From Stellar Clusters to Cosmic Horizons: Telescopes are Mapping the Edge of the Observable Universe

“The Exotic Stellar Population of Westerlund 1 (NIRCam)”. Image: ESA/Webb

By Mariana Meneses

The European Space Agency (ESA) has released the first part of an unprecedented 3D map of the universe, captured by their Euclid Space Telescope which launched in 2023. These observations form part of our ever-expanding view of the observable universe – a vast cosmic horizon, representing everything we can theoretically detect from our vantage point in space and time.

“The huge mosaic released by ESA’s Euclid space telescope on 15 October 2024 accounts for 1% of the wide survey that Euclid will capture over six years. The location and actual size of the mosaic on the Southern Sky is shown in yellow.” Credit: European Space Agency

The featured image for this article features the Stellar Population Westerlund 1, which is packed with massive, evolved stars in the late stages of their life cycles. Over the next 40 million years, it will erupt with more than 1.500 supernovae as the stars explode, offering us an opportunity to study some of the most extreme phenomena in the universe. By observing this cluster, we can uncover how these giant stars live, explode, and shape the birth of new baby stars around them.

“Westerlund 1 is an incomparable natural laboratory for the study of extreme stellar physics, helping astronomers to find out how the most massive stars in our Galaxy live and die.” – ESA/Webb

According to the official ESA/Webb website, Westerlund 1 is special because it is one of the few remaining super star clusters in the our Milky Way galaxy, containing over 10.000 times the mass of the Sun in a small, dense region. This cluster offers valuable insights into how stars, especially massive ones, formed in our galaxy’s past, during a period when star production was at its peak.

“In ancient Greek mythology, Ara was identified as the altar where the gods first made offerings and formed an alliance before defeating the Titans” ( Wikipedia ). Image: IAU and Sky & Telescope.

Westerlund 1 is roughly 12.000 light-years away in the southern constellation Ara (the Altar). By mapping cosmic structures with high-resolution telescopes, such as Euclid, Hubble and the James Webb, humans can visualize and locate, among other things, these rare stellar nurseries within our galaxy.

Westerlund 1 is a small piece of a much larger puzzle: the observable universe. Like a cosmic map, the observable universe encompasses every point in space from which we can detect light, with a diameter of about 93 billion light-years.

In Dark Energy Explorers: A Citizen Science Project Looks Over 9 Billion Years Into the Past, TQR featured the work of University of Texas PhD student Lindsay House, who is working on classifying millions of galaxies for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). Her project, Dark Energy Explorers, has engaged over 11,000 volunteers from 85 countries who have contributed nearly 4 million galaxy classifications since 2021. She leverages citizen science classifications as training data for machine learning algorithms, and her goal is to help create one of the largest maps of the universe, focusing on galaxies 9 to 11 billion light-years away. This will provide crucial insights into dark energy and the universe’s expansion rate.

Recently, the European Space Agency unveiled the first segment of what will become the most extensive 3D map of the universe, captured by the Euclid Space Telescope, which was launched in 2023. This initial mosaic, composed of 208 gigapixels, represents just 1% of the planned final map and already encompasses 14 million galaxies and tens of millions of stars.

“During this survey, the telescope observes the shapes, distances and motions of billions of galaxies out to 10 billion light-years. By doing this, it will create the largest cosmic 3D map ever made.” Credit: European Space Agency.

According to the Max Planck Institute for Astronomy, the Euclid mission aims to map one-third of the sky over six years, ultimately capturing billions of galaxies extending up to 10 billion light-years away. The primary scientific objective is to better understand the universe’s evolution and shed light on dark energy and dark matter, which collectively constitute 95% of the universe’s composition.

The released data demonstrates unprecedented resolution, revealing intricate details previously invisible, such as faint galactic cirrus clouds and individual stars within the Milky Way. According to The Guardian, Professor Mat Page from University College London highlighted the mission’s significance, noting that this is the first time such a large area of sky has been imaged at such high resolution.

The observable universe represents the cosmic horizon of what we can detect. This horizon is limited by the 13.8-billion-year age of the universe (i.e., we cannot see anything that happened before the Big Bang or the first 300 million years after), and also by the speed of light (i.e., for us to see something, its light must have had time to reach Earth, and light from far-away objects may not have reached us yet). However, while light is traveling, the universe is expanding; that’s why we can see things up to about 46.5 billion light-years away, and why the observable universe is constantly growing as more light from distant objects reaches us.

This spherical region extends in all directions from Earth, stretching to the cosmic microwave background radiation – the oldest detectable electromagnetic radiation. This region includes countless galaxies and celestial objects that we haven’t yet observed, representing everything that could theoretically be detected from our vantage point.

Artistic conception of the observable universe. Image: Pablo Carlos Budassi, featured in NASA website.

This image, initially created in 2012 by Pablo Carlos Budassi, an Argentinian designer and artist, uses a logarithmic scale to visually depict astronomical objects across different scales.

A Logarithmic Tour across the Observable Universe | Observable Universe

Centered on the Sun to highlight the solar system’s structure, it spirals outward to show planets, moons, asteroids and comets, then transitions to nearby stars, star clusters, and the Milky Way’s satellite galaxies, which are smaller galaxies that orbit around the Milky Way. Beyond our galaxy, it also features prominent galaxies and cosmic structures, gradually showing the large-scale cosmic web and the “end of greatness”, a scale in the universe where galaxies appear evenly distributed (about 300 million light-years from the Big Bang). The outer edge represents the cosmic microwave background, the oldest detectable light, marking the observable universe’s limit.

