The heat death of the universe will mark the perceptive end of time

Due to dark energy, the universe is expected to end as a dark, sparse place

While the sun is expected to continue shining for another five billion years, the expansion of the universe will cause nearby stars and galaxies to become ever more distant, until they are too far to observe. Once the last old, red, dim stars cool into darkness after trillions of years, the universe will settle into a state of eternal, dark emptiness.

The arrow of time—the concept that time only moves in the forward direction—is thought to result from the universe progressing toward greater disorder, or entropy.

It is impossible for a telescope to ever see the Big Bang and the beginning of time because light in the early universe was trapped in a dense 'cosmic fog' of elementary particles.

Time seems to stop for objects at the event horizon of black holes

At that location, the warping of space-time is so severe that the entirety of an object's space-time motion is solely through space, leaving it immobile in the time dimension. Faraway observers looking at the object would see it appear frozen before slowly dimming and reddening.

Dark energy dominance represents the final era of the universe

As the universe expanded after the Big Bang, matter and energy diluted at different rates, with light once the most dense substance in the cosmos, followed by a period when matter was the most dense component. About five billion years ago, the density of dark energy was higher than that of both light and matter, which will continue to dilute.

Time disappears when encompassing all quantum configurations across the universe

The Wheeler-DeWitt equation, a modified version of the Schödinger equation—the formula that describes the evolution of the quantum configuration of a system—describes the quantum state of space-time across the universe. The transformation causes the variable of time to disappear, suggesting it is not a fundamental, physical parameter of the cosmos.

Under the Big Rip scenario, dark energy becomes strong enough to tear atoms apart

If the density of dark energy, which produces a repulsive effect through the expansion of space, increases with time, then all structures of matter, including galaxies, stars, planets, and atoms, will be pulled apart once the strength of dark energy surpasses the magnitude of the forces holding those structures together, including the gravitational and electromagnetic forces.

Even in an infinite universe, its limited age prevents a sky full of starlight

Olbers' paradox asks why the sky would ever appear dark if the universe is full of stars in all directions emitting light everywhere. Light from the farthest stars requires more time to reach our skies than the universe's age.

If dark energy is weakening, the universe may collapse under its own gravity

Scientists using the Dark Energy Spectroscopic Instrument (DESI) analyzed light from 15 million galaxies and quasars to identify that the universe's accelerated expansion is diminishing. If this continues, the universe might reverse course and collapse in as little as 20 billion years.

Understanding how and when time began requires a theory of quantum gravity

Rewinding the universe to the big bang compresses all matter, energy, space, and time into a single point, necessitating a model that combines general relativity with quantum mechanics. Although physicists understand time to have begun when the universe did, a clear understanding of the mechanisms involved remains unclear.

Because some models of the universe do not incorporate time, it may be an emergent property born from our limited perception of the universe, rather than a fundamental property of reality.

The underlying fabric of the universe—spacetime—changes shape in the presence of mass and energy, which affects measurements of space and time.

The ultraviolet catastrophe is the infinite energy prediction of classical physics

When scientists first tried calculating the radiation emitted by a heated object, their assumption was that energy was equally distributed across wavelengths and produced infinite energy at low wavelengths. This contradicted observations, which showed near-zero energy intensity at those values.

The observable universe has a mass of 100 sexdecillion kilograms

The amount of visible matter in the universe is most accurately determined by measuring the cosmic microwave background. Quantities of dark matter and dark energy are calculated through the observed bending of light around galaxies and the expansion of the universe, respectively.

If the universe is infinite, it may be impossible to say what's beyond it

The universe is defined as all that exists, including space and time. If the universe is infinite, as observations currently suggest, there is no boundary from which we can ask what lies beyond.

If the sun disappeared, we would not know about it for 8 minutes and 20 seconds, at which point the Earth would suddenly move in a straight line on its way out of the solar system.

Why Earth will be made uninhabitable by the sun far sooner than when the sun dies

The star has enough fuel to continue fusing hydrogen into helium for another 5 billion years, but the gradual increase in core temperature from these reactions will brighten the sun by 10% every billion years. Some models predict that the increased radiation will be enough to vaporize all of Earth's surface water about 1 billion years from now, long before the sun becomes a red giant and disintegrates the planet.

