Based on everything we currently measure, yes. The universe will almost certainly expand forever. Every major line of evidence, from the afterglow of the Big Bang to the brightness of distant exploding stars, points toward a cosmos that keeps stretching indefinitely, growing colder and emptier over trillions of years. But there’s a twist: recent findings have introduced just enough uncertainty to keep the question scientifically alive.
Why Physicists Think Expansion Won’t Stop
The answer comes down to two things: geometry and energy. The Planck satellite, which mapped the oldest light in the universe with extraordinary precision, found that space is flat to within a fraction of a percent. The technical measurement of spatial curvature came in at 0.0007 ± 0.0019, essentially zero. A flat universe doesn’t have enough matter or energy to curve back on itself and collapse. It just keeps going.
Then there’s dark energy, the mysterious force accelerating the expansion. The standard model of cosmology, called Lambda-CDM, describes a universe made of roughly 71% dark energy, 24% dark matter, and about 5% ordinary matter. Dark energy acts like a kind of anti-gravity, pushing space apart faster over time. As the universe expands and matter thins out, dark energy’s share of the total energy budget only grows, meaning its influence gets stronger, not weaker. Under these conditions, expansion doesn’t just continue. It accelerates.
The total density of the universe sits at about 1.02 ± 0.02 times the critical density, the exact threshold between eventual collapse and endless expansion. That value is so close to the tipping point that physicists consider it effectively balanced on the side of perpetual growth.
The Hubble Tension: A Crack in the Picture
There is, however, a persistent puzzle. Two different ways of measuring the universe’s expansion rate produce two different answers, and nobody can fully explain why. Measurements based on the cosmic microwave background (the afterglow radiation from shortly after the Big Bang) give a rate of about 67.4 km/s per megaparsec. Measurements based on nearby supernovae and pulsating stars give about 73 km/s per megaparsec. That gap, known as the Hubble Tension, has reached a statistical significance of 5 to 6 sigma, making it very unlikely to be a fluke.
In 2023 and 2024, the James Webb Space Telescope confirmed that the higher, nearby measurements from the Hubble Space Telescope were accurate. This didn’t resolve the tension. It deepened it. As Nobel laureate Adam Riess put it, “Hubble measurements remain reliable as we climb farther along the cosmic distance ladder.” Something about our understanding of the universe, possibly the behavior of dark energy itself, may be incomplete. The discrepancy doesn’t necessarily mean the universe will stop expanding, but it does suggest the standard model might need revision.
What “Expanding Forever” Actually Looks Like
If the universe does expand forever, the long-term forecast is bleak in a way that’s hard to grasp at human timescales.
In a couple of trillion years, expansion will have carried distant galaxies so far away that their light can no longer reach us. The Milky Way, by then merged with its neighboring galaxies, will appear to float alone in an observable universe that looks nearly empty. About 100 trillion years from now, all star formation will cease. The raw materials, hydrogen and helium gas, will have been used up or scattered too thinly to collapse into new stars. This marks the end of what astronomers call the Stelliferous Era, the age of stars we live in now.
After that comes the Degenerate Era. Galaxies as we know them will be gone. The universe will contain only the remnants of dead stars: white dwarfs cooling toward absolute zero, neutron stars, and black holes. If protons eventually decay (with a theoretical half-life around 10^30 years), even those remnants will dissolve into a thin soup of electrons and their antimatter counterparts. That plasma would be too sparse to interact meaningfully, leaving a universe with almost no structure at all.
The final state is sometimes called heat death. It’s not a dramatic event. It’s the opposite: a universe that has reached maximum entropy, where all useful energy has been converted to waste heat spread so thin across such vast distances that nothing can happen anymore. No work, no change, no complexity. Just an ever-expanding void at a temperature barely above absolute zero.
Could the Universe Collapse Instead?
For most of the 20th century, the big question was whether gravity would eventually slow expansion enough to reverse it, pulling everything back into a “Big Crunch.” The discovery of dark energy in 1998 seemed to rule that out. But the idea hasn’t completely disappeared.
A November 2025 study suggested that the universe’s expansion may actually be slowing, not accelerating, which would challenge one of the most celebrated findings in modern physics. If dark energy is not constant but instead weakening over time, and if it eventually turns negative, the universe could theoretically reverse course and contract. That scenario would replace eternal expansion with a collapse back to extreme density.
This remains speculative. The evidence for accelerating expansion is strong and built on decades of independent observations. But the possibility that dark energy changes over time is something cosmologists are actively investigating, particularly with new data from instruments like the Dark Energy Spectroscopic Instrument (DESI).
The Big Rip: A More Violent Alternative
There’s also a scenario where expansion doesn’t just continue but runs away catastrophically. If dark energy intensifies over time rather than staying constant, it could eventually overpower every force in nature. First it would tear apart galaxy clusters, then individual galaxies, then solar systems, then planets, then atoms themselves. This is called the Big Rip.
The Big Rip depends on a single parameter in the equations describing dark energy: a value physicists call “w.” If w equals exactly negative one, dark energy stays constant and the universe expands forever at an accelerating but manageable rate. If w drops below negative one, dark energy grows without limit and eventually rips apart all matter in a finite amount of time. Current measurements place w very close to negative one, consistent with a constant dark energy. The Big Rip is not ruled out, but it’s not favored by existing data.
What We Know for Certain
The universe is expanding right now, and that expansion is accelerating. Space is geometrically flat. Dark energy dominates the energy budget. Every mainstream measurement supports a universe that will expand forever, growing colder and darker until it reaches a state of maximum entropy with no usable energy left. That’s the most probable outcome based on current physics.
The caveats are real but narrow. The Hubble Tension hints that our model is incomplete. A handful of recent studies raise the possibility that dark energy isn’t perfectly constant. If dark energy weakens or reverses, the long-term forecast changes dramatically. But those are open questions, not established results. For now, the best answer cosmology can give is: yes, the universe will very likely expand forever, and the ending will be quiet, cold, and extraordinarily slow.