To see a world in a grain of sand
And a heaven in a wild flower,
Hold infinity in the palm of your hand,
And eternity in an hour.
— William Blake
One of the most iconic contemporary telescopes in astronomy, the Hubble space telescope, has provided us with images that could be described as both science and art. Part of this long-lasting and celebrated legacy is undoubtably the telescope’s three Deep Field images: the Hubble Deep Field taken in 1995, the Hubble Ultra-Deep Field in 2004, and the Hubble Extreme Deep Field in 2012. These are consecutively the deepest-ever views of the Universe, as of yet, in visible and near-infrared light. Since looking farther into space means looking further back in time, these images allowed astronomers and cosmologists to observe objects as far as 13.2 billion light-years, revealing a time when the Universe was just 500 million years old, and some of the first galaxies were still evolving.
Yet with the Extreme Deep Field, even Hubble reached the limits of its observing capability. The astronomical community, not feeling content on waiting for the launch of the next-generation James Webb Space Telescope, asked a simple question: How can we look even further today? The answer: NASA’s Frontier Fields.
The Frontier Fields is an ambitious 3-year-long observing campaign that utilises NASA’s Great Observatories—Hubble, Spitzer, and Chandra space telescopes—to create six new Deep Fields centered around specific galaxy clusters and allow astronomers to delve even deeper into the Universe, using gravitational lensing as a helping tool.
Gravitational lensing is a term that came into the spotlight with Albert Einstein’s general theory of relativity in the early 20th century. What Einstein showed was that, since every mass in the Universe has gravity, this gravity can “warp” space-time, causing the light of a more distant object to be bent and refocused somewhere else. The greater the mass, the greater its gravity and the greater its ability to bend light. In essence, gravitational lensing can be described as a magnifying lens that amplifies the light of distant, background objects that can’t be observed directly, thus bringing them into view. And galaxy clusters are among the best of the Universe’s such magnifying lenses.
During the course of the program, Hubble will use its Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) in parallel, with one camera focused at a certain galaxy cluster and the other at a “blank” patch of space very near the cluster. Six months later, when the Earth will be at the opposite side of the sky in its orbit, Hubble will have an 180-degree opposite orientation. As an effect, the cameras will “swap” places, each camera now observing the other’s previous spot on the same cluster, thus giving us a more detailed, overlapping, and complete set of observations. The first target on which Hubble has already started observations is the supercluster Abell 2744 (also known as the Pandora’s Cluster), which is believed to be the result of a collision between four smaller galaxy clusters, located at a distance of 3.5 billion light-years away. Superclusters like Abell 2744 are the largest of structures in the Universe, containing hundreds of galaxies. By observing them, astronomers can yield valuable insights into the distribution of matter in the Universe on the largest of scales and the Universe’s overall structure.
By the end of the 3-year-long period of observations for the Frontier Fields, Hubble will have devoted 840 of its orbits to the project and nearly 2 million seconds of total exposure time. This will be equal to the Hubble’s Extreme Deep Field total exposure time, an image that was obtained over the course of a decade without the help of any gravitational lensing.
While Hubble will make its observations in visible and near-infrared light, it will be accompanied by Spitzer, observing in the infrared and Chandra observing at X-ray wavelengths. Utilising all of NASA’s Great Observatories, astronomers hope to not only measure with great accuracy the clusters’s age, mass, and distance, but also to map the number of black holes that lie inside them. Also, astronomers hope to shed some light to the ever-elusive nature of dark matter that is believed to permeate galaxy clusters. The ultimate goal of the project, of course, is to take advantage of the effects of gravitational lensing that the clusters will provide. It is hoped that the telescopes will gain up to a 100-fold increase to their observing power, bringing far-distant and previously unobservable areas of the Universe into clear focus.