Dark stars are hypothetical stars that would have existed in the early universe

Primarily composed of visible matter, this celestial object would also contain a significant amount of dark matter that would annihilate with itself, converting its mass into energy. This energy would prevent the star from collapsing under its own gravity and from triggering nuclear fusion until its dark matter fuel was exhausted.

View a timeline of the universe's major events, according to the Big Bang model.

As of 2025, the best cosmological model of the universe includes dark energy

The Lambda cold dark matter model uses the Greek letter to denote the presence of a cosmological constant version of dark energy, alongside slow-moving dark matter and ordinary visible matter. It can account for the observed distribution of galaxies, the structure of the cosmic microwave background, the acceleration of the universe, and the abundance of hydrogen.

Watch a simulation of a collapsing star becoming a black hole

When a massive star can no longer release enough energy through fusion in its core to fight off gravitational collapse, the dense core collapses first, producing an event horizon that traps light within it, including some light produced within the star at the time of collapse. As more material falls into the growing event horizon, the star will appear to eclipse itself from the inside.

Through gravitational redshift, light loses energy as it leaves a massive object

Just as one becomes exhausted as they walk up a flight of stairs against gravity, light is predicted to lose energy as it travels out of a gravitational well created by an object warping spacetime. This is observable through a decrease in the frequency of light toward longer, redder wavelengths.

General relativity describes a singularity as the final moment in time

Using Penrose diagrams—mathematical structures that compress all of space and time—physicists can observe the path of light and which events can affect one another. The space-time maps help visualize the structure of black holes and describe what happens when objects fall in.

The Planck time, equal to 5.39 × 10⁻⁴³ seconds or less than a millionth of a trillionth of a trillionth of a trillionth of a second, is the smallest unit of time that could ever be theoretically measured.

Explore an interactive forecast of the future of the universe, from the return of Halley's Comet to the last light in the cosmos.

Oscillating matter in the early universe left behind patterns detailing cosmic expansion

As gravity tried to bring matter closer together in the early universe, its high temperature produced an outward pressure that caused it to expand. These competing forces caused matter to vibrate and produce the equivalent of sound waves, which spread outward as the universe expanded and cooled, serving as relics of dark energy's history.

The cosmic dark ages: before stars existed

The theories behind the Big Bang suggest the process of the universe beginning to the creation of stable, neutral atoms took just half a million years. But the transition from such atoms to star formation took surprisingly much longer, upwards of 100 million years. Why? This deep dive explores the science behind the unique physical composition of the so-called cosmic dark ages.

From the Big Bang to present—a timeline

A timeline for everything, from the Big Bang to the beginnings of life on Earth. This article lays out, in spans of billions of years, what was happening in the universe as it became the universe. Starting from the Planck era, the earliest known meaningful time, to the probable "Big Crunch" 30 billion years from now, see how the universe has existed and persisted.

The universe might have emerged from a quantum fluctuation—not absolute nothing

Traditional physics breaks down at the moment of the Big Bang, but quantum theory suggests that “empty” space is never truly empty. In 1973, physicist Edward Tryon proposed that our universe began from a quantum energy blip in the quantum vacuum.

Under a model of quintessence, the strength of dark energy should change over time

The theory suggests that dark energy permeates space and may drive the universe to expand or contract throughout its history. The model's flexibility allows it to fit every possible observed behavior of the universe's expansion, rendering it unfalsifiable and untestable.

The cosmic microwave background radiation is the oldest light in the universe

Just after the Big Bang, the high-density universe prevented light from traveling very far before being absorbed and emitted repeatedly by free-floating particles. As the universe expanded, electrons and protons combined to form hydrogen, thereby freeing up space for light to travel unimpeded, creating the background light we observe today.

The weakening of dark energy suggests a dynamic universe with evolving forces

The discovery, which came from analyzing extensive data from a major cosmological survey, challenges the assumed constancy of dark energy in modern physics. Understanding dark energy's behavior is crucial for predicting the universe's long-term evolution.

Even in a perfect vacuum, empty space is full of quantum activity

Scientists have tried to create “nothing” by removing all matter, energy, and heat from a container, but the uncertainty principle and relativity prevent such conditions within any period of time. This means the universe is full of quantum foam, which can produce forces from nothingness.

Neutron stars, the remains of some massive stars after they explode in a supernova, are the densest directly observable objects in the universe.

