Indirect evidence is emerging, yet the 'dark' universe remains unclear

Indirect evidence is emerging, yet the 'dark' universe remains unclear

A composite image of the Bullet Cluster, formed after the collision of two large groups of galaxies. Most of the matter in the clusters (blue) is clearly different from normal matter (pink), providing evidence that nearly all of the matter in the clusters is dark.

A composite image of the Bullet Cluster, formed after the collision of two massive groups of galaxies. Most of the matter in the clusters (blue) is clearly different from normal matter (pink), providing evidence that nearly all of the matter in the clusters is dark. | Photo credit: NASA/CXC/CfA/M. Markevich

The general theory of relativity has been very successful in explaining gravity and a surprising array of other related phenomena, such as gravitational waves, gravitational lensing, gravitational red shift, the existence of black holes, and time dilation. This theory refines Isaac Newton's laws and provides a unified description of gravity as a geometric property of spacetime.

We've seen gravity work at different scales, from the microscopic to the macroscopic. But when we zoom out to look at the universe as a whole, it seems space is permeated with a mysterious form of energy that defies gravity. This so-called dark energy – which physicists believe accounts for 70% of the energy destroyed by the Big Bang 13.8 billion years ago – creates a kind of negative pressure that stretches the fabric of spacetime and allows celestial objects like stars and galaxies to move apart. This is the opposite of the Newtonian idea of ​​gravity: as an attractive force that pulls objects closer to each other.

Where there is a lot of matter, the effect of gravity is more than that of dark energy. But when there is no matter in space, dark energy dominates.

A 'hidden' universe

Similarly, based on certain cosmic observations, researchers have proposed the presence of an invisible form of matter called dark matter. In fact, 44 years ago this month, astronomer Vera Rubin discovered dark matter. published his famous research paper With indirect evidence about the necessity of dark matter.

Theories of gravity say that galaxies have the highest rotation rates near their centers and the lowest at the outer edge. Yet scientists like Dr. Rubin found many rotating galaxies in which the velocity of stars did not decrease as they moved away from the center of the galaxy. One way to explain this is that if there was more matter in the galaxy than was visible, a greater gravitational force was exerted, which caused stars at the rim to rotate more rapidly than they would otherwise. This extra matter is dark matter.

Dark matter and dark energy are both hypotheses. They have a very strong hypothetical basis, but we have not been able to find physical evidence for them. Scientists have hypothesized the existence of these two entities so that they can explain their observations without breaking the general theory of relativity.

Not all scientists agree with this view. Some have attempted to create an alternative paradigm of gravity – in which some unknown property of the force can cause the observed phenomena without using dark matter or dark energy.

However, these alternatives have a significant problem: they do not explain all the inequalities, whereas the dark matter and dark energy hypotheses do.

What have we found?

If we have to fully understand the general theory of relativity, we have to find out what dark matter and dark energy are. Many researchers around the world, including India, are working on this.

Their studies make heavy use of simulations to understand what the universe would look like if there were certain kinds of dark matter or dark energy. For example, one study Published on April 16 In Monthly Notices of the Royal Astronomical Society The US researchers said they were able to explain the observed behaviour of real galaxies, and the motions of their stars and gas, in simulations that assumed dark matter was present in galaxies.

We also have telescopes that are constantly making new observations of space. They are becoming more sophisticated, allowing scientists to collect more refined data that they can use to improve their theories. For example, a 11th April paper In Astrophysical Journal Letters The James Webb Space Telescope has observed indirect evidence of normal regular and dark matter in the ring of an old galaxy called JWST-ER1g, the report said.

When looking for something that's really hard to find, it's also useful if researchers share information about where they are. could not do to find dark matter, which will help others focus on places where it might be. For example, on March 28, scientists First results published Initial data from the Broadband Search for Dark Photon Dark Matter (BREAD) experiment ruled out the possibility of dark-matter particles in a certain mass range.

Turning on Lambda

Similarly, the Dark Energy Spectroscopic Instrument (DESI) in Arizona, US, is attempting to create the largest 3D map of the universe. This mountaintop telescope is equipped with 5,000 tiny robots that help it look back 11 billion years with greater precision than ever before. So far, the data from DESI agrees at a fundamental level with the ΛCDM model of the universe, which is our best mathematical model to explain the Big Bang and today's universe. 'CDM' is short for 'cold dark matter'.

Λ (lambda) is the cosmological constant: it characterizes the energy density of space and is closely related to dark energy. It appears in the equations of the general theory of relativity. Some studies have found that dark energy can change over time, which contradicts the assumptions of the ΛCDM model.

In fact, Λ also makes a surprising appearance in modified theories of gravity that some researchers are working on. One of them is MOND, short for 'modified Newtonian dynamics'. It does not require the existence of dark energy; instead, it proposes that when gravity is weak, such as at the outer edges of large galaxies, it also behaves differently. While it enjoys some popularity, a research group reported on April 5 that data from the Cassini mission (1997-2017) showed no signs that Saturn's orbit had the slight deflection that MOND says it should have.

By mapping the positions of thousands of galaxies over many years, we can measure how fast the universe is expanding due to dark energy. But for now, we have no choice but to draw all our conclusions about dark matter and dark energy from only indirect evidence.

Qudsia Ghani is Assistant Professor in the Department of Physics at Government Degree College Pattan, Baramulla.

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