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Starts With A Bang

What Is The Ultimate Fate Of The Loneliest Galaxy In The Universe?

In the middle of a great cosmic void, a single, isolated galaxy persists amidst the darkness. It’s about to get a lot lonelier.

Here in our own cosmic backyard, the Milky Way is just one galaxy among many. A slew of satellites galaxies accompany us on our journey through the Universe, and our nearby neighbor Andromeda outclasses us in terms of mass, stars, and even physical extent. All told, we’re just one of perhaps ~60 galaxies bound in our local group, which itself is a modestly small galaxy group on the outskirts of the enormous Virgo Cluster.

Not every galaxy is so fortunate, however. While galaxies are most commonly found bound together in large numbers, there are enormous cosmic voids separating the rich structures found throughout the Universe, with only tiny amounts of matter inside. One remarkable example is the galaxy MCG+01–02–015, which is the only one around for some 100 million light-years in all directions. It’s the loneliest galaxy in the known Universe, and we can scientifically predict its ultimate fate.

Our local supercluster, Laniakea, contains the Milky Way, our local group, the Virgo cluster, and many smaller groups and clusters on the outskirts. However, each group and cluster is bound only to itself, and will be driven apart from the others due to dark energy and our expanding Universe. After 100 billion years, even the nearest galaxy beyond our own local group will be approximately a billion light years away, making it many thousands, and potentially millions (when you take the different stellar populations that will be inside) of times fainter than the nearest galaxies appear today. (ANDREW Z. COLVIN / WIKIMEDIA COMMONS)

To understand what this galaxy is going to do, first we have to understand what it’s like from the inside out. When the Universe was much younger than it is today, it was almost perfectly uniform, with regions that are only slightly overdense or underdense compared to the large-scale average. The regions with more matter than average will self-gravitate, drawing in matter from the surrounding volumes of space and eventually leading to the formation of stars, galaxies, and groups and clusters of galaxies on even larger scales.

Regions that are underdense, however, tend to give up their matter to the surrounding overdense regions, leading to vast cosmic voids between the strands of the cosmic web. Contrary to popular belief, however, even the regions of below-average density still tend to hang on to some amount of matter — both normal and dark — and with enough time, that matter will collapse to form structures, too.

Streams of dark matter drive the clustering of galaxies and the formation of large-scale structure, as shown in this KIPAC/Stanford simulation. While the locations where stars, galaxies, and clusters of galaxies emerge are most notable, the enormous cosmic voids separating the matter-rich structures are just as important to understanding our Universe. (O. HAHN AND T. ABEL (SIMULATION); RALF KAEHLER (VISUALIZATION))

The overwhelming majority of galaxies, today, can be found along the filaments of our cosmic large-scale structure, with enormous concentrations of galaxies existing at the nexus points of multiple filaments. It’s dark matter that drives the formation of this cosmic web — outmassing normal matter by a relatively consistent 5-to-1 ratio — while it’s the normal matter that collides, heats up, sheds momentum, and forms stars.

The matter remaining in a cosmic void, rather than undergoing a complicated story of gravitational growth from a series of mergers, will instead tend to form a large, isolated single galaxy via monolithic collapse. From a distance, a galaxy that forms like this might appear very similar to any other spiral galaxy, such as Andromeda, but there are important additional properties that only a more detailed investigation will reveal.

In between the great clusters and filaments of the Universe are great cosmic voids, some of which can span hundreds of millions of light-years in diameter. While some voids are larger in extent than others, the void that houses MCG+01–02–015 is special because it is so low in density that, rather than having only a few galaxies, it only contains this one known galaxy at all. It is possible, however, that small, low surface brightness galaxies may exist in this region after all, albeit below our present detection threshold. (ANDREW Z. COLVIN (CROPPED BY ZERYPHEX) / WIKIMEDIA COMMONS)

An extremely isolated galaxy, unlike their more common, more clustered counterparts, forms as follows.

