The study of the effects of dark matter on the formation of large-scale structure in the universe becomes imperative to discuss how the universe that we know today was created in a massive explosion popularly known as the big bang theory. According to this theory, all the matter that is present and that ever was present was created some 13.8 BYA (billion years ago).
The big bang tells us the universe began in a hot and dense state, and over time structures such as galaxies and galaxy clusters formed. However, the ordinary matter couldn’t alone explain the formation of such large-scale structures like the clusters of galaxies so the concept of dark matter was introduced, and it ought to make up the majority of the large-scale structure of the present universe.
This article explores the effects of dark matter and dark energy in the formation of large-scale structures in the universe forms. So, delve with us on this journey and explore the dark secrets of the universe.
1. Basic Observations and Rise of the Concept of Cosmic Microwave Background
Here we examine the phenomenon from a cosmological perspective. In order to do so we need the creation of a suitable model for the universe as well as make a suitable assumption concerning physical processes which dominate cosmology. The two main observations we can use when addressing our own large-scale universe structure are Hubble’s expanding universe and cosmic microwave background radiation. A simple observation that has helped in determining the structure and magnitude of the universe has led us into determining if the spectral lines from planetary objects are directed toward the red side of the spectrum. The shifting of planetary spectral lines confirms the expansion of the universe.
It was the Hubble Space Telescope that provided evidence of this expansion in 1998. The second observation that explained the large-scale structure of the universe came from cosmic microwave background radiation. These radiations are highly isotropic, with high energy density, hence providing a clue for the universe to be isotropic.
1.1. Planck Epoch
The time from the big bang till now has been divided into different timelines or epochs, starting from what scientists called a singularity. Singularity also known as the Planck epoch was the earliest period of the universe and reportedly extended from 0 to 10-43 seconds. It is called the Planck epoch because it can be only measured at the Planck time scale.
1.2. Inflation Epoch
With the advent of the Planck epoch, the universe entered an inflation epoch, starting from the big bang reportedly it lasted for about 10-32 seconds. By this time the Plancks epoch had already led to the foundation of fundamental forces. The inflation epoch was the time like a roller coaster ride. It was the time when the universe had high energy density, high dark matter density, high temperature and pressure, and simultaneous cooling of the universe.
During this epoch, the expansion of the universe went about exponentially. The rapid expansion of the universe is thought to have smoothened out the initial irregularities in the distribution of matter and energy providing the seeds for the formation of large-scale structures such as galaxies and clusters of galaxies. The epoch is also suspected to be the primordial soup of fundamental particles like electrons, protons, neutrons, and quarks.
As the universe expanded further and as the temperature and density decreased the energy of each particle decreased. It was allegedly after 10-11 seconds from the big bang that the universe started cooling tremendously, which allowed fundamental particles like electrons and protons to combine to form neutral atoms like hydrogen and helium.
Several thousand years after the big bang, the nuclei reacted with electrons creating cosmic microwave background radiation (CMB) and is said to be a remnant of the big bang and provides information about the beginning of the universe even today (the oldest light in the universe).
1.3. The Discovery of Cosmic Microwave Background
As the universe expanded and cooled this radiation has been stretched to longer wavelengths becoming microwave radiation as detected by radio telescopes today.
The discovery of cosmic microwave background by Arno Penzias and Wilson was one of the most significant discoveries as it provided a crucial tool for cosmologists to understand the universe’s composition, evolution, and structure formation. Its uniformity and slight temperature fluctuations provide insights into the universe’s initial conditions while the map of the temperature fluctuations can help determine the distribution of matter in the universe.
Over a period of time, the denser materials become gravitationally attracted to give shape to the modern universe, which is closely related to the amount of distribution of matter, be it ordinary matter or dark matter in the universe. Dark matter constitutes around 85% of the universe.
