Eight New Quadruple Lenses Aren’t Just Gorgeous, They Reveal Dark Matter’s Temperature
“Ever since astronomers first realized that the Universe required the existence of dark matter to explain the cosmos that we see, we’ve sought to understand its nature. While direct detection efforts have still failed to bear fruit, indirect detection through astronomical observations not only reveal the presence of dark matter, but this novel method of using quadruply lensed quasar systems has given us some very strong, meaningful constraints on just how cold dark matter needs to be.
Dark matter that’s too hot or energetic cannot form structures below a certain scale, and the observations of these ultra-distant, quadruple-lens systems show us that dark matter must form clumps on very small scales after all, consistent with them being born as arbitrarily cold as we can imagine. Dark matter’s not hot, nor can it even be very warm. As more of these systems come in and our instruments go beyond what even Hubble’s capabilities are, we might even discover what cosmologists have long suspected: dark matter must not only be cold today, but it must have been born cold.”
We might not yet know the nature of dark matter, as we’ve never been able to detect the particle responsible for it directly. But we know it clumps and gravitates together, with the exact way it would do so dependent on the amount of kinetic energy it had when it was born relative to its mass. Dark matter could have been extremely hot, such as a scenario where it was made from neutrinos, cold, such as from a very heavy WIMP particle (or a born-super-cold axion), or anywhere in between.
Thanks to a new technique involving quadruple-lens systems, we’ve just learned how cold dark matter needs to be. Get the (beautiful) story today!
Antimatter Mystery Likely Due To Pulsars, Not Dark Matter
“Whenever there’s an unexplained phenomenon that we’ve measured or observed, it presents a tantalizing possibility to scientists: that perhaps there’s something new at play beyond what’s presently known. We know there are mysteries about our Universe that require new physics at some level — mysteries like dark matter, dark energy, or the cosmic matter-antimatter asymmetry — whose ultimate solution has yet to be discovered.
However, we cannot claim evidence for a new discovery until everything that represents what’s already known is quantified and accounted for. By factoring in the effect of pulsars, the positron excess observed by the Alpha Magnetic Spectrometer collaboration may turn out to be explicable entirely by conventional high-energy astrophysics, with no need for dark matter. Right now, it appears that pulsars may be responsible for 100% of the observed excess, requiring scientists to go back to the drawing board for a direct signal that reveals our Universe’s elusive dark matter.”
Over the past decade, the Alpha Magnetic Spectrometer experiment aboard the International Space Station has taken the best-ever measurements of cosmic rays directly from space. One of the surprises they saw was an excess of positrons, the antimatter counterpart of electrons, in unexpectedly large abundances at high energies. It offered a tantalizing possibility that, just perhaps, dark matter might be the culprit in this cosmic mystery.
However, a new study offers evidence that a much more mundane explanation, pulsars, might be the cause instead. Come get the full story today.
What Is The Ultimate Fate Of The Loneliest Galaxy In The Universe?
“After an extraordinary amount of time has passed, googols of years or even more, the loneliest galaxy in the Universe will appear completely empty. No stars, stellar remnants, planetary corpses or even black holes ought to remain. And yet, it will still exist. Someone who could measure the spacetime curvature of the Universe or somehow detect dark matter or ultra-low energy neutrinos would encounter an enormous, diffuse halo of mass that will persist for far longer than any bound structure made of normal matter.
Eventually, dependent on the actual (and yet unknown) masses of individual dark matter particles and neutrinos, this remnant dark halo will decay, ejecting itself particle-by-particle until none remain. Until the masses and properties of those particles are known, however, we cannot calculate that timescale; we can only know it will persist longer than any normal matter will. The eventual fate of the last galaxies in the Universe will be a skeletal dark matter/neutrino halo, far outlasting anything else we’ve ever observed.”
Most of the galaxies we find in the Universe aren’t found in isolation, but exist bound to other galaxies, whether in a small group like our own or in an enormous grouping like the galaxies of the Virgo cluster. But out there, hundreds of millions of light-years away, galaxy MCG+01-02-015 exists in true isolation, with no other galaxies surrounding it for some 100 million light-years in all directions. Whereas we have tens of thousands of galaxies within that distance of ourselves, it has not even one. As a result, it’s a much cleaner astronomical laboratory, and we can predict its future far more certainly than we can predict our own.
So what is the ultimate fate of this galaxy: the loneliest one in the Universe? Come find out, with implications for everything else we know of, too!
Why Humans Should Be Thankful That Our Universe Has Dark Matter
“In a Universe without dark matter, we might still have stars and galaxies, but the only planets would be gas giant worlds, with no rocky ones to speak of. Without carbon, there are no organic molecules; without oxygen, there is no liquid water; without a whole slew of elements from the periodic table, biochemical life would be completely impossible.
Only with the presence of massive dark matter halos, surrounding galaxies and driving the growth of the cosmic web, can a planet like Earth or carbon-based life like we find terrestrially be formed. As we’ve come to understand what makes up our Universe and how it grew to be this way, one inescapable conclusion emerges: dark matter is fundamentally necessary for life to arise. Without it, the chemistry that underlies all life could never have occurred. Today and every day, we should be thankful for every part of the cosmic story that allowed us to exist. Even dark matter.”
Today marks American Thanksgiving, a holiday where we give thanks for all the positive things that have impacted our lives and the bountiful harvest that nature provides in order for us to survive through the harsh winters. But one of the things that’s not only unappreciated, but often derided in the popular media is dark matter, a substance which interacts gravitationally but not through any other known force.
Yet, without dark matter, humans, chemical-based life, or even rocky planets wouldn’t be able to exist in our Universe. Here’s why you, and everyone, should be thankful for it.
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.