What Is: Cosmic Event Horizon

The Final Boundary
An illustration of the Cosmic Event Horizon. Unlike the Observable Universe, which is defined by light that has reached us, this horizon marks the limit of causal contact. Beyond this line, space expands faster than the speed of light, meaning no signal sent from Earth today could ever overtake the expansion to reach galaxies in these regions.
Image Credit: Scientific Frontline

Scientific Frontline: Extended "At a Glance" Summary

• The Core Concept: A theoretical boundary in the universe separating events that can ever causally affect an observer from those that never will; effectively, it marks the absolute limit of future visibility.
• Key Distinction/Mechanism: Unlike the Particle Horizon (which defines the observable past) or the Hubble Sphere (a kinematic boundary where recession velocity equals the speed of light), the Event Horizon is a strict causal limit determined by the accelerating expansion of space. Light emitted from galaxies beyond this horizon at the present moment will never reach Earth, regardless of how much time passes.
• Origin/History: Rooted in the standard \(\Lambda\)CDM model of cosmology; current interest is driven by the "Crisis in Cosmology" regarding Dark Energy and the Cosmological Coupling hypothesis, which suggests a link between black hole growth and cosmic expansion.
• Major Frameworks/Components:
• \(\Lambda\)CDM Model: The standard framework involving Dark Energy and Cold Dark Matter that predicts the horizon's existence.
• FLRW Metric: The geometry of spacetime describing an expanding universe.
• Cosmological Coupling: A recent hypothesis positing that black holes are the source of Dark Energy.
• Black Hole Cosmology: A theoretical model suggesting our observable universe may be the interior of a black hole within a larger parent universe.
• Branch of Science: Cosmology, Astrophysics, Theoretical Physics.
• Future Application: Critical for refining models of Dark Energy and testing the limits of General Relativity; ultimately essential for predicting the long-term fate of the universe (e.g., "Cosmic Solitude").
• Why It Matters: It defines the fundamental limits of our reality and causal connection to the rest of the cosmos. Recent theories connecting this horizon to black hole physics could radically alter our understanding of the Big Bang, suggesting our universe is a "cell" within a larger multiverse rather than an isolated expanse.

The Most Precise Accounting Yet of Dark Energy and Dark Matter

G299 was left over by a particular class of supernovas called Type Ia. 
Credit: NASA/CXC/U.Texas

 Astrophysicists have performed a powerful new analysis that places the most precise limits yet on the composition and evolution of the universe. With this analysis, dubbed Pantheon+, cosmologists find themselves at a crossroads.

Pantheon+ convincingly finds that the cosmos is composed of about two-thirds dark energy and one-third matter — mostly in the form of dark matter — and is expanding at an accelerating pace over the last several billion years. However, Pantheon+ also cements a major disagreement over the pace of that expansion that has yet to be solved.

By putting prevailing modern cosmological theories, known as the Standard Model of Cosmology, on even firmer evidentiary and statistical footing, Pantheon+ further closes the door on alternative frameworks accounting for dark energy and dark matter. Both are bedrocks of the Standard Model of Cosmology but have yet to be directly detected and rank among the model's biggest mysteries. Following through on the results of Pantheon+, researchers can now pursue more precise observational tests and hone explanations for the ostensible cosmos.

"With these Pantheon+ results, we are able to put the most precise constraints on the dynamics and history of the universe to date," says Dillon Brout, an Einstein Fellow at the Center for Astrophysics | Harvard & Smithsonian. "We've combed over the data and can now say with more confidence than ever before how the universe has evolved over the eons and that the current best theories for dark energy and dark matter hold strong."

The early cooling of the universe

A look into the past: Cosmic microwave radiation (left) was released 380,000 years after the Big Bang and serves as the background for all galaxies in the universe. The starburst galaxy HFLS3 (centre) is embedded in a cloud of cold-water vapor and appears as it did 880 million years after the Big Bang. Because of its low temperature, the water casts a dark shadow on the microwave background (detail enlargement on the left). This represents a contrast about 10,000-fold stronger than its intrinsic variations of only 0.001% (light/dark spots). 
Credit: ESA and the Planck Collaboration; zoom-in panel: Dominik Riechers/University of Cologne; image composition: Martina Markus/University of Cologne

The shadow of a cosmic water cloud reveals the temperature of the young universe

A telescope in the French Alps has allowed researchers to peer deep into the past of the universe. For the first time, they were able to observe an extremely distant hydrogen cloud that shadows the cosmic background radiation created shortly after the Big Bang. The shadow is created because the colder water absorbs the warmer background radiation on its way to Earth. This provides information about the temperature of the cosmos just 880 million years after the Big Bang. To measure the early history of the universe, an international team used the Northern Extended Millimeter Array (NOEMA), the most powerful radio telescope in the northern hemisphere.

The universe came into being around 13.8 billion years ago with the Big Bang. At that time, a hot, dense fog of radiation and elementary particles wafted in space, which was rapidly expanding. The density and temperature decreased just as quickly, and the light particles (photons) lost increasingly more energy. After about 380,000 years, this plasma had cooled down to 3000 Kelvin. It was then possible for stable atoms to be created. And the photons had a free path and spread out into space. The cosmos became transparent so to speak.

Dark Energy Spectroscopic Instrument (DESI) Creates Largest 3D Map of the Cosmos

DESI’s three-dimensional “CT scan” of the Universe. The earth is in the lower left, looking out over 5 billion light years in the direction of the constellation Virgo. As the video progresses, the perspective sweeps toward the constellation Bootes. Each colored point represents a galaxy, which in turn is composed of hundreds of billions of stars. Gravity has pulled the galaxies into a “cosmic web” of dense clusters, filaments and voids.
Credit: D. Schlegel/Berkeley Lab using data from DESI

The Dark Energy Spectroscopic Instrument (DESI) has capped off the first seven months of its survey run by smashing through all previous records for three-dimensional galaxy surveys, creating the largest and most detailed map of the universe ever. Yet it’s only about 10% of the way through its five-year mission. Once completed, that phenomenally detailed 3D map will yield a better understanding of dark energy, and thereby give physicists and astronomers a better understanding of the past – and future – of the universe. Meanwhile, the impressive technical performance and literally cosmic achievements of the survey thus far are helping scientists reveal the secrets of the most powerful sources of light in the universe.

DESI is an international science collaboration managed by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) with primary funding for construction and operations from DOE’s Office of Science.

Cosmic lights in the forest

TACC’s Frontera, the fastest academic supercomputer in the US, is a strategic national capability computing system funded by the National Science Foundation.
Photo Credit: TACC.

Like a celestial beacon, distant quasars make the brightest light in the universe. They emit more light than our entire Milky Way galaxy. The light comes from matter ripped apart as it is swallowed by a supermassive black hole. Quasar light reveals clues about the large-scale structure of the universe as it shines through enormous clouds of neutral hydrogen gas formed shortly after the Big Bang on the scale of 20 million light years across or more. 

Using quasar light data, the National Science Foundation (NSF)-funded Frontera supercomputer at the Texas Advanced Computing Center (TACC) helped astronomers develop PRIYA, the largest suite of hydrodynamic simulations yet made for simulating large-scale structure in the universe.

“We’ve created a new simulation model to compare data that exists at the real universe,” said Simeon Bird, an assistant professor in astronomy at the University of California, Riverside. 

Bird and colleagues developed PRIYA, which takes optical light data from the Extended Baryon Oscillation Spectroscopic Survey (eBOSS) of the Sloan Digital Sky Survey (SDSS). He and colleagues published their work announcing PRIYA in the Journal of Cosmology and Astroparticle Physics (JCAP). 

Discovery of Unexpected Ultramassive Galaxies May Not Rewrite Cosmology, But Still Leaves Questions

Infrared view of the universe captured by the James Webb Space Telescope.
Image Credit: NASA, ESA, CSA and STScI.

Ever since the James Webb Space Telescope (JWST) captured its first glimpse of the early universe, astronomers have been surprised by the presence of what appear to be more “ultramassive” galaxies than expected. Based on the most widely accepted cosmological model, they should not have been able to evolve until much later in the history of the universe, spurring claims that the model needs to be changed.

This would upend decades of established science.

“The development of objects in the universe is hierarchical. You start small and get bigger and bigger,” said Julian Muñoz, an assistant professor of astronomy at The University of Texas at Austin and co-author of a recent paper that tests changes to the cosmological model. The study concludes that revising the standard cosmological model is not necessary. However, astronomers may have to revisit what they understand about how the first galaxies formed and evolved.

Cosmology studies the origin, evolution and structure of our universe, from the Big Bang to the present day. The most widely accepted model of cosmology is called the Lambda Cold Dark Matter (ΛCDM) model or the “standard cosmological model.” Although the model is very well informed, much about the early universe has remained theoretical because astronomers could not observe it completely, if at all.

The key to why the universe exists may lie in an 1800s knot idea science once dismissed

The model suggests a brief “knot-dominated era,” when these tangled energy fields outweighed everything else, a scenario that could be probed through gravitational-wave signals.
Image Credit: Courtesy of Muneto Nitta/Hiroshima University

In 1867, Lord Kelvin imagined atoms as knots in the aether. The idea was soon disproven. Atoms turned out to be something else entirely. But his discarded vision may yet hold the key to why the universe exists.

Now, for the first time, Japanese physicists have shown that knots can arise in a realistic particle physics framework, one that also tackles deep puzzles such as neutrino masses, dark matter, and the strong CP problem. Their findings, in Physical Review Letters, suggest these “cosmic knots” could have formed and briefly dominated in the turbulent newborn universe, collapsing in ways that favored matter over antimatter and leaving behind a unique hum in spacetime that future detectors could listen for—a rarity for a physics mystery that’s notoriously hard to probe.

“This study addresses one of the most fundamental mysteries in physics: why our Universe is made of matter and not antimatter,” said study corresponding author Muneto Nitta, professor (special appointment) at Hiroshima University’s International Institute for Sustainability with Knotted Chiral Meta Matter (WPI-SKCM2) in Japan.

“This question is important because it touches directly on why stars, galaxies, and we ourselves exist at all.”

Scientists find first observational evidence linking black holes to dark energy

Artist’s impression of a supermassive black hole. Cosmological coupling allows black holes to grow in mass without consuming gas or stars.
Image Credit: UH Manoa

Searching through existing data spanning 9 billion years, a University of Michigan physicist and colleagues have uncovered the first evidence of “cosmological coupling”—a newly predicted phenomenon in Einstein’s theory of gravity, possible only when black holes are placed inside an evolving universe.

Gregory Tarlé, U-M professor of physics, and researchers from the University of Hawaii and other institutions across nine countries studied supermassive black holes at the heart of ancient and dormant galaxies to develop a description of them that agrees with observations from the past decade. Their findings are published in two journal articles, one in The Astrophysical Journal and the other in The Astrophysical Journal Letters.

The first study found that these black holes gain mass over billions of years in a way that can’t easily be explained by standard galaxy and black hole processes, such as mergers or accretion of gas. According to the second paper, the growth in mass of these black holes matches predictions for black holes that not only cosmologically couple, but also enclose vacuum energy—material that results from squeezing matter as much as possible without breaking Einstein’s equations, thus avoiding a singularity.

The Largest Suite of Cosmic Simulations for AI Training

The CAMELS project (Cosmology and Astrophysics with MachinE Learning Simulations) combines over 4,000 cosmological simulations, millions of galaxies, and 350 terabytes of data to decipher secrets of the universe.

Totaling 4,233 universe simulations, millions of galaxies and 350 terabytes of data, a new release from the CAMELS project is a treasure trove for cosmologists. CAMELS — which stands for Cosmology and Astrophysics with MachinE Learning Simulations — aims to use those simulations to train artificial intelligence models to decipher the universe’s properties.

Scientists are already using the data, which is free to download, to power new research, says project co-leader Francisco Villaescusa-Navarro, a research scientist with the Simons Foundation’s CMB (Cosmic Microwave Background) Analysis and Simulation group.

Villaescusa-Navarro leads the project with associate research scientists at the Flatiron Institute’s Center for Computational Astrophysics (CCA) Shy Genel and Daniel Anglés-Alcázar, who is also a UConn Associate Professor of Physics.

“Machine learning is revolutionizing many areas of science, but it requires a huge amount of data to exploit,” says Anglés-Alcázar. “The CAMELS public data release, with thousands of simulated universes covering a broad range of plausible physics, will provide the galaxy formation and cosmology communities with a unique opportunity to explore the potential of new machine-learning algorithms to solve a variety of problems.”

Astronomers suggest radiation, not supernovae, drives superwinds in some galaxies

This image zooms in on the Mrk 71 region in the galaxy NGC 2366. The red, blue and green colors reflect the emission of oxygen and helium ions. The observations were made from the Hubble Space Telescope. Image credit: Sally Oey

The finding could provide insight into how the universe became transparent

When astronomers observe superwinds traveling at extremely high speeds from super star clusters, or “starbursts,” they previously assumed the winds were driven by supernovae, the explosions of stars.

This was the case for a starburst called Mrk 71 in a nearby galaxy. Astronomers had observed incredibly fast superwinds—traveling at about 1% of the speed of light—emanating from the cluster, and classic reasoning suggested the blasts from many supernovae drive the gas to such a high rate of speed.

But University of Michigan astronomers think supernovae aren’t the reason: the cluster is too young to have supernovae. They suspect a different mechanism is behind the superwind.

By studying the wind and starburst properties, the astronomers established that ultraviolet radiation from the compact starburst itself drove the superwind. Their findings, published in the journal Astrophysical Journal Letters, may help explain one chapter of the universe’s beginnings.

Expansion of universe directly impacts black hole growth

First rendered image of a black hole, illuminated by infalling matter
(Image credit: Jean-Pierre Luminet)

Over the past 6 years, gravitational wave observatories have been detecting black hole mergers, verifying a major prediction of Albert Einstein’s theory of gravity. But there is a problem—many of these black holes are unexpectedly large. Now, a team of researchers from the University of Hawaiʻi at Mānoa, the University of Chicago, and the University of Michigan at Ann Arbor, have proposed a novel solution to this problem: black holes grow along with the expansion of the universe.

Since the first observation of merging black holes by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015, astronomers have been repeatedly surprised by their large masses. Though they emit no light, black hole mergers are observed through their emission of gravitational waves—ripples in the fabric of spacetime that were predicted by Einstein’s theory of general relativity. Physicists originally expected that black holes would have masses less than about 40 times that of the Sun, because merging black holes arise from massive stars, which can’t hold themselves together if they get too big.

Comparison of black hole merger observations with predictions from the new model. The horizontal axis shows the total mass of both black holes in any individual merger, relative to the Sun’s mass.

The LIGO and Virgo observatories, however, have found many black holes with masses greater than that of 50 suns, with some as massive as 100 suns. Numerous formation scenarios have been proposed to produce such large black holes, but no single scenario has been able to explain the diversity of black hole mergers observed so far, and there is no agreement on which combination of formation scenarios is physically viable. This new study, published in the Astrophysical Journal Letters, is the first to show that both large and small black hole masses can result from a single pathway, wherein the black holes gain mass from the expansion of the universe itself.

How football-shaped molecules occur in the universe

Graphic Credit: Shane Goettl/Ralf I. Kaiser

For a long time, it has been suspected that fullerene and its derivatives could form naturally in the universe. These are large carbon molecules shaped like a football, salad bowl or nanotube. An international team of researchers using the Swiss SLS synchrotron light source at PSI has shown how this reaction works. The results have just been published in the journal Nature Communications.

“We are stardust, we are golden. We are billion-year-old carbon.” In the song they performed at Woodstock, the US group Crosby, Stills, Nash & Young summarized what humans are essentially made of: star dust. Anyone with a little knowledge of astronomy can confirm the words of the cult American band – both the planets and we humans are actually made up of dust from burnt-out supernovae and carbon compounds billions of years old. The universe is a giant reactor and understanding these reactions means understanding the origins and development of the universe – and where humans come from.

In the past, the formation of fullerenes and their derivatives in the universe has been a puzzle. These carbon molecules, in the shape of a football, bowl or small tube, were first created in the laboratory in the 1980s. In 2010 the infrared space telescope Spitzer discovered the C60 molecules with the characteristic shape of a soccer ball, known as buckyballs, in the planetary nebula Tc 1. They are therefore the biggest molecules to have been discovered to date known to exist in the universe beyond our solar system.

Astronomers reveal new map of dark matter, mass in universe

Victor M. Blanco 4-meter Telescope, left, at the Cerro Tololo Inter-American Observatory in Chile houses the camera used by the Dark Energy Survey.
Image Credit: Dark Energy Survey

For decades, cosmologists have mapped the distribution of mass in the universe, both visible material and the mysterious dark matter, in an effort to improve our understanding of these fundamental building blocks. Astronomer Eric Baxter from the University of Hawaiʻi Institute for Astronomy co-authored new research that traces the mass distribution in the universe in three dimensions. The updated analysis was published in Physical Review D.

