation and dark matter, but the relationship between baryons and dark matter.
The way a galaxy forms stars over its history matters tremendously for what we get today, and by simulating it all together, it adds up to one stunning conclusion: success for dark matter in an entirely new way!