Will our Galaxy merge with another Galaxy in the future? What are the potential consequences of this event?

 The Andromeda Galaxy is hurtling toward the Milky Way at 110 kilometers per second. Yet when they collide, virtually not a single star will touch another.

Long modeled as an inevitable head-on impact in four to five billion years, recent astrometric data from the Gaia space telescope shows the encounter might be a glancing blow or delayed, though the two giants will eventually coalesce. Even earlier, the Milky Way is expected to absorb smaller neighbors, including the Large Magellanic Cloud in roughly 2.4 billion years.

Galaxies, after all, are mostly empty space. If the Sun were scaled down to the size of a ping-pong ball, the nearest neighboring star would be another ball roughly a thousand kilometers away. Because stellar distances are so vast compared to stellar diameters, the two stellar populations will pass through each other like ghostly swarms without their individual suns crashing together.

The consequences will instead be gravitational and atmospheric. Interstellar gas clouds, unlike stars, are diffuse and sprawling; when they collide, the resulting friction and compression trigger a period of rapid star birth known as a starburst. Over several passes spanning hundreds of millions of years, mutual gravitational tides will strip away the familiar spiral arms of both galaxies, hurling streamers of stars across intergalactic space. The Solar System has about a fifty percent chance of being flung into the outer tidal halo, but the planets themselves will remain firmly tethered in their orbits around the Sun.

Eventually, the two central supermassive black holes—Sagittarius A* and Andromeda's far larger monster—will sink toward the combined galactic core through dynamical friction. After orbiting each other for millions of years, they will merge into an even larger gravitational anchor, releasing gravitational waves.

The final result will be a single, smooth elliptical galaxy, often dubbed "Milkomeda." Any distant observers watching the event would see the night sky transform from a quiet ribbon of light into a chaotic blaze of young blue stars, before fading into an ambient, featureless glow. By that point, however, Earth will be long uninhabitable, as the Sun's steadily increasing luminosity will have boiled off the oceans billions of years earlier.

An artist impression of the night sky as the Andromeda Galaxy and the Milky Way begin their merger. Source: Wikimedia Commons.

How do black holes merge? What happens after it does?

 When black holes collide, the mass of entire suns vanishes in a split second. The merger releases more energy than every star in the observable universe combined, yet emits no light.

The process begins long before the actual impact. Black holes that find themselves trapped in each other’s gravitational pull enter a phase called the inspiral. As they orbit, their immense mass churns the fabric of spacetime, creating ripples known as gravitational waves. These waves carry energy away from the binary system. As the system loses energy, the black holes are drawn closer together, orbiting faster and faster. By the time they are within a few hundred miles of each other, they are moving at a significant fraction of the speed of light.

When the two event horizons finally touch, they fuse to create a single, larger black hole. This new object is not immediately stable. For a brief moment, it is deformed and asymmetrical. To settle into a stable shape—a perfect sphere, or an oblate spheroid if it is spinning rapidly—the newly formed black hole must shed its imperfections. It undergoes a "ringdown" phase, vibrating like a struck bell and radiating the last of its distortions away as a final burst of gravitational waves.

The resulting single black hole is surprisingly lighter than its two parent objects. The missing mass is converted entirely into the energy of those spacetime ripples, following Einstein's $E=m{c}^2$. When the LIGO observatory detected the first black hole merger in 2015, the measurements revealed that two black holes weighing 36 and 29 solar masses combined to form a single 62-solar-mass black hole. In that fraction of a second, three solar masses were converted directly into pure wave energy.

Sometimes, the waves are not emitted equally in all directions. If the original black holes were different sizes or spinning at odd angles, the resulting gravitational waves can shoot out asymmetrically. This imparts a massive recoil velocity—a "kick"—to the final black hole. In extreme cases, this kick can reach thousands of miles per second, enough to completely eject the newly formed black hole from its host galaxy to wander intergalactic space alone.

A simulation of the first detected black hole merger, GW150914, showing how the intense gravity of the two black holes warps the starlight behind them. Photo by Simulating eXtreme Spacetimes Lensing (SXS) is licensed under CC BY-SA 4.0.

When two black holes merge, what will happen to their singularities?

 When two black holes merge, their event horizons touch first. For a fraction of a second, a single giant black hole contains two distinct singularities on an inescapable collision course.

To understand how this happens, you have to separate the "black hole" from the "singularity." The event horizon is simply the boundary where gravity becomes so strong that light cannot escape. The singularity is the actual physical object at the center—a region where, according to classical general relativity, matter is crushed to infinite density and spacetime curvature becomes infinite.

As the merging event horizons connect, they form a single, rapidly shifting, peanut-shaped boundary that quickly settles into a sphere.

Inside an event horizon, space and time behave differently. Space itself flows inward toward the center faster than the speed of light. Because all physical paths inside a black hole lead inexorably inward, the two singularities cannot orbit each other indefinitely or remain apart. They are dragged together and collide, merging into a single, more massive singularity. If the original black holes were spinning—which they almost always are—the newly combined singularity will not be a point, but a rapidly rotating ring, known as a ring singularity.

This new singularity does not contain the exact combined mass of the original two. In the final moments before they merge, the black holes whip around each other at a significant fraction of the speed of light, churning the fabric of spacetime. This violent acceleration converts a portion of the black holes' mass into pure energy, which radiates outward across the universe as gravitational waves. When the LIGO observatory detected the first black hole merger in 2015, the resulting singularity was three solar masses lighter than the two original black holes combined. Three entire suns' worth of mass had been transformed into gravitational ripples in a fraction of a second.

There is one major caveat to this process: quantum physics. Most physicists believe that true singularities—points of literally infinite density—do not actually exist. The concept of a singularity is likely just a placeholder indicating where current mathematical models of gravity break down. If modern theories like string theory or loop quantum gravity are correct, the centers of black holes might actually be hyper-dense quantum states, such as fuzzballs or Planck stars. If so, those two exotic objects simply smash together to form a larger, immensely dense core.

An artist's illustration of gravitational waves radiating from the merger of two massive objects. Photo by NOIRLab is licensed under CC BY 4.0.