The key to unlocking dark energy
New research could help us understand the force that we think is accelerating the expansion of the Universe. As Colin Stuart explains, it’s all to do with the number 0.007297351
The mysterious force that might speed up the Universe’s expansion
There’s an often-overlooked number that cosmologists believe holds the key to unlocking some of the deepest mysteries in the Universe. According to Richard Feynman: “All good theoretical physicists put this number up on their wall and worry about it.” Equal to approximately 1/137, it’s called the ‘fine structure constant’ and it describes the way light interacts with atoms. Except, according to new research, it might not be so constant after all. That in turn could help us to explain why the expansion of the Universe appears to be speeding up, and to predict the ultimate fate of the cosmos.
It was 1998 when our understanding of the Universe began to turn upside down. The cosmos has been expanding since its creation in the Big Bang nearly 14 billion years ago. Most astronomers assumed that this expansion was slowing down as the energy from that initial event petered out. Except new measurements using distant exploding stars called supernovae showed the opposite to be true. Over the last few billion years, it
seems, the Universe has been growing at an ever-accelerating pace (see box, page 37).
Dark energy rush
This unexpected twist is usually put down to a shadowy entity called dark energy, which makes up around 68 per cent of all the stuff in the Universe. It acts as a form of anti-gravity and is pushing the Universe apart. But why did its influence only start to ramp up long after the Big Bang? Traditionally, dark energy has been seen as a constant. As it pushes galaxies apart, their gravitational attraction wanes; this means they find it harder to resist the effects of dark energy and the expansion quickens, weakening their attraction yet further – creating a vicious cycle. So it’s not that dark energy itself is getting stronger, rather that its ability to accelerate the expansion of space is gradually ramping up.
There is another, less mainstream option. Perhaps the strength of dark energy itself has changed over the lifetime of the Universe and it’s only now becoming more potent. If that’s true then it would rule out the majority of the current leading theories for what dark energy is made of (see box, page 37). It’s that special number – the fine structure constant – that provides a possible way to test this controversial idea. To understand the significance of the fine structure constant – also known by the Greek letter alpha (α) – you need to dive inside the atom. A force known as electromagnetism – the same force that attracts the North Pole of one magnet to the South Pole of another – keeps particles called electrons whizzing around a central nucleus. The fine structure constant sets the strength of electromagnetic force, in turn dictating the sizes of the electrons’ orbits. If it were stronger, for example, then the electron orbits would be crowded more tightly around the nucleus.
Yet if the strength of dark energy has changed over the Universe’s history, it could have had a significant effect on the fine structure constant too. “If dark energy is not a constant then it must be some kind
of field,” says Carlos Martins, from the University of Porto. In physics, a field is a region over which a force has an influence – for example the Earth’s gravitational field. The dark energy field would in turn affect the electromagnetic field within atoms. “The two fields are so interconnected that a change in one triggers a change in the other,” says Martins. “In any model in which dark energy varies, you unavoidably expect some variation in the fine structure constant.” In other words, find that alpha varies and you can be confident dark energy varies too. “Traditional models of dark energy don’t produce variations in the fine structure constant,” says Rubén Arjona from the Autonomous University of Madrid.
Significant number
The value of alpha can be exquisitely measured in a laboratory here on Earth, making it one of the most accurately determined numbers in physics. It is equal to 0.007297351, give or take 6 on that last digit. According to research published in February it can even be measured by looking at the signals pinging between our satellite navigation systems. To see if alpha maintains this value across the vastness of space you need some way of measuring it far across the Universe, and this is where quasars come in. Their name is a contraction of ‘quasi-stellar object’ and they look like stars, but in fact they are the bright cores of some of the first galaxies to form. “We can use them to reach back to within 800 million years of the Big Bang,” says John Webb, from the University of New South Wales in Sydney, Australia.
