Proof that Dark Matter Exist!

Introduction:

About 85 per cent of the universe’s mass is believed to be made up of dark matter, a hypothetical type of stuff. Dark matter is so-called because it doesn’t interact with the electromagnetic field, meaning it doesn’t emit electromagnetic radiation like light or absorb electromagnetic radiation like it does. As a result, it is hard to detect. Numerous astrophysical observations support the existence of dark matter, including gravitational effects that cannot be described by the gravity theories currently in use without the presence of more matter than can be observed. Because of this, most scientists believe that dark matter is prevalent in the universe and has significantly influenced its structure and evolution.

Calculations demonstrating that many galaxies would behave entirely differently if they did not include a significant amount of unseen substance are the main source of evidence for dark matter. Galaxies would not have formed in many cases, and some would not have moved in the manner that they do now. Along with astronomical observations of the structure of the observable universe, the formation and evolution of galaxies, the location of mass during galactic collisions, and the motion of galaxies within galaxy clusters, gravitational lensing, and cosmic microwave background observations are other lines of evidence. 

According to the conventional Lambda-CDM model of cosmology, the universe’s overall mass-energy composition is made up of 68 per cent dark energy, 27 per cent dark matter, and 5 per cent of ordinary matter and energy. Dark energy and dark matter together make up 95 per cent of the total mass-energy content, meaning that dark matter makes up 85 per cent of the total mass.       

Dark matter must hardly interact with regular baryonic matter and radiation other than through gravity because no one has yet actually witnessed it if it even exists. The majority of dark matter is assumed to be non-baryonic; it might consist of some subatomic particles that have not yet been identified. The main hypothesis for dark matter is an as-yet-unidentified new type of elementary particle, particularly weakly interacting massive particles (WIMPs), while axions have gained new interest as a result of investigations not finding WIMPs. 

Although numerous experiments are actively being conducted, none have yet been successful in directly detecting and studying dark matter particles. Depending on its velocity, dark matter is categorized as ”cold,” ”warm,” or ”hot” (more precisely, its free streaming length). Current theories prefer a cold dark matter scenario in which structures develop over time as particles accumulate

Even though the majority of scientists agree that dark matter exists, some physicists have proposed numerous modifications to the basic rules of general relativity in response to some facts that are difficult to explain by ordinary dark matter. These include tensor-vector-scalar gravity, modified Newtonian dynamics, or entropic gravity. Without introducing additional non-baryonic matter, these models aim to explain all data.

History:

There is a long history behind the dark matter theory. Lord Kelvin calculated the number of dark bodies in the Milky Way from the observed velocity dispersion of the stars around the galaxy’s core in a lecture given in 1884. He calculated the mass of the galaxy using these data, which he found to be distinct from the mass of the visible stars. As a result, Lord Kelvin came to the conclusion that ”many of our stars, maybe a great proportion of them, maybe black bodies.” Henri Poincare discussed Kelvin’s ideas in 1906 in ”The Milky Way and Theory of Gases,” using the French name matiere obscure (translated as ”dark matter”).

Dutch astronomer Jacobus Kapteyn was the first to postulate the presence of dark matter using star velocities in 1922. According to a 1930 article, Swedish physicist Knut Lundmark was the first to realize that the universe must have much more mass than what we can see. Jan Oort, a Dutchman and pioneer of radio astronomy, also proposed the idea of dark matter in 1932. Oort discovered that the mass in the galactic plane must be higher than what was observed while examining stellar motions in the immediate galactic neighbourhood, however, this measurement was later proved to be incorrect.

Fritz Zwicky, a Swiss astronomer who worked at the California Institute of Technology and investigated galaxy clusters, came to a similar conclusion in 1933. Zwicky discovered evidence of an invisible mass he named ”dark matter” by applying the virial theorem to the Coma Cluster. Zwicky calculated its mass based on the movements of galaxies close to its edge and contrasted it with a calculation based on the galaxy’s brightness and population. He calculated that the cluster had around 400 times more mass than could be seen optically. Since the gravity of the visible galaxies was much too weak for such rapid orbits, the material had to be concealed. 

