Matter is the most inevitable entity by which the whole universe seems to exist. Every existence is enclosed by matter and we can’t think of anything without that also. Within that context we generally come to hear the word ” DARK MATTER “. But that thing also comprised of some mystical concepts as we aren’t that much capable of revealing it due to many limitations.
But dark matter exists because its gravity tugs on the stars and galaxies around it, altering their movement. Dark matter also tugs on light as it passes, bending its path, a phenomenon called gravitational lensing. And now, by studying where that lensing appears in the sky, an international team of scientists have released a detailed, 3D map of dark matter.
The biggest advantage of the cosmic map, which was published Monday (Sept. 24) in the preprint journal arXiv, is that it will help scientists figure out precisely how and where dark energy — an unseen energy that suffuses the universe, accelerating its expansion — operates in space, researchers said in a statement
“Our map gives us a better picture of how much dark energy there is and tells us a little more about its properties and how it’s making the expansion of the universe accelerate,” Rachel Mandelbaum, an astronomer at Carnegie Mellon University in Pittsburgh who was involved in the survey, said in the statement.
WHAT ACTUALLY DARK MATTER IS ?
Dark matter is a hypothetical form of matter that is thought to account for approximately 85% of the matter in the universe, and about a quarter of its total energy density. The majority of dark matter is thought to be non-baryonic in nature, possibly being composed of some as-yet undiscovered subatomic particles.
Its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained unless more matter is present than can be seen. For this reason, most experts think dark matter to be ubiquitous in the universe and to have had a strong influence on its structure and evolution. The name dark matter refers to the fact that it does not appear to interact with observable electromagnetic radiation, such as light, and is thus invisible (or ‘dark’) to the entire electromagnetic spectrum, making it extremely difficult to detect using usual astronomical equipment.
HOW DARK MATTER EXISTS ?
Galaxy rotation curves
The arms of spiral galaxies rotate around the galactic center. The luminous mass density of a spiral galaxy decreases as one goes from the center to the outskirts. If luminous mass were all the matter, then we can model the galaxy as a point mass in the centre and test masses orbiting around it, similar to the Solar System. From Kepler’s Second Law, it is expected that the rotation velocities will decrease with distance from the center, similar to the Solar System. This is not observed. Instead, the galaxy rotation curve remains flat as distance from the center increases.
If Kepler’s laws are correct, then the obvious way to resolve this discrepancy is to conclude that the mass distribution in spiral galaxies is not similar to that of the Solar System. In particular, there is a lot of non-luminous matter (dark matter) in the outskirts of the galaxy.
Stars in bound systems must obey the virial theorem. The theorem, together with the measured velocity distribution, can be used to measure the mass distribution in a bound system, such as elliptical galaxies or globular clusters. With some exceptions, velocity dispersion estimates of elliptical galaxies do not match the predicted velocity dispersion from the observed mass distribution, even assuming complicated distributions of stellar orbits.
As with galaxy rotation curves, the obvious way to resolve the discrepancy is to postulate the existence of non-luminous matter.
Galaxy clusters are particularly important for dark matter studies since their masses can be estimated in three independent ways:
- From the scatter in radial velocities of the galaxies within clusters
- From X-rays emitted by hot gas in the clusters. From the X-ray energy spectrum and flux, the gas temperature and density can be estimated, hence giving the pressure; assuming pressure and gravity balance determines the cluster’s mass profile.
- Gravitational lensing (usually of more distant galaxies) can measure cluster masses without relying on observations of dynamics (e.g., velocity).
Generally, these three methods are in reasonable agreement that dark matter outweighs visible matter by approximately 5 to 1.
HOW 3D MAP OF DARK MATTER WORKS?
In the making of this dark matter map, the researchers carefully studied the shapes of up to 10 million galaxies, including those from very far away in space, from which light created billions of years ago, during the early universe, is only now reaching Earth. [The 11 Most Beautiful Mathematical Equations]
They measured how much those galaxies’ shapes seemed to be distorted from what astronomers expect, and then teased out how much of that distortion was due to dark matter lensing, rather than the effects from the atmosphere or the telescope and detector used. That difference allowed researchers to infer how much dark matter the light had to pass through before reaching Earth.
This map is drawn from just the first of five years’ worth of observations from the Japanese Subaru Telescope in Hawaii, as part of a project called the Hyper Suprime-Cam survey (HSC). The HSC will continue to peer through space for four more years to make its map more precise and complete.
One early result: The HSC found evidence for a bit less dark energy in the universe than another survey, conducted previously in Europe, called the Planck survey. That survey looked at the faint traces of the Big Bang left behind in electromagnetic radiation, known as the cosmic microwave background. The slight difference is small enough that it’s not statistically significant, meaning there could be no true difference at all, but the difference is tantalizing, they said.
Because dark matter remains to be conclusively identified, many other hypotheses have emerged aiming to explain the observational phenomena that dark matter was conceived to explain. The most common method is to modify general relativity. General relativity is well-tested on solar system scales, but its validity on galactic or cosmological scales has not been well proven. A suitable modification to general relativity can conceivably eliminate the need for dark matter.
SOURCE – livescience