Astronomy Buff

What is Dark Matter and Dark Energy?

by Tony on February 10th, 2007

Here is my sixth post for Just Science Week.

It is one of the biggest successes of cosmology that astronomers are now confident about what makes up the entire universe. It is made of:

• 5% ordinary matter (atoms, protons, neutrons, electrons). Known as baryonic matter.
• 25% dark matter. Unseen particles that hang around galaxies and clusters of galaxies.
• 70% dark energy. An unknown type of energy that is spread evenly throughout the universe, causing the expansion of the universe to accelerate.

It is truly remarkable that cosmologists have been able to do this. Of course, these are the broad strokes, the devil is in the details. The true nature of dark matter is still unknown, although it has been detected around galactic clusters, but at least it has been seen.

Even less is known about dark energy.

In this post, I thought I’d give a brief summary of what is known so far about dark matter and dark energy. The two largest constituents of the universe we live in.

Technorati Tags: cosmology, dark matter, Just Science Week

Dark Matter

Dark matter makes up an estimated 25% if the universe. How do we know? We can see the effect it has on the motions of galaxies and clusters of galaxies. Long ago Vera Rubin noticed that galaxies didn’t rotate as they should. When all of the mass that one could see was added up, the rotation of the galaxy should follow a specific curve and the galaxies Rubin measured did not follow that curve. They rotated as if there was more mass in the galaxies than could be seen. The conclusion was that there must be some kind of matter affecting the rotation rates of these galaxies.

The characteristics of dark matter particles:

• They are electrically neutral, meaning they have no charge. We know this because dark matter doesn’t interact with light as charged particles would. When charged particles interact with light, they radiate, making them non-dark.
• They are slow moving. Dark matter is concentrated in and around galaxies and clusters of galaxies, they do not move fast enough to escape from the gravitational pull. Because of this slow speed, cosmologists call dark matter particles cold.

Many have suggested that neutrinos are dark matter particles. They are abundant and electrically neutral, but they are not slow, they move at the speed of light. It remains to be discovered just what exotic particle could make up dark matter.

Dark Energy

Even less is known about dark energy. So far, it is known to be:

• Spread throughout the universe evenly.
• It maintains a constant density as the universe expands.

Which means:

• It causes the universe to accelerate.
• It affects the curvature of spacetime.

The biggest evidence for the existence of dark energy is the fact that the expansion of the universe is accelerating. The energy required to accelerate the universe can be inferred from measurements of the redshifts of distant galaxies. These redshifts tell us how far away a galaxy is and how fast it is flying away from us.

Further, the rate of expansion of the universe depends on how stars, galaxies, and anything else with mass is distributed across the cosmos. This mass can be converted into energy density using E=mc^2 and is an important measure for determining HOW the universe gets bigger.

If the energy density is below some number, the universe collapses; if greater, then it flies apart at the same rate; if it’s at some critical density, the universe stops expanding altogether.

In order to accelerate, there needs to be an additional push acting on spacetime. This force comes in the form of a constant energy density from somewhere, and this as yet unknown source is called dark energy.

In addition to accelerating the expansion, dark energy affects the shape of the universe too. Its effects have been deduced from mapping the Cosmic Microwave Background. Recent measurements of the temperature fluctuations in the CMB have allowed cosmologists to deduce that the curvature of space is essentially flat, like a big rubber sheet getting stretched all the time.

Since the amount of light and dark matter in the universe isn’t enough to make the curvature of the universe flat, the rest must be made up for by dark energy. That’s how the 70% number listed above was arrived at.

I realize this is a simplistic outline, but this is a blog after all, not a text book. I would urge you to go here for more detailed information:

Sean Carroll’s Cosmological Primer
Ned Wright’s Cosmological Tutorial

Share This

Related Stories

Dark Energy is a Misnomer
Six Characteristics of Dark Energy
Edge of the Universe May Be Creating Dark Energy
Life Changes
How To Weight the Entire Universe All At Once

POSTED IN: cosmology