Cosmic bubble theory for dark energy disproven

A team headed by Adam Riess of Hopkins and the Space Telescope Science Institute has eliminated an alternative theory to dark energy, the phenomenon believed to be responsible for the constant expansion of the universe.

Dark energy became the term used to describe the mysterious force that was acting in opposition to the pull of gravity, causing galaxies to slowly move away from each other.

Using NASA’s famous Hubble Space Telescope the scientists calculated the rate of expansion of the universe with an unprecedentedly low 3.3 percent uncertainty, 30 percent smaller than the next best estimate made using Hubble measurements two years ago.

They calculated their measurements by observing changes in light intensity from distant stars. These stars were used as “cosmic yardsticks,” since their luminosity corresponds to how far they are from Earth, and changes in their luminosity can be used to calculate how fast they are moving away from Earth.

The researchers calculated that rate of expansion to be 73.8 kilometers per second per megaparsec. This means that for every additional million parsecs (equal to 3.26 million light-years, or the distance light travels in 3.26 million years) a galaxy is from Earth, it appears to be moving away at a rate of 73.8 kilometers per second.

This number effectively debunks the cosmic bubble hypothesis for the expansion of the universe. The cosmic bubble hypothesis proposed that a bubble of relatively empty space, approximately eight billion light years across, exists in the vicinity of our galaxy, the Milky Way.

Because the bubble is less dense than the universe around it, it would expand at a much faster rate. To an observer in the bubble, or in the close vicinity of it, this would create the illusion of a rapidly expanding universe.

However, the bubble theory relies on two variables. First it requires Earth to be positioned close to the bubble to explain why astronomers have always observed the universe as expanding. In a press release by NASA, Lucas Macri of Texas A&M University stated that the chances of Earth being situated close to the bubble has a “one in a million chance of occurring.”

Furthermore, the bubble theory requires the rate of the universe’s expansion to be much slower than that calculated by Reiss and his team from SHOES, the Supernova H0 for the Equation of State — about 60 and 65 kilometers per second per megaparsec. This is well outside the 3.3 percent range of uncertainty.

The bubble theory was proposed after the initial discovery of dark matter by Reiss and others in 1998. At that time scientists believed that cosmic expansion was in decline, due to the gravitational pull that individual galaxies would exert on each other.

But when observing distant Type Ia supernovae — the release of light energy that occurs upon the death of a star — Reiss realized that they were dimmer than expected.

The decrease in intensity implied that the supernovae were moving farther away from Earth at a faster rate than previously thought. Reiss therefore hypothesized that the only explanation for this occurrence was that the rate of expansion had sped up in the recent past.