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.
Forget WIMPs, Axions And MACHOs: Could WIMPzillas Solve The Dark Matter Problem?
“But what, exactly, is dark matter? And, moreover, can we be certain it exists? There are a huge suite of detectors and experiments out there searching for it, and yet no robust, verified, direct detection has ever been reported. There is no smoking gun we can point to and say, “this was an event caused by an interaction with dark matter.” The overwhelming majority of detectors out there are looking for WIMP-type dark matter, with a small contingent also looking for axions. (MACHOs, or other sources of “normal” dark matter, have been ruled out.) But all of this may be misguided. Dark matter might not be any of those things we’re looking for. In fact, it’s arguable that the candidate with the best motivations for it have no experiments to their name at all: WIMPzillas!”
Many detractors of dark matter point to the fact that we haven’t directly detected it yet as evidence that dark matter doesn’t exist. Yet practically every dark matter search that’s ever been performed has focused on just one particular class of dark matter: WIMPs. Well, WIMPs have been constrained very tightly, and we’re no closer to seeing WIMP dark matter than we were decades ago. But in that same time span, we’ve seen a number of exciting discoveries, including neutrino masses, come to fruition. Related to neutrino masses is the idea that there would be super-heavy right-handed neutrino counterparts to the light ones we see today. Could these “WIMPzillas” be the dark matter we’ve been looking for?
The theoretical motivation is compelling, and yet there are no detection experiments looking for one. Perhaps we need to fix that!
What Astronomers Wish Everyone Knew About Dark Matter And Dark Energy
“It wasn’t always apparent that this would be the solution, but this one solution works for literally all the observations. When someone puts forth the hypothesis that “dark matter and/or dark energy doesn’t exist,” the onus is on them to answer the implicit question, “okay, then what replaces General Relativity as your theory of gravity to explain the entire Universe?” As gravitational wave astronomy has further confirmed Einstein’s greatest theory even more spectacularly, even many of the fringe alternatives to General Relativity have fallen away. The way it stands now, there are no theories that exist that successfully do away with dark matter and dark energy and still explain everything that we see. Until there are, there are no real alternatives to the modern picture that deserve to be taken seriously.”
If you yourself are not a professional astronomer or astrophysicist, you might feel in your gut that dark matter and dark energy just can’t be right. How could we possibly conclude that only 5% of our Universe is made out of the matter that everything we know of is composed of, and that 95% of the Universe is made of some dark substance we’ve never successfully detected. It sounds crazy, and therefore, you wonder, isn’t there some simpler explanation that’s likely correct? It’s not just you: the professionals have wondered this, too. And if there is, it means we have to throw out all of the successes we’ve achieved so far. So goodbye, General Relativity. Goodbye to the Big Bang, to astrophysics, to structure formation, to what we know about stars and galaxies, to gravitational lensing, to gravitational waves, and so much more. To get rid of dark matter and dark energy, we have to throw out the baby with the bathwater.
There are countless lines of evidence that support the dark matter and dark energy picture, and astronomers wish the public (and some of their contrarian colleagues) knew about this. Now, you can know for yourself.
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!
Ask Ethan: If Dark Matter Is Everywhere, Why Haven’t We Detected It In Our Solar System?
“All the evidence for dark matter and dark energy seem to be way out there in the cosmos. It seems very suspicious that we don’t see any evidence of it here in our own solar system. No one has ever reported any anomaly in the orbits of the planets. Yet these have all been measured very precisely. If the universe is 95% dark, the effects should be locally measurable.”
You know the deal with dark matter: it makes up 85% of the mass of our Universe, it has gravitational effects but no collisions with normal matter or itself, and it explains a whole slew of cosmological observations. But why, then, if it’s everywhere, including in an enormous, diffuse halo around our Milky Way, doesn’t it affect the motion of our Solar System in an observable way? Surely, when you say that matter is distributed all throughout our galaxy, that will include the Sun’s neighborhood, right? The truth is, it actually does! Dark matter must exist throughout the Solar System, but that doesn’t mean its effects are observable. Contrariwise, you have to do the calculation to know what its density is, and to quantify the effects it would have on the planets. We can actually do this ourselves, and the results we find tell us, under a variety of conditions, exactly what we’d expect. Dark matter should be in our Solar System, and our best observations aren’t yet able to test whether it exists or not!
