Space News & Blog Articles

Dark matter rules every galaxy. But what exactly is it? Astronomers believe it to be some kind of new, exotic particle. You may have heard some terms tossed around, like WIMPs or axions. Let’s explore what those terms actually mean.

First off, there’s a few things that we know about dark matter. Astronomers believe that dark matter is some kind of particle, previously unknown to physics. Whatever it is, it makes up about 80% of the mass of the universe. It barely interacts with light, if at all. It barely interacts with normal matter, if at all. It barely interacts with itself, if at all. We also know that it’s “cold”, which means that the individual particles don’t have very high velocities.

It basically just sits there and gravitates. But that gravity is essential: it keeps galaxies glued together and provides the scaffolding for the entire large-scale structure of the universe.

One of the earliest candidates for the dark matter particle are the WIMPs, for weakly-interacting massive particles. It’s not so much a name as a catch-all category. In this case, “weakly-interacting” means “interacts via the weak nuclear force” (although that interaction is also literally weak). WIMPs would be a new kind of particle that only talk to normal matter via the weak force, which would explain why we only rarely see it. In this scenario, WIMPs flood the universe – and might even be traveling through you right now, though you would never know except for their gravity.

The axion, on the other hand, is another hypothetical particle (or really, category of possible particles) that was motivated by theoretical explorations of various symmetry laws in the universe. It just so turned out that this hypothetical particle, if it existed in sufficient number, would operate exactly like we know the dark matter should.

We find examples of fractals everywhere in nature. Tree branches, snowflakes, river deltas, cloud formations, and more. So it’s natural to ask the ultimate question: is the entire universe one giant fractal? The answer is…no, but sorta yes.

Benoit Mandelbrot, who pretty much everyone agrees introduced the modern concept of fractals into the world (and even coined the term), was the first to wonder if our universe might be in the form of a fractal. At the time, astronomers had just begun constructing extensive deep-space catalogs of galaxies, and were just beginning to piece together the large-scale structure of the universe.

Since fractals are everywhere, maybe everywhere is a fractal. Maybe when you zoom out and see a particular pattern of galaxies, you can zoom out even further and find the same pattern repeated. And so on and so on, all the way to infinity.

Alas, comprehensive galaxy surveys would reveal that our universe is not best described as a fractal. There’s a limit, known as the homogeneity scale, where one patch of the universe looks pretty much like any other patch of the same scale. That scale is about 100 megaparsecs. Zoom out further, and you just see a bunch of the same patches side-by-side, with no larger pattern.

What’s more, there’s no simple fractal description of the pattern of galaxies on the way up to the homogeneity scale. While Mandelbrot’s idea was really cool, it just didn’t hold up to observational scrutiny.

Having eyes in the sky is useful for a variety of activities.  Everything from farming to military operations has benefited from the boom in drone usage, as the small aircraft track the progress of crop disease, enemy movements, or how awesome a professor skier looks going down a mountain.  Now the benefits of aerial surveillance has spread to other worlds as Perseverance is starting to map out its path with help from Ingenuity.

It has always been difficult to guide rovers over the Martian surface from over 200 million miles away.  NASA, and now China, have done surprisingly well so far by not having their rovers being stuck for any significant amount of time during their normal mission operation time.  However, campaigns such as “Free Spirit” show the devastating effect terrain can have on a rover.  So it is all the more important for rover handlers to know what they are driving over and how it might affect the rover itself.

The last image from Spirit before it lost contact with Earth.
Credit – Marco Di Lorenzo, Kenneth Kremer, NASA / JPL / Cornell

Normally this is done by very careful pathfinding.  There is an entire team at NASA that is dedicated solely to finding the best path forward for Perseverance and Curiosity.  In the past, such as with Spirit and Opportunity, rover navigators had to rely on space based images such as those provided by HiRISE and other orbiting satellites.  While useful, the resolution was less than desired, as they were only able to capture features measured about a meter.  Even smaller obstructions could prove fatal to the rovers, so drivers had to also rely on the cameras on the rovers themselves to ensure they weren’t falling into any sand pits or similar hazards.

That resolution problem has now been solved with Ingenuity. After it’s successful first few flights, which were focused on proving the idea of a helicopter working on another planet, the miniature aircraft switched roles to try to prove its usefulness as a scout for its larger rover companion.  

Ingenuity’s view of the Séítah dune field on it’s ninth flight. Part of the helicopter’s landing gear can be seen on the left side of the screen.
Credit – NASA / JPL – Caltech

On its ninth flight, Ingenuity successfully transmitted back some images of the tracks Perseverance had already made, as well as some new areas the rover is approaching.  Some of those areas will actually be inaccessible to the rover, making Ingenuity likely the only close up viewpoint that we will ever get of them.  A dune field, nicknamed Séítah, for example, is too difficult to traverse with the rover, but captured beautifully as Ingenuity flew right over it.