If dark energy is accelerating the expansion of the universe, how can we identify its signature? Researchers at the University of Hawaii have been using microwaves to detect what they believe to be dark energy at work. If their work stands up, it will be a useful step for cosmology, but also a potential boon for those of us with interstellar travel in mind. We obviously want to understand a force that may one day have propulsion implications, and it’s possible that the universe is offering a set of useful clues. Here cosmology and propulsion science share a common interest.
Led by István Szapudi, the researchers zeroed in on galactic superclusters — the largest structures in the universe — and so-called ‘supervoids,’ vast areas with few galaxies in them. Remember the prefix ‘super’ here, for conventional galactic clusters are some ten times smaller and held together by gravity, while the Hawaii team believes galaxies in the supervoids and superclusters are more affected by dark energy than gravity. See the team’s Web site for more on the nature of supervoids and superclusters and its use of the Sloan Digital Sky Survey.
The work proceeded by imposing supervoid and supercluster information on a map of the Cosmic Microwave Background, the most distant light visible to us, stretched by the expansion of the universe into the part of the spectrum we associate with radio rather than light. As microwaves pass through them, the superclusters and supervoids have a decided effect. Says Szapudi:
“When a microwave enters a supercluster, it gains some gravitational energy, and therefore vibrates slightly faster. Later, as it leaves the supercluster, it should lose exactly the same amount of energy. But if dark energy causes the universe to stretch out at a faster rate, the supercluster flattens out in the half-billion years it takes the microwave to cross it. Thus, the wave gets to keep some of the energy it gained as it entered the supercluster.”
The image above gives an idea of the result. Microwaves passing through a supercluster are somewhat stronger than those passing through a supervoid. The team believes we are seeing dark energy at work as it stretches supervoids and superclusters to cool or heat light.
Image: These two images from the team’s paper produce spots that are highly significant; taken together, the spots have only a 1-in-200,000 chance of occurring randomly. This is arguably the clearest detection of the ISW effect (see below) to date. It has been detected before at about the same statistical significance, but those detections involve a somewhat cumbersome combination of galaxies from various heterogeneous galaxy samples (the team used a single sample). Credit: István Szapudi/University of Hawaii.
By ‘ISW,’ the team refers to the Integrated Sachs-Wolfe effect, which is responsible for the heating or cooling of photons as they pass through the supercluster and supervoid areas — if interpreted correctly, this is a direct signal of dark energy. You can read a University of Hawaii news release here, while the paper to study first is Granett et al., “Dark Energy Detected with Supervoids and Superclusters,” a look at the investigation that will be reported in the Astrophysical Journal in shorter form.