  1. The regions that fail to give up all of their matter to the filamentary network that comprises our large-scale structure will gravitate towards their mutual center-of-mass, determined by the presence of both dark matter and normal matter.
  2. The dark matter forms a large, diffuse halo of mass, while the normal matter sinks to the center, colliding with other normal matter particles and collapsing in the shortest dimension first.
  3. The normal matter “pancakes,” which is the scientific term for “goes splat,” and forms a disk that starts rotating.
  4. Inside the disk, stars form, leading to the familiar spiral structure we recognize.
  5. Dark matter gets dynamically heated, changing its density profile somewhat, while low-mass neutrinos eventually fall into the halo, adding to the mass.

Subsequently, the normal matter goes through the normal stellar life cycle, leading to the isolated galaxies we see today.

The galaxy shown at the center of the image here, MCG+01–02–015, is a barred spiral galaxy located inside a great cosmic void. It is so isolated that if humanity were located in this galaxy instead of our own and developed astronomy at the same rate, we wouldn’t have detected the first galaxy beyond our own until we reached technology levels only achieved in the 1960s. This galaxy must be surrounded by an enormous, diffuse halo of both dark matter and neutrinos, in addition to the gas, plasma, dust, and stars found in the plane of the disk. (ESA/HUBBLE & NASA AND N. GORIN (STSCI); ACKNOWLEDGEMENT: JUDY SCHMIDT)

But the Universe is just getting started. Dominated by dark energy, distant galaxies will not only recede from one another, but their apparent recession speeds will increase faster and faster as time goes on. For galaxies like our own, we’ll remain bound to our local group, including Andromeda, Triangulum, and about 60 additional galaxies, until they all merge together many billions of years in the future. Galaxies beyond our gravitationally bound group, like those in the Virgo cluster, will remain bound to their own parent groups, but will accelerate in their recession from our own.

For an isolated, lonely galaxy, however, all of the galaxies and galactic groups will accelerate away. A galaxy like MCG+01–02–015 will remain isolated, forming stars in bursts lining its spiral arms for as long as new material to form new generations of stars remains.

The spiral galaxy NGC 6744, part of the LEGUS survey, showcases new star formation along the spiral arms, where gas and dust are plentiful, but none in the galactic center, which is overwhelmed with stars and contains little gas. Over relatively short timescales, as we look to the far future, practically all galaxies will see their star formation rates effectively asymtote to zero. (NASA, ESA, AND THE LEGUS TEAM)

Over the next few tens of billions of years, all the galaxies that can be seen will accelerate away, leaving only some highly redshifted photons behind. Besides those, 100 billion years from now, there will be no indication that any other galaxies ever existed within our visible Universe.

Star formation rates will continue dropping inside each galaxy, with Sun-like stars burning out and only the least massive stars — the red dwarfs and their failed-star (brown dwarf) counterparts — continuing to shine. As billions of years turn into trillions or even hundreds of trillions of years, even these stars will burn through all of their fuel. White dwarfs, the dead remnants of most stars, will eventually fade away to become black dwarfs, as they cool down to become completely invisible.

An accurate size/color comparison of a white dwarf (L), Earth reflecting our Sun’s light (middle), and a black dwarf (R). When white dwarfs finally radiate the last of their energy away, they will all eventually become black dwarfs. The degeneracy pressure between the electrons within the white/black dwarf, however, will always be great enough, so long as it doesn’t accrue too much mass, to prevent it from collapsing further. This is the fate of our Sun after an estimated 1⁰¹⁵ years. (BBC / GCSE (L) / SUNFLOWERCOSMOS (R))

After around a quadrillion (10¹⁵) years have passed, the last stellar remnants will have burned out, darkening the Universe. Only the occasional merger of multiple objects, such as brown dwarfs, will cause a temporary reignition of nuclear fusion, creating starlight for tens of trillions of years at a time. Those events will not only be rare, but will have to fight against a competing process.

