By Massimo Robberto, Observatory Scientist at STScI
In 1994, when I was a young Assistant Astronomer in Italy, I started regularly visiting STScI for a collaboration on stellar coronagraphy with Mark Clampin and Francesco Paresce. Those were the months immediately following the first Hubble Servicing Mission and excitement was in the air at the Institute. New pictures comparing the “pre” and “post’ performance of the telescope were posted daily, as testimony for NASA’s spectacular achievement and the bright future ahead. Still, traces of the shock caused by the initial failure (spherical aberration) were evident. In particular, in the control room of the first floor an entire wall had been covered with hundreds of cartoons from all over the world mocking NASA for the Hubble primary mirror disaster. STScI staff had diligently collected and posted all of them, regardless on their quality or even language. A bit of irony helps keeping things in the right perspective when a crisis strikes, and staying focused, to work on a solution.
One of those cartoons attracted my attention. It showed a group of perplexed experts, the “Hubble Design Group”, wondering if “outer space really is blurry and out of focus” (Figure 1). I liked it immediately and discretely made my copy. The cartoon was different from the others because, I thought, it was a cartoon about us, about how our intelligence works. We always look for an explanation, for a reason, and when all good possibilities fail, we start considering the bizarre and the improbable. We feel that saying “I don’t know” and stop wondering is even worse than opening the door to what may look absurd. And our secret hope, as scientists, is to discover that something apparently absurd is actually real.
Figure 1: Ralph Dunagin’s strip, Dunagin’s People, ran from 1960 to 2001. The Hubble Servicing Mission occurred in 1993.
Fast-forward 20 year: today, the Hubble Space Telescope is still fully operational at its best, acquiring data that are transforming our knowledge of the Universe. In particular, a Hubble Legacy Program is underway to study one of the most spectacular phenomena gloriously unveiled by the Hubble images: gravitational lensing (Figure 2). As predicted by Einstein, mass warps space-time. When a large mass is present, as in the case of the dark matter halos surrounding clusters of galaxies, the light of more remote objects along our line of sight gets distorted. The signal is so strong (“strong gravitational lensing”) that we can use it to map the distribution of dark matter in these halos. This is at present the only direct method we have to probe the effects of what may be the most enigmatic particles in physics.
Figure 2. This HST/ACS image, obtained in 2009 immediately after the last Hubble Servicing Mission, shows a gravitationally lensed background galaxy in the field of the Arp 370 galaxy cluster.
If the alignment conditions are favorable, the brightness of some remote galaxy may be magnified: the gravitational lens effect can make visible objects that would have otherwise remained beyond Hubble’s reach. One can say that the so-called Frontier Fields program is using two telescopes in a series, one made by us (Hubble) and one provided by Nature (gravitational lensing), to search for the most distant galaxies and supernovae. With this “trick” Hubble can give us a glimpse of the type of science that will be routinely carried out by the James Webb Space Telescope.
As one moves further away from the center of a galaxy cluster, the gravitational lens effect becomes less pronounced. No matter where we look in the sky, the shapes of thousands of galaxies are all slightly distorted in some way (“weak gravitational lensing”). It is a vanishingly small signal, but it is correlated for all the galaxies that are at similar distances, so it is possible to detect it by applying statistical methods to the most exquisite wide-field images. Future space missions such as Euclid and WFIRST are designed so as to carry out this type of study, which is critical to understand the build-up of giant cosmic structures over time and the process of galaxy formation.
Another fascinating aspect is that amplification can be caused by the random and temporary alignment of stars in crowded fields. In this case the brief light amplification (micro-lensing) can be used to unveil the presence of planets like Earth. WFIRST has the capability of monitoring billions of stars in the Galactic Bulge, where lensed planets could flash like lights on a Christmas tree. Crafting a suitable cadence of observations to exploit this capability is one of the main challenges faced by WFIRST.
A 10m-class space telescope like the proposed ATLAST will eventually produce images of incredible sensitivity and spatial resolution. At that point the ubiquitous gravitational lensing will become more obvious and, perhaps, just another ordinary aspect of our perception of the Universe.
Twenty-five years after launch, the Hubble Space Telescope is showing us that because of gravitational lensing the Universe is really somewhat blurry and out of focus. The intuition of a cartoonist has anticipated one of the most spectacular discoveries of all time. Let’s keep an open mind to more surprises. What today looks like an absurd concept, imagined only in the mind a creative artist, may become tomorrow part of our understanding of this beautiful, and very extravagant, Universe.