What is dark energy, and how are astronomers trying to find it?
Govert Schilling investigates dark matter, what we've learned about it, and what we might discover in the future.
As physicists understand it, dark energy is a property of empty space. Like some sort of anti-gravity, it pushes empty space away from itself, accelerating the expansion that started 13.8 billion years ago with the Big Bang, and creating ever more space in the process.
More space means more dark energy, so the effect is self-reinforcing.
The discovery of the accelerating expansion of the Universe, by two competing teams of astronomers in 1998, was awarded the 2011 Nobel Prize in Physics.
But after two decades of additional research, the true nature of dark energy is still a complete mystery.
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When the Hubble Space Telescope was launched, no one had ever heard about dark energy.
But, in part thanks to Hubble, astronomers have come to realise that we live in an accelerating Universe, in which empty space is expanding ever faster and faster, thanks to this mysterious dark ingredient.
A unique instrument mounted on a venerable telescope in Arizona has started compiling the most detailed 3D-map of the cosmos ever.
The goal is to further our understanding of the expansion history of the Universe, and – hopefully – to uncover the true nature of dark energy.
But how do you discover that the present-day Universe is expanding faster than it did one or two billion years ago?
The trick is to study galaxies a couple of lightyears away. The light of these distant galaxies arrives on Earth with a longer (redder) wavelength, because the space it’s been travelling through has been expanding, stretching the light along with it.
Galaxy redshifts therefore provide you with their light travel times. If you also know their corresponding distances, it becomes possible to reconstruct the cosmic expansion history.
Back in 1998, teams led by astronomers Saul Perlmutter, Brian Schmidt and Adam Riess used a specific type of supernova explosion to gauge distances.
These Type Ia supernovae have well-known luminosities and by comparing those to their observed apparent brightness in the sky, it becomes possible to derive their distances.
The Hubble Space Telescope greatly added to this work by discovering and measuring dozens of extremely remote supernovae.
However, supernovae are difficult to study as they are so far away and cannot be summoned at will, so astronomers can only study where they happen to appear.
If astronomers really want to understand this mysterious effect, then they need a global map to track how the Universe’s acceleration changed over time – not just over the last few billion years, but right back to the Big Bang.
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That’s where the new instrument comes in. Called DESI (for Dark Energy Spectroscopic Instrument), it uses a very different cosmic yardstick that can be studied throughout the observable Universe.
All you need to do is create a detailed 3D map of tens of millions of galaxies.
And that’s exactly what DESI’s job is over the next five years, using the light-gathering power of the old 4m Mayall Telescope at Kitt Peak National Observatory (KPNO) in southern Arizona.
"DESI is giving the 47-year-old telescope a new lease of life," says KPNO telescope scientist Dick Joyce.
It will do this by looking for something called baryon acoustic oscillations (BAO).
Right after the Big Bang, sound waves propagated through the hot, dense primordial soup, powered by energetic radiation that was still strongly interacting with the electrically charged plasma.
But, explains DESI co-spokesperson Daniel Eisenstein of Harvard University, after some 380,000 years, neutral atoms formed, and the Universe became transparent.
Radiation no longer pushed matter around, and the sound wave pattern became ‘frozen in’.
When slightly denser regions subsequently started to attract more and more matter, galaxies preferentially formed along this (expanding) cosmic scaffolding.
The end result is that the current distribution of galaxies in the Universe is not completely random.
"The effect is much too small to detect by eye," says Eisenstein, "but by statistically studying millions of galaxies, it becomes evident."
These fluctuations are BAOs. As the pattern changes over time, they can be used to gauge cosmic distances, which can then be compared to redshift measurements to disentangle the expansion history of the Universe and the effects of dark energy.
Using the 2.5m Sloan Digital Sky Survey telescope in New Mexico, Eisenstein and his colleagues have already produced a 3D-map of over 1.5 million galaxies that clearly showed the BAO signal.
Their BOSS programme (for Baryon Oscillation Spectroscopic Survey) was completed in 2014. However, it was a slow and cumbersome process.
For each new exposure, a thousand optical fibres had to be manually positioned on a custom-made focal plane plate, in which holes had been drilled at specific positions to catch the light of the many galaxies in the field of view. As a result, at most three exposures could be made on any clear night.
KPNO’s Dick Joyce calls DESI a major step forward. It could even be described as BOSS on steroids.
Mounted on a much larger telescope, it uses 10 sensitive spectrographs, each of which can dissect the light of 500 galaxies at once.
Most importantly, the 5,000 optical fibres that feed the spectrographs are positioned robotically within just two or three minutes, during the time the telescope slews to a new field.
On a clear night, between 20 and 30 20-minute exposures can be made, yielding 10 terabytes of raw observational data.
To achieve a large 3.2˚ field of view (as wide as six full Moons), the Mayall Telescope has been outfitted with a custom-made corrector: a 3-tonne, barrel-shaped assembly of 6 lenses, the largest of which measures 1.1m in diameter.
Like a giant pizza, the 0.8m-diameter focal plane of the telescope is divided into 10 wedges with 500 optical fibres each.
In total, a whopping 240km of optical fibre guides the light of thousands of galaxies in the field of view to the sensitive spectrographs.
Before being spread out into a detailed spectrum, the light is split into three broad wavelength bands: blue, red and infrared.
To prepare for the DESI project, telescopes at Kitt Peak and Cerro Tololo (Chile) have carried out large photographic sky surveys over the past years.
These have been combined into the DESI Legacy Imaging Surveys project, which is freely available on www.legacysurvey.org.
On the basis of this vast collection of deep-sky images, in three colour bands, project scientists have selected the galaxies and quasars (the luminous cores of very distant galaxies) that will be spectroscopically observed by DESI.
According to Eisenstein, DESI is the next big step in the study of the large-scale structure of the Universe, and of BAOs in particular.
It will map one third of the celestial sky (14,000 square degrees), and collect spectra for 35 million galaxies and 2.4 million quasars, out to distances of 11 billion lightyears.
The resulting 3D-map of the Universe is by far the largest ever made. And by studying the characteristic size of the BAOs at various redshifts, astronomers will be able to reconstruct the expansion history of the Universe and the evolving role of dark energy.
First light for DESI was achieved on 22 October 2019. After a commissioning phase, the survey is now in full swing.
"Until 2025, the Mayall Telