For years, astronomers have been chasing a cosmic riddle. The expansion of the universe just doesn’t add up neatly. Early measurements suggest a slower rate than what local readings now show. This ongoing mismatch, called the Hubble tension, still baffles scientists.
Now, a fresh idea is stepping into the debate. A group of researchers is exploring a possibility most have ignored. What if dark matter — that invisible mass shaping the cosmos — actually changes over time?
A Shift in Focus
Until now, theories have focused on modifying gravity or scrapping dark matter entirely. Some questioned time’s flow or imagined a rotating universe. Many researchers instead explored evolving dark energy, which drives the cosmos apart. But evolving dark matter? That’s been mostly overlooked.
This is partly because dark matter models work remarkably well. They explain galaxies’ motion and other observations, even if we haven’t directly detected the particles. Critics of dark matter usually aim to get rid of it, not tweak it. That makes this new approach a refreshing shift.
A Universe That Oscillates
In their study, the scientists examined both evolving dark energy and evolving dark matter. Their analysis showed that changing dark matter fits the data more closely.
They note that the two models — evolving energy or evolving matter — often appear similar. That’s because cosmic evolution depends on the balance of energy and matter. A universe with one fixed and the other changing can resemble one where the roles are reversed.
The team suggests a model where a fraction of dark matter behaves oddly. About 15% of it may have an equation of state that shifts with time. In simple terms, this means the properties of that portion would oscillate — rise and fall like a wave.
This isn’t entirely new territory. Neutrinos, which are nearly invisible particles, also have mass and shift states. They can’t account for all dark matter, but they show such behaviour is possible. If cold dark matter — the standard kind — also oscillates, the numbers begin to line up.
A Step Towards New Ideas
Their findings suggest a mix of oscillating and regular dark matter may help solve the Hubble tension. It also stays consistent with what we already observe about dark matter’s role in the cosmos.
The researchers stress this is only a toy model. It doesn’t pin down exact particle properties or behaviours. Still, they believe it's a step worth taking.
At the very least, evolving dark matter gives theorists something new to work with. If the Universe won’t give up its secrets easily, perhaps it’s time we start asking different questions.
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