What's Dark Energy Up to?
- kieronconway
- Jan 8
- 5 min read
Updated: 1 day ago
© 2025 Kieron Conway - All rights reserved.
A Cosmic Mystery for the Next Generation of Scientists
Recent research from South Korea (2025), has raised new questions about one of the biggest mysteries in modern physics: dark energy. Understanding what dark energy is, and what it might do in the future, could determine the ultimate fate of our universe and there may be new evidence that we might have got it wrong!
The Expanding Universe: A Quick History
Before the 1990s
Modern cosmology tells us that the universe began at T = 0, often incorrectly called the 'Big Bang'. Very shortly after this moment, the universe went through a very brief but dramatic period known as inflation, during which space itself expanded extremely rapidly.
This expansion of space meant that distant regions of the universe moved away from each other faster than the speed of light, something allowed by Einstein’s theory of relativity because it is space itself that expands, not objects moving through space which are limited to lower than light speed.
After inflation ended, the expansion continued but gradually slowed down due to the gravitational pull of all the matter in the universe.
A Shock Discovery in the Late 1990s
In 1998, astronomers studying distant Type Ia supernovae made a surprising discovery. These exploding stars appeared dimmer than expected, which meant they were farther away than they should have been in a universe slowing-down.
The conclusion was astonishing: The expansion of the universe is speeding up, not slowing down.
This discovery overturned decades of thinking and led to the award of the Nobel Prize in Physics in 2011.
Enter Dark Energy
To explain this accelerating expansion, scientists introduced the idea of dark energy:
It cannot be seen directly – so it's dark.
Its nature is unknown – no one knows what it is.
It makes up about 70% of the total energy content of the universe.
In the most successful cosmological model, called Lambda–CDM (ΛCDM), dark energy is treated as a cosmological constant: a form of energy that fills space evenly and remains constant as the universe expands.
As the universe grows larger:
Matter becomes more spread out – the density of matter decreases.
Dark energy stays the same per unit volume – the density of dark energy is constant.This causes dark energy to dominate and drive acceleration.
What if?
In Part 2 of A Journey into Modern Physics, we looked at how the universe might meet its end. One of the things that came out of this was: what dark energy does in the future could decide the fate of the universe, depending on what happens to the density of dark energy. These predictions were considered to be highly speculative and most unlikely.
But has modern research changed all that?
What if the density of dark energy increases in the future?
If dark energy becomes stronger in the future, the expansion of the universe could accelerate without limit. Even gravity would not be able to hold the structures of the universe together.
If the density of dark-energy were to increase unchecked;
Galaxies would start to fall apart.
Then stars and planets would fall apart.
Eventually even atoms themselves would fall apart.
This dramatic scenario is known as the Big Rip and was regarded as a very remote possibility.
What if the density of dark energy decreases in the future?
If dark energy weakens or decreases in the future, gravity might eventually halt the expansion and reverse it. The universe could contract back into an extremely dense state.
This scenario is known as the Big Crunch and was also regarded as a very remote possibility,
Some theories even suggest that this could lead to a new expansion, producing a cyclic or “bouncing” universe.
A Historical Detour: Steady-State and Cyclic Universes
In the 1950s, astronomer Fred Hoyle proposed the steady-state theory, in which the universe expands but maintains constant density through continuous creation of matter.
Although this idea was ruled out by observations, it later evolved into cyclic universe models, where the universe undergoes repeated expansions and contractions.
However, these ideas were largely set aside, but recent discoveries may bring them back into discussion.
A New Twist from 2025
In 2025, scientists at Yonsei University in South Korea re-examined how astronomers measure cosmic distances using Type Ia supernovae, known as standard candles. The brightness of Type 1a falls off as the square of the distance to it. So, if you double the distance, the brightness falls off by a quarter. By comparing the measured brightness to the standard, you can determine distance to the 1a.
The Koreans' improved methods suggest that these supernovae may not all shine with exactly the same brightness. Instead, their brightness may also depend on the age of the galaxy in which they occur.
This means that earlier measurements may need small corrections.
Why Type 1a Supernovae Are So Important
Type 1a supernovae occur when a white dwarf star, feeding off a companion object, becomes unstable and explodes in a powerful, runaway fusion reaction. Because this happens at a well-defined mass, these explosions produce nearly the same peak brightness and duration.
This makes them excellent standard candles, objects of known brightness that allow astronomers to measure distances across the universe using the inverse-square law.
What the New Research Suggests
If supernova brightness depends on the age of the host galaxy, then distance estimates must be adjusted. When this correction is applied, the results may suggest that:
The density of dark energy is not constant after all, but may even be decreasing.
This is not yet confirmed and remains an active area of research, but if found to be correct, the implications are huge: the density of dark energy can change over time and may be decreasing.
Could the Universe Bounce?
If the density of dark energy is decreasing and therefore it is weakening, then gravity might eventually slow the expansion and cause contraction. This would re-open the possibility of a cyclic universe, once considered obsolete.
Whether this idea survives depends on further observations and independent confirmation of the results obtained in South Korea.
The Big Picture
Dark energy is one of the greatest unsolved mysteries in physics. New measurements and better data are challenging long-held assumptions, reminding us that even our best theories may need revision in the light of new measurements.
For students entering science today, the mystery of dark energy is wide open.
The universe may still have some very big surprises in store.
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