What are dark energy and dark matter?

Dr. Carla Frohlich is an astrophysicist and assistant professor of physics at N.C. State
Dr. Carla Frohlich is an astrophysicist and assistant professor of physics at N.C. State

Dr. Carla Frohlich is an astrophysicist and assistant professor of physics at N.C. State. Here she sheds light on a dark subject. Questions and answers have been edited.

Q: What is dark energy?

Almost 15 years ago, two teams of scientists studied distant supernova explosions to measure how the universe has expanded over its lifetime. They expected to find that the expansion of the universe would be constant or even be slowing – due to galaxies pulling toward each other with their gravity. But to their surprise, the scientists found quite the opposite: The universe is expanding faster and faster. What causes this?

The answer is dark energy, which seems to stretch space. But what exactly dark energy is remains one of the biggest mysteries in astrophysics.

Q: What is dark matter? What is the difference between the two?

We know of galaxies that zoom through space so fast that their own gravity should not be able to hold them together; they should break apart. But this does not happen. Something is holding them together. Dark matter is invisible matter that provides this extra gravity that holds together galaxies. It is called “dark” because astronomers cannot see it in any wavelength of light. Scientists have different theories what dark matter is: It could be normal objects such as cold gas, dark galaxies or MACHOS (massive compact halo objects). Or it could be strange particles such as axions, WIMPS (weakly interacting massive particles) or neutrinos. We also know about the dark matter in the infant universe from the afterglow of the Big Bang.

Q: I remember being fascinated by stories of black holes as a child – where do they fit into these phenomena?

Black holes occur when a star that is at least 10 times as massive as the sun dies. The star dies in a large supernova explosion and leaves behind a burned-out remnant collapsed on itself. The force of gravity around such an object is so high that nothing can escape, not even light. This is why it is called a black hole.

Q: If we can’t see dark energy and dark matter, how do we know they exist?

Almost all our information about the universe comes from light (electromagnetic radiation): Stars and galaxies shine; the moon and the planets are illuminated by the sun. If something does not shine, we have to look for its effects on other objects that shine and that we can see. For example, the force of gravity of a black hole changes the orbits of stars close to the black hole. By carefully measuring the orbits of these stars, we know that there is a black hole. In the same way, we know about dark energy and dark matter: We observe their effects on stars and galaxies.