NASA’s Roman mission could tell us if the universe will eventually tear itself apart

Universe expansion concept

NASA Nancy Grace Roman Space Telescope

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 exoplanet science. HLSS is a high-volume precision survey of millions of galaxies dating back billions of years. The Survey’s primary goal is to study universal expansion over the Universe’s history. The HLSS is so deep and wide that it’ll enable science that isn’t possible with other existing telescopes.

“While this survey is designed to explore cosmic acceleration, it will also offer clues about many other tantalizing mysteries,” said Wang. “It will help us understand the first generation of galaxies, allow us to map dark matter, and even reveal information about structures that are much closer to home, right in our local group of galaxies.”

Roman vs Hubble Field of View

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.

Evolution of the Universe Sketch

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:

  1. Is cosmic acceleration caused by a new energy component or by the breakdown of general relativity (GR) on cosmological scales?
  2. 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.

<p>Cosmologists have already mapped the large-scale structure, but the Roman Telescope's HLSS will take this mapping one step further. The HLSS will tell us the distances to about two million galaxies when the Universe was only two to three billion years old. This has never been done before and will be new data.</p>
<p>It boils down to measuring as many things as accurately as possible. If the Roman telescope can add new depth and breadth to our understanding of the large-scale structure of the universe over time, we can understand the history of the expansion of the universe. Then we might finally have our answer.</p>
<p>"Roman will determine the expansion history of the universe to test possible explanations for its apparently accelerated expansion, including dark energy and modification of Einstein's gravity," the authors write in their article. "Roman will determine the growth history of the largest structures in the universe to test the possible explanations for their apparently accelerated expansion, including dark energy and modification of Einstein's gravity…"</p>

This video resolves between the entire collection of redshift cubes in 55 seconds. As the universe expands, the density of galaxies in 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 congregated along vast strands of gas separated by vast voids, a foam-like structure that resonated on large cosmic scales in today’s Universe. This visualization shows the number and abundance of simulated galaxies at different cosmic ages ranging from 4% to 43% of the current age of the Universe of 13.8 billion years. Each cube represents a fixed volume of space, about 100 million light-years per side. As the sequence progresses, the expansion of the universe rapidly reduces the density of galaxies. Each die shows a specific cosmological redshift from 9 to 1, with previous dies cast in redder shades.

The last sentence describes where we are now. The universe is expanding and the expansion is accelerating. That shouldn’t be the case, because the gravity of all matter in the universe should impede this expansion. The acceleration means that Einstein’s theory of gravity is not entirely correct. Or it means we need to add a new energy component to the universe: dark energy.

As explained in his TGR, Einstein’s gravity is accurate up to a point. So it was with Newton until we could observe larger parts of the universe. Newton’s gravitation accurately describes what happens to gravitation on local scales, and Einstein’s gravitation accurately explains what happens on an even larger scale. But now we face the whole universe and our understanding is insufficient.

This study simulates what the Roman can contribute to this topic. The Roman Telescope’s vast and deep 3D images of the universe are a new opportunity to distinguish between the leading theories attempting to explain cosmic acceleration: a modified gravity or dark energy theory.

Science can only win. Every result brings us closer.

“To shed light on the unknown nature of cosmic acceleration, we need to measure two free-time functions: the history of cosmic expansion and the growth rate of large-scale structures,” the authors write. “These can tell us whether dark energy varies with time and whether it is an unknown energy component (e.g. a cosmological constant) or the consequence of modifying general relativity as the theory of gravitation.”

Roman Space Telescope Redshift Cosmological Infographic

This graphic illustrates how the cosmological redshift works and how it provides information about the evolution of the Universe. The universe is expanding, and that expansion is expanding the light that travels through space. The more it’s stretched, the greater the redshift and the greater the distance the light has traveled. So we need telescopes with infrared detectors to see the light from the first, most distant galaxies. Image credits: NASA, ESA, Leah Hustak (STScI)

“In any case, we can look forward to new physics – whether we learn that cosmic acceleration is caused by dark energy or whether we find that we need to modify Einstein’s theory of gravity,” Wang said. “Roman will test both theories at the same time.”

The authors note that their reference HLSS is an example of how the High Latitude Wide Area Spectroscopic Survey might be implemented on Roman. “The actual survey that Roman will conduct will be defined prior to launch in an open community process, taking into account the landscape of dark energy projects and their synergies,” they write.

Will we ever know how the universe will end? Maybe someday we will, and we can talk about it at cocktail parties. And we can talk about how the Roman Nancy Gracy Space Telescope helped us find our answer.

Originally published on Universe Today.

For more on the subject, see Dark Energy Vs. Modified Gravity: NASA’s Roman mission will test competing theories of cosmic acceleration. NASA’s Roman mission could tell us if the universe will eventually tear itself apart

Russell Falcon

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