How Big Will Our Universe Be in 100 Years?

How much wider than today’s 93 billion light-years will the universe be a hundred years from now?

Welcome to FreeAstroScience. A year ago we answered this question with a growth figure we can no longer defend, so this August 2026 rewrite redoes the arithmetic in the open and corrects three of its numbers, then turns to the missions that will redraw the map faster than expansion ever could.

Over the next 100 years the observable universe will widen by roughly 900 light-years, about 9 parts in a billion of its 93-billion-light-year diameter. The gain comes from old light newly arriving plus cosmic expansion pushing the visible horizon outward at just over three times the speed of light.

Everything we can see has an edge, and that edge is racing away at about 3.2 times the speed of light. Nothing in physics forbids it, because Einstein’s limit applies to objects moving through space, not to space itself stretching. Accept that one strange fact and the question of how fast the universe is expanding, and what a single century adds to it, becomes arithmetic you can check at a kitchen table.

The map grew almost a million-fold in one century

In the early 1920s the settled answer to the size of everything was the Milky Way, about 100,000 light-years across. Edwin Hubble broke that ceiling twice within a decade. First he showed the Andromeda spiral sits far beyond our galaxy, and then, in a 1929 paper built on just 24 galaxies, he found that the farther a galaxy lies, the faster it recedes. His original rate came out near 500 km/s per megaparsec, roughly seven times today’s value, because his distance calibration was badly off. The relation held anyway.

Divide today’s 93-billion-light-year map by the 1920 one and the century’s growth factor lands near a million.

How fast is the universe expanding right now?

Two methods disagree. Planck’s map of the cosmic microwave background, filtered through standard cosmology, gives 67.4 ± 0.5 km/s/Mpc. Adam Riess and the SH0ES team, who calibrated Cepheids and 42 Type Ia supernovae across more than 1,000 Hubble Space Telescope orbits, measure 73.04 ± 1.04. Since 2022 the gap has stood at 5 sigma, odds of about one in three and a half million that chance alone produced it.

Table 1 — The two headline measurements of the expansion rate (Planck 2020 and SH0ES 2022)

Since March 2025 a second argument sits on the table. The DESI collaboration’s second data release, built from more than 14 million galaxies and quasars, prefers a dark energy that weakens over time, at 2.8 to 4.2 sigma depending on which supernova sample joins the fit. We covered the force behind all this in our explainer on dark energy and whether it could rip the universe apart; the short version is that even the engine of the acceleration is now in play.

What one more century actually buys

We rewrote this page in August 2026 partly to correct it.

Our 2025 version claimed the observable universe would grow by about 200 light-years in a century, and we cannot reconstruct where that figure came from, because the honest arithmetic gives a bigger number. Today’s map, the one we walked through in our explainer on why the observable universe spans 93 billion light-years, has a radius of about 46.5 billion light-years. That earlier piece answers how big the view is now. This one moves the clock forward.

growth per year = c + H₀ × R

In words: each year the visible radius gains one light-year of newly arrived light, c, plus whatever the expansion adds, which is the Hubble rate H₀ multiplied by the radius R of 46.5 billion light-years. Feed in Planck’s 67.4 km/s/Mpc and the expansion term comes to about 3.2 light-years per year. Use the SH0ES value of 73.04 instead and it rises to about 3.5. Either way the radius gains between 4.2 and 4.5 light-years a year, which compounds to 420 to 450 light-years of new radius per century, or roughly 900 light-years of new diameter.

Now shrink it. Nine hundred light-years against 93 billion works out to about 9 parts in a billion per century. Apply that same fraction to Earth and our planet would widen by roughly 12 centimeters in a hundred years, about the length of your palm. Nobody will measure the difference. Not from inside the map.

The instruments will outrun the expansion

Measured in light-years, a century of expansion buys almost nothing — measured in instruments, the same century is enormous. NASA’s Nancy Grace Roman Space Telescope had a formal commitment to launch no later than May 2027, and in April 2026 the agency moved the target up to early September 2026 on a Falcon Heavy. Roman’s program executive Lucas Paganini expects first images by the end of this year, which makes the month we are writing this, August 2026, the last quiet one before the data starts.

ESA’s LISA follows in 2035: three spacecraft flying a triangular formation with arms 2.5 million kilometers long, built to catch gravitational waves from merging supermassive black holes. Back in 2025 we dated it 2037, and the adopted mission plan says 2035, so we fixed that too.