All of the collapsed objects, which is where the normal matter will overwhelmingly wind up, will gravitationally interact. The random close encounters between masses will, over time:

  • lead to gravitational interactions and momentum exchange,
  • ejecting the lightest ones, hurling them into intergalactic oblivion,
  • and causing the heavier-mass objects to sink towards the center, losing momentum in a process known as violent relaxation.
Once star formation has completed in a galaxy, all of the gas and dust will be gone and locked up in individual bound objects, such as stars and stellar remnants. Over long enough timescales, not only will each and every star die, becoming a black hole, neutrons star, or white (and then eventually black) dwarf, but mutual gravitational interactions will either kick the stars/stellar remnants out of the galaxy or funnel them into the center, where they will merge into a single object. (NASA, ESA AND WOLFGANG BRANDNER (MPIA), BOYKE ROCHAU (MPIA) AND ANDREA STOLTE (UNIVERSITY OF COLOGNE))

After enough time has passed, somewhere around 10¹⁹ or 10²⁰ years, only a small percentage of those masses composed of normal matter will remain, largely in the form of black holes or stellar remnants. Yet the large, diffuse halo of non-normal matter — dark matter and massive neutrinos — will remain largely unchanged; the evolution of normal matter should have only negligible effects here.

As we add more zeroes to the age of the Universe, the central black hole will grow by devouring matter, flaring when it does. The planets that remain in orbit around dead stellar remnants will see their orbits decay via gravitational radiation, spiraling into their remnants. Eventually, all the normal matter will be either ejected or concentrated into massive and supermassive black holes. Still, that halo of dark matter and neutrinos will remain.

The simulated decay of a black hole not only results in the emission of radiation, but the decay of the central orbiting mass that keeps most objects stable. Black holes are not static objects, but rather change over time. For the lowest-mass black holes, evaporation happens the fastest, but even the greatest-mass black hole in the Universe won’t live past the first googol (1⁰¹⁰⁰) years. (EU’S COMMUNICATE SCIENCE)

As the aeons tick past and the Universe ages even more severely, black holes themselves will decay through the quantum process of Hawking radiation. Stellar mass black holes will evaporate on timescales of around 10⁶⁷ years, while the most massive black holes in today’s Universe might persist for around 10¹⁰⁰ years. If we were to examine the most isolated galaxy of all, its black hole is likely to last 10⁸⁰ to 10⁹⁰ years, but no more.

Yet even when that much time has passed, and the last black hole in the most isolated galaxy we know of has decayed away, dark matter and neutrinos will still exist in the same enormous halo-like configuration they always did. Even without normal matter to absorb or emit radiation, the skeletal structure of the galaxy — the dark matter and neutrinos that don’t interact with photons — will still persist.

Our galaxy is thought to be embedded in an enormous, diffuse dark matter halo, indicating that there must be dark matter surrounding everything from our solar system to nearby dwarf galaxies. For an isolated galaxy (or our own local group in the far future), the normal matter remnants will be ejected, will merge, and will decay, but the halo of dark matter and neutrinos will persist for far longer. These halos will be the last remaining structures in the Universe. (ROBERT CALDWELL & MARC KAMIONKOWSKI NATURE 458, 587–589 (2009))

After an extraordinary amount of time has passed, googols of years or even more, the loneliest galaxy in the Universe will appear completely empty. No stars, stellar remnants, planetary corpses or even black holes ought to remain. And yet, it will still exist. Someone who could measure the spacetime curvature of the Universe or somehow detect dark matter or ultra-low energy neutrinos would encounter an enormous, diffuse halo of mass that will persist for far longer than any bound structure made of normal matter.

Travel the Universe with astrophysicist Ethan Siegel. Subscribers will get the newsletter every Saturday. All aboard!

Eventually, dependent on the actual (and yet unknown) masses of individual dark matter particles and neutrinos, this remnant dark halo will decay, ejecting itself particle-by-particle until none remain. Until the masses and properties of those particles are known, however, we cannot calculate that timescale; we can only know it will persist longer than any normal matter will. The eventual fate of the last galaxies in the Universe will be a skeletal dark matter/neutrino halo, far outlasting anything else we’ve ever observed.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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