The main difference between asteroids and comets is their composition and origin. Asteroids are mostly found in the asteroid belt between Mars and Jupiter, composed mainly of silicate rock and metal. Comets, which come from colder, distant regions like the Kuiper Belt or Oort Cloud, contain ice, dust, and rock. As comets approach the Sun, their ice vaporizes, forming a glowing coma and tail (Space).

The finite speed of light means that when we look farther into space, we are also looking further back in time. The deeper we look, the less expanded the universe appears, revealing a denser, hotter state. Eventually, we reach a point when the universe was so dense and hot that light couldn’t travel freely because space was filled with plasma, which scatters light. This created a “fog” in the early universe, making it impossible to see beyond that point. What we can observe is the surface of this fog, marking the limit of our vision into the past.

Technological Advances in Observing the Universe

Advances in telescopic technology have revolutionized our ability to observe the universe, significantly expanding our understanding of cosmic phenomena and the observable universe’s boundaries.

Stephan’s Quintet. Image: Hubble/NASA.

The Hubble Space Telescope set a high standard for deep-space observation by capturing detailed images of galaxies, nebulae, and star clusters billions of light-years away, helping astronomers refine their estimates of the universe’s age and its rate of expansion.

Hubble’s Inside the Image: Stephan’s Quintet | NASA Goddard

The James Webb Space Telescope (JWST), launched in 2021, has taken these observations further by using infrared capabilities to peer through cosmic dust and reveal previously hidden structures. JWST’s advanced imaging allows us to study detect and study some of the earliest galaxies formed after the Big Bang, as well as the atmospheres of exoplanets, providing clues about their habitability.

The James Webb Space Telescope (JWST) has improved our understanding of the early universe by revealing unexpected characteristics of galaxies and cosmic structures that existed just hundreds of millions of years after the Big Bang. Unlike previous telescopes, JWST’s infrared capabilities have allowed astronomers to observe incredibly bright and massive galaxies that challenge existing theoretical models of cosmic evolution.

Quanta Magazine reports that scientists are grappling with multiple potential explanations for these unprecedented observations, including the possibility that early stars were fundamentally different from contemporary stars, that star formation was extraordinarily efficient, or that galactic processes worked in ways not previously understood.

The telescope has uncovered mysterious phenomena, like supermassive black holes in the early universe that seem to have formed and grown at rates that defy current scientific understanding. These are opportunities to refine our comprehension of cosmic formation and evolution. The telescope continues to stream new data daily.

This observation combines mid-infrared light from NASA’s James Webb Space Telescope, and ultraviolet and visible light from NASA’s Hubble Space Telescope. The smaller spiral on the left, cataloged as IC 2163, passed behind NGC 2207, the larger spiral galaxy at right. Image: NASA, ESA, CSA, STScI.

A Tour of IC 2163 and NGC 2207 | James Webb Space Telescope

Looking ahead, the upcoming Nancy Grace Roman Space Telescope, set to launch in the mid-2020s, promises to further enhance our view of the universe. Equipped with a wide-field instrument, the Roman Telescope will be capable of imaging wide areas of the sky at resolutions similar to Hubble’s. This will enable astronomers to survey millions of galaxies, contributing to our understanding of dark energy, dark matter, and the large-scale structure of the cosmos.

“To learn more about dark energy, Roman will use its powerful 2.4-meter mirror and Wide Field Instrument to do two things: map how matter is structured and distributed throughout the cosmos and measure how the universe has expanded over time. In the process, the mission will study galaxies across cosmic time, from the present back to when the universe was only half a billion years old, or about 4 percent of its current age.” (NASA – Nancy Grace Roman Space Telescope)

Additionally, with ground-based telescopes like the Extremely Large Telescope and the Giant Magellan Telescope, which feature adaptive optics to counteract atmospheric distortion, we are moving closer to unlocking unprecedented details about the universe, including directly imaging exoplanets. These telescopes will complement space-based observatories by providing sharper images and higher resolution in certain wavelengths.

The Giant Magellan Telescope is expected to be completed in the early 2030s. Image: Giant Magellan Telescope.

Theoretical Implications of the Observable Universe’s Limit

Given that the observable universe is constrained by the speed of light and the universe’s age, we can only see regions whose light has had time to reach us. Yet, due to the universe’s expansion, this observable boundary stretches, beyond which regions recede too quickly for their light to ever reach us. As expansion continues, some areas will be permanently “lost” to observation, leaving vast, unexplored parts of the cosmos inaccessible. This prompts intriguing questions about what lies beyond—whether physical laws and cosmic structures persist, or entirely different realms exist.

Could the Universe be infinite? with Neil deGrasse Tyson | CuriousCosmos

The cosmological principle assumes that the same physical laws and structures apply universally, but we cannot verify this for areas beyond the reach of our observations. A 2023 study, led by Dr. Pavan Kumar Aluri, from the Indian Institute of Technology, questions this assumption. The team argue that discrepancies could imply that the universe might not be as homogeneous as previously thought, leading scientists to reconsider or refine current models and the assumptions behind them. In The Big Ring: Is the Newly Found Cosmic Megastructure the End for the Cosmological Principle?, we discussed more of these studies.

“Astronomers spotted something unexpected in Westerlund 1 cluster. Around one of the stars — known as W26, a red supergiant and possibly the biggest star known— they have discovered clouds of glowing hydrogen gas, shown as green features in this image.” Image: ESO/VPHAS+ Survey/N. Wright.

Advancements in telescopic technology are continuously allowing us to see farther, probe deeper, and understand more about the universe’s history and structure. However, these tools are still limited by the very nature of the universe itself. While instruments like the JWST, Euclid and the upcoming Nancy Grace Roman Space Telescope will enable us to observe previously hidden regions, they can only reveal what is within our observable reach, constrained by the cosmic boundaries that continue to move beyond our grasp.

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