Controversial ‘Dark Matter Free Galaxy’ Passes Its Most Difficult Test
“In theory, all galaxies should contain copious amounts of dark matter, with one exception. Galactic mergers, interactions, or gas stripping events can isolate large amounts of normal matter. These liberated clumps should gravitate and recollapse, creating dark matter-free galaxies. Detractors argued their absence proved dark matter’s non-existence. However, 2018 and 2019 saw scientists announce two dark matter-free galaxies: NGC 1052-DF2 and NGC 1052-DF4.”
One of the most counterintuitive predictions of dark matter is that, owing to the differing forces that normal matter and dark matter experience in environments rich in matter and radiation, it should be separable from normal matter. Therefore, when major galaxy mergers or interactions occur, it should be possible to strip normal matter out of the dark matter halos they’re bound to, creating dark matter-free galaxies.
Long predicted by theory but never discovered, they were used by dark matter detractors to demonstrate the insufficiency of dark matter. But in 2018, the galaxy NGC 1052-DF2 was measured well enough to conclude it was devoid of dark matter; in 2019, it was joined by NGC 1052-DF4. While a different team claimed these galaxies were closer and therefore not dark matter-free, the original researchers turned to Hubble to settle the matter.
NGC 1052-DF4 has now been measured better than ever before, and it’s at the original (farther) distance, implying that it really is dark matter-free. Come get the full story today.
Dark Matter’s Biggest Problem Might Simply Be A Numerical Error
“If this new paper is correct, however, the only flaw is that cosmologists have taken one of the earliest simulation results — that dark matter forms halos with cusps at the center — and believed their conclusions prematurely. In science, it’s important to check your work and to have its results checked independently. But if everyone’s making the same error, these checks aren’t independent at all.
Disentangling whether these simulated results are due to the actual physics of dark matter or the numerical techniques we’ve chosen could put an end to the biggest debate over dark matter. If it’s due to actual physics after all, the core-cusp problem will remain a point of tension for dark matter models. But if it’s due to the technique we use to simulate these halos, one of cosmology’s biggest controversies could evaporate overnight.”
On large cosmic scales, cold dark matter provides the perfect answer to a number of puzzles. Without it, the cosmic microwave background, the large-scale galaxy clustering seen in the Universe, the absorption properties of gas clouds intercepted by background quasar light, gravitational lensing and much more cannot be explained. However, on small scales, the simulations of dark matter all produce expected dark matter halos whose properties don’t align with the small galaxies we actually see. For decades, dark matter’s detractors have latched onto this as the biggest flaw with dark matter. But a new study says it might be a flaw of the simulation methods used, not of the theory at all.
If so, it might turn out that dark matter’s biggest apparent problem is simply a numerical error, resolving one of cosmologies greatest controversies.
Was Dark Matter Really Created Before The Big Bang?
“So if that’s what the observational data points towards, what can we say about where dark matter comes from? A recent headline that made quite a splash claimed that dark matter may have originated before the Big Bang, and many people were confused by this assertion.
It might seem counterintuitive, because the way most people conceive of the Big Bang is as a singular point of infinite density. If you say the Universe is expanding and cooling today, then you can extrapolate it back to a state where all the matter and energy was compressed into a single point in space: a singularity. This corresponds to an initial start time for our Universe — the beginning of our Universe — and that’s the Big Bang.
So how could something that exists in our Universe, like dark matter, have originated before the Big Bang? Because the Big Bang wasn’t actually the beginning of space and time.”
Last month, a paper came out claiming that dark matter may have been created before the Big Bang. Although it might sound implausible, it’s absolutely a possibility that we cannot rule out, although it might be an idea that’s extraordinarily difficult to test when we compare it up against the other options. We have to keep every scenario that hasn’t been ruled out in mind, and understand that despite all we don’t know about dark matter, there’s a ton of indirect evidence brought to us by the full suite of observations at our disposal.
Could dark matter have been created before the Big Bang? Yes, but three other possibilities are maybe even more viable. Come find out why today.