2. Types of Dark Matter
2.1. Cold Dark Matter
It is composed of particles that move relatively slowly and don’t have much kinetic energy. They move very slowly relative to the speed of light which is why the term cold. In the cold dark matter theory, the structure grows hieratically with small objects collapsing into their own gravity and forming larger structures. The presence of cold dark matter explained several observations of the universe such as the large-scale structure and dynamics of galaxies and clusters of galaxies.
Several cold dark matter candidates have been proposed which include weakly interacting massive particles (WIMPS) Axions, and sterile neutrinos. These particles are theoretically predicted but their existence has not been confirmed through experimentation.
2.2. Hot Dark Matter
Hot dark matter is a theoretical type of dark matter particle that was proposed in the 1980s. Unlike cold dark matter particles, hot dark matter particles are characterized by having very high velocities and very low masses, which means that they would be moving so fast that they would not be able to clump together and form structures like galaxies and galaxy clusters.
The possible dark matter candidates for the hot dark matter are the neutrinos. Neutrinos are subatomic particles that have very small masses and interact weakly with other matter.
2.3. Warm Dark Matter
These dark matter particles should have a mass range between a few keV (kilo electron volt) to several tons of keV, which is larger than the mass of hot dark matter particles but smaller than the cold dark matter particles. The possible dark matter candidates for the warm dark matter are sterile neutrinos and graviton.
3. Evidence of Dark Matter
There is substantial evidence to suggest that dark matter constitutes a significant portion of matter in the universe, as discussed previously, comprising around 85% of the universe. In this part, we will be discussing the pieces of evidence of each dark matter.
3.1. Evidence for Hot Dark Matter
The presence of observed large-scale structure and cosmic microwave background radiation also bear evidence of the presence of hot dark matter particles. The other evidence comes from the specter of Quasars. The Lyman- alpha forest absorption lines are the fine lines seen in the specter of Quasars.
The absorption lines are thought to be caused by clouds of hydrogen gas between the Quasar and the Earth. The observed distribution of these clouds can be used to study the large-scale structure of the universe and suggests the presence of hot dark matter. Neutrinos as discussed are thought to be part of dark matter and therefore, provide evidence for hot dark matter. Similar evidence proves the existence of warm dark matter.
4. Dark Matter
Now that we have come to know about dark matter and its types, it is time to see how dark matter influenced the structure formation of the universe in brief. For the study of the effects of dark matter on the formation of large scale structure in the universe, a discussion on dark matter becomes inevitable.
The formation of the structure by dark matter proceeded in a hierarchical fashion, with smaller structures forming first and merging to form larger structures. This process is known as a bottom-up scenario, in which larger structures form first and fragment into smaller ones is known as a top-down scenario. Dark matter halos emerged at first.
5. Structure Formation
As the dark matter halos grew they provided the gravitational wells that allowed gas and ordinary matter to come together in their centers eventually forming galaxies. The role of dark matter in structure formation can be seen in the distribution of matter in the universe today. Observations of the large-scale structure of the universe such as galaxies and galaxy clusters reveal a web-like pattern of dark matter (cosmic web) that spans the entire observable universe.
This structure is thought to have been formed through the hierarchical growth of dark matter halos and the mergers of these halos over billions of years.
This structure is thought to have formed through the hierarchical growth of dark matter halos and the mergers of these halos over billions of years. Interestingly, if we look at a possibility of a Universe without Dark Matter, it complicates our understanding of the topic.
6. Recent Developments
Recent advancements in observational techniques and numerical simulations have allowed researchers to probe the effects of dark matter on the large-scale structure of the universe in unprecedented detail. The results of these studies have provided evidence for the existence of dark matter and have helped to refine our understanding of the processes that shape our universe
Despite these advances, many questions about the nature of dark matter, and its types remain in some mystery. Any attempt to look into the study of the effects of dark matter on the formation of large-scale structures in the universe needs to acknowledge that and prepare for the potential for new discoveries in the coming years.
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