Every ten years, the US astronomy community produces a roadmap outlining the most compelling questions in astronomy and the projects needed within the next decade to answer them. The most recent roadmap, Pathways to Discovery in Astronomy and Astrophysics for the 2020s, released in 2021, developed three broad scientific themes: Worlds and Suns in Context, New Messengers and New Physics, and Cosmic Ecosystems. The report recognized the development of a strong US Extremely Large Telescope Program (US-ELTP) as the most important ground-based optical capability needed to advance the state of knowledge in each of these themes. The US-ELTP will enable astronomers to address these priorities and search for signs of life in distant extrasolar planets, unravel the mysteries of dark matter and dark energy, and discover the interconnected evolution of galaxies, black holes, and their environments.
Worlds and Suns in Context: Pathways to Habitable Worlds
— The study of the formation and evolution of planetary systems around other stars and the interconnections between planets and their host stars
Is there life outside our Solar System?
Discovering the ubiquity and diversity of planetary systems is one of the great achievements of modern astronomy. At the end of 1991, the total number of known planets around stars other than the Sun (exoplanets) was precisely zero. A little more than three decades later, that number has ballooned to more than 5000. Astronomers now estimate that, on average, there is at least one planet per star in the Milky Way.
The observed exoplanets come in a variety of configurations. There are Jupiter-like planets close to their host suns, complex systems much like our Solar System, and super-Earths at the right distance from their host stars to support liquid water and possibly life as we know it. Studying these planets will help us understand the formation of our own Solar System as well as the origin and ultimate fate of the Earth. Precise observations of planets that could host liquid water may also reveal biosignatures — signs of life in the planets’ atmospheres — if they exist. Such a discovery will allow us to scientifically address the question: Are we alone?
Exoplanets are inherently difficult to observe. Firstly, detailed studies of exoplanets are easier if they pass in front of (or ‘transit’) their host star as viewed from the Earth and if we can observe them during that transit. Capturing such an alignment from Earth is rare, however. Secondly, even if one finds a suitable exoplanet, carrying out the necessary observations is challenging because exoplanets are small, far away, and positioned too close to their bright host stars to be easily isolated by current telescopes. Finally, even if one successfully observes an exoplanet, the more involved task of detecting biosignatures is one of the most challenging observations to make in all of astronomy.
The US-ELTP telescopes will overcome many of these challenges. With its full-sky coverage and extended viewing night, the US-ELTP system is much more likely to catch transits wherever and whenever they occur. In addition, their instruments — many of which were specifically designed for exoplanet observations — will significantly elevate our capabilities to study exoplanets. Lastly, their highly capable adaptive optics systems will provide the extreme precision and sensitivity required to detect biosignatures in exoplanet atmospheres. Whether there is life beyond Earth or not, the US-ELTP will push the boundaries of exoplanet exploration and help us understand our own Solar System, the origin and fate of Earth, and our place in the Universe.
New Messengers and New Physics: New Windows on the Dynamic Universe
— The application of multi-messenger astrophysics and time-domain astronomy to address fundamental questions in cosmology and physics
What is the nature of the Universe?
In the early 1930s, astronomers postulated the existence of dark matter, a mysterious mass needed to explain the observed effects of gravity. In the early 1990s, astronomers developed the concept of dark energy to explain the observed acceleration in the Universe’s expansion. Now, astronomers generally believe that these mysterious entities—dark matter and dark energy—make up the vast preponderance of the Universe although neither has been directly observed. Current models suggest that the Universe is made of approximately 95% dark matter and dark energy, while only 5% of it is the normal matter and energy that we can directly detect.
Since the concepts of dark matter and dark energy were introduced, astronomers have been developing myriad models and proposing corresponding observations needed to test them. However, previous observations have been unable to resolve the mystery. With the US-ELTP system, we can conduct observations with unsurpassed sensitivity and angular resolution. These capabilities will advance our understanding of these dark mysteries and possibly reveal hidden additional new physics. In particular, the US-ELTP system will observe gravitational lenses and dwarf galaxies, two classes of targets expected to reveal much about the nature of dark matter and dark energy.
What are gravitational waves telling us about the nature and structure of the Universe?
Besides illuminating our understanding of the dark Universe, the US-ELTP system will also observe the phenomena associated with the production of gravitational waves. For most of human existence, we have observed the Universe exclusively via visible light, the wavelengths of light that our eyes have evolved to see. Within the last century, we developed ways to ‘see’ the Universe with invisible light, namely in the infrared, ultraviolet, radio, and X-ray wavelengths. Each new way of viewing the Universe led to new discoveries, and more interestingly, to new questions.
With the advent of observatories that can detect waves not of light, but of gravity — new messengers of cosmic information — we can view the Universe with incredibly different eyes. Since 2016 we have detected more than 100 events through their emission of gravitational waves. The analysis of these gravitational wave events in both gravitational and electromagnetic waves could reveal new physics via novel information about their structure and dynamics only characterized by their gravitational-wave emission. Studying their environments may even shed light on the nature of dark matter and dark energy.
So far, we have only observed light from a small percentage of these gravitational wave sources. With the increased sensitivity and resolution of the US-ELTP telescopes, we will be able to identify more sources and get better characterization through the light they emit in the optical and infrared. These extremely large telescopes are also well-suited to tracking the variability of these objects, revealing important clues to their nature. Finally, these telescopes offer all-sky coverage and more observing hours per 24-hour day, allowing us to observe these sources despite their unpredictability and often short observation windows.
Cosmic Ecosystems: Unveiling the Hidden Drivers of Galaxy Growth
— The integrated study of gas, stars, and black holes within galaxies, their interconnections across a range of scale and density, and the processes that shape the evolving life cycle of the Universe
What is the relationship between black holes and galaxies and how do they evolve with time?
The development of substructures that connect and surround galaxies in the Universe appears to strongly relate to galaxy formation in the early Universe. Material streaming between and around galaxies feeds growing galaxies and their central black holes, but now many of our previous ideas about the growth of galaxies in the early Universe are being shown by JWST observations to be inaccurate. Higher spatial resolution observations through the US-ELTP are needed to improve our understanding.
Using the US-ELTP telescopes, astronomers will be able to study the centers of a large variety of galaxies in the early Universe, and ultimately unravel the complex processes behind how galaxies came to be. The US-ELTP telescopes will use their immense capabilities to vastly extend the observed number and sizes of galaxies that house black holes. In particular, they will be able to measure black hole masses up to 10 times smaller in galaxies more than 20 times farther away than is currently possible, thus greatly extending the number of measurable black hole masses. The US-ELTP’s larger sample of black holes with a larger range of masses and ages will be key to understanding cosmic ecosystems.
Unexpected Discoveries Anywhere in the Sky
— Things we do not know we do not know
What else is out there?
Every time astronomers built a new generation of telescopes, used a new detector or instrument, or looked at the sky in a new way, they found the unexpected. The Universe — in all its vastness — seems to have a rule that if something is not prohibited, then it must exist. We only have to expand our tools, open our eyes, and find it. Time after time, the Universe never fails to astonish, and the US-ELTP will be ready.