“Our study forecasts the science Roman’s spectroscopy survey will enable and shows how various adjustments could optimize its design,” said lead author Wang.
NASA’s Wide-Field Infrared Survey Telescope (WFIRST) is now named the Nancy Grace Roman Space Telescope, after NASA’s first Chief of Astronomy. Credit: NASA
The Roman will conduct a High Latitude Wide Area Survey (HLWAS). The High Latitude Spectroscopic Survey (HLSS) is the spectroscopic part of the HLWAS outlined in this study. The HLWAS is one of the telescope’s featured science objectives, along with novel approaches to
The Roman Space Telescope’s field of view will dwarf the Hubble’s. (No disrespect to the venerable Hubble, The Bringer of Knowledge.) Credit: NASA/GSFC/JPL
Roman’s HLSS relates to Universal expansion, Dark Energy, and Einstein’s Theory of General Relativity (TGR). Obviously, those are all deep and detailed topics, and they won’t fit in a Kurzgesagt-sized nutshell, but here’s how they fit together.
In 1915, when Einstein first put forth his TGR, nobody thought the Universe was expanding. TGR succeeded in explaining things Newtonian Gravity couldn’t. But it had a flaw. Einstein himself realized that his theory predicted that a static Universe was unstable, and it either has to expand or contract to be stable. But he rejected that, and he tripped himself up by introducing the now-notorious ‘cosmological constant’ to compensate. He used it to counteract the effect of gravity and achieve a static Universe. Einstein later called this his greatest blunder.
Then in the 1920s, astronomers discovered that the Universe is expanding. Bye-bye cosmological constant. American astronomer Edwin Hubble played a prominent role in the discovery, and the rule describing the expansion is called Hubble’s Law. (Sidebar: Belgian scientist and priest Georges Lemaître did earlier work on expansion, but he published his work in an obscure journal. Now Hubble’s Law is increasingly referred to as the Hubble–Lemaître law.) They discovered that galaxies are all moving away from each other, with only a very few exceptions. The Universe is expanding.
The expansion of the Universe was and is a mystery. Scientists have a placeholder name for the force that must be driving the expansion: Dark Energy.
For a long time, cosmologists thought the expansion was slowing. But it turns out that’s not true.
In 1998 scientists discovered that the Universe’s rate of expansion is accelerating. It shouldn’t be because the gravity from all the matter should slow the expansion down. With that discovery, the cosmological constant came back into play. It’s now the simplest explanation for the accelerating expansion. The cosmological constant is represented by the Greek capital letter lambda: Λ.
This image shows the expansion of the Universe accelerating. Time flows from bottom to top. Credit: Ann Feild (STScI)
Wouldn’t it be nice if the interminable guessing over the fate of the Universe was over? Wouldn’t it be fun to know how the Universe will end? (Lawrence Krauss thinks so.) It’d be as much fun as knowing what triggered its beginning. Imagine how popular you’d be at cocktail parties.
This brings us to the Roman Telescope and its High Latitude Spectroscopic Survey. The HLSS might be able to tell us about the future of the Universe’s expansion and if the Universe will continue to expand faster and faster and end in a Big Rip.
In their paper, the authors clarify the overall goal of the Survey. There are two top-level questions:
Is cosmic acceleration caused by a new energy component or by the breakdown of general relativity (GR) on cosmological scales?
If the cause is a new energy component, is its energy density constant in space and time, or has it evolved over the history of the universe?
There’s no magic to this. In a way, there’s brute force involved. The more of the Universe you can measure, and the more precisely you can measure it, the more accurate your conclusions are likely to be. This is behind the drive for larger, more precise telescopes like the Roman Space Telescope. The answers to our questions are more complex and harder to find.
In the paper, the authors present a reference design for the HLSS. The Roman’s HLSS will cover nearly 2,000 square degrees or about 5% of the sky in about seven months. This is a considerable improvement over other telescopes like the Hubble. “Right now, with telescopes like Hubble, we can sample tens of high-redshift galaxies. With Roman, we’ll be able to sample thousands,” explained Russell Ryan, an astronomer at STScI.