Baxter and his University of Chicago collaborators, Chihway Chang and Yuuki Omori, compiled data using two different sky surveying methods. This new analysis shows that there is six times as much dark matter in the universe compared to matter that is visible—a finding that was already well-known. However, the team also found that the matter is not as clumpy as previously expected when compared to the current best model of the universe.

The researchers claim the findings could add to a growing body of evidence that there may be something missing from the existing standard model of the universe.

NASA Reveals New Details About Dark Matter’s Influence on the Universe

Created using data from NASAs Webb telescope in 2026 (right) and from the Hubble Space Telescope in 2007 (left), these images show the presence of dark matter in the same region of sky. Webb's higher resolution is providing new insights into how this invisible component influences the distribution of ordinary matter in the universe.
Image Credit:NASA/STScl/A Pagan

Scientific Frontline: Extended "At a Glance" Summary

The Core Concept: A highly detailed map of dark matter distribution created using data from the James Webb Space Telescope (JWST), revealing the invisible "scaffolding" that structures the universe.

Key Distinction/Mechanism: Unlike previous, blurrier maps, this new visualization is twice as sharp and provides empirical confirmation that dark matter and ordinary matter are tightly interlocked. It utilizes gravitational lensing—observing how dark matter's mass warps space and bends light from distant galaxies—to trace invisible structures with unprecedented precision.

Major Frameworks/Components:

• Gravitational Lensing: The primary method used to detect non-luminous dark matter by measuring how it distorts background light.
• Cosmic Evolution Survey (COSMOS): The specific region of the sky (in the constellation Sextans) observed for this study.
• Mid-Infrared Instrument (MIRI): A key JWST instrument used to measure galactic distances and penetrate cosmic dust.
• Matter Correlation: The study confirms a direct spatial overlap between "clumps" of dark matter and clusters of ordinary (baryonic) matter.

Branch of Science: Astrophysics, Cosmology.

Future Application: These detailed maps will help refine models of cosmic evolution, specifically clarifying how early dark matter structures accelerated the formation of the first stars and galaxies, thereby enabling the creation of planetary systems.

Why It Matters: It validates the theory that dark matter acts as the gravitational anchor for the visible universe. By proving that dark matter grew alongside ordinary matter, scientists can better understand the timeline of the universe's development, including the conditions that allowed for the emergence of planets like Earth.

Astrophysics: In-Depth Description

An illustration of the vast and complex field of astrophysics, featuring elements that represent celestial objects and phenomena.
Image Credit: Scientific Frontline / stock image

Astrophysics is the branch of physics that applies physical laws and theories to understand the origin, evolution, structure, and behavior of celestial objects and phenomena. Its primary goal is to use the principles of physics to explain the universe and everything within it, from stars and planets to galaxies and the entirety of the cosmos.

Uncovering the secrets of ultra-low frequency gravitational waves

An artist's impression of the colliding bubbles that can produce extremely low frequency gravitational waves during a cosmological phase transition in the early Universe.
Image credit: Riccardo Buscicchio.

New methods of detecting ultra-low frequency gravitational waves can be combined with other, less sensitive measurements to deliver fresh insights into the early development of our universe, according to researchers at the University of Birmingham.

Gravitational waves - ripples in the fabric of Einstein's spacetime - that cross the universe at the speed of light have all sorts of wavelengths, or frequencies. Scientists have not yet managed to detect gravitational waves at extremely low ‘nanohertz’ frequencies, but new approaches currently being explored are expected to confirm the first low frequency signals quite soon.

The main method uses radio telescopes to detect gravitational waves using pulsars – exotic, dead stars, that send out pulses of radio waves with extraordinary regularity. Researchers at the NANOGrav collaboration, for example, use pulsars to time to exquisite precision the rotation periods of a network, or array, of millisecond pulsars – astronomers’ best approximation of a network of perfect clocks - spread throughout our galaxy. These can be used to measure the fractional changes caused by gravitational waves as they spread through the universe.

