A composite image of the Bullet Cluster, formed after the collision of two large clusters of galaxies. Most of the matter in the clusters (blue) is clearly separate from the normal matter (pink), giving evidence that nearly all of the matter in the
clusters is dark.
| Photo Credit: NASA/CXC/CfA/M. Markevitch
The general theory of relativity has been very successful at explaining gravity and an astonishing 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 have observed gravity operating at different scales, from microscopic to macroscopic. But as we zoom out to look at the universe as a whole, it seems as if space is permeated with a mysterious form of gravity-defying energy. This so-called dark energy — which physicists have come to believe made up 70% of energy that the Big Bang blew out 13.8 billion years ago — creates a sort of negative pressure that stretches the fabric of spacetime and allows celestial objects like stars and galaxies to drift apart. This is in contrast to the Newtonian idea of gravity: as an attractive force that causes objects to come closer together.
In places with lots of matter, gravity has more of an 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 about the need for dark matter.
Theories of gravity say the rotation rate is highest near the galaxies’ centre and lowest at the outer rim. Yet scientists like Dr. Rubin found many rotating galaxies in which the velocities of the stars didn’t decrease away from the galactic centre. One way to explain this is if the galaxy had more matter than was visible, exerting more gravitational force that pushed stars at the rim to move faster than they would otherwise. This additional matter is dark matter.
Both dark matter and dark energy are assumptions. They have a very strong hypothetical basis but we haven’t been able to find physical evidence of them. Scientists postulated the existence of these two entities so that they can 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 alternate paradigm of gravity — one in which some unknown properties of the force could cause the observed phenomena without invoking dark matter or dark energy.
However, these alternatives suffer from an important problem: they don’t explain away all the disparities, whereas the dark matter and dark energy hypotheses do.
What have we found?
If we need to fully understand the general theory of relativity, we need to figure out what dark matter and dark energy are. Many researchers are working on this around the world, including in India.
Their studies make heavy use of simulations to understand how the universe would look if there were certain kinds of dark matter or dark energy. For example, a study published on April 16 in the Monthly Notices of the Royal Astronomical Society by researchers in the U.S. reported being able to explain the observed behaviour of real galaxies and the motions of their stars and gases in simulations that assumed the galaxies contain dark matter.
We also have telescopes constantly making new observations of space. They have been becoming more sophisticated, allowing scientists to collect more fine-tuned data they can use to improve their theories. For example, an April 11 paper 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 named JWST-ER1g.
When looking for something that is really hard to find, it’s also useful if researchers share information about where they couldn’t find dark matter, allowing others to focus on places where it can be. On March 28, for example, scientists published the first results of the Broadband Search for Dark Photon Dark Matter (BREAD) experiment. The preliminary data ruled out dark-matter particles in a certain mass range.
Turning on lambda
Similarly, the Dark Energy Spectroscopic Instrument (DESI) in Arizona, in the U.S., is attempting to make the largest 3D map of the universe. This mountain-top telescope is fit with 5,000 small robots that help it look 11 billion years into the past with greater precision than before. So far, data from DESI has agreed 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 associated with dark energy. It appears in equations of the general theory of relativity. Some studies have found that dark energy might be changing with time, which is at odds with 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 of ‘modified Newtonian dynamics’. It doesn’t require the existence of dark energy; instead, it proposes that when gravity is weak, such as at the outer rims of large galaxies, it also behaves differently. While it enjoys some popularity, one research group reported on April 5 that data from the Cassini mission (1997-2017) showed no sign that Saturn’s orbit had a slight deviation that MOND says there should be.
By mapping the position of thousands of galaxies over many years, we can keep measuring how much the universe’s expansion due to dark energy is accelerating. But for now, we have no choice but to draw all our inferences about dark matter and dark energy from indirect evidence alone.
Qudsia Gani is an assistant professor in the Department of Physics, Government Degree College Pattan, Baramulla.
