The Last Astronomers Will See a Different Universe

Point a telescope at the dark between the stars and you'll find galaxies, hundreds of billions of them, and almost every one is racing away from us. That motion is one of the ways we know the universe had a beginning. In the far future, it will be gone. In 2007, physicists Lawrence Krauss and Robert Scherrer worked out what happens to the evidence for the Big Bang in a universe whose expansion keeps speeding up. In this video we follow the three clues that told us the universe began (Slipher's and Hubble's receding galaxies, the afterglow Penzias and Wilson heard as a hiss in 1965, and the hydrogen-helium recipe of the first few minutes), the 1998 surprise that the expansion is accelerating, and the cosmic event horizon that will carry every other galaxy out of sight. Then we take the clues away one at a time, and meet the last astronomers: careful, brilliant, living under red dwarf suns trillions of years from now, who will measure everything right and still conclude that their galaxy is all there is. Chapters 0:00 The empty sky 0:56 I. The three clues 3:47 II. The surprise 4:45 III. The horizon 6:13 IV. The erasure 7:54 V. The last astronomers 9:41 VI. The window What do you think we're already missing? Tell me in the comments. A note on what this is: the far-future timeline follows Krauss and Scherrer's paper, which assumes dark energy stays constant. Recent DESI measurements hint that it may be changing (about 3 sigma, not yet a discovery), so the ending of this story isn't settled. The timing of the Milky Way–Andromeda merger is also uncertain: one 2025 study puts the chance of a merger within 10 billion years at about 50%, a 2026 study at about 90%; either way the pair stays bound. The deep field, the diagrams and the readouts are illustrations, not real data. Sources Krauss & Scherrer (2007), The Return of a Static Universe and the End of Cosmology, Gen. Rel. Grav. 39, 1545 https://arxiv.org/abs/0704.0221 Loeb (2011), Cosmology with hypervelocity stars, JCAP 04, 023 https://arxiv.org/abs/1102.0007 Loeb (2002), The long-term future of extragalactic astronomy, Phys. Rev. D 65, 047301 https://arxiv.org/abs/astro-ph/0107568 Hubble (1929), A relation between distance and radial velocity among extra-galactic nebulae, PNAS 15, 168 https://doi.org/10.1073/pnas.15.3.168 Alpher & Herman (1948), Evolution of the Universe, Nature 162, 774 https://doi.org/10.1038/162774b0 Penzias & Wilson (1965), A Measurement of Excess Antenna Temperature at 4080 Mc/s, ApJ 142, 419 https://doi.org/10.1086/148307 Riess et al. (1998), Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant, AJ 116, 1009 https://arxiv.org/abs/astro-ph/9805201 Perlmutter et al. (1999), Measurements of Omega and Lambda from 42 High-Redshift Supernovae, ApJ 517, 565 https://arxiv.org/abs/astro-ph/9812133 Laughlin, Bodenheimer & Adams (1997), The End of the Main Sequence, ApJ 482, 420 https://doi.org/10.1086/304125 Sawala et al. (2025), No certainty of a Milky Way–Andromeda collision, Nature Astronomy https://doi.org/10.1038/s41550-025-02... Wu, Huang, Zhang, Sun & Shao (2026), The Fate of the Milky Way–Andromeda System: To Merge or Not? https://arxiv.org/abs/2603.22863 DESI Collaboration (2025), DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints https://arxiv.org/abs/2503.14738 #space #cosmology #astronomy