Gravity is the glue that holds the universe together. It keeps us anchored to Earth, keeps the planets in orbits around the sun, keeps the sun in orbit around the galactic center and keeps our Milky Way galaxy ensconced in the local galactic cluster.
According to Newton’s law of gravity, its effects should get smaller as the distance from a large mass increases. The squared relationship means that doubling the distance decreases the force by a factor of four. Tripling the distance decreases it by a factor of nine. Four times decreases by sixteen, and so on.
Gravity is a very weak force. As such, the effects are miniscule unless one of the masses is large. It takes the mass of Earth — a staggering 600 billion quadrillion kilograms — to exert the 185 pounds of force needed to keep your columnist anchored to the planet.
Until the 1970s, astronomers thought Newton’s gravity adequately described the motion of stars in distant galaxies. Then, with the use of telescopes that are more powerful and finer spectrographic tools, astronomers discovered that stars in the outer regions of galaxies were moving too fast.
Like planets in the solar system, stars in orbit around galactic centers should orbit slower the farther out they are. Yet, the measurements showed that stars at the edge of galaxies had the same angular rotation speeds as those near the center. Could it be that Newton’s gravity was incorrect? Or was gravity different outside our own immediate gravitational environment?
If Newton’s gravity was incorrect, it would negate 350 years of physics and call into question many of astronomy’s advances. On the other hand, if the laws of physics were different elsewhere in the universe, it would violate a revered axiom of natural science that natural laws are the same everywhere and every when.
Another explanation lay waiting in the wings, one that would upend astrophysics while keeping Newton’s gravity and the universality axiom intact.
The proposed solution was that some invisible yet massive presence permeated the galaxies and exerted the missing but required gravitational force.
The concept of dark matter was born. When astronomers measure the effects of dark matter’s gravity on stars that we can see, they estimate that dark matter amounts to 23 percent of the universe compared with just 4.6 percent represented by ordinary matter.
Galactic clusters and superclusters distort space-time with their immense mass. The distorted space-time acts as a gravitational lens that bends and converges light rays emanating from a distant object behind a cluster. The greater the bend, the more massive the lens, allowing astronomers to confirm that galactic clusters indeed have masses that exceed those measured by luminous matter. This second type of data has provided additional confirmation of the existence of dark matter.
Although there are theories about what dark matter might be, there is no conclusive evidence to back any of them.
No one has yet to see an individual particle of dark matter, although several experiments are in operation, including on the International Space Station and thousands of feet underground to try to capture a trace of one.
Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Friday of the month. Email questions and comments to brill@hawaii.edu.
