INTRODUCTION TO THE DEEP COSMOS
The rest of the universe appears to be made of a mysterious, invisible substance called dark matter (25 percent) and a force that repelsgravity known as dark energy (70 percent). Scientists have not yet observed dark matter directly.
Dark matter is a type of matter thought to be responsible for much of the mass in the universe. The idea arose when astronomers found the mass of large astronomical objects got from their gravitational effects was much greater than the mass calculated from the “luminous matter” they contain: stars, gas, and dust.
Is dark matter and dark energy the same?
The visible universe—including Earth, the sun, other stars, and galaxies—is made of protons, neutrons, and electrons bundled together into atoms. Perhaps one of the most surprising discoveries of the 20th century was that this ordinary, or baryonic, matter makes up less than 5 percent of the mass of the universe.
The rest of the universe appears to be made of a mysterious, invisible substance called dark matter (25 percent) and a force that repels gravity known as dark energy (70 percent).
Scientists have not yet observed dark matter directly. It doesn’t interact with baryonic matter and it’s completely invisible to light and other forms of electromagnetic radiation, making dark matter impossible to detect with current instruments. But scientists are confident it exists because of the gravitational effects it appears to have on galaxies and galaxy clusters.
For instance, according to standard physics, stars at the edges of a spinning, spiral galaxy should travel much slower than those near the galactic center, where a galaxy’s visible matter is concentrated. But observations show that stars orbit at more or less the same speed regardless of where they are in the galactic disk. This puzzling result makes sense if one assumes that the boundary stars are feeling the gravitational effects of an unseen mass—dark matter—in a halo around the galaxy.
Many scientists have also pointed out that the known properties of dark energy are consistent with a cosmological constant, a mathematical Band-Aid that Albert Einstein added to his theory of general relativity to make his equations fit with the notion of a static universe. According to Einstein, the constant would be a repulsive force that counteracts gravity, keeping the universe from collapsing in on itself. Einstein later discarded the idea when astronomical observations revealed that the universe was expanding, calling the cosmological constant his “biggest blunder.”
Now that we see the expansion of the universe is accelerating, adding in dark energy as a cosmological constant could neatly explain how space-time is being stretched apart. But that explanation still leaves scientists clueless as to why the strange force exists in the first place.