These discoveries led Zwicky to conclude that the mass and gravitational attraction needed to hold the cluster together were given by unseen stuff. Due mostly to an outdated Hubble constant value, Zwicky’s estimations were incorrect by more than an order of magnitude; the same calculation performed now reveals a smaller fraction using higher values for luminous mass. Zwicky did, however, accurately deduce from his calculations that the majority of the stuff was dark.

Measurements of galaxy rotation curves provided more proof of irregularities in the mass-to-light ratio. The mass-to-luminosity ratio appeared to rise radially, according to Horace W. Babcock’s 1939 report on the rotation curve for the Andromeda nebula (now known as the Andromeda Galaxy). Instead of the missing stuff he had discovered, he attributed it to either light absorption within the galaxy or altered dynamics in the outer sections of the spiral. Jan Oort found and published about the vast non-visible halo of NGC 3115 in 1940 as a result of Babcock’s 1939 observation of unexpectedly rapid rotation in the Andromeda galaxy’s periphery and a mass-to-light ratio of 50.

Additional persuasive evidence was provided by the 1960s and 1970s study of Vera Rubin, Kent Ford, and Ken Freeman, which also used galaxy rotation curves. A novel spectrograph was used by Rubin and Ford to more precisely measure the velocity curve of edge-on spiral galaxies. In 1978, this outcome was verified. The findings of Rubin and Ford were published in a significant paper in 1980. As a result, by about 1980, the seeming necessity for dark matter was widely acknowledged as a significant unsolved problem in astronomy. They demonstrated that most galaxies must have roughly six times as much dark as visible mass.

Radio astronomers were using new radio telescopes to study the 21 cm line of atomic hydrogen in nearby galaxies at the same time that Rubin and Ford were investigating optical rotation curves. The sampling of rotation curves, and consequently of the total mass distribution, is extended to a new dynamical domain by the radial distribution of interstellar atomic hydrogen (H-I), which frequently covers much wider galactic radii than those reachable by optical research. The H-I rotation curve did not follow the anticipated Keplerian fall, as was evident from the early mapping of Andromeda with the 300-foot telescope at Green Bank and the 250-foot dish at Jodrell Bank. 

Morton Roberts and Robert Whitehurst were able to go much beyond the optical readings and track the rotating velocity of Andromeda to 30 pcs as more sensitive receivers became available. Figure 16 of that paper combines optical data (the cluster of points at radii of less than 15 pcs with a single point further out) and H-I data between 20 and 30 pcs, demonstrating the flatness of the outer galaxy rotation curve. The solid curve peaking at the centre is the optical surface density, while the other curve displays the cumulative mass, still rising linearly at the outermost measurement. Interferometric arrays were being developed concurrently for extragalactic H-I spectroscopy. 

The H-I rotation curves of five spirals that were mapped with the Owens Valley interferometer were published by David Rogstad and Seth Shostak in 1972. All five rotation curves were extremely flat, indicating extremely high values of mass-to-light ratio in the outer regions of their extended H-I discs.

In the 1980s, several observations—such as gravitational lensing of background objects by galaxy clusters, the temperature distribution of hot gas in galaxies and clusters, and the pattern of anisotropies in the cosmic microwave background—supported the existence of dark matter. The majority of cosmologists agree that dark matter is made up of an unidentified form of a subatomic particle. One of the main initiatives in particle physics is the search for this particle using several techniques.

Galaxy Rotation Curves:

Spiral galaxies’ arms revolve around the galactic core. From the centre to the periphery of a spiral galaxy, the luminous mass density drops. If the galaxy were made up entirely of luminous mass, we could represent it as having a central point mass with test masses rotating around it, much like the solar system. According to Kepler’s Second Law, it is anticipated that, like in the Solar System, the rotational velocities will diminish with increasing distance from the centre. This isn’t noticed. As you get further away from the galaxy’s centre, the rotation curve stays flat.

The obvious conclusion to draw if Kepler’s laws are accurate is that the mass distribution in spiral galaxies is different from that of the solar system. Particularly, the galaxy’s periphery contains a significant amount of dark matter or non-luminous stuff.

