fr gravity

A scale that weighs not a body, but the very space around it. That's how Einstein's equations work in general relativity: they link the curvature of spacetime to what resides within it.

At the center of attention is a monopole, a special clump of a non-Abelian field (a field where the order of operations changes the result), resembling a magnetic charge without a pair. In this work, it is placed in curved spacetime described by Einstein's equations with a Gauss-Bonnet addition (a correction important at small distances).

An engineering blueprint of a skyscraper always includes a number: the maximum load on the foundation — say, 500 kilopascals. Einstein's field equations in curved spacetime impose a similar limit on the 'force load' inside stars, planets, and any other clumps of matter.

Spring scales in a shop show weight simply: the load presses on a spring, it compresses, and the needle moves along the scale. The relation between force and compression is direct, like Hooke's law. Now imagine a scale with a tricky platform: the force is transmitted through levers, and the reading depends not only on the weight but also on how the platform itself bends.

In the first moments after its birth, the universe expanded faster than a match lights. This process is called inflation. It explains why today's cosmos is so homogeneous and flat. But what happened before it? Perhaps the laws of quantum gravity were at work then, and their traces can be found in the data.

A thousand years ago, Vikings took Iceland spar to sea. The translucent crystal split light into two rays. By their brightness, they could find the sun even in fog. Birefringence — the physical term for this phenomenon — now helps hunt for the most elusive force in the cosmos.

In August 2019, the LIGO and Virgo detectors picked up gravitational waves from the merger of two compact objects in a binary star. One weighed 23 solar masses — a standard black hole. The other came in at 2.5–2.67 M⊙.

The first three minutes after the birth of the world are a cosmic kitchen where nature cooked the lightest atomic nuclei. The recipe is set by gravity: it controls the cooling rate of the hot 'broth' of particles. If gravity worked differently than we are used to, the dish would have turned out different — with different amounts of helium and deuterium.

When two black holes merge far away, we receive a tremor of space — gravitational waves. Usually, they are described by Einstein's equations, but what if gravity works differently at the tiniest distances? The authors took one such hypothesis — Hořava–Lifshitz gravity.

Gravity isn't just the force that keeps us on the floor. On cosmic scales, it conducts galaxies, bends light rays, and decides the fate of everything. A century ago, Einstein described it as the curvature of space and time.

In the depths of space, gravity is mastering the art of disguise. For a quarter century, astrophysicists have been trying to decipher the nature of dark energy, which makes the Universe expand at an accelerating rate.

A snowflake grows, an oil slick spreads, a crowd moves — each case has its own equations. But the end result is often the same: shape, speed, and pattern follow a simple scheme that repeats across scales. This is universality — when a common law hides behind the details.

A new tuning fork for the cosmic symphony sounded from the Chilean highlands: the ACT telescope refined the note of primordial perturbations — the spectral index nₛ shifted upward, disrupting the harmony of many previous models.

A builder checks the strength of an arch not by running through all possible shapes in their head, but by mentally nudging it and seeing how much force is expended. That's essentially how the principle of virtual work works in mechanics. The authors of a new study have applied this approach to the very geometry of spacetime. They constructed a covariant framework of virtual work for the metric.

The Universe has a rate at which it expands — called the Hubble constant. The problem is that the two most precise ways to measure it give different numbers. One method, using the early light of the Universe (the cosmic microwave background), gives one value.

In the classical picture of the world, gravity is the smooth bending of space and time. But at the microscopic level, where quantum mechanics rules, these bends should turn into a grainy ripple. The particles of such a ripple are called gravitons. The trouble is that straightforward calculations with gravitons always run into infinities — the theory loses its meaning.

At the very beginning of the universe, when all of space was compressed into a point, the laws of physics break down. This point is called a singularity — an edge where our equations lose meaning. But what if it's not an end, but a special type of boundary? The authors of this work propose viewing the singularity as a free boundary.

In the microcosm, long-range forces, gravity or electricity, do not switch off even at enormous distances. Therefore, any particle drags behind it a trail of soft (i.e., very low-energy) photons or gravitons. This trail is called the "dress"; without it, calculations fall apart and infinities appear.

A supermassive black hole drifts through a cloud of dark matter. In the usual picture, this invisible substance just adds mass and barely interacts with ordinary matter. But dark matter particles could have intrinsic rotation — spin.

A neutron star is the leftover core of an exploded giant, compressed to the size of a city. A teaspoon of its matter weighs a billion tons. Inside such a star, particles are packed so tightly that quantum effects start to show. One of them is the intrinsic rotation of particles, or spin.

The light cone is a blueprint of causality, laser-cut onto the fabric of the universe. In classical physics, it is as sharp as a scalpel: events inside are linked by invisible threads of influence, while outside lies eternal isolation, sealed by the ultimate speed limit.

Toss a handful of coins on a table: each one lands heads or tails on its own. Now imagine the coins start flipping over, nudging each other. That's roughly how neutrinos behave in a very dense environment.

Some solutions to Einstein's equations allow closed timelike curves — loops that let you travel to the past. Two well-known designs, the Ori geometry and the Ahmed geometry, do without exotic matter, catching physicists' eyes.

Imagine an air mattress being stretched, with a whirlpool of water spinning on it. As you stretch, the whirlpool tightens: its size relative to the whole surface shrinks.

Baking bread starts with nearly homogeneous dough, in which tiny gas bubbles are already hidden. They grow, and the rate of their growth determines the future structure of the crumb. In cosmology, the role of such bubbles is played by the seeds of galaxies — barely noticeable density enhancements in the hot 'soup' of the early universe.

Gravity is a cosmic net: massive bodies stretch it, and lighter objects roll along its curves. But in the world of atoms, this net begins to tremble chaotically, producing infinite values in the equations. Spacetime curvature ceases to be smooth. Usually, quantum corrections to gravity explode into infinities.

Imagine a giant orchestra where every musician is a particle born in the fire of the Big Bang. When time's baton gives the downbeat, these particles begin to decay into lighter ones. Each decay is accompanied by a quiet chord — the emission of gravitons, tiny quanta of gravity.

Right after the Big Bang, space swelled at an unimaginable speed — that's inflation. Now expansion has slowed, but galaxies are moving apart faster and faster. The culprit behind the acceleration has been named dark energy.

Stephen Hawking imagined that the universe was born not from an explosion, but by smoothly emerging from a quantum 'nothing.' In worlds with concave geometry (anti-de Sitter space), such a birth acquires a rim, like the hoop of a drum.

A guitar string decides for itself how tightly to stretch when a musician plays a chord in different octaves. That's roughly the picture physicists paint when they try to unite gravity, the weakest but most pervasive force, with the quirky laws of the microcosm.

A warp drive creates a bubble around the ship, compressing space in front and stretching it behind. This is a clever way to bypass the speed-of-light limit without violating Einstein's theory.

Einstein's theory of gravity explains how mass warps space and time. Its predictions have been tested near Earth, but the most grueling tests lie around black holes, where distortions reach their breaking point. Until now, astronomers saw them as blurry smudges on an underexposed photo.

In the first second after the Big Bang, the universe was a boiling cauldron of energy. Particle-antiparticle pairs were born and instantly annihilated each other. But for every billion such pairs, one particle of matter survived, and that leftover became stars, planets, and people. New research blames modified gravity for this.

A stone falls to the ground along the most economical route — the one that expends the least time and energy. This principle of least action governs all of classical physics. But in the microworld, everything jitters: fields fluctuate, and you can't say precisely where anything is.

A rubber band stretched in different directions behaves differently from a whole sheet of paper. If you pull on an edge, the deformation does not spread evenly but seems to linger in certain spots.

A distant orchestra plays a symphony, but the recording is full of noise. The melody is written for two instruments, yet a third voice seems to break through the static — quiet and unfamiliar. This is how astrophysicists listen to gravitational waves, trying to catch the slightest deviations from Einstein's predictions.

A stone thrown into a pond creates ripples that spread far. But if you throw it into thick jelly, the waves die out almost immediately. That's roughly how an additional gravity field behaves under different mathematical descriptions. The authors built a unified approach to calculate how such a field manifests itself in the Solar System.

A snowdrift grows by strict rules: falling snowflakes form bumps, those attract more snow, and the bumps amplify. The same mathematics describes city growth, patterns on bark, ripples on water, and clusters of cosmic dust. Physicists call this uniformity universality.

In one hospital ward it's +40°C, in another +10°C, and on average it's +25°C. If temperature affected something nonlinearly (say, the rate of bacterial growth) the average would give a wrong prediction. The same problem arises when cosmologists describe a universe where galaxies are packed into clusters and voids yawn between them.

Light from distant galaxies passes through clumps of matter, and the gravity of those clumps slightly bends its path, like a lens. By measuring how much the images of distant galaxies are distorted, we can infer the distribution of matter and how gravity affects light.