The discovery of dark matter

The 2019 Nobel honours the discovery of dark matter and the creation of techniques for discovering exoplanets. It is split three ways. Canadian cosmologist, James Peebles gets half the monetary award of Kronor 900,000 (about US $ 909,000). Swiss scientists, Michel Mayor and Didier Queloz shared half. The Royal Swedish Academy of Sciences said the three “transformed our ideas about the cosmos”.

Dark matter became a speculative “solution” to problems in astrophysical calculations in the 1960s. In 1916, when Einstein released his general theory of relativity, he inserted a “cosmologial constant” to adjust for his belief the universe was static in size. But by the 1920s, it was known that the universe was expanding. Einstein regarded the cosmological constant as a blunder and removed it from his equations. 

In 1964, a breakthrough occurred, by accident. The 1978 Nobel Laureates Arno Penzias and Robert Wilson were examining microwave radiation. They kept picking up “noise”, which they initially assumed to be instrument error.  James Peebles made theoretical calculations when he realised the “noise” provided information about the amount of matter created in the Big Bang. The uneven noise showed minute differences in temperatures in different regions and indicated how matter clustered to form galaxies. 

That led to another puzzle: most of that matter was undetectable by other means. Hence, the name “dark matter”. Measurements of cosmic background radiation and the resultant theory showed the matter we can detect is only about 31 per cent of what should be there. About 5 per cent is ordinary matter and 26 per cent is dark matter, inferred via calculations matching observations. 

About 69 per cent is absolutely undetectable. In 1984, Peebles was one of those who revived Einstein’s cosmological constant, using it to represent undetectable energy. This is called dark energy and assumed to account for the “missing” 69 per cent, which was somehow converted to energy.  

In April 1992, John Mather and George Smoot presented an image of the first light in the universe (Nobel Prize in Physics 2006). This helped refine calculations and suggested Peebles was correct. In 1998, Saul Perlmutter, Brian Schmidt and Adam Riess showed that the universe’s expansion was accelerating (Nobel Prize in Physics 2011). That indicated an unknown, undetectable dark energy was indeed, responsible for pushing the expansion and Peebles’ theoretical calculations were confirmed. This leaves a big mystery of course, since almost nothing is known of dark matter and dark energy. 

Michel Mayor and Didier Queloz developed methods to observe what we have long known to be true. Other stars must have planetary systems orbiting them, like our own. But planets are very small objects on a galactic scale. Nor do they have their own light source, only reflecting light form their parent star, or stars. 

The radial velocity method is used to find exoplanets. This measures small changes in the velocity of a star (velocity is a measure of both speed and direction) caused by the gravity of its planets. 

Radial velocity is measured using Doppler effects. Light from an object moving towards us tends towards the blue end of the spectrum, while objects moving away from us, tend towards red. By measuring apparent changes in wavelength, it is possible to calculate velocity of a star. 

But even a planet as large as Jupiter (1300x Earth volume and 300x Earth mass) causes the sun (a relatively small star) to shift just 12m/s. Earth itself causes just about 0.09 m/s shift. Scientists measure thousands of wavelengths to work out radial velocity shifts and then look for the planet, or planets, causing perturbation. 

Mayor mounted his first spectroscope in 1977 but it could only detect gross changes of about 300m/s which wasn’t enough to detect planets in faraway stars. In the 1990s, his doctoral student, Queloz, worked with his research group to refine the technology and the calculation techniques. 

In October 1995, they revealed the first exoplanet, a huge gas giant like Jupiter, orbiting very close to its star, 51 Pegasi. Since then, using similar techniques, over 4,000 planets have been discovered, including many that are Earth-like. This has helped astronomers improve their models for planetary development since some of the results, including the very first, showed the earlier understanding of planet formation was wrong. 

These discoveries open up new areas of research. There are many scientists working to get to grips with dark matter and dark energy. Others are examining data about exoplanets. This could eventually lead to better understanding of, among other things, places where life is most likely to develop.


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