Velocity Dispersion:

The virial theorem must be followed by stars in confined systems. The theorem can be used to determine the mass distribution in a confined system, such as elliptical galaxies or globular clusters, along with the velocity distribution that has been observed. Even when complex distributions of star orbits are taken into account, the velocity dispersion estimates of elliptical galaxies generally do not

match the anticipated velocity dispersion from the observed mass distribution.

Similar to galaxy rotation curves, postulating the existence of non-luminous matter is the apparent method to explain the disparity.

Galaxy Clusters:

Since the masses of galaxy clusters may be determined in three different ways, they are extremely significant for dark matter investigations.

1. Based on the clusters’ galaxies’ scattered radial velocities.
2. From the heated gas in the clusters’ X-rays. Assuming pressure and gravity balance dictate the cluster’s mass profile, it is possible to estimate the gas temperature and density from the X-ray energy spectrum and flux. This results in pressure.
3. Without relying on studies of dynamics, gravitational lensing can measure cluster masses (often of more distant galaxies) (e.g., velocity). 

These three techniques generally agree that dark matter surpasses visible stuff by a factor of about 5 to 1.

Copyright: Wikipedia Commons

Gravitational Lensing:

As a result of general relativity, large objects like a galaxy cluster that are situated between a distant source like a quasar and the viewer should function as a lens, bending the light coming from this source. The amount of lensing detected increases with object mass.

When light from background galaxies passes through such a gravitational lens, strong lensing, or arc distortion, is detected. Abell 1689 is just one among the far-off groupings where it has been seen. The mass of the intervening cluster can be determined by measuring the distortion geometry. The mass-to-light ratios obtained in the several instances when this has been done match the dynamical dark matter measurements of clusters. An image may be duplicated as a result of lensing. Scientists have calculated and mapped the distribution of dark matter around the MACS J0416.1-2403 galaxy cluster by examining the dispersion of several picture copies.

Weak gravitational lensing analyses statistical data from extensive galaxy surveys to study minute distortions of galaxies. The mean distribution of dark matter can be identified by analyzing the apparent shear deformation of the nearby background galaxies. According to dark matter densities predicted by

other large-scale structure measurements, the mass-to-light ratios are in agreement. Spacetime is bent by mass, in this case, the mass of the dark matter, not by dark matter itself, which does not bend light. The lensing effect is the result of light following the curvature of spacetime.

The Dark Energy Survey Collaboration unveiled a new, comprehensive map of dark matter in May 2021. By utilising a machine learning technique, the map also showed previously unknown filamentary structures connecting galaxies.

Cosmic Microwave Background:

Even though dark matter and regular matter are both forms of matter, their behaviours are different. Particularly in the early universe, ordinary matter was ionized and experienced significant Thomson scattering interactions with radiation. Despite not directly interacting with radiation, dark matter has an impact on the cosmic microwave background (CMB) through its gravitational potential (mostly on large scales) and impacts the density and velocity of conventional matter. Dark matter perturbations differ from regular perturbations in how they develop over time and how they affect the CMB.

Although the temperature anisotropies in the cosmic microwave background are incredibly small—a few parts in 100,000—they are nonetheless very near to becoming a perfect blackbody. An anisotropic sky map can be broken down into an angular power spectrum, which is seen to have several acoustic peaks with varying heights but nearly equal spacing. Modern computer algorithms like CMBFAST and CAMB can anticipate the series of peaks for any posited set of cosmic parameters, constraining cosmological parameters by comparing theory to data. While the third peak mostly refers to the density of dark matter, measuring both the density of matter and the density of atoms, the first peak primarily displays the density of baryonic matter.

COBE made the initial discovery of the CMB anisotropy in 1992, but its resolution was too low to identify the auditory peaks. Following the balloon-borne BOOMERanG experiment’s discovery of the first acoustic peak in 2000, the power spectrum was meticulously monitored by WMAP from 2003 to 2012 and even more meticulously by the Planck spacecraft from 2013 to 2015. The Lambda-CDM model is supported by the results.

The precise structure of the observed CMB angular power spectrum is well described by the Lambda-CDM model but is challenging to replicate with any competing model, such as modified Newtonian dynamics. This provides compelling evidence for the existence of dark matter (MOND). Structure formation.