Ask Ethan: Can Black Holes And Dark Matter Interact?
“If you do the math, you’ll find that black holes will use both normal matter and dark matter as a food source, but that normal matter will dominate the rate of growth of the black hole, even over long, cosmic timescales. When the Universe is more than a billion times as old as it is today, black holes will still owe more than 99% of their mass to normal matter, and less than 1% to dark matter.
Dark matter is neither a good food source for black holes, nor is it (information-wise) an interesting one. What a black hole gains from eating dark matter is no different than what it gains from shining a flashlight into it. Only the mass/energy content, like you’d get from E = mc2, matters. Black holes and dark matter do interact, but their effects are so small that even ignoring dark matter entirely still gives you a great description of black holes: past, present, and future.”
You might not be able to make a black hole out of dark matter entirely, but once a black hole exists, anything that falls past its event horizon will add to its mass, whether it’s particles, antiparticles, radiation or dark matter. And the longer black holes sit in the galaxy, the more and more dark matter will eventually fall in.
The question isn’t whether dark matter contributes to black holes; it’s how and how much. Let’s give you the answer on this edition of Ask Ethan!
This One ‘Anomaly’ Is Driving Physicists To Search For Light Dark Matter
“If the result is robust, one potential explanation is the existence of a new particle with a specific mass: about 0.017 GeV/c^2. This particle would be heavier than the electron and all of the neutrinos, but lighter than every other massive, fundamental particle ever discovered. Many different theoretical scenarios have been proposed to account for this measurement, and various ways to look for an experimental signature have also been devised.
When you hear about experiments looking for a dark photon, a light vector boson, a protophobic particle, or the force-carrying particle for a new, fifth force, they’re all looking for variants that could explain this Atomki anomaly. Not only that, but many of them also seek to solve one of the big puzzles with this particle: the dark matter puzzle. There’s no harm in shooting for the Moon, but every measurement has met with the same disappointment: null results.”
You have to go where the data point you, even if there’s every reason to believe that what you’re undertaking is nothing but a fool’s errand. There has been an enormous increase in the experiments that are deciding to search for light dark matter: dark matter particles heavier than an electron but lighter than the other Standard Model particles. We’ve been probing this energy range for decades, finding nothing, but there’s one nuclear physics experiment that indicated an anomaly back in 2015-2016, and that’s enough evidence to alter the direction of a field!
If you’ve ever wondered why physicists care about light dark matter, this is one read you won’t want to miss. It’s all null results so far, but that’s just further motivation to deepen the search.
Happy Birthday To Vera Rubin: The Mother Of Our Dark Matter Universe
“Dark matter should drive the formation of structure on all large scales, with every galaxy consisting of a large, diffuse halo of dark matter that is far less dense and more diffuse than the normal matter. While the normal matter clumps and clusters together, since it can stick together and interact, dark matter simply passes through both itself and normal matter. Without dark matter, the Universe wouldn’t match our observations.
But this branch of science truly got its start with the revolutionary work of Vera Rubin. While many, including me, will deride the Nobel committee for snubbing her revolutionary science, she truly did change the Universe. On what would have been her 91st birthday, remember her in her own words:
“Don’t let anyone keep you down for silly reasons such as who you are, and don’t worry about prizes and fame. The real prize is finding something new out there.”
50 years later, we’re still investigating the mystery Vera Rubin uncovered. May there always be more to learn.”
Today, dark matter is practically accepted as a given, owing to an overwhelming suite of evidence that points to its existence. Without adding dark matter as an ingredient, we simply can’t explain the Universe, from gravitational lensing to large-scale structure to Big Bang nucleosynthesis to the cosmic microwave background and much more. But throughout the 1930s, 40s and 50s, no one would even give the idea a second thought. Until, that is, Vera Rubin came along and changed everything.
Today would have been her 91st birthday, and it’s about time you got the scientific story to celebrate what she taught us all.
