modified gravity vs dark matter

An interpreter gets distracted by every rustle in the hall, and a one-hour meeting stretches into two. A neutron star compresses a solar mass into a sphere the size of a city, and the matter in its core is denser than an atomic nucleus. Physicists cannot probe its structure in the lab: the required pressures are unattainable.

Think of a merry-go-round: the farther you sit from the center, the faster you're carried around. For a long time, astronomers assumed galaxies work the same way — stars at the edges should orbit more slowly than those near the center. But back in the 1970s, it turned out that in many spiral galaxies, the speed of stars barely drops with distance from the center.

The most massive stars are like overinflated balloons: their envelope stops stretching and instead begins to furiously vent material. At a certain stage, the mass loss becomes so intense that the star stops growing in size. Astronomers see this moment as a kink in the diagram where stars are sorted by luminosity and temperature.

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⊙.

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.

After the Big Bang, dark matter and ordinary matter (mostly hydrogen) went their separate ways in terms of speed. Their gravitational pull acts like a bow gliding across a string: when frequencies match, resonance occurs, and sound waves start to rapidly grow in ordinary matter.

In a large gas cloud where stars are born, the magnetic field lines are roughly aligned in one direction, like the fibers in a piece of felt. But look inside, into the dense clumps that will eventually become stars, and the picture changes: there the lines are twisted every which way, like a ball of yarn after a kitten has played with it.

Previously, astronomers calculated galaxy mass as if they were perfect spheres. But a galaxy is a flat pancake. The new 'Lost and Found' model takes the actual shape into account: it sums up the gravitational pull from all parts of the disk, not just the central bulge. The result is stunning: many galaxies need a third less invisible dark matter than previously thought.

When stars at the edges of galaxies rotate faster than calculations based on visible mass predict, standard cosmology invokes the hypothesis of hidden dark matter — invisible substance that reveals itself only through gravity. A new study offers an alternative that requires no new particles.

Stars on the outskirts of galaxies rotate unexpectedly fast. According to Newton, the farther from the center, the slower the motion should be, like in a round dance, where the outer dancers move more slowly. But in reality, stellar outskirts race along almost as fast as the core, resembling a giant carousel: on it, the outer horses fly faster so as not to fall behind the inner ones.

A galaxy is like a giant merry-go-round: stars and gas whirl around the center, and their speeds reveal how much stuff is on the ride. If we only saw the luminous matter, the outer parts would spin slower than observations show. So there must be invisible mass, dark matter, that holds the galaxy together.

At the center of our Galaxy lies a supermassive black hole, with stars orbiting around it. Their motion resembles a cosmic dance: some race along elongated orbits, others follow nearly circular paths. If invisible dark matter exists near the black hole, it slightly distorts these orbits, like an unseen hand nudging a star off course.

The Gaia space telescope has measured how fast more than a hundred thousand stars spin. It turns out that a star's rotation speed depends on its temperature. For stars cooler than 6500 degrees, the surface moves at 10–20 kilometers per second.

Three distant galaxies spin like giant carousels. Their stars and gas move along spiral arms, and from the speed of this motion you can calculate how much total matter the galaxy contains — visible and invisible. Astronomers combined two views: radio telescopes revealed cold gas, while the James Webb Space Telescope showed the stars.