modified gravity

A light ray passing near a massive body bends around it, like a runner rounding the corner of a stadium. For a neutron star, this deflection angle is usually assumed to be the same as for a point mass. But if the star is wrapped in an invisible halo of dark matter, the picture changes: light passing through the halo is deflected less than the total mass would predict.

As planets orbit the Sun, they don't just follow their paths: they give each other a slight tug. It's like a dance where each partner feels the movements of the others. Scientists set out to determine how strongly this mutual attraction alters the planets' trajectories. It turns out there are small corrections that were previously overlooked.

A galaxy's speedometer is pegged. Outer stars orbit so fast that, by all laws of gravity, they should fly off like spray from a spinning wheel.

Stare long enough into the night sky, and sooner or later your gaze hits a blackness beyond which nothing is visible. This is not just a distant fence: it is the edge of the universe accessible to us, the cosmic horizon. Light from more distant stars simply hasn't had time to reach us.

An invisible scaffold holds galaxies together, keeping them from flying apart. This is dark matter — matter that doesn't shine and barely interacts with ordinary stuff. Astrophysicists have long debated what it's made of.

The cosmological standard model is like a recipe for a cake where everything is strictly measured: dark matter is like flour without a single lump (zero pressure), and dark energy is like the perfect leavening agent (equation of state exactly minus one). But what if the flour is a bit damp and the leavening agent has lost its oomph?

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.

In 2017, detectors picked up a gravitational wave from the collision of two neutron stars. Just 1.7 seconds later, telescopes saw a flash of gamma-ray burst from the same spot. That tiny delay became a treasure for physicists.

In the vast cosmic ocean, dark matter gathers into clumps and filaments, like plankton in currents. But if gravity doesn't work exactly as Einstein said, these patterns change — barely noticeably, like ripples from an invisible wind. You can spot such ripples by comparing sky maps with computer simulations.

In the vast cosmic ocean, dark matter gathers into clumps and filaments, like plankton in currents. But if gravity doesn't work exactly as Einstein said, these patterns change — barely noticeably, like ripples from an invisible wind. You can spot such ripples by comparing sky maps with computer simulations.

Cosmologists are like cartographers — only instead of mountains and rivers, they map invisible forces. Dark energy, which drives the accelerating expansion of the universe, remains a mystery. One idea is the 'kinetic gravity braiding' (KGB) model. In it, dark energy interlocks with gravity. The celestial map's paper becomes elastic. It alters the path of light rays.

A stone falls into still water — ripples spread across the surface. A black hole behaves similarly when something falls into it or when it merges with another black hole. The hole begins to oscillate, and these oscillations propagate through space as gravitational waves.

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).

The Standard Model of particle physics is a catalog of all known building blocks of the universe: electrons, quarks, and others. But it is silent about dark matter and doesn't get along with gravity.

Gravity is a remarkably weak force. A small magnet holds a paperclip against the pull of an entire planet. Physicists have long sought the reason for this weakness, and one guess leads us into hidden dimensions. Perhaps gravity 'leaks' into them, losing strength in our three-dimensional world.

Two galaxies (our Milky Way and its satellite, the Large Magellanic Cloud) slowly circle each other. Their gravitational pull stirs the distant outskirts of the Milky Way, like a pebble thrown into a pond makes the water ripple.

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⊙.

A neutron star is a city the size of Manhattan that weighs as much as half a million Earths. A teaspoon of its material would outweigh Mount Everest. Now, into this superdense sphere, something invisible and elusive has been mixed — dark matter particles. That's exactly what physicists did in a new study.

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.

A light ray passing by a massive body gets deflected: this is the effect of gravitational lensing, familiar from images of distant galaxies. The astrophysicists started with a rotating black hole (Kerr spacetime) and added a tiny correction to how light itself propagates.

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.

Two figure skaters on a lake frozen to the horizon. They are so far apart that their silhouettes turn into dots, and their mutual attraction becomes almost an article of faith. By Newton's law, they should circle in an unimaginably slow waltz. But telescopes see otherwise.

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 lead sphere hangs above another lead sphere. According to the usual laws of gravitation, they attract each other exactly along the vertical — along the line connecting their centers.

Hot gas spirals inward, falling onto an invisible superdense object. Such a disk resembles a giant whirlpool, but it is often tilted: its axis does not align with the rotation axis of the central body. The gravity of the massive object makes the disk wobble slightly, like a spinning top losing its balance.

The world around us is three-dimensional, but perhaps that's just an illusion. Some theories allow for extra dimensions of space, curled up or stretched in special ways. One such model depicts a five-dimensional universe with a "diluted" dimension (called a linear dilaton).

An endless ocean where every wave carries information about what happens in the depths. That is roughly how our Universe works under the holographic principle: the physics inside a volume can be described by processes on its boundary.

Dwarf galaxies orbiting the Milky Way look as if they’ve survived a gravitational storm. Their stellar bodies are stretched, scattered by tidal tails, and heated from within.

In the first moments after the Big Bang, the universe underwent a rapid expansion — inflation. Like a balloon inflated not uniformly but with tiny pulsations, it stretched billions of times over a fraction of a second. These pulsations froze into density ripples, from which galaxies later grew.

In the ordinary world without dark energy, the mass of any object is always positive. That seems obvious: positive mass attracts, creating gravity. But the real universe has dark energy — a mysterious force making the cosmos expand faster and faster.

Almost a century after Фриц Цвикки suspected something was amiss in the motion of galaxies in the Coma cluster, and Вера Рубин measured stellar velocities in spiral arms, the nature of тёмной материи remains one of the most vexing mysteries.

A teaspoon of neutron star matter weighs as much as a mountain. But this mountain may have an invisible twin. Deep inside a neutron star, ordinary matter is compressed to unimaginable densities. There, protons and neutrons can dissolve into a quark soup.

At the centers of many galaxies lurk black holes — regions with gravity so strong that even light cannot escape. Stars often orbit them, and sometimes one of those stars is a pulsar, a cosmic lighthouse sending out strictly periodic radio pulses. When such a pulsar moves behind a black hole, its signals reach us with a delay.

A large underground tank of liquid xenon waits for a rare event — a nudge from a dark matter particle. Normally, gravity is too weak to notice such a nudge. But if space has hidden dimensions, everything changes: at tiny distances, gravity suddenly strengthens, and the chance of catching an elusive particle grows.

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.

The vacuum is not as empty as it seems. If an observer moves with acceleration, the vacuum around them heats up: this was predicted by William Unruh back in the 1970s. The effect is akin to black hole radiation, but in flat space.

The Universe behaves like a soap bubble swelling in a multidimensional foam. The dark bubble model explains why it's expanding ever faster: the spacetime bubble grows, and its membrane gains a tiny thickness — about a micron, like a bacterium. At such distances, gravity 'blurs' across this thickness and weakens.

A flashlight beam travels straight into the dark: that's how we usually think of light. But near a massive body, like a star or a black hole, space is curved, and the beam bends smoothly, as if guided by an invisible hand. The authors of a new study have worked out exactly how gravity alters the familiar laws of electricity and magnetism.

Drop a pebble into a pond — ripples spread across the water. In much the same way, a massive body circling a black hole perturbs the fabric of spacetime, generating gravitational waves. But what if the pond itself isn't flat, but has hidden folds — extra dimensions invisible to the eye?

Our universe isn't just expanding: it's doing so faster and faster. It's like raisin bread dough in the oven: the raisins move apart, and the farther they've already separated, the faster they continue to recede. The strange force pushing the cosmos apart is called dark energy. Most scientists think it's constant, like a permanent spring built into the very fabric of space.