South Korean astronomers are challenging the notion that the universe’s expansion is accelerating, an observation in the 1990s that lead to the theory of dark energy. This is currently very controversial, and may simply fizzle away or change our understanding of the fate of the universe.

In the 1990s astronomers used data from Type Ia supernovae to determine the rate of the expansion of the universe. Type Ias are known as standard candles because they put out the exact same amount of light. The reason for this is the way they form. They are caused by white dwarfs in a double star system – the white dwarfs might pull gas from their partner, and when that gas reaches a critical amount its gravity is sufficient to cause the white dwarf to explode. Because the explosions occur at the same mass, the size of the explosion, and therefore its absolute brightness, is the same. If we know the absolute brightness of an object, and we can measure its apparent brightness, then we can calculate its exact distance.

The astronomers used data from many Type Ia supernova to essentially map the expansion of the universe over time. Remember – when we look out into space we are also looking back in time. They found that the farther away galaxies were the slower they were moving away from each other, as if the universal expansion itself were accelerating over time. This discovery won them the Nobel Prize. The problem was, we did not know what force would cause such an expansion, so astronomers hypothesized the existence of dark energy, as a placeholder for the force that is pushing galaxies away from each other. This dark energy force would have to be significant, stronger than the gravitational force pulling galaxies together.

The South Korean astronomers, however, are challenging this conclusion. They hypothesize that perhaps Type Ia supernovae are not all created equally. Perhaps the age of the star affects the brightness, with older white dwarfs creating brighter supernova than younger ones. To determine if this is correct they analyzed over 300 Type Ias using data from the Dark Energy Spectroscopic Instrument (DESI) in Arizona. They claim, with high statistical significance, that the data supports the conclusion that older Type Ia supernova are brighter. If you then plug their correction into the analysis of the expansion of the universe, it turns out that the universe is currently decelerating, not accelerating.

This would not necessarily mean that dark energy does not exist. Rather they think that dark energy is weakening over time. We are already passed the point where gravity is stronger than dark energy. If true, this means the universe will not expand forever, but will eventually come back together in what is called the “Big Crunch.”

However – the rest of the astronomy community is skeptical, to varying degrees. Some argue that, while significant, the effect size is tiny and it is very easily an artifact of the analysis. This same group also made similar claims before, and those prior claims did not stand up to scrutiny. So their track record does not instill confidence.

This kind of debate among scientists is healthy. One study should not be enough to reverse a longstanding conclusion. But at the same time scientists need to be open to such challenges. In the end – the evidence will reign supreme, and will determine the consensus that emerges among astronomers. In the end, it’s hard to argue with the evidence.

The good thing about astronomy is that you can simply make more observations  This is what needs to happen, more and more detailed observations will either confirm or refute the conclusions of the South Korean researchers. Their paper will then either fade into obscurity or become a seminal paper, and perhaps even the basis of a future Nobel Prize.

Meanwhile, the debate about the ultimate fate of the universe continues. I have followed this question for decades, and it remains a fascinating question. There are no implications for us in the near term, of course, we are talking about what will happen billions or trillions of years in the future. But it is important for our understanding of the universe, and it is interesting to contemplate the ultimate fate of everything.

These are two very different vision of the future. In the Big Crunch scenario, the expansion of the universe continues to slow and eventually stops. And then the universe will slowly start coming back together. This process will accelerate until you have the opposite of the Big Bang – the entire universe collapses into a singularity. This, of course, raises the question about what happens next – will this lead to another Big Bang in an endless cycle? There is something intriguing about this.

The other possibility is that the universe simply continues to expand forever. Eventually we will experience the heat death of the universe, when there is no more energy to do anything. It is also possible that the accelerated expansion will get so great that even atoms come apart in a “Big Rip”. The big difference in this scenario is that there is no cycle – the universe is a one-off. Perhaps there are many universes, and there is a greater cycle, but our universe will die.

This question has gone back and forth over my lifetime, and perhaps it will again. This is partly because, when we look at the mass-energy of the universe it is very close to being right at the equilibrium point, the point at which expansion will slow asymptotically to zero, but not contract or rip apart. Perhaps this is because that is the actual fate of the universe – balanced right on the edge between endless expansion and the Big Crunch.

At this point I think it is reasonable to say that we don’t know. At least there is significant uncertainty, enough that subtle changes to our understanding of phenomena like Type Ia supernova can change the conclusion. But that also makes it an exciting science story to follow.