First-of-its-kind measurement of Universe’s expansion rate weighs in on longstanding astronomy debate

Image Credit: Patrick Kelly, University of Minnesota

Thanks to data from a magnified supernova, a team led by University of Minnesota researchers has successfully used a first-of-its-kind technique to measure the expansion rate of the Universe. Their data provide insight into a longstanding debate in the field of astronomy and could help scientists more accurately determine the Universe’s age and better understand the cosmos.

The work is divided into two papers, published in Science, one of the world’s top peer-reviewed academic journals, and The Astrophysical Journal, a peer-reviewed scientific journal of astrophysics and astronomy.

In astronomy, there are two precise measurements of the expansion of the Universe, also called the “Hubble constant.” One is calculated from nearby observations of supernovae, and the second uses the “cosmic microwave background,” or radiation that began to stream freely through the Universe shortly after the Big Bang. 

However, these two measurements differ by about 10%, which has caused widespread debate among physicists and astronomers. If both measurements are accurate, that means scientists’ current theory about the make-up of the universe is incomplete.

A speed camera for the universe

The stars (or rather galaxies) of the show.
A montage of eight time-delay gravitational lens systems. There’s an entire galaxy at the center of each image, and the bright points in rings around them are gravitationally lensed images of quasars behind the galaxy. These images are false-color and are composites of data from different telescopes and instruments.
Image Credit: ©2025 TDCOSMO Collaboration et al.
(CC BY-ND 4.0)

There is an important and unresolved tension in cosmology regarding the rate at which the universe is expanding, and resolving this could reveal new physics. Astronomers constantly seek new ways to measure this expansion in case there may be unknown errors in data from conventional markers such as supernovae. Recently, researchers including those from the University of Tokyo measured the expansion of the universe using novel techniques and new data from the latest telescopes. Their method exploits the way light from extremely distant objects takes multiple pathways to get to us. Differences in these pathways help improve models on what happens at the largest cosmological scales, including expansion.

Astronomers discover a superheated star factory in the early universe

Glowing deep red from the distant past: galaxy Y1 shines thanks to dust grains heated by newly-formed stars (circled in this image from the James Webb telescope).
Image Credit: NASA, ESA, CSA, STScI, J. Diego (Instituto de Física de Cantabria, Spain), J. D’Silva (U. Western Australia), A. Koekemoer (STScI), J. Summers & R. Windhorst (ASU), and H. Yan (U. Missouri)

Astronomers have uncovered a previously unknown, extreme kind of star factory by taking the temperature of a distant galaxy using the ALMA telescope. The galaxy is glowing intensely in superheated cosmic dust while forming stars 180 times faster than our own Milky Way. The discovery indicates how galaxies could have grown quickly when the universe was very young, solving a long-standing puzzle for astronomers.  

The first generations of stars formed under conditions very different from anywhere we can see in the nearby universe today. Astronomers are studying these differences using powerful telescopes that can detect galaxies so far away their light has travelled towards us for billions of years.   

Scientists reveal distribution of dark matter around galaxies 12 billion years ago–further back in time than ever before

 The radiation residue from the Big Bang, distorted by dark matter 12 billion years ago.
Credit: Reiko Matsushita

A collaboration led by scientists at Nagoya University in Japan has investigated the nature of dark matter surrounding galaxies seen as they were 12 billion years ago, billions of years further back in time than ever before. Their findings, published in Physical Review Letters, offer the tantalizing possibility that the fundamental rules of cosmology may differ when examining the early history of our universe.

Seeing something that happened such a long time ago is difficult. Because of the finite speed of light, we see distant galaxies not as they are today, but as they were billions of years ago. But even more challenging is observing dark matter, which does not emit light.

Consider a distant source galaxy, even further away than the galaxy whose dark matter one wants to investigate. The gravitational pull of the foreground galaxy, including its dark matter, distorts the surrounding space and time, as predicted by Einstein’s theory of general relativity. As the light from the source galaxy travels through this distortion, it bends, changing the apparent shape of the galaxy. The greater the amount of dark matter, the greater the distortion. Thus, scientists can measure the amount of dark matter around the foreground galaxy (the “lens” galaxy) from the distortion.

However, beyond a certain point scientists encounter a problem. The galaxies in the deepest reaches of the universe are incredibly faint. As a result, the further away from Earth we look, the less effective this technique becomes. The lensing distortion is subtle and difficult to detect in most cases, so many background galaxies are necessary to detect the signal.