Cosmology astro-ph.CO

In the first moments after the Big Bang, the Universe underwent rapid expansion: inflation. But perhaps it was contracting before that, and then bounced. Both scenarios leave nearly identical imprints on the cosmic microwave background, and cosmologists have debated for decades which one is correct.

The galaxy NGC 1560 is a modest island of stars and gas ten million light-years away. Its rotation curve, first measured in 1991, became an icon for those seeking an alternative to dark matter. The plot of rotation speed versus radius shows bends that seem to exactly mirror the bends in the distribution of ordinary matter.

Two telescopes look at the same Universe and see different expansion rates. Local measurements from supernovae give 73.04 ± 1.04 km/s/Mpc, while cosmic microwave background data within the standard model predict 67.36 ± 0.54.

In the dark room of the early Universe, the first spark flickered. A few hundred million years after the Big Bang, when space was filled only with hydrogen and helium, Population III stars were born: metal-free giants whose light first tore through the primordial darkness.

In April 2023, the LIGO, Virgo, and KAGRA detectors caught a gravitational wave from the merger of two black holes. With masses of 137 and 103 solar masses, GW231123 became the most massive event ever observed.

The universe is expanding, and the pace of that expansion is set by dark energy, the mysterious force accelerating the recession of galaxies. In the standard cosmological model, its density is constant: a cosmological constant, an unwavering metronome since the birth of the cosmos.

In laboratories on Earth, Bose-Einstein condensates are created by cooling a cloud of atoms to billionths of a kelvin. The atoms lose their individuality and merge into a single quantum wave that behaves as one entity. But in space, such conditions are not needed: gravity itself creates deep wells where matter compresses and cools into a state of quantum fog.

The Universe after the Big Bang was almost perfectly homogeneous. Almost, because tiny density perturbations stretched by inflation became the seeds of galaxies. But what if these same perturbations could give birth not only to stars but also to black holes?

An electron flies out of a tritium nucleus with an energy of 18.57 keV, leaving behind an invisible partner: a neutrino. If that neutrino has access to a fifth dimension, its mass splits into a whole tower of states, and each one takes its own bite out of the spectrum.

The ARCADE2 radiometer on a high-altitude balloon detected an excess radio emission, while the ground-based EDGES experiment found an anomalously deep trough in the hydrogen spectrum at 21 cm. Both signals do not fit the standard cosmological model and point to an unknown source of soft photons.

Two black holes circle in a tight dance, and around them something remarkable is born: a gravitational molecule. A cloud of ultralight bosons, particles with masses below an electronvolt, envelops the pair like the electron cloud in an H2+ ion.

The cosmological community has been living in a state of quiet civil war for several years. Two camps measure the expansion rate of the Universe and get different answers. Planck, peering at the cosmic microwave background, says 67.4 km/s/Mpc. SH0ES, climbing the distance ladder from Cepheids to supernovae, insists on 73.2.

The ACT telescope in Chile's Atacama Desert stared at the sky from 2007 to 2022. Its target: the cosmic microwave background, light that has been traveling toward us for 13.8 billion years. This light is polarized: its waves oscillate in specific planes, like strings stretched in different directions.

Deep inside a cryostat, beneath layers of superconducting coils, lies a stack of hundreds of semiconductor layers. Each layer is just a few nanometers thick: gallium arsenide, then aluminum gallium arsenide. The electrons within these layers are confined to a plane, like ants on a sheet of paper.

A snowflake falling from a cloud chooses between a left-handed and a right-handed form. The difference between them is almost imperceptible, like a breath of wind, yet it is that very breath that decides which pattern will grow.

A century ago, magnetic observatories recorded only the slow breathing of Earth's core. Today, their data have become hunting grounds for particle physics. At the Eskdalemuir Observatory in northern England, fluctuations of the magnetic field are recorded every hundredth of a second.

The universe is putting on a show, and the lead role goes to dark energy. For a long time, the actor seemed unchanging: the cosmological constant (the vacuum energy that accelerates cosmic expansion) set the same tone throughout.

NGC 3627, a spiral beauty in the constellation Leo, harbors a trail that fits no standard scenario. A narrow band of cold gas, a molecular trail, stretches more than 8 kiloparsecs (thousands of parsecs, each about 3260 light-years) with a width of about 200 parsecs. That is like a thread as long as a football field and as thick as a pencil.