Combined, everything within the orbit of Neptune only adds the mass of a large asteroid; that’s not nearly enough. Come get the full story on this week’s Ask Ethan!
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!
Ask Ethan: Could Dark Matter Not Be A Particle At All?
“If dark energy can be interpreted as an energy inherent to the fabric of space itself, could it also be possible that what we perceive as “dark matter” is also an inherent function of space itself – either tightly or loosely coupled to dark energy? That is, instead of dark matter being particulate, could it permeate all of space with (homogeneous or heterogeneous) gravitational effects that would explain our observations – more of a “dark mass”?”
When it comes to all the matter and radiation in the Universe that we know of, at a fundamental level, every bit of it is made out of particles. From photons to neutrinos to leptons and quarks, there’s a quantum of energy for every type of energy we know of. Except, that is, for dark energy, which appears to be inherent to space itself, and doesn’t have a particle counterpart. There’s no evidence for clumping, inhomogeneities, or changes in dark energy over time. Well, what about dark matter, then? Is it possible that the most elusive form of mass in our Universe isn’t a particle at all, but rather can be interpreted as some sort of function inherent to space itself? While it does need to clump, and drives the formation of galaxies and the other structure in the Universe, it doesn’t necessarily need to be particle-based in nature.
It could, in fact, behave as a perfect cosmological fluid! What are the alternatives, constraints, and how do we know? Find out on this week’s Ask Ethan!
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.
New Dark Matter Physics Could Solve The Expanding Universe Controversy
“If either photons, neutrinos, or some new type of dark radiation (that interacts with dark matter but not any of the normal particles) has a non-zero cross-section with dark matter, it could bias measurements of the Hubble rate to an artificially low value, but only for one type of measurement: the kind that you get from measuring these leftover relics. If interactions between dark matter and radiation are real, they might not only explain this cosmic controversy, but could be our first hint of how dark matter might directly interact with other particles. If we’re lucky, it could even give us a clue to how to finally see dark matter directly.”
One of the biggest controversies in physics today is over the expanding Universe. Despite attempts to measure the Hubble rate for nearly 100 years, we still don’t know exactly how fast the Universe expands. Two independent classes of methods, from the cosmic distance ladder and the Big Bang’s leftover relic, give two very precise and incompatible results: 73 km/s/Mpc and 67 km/s/Mpc, respectively. There’s always the possibility that one class of methods gives a biased answer, and we simply haven’t uncovered the bias. But it’s also possible that new physics is responsible, that both teams are right, and that the discrepancy is a hint of the next great leap forward in our understanding of the fundamental properties of the Universe itself.
One exciting possibility is that dark matter has a new interaction with radiation: either photons, neutrinos, or a new type of ‘dark radiation.’ Come learn more about it today!
Ask Ethan: Which Fundamental Science Question Is The Most Important?
If you could have a complete answer to one of these 5 questions what would it be?
1.) Did cosmic inflation happen or was there another process?
2.) Is earth the only place in the cosmos with life?
3.) How [can we] merge general relativity and quantum mechanics?
4.) What is dark energy and dark matter?
5.) How did life begin on Earth?
There are a very large number of unsolved mysteries in the Universe, many of which would revolutionize our understanding of what it all is… and what it all means. If you could know the answer to only one of these questions, but know it immediately and fully, which one would you pick? Would you want to know more about the origin of the Universe, pushing things back before the Big Bang? Would you want to know about life elsewhere in the Universe, far beyond Earth? Would you choose quantum gravity, or how to merge our two great, incompatible theories of how everything works? Would you want to know what dark matter and dark energy truly are? Or would you go for the origin of life on Earth?
There’s no right answer, which is to say there are five great answers, but not every answer is equal. Come take a deep consideration on this week’s Ask Ethan!