How Much Of The Dark Matter Could Neutrinos Be?
“If we restrict ourselves to the Standard Model alone, we simply cannot account for the dark matter that must be present in our Universe. None of the particles we know of have the right behavior to explain all of the observations. We can imagine a Universe where neutrinos have relatively large amounts of mass, and that would result in a Universe with significant quantities of dark matter. The only problem is that dark matter would be hot, and lead to an observably different Universe than the one we see today.
Still, the neutrinos we know of do behave like dark matter, although it only makes up about 1% of the total dark matter out there. That’s not totally insignificant; it equals the mass of all the stars in our Universe! And most excitingly, if there truly is a sterile neutrino species out there, a series of upcoming experiments ought to reveal it over the next few years. Dark matter might be one of the greatest mysteries out there, but thanks to neutrinos, we have a chance at understanding it at least a little bit.”
Dark matter is a form of matter that gravitates, but neither absorbs nor emits light, and has been frustratingly difficult to pin down and directly detect. There’s a known particle that has exactly those same properties: the neutrino! You might wonder, then, if perhaps neutrinos had the right value of mass and number, if they could make up the dark matter? And if not all of it, could they at least make up part of it? This is a question that astronomers and physicists have pondered for decades, and we might be closer than ever to the actual answer.
How much of the dark matter can neutrinos actually be? Find out today!
The ‘WIMP Miracle’ Hope For Dark Matter Is Dead
“Our hunt for dark matter in the lab, through direct detection efforts, continues to place important constraints on what physics may be present beyond the Standard Model. For those wedded to miracles, though, any positive results now appear increasingly unlikely. That search is now reminiscent of the drunk looking for his lost keys beneath the lamppost. He knows they’re not there, but it’s the only place where the light enabling him to look shines.
The WIMP miracle may be dead and gone, as particles interacting through the weak force at the electroweak scale have been disfavored by both colliders and direct detection. The idea of WIMP dark matter, however, lives on. We just have to remember, when you hear WIMP, we include dark matter that’s weaker and wimpier than even the weak interactions will allow. There is undoubtedly something new out there in the Universe, waiting to be discovered.
But the WIMP miracle is over. But we still might get the best miracle of all: if these experiments turn up something beyond a null result. The only way to know is to look.”
We have all sorts of enormous experiments built on the hope that dark matter would be a very specific type of particle: a WIMP. If it has a mass of around the electroweak scale, and doesn’t interact with the strong or electromagnetic forces but does have a nuclear recoil, these experiments should see it. This was well-motivated by a slew of theories of new physics at the electroweak scale. But with the limits we presently have, recently improved by LUX and XENON1T, those WIMPs, motivated by a theoretical scenario called the “WIMP Miracle,” are now ruled out.
Dark matter may be wimpier than the weak interaction, but our original motivation for building these detectors is dead and gone.
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!
The Galaxy That Challenged Dark Matter (And Failed)
“It’s fair to say that galaxies come in a great variety of shapes, sizes, densities and masses. Despite all that we know, there is so much we’re still learning as far as how they form, evolve, and grow up in the Universe. But whenever you have a surprising observation, the first thing you need to check is whether the conclusion it leads you to holds up when you make your observation using a different method.
These new observations don’t prove that dark matter exists, but they do remove a primary reason for doubting it. Instead of a single object that lacks a cosmic explanation, we now have an object that’s consistent with the observations of many similar objects in the same class. NGC 1052-DF2 is an interesting object that deserves further study, but it’s unlikely to have no dark matter at all. Although observations will always be our guide, this result highlights how important it is to independently verify your work before drawing grand, revolutionary conclusions.”
Earlier this year, a study came out identifying an ultra-diffuse galaxy 65 million light years away. While it was the size of the Milky Way, it had only 0.5% of the stars our galaxy has inside of it. When scientists measured the motions of the globular clusters around it, they found that they weren’t moving relative to one another at all, indicating an extremely low mass for the galaxy: a mass so low that it implied the galaxy had no dark matter. This shouldn’t be possible! Did this mean that there was a flaw with the measurements? That dark matter didn’t exist?
No; it meant we needed a better, independent method to draw a responsible conclusion. Well, that’s what science just delivered! Happy end-of-2018 to us all!
Physicists Used Einstein’s Relativity To Successfully Predict A Supernova Explosion
“When the lens and a background source align in a particular fashion, quadruple images will result. With slightly different light-travel paths, the brightness and arrival time of each image is unique. In November 2014, a quadruply-lensed supernova was observed, showcasing exactly this type of alignment. Although a single galaxy caused the quadruple image, that galaxy was part of a huge galaxy cluster, exhibiting its own strong lensing effects. Elsewhere in the cluster, two additional images of the same galaxy also appear.”
We normally think of light traveling in a straight line, but that’s only true if your space is flat. In the real Universe, mass and matter not only exist, but clump together into massive structures like galaxies, quasars, and galaxy clusters. When a background source of light passes through these foreground masses, the light can get bent and distorted into multiple images that are magnified and arrive at slightly different times. If an event occurs in one such image, we can predict, based on General Relativity, cluster dynamics, and dark matter, when that event will appear in the other images.
In November 2014, we discovered a multiply-lensed supernova, and predicted where and when it would appear in the other images. Einstein and dark matter both win again!
