Our first story of the day is one of astronomers using gravity as a telescope; non-pointable telescopes, but telescopes nonetheless. While light has no mass, it has energy, and that good ol’ E=mc^2 means that energy and mass are the same as far as gravity is concerned. As light passes by massive objects, its path gets bent. We can see this – like you and I can test this for ourselves! – during solar eclipses, when the light from distant stars that are located near the Sun’s edge in the sky appear shifted even closer to the edge. Lenses can shift light around too, and if a lens or a mass is big enough, it can focus light into a brighter object than we might otherwise see.
With gravity, light that would otherwise fly toward some other part of the universe can be bent in our direction, allowing us to see more light than normal. The resulting bright, and often distorted, images are called gravitational lenses. The distortion comes from the fact that the masses causing the lensing are often lumpy galactic clusters, and just like a lumpy lense would create a funhouse image, a lumpy mass will create a distorted image.
Since they were first predicted more than 100 years ago, folks have been combing the skies, looking for light from the most distant objects to appear as distortions near galaxy clusters. The most famous example may be Abell 370, which has a myriad of lensed objects warping through deep images of this system. Unfortunately, since the first one was found in 1979, very few have been located… until now. The Dark Energy Survey has been systematically mapping the sky at a resolution and depth combination that has never before been achieved in a survey. By depth we mean, how faint can be seen. In their newest paper, this collaboration has announced the discovery of 335 strong lensing candidates – distant objects that are massively distorted as their light is magnified. While each distorted blob will need follow-up observations to confirm they are truly distant and how the images do or don’t relate to each other, this discovery tells us there are up to 335 new objects whose light has been traveling toward us since early in the universe’s history and that we can use to study galaxy and star formation in the first days of the universe. That’s not all. The reason the Dark Energy Survey is investing so much time and energy into finding lensed objects is that sometimes the alignments of mass and light allow the same distant object to be lensed into multi-images, just like multiple mirrors can allow you to see the same thing repeated multiple times. Since the light path for each version of the same object is slightly different, we can use geometry and a few other tricks to figure out the difference in each path’s lengths to directly measure the changing size of the universe. This technique gives us another way to determine if Dark Energy is real, and if it is, how it has changed (or not) over the course of time. This work appears in the Astrophysical Journal Letters and was led by Xiaosheng Huang.
This is just a start. The instrument they used, the Dark Energy Spectroscopic Instrument, or DESI, isn’t fully up to operational speed yet, and when it is, these 335 new objects will hopefully turn out to be the tip of an iceberg that crushes the problem of Dark Energy.
- Berkeley Lab news release
- “Finding Strong Gravitational Lenses in the DESI DECam Legacy Survey,” X. Huang et al., 2020 May 7, The Astrophysical Journal (Preprint on arxiv.org)