dark energy

The words sound ominous - dark energy. Scientists are slowly coming to understand this mysterious feature of the universe.

The matter we can see, composed of protons and neutrons ('baryonic matter'), makes up only about 10-20% of all matter in the universe. The remainder is so-called "dark matter", because it neither emits nor reflects light and so cannot be seen. No-one yet knows for sure what it is (they think it may be small unreactive particles), but scientists know it exists because its gravitational impact can be measured. So there is much more to the universe than we can see, and in fact there is much more than matter - "dark energy" makes up perhaps 70%.

But dark energy is even harder to detect. Quantum physics concerns the interactions of very small particles, which behave quite differently to the objects we can see, and quite strangely to our way of thinking. One aspect of quantum physics is that where there is a quantum field, particles can appear and disappear.

 Quantum fluctuations in apparently empty space cause particles to appear and disappear rapidly. (I use the words "apparently empty" to describe space because particle physicists, who study quantum effects, say that where there is a quantum field, it is far from empty.) This causes empty space to have energy (known as vacuum energy or "dark energy") and to exert a gravitational force. The effect of dark energy can also be measured by its gravitational impact.

Dark energy acts in the opposite way to gravity - it tends to push things apart rather than pull them together, and is responsible for the expansion of the universe. In fact the amount of dark energy is within a very small range that allowed the universe to avoid either blowing apart by expanding too fast, or collapsing in on itself. Dark energy consists of large amounts of both positive and negative energy, and the two cancel out to 119 decimal places, leaving a very small number in the 120th place, allowing the universe to expand relatively slowly. Cosmologist Leonard Susskind comments: "To make the first 119 decimal places of the vacuum energy zero is most certainly no accident." This is one basis of the so-called "fine-tuning" argument that only a designer God could have been responsible for this, although Susskind, in common with many scientists, prefers the multiverse hypothesis.

Stephan's Quintet group of galaxies (Photo: NASA)

Now, Astronomy magazine reports,  scientists have been able to measure the effects of dark energy in areas of space where there is either a high density of galaxies (superclusters) or a dearth of galaxies (supervoids). The dark energy stretches these areas, and this changes slightly the properties of microwave radiation passing through them. Those changes have been measured, and they demonstrate that dark energy exists, although it is still unclear exactly what it is.

Cosmology continues to be an interesting and exciting pursuit to follow.