Risa Wechsler: Mapping the Invisible
By: Ellie Griffin
Even if you do not know much about cosmology, you have probably heard of the two most famous cosmologists: Carl Sagan and Neil DeGrasse Tyson. Their work in popular culture, such as with well-known tv series, has brought the field of cosmology more into the public eye. Still, however, many confuse the name cosmology with similar sounding cosmetology or astrology, though cosmology is a subfield of physics. While similar to astrophysics, cosmology focuses on questions of a larger scale: the size of galaxy clusters and up to the scale of the universe or multiverse. One of the largest charges for cosmologists of our time is to learn more about dark matter and dark energy, which combined make up about 97% of our universe and of which very little is known. Additionally, cosmologists address many theoretical problems, such as the Black hole information paradox, the possibility of wormholes, the multi-universe theory, and the fate and shape of the universe. Cosmology is a lesser-known and smaller field of physics asking what many refer to as the “big-questions.” Cosmologists aim to answer these questions, and this attracted Risa Wechsler to the field.
In high school, Risa Wechsler began to think about “big-questions” of what the universe is made of and how it came to its current state. These questions led her to the field of cosmology, and she now works at Stanford University, including at the SLAC (an acronym for Stanford Linear Accelerator Center) National Accelerator Laboratory. From 2014-2018, Wechsler was a co-spokesperson of the Dark Energy Spectroscopic Instrument (DESI). Along with this position, she is a founding member of the Dark Energy Survey (DES) and the Large Synoptic Survey Telescope (LSST) Dark Energy Science Collaboration.
DES works to detect supernovae and other cosmic structures while DESI works to create a 3D map of the universe spanning nearby areas to those 11 billion light years away. The goal of this mapping is to measure the effect of dark energy on the expansion of the universe. DESI will, according to the DESI website, “obtain optical spectra for tens of millions of galaxies and quasars.” DESI employs optical fibers and 5,000 robotic positioners to determine the redshift of various objects in the universe. The Doppler effect explains redshift: as the propagator of a wave, in this case the wave is light, moves away from the receptor of that wave, the wavelength appears longer than in actuality. For light, a longer wavelength causes the light being emitted from areas far away from us in the universe to appear redder. The farther away from us, the faster that area of the universe travels away. With this knowledge of the nature of the universe’s expansion, the greater the redshift of light from an element, the farther away it is. Thus, DESI’s 3D map of the universe can be constructed based on an object’s distance from our location.
Since 1929 when Edwin Hubble determined that the universe was expanding, and that the rate of expansion was proportional to the distance away from us, cosmologists have puzzled over why the universe acting this way, instead of behaving the opposite way. The gravitational field of matter attracts matter, so, theoretically, all the matter in the universe should be attracted to and so eventually compress into a single point-- a theory known as the Big Crunch. As an answer, cosmologists surmise that dark energy must exist. Dark energy supposedly acts much like the opposite of gravity: it repulses objects in the universe and accelerates the expansion of the universe. Dark energy is uniform across space and has a very small density. Cosmologists do not know much more about dark energy, so the topic currently dominates the field.
Risa Wechsler’s work with organizations mapping the universe in as much detail as possible will help cosmologists better understand the mysterious and invisible dark energy. Dark energy poses many more questions about our universe, even becoming support for the multi-universe theory, or idea that multiple universes exist. Alone, it makes up about three fourths of our universe. And Wechsler uses telescopes to map this invisible, omnipresent dark energy.