Everything that we have ever observed in the universe, frommatter before radiation, can be decomposed into the smallest components. Everything in this world consists of atoms, which consist of nucleons and electrons, and nucleons are divided into quarks and gluons. Light also consists of particles: photons. Even gravitational waves, in theory, are composed of gravitons: particles that we once, with luck, will 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 accustomed to believing that dark matter is composed of particles, and hopelessly trying to detect them. But what if we are looking for the wrong things?
If dark energy can be interpreted asthe energy inherent in the fabric of space, can it be that the “dark matter” is also an internal function of space itself - closely or remotely related to dark energy? And what instead of dark matter, gravitational effects that could explain our observations will be more due to the "dark mass"?
Well, especially for you physicist Ethan Siegel put our theoretical approaches and possible scenarios on the shelves.
One of the most interesting features of the universelies in a one-to-one relationship between what is in the Universe and how the rate of expansion changes over time. Thanks to many careful measurements of many disparate sources - stars, galaxies, supernovae, the cosmic microwave background and large-scale structures of the Universe - we were able to measure both of them, determining what the Universe consists of. In principle, 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, we now know that the Universe is made of the following:
- 68% of the dark energy that remains at a constant energy density even with the expansion of space;
- 27% of dark matter, which exhibits a gravitational force, is washed away with increasing volume and does not allow to measure itself using any other known force;
- 4.9% of ordinary matter, which exerts all its strength, is washed away with increasing volume, stray into lumps and consists of particles;
- 0.1% neutrinos that exhibit gravitational andelectroweak interactions, are composed of particles and get together only when they slow down enough to behave like matter, not radiation;
- 0.01% of photons that exhibit gravitational and electromagnetic effects behave like radiation and are washed away both with increasing volume and with stretching wavelengths.
Over time, these various components become relatively more or less important, and this percentage represents what the universe is made up of today.
Dark energy, as follows from our bestmeasurements, has the same properties anywhere 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 everywhere and always the same. As far as we can judge, dark energy does not need particles; it can easily be a property inherent in the fabric of space.
But dark matter is fundamentally different.
To form the structure that we seein the Universe, especially on a large cosmic scale, dark matter must not only exist, but also come together. She cannot have the same density everywhere in space; rather, it should be concentrated in regions of high density and should have a lower density, or none at all, in regions of low density. We can actually say how much of all the matter is in different areas of space, guided by observations. Here are the three most important ones:
The power spectrum of matter. Map matter in the universe,look at what scale it corresponds to galaxies - that is, with what probability you will find another galaxy at a certain distance from the galaxy with which you start - 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 did not collect early enough, the structure would be destroyed on a small scale. The energy power spectrum 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 the rest mass.
Gravity lensing. Take a look at the massive object. Suppose a quasar, galaxy or cluster of galaxies. See how the background light is distorted by the presence of an object. Since we understand the laws of gravity, which are governed by Einstein's general theory of relativity, the way light is curved 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 glowradiation of the Big Bang, you will find that it is approximately uniform: 2,725 K in all directions. But if you take a closer look, you will find that tiny defects are observed on a scale of tens to hundreds of microkelvins. They tell us several important things, including the energy density 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 current age. The answer is that the densest region was only 0.01% denser than the least dense region. In other words, dark matter began with a homogeneous state and, as time went on, got lumpy.
Combining all this, we conclude thatdark matter should behave like a liquid filling the Universe. This liquid has a negligible pressure and viscosity, responds to radiation pressure, does not collide with photons or ordinary matter, was born cold and nonrelativistic, and is knocked together 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 with time.
Here's what we can say about her in largescale because they are related to observations. On small scales, we can only assume, not being completely sure that dark matter consists of particles with properties that make it behave in this way on a large scale. The reason we assume this is because the Universe, as far as we know, consists of particles at its core, and that’s all. If you are a substance, if you have mass, a quantum analog, then you must inevitably consist of particles at a certain level. But until we find 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 as particles should.
That’s why it’s so important to try.direct detection of dark matter. It is impossible to confirm or refute the fundamental component of dark matter in theory, only in practice, backed up by observations. Apparently, dark matter is in no way connected with dark energy.
Does it consist 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 is reasonable to assume that dark matter will be the same.