New Method For Tracing Dark Matter Reveals Its Location, Abundance As Never Before
“By measuring the distorted light from distant galaxies behind a galaxy cluster, scientists can reconstruct the total cluster mass. In every galaxy cluster, the majority of the mass is outside of the galaxies: there is a huge dark matter halo. The intracluster gas, however, may be distributed differently, as normal matter can collide and heat up, emitting X-rays. But individual stars, ejected from galaxies, should trace the same path as the dark matter. In a cosmic first, scientists measured this intracluster light, and found it traces out the dark matter perfectly.”
If you want to know where the dark matter is located in the Universe, you had to infer its presence and abundance by measuring the gravitational effects it had on space. When it comes to large-scale structures, like galaxy clusters, this often involved exceedingly difficult reconstructions involving gravitational lensing, and relied on serendipitous alignments of observable background structures. But a new study has concocted an alternative method that works extremely well: just measure the intracluster light from stars that have been ejected from the component galaxies.
Well, with the first two clusters down, we have a verdict: it’s the best dark matter-tracer of all time. Come get the remarkable story today!
This Is How Mastering Dark Matter Could Take Us To The Stars
“Because dark matter is everywhere, we wouldn’t even need to carry it with us as we traversed the Universe. As far as we understand it — and admittedly, we need to understand it a lot farther — dark matter could truly deliver our dream of the ultimate fuel. It’s abundant all throughout our galaxy and beyond; it should have a non-zero annihilation cross-section with itself; and when it does annihilate, it should produce energy with 100% efficiency.
Perhaps, then, most of us have been thinking about experiments seeking to directly detect dark matter all wrong. Yes, we want to know what makes up the Universe, and what the physical properties of its various abundant components truly are. But there’s a science-fiction dream that could come true if nature is kind to us: unlimited, free energy just waiting there for us to harness, no matter where in the galaxy we go.
Mastering dark matter is the endeavor that just might make it so.”
When we talk about our dreams of traveling to the stars, it normally involves a mythical, futuristic form of travel that goes beyond the known laws of physics. Why’s that? Because even if you increase the efficiency of your rocket fuel far beyond the limitations of any chemical-based reaction we know of, you’d still be limited in how far you could go by the mass of your spacecraft and the fuel you were able to take with you on board. You’d still have to accelerate (and decelerate) all the fuel you brought with you, until you ran out. If only there were a 100%-efficient fuel source that was ubiquitous all throughout the galaxy and beyond.
There is: dark matter. Here’s why it’s so important to study, understand, and eventually, fulfill the dream of harnessing it!
This Is Why Every Galaxy Doesn’t Have The Same Amount Of Dark Matter
“It isn’t the properties of one or two galaxies that will be the ultimate test of dark matter, however. Whether these galaxies are generic dwarf galaxies or our first examples of dark matter-free galaxies isn’t the point; the point is that there are hundreds of billions of these dwarf galaxies out there that are presently below the limits of what’s observable, detectable, or having their properties measured. When we get there, especially in the distant Universe and in post-interaction environments, we can fully expect to truly find this yet-unconfirmed population of galaxies.
If dark matter is real, it must be separable from normal matter, and that works both ways. We’ve already found the dark matter-rich galaxies out there, as well as isolated intergalactic plasma. But dark matter-free galaxies? They might be right around the corner, and this is why everybody is so excited!”
When the Universe was first born, everything was uniform. There was dark matter and normal matter everywhere, in the same 5-to-1 ratio in all structures. But then the Universe had to go and get messy. It formed stars and galaxies of different masses and sizes, and that’s where the trouble started. In large, massive galaxies, even cataclysms like supernovae or active supermassive black holes don’t eject very much normal matter. But in small galaxies, significant amounts of normal matter can get ejected, upping that ratio to dozens or event hundreds to one. That ejected matter doesn’t just go away, but can itself, at least in theory, form dark matter-free galaxies. Where are we in our understanding of galaxies, dark matter, and gravitation?
It’s just a small piece of the puzzle, but this explains why not every galaxy has the same 5-to-1 ratio you might naively expect!
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.