Could the whole thing be infinite?

Planck’s geometry test finds space flat to within measurement error, a curvature parameter of 0.001 ± 0.002, and perfect flatness is exactly what an infinite universe would look like. It is equally what a finite universe far larger than our view would look like. No observation now on the books can split those two outcomes, and we will not pretend otherwise.

A floor does exist. Mihran Vardanyan, Roberto Trotta, and Joseph Silk showed in 2011 that if space curves at all, its curvature radius exceeds 42 gigaparsecs, about 137 billion light-years, at 99 percent confidence, which packs at least 251 Hubble volumes into the whole, several times everything we can see. Readers of the 2025 text met a minimum width of 37 trillion light-years here instead, and we removed that claim because we could not trace it to any published measurement. Where such a floor comes from, and why a flat, uniform cosmos needs an inflationary burst at the start, is a story we told in our piece on why the Big Bang needs cosmic inflation.

One more cut: the multiverse section is gone. Nothing this article measures touches it, and the idea deserves a piece of its own rather than a paragraph of garnish.

Three numbers here could still move

Our growth figure inherits the Hubble tension whole: until 67.4 versus 73.04 resolves, the honest answer spans 420 to 450 light-years of new radius per century, and we quoted the spread rather than picking a side. DESI’s preference for evolving dark energy sits at 2.8 to 4.2 sigma, below the 5-sigma bar physics treats as discovery, so read it as a lead worth following rather than a verdict. Launch dates move as well, and Roman’s September 2026 window was itself shifted from a May 2027 commitment, in the rare happy direction.

A map that outgrows its own expansion

What this rewrite established: a century adds roughly 900 light-years to a 93-billion-light-year map, a change of 9 parts in a billion that no instrument will register, and the rate behind that arithmetic is still a 5-sigma standoff between 67.4 and 73.04. Meanwhile Roman flies next month and LISA in 2035, so the map will be redrawn by hardware long before expansion budges it.

We tore up our own 2025 answer for you because a wrong number left standing quietly cheats every reader who trusted it, and putting hard cosmology into words you can check for yourself is the entire point of FreeAstroScience. So check us: run the kitchen-table arithmetic and tell us if you land somewhere else, since a mind that keeps testing what it reads is the one tool no telescope replaces. Come back when Roman’s first images arrive around the end of 2026; we will rerun these numbers against whatever it sees. FreeAstroScience, Rimini. Gerd Dani.

Sources

1. Hubble, E. (1929). A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae. Proceedings of the National Academy of Sciences, 15(3), 168-173. DOI 10.1073/pnas.15.3.168. https://www.pnas.org/doi/10.1073/pnas.15.3.168
2. Planck Collaboration; Aghanim, N., et al. (2020). Planck 2018 results. VI. Cosmological parameters. Astronomy and Astrophysics, 641, A6. DOI 10.1051/0004-6361/201833910. https://www.aanda.org/articles/aa/full_html/2020/09/aa33910-18/aa33910-18.html
3. Riess, A. G., Yuan, W., Macri, L. M., et al. (2022). A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km/s/Mpc Uncertainty from the Hubble Space Telescope and the SH0ES Team. The Astrophysical Journal Letters, 934, L7. DOI 10.3847/2041-8213/ac5c5b. https://arxiv.org/abs/2112.04510
4. DESI Collaboration; Abdul-Karim, M., et al. (2025). DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints. Physical Review D, 112, 083515. DOI 10.1103/tr6y-kpc6. https://arxiv.org/abs/2503.14738
5. Vardanyan, M., Trotta, R., and Silk, J. (2011). Applications of Bayesian model averaging to the curvature and size of the Universe. Monthly Notices of the Royal Astronomical Society: Letters, 413(1), L91-L95. DOI 10.1111/j.1745-3933.2011.01040.x. https://arxiv.org/abs/1101.5476
6. LISA Consortium. Mission overview: launch expected 2035. https://www.lisamission.org/mission/
7. Aerospace America, AIAA (2026). NASA targeting September to launch Nancy Grace Roman Space Telescope. Published April 22, 2026. https://aerospaceamerica.aiaa.org/nasa-targeting-september-to-launch-nancy-grace-roman-space-telescope/