This Is The Real Reason We Haven’t Directly Detected Dark Matter
“So we keep looking, we keep thinking of new possibilities for what it could be, and we keep thinking of new ways to search for it. That’s what science at the frontiers is like. Personally, I don’t expect these direct detection attempts to be successful; we’re stabbing in the dark hoping we hit something, and there are little-to-no good reasons for dark matter to be in these ranges. But it’s what we could see, so we go for it. If we find it, Nobel Prizes and new physics discoveries for everyone, and if we don’t, we know a little more about where the new physics isn’t. But just as you shouldn’t fall for the hyper-sensationalized claims that dark matter has been directly detected, you shouldn’t fall for the ones that say “there’s no dark matter” because a direct detection experiment failed.”
At some point, when you’re looking for an unknown, you have to give up and declare it isn’t there. Sometimes you’re right, and other times, you discover that you either weren’t looking in the right place, or weren’t looking in the right way. It took over 25 years to find the neutrino from when Pauli first proposed it; over 50 years to find the Higgs boson from when it was first theorized; and over a century to find the first gravitational wave, first predicted by Einstein’s theory in 1915. So why, then, would we give up so quickly after not finding dark matter, after only a few decades of looking under a particular set of assumptions?
Dark matter isn’t easy to find, but it isn’t supposed to be. Absence of evidence is not evidence of absence. Learn the real reason we haven’t detected it, yet, today.
The Most Important X-Ray Image Ever Taken Proved The Existence Of Dark Matter
“Yet the most important X-ray image of all time was an incredible surprise. This is the Bullet Cluster: a system of two galaxy clusters colliding at high speeds. As the gaseous matter inside collides, it slows, heats up, and lags behind, emitting X-rays. However, we can use gravitational lensing to learn where the mass is located in this system. he bending and shearing of light from background galaxies shows it’s separated from the matter’s and X-rays’ location. This separation is some of our strongest evidence for dark matter.”
There are many different lines of evidence for dark matter, but one of the biggest contentions of those who disbelieve it is that a direct empirical proof of its existence is needed. If it exists in a large, diffuse halo around every galaxy, cluster, and component of large-scale structure in the Universe, you should be able to prove it. Starting more than 10 years ago, astronomers have been able to do just that. When galaxy clusters collide, the overwhelming majority of normal matter, residing in the intracluster medium, should smash together, heat up, and emit X-rays. It does! But the biggest deal is that the gravitational mass, reconstructed through lensing, doesn’t coincide with the normal matter.
There must be some other type of matter from the normal, baryonic matter. Ergo, dark matter. Here’s (IMO) the most important X-ray story of all-time.
Ask Ethan: When Were Dark Matter And Dark Energy Created?
“Today [normal matter] is only 4.9% while Dark Matter and Dark Energy takes the rest. Where did they come from?”
The Universe, as we know it, got its start in earnest when the hot Big Bang began. Space was filled with all the particles and antiparticles of the Standard Model, up at tremendous energies, while the Universe then expanded, cooled, and gave rise to all we know. But when did dark matter and dark energy, which make up 95% of the Universe we know today, come into the picture? Was the Universe born with these components of energy? Or were they created at a later time? We have some inklings that dark matter was likely created in the extremely early stages, but may not have been present from the Universe’s birth. On the other hand, all theoretical signs point to dark energy always existing, but observationally, we have about 4 billion years where we cannot measure its presence at all.
Where do dark matter and dark energy come from? It’s a great cosmic mystery, but we do know something about it. Find out where we are today!
Modified Gravity Could Soon Be Ruled Out, Says New Research On Dwarf Galaxies
“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.”
If you have two galaxies in the Universe that look the same, you’d expect them to behave the same. After all, the laws that govern them ought to be identical, and so if their properties are identical, so should their behavior. The Draco and Carina dwarf galaxies are roughly the same mass, the same size, and have the same distribution of starlight. The only discernible difference is that one galaxy has only old stars, while the other has a mix of old and new stars. And yet, when we look at the gravitational effects of the mass on the stars, their behaviors are incredibly different. One seems to indicate a large unseen mass source in the center, and the other doesn’t.
In modified gravity, this makes no sense. But in dark matter theories, simple heating due to star formation could explain it all. Keep your ear to the ground, because this could lead to the death-knell for modified gravity!
How The Planck Satellite Forever Changed Our View Of The Universe
“Most importantly, a spectacular agreement to a never-before-achieved precision now exists between the CMB we observe and the theoretical predictions of a Universe with 5% normal matter, 27% dark matter, and 68% dark energy. There might be wiggle room of 1-2% in some of those numbers, but a Universe without dark matter and dark energy, both, in great abundance, is a no-go in the face of these observations. They’re real, they’re necessary, and their predictions match the full suite of data perfectly.
Inflation, neutrino physics, and the Big Bang have additional pieces confirming them, while alternatives and specific variants are better constrained. Most definitively, the Planck collaboration states, “We find no compelling evidence for extensions to the base-ΛCDM model.” At last, we can state, with extraordinary confidence, what the Universe is made of.”
For centuries, the question of what the Universe was made of was one of the most unknowable wonders of existence. This week, the Planck collaboration, whose team made the most accurate, precision measurements of the Big Bang’s leftover glow, released their final results, providing unprecedented answers to that question. We now know what the Universe is made of, how old it is, how fast it’s expanding, and a whole suite of other information about it, better than we’ve ever known before. The Planck satellite has revolutionized our view of the Universe, and we’re unlikely to ever do better using this line of inquiry.
How has our view of the Universe changed as a result of Planck? Come find out today!