Discovering the History of the Universe

Elisa Visher | elisa.visher@yale.edu April 1, 2013

Discovering the History of the Universe

A picture of the Chile telescope used to find close supernova. Courtesy of Charles Baltay.

The current Wide Field Infrared Survey Telescope (WFIRST) project has been around for years, changing names, technology, and levels of design as it develops. Originally, it was proposed when a team of researchers, including Charles Baltay of Yale University, wanted to create a satellite telescope in space that would enable them to measure the history of the expansion of the universe. It has been understood for a while now that the universe is expanding; however, it was only recently discovered that this expansion was accelerating. Based on everything known about physics up to that point, scientists would have expected the universe’s expansion to be decelerating, reasoning that the combined mass within the universe would exert its gravitational force and pull the expansion back from the initial velocity provided by the Big Bang. Instead, the universe was expanding and breaking all kinds of physics laws in the process.

The plans for the original SNAP telescope. Courtesy of Charles Baltay.

Why is the Universe Accelerating?

Two theories have presented explanations for the acceleration of the universe. One, there is some new property of gravity that Einstein’s theory of general relativity does not explain. Or two, there is a completely new component in the universe that has repulsive gravity, causing it to push the universe apart at a faster and faster rate. Physicists have a special name for this rule-breaking new component: dark energy. Dark energy is unrelated to dark matter — while both are termed dark because they do not produce light, dark matter does not break any of the classical laws of physics. Dark energy, however, breaks them all and comprises around three-quarters of the known mass of the universe.

The WFIRST project hopes to provide evidence for one of the two theories and prove either the breakdown of general relativity or the existence of dark energy, and, if dark energy exists, whether it is constant over time or changing in some way. These two models predict different behaviors for the expansion of the universe. They also predict different patterns for its acceleration: it may have always existed and be constant, it could be increasing, or it could be decreasing. The goal of the project is to look back into the past to see how acceleration has occurred over the history of the universe.

The WFIRST data hopes to distinguish between different possible histories of the expansion of the universe. The data points measured by the telescope will show how the universe has been accelerating in the past by adding points towards the left of the graph to distinguish between possible curves. Courtesy of Charles Baltay.

A History of the WFIRST Project

The first iteration of the WFIRST project was called SNAP, or the Super Nova Acceleration Probe, because the expansion of the universe can be measured by measuring the acceleration of supernovas (exploding stars). These stars are windows into the past; they are so far away that the light we see from them today was emitted billions of years ago and took all this time, travelling at the finite speed of light, to reach us.

The SNAP researchers approached the Department of Energy for funding. However, when NASA heard of the project, they immediately wanted to be involved in it as well; as Professor Baltay put it, “You can’t just let a bunch of college kids shoot things into space.” Eventually, the two organizations agreed to create a joint project, which was then named the JDEM, or the Joint Dark Energy Mission. The proposed project would cost around one $1–1.5 billion to implement, an amount of funding which is understandably hard to obtain. At this time, however, a ten-year “decadal” review for the whole field of astrophysics called “New Worlds, New Horizons” was underway. Through this review, JDEM was compared to other large-budget projects, rated as the highest priority large space project, and renamed WFIRST.

The quality of data that WFIRST should be able to provide is significantly better than previous measurements. Courtesy of Charles Baltay.

A New Opportunity for the Project

As a response to this recommendation, NASA set up a panel of experts to design the WFIRST space mission. “At this point we were proposing to do the project with an approximately one-meter diameter mirror for the telescope because this was all we thought we could afford, and it would be sufficient do the job,” Baltay said. “Then, NASA got a call from the NRO (the National Reconnaissance Organization) saying that they had two 2.4-meter telescopes and asking if we wanted them.” They did, of course — to provide a point of comparison, these telescopes were of same quality as the Hubble space telescope.

Thus began a new round of design for the project. Once installed, these new mirrors will allow the telescope to measure supernovas that are much further away and to make more accurate measurements on closer ones, leading to the smaller margins of error that may be integral to showing the existence of dark energy and uncovering the history of the acceleration of the universe.

A picture of the Orion Nebula taken by one of the earth-based telescopes associated with the project. Courtesy of Charles Baltay.

Why Do This from Space?

Whenever something can be done from the earth, people want to do it that way. “’Why space?’ is the first question any congressman will ask,” explains Baltay. “To launch a toilet seat into space — that’s a few hundred million dollars. To have it flush — that’s extra.” As evidenced by the $1.5 billion budget of the project, anything done in space is very expensive. However, we can only see the distant supernovas that show us what was happening in the far past from space, because their light is very faint and not visible from within Earth’s atmosphere. The closer supernovas are much brighter and can be seen from Earth, specifically Chile, where Yale researchers are already working to find and measure them through the La Silla-QUEST Variability Survey. These nearby supernova are both easier and harder to discover, because while they can be seen from the Earth, there are much fewer of them.

What Will the Telescope Be Doing?

The WFIRST telescope will make a relatively simple measurement: the wavelengths of light emitted from distant supernova. These wavelengths will then be compared to the wavelengths of light emitted from much closer supernovas. The light from these nearby supernovas can tell us the typical wavelength of light emitted during a supernova explosion, and can thus be used as a base wavelength to compare to the light from distant supernovas measured by WFIRST. When this comparison is made, the distant supernovas’ light will be red-shifted to different degrees, meaning that the light seen by the telescope will appear more red than the light actually emitted by the supernovas as they exploded. This is caused by the Doppler Effect: Because light moves at a constant speed, its wavelength will be stretched if something is moving away (appearing more red) or compressed if something is moving closer (appearing more blue).

Through this method, the WFIRST telescope will be able to measure supernovas at different distances from Earth to find out how quickly each one is moving away. Since each supernova represents a different point in the universe’s history, these velocities can then be used to find the acceleration of the universe at different times in its history. Thus, through many careful measurements and calculations, the WFIRST researchers will be able to describe the history of acceleration in the universe and answer important questions about the nature of dark energy and the universe’s future.

The Future of the Project

WFIRST still has a long way to go. New plans incorporating the NRO telescope will be submitted soon and then final approval for the project must be obtained. After this, the project must be built and launched into space. Baltay says that he hopes the satellite will be up in space by 2021, at which point the project’s team can immediately start collecting data and answering questions about the history of the universe and what it is likely to look like billions of years in the future.

About the Author: Elisa Visher is a junior in Trumbull College studying Anthropology and Biology. She works in Brenda Bradley and Paul Turner’s labs, where she uses non-invasive sampling methods to determine population structure in primates and uses experimental evolution to determine methods of heat tolerance evolution in viruses, respectively.

Acknowledgements: The author would like to sincerely thank Charles Baltay for all of his time and help understanding the project, its history, and its science.