The key is to look for ancient quasar light that has passed through a nearby cloud of gas and dust before setting out on the long trek to Earth. Astronomers use a device called a spectrometer to break the light up into a spectrum of its constituent colours – a bit like a raindrop splitting sunlight to create a rainbow. The otherwise colourful spectrum is full of dark, black bands known as absorption lines. These are simply colours that are missing because electrons in the gas cloud swallowed that part of the quasar’s light, using it to jump up to a higher orbit around the atom’s nucleus. If the fine structure constant was different back then, so was the size of that jump. That means a different part of the light will be missing and the black absorption line will appear in a slightly different part of the spectrum.
This is exactly the sort of experiment that astronomers have been performing for the last few years, with tantalising results. Their findings suggest that the fine structure constant was different in the past. Then, in April this year, John Webb and his colleagues published four measurements of a single quasar – J1120+0641 – pushing measurements of alpha back to 13 billion years ago. “That’s half as far again as any previous measurement,” says Webb. When combined with existing quasar data, these new results support the idea that alpha differs from its Earthly value by two parts in 100,000. “It is a very tentative result, but it is also very suggestive,” says Webb.
To put this work on firmer ground we need to make more quasar observations and that’s already in the pipeline in the months and years ahead. “There are
30 more known quasars that we can study at the moment,” says Webb. An upcoming instrument – the European Extremely Large Telescope (E-ELT) will be
well suited to studying them in this way when it sees first light in 2025. It will be positioned in the Atacama Desert in Chile, 20km from the existing Very Large Telescope, which Webb used to make his potentially revolutionary quasar measurements. “This topic is one of the drivers for building the E-ELT,” says Webb. Carolos Martins agrees that it will be gamechanger for this field of enquiry. “It will have a huge impact,” he says.
In the meantime, Rubén Arjona has been measuring alpha when the Universe was a few billion years old by analysing the cosmic microwave background (CMB), the left-over radiation from the Big Bang. A map of this radiation (see above) is riddled with speckles – tiny spots that are slightly hotter or cooler than the background temperature. Variations in alpha would have an effect on these speckles. Usually you have to base your analysis of the CMB on a particular theory of dark energy to account for how much the Universe has expanded. However, Arjona used a technique called machine learning, where computer algorithms look for patterns in the data instead. “One of the advantages of using machine learning is that you don’t have to assume any dark energy model,” he says. “It ensures you’re not biased.” His work, published in February, found no variation in alpha and is consistent with dark energy being constant.
Looking at our Galaxy’s heart
Other astronomers have been probing the variability of alpha even closer to home, in our own Galaxy. A team led by Aurélien Hees from the Paris Observatory has been scrutinising the stars orbiting Sagittarius A*, the supermassive black hole at the heart of the Milky Way. Their work was published in February. “It’s the first time variations in alpha have been looked for around such a compact object,” he says. In contrast to the quasar observations, Hees found no evidence that
alpha varies from its Earthly value. “If any variation is there it must be smaller than one part in 100,000,” he says. Otherwise he’d have seen it.
Where will it all end?
So what does this all mean for the fate of the Universe? It could – and that’s a big could – stop us all being ripped apart. If dark energy is constant, as traditional models suggest, then its influence grows uncontrollably as the Universe expands. This would lead to what astronomers call the ‘Big Rip’. Eventually, even the space between and within atoms is stretched to such an extent that they are torn apart. The Universe becomes an empty sea of shrapnel. No stars, planets, life forms or even atoms. This could all happen in as little as 22 billion years.
Yet if dark energy and alpha have varied over the Universe’s history – as these results suggest – there is a glimmer of hope. “It makes the future harder to predict,” says Martins. “The acceleration of the Universe could stop or speed up.” Only by building giant telescopes like the E-ELT will we know once and for all if we’re being granted a stay of execution, or whether the lights will go out on this Universe sooner rather than later.
If dark energy is constant, as traditional models would have it, then its influence grows uncontrollably as the Universe expands. This would lead to what astronomers call the ‘Big Rip’