A black hole with the mass of the observable universe would be larger than the observable universe, leading some theorists to suggest that we may live inside a black hole.

View the composition of the universe, dominated by dark energy and dark matter

About 95% of the observable universe is made of substances whose composition is unknown, but most of what remains (4%) is in the form of clouds of hydrogen and helium, followed by gases and plasma within stars (0.5%) and neutrinos created in nuclear reactions (0.3%).

Quantum mechanics-based technology may explain the Fermi paradox

It describes the discrepancy between the high likelihood of life beyond Earth given the vastness of the universe and the lack of evidence for such life, since extraterrestrial civilizations have not communicated with us. However, quantum communication—a more information-dense and effective method than classical means—requires massive receivers and very narrow signals, which would easily miss Earth, making a lack of signals expected.

The universe's expansion is accelerating due to a mysterious force called dark energy

Astronomers discovered in the late 1990s that galaxies move apart faster over time, contradicting expectations that gravity should slow cosmic expansion. This prompted the revival of the "cosmological constant," a form of repulsive gravity first proposed by Albert Einstein.

The black hole information paradox comes to an end

Quanta Magazine provides a deep dive into black holes and the black hole information paradox. For a very long time it was believed nothing could escape a black hole. Therein lies the paradox. Per quantum mechanics, whatever falls into a black hole should not necessarily be lost forever, and may eventually return. In a landmark series of calculations, physicists have proved that black holes can shed information.

The Big Bang theory explains the universe’s expansion, not its origin

While we have strong evidence that the universe expanded from an extremely hot, dense state, we can only trace events back to a tiny fraction of a second after that point. Earlier stages, like inflation and potential force unification, remain unproven and unobservable with current tools.

A naked singularity is an infinitely dense point without a black hole's event horizon

Their existence would reveal where Einstein’s theory breaks down and quantum gravity takes over. The prevailing idea—cosmic censorship—suggests these singularities are always hidden behind black hole horizons, shielding the rest of the universe from their unpredictability.

Antimatter's rarity across the cosmos is one of the biggest problems in modern physics because the high-energy environment of the early universe should have produced equal amounts of matter and antimatter.

One model of the universe expresses all constants using only time

About seven base units, such as meters for length, have historically been used to describe everything in our physical reality. However, researchers have shown that, through relativistic space-time, clocks can serve as replacements for rulers, scales, and other instruments, allowing everything to be measured in terms of time.

The cosmic microwave background is the faded light of when the universe glowed orange

About 400,000 years after the Big Bang, the universe cooled enough for atoms to form, releasing a burst of orange-hued thermal radiation that has since redshifted into microwaves we now detect as the CMB. No matter where you point a detector in space, the CMB signal is always there.

As of 2025, the Standard Model has at least five unresolved problems

Among the issues with the most successful theory of the universe at subatomic scales is its inability to explain the extreme imbalance observed between visible matter and antimatter, since both should be roughly equal. The Standard Model also omits gravity and cannot account for the observed rate of cosmic expansion.

The finite speed of light means we can only perceive the world's past

When objects emit or reflect light, it takes time for the light to reach an observer and be detected. During the travel time, the object may have changed, limiting detections to describing the conditions of the object when the light was first emitted or reflected, before it was detected.

The sun will become a red giant after it exhausts the hydrogen fuel in its core, causing gravity to collapse the core and heat it enough for the surrounding hydrogen shell to ignite, releasing enough energy to cause the star to swell.

View a simulation of the dark matter filaments that make up the cosmic web

The distribution of matter in the universe is not uniform, with interconnected threads of galaxy clusters surrounded by voids of empty space. Simulations of the universe show this is only possible within 13.8 billion years—the universe's age—if a foundational network of seeds—dark matter halos—were present to gravitationally attract visible matter.

Black holes are formed from massive stars collapsing under their own gravity

This creates a region where spacetime curves infinitely, known as a singularity. Black holes' intense mass and energy allow them to trap light, distort time and space, and present challenges with reconciling Einstein's theory with quantum mechanics.

String theory replaces black hole singularities with fuzzballs of tangled strings

Black holes contain singularities where information about the material that fell in is lost, violating quantum mechanics. Instead, modeled as a bundle of strings, information is stored and slowly released as the black hole evaporates.