How can we find out what dark matter is made of? The technique we can use depends on its composition. Consider the possibility that some dark matter is made up of normal particles: protons, neutrons, and electrons. Suppose these particles. If black holes didn’t have accretion disks, they would be invisible to us. White and brown dwarfs emit some radiation, but their luminosity is so weak that they cannot be seen at distances of more than a few thousand light-years.
However, we can look for such compact objects because they can act as gravitational lenses. For example, suppose the dark matter in the halo of the Milky Way is made up of black holes, brown dwarfs, and white dwarfs. These objects were whimsically called MACHOs (Massive Compact Halo Objects). When an invisible MACHO passes directly between a distant star and the Earth, it acts like a gravitational lens and focuses the distant star’s light. This is what the star looks likeglow within a period of a few hours to several days before returning to normal brightness. Since we cannot predict when a particular star might shine like this, we have to observe a large number of stars to catch one on the spot. There are not so much astronomers to continue to monitor so many stars, but today’s automated telescopes and computer systems can do it for us.
Research teams observing millions of stars in the nearby galaxy called the Large Magellanic Cloud have reported several examples of the type of brightness expected when MACHO is present in the halo of the Milky Way. However, there are not enough MACHO in the halo of the Milky Way to explain the mass of dark matter in the halo.
This result, together with a large number of other experiments, leads us to the conclusion that the types of matter known to us may only make up a small part of dark matter. Another possibility is that dark matter is made up of a new type of particle that researchers are now trying to detect in laboratories here on Earth.
The types of dark matter particles proposed by astronomers and physicists can generally be divided into two main categories: hot and cold dark matter. The terms hot and cold do not refer to the true temperatures, but to the average velocities of the particles, analogous to how we could imagine air particles that are currently moving in your room. On average, air particles move more slowly in a cold room than in a warm room.
If particles of dark matter in the early universe moved easily fast and far compared to the lumps & bumps of ordinary matter that eventually grew into larger galaxies and structures, we call those particles hot dark matter. In this case, the movements of the particles would detect smaller lumps and bumps, which means that fewer small galaxies would be made.
If, on the other hand, the particles of dark matter move slowly and cover only short distances compared to the lump sizes in the early universe, we call this cold dark matter. Their slow speeds and energy would mean that even the tiniest chunks of ordinary matter would survive to become tiny galaxies. By looking at when galaxies formed and how they evolved, we can use the observations to distinguish between the two types of dark matter. more consistent with models based on cold dark matter.
Solving the dark matter problem is one of the greatest challenges for astronomers. After all, we can hardly understand the evolution of galaxies and the long history of the universe without understanding what its most massive component is made up of. For example, we need to know what role dark matter played in the formation of the higher density “seeds” that led to the formation of galaxies. And since many galaxies have large halos of dark matter, how does this affect their interactions with one another and Shapes and types of galaxies that create their collisions?
Astronomers-Armed with various theories, astronomers are working hard to create models of the structure and evolution of galaxies that properly account for dark matter. While we don’t know what dark matter is, we have some clues as to how it affected formation of very-first galaxies. As we’ll see in The Big Bang, careful measurements of the microwave radiation left over after the Big Bang have allowed astronomers to put very tight limits on the actual sizes of those early seeds that led to the formation of the large galaxies we see. in today’s universe. Astronomers have also measured the relative numbers and distances between galaxies and clusters of different sizes in the universe today.So far, most of the evidence seems to weigh heavily in favor of cold dark matter, and most current models of galaxy formation and large-scale structures formation use cold dark matter as a major component.
As if the presence of dark matter, a mysterious substance that exerts gravity and surpasses all known stars and galaxies in the universe but does not emit or absorb light, was not enough, there is an even more enigmatic and equally important component of the universe that it has recently become discovered: We called it dark energy parallel to dark matter. We’ll talk more about this and examine its impact on the evolution of the universe in The Big Bang. For now, we can complete our inventory of the contents of the universe by realizing that the entire universe appears to contain some mysterious energy that separates space-time, bringing with it galaxies and larger structures made of galaxies. Observations show that dark energy becomes more important relative to gravity as the universe ages. As a result, the expansion of the universe is accelerating, and that acceleration appears to be mainly because the universe was about half its current age today.
What we see when we look into the universe, the light of trillions of stars in hundreds of billions of galaxies, shrouded in intricate veils of gas and dust, is therefore really just a pinch of icing on the cake: As we are in The Big Bang see, when we see outside of the galaxies and galaxy clusters in the universe as a whole, astronomers find that for every gram of normal luminous matter such as protons, neutrons, electrons and atoms in the universe, there are about 4 grams of non-luminous matter. normal matter, mainly intergalactic hydrogen and helium. There is about 27 grams of dark matter, and the corresponding energy, even more dark energy. Science is always a “progress report,” and we often find ourselves in areas to which we have more questions than answers.
Next, let’s summarize all of these clues in order to trace the life history of galaxies and the large-scale structure in the universe. What follows is the current consensus, but research in the area is advancing rapidly and some of these ideas are likely to change as new observations are made.