The Local Group

Everything humanity will ever physically reach, across the entire remaining lifespan of the universe, lies inside a 10-million-light-year bubble of roughly 80 galaxies. Cosmic expansion is already carrying the rest beyond any conceivable horizon. This bubble is the Local Group, and it is the whole future.

The shape of the neighborhood

Two large spirals dominate: the Milky Way (~200-400 billion stars) and Andromeda (M31, ~1 trillion stars, 2.537 million light-years away). Triangulum (M33) is a distant third at 2.73 million light-years and ~40 billion stars. Everything else is a dwarf — about 60 confirmed satellites of the two giants, plus a long tail of ultra-faint dwarfs the Dark Energy Survey and DES-like sky surveys have been pulling out of the noise since 2015. The full census is not closed; new dwarfs keep showing up.

Total mass is roughly 2-3 trillion solar masses, the overwhelming majority of which is dark matter. The barycenter sits somewhere between the Milky Way and Andromeda, both of which are falling toward it at about 110 km/s in mutual approach.

Galaxy	Type	Distance (ly)	Stars	Notes
Milky Way	SBbc barred spiral	0	200-400 billion	Home
dest andromeda (M31)	SA(s)b spiral	2,537,000	~1 trillion	Largest member, approaching
Triangulum (M33)	SA(s)cd spiral	2,730,000	~40 billion	Probable Andromeda satellite
dest large magellanic cloud	SBm irregular	160,000	~30 billion	Naked-eye satellite
Small Magellanic Cloud	SBm irregular	200,000	~3 billion	Tidal companion of LMC
Sagittarius Dwarf	dSph	70,000	~10 million	Being eaten by Milky Way
~50+ ultra-faint dwarfs	dSph/UFD	30k-1M	<100,000 each	Census still growing

Travel times, honestly

At 0.9c, a relativistic ship reaches the LMC in 77,500 years of ship time (178,000 years Earth time). Andromeda takes 1.23 million ship-years. The edge of the Local Group costs 4.84 million ship-years. These numbers assume technology that doesn't exist and energy budgets nobody can produce — at 0.9c, the kinetic energy of a 1-ton ship is roughly 10²⁰ joules, several thousand times annual world energy use.

Destination	Distance (ly)	0.9c Earth time	0.9c ship time
LMC	160,000	178,000 yr	77,500 yr
Andromeda	2.54M	2.82M yr	1.23M yr
Triangulum	2.73M	3.03M yr	1.32M yr
Local Group edge	~10M	11.1M yr	4.84M yr

For comparison: mission voyager 1, the fastest object humanity has ever launched on a permanent trajectory away from Earth, has covered 0.05% of the distance to Proxima Centauri after 48 years. Intergalactic travel as a biological project is not a hard problem; it is a category error. The only candidates are von Neumann probes on million-year timescales, tech alcubierre drive-class spacetime engineering, or whatever a post-biological civilization decides to send.

The one trip that doesn't need a ship

In roughly 4.5 billion years, the Milky Way and Andromeda will collide. The merger plays out over 1-2 billion years and produces an elliptical galaxy informally called Milkdromeda. Individual star systems are statistically safe — the mean distance between stars is so large that direct stellar collisions are essentially zero. The Sun's planetary system is more threatened by the Sun's own red-giant phase, which will likely sterilize Earth before the merger completes.

This is the only intergalactic event that requires no propulsion. The destination is coming to us.

What's contested

The total mass of the Local Group is uncertain by a factor of nearly two. Estimates from the "timing argument" (using the Milky Way-Andromeda approach velocity) give ~5 × 10¹² solar masses; estimates from satellite kinematics give closer to 2 × 10¹². The discrepancy matters because it changes the inferred dark matter halo structure and the predicted merger timing by hundreds of millions of years.

The dwarf galaxy census is also open. The "missing satellites problem" predicted hundreds more Milky Way satellites than were observed; surveys since 2015 have closed part of the gap, but the count of ultra-faint dwarfs is still rising and nobody is confident about the floor. Whether the Local Group is genuinely an isolated bound system or a transient configuration inside a larger flow is itself debated — Peebles and others have argued the Local Sheet structure complicates the "island universe" picture.

Why this has to do with other realms

The Local Group is the hard ceiling on any concept fermi paradox resolution that involves physical contact. If interstellar travel is already a million-year project at concept-relativistic-velocity-limits, then intergalactic contact requires either propagation timescales that dwarf the age of complex life on Earth, or a kind of civilization that has decoupled from biological substrate entirely. The Local Group also sets the boundary on cosmological eschatology: in roughly 100 billion years, accelerating expansion will redshift everything outside the merged Milkdromeda into invisibility. Future astronomers in that galaxy will look out and see only their own stars, and will have no empirical access to the existence of other galaxies at all. The observable universe shrinks to one's own neighborhood — which means the wiki entry on the Local Group is, in a long enough timeframe, the wiki entry on everything.

An open question

If a future civilization is forever confined to a single merged galaxy with no observational evidence of any others, does the concept of "universe" survive in their cosmology — or does it quietly disappear, the way we no longer talk about the celestial spheres?

Key sources

The Local Group of Galaxies by Sidney van den Bergh (2000, Cambridge) — the canonical reference text, dated but foundational.
van der Marel et al. (2012, ApJ) — proper motion measurement of M31 establishing the head-on collision trajectory.
Cautun et al. (2019, MNRAS) — revised Local Group mass from Gaia DR2 satellite kinematics.
to verify: recent Dark Energy Survey papers on ultra-faint dwarf discoveries (2015-2023 cadence).
Peebles & Nusser (2010, Nature) — Local Sheet flow context for the Local Group's dynamical state.

Abhishek