Structure Formation:

After the Big Bang, when density perturbations caused stars, galaxies, and clusters to form, this time is referred to as the structure formation phase. The general relativity Friedmann solutions represent a homogeneous cosmos before structure forms. The homogenous universe was later condensed into stars, galaxies, and larger structures as minuscule anisotropies increased and expanded. Radiation, which was the primary constituent of the universe in its early stages, has an impact on ordinary matter. The resulting washout of its density disturbances prevents them from condensing into the structure. The universe would not have had enough time to allow density perturbations to form the galaxies and clusters that are seen today if there had only been ordinary stuff.

Due to its immunity to radiation, dark matter offers a remedy to this issue. Its density perturbations can therefore grow first. The ensuing gravitational potential accelerates the creation of structures by acting as an attractive potential well for subsequently collapsing ordinary matter.

Bullet Cluster:

The next most plausible answer, if dark matter does not exist, is that general relativity, the dominant theory of gravity is flawed and has to be revised. Because of the Bullet Cluster’s apparent centre of mass’s great distance from the baryonic centre of mass, modified gravity theories are put to the test. The Bullet Cluster is the outcome of a recent collision between two galaxy clusters. Modified gravity has a significantly harder time explaining this result than standard dark matter models, especially since the observational evidence is model-independent.

Type Ia Supernova distance measurements:

Type Ia supernovae serve as reference candles for extra galactic distance measurements, which can be used to determine how quickly the universe has previously expanded. According to data, the universe is expanding faster than ever, and dark energy is typically blamed for this. Since observations indicate the universe is almost flat, it is expected the total energy density of everything in the universe should sum to 1 (tot 1). The measured dark energy density is 0.690; the observed ordinary (baryonic) matter-energy density is b 0.0482 and the energy density of radiation is negligible. This leaves a missing dm 0.258 which nonetheless behaves like matter (see technical definition section above) – dark matter.

Sky Surveys and Baryon Acoustic Oscillations:

Large-scale variations in the density of the universe’s observable baryonic matter (normal matter) are known as baryon acoustic oscillations (BAO). These can be seen in the angular power spectrum of the cosmic microwave background and are predicted to form under the Lambda-CDM model as a result of acoustic oscillations in the photon-baryon fluid of the early cosmos. A recommended length scale for baryons is established by BAOs. The effect of the recombination of dark matter and baryons is much less pronounced in the galaxy distribution in the nearby universe, but it can still be seen as a slight (1 per cent) preference for pairs of galaxies to be separated by 147 Mpc as opposed to those separated by

130-160 Mpc. The Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey, two significant galaxy redshift surveys, made the discovery of this feature in 2005 after it had been anticipated theoretically in the 1990s. The Hubble constant and the typical universe matter density can be precisely estimated by combining CMB observations with BAO values from galaxy redshift surveys. The LambdaCDM model is supported by the results.

Redshift-Space Distortions:

The distribution of galaxies may be mapped out in three dimensions using large galaxy redshift surveys. Because distances are calculated using observed redshifts, which also include a contribution from the galaxy’s alleged unusual velocity in addition to the dominating Hubble expansion term, these maps include some minor distortions. Due to their gravity, superclusters are often expanding slower than the cosmic mean, whereas voids are expanding more quickly. In a redshift map, galaxies ahead of a supercluster have excess radial velocities in its direction and redshifts that are a little bit higher than their distance would suggest, while galaxies behind the supercluster have redshifts that are a little bit lower than would be expected given their distance. 

Superclusters appear to be compressed in the radial direction as a result of this phenomenon, and voids appear to be stretched. They maintain their original angular locations. Since the true shape of the influence is unknown, no one structure can be used to identify it. However, numerous structures can be averaged to measure the effect. Nick Kaiser made a quantitative prediction about it in 1987, and the 2dF Galaxy Redshift Survey made the first conclusive measurement of it in 2001. The outcomes match the predictions of the Lambda-CDM model.

Lyman-Alpha Forest:

The Lyman-alpha forest in astronomical spectroscopy is the total of the absorption lines resulting from neutral hydrogen’s Lyman-alpha transition in the spectra of far-off galaxies and quasars. Observations from the Lyman-alpha forest can also restrict cosmic models. These restrictions line up with those discovered using WMAP data.