Gigastructures

Gigascale engineering is the construction of galactic megastructures on a scale of at least 1 million km in length (which is 10⁹ metres hence the prefix "giga"). These biospheres can only be made by Type III civilizations or beyond.


Birch planet	A Birch planet would be a shell world with a diameter of roughly 1 light year with Earth-like gravity which surrounds a supermassive black hole that is 1.5 trillion times more massive than the Sun. Material needed would be all 400 billion stars and all of the planets in the Milky Way galaxy, to build a single planet which has more surface area than all of the planets in the Milky Way combined. The crust region is composed of 280 million layered shell worlds totalling 355 septillion Earths-worth of surface area, which is equivalent to about a million fully Dyson-sphered Type III galaxies.
Megaring	A Megaring is a mammoth ringworld stretching twenty light-years in diameter. The flat habitable surface is a million km wide, with 60,000-km-high atmospheric retaining walls at the edges. The surface gravity of 1 milligee is created by the 10%c rotation of the ring, so a man weighing 70 kg on Earth would weigh 70 grams on Megaring. If he could jump 1 meter on Earth, he can leap at least a kilometer on Megaring, and effectively fly without any technological aid. Most of its mass would consist of more than 100,000 suns circling the Megaring in helical orbits at a distance of 1 AU. The total energy cost of this project is up to 10⁴⁶ J, allowing a galactic culture to create this exotic habitat with a livable surface area of more than a trillion Earths. A Big Megaring is a ringworld a thousand light-years in radius, spinning at 10%c along the circumference. It looks like a hollowed-out Alderson disc, 600 million km wide and a million km deep with kilometer-thick walls. The Big Megaring structure takes only 34 million solar masses, but 3 billion solar masses of air are needed to provide a 1 atm base pressure. The estimated assembly energy is 10⁵¹ J, with a quasar at the center for heat and light. This is a truly gigantic space habitat which affords a living area equivalent to 130,000 trillion Earths or more than 240 million Dyson Spheres.
Megadisk	A Megadisk is a scaled up Megaring. The radius of the central hole is 1 light-year, and the distance to the outer edge is 10 light-years. The base structure is 100 km thick and the total weight is 11 galactic masses (1M_G = 3 x 10⁴¹ kg), of which 10 are in the Megadisk frame and 1 is air, providing a 1 atm pressure on both sides of the disk. The energy required for assembly is about 10⁵⁴ J, so this might be handled by cooperating early Type III civilizations or a late Type III civilization that has already colonized thousands of galaxies and can acquire the 11 galaxies of mass to build such a goliath. The Megadisk would provide a living area equivalent to 10²⁰ Earths or nearly a thousand Big Megarings.
Megasphere	The Megasphere is the ultimate feat in gigascale engineering. It is a Dyson Sphere 200 light-years in diameter, with a surface area of more than 130,000 square light-years. The base structure is 10 km thick, and both sides are equally habitable. About 1300 galactic masses are needed as raw materials, with 10⁵⁶ J required for assembly, and another 10⁵⁶ J to maintain the Megasphere at a comfortable 25 ºC for a billion years. This is a Type IV endeavor.