For centuries, three branches of physics (cosmology, particle physics, and nuclear physics) worked like isolated shores.

Astronomers have found an invisible object in the Universe weighing just a million Suns — for cosmic scales, that’s a mere trifle, a hundred times lighter than anything they’ve been able to “weigh” at such distances. The object itself doesn’t shine, but it gave itself away by bending the light from an even more distant galaxy.

Gamma-ray bursts are the most powerful explosions in the Universe: a dying star collapses into a black hole, ejecting streams of energy. Often this happens after a supernova flare. But astronomers caught a strange rhythm — in the emission of 34 such bursts, clear pulsations appeared, like beats of a cosmic heart.

Astronomers use Type Ia supernovae — thermonuclear explosions of white dwarfs — as 'standard candles': their peak luminosity was long thought to be the same everywhere in the universe.

The early universe was far from perfect: some regions stretched faster than others, creating folds and warps. But a quantum bounce — the transition from contraction to expansion, driven by the laws of the microworld — fixes everything.

The Universe is a grand highway where galaxies, like cars, are moving apart faster and faster. The gas pedal is controlled by mysterious dark energy.

Dark matter is the invisible scaffold of the Universe, revealing itself only through gravity. Vera Rubin, studying the rotation of galaxies, found that stars at the periphery move too fast, as if held by an unseen mass. This became compelling evidence for the existence of dark matter, yet its particles still elude direct detection.

The Universe is filled with invisible mass, dark matter, which reveals itself only through gravity. One hypothesis suggests that dark matter consists of primordial black holes — objects formed in the extreme conditions just after the Big Bang.

The James Webb Space Telescope gathered thousands of spectra from distant galaxies — colorful fingerprints of their light. To avoid drowning in this diversity, astronomers tasked a program with sorting through the finds.

In the first moments after the Big Bang, the universe may have given rise to objects unlike the familiar stars — dark stars.

Particles of dark matter can behave like molasses: in one model they don't interfere with each other; in another, they gently jostle, creating viscosity. This determines how quickly giant black holes at the centers of galaxies fall into each other and merge.

When Edwin Hubble discovered the expansion of the universe, and Vera Rubin discovered dark matter, the framework of cosmology seemed solid.

Fast radio bursts are brilliant cosmic flashes that, like a starting signal, set off a race of radio waves across the universe. Astronomers used them as a natural laboratory to test whether the photon, the quantum of light and the carrier particle of the electromagnetic interaction, has mass.

Right after the Big Bang, primordial black holes floated in the hot plasma — clumps of mass the size of a mountain squeezed into the volume of an atom. According to Hawking's prediction, they slowly evaporated, emitting radiation like tiny stoves.

Scientists have described a new type of invisible star that behaves like a breathing lens. Such a star is made of ultralight boson particles (not yet discovered). Its gravity causes spacetime curvature — this is the lens, which rhythmically expands and contracts, as if breathing.

On a high-speed highway, a steady flow of cars sometimes clumps into a traffic jam because of random slowdowns. In the young Universe, the overall expansion of galaxies after the Big Bang resembled this orderly movement. But each galaxy also performed its own maneuvers — braking or accelerating under the pull of neighboring galaxies.

Dark matter is the invisible scaffold of the cosmos, its existence betrayed by whirls of galaxies to Fritz Zwicky and Vera Rubin. What it is made of remained a mystery for half a century. A new hypothesis paints an unusual picture.

In the 1930s, Fritz Zwicky, observing the Coma cluster, noticed anomalously high galaxy velocities: their motion pointed to a huge hidden mass gravitationally binding the system.

The universe exists in a state of false vacuum, like water behind a dam. Behind it lies the true vacuum — an abyss with different laws. Normally, water can only seep through the pores in a thin trickle (a quantum effect), barely changing the level.

In the familiar world, we have three spatial dimensions. However, theoretical physics allows for additional, hidden directions — compactified, that is, curled up to microscopic sizes.

Data from the DESI galaxy survey hint: dark energy was more powerful in the past than today. This 'phantom' behavior threatens the Universe with a Big Rip — a finale where even atoms disintegrate. But the KMIX model explains this as an illusion. Two fundamental fields — like a distorting mirror — warp our picture: constant energy appears variable.

Type Ia supernovae are the 'standard candles' of cosmology: their peak luminosity is nearly identical, allowing distances to be determined from apparent brightness — the fainter the object, the farther away it is.