Introduction
Discovery
Confirmation
Dark Energy vs. Dark Matter
What is Dark Energy?
How Can We Find It?
Studying Supernovae
Sound Waves Method
Gravitational Lensing

Sound Waves

Ripples from the primordial fireball tell dark energy’s secrets

Sci-fi movies notwithstanding, you can’t hear sounds in the vacuum of space. But you should be able to "see" the sounds of the early universe preserved in the way galaxies are distributed through space. That pattern should reveal important details about the epoch shortly after the Big Bang — including the composition of dark energy.

Until about 400,000 years after the Big Bang, the universe was a dense, hot cauldron of particles of matter and energy.

Disturbances in this dense soup set off sound waves, like the ripples when you throw a stone into a pond. These sound waves helped matter begin to clump together, forming the first structure in the universe. We see the result of this clumping in the cosmic microwave background radiation — the "afterglow" of the Big Bang.

But we should also see it in the way galaxies are distributed at different times in the history of the universe. The ripples in the early universe established a basic "yardstick" for the distribution of matter. As the universe expands, the yardstick expands with it. By measuring the size of the yardstick at different times in the history of the universe, astronomers can plot how the rate of expansion of the universe has changed.

The yardstick was preserved as the universe expanded and cooled further and the first galaxies took shape, but it’s not easy to detect. A look at even a tiny region of the sky reveals millions of galaxies distributed through space and time — some of them are quite close, which means we see them as they appeared just a few million years ago, while others are billions of light-years away, so we see them as they appeared when the universe was young.

So astronomers must measure millions of galaxies, isolate them by distance (which means at different ages of the universe), then analyze the patterns at different epochs. The HETDEX project, for example, will produce a 3-D map of at least one million galaxies that are roughly 9 billion to 11 billion light-years away, which means we see them as they looked when the universe was as little as 20 percent of its current age.

From that, astronomers will measure distances between individual galaxies to look for patterns in their arrangement. They should find that at different times in the history of the universe, galaxies "prefer" a particular distance from one another — like the crests of the waves on a pond. Astronomers then compare this distance to the yardstick imprinted in the early universe to determine how the expansion rate of the universe has changed over the eons.

Measuring the expansion rate at different times shows the "strength" of dark energy at different epochs. Different models of dark energy predict different changes in the expansion rate, so measuring the length of a basic cosmic yardstick — the imprint of ancient sound waves — may reveal the nature of dark energy.

Gravitational Lensing

Studying Supernova

Determining Scale Length

HETDEX and other searches will map galaxies at different times in the history of the universe to try to find the "imprint" of sound waves from the very early universe. Left: A field of galaxies for study. Center: Astronomers will measure the distances between millions of galaxy pairs, and use statistics to find a common "scale length." Right: This common length is the imprint of the early sound waves, which shows up as "ripples" in the distribution of galaxies. The scale length changes as the universe ages, so determining that length at different epochs will reveal how the expansion rate of the universe has changed over time. [Tim Jones/Damond Benningfield]