Indirect evidence builds, yet the ‘dark’ universe remains dark

A composite image of the Bullet Cluster, formed after the collision of two large galaxy clusters. Most of the matter in the clusters (blue) is clearly separated from the normal matter (pink), providing evidence that nearly all the matter in the clusters is dark. | Photo Credit: NASA/CXC/CfA/M. Markevitch

General relativity has been very successful in explaining gravity and a number of other related phenomena, such as gravitational waves, gravitational lensing, gravitational redshift, the existence of black holes, and time dilation. This theory elaborates Isaac Newton’s laws and provides a unified description of gravity as a geometric property of spacetime.

We have observed gravity acting on different scales, from the microscopic to the macroscopic. But as we zoom in to see the universe as a whole, it seems as if space is permeated with a mysterious form of energy that defies gravity. This so-called dark energy – which physicists have come to believe makes up 70% of the energy that exploded in the Big Bang 13.8 billion years ago – creates a kind of negative pressure that stretches the fabric of space-time and allows celestial objects like stars and galaxies to leave. This contrasts with the Newtonian idea of ​​gravity: as an attractive force that causes objects to come closer together.

In matter-rich places, gravity has more effect than dark energy. But when space is empty of matter, dark energy dominates.

A ‘hidden’ universe

Similarly, based on some cosmological 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 published her famous paper with indirect evidence for the need for dark matter.

Theories of gravity state that the rotation rate is highest near the center of galaxies and lowest at the outer rim. However, scientists like Dr. Rubin found many rotating galaxies in which the velocities of the stars did not decrease away from the galactic center. One way to explain this is if the galaxy had more matter than was visible, exerting more gravitational force that caused the stars at the edge to move faster than they otherwise would. This extra matter is dark matter.

Both dark matter and dark energy are conjectures. They have a very strong hypothetical basis, but we have not been able to find physical evidence for them. Scientists postulated the existence of these two entities so that they could explain their observations without having to break the general theory of relativity.

Not all scientists agree with this approach. Some have attempted to create an alternative paradigm of gravity—one in which some unknown force property could cause the observed phenomena without invoking dark matter or dark energy.

However, these alternatives suffer from an important problem: they do not explain all the disparities, whereas the dark matter and dark energy hypotheses do.

What have we found?

If we are to fully understand general relativity, we need to understand what dark matter and dark energy are. Many researchers are working on this worldwide, including India.

Their studies use many simulations to understand what the universe would look like if it had certain types of dark matter or dark energy. For example, a study published on April 16 in Monthly Notices of the Royal Astronomical Society by researchers in the US reported that they were able to explain the observed behavior of real galaxies and the motions of their stars and gas in simulations that assumed the galaxies contained dark matter.

We also have telescopes that are constantly making new observations of space. They have become more sophisticated, allowing scientists to collect more fine-tuned data that they can use to refine their theories. For example, a newspaper of April 11 in The Astrophysical Journal Letters reported that the James Webb Space Telescope had observed indirect evidence of normal regular and dark matter in the ring of an old galaxy called JWST-ER1g.

When you’re looking for something that’s really hard to find, it’s also helpful if researchers share information about where they are could not find dark matter, allowing others to focus on the places where it might be. On March 28, for example, scientists published the first results of the Broadband Photon Dark Matter Search (BREAD) experiment. Preliminary data ruled out dark matter particles in a certain range of masses.

Turning on the lambda

Similarly, the Dark Energy Spectroscopic Instrument (DESI) in Arizona, USA, is trying to make the largest 3D map of the universe. This mountaintop telescope is fitted with 5,000 tiny robots that help it look 11 billion years into the past with greater precision than ever before. So far, data from DESI agree at a basic level with the ΛCDM model of the universe, our best mathematical model to explain the Big Bang and the universe today. ‘CDM’ is short for ‘cold dark matter’.

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

In fact, Λ also makes a surprising appearance in the modified theories of gravity that some researchers have been working on. One of them is MOND, an acronym for ‘modified Newtonian dynamics’. It does not require the existence of dark energy; instead, he 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 sign that Saturn’s orbit had the slight deviation that MOND says it should.

By mapping the position of thousands of galaxies over many years, we can continue to measure how much the expansion of the universe is accelerating due to dark energy. But for now, we have no choice but to draw all our conclusions about dark matter and dark energy from indirect evidence alone.

Qudsia Gani is an Assistant Professor in the Department of Physics, Pattan Government Diploma College, Baramulla.