The XENON1T detector is now on-line in Italy, looking for particles that may be the mysterious “dark matter” that seems to make up most of the universe’s matter. 3.5 metric tons of liquid Xenon cooled to -95°C detect interactions between particles passing through the tank of ultra-pure water surrounding the Xenon detector, making this the largest, most sensitive dark matter experiment to date.
Although its first 30 days of operation have not yielded any big discoveries, this isn’t unexpected. The particles it’s looking for are called WIMP’s – Weakly Interacting Massive Particles. By definition they are difficult to detect – they are “weakly interacting” and detectable only through gravity and the weak force.
WIMP’s are the most widely accepted hypothesis as to the nature of dark matter, but it’s just that – a hypothesis. We know that our standard model of cosmology, based on our observations of the universe, would require that only 4.9% of it is made of the ordinary matter that humans and the detectors we’ve built can easily see. 26.8% is “dark matter” that has a measurable gravitational influence on ordinary matter but can’t be seen, while 68.3% of the universe is the even stranger “dark energy.”
“Dark matter” is a bit of a misnomer – we don’t know for sure it’s matter at all. “Dark gravity” is a better term, since we only postulate its existence through its gravitational effects.
XENON1T is an example of what humanity can do when nations come together in the pursuit of scientific knowledge. Big experiments are expensive, but their results benefit the world’s knowledge. XENON1T is produced by a consortium of scientists from the US, Germany, Italy, Switzerland, Portugal, France, the Netherlands, Israel, Sweden and the United Arab Emirates.
Solving the mystery of the nature of dark matter would be a huge step forward in our understanding of the universe we live in. Should XENON1T find the WIMP’s it’s looking for, the universe will make a lot more sense. And if it doesn’t, it may be a sign that we need to explore even more imaginative hypotheses. The human brain and its senses have only evolved to facilitate our survival on Earth, and there is some audacity in attempting to understand the nature of the universe on scales that are much larger or much smaller than the environment we’re made to operate within. The fact we can do this at all, and build things like XENON1T in response, is part of the wonder of science to me.