No. The universe is not slowing down. Its expansion is speeding up, and it has been for billions of years. This was one of the most surprising discoveries in modern science, and it overturned what physicists had assumed for most of the 20th century. The expectation was that gravity from all the matter in the universe would gradually put the brakes on expansion. Instead, something is pressing the accelerator.
Why Scientists Expected It to Slow Down
After the Big Bang roughly 14 billion years ago, the universe began expanding in all directions. For decades, the central question in cosmology was not whether expansion would slow down, but by how much. In a universe full of matter, every galaxy gravitationally pulls on every other galaxy. That mutual attraction should act like friction, steadily decelerating the expansion over time. The only real debate was whether gravity would eventually stop expansion altogether and pull everything back into a “Big Crunch,” or merely slow it to a crawl.
And for the first several billion years, that is exactly what happened. Matter was dense enough and close enough together that gravity dominated. The expansion rate did decrease. If you had measured the universe’s behavior during that early era alone, you would have concluded it was slowing down.
The 1998 Discovery That Changed Everything
In 1998, two independent research teams set out to measure how quickly the expansion was decelerating. They used a specific type of exploding star called a Type Ia supernova, which burns at a known brightness and works like a cosmic mile marker. By measuring how dim these supernovae appeared at great distances, the teams could calculate how much the universe had expanded since the light left those stars.
The results were baffling. Over 50 distant supernovae were dimmer than they should have been in a decelerating universe. The light had traveled farther than expected, meaning the space between us and those explosions had stretched more than gravity should have allowed. The only explanation: the expansion of the universe wasn’t slowing down at all. It was accelerating. The three lead researchers received the 2011 Nobel Prize in Physics for this finding.
Physicists quantify this with something called the deceleration parameter. A positive value means expansion is slowing. A negative value means it’s speeding up. The best current estimate puts that parameter at roughly negative 0.55, firmly in accelerating territory.
What Is Pushing the Universe Apart
The force behind this acceleration is called dark energy, and it makes up approximately 68 to 70% of the total energy content of the universe. Despite being the dominant ingredient in the cosmos, dark energy remains deeply mysterious. No one has directly detected it or identified what it actually is.
What scientists do know is how it behaves. Dark energy acts like a negative pressure woven into the fabric of space itself. As the universe expands and matter thins out, gravity weakens over distance. But dark energy doesn’t dilute in the same way. It appears to maintain a roughly constant energy density even as space stretches. So the larger the universe gets, the more dark energy there is relative to gravity’s pull, and the faster expansion accelerates. This is why the universe decelerated early on, when matter was packed tightly together, but switched to accelerating several billion years ago as matter spread out and dark energy took the upper hand.
How Scientists Measure the Expansion Rate
The speed of expansion is described by the Hubble constant, which tells you how fast a distant galaxy is moving away from you based on its distance. A galaxy one megaparsec away (about 3.26 million light-years) should be receding at a speed equal to the Hubble constant, measured in kilometers per second.
The problem is that two main methods of measuring this number give different answers. When scientists calculate it from the cosmic microwave background, the faint afterglow of the Big Bang, they get about 67.4 km/s/Mpc. When they measure it using nearby supernovae and other direct observations, they consistently get a higher value near 73 km/s/Mpc. This gap, known as the Hubble tension, has persisted for over a decade.
In 2024, the James Webb Space Telescope conducted its largest study of this discrepancy and confirmed that the mismatch is not caused by errors in the Hubble Space Telescope’s measurements. As Nobel laureate Adam Riess, who led the study, put it: the tension likely points to something genuinely missing from our understanding of the universe. Possible explanations range from an unknown form of matter that gave the universe an extra push after the Big Bang, to exotic particles or properties of dark matter that current models don’t account for.
Is Dark Energy Constant or Changing?
One of the biggest open questions is whether dark energy has always exerted the same force or whether it evolves over time. If dark energy is truly constant (as described by Einstein’s cosmological constant), the universe will keep accelerating at a steady pace. If it changes, the future could look very different.
New data from the Dark Energy Spectroscopic Instrument (DESI), which released its first-year results in 2024, offered an intriguing hint. When DESI’s measurements of the universe’s expansion history were combined with data from the cosmic microwave background and supernovae, the results showed a preference for dark energy that changes over time, at a statistical significance of up to 3.9 sigma depending on which supernova dataset was used. That’s notable but not yet conclusive. Physicists typically require 5 sigma before declaring a discovery. If confirmed by future data, a time-varying dark energy would fundamentally reshape cosmology.
What Acceleration Means for the Universe’s Future
If dark energy remains constant, the most likely outcome is what physicists call the Big Freeze, or Heat Death. The universe continues expanding at an increasing pace. Galaxies drift apart. New stars gradually stop forming as the raw materials thin out. Existing stars burn through their fuel and die. Over incomprehensibly long timescales, the universe reaches a state of maximum entropy: uniform, cold, and dark, with no energy gradients left to drive any physical process.
If dark energy strengthens over time, a more dramatic fate is possible. In the Big Rip scenario, dark energy’s repulsive force eventually overwhelms not just the gravity holding galaxy clusters together, but the forces binding individual galaxies, star systems, planets, and ultimately atoms. Spacetime itself would tear apart at the most fundamental level. This scenario remains speculative, but the DESI hints about evolving dark energy have kept it in the conversation.
Your Corner of the Universe Is Not Expanding
One important nuance: cosmic expansion does not affect everything equally. Galaxy clusters, which can span tens of millions of light-years and contain hundreds to thousands of galaxies, are the largest objects in the universe still held together by their own gravity. Anything smaller than a galaxy cluster, including our Milky Way, our solar system, and certainly your body, is bound tightly enough by gravity (and the other fundamental forces) that expansion has no measurable effect.
The acceleration only matters on scales larger than galaxy clusters, where the distances are vast enough and gravity weak enough for dark energy to dominate. So while the observable universe is flying apart at an accelerating rate, the distance between you and the nearest star is not changing because of it. The expansion is real and measurable, but it operates on a canvas far larger than everyday experience.