A Cleaner Cosmological Ruler Could Shed Light on Dark Energy

A 12-inch ruler isn’t much help when you’re trying to trying to measure the universe. To handle the enormous distances between planets, stars, galaxies, and groups of galaxies, astronomers have developed a whole set of measuring tools and units of measurement. In an upcoming issue of the American Physical Society’s journal Physical Review Letters, a team of scientists is proposing a pristine new tool that could help us unravel the nature of dark energy.

This is one slice through a map of the large-scale structure of the universe from the Sloan Digital Sky Survey. Each dot in this picture indicates the position of a galaxy 6 billion years into the past. The image covers about 1/20th of the sky.
Image credit: Daniel Eisenstein and the SDSS-III collaboration.

Twenty years ago, astronomical observations revealed that the universe is expanding more quickly today than in the past. This was a big surprise. Most scientists thought the universe was expanding more slowly than ever—that’s the logical conclusion when you consider that all of the things in the universe are attracted to each other by gravity. The acceleration, scientists reasoned, must be due to an unknown type of energy opposing standard attractive gravity. This unknown energy is now called dark energy.

By default, the nature of dark energy is tied up with the history of the universe. The more we know about the origin of the universe and how it has evolved over time, the more we know about the role dark energy plays.

Since there’s no scrapbook of the cosmos’ early days to look back at, unraveling the story of the universe requires working backward from present-day observations with the help of theoretical models, computer simulations, and experiments. A key part of this process is exploring the cosmic web, the large-scale structure of the universe. This is what you would see if you zoomed way out from your screen, your city, the Earth, the solar system, our interstellar neighborhood, the Milky Way galaxy, the Virgo Supercluster, and beyond—to outside of the observable universe. Scientists have mapped the cosmic web using measurements from projects like the Sloan Digital Sky Survey.

One of the keys to extracting information from the cosmic web is determining the relationship between its elements—the distances between galaxies, for example. The standard tool for this is based on something called baryon acoustic oscillations.

Back in its early days, the universe had a nearly smooth distribution of dark matter, regular matter, and photons. The word “nearly” is essential, though: at a certain point in time, regions with a slightly higher density of dark matter began to collapse under gravity. This caused ripples that traveled outward in space, kind of like ripples from a rock dropped in a pond.

The gravitational collapses across the universe all took place at about the same time, so the ripples all have the same radius today. The size of this circle measured in the early universe is called the sound horizon. Since the ripples correspond to extra dense regions and galaxies usually form in such regions, we can measure the same sound horizon scale in the distribution of galaxies. This sound horizon scale then serves as a standard cosmological ruler.

“Our measurement of the sound horizon scale in the distribution of galaxies depends on the distance of the galaxy survey from us, this quantity depends on dark energy,” says Stefano Anselmi, a cosmologist working both at the Paris Observatory and at the Paris Institute of Astrophysics. By comparing the sound horizon measured in the early universe (from cosmic microwave background measurements) to what we see today, we can learn about the properties of dark energy.

Sounds good, right? Here’s the problem. The sound horizon has changed length slightly over the last 14 billion years. As cosmological measurements get more precise, this slight length change can lead to systematic errors in distance estimates. Scientists have introduced techniques to correct for this, but the techniques involve model dependencies that wouldn’t be there if the ruler were pristine.

This bothered Anselmi. As a graduate student, he became obsessed with understanding what cosmologists meant by a cosmological standard ruler. Eventually he realized that his confusion wasn’t from lack of understanding, his confusion was the result of inaccuracies in what cosmologists were writing and saying about the issue. They were referring to a pristine ruler, but what they actually measured was an imperfect ruler.

Now, a team led by Anselmi is proposing a new, pristine ruler. Along with collaborators from Case Western University, the Observatory of Paris, and the University of Pennsylvania, he uncovered something called the linear point, a length scale slightly smaller than the sound horizon. The linear point is still created by the baryon acoustic oscillations, but it isn’t influenced by the changes in sound horizon length.

In this new research article, the team showed how to use the linear point to estimate cosmic distances using data from galaxy surveys. Their results precisely matched other estimates using the sound horizon.

“The Linear Point, since it does not need to be corrected theoretically, can provide a less ‘model-dependent’ study of dark energy,” explains Anselmi. “Cosmologists now have a different and complementary ruler that allows us to cross-check the traditional sound horizon ruler results. Therefore, if in the future the two rulers give incompatible outcomes, that will be a good starting point to ask new questions that can help us on the road toward the discovery of the nature of dark energy,” he says.

The process of reaching this more straightforward ruler was anything but. The journey involved random lucky discoveries, many wrong paths, and years of blindness, says Anselmi. However, the journey to a fundamental understanding is rarely easy, especially when it challenges standards that are widely accepted.

Kendra Redmond

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