“Although Roman could execute a shallow and wide-area survey comparable to Euclid’s in approximately 1 yr of observing time, the deeper survey proposed here is a better complement to other surveys and more effectively exploits the capabilities of Roman’s larger aperture,” the paper states. “Per unit observing time, Roman is an extraordinarily efficient facility for slitless spectroscopic surveys, so it is well-positioned to respond to developments in experimental cosmology between now and mission launch in the mid-2020s.”
The new study shows that Roman’s HLSS should precisely measure 10 million galaxies from when the Universe was between three to six billion years old. Astronomers will use that data to map the large-scale structure of the Universe.
Cosmologists have already mapped the large-scale structure, but the Roman Telescope’s HLSS will take that mapping a step further. The HLSS will tell us the distances to about two million galaxies from when the Universe was only two to three billion years old. That’s never been done before and will be new data.
It boils down to measuring as many things as we can as accurately as we can. If the Roman Telescope can bring new depth and breadth to our understanding of the Universe’s large-scale structure over time, we can understand the history of the Universe’s expansion. Then, maybe, we’ll finally have our answer.
“Roman will determine the expansion history of the universe in order to test possible explanations of its apparent accelerating expansion, including dark energy and modification to Einstein’s gravity,” the authors write in their paper. “Roman will determine the growth history of the largest structures in the universe in order to test the possible explanations of its apparent accelerating expansion, including dark energy and modification to Einstein’s gravity…”
This video dissolves between the entire collection of redshift cubes in 55 seconds. As the Universe expands, the density of galaxies within each cube decreases, from 528,000 in the first cube to 80 in the last. Each cube is about 100 million light-years across. Galaxies assembled along vast strands of gas separated by immense voids, a foam-like structure echoed in the present-day Universe on large cosmic scales. This visualization shows the number and clustering of simulated galaxies at different cosmic ages, ranging from 4% to 43% of the Universe’s current age of 13.8 billion years. Each cube represents a fixed volume of space, about 100 million light-years per side. Over the sequence, the expansion of the Universe quickly lowers the density of galaxies. Each cube shows a specific cosmological redshift, from 9 to 1, with earlier cubes cast in redder shades.
That last sentence describes where we’re at now. The Universe is expanding, and the expansion is accelerating. That shouldn’t be the case because the gravity of all the matter in the Universe should be a drag on that expansion. The acceleration means that Einstein’s theory of gravity isn’t exactly correct. Or it means that we need to add a new energy component to the Universe: Dark Energy.
As explained in his TGR, Einstein’s gravity is accurate, to a point. So was Newton’s until we could observe larger portions of the Universe. Newton’s gravity accurately describes what happens with gravity on local scales, and Einstein’s gravity accurately explains what happens on an even larger scale. But now we’re confronting the entire Universe, and our understanding is inadequate.
This study simulates what the Roman can bring to the issue. The Roman Telescope’s vast and deep 3D images of the Universe are a new opportunity to discern between the leading theories that attempt to explain cosmic acceleration: a modified theory of gravity or Dark Energy.
Science can only win. Either result gets us closer.
“In illuminating the unknown nature of cosmic acceleration, we need to measure two free functions of time: the cosmic expansion history and the growth rate of large-scale structure,” the authors write. “These can tell us whether dark energy varies with time and whether it is an unknown energy component (eg, a cosmological constant), or the consequence of the modification of general relativity as the theory of gravity.”
This graphic illustrates how cosmological redshift works and how it offers information about the universe’s evolution. The universe is expanding, and that expansion stretches light traveling through space. The more it has stretched, the greater the redshift and the greater the distance the light has traveled. As a result, we need telescopes with infrared detectors to see light from the first, most distant galaxies. Credit: NASA, ESA, Leah Hustak (STScI)
“We can look forward to new physics in either case – whether we learn that cosmic acceleration is caused by dark energy or we find that we have to modify Einstein’s theory of gravity,” Wang said. “Roman will test both theories at the same time.”
The authors point out that their HLSS reference is an example of how could implement the High Latitude Wide Area Spectroscopic Survey on Roman. “The actual survey that Roman will execute will be defined in an open community process prior to launch, taking into consideration the landscape of dark energy projects and their synergies,” they write.
Will we ever know how the Universe will end? Maybe one day we will, and we can chat about it at cocktail parties. And we can talk about how the Nancy Gracy Roman Space Telescope helped us find our answer.