Everything that we have ever observed in the Universe, from matter to radiation, can be decomposed into the smallest components. Everything in this world consists of atoms, which are composed of nucleons and electrons, and nucleons are divided into quarks and gluons. Light also consists of particles: photons. Even gravitational waves, in theory, are made up of gravitons: particles that we someday, with luck, find and fix. But what about dark matter? Indirect evidence of its existence cannot be denied. But should it also be composed of particles?
We are used to thinking that dark matter is made up of particles, and we hopelessly try to detect them. But what if we are looking in the wrong place?
If dark energy can be interpreted as energy inherent in the very fabric of space, could it be that “dark matter” is also an internal function of space itself - closely or remotely related to dark energy? And that instead of dark matter, the gravitational effects that might explain our observations will be more due to "dark mass"?
Well, especially for you, physicist Ethan Siegel has laid out our theoretical approaches and possible scenarios.
One of the most interesting features of the universe is the one-to-one relationship between what is in the universe and how the rate of expansion changes over time. Through many careful measurements of many disparate sources - stars, galaxies, supernovae, the cosmic microwave background, and the large-scale structures of the Universe - we were able to measure both, determining what the Universe is made of. Basically, there are many different ideas about what our Universe can consist of, and they all have different effects on cosmic expansion.
Thanks to the data obtained, we now know that the universe is made of the following:
- 68% dark energy, which remains at a constant energy density even with the expansion of space;
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- 27% of dark matter, which exhibits gravitational force, is blurred as the volume increases and does not allow itself to be measured using any other known force;
- 4.9% of ordinary matter, which manifests all its forces, is blurred as the volume increases, knots into lumps and consists of particles;
- 0.1% neutrinos, which exhibit gravitational and electroweak interactions, are made up of particles and knock together only when they slow down enough to behave like matter, not radiation;
- 0.01% of photons, which exhibit gravitational and electromagnetic influences, behave as radiation and are blurred both with increasing volume and with stretching of wavelengths.
Over time, these various components become relatively more or less important, and this percentage represents what the universe is made of today.
Dark energy, as follows from our best measurements, has the same properties at any point in space, in all directions of space and in all episodes of our cosmic history. In other words, dark energy is both homogeneous and isotropic: it is the same everywhere and always. As far as we can tell, dark energy doesn't need particles; it can easily be a property inherent in the fabric of space.
But dark matter is fundamentally different.
For the structure that we see in the Universe to form, especially on a large cosmic scale, dark matter must not only exist, but also come together. It cannot have the same density throughout space; rather, it should be concentrated in regions of higher density and should be less dense, or absent altogether, in regions of lower density. We can actually tell how much of the total matter is in different regions of space, guided by observations. The three most important are:
Power spectrum of matter
Map the matter in the universe, see at what scales it corresponds to galaxies - that is, how likely you are to find another galaxy at a certain distance from the galaxy you start with - and study the result. If the universe consisted of a homogeneous substance, the structure would be smeared. If there were dark matter in the universe that didn't collect early enough, the structure on a small scale would be destroyed. The power spectrum of energy tells us that approximately 85% of the matter in the Universe is represented by dark matter, which is seriously different from protons, neutrons and electrons, and this dark matter was born cold, or its kinetic energy is comparable to rest mass.
Gravitational lensing
Take a look at the massive object. Let's say a quasar, galaxy or galaxy clusters. See how background light is distorted by the presence of an object. Since we understand the laws of gravity governed by Einstein's theory of general relativity, how light is bent allows us to determine how much mass is present in each object. Through other methods, we can determine the amount of mass that is present in ordinary matter: stars, gas, dust, black holes, plasma, etc. And again we find that 85% of the matter is represented by dark matter. Moreover, it is distributed more diffusely, cloudy, than ordinary matter. This is confirmed by weak and strong lensing.
Cosmic microwave background
If you look at the remaining glow of the Big Bang radiation, you will find that it is roughly uniform: 2.725 K in all directions. But if you take a closer look, you can find that tiny defects are observed on scales from tens to hundreds of microkelvin. They tell us a few important things, including the energy densities of ordinary matter, dark matter and dark energy, but most importantly, they tell us how homogeneous the universe was when it was only 0.003% of its present age. The answer is that the densest region was only 0.01% denser than the least dense region. In other words, dark matter started out in a homogeneous state and clumped together as time went on.
Putting it all together, we come to the conclusion that dark matter should behave like a liquid that fills the universe. This fluid has negligible pressure and viscosity, reacts to radiation pressure, does not collide with photons or ordinary matter, was born cold and nonrelativistic, and bunches up under the influence of its own gravity over time. It determines the formation of structures in the Universe on the largest scales. It is highly heterogeneous, and the magnitude of its heterogeneity increases over time.
Here's what we can say about it on a large scale, as they relate to observations. On small scales, we can only assume, not completely sure, that dark matter is composed of particles with properties that make it behave this way on a large scale. The reason we assume this is because the universe, as far as we know, is made up of particles at its core, that's all. If you are a substance, if you have a mass, a quantum analogue, then you must inevitably consist of particles at a certain level. But until we have found this particle, we have no right to exclude other possibilities: for example, that this is a kind of liquid field that does not consist of particles, but affects space-time in the way that particles should.
This is why it is so important to try to directly detect dark matter. It is impossible to confirm or deny the fundamental component of dark matter in theory, only in practice, backed up by observations. Apparently, dark matter has nothing to do with dark energy.
Is it made of particles? Until we find them, we can only guess. The universe manifests itself as quantum in nature when it comes to any other form of matter, so it's reasonable to assume that dark matter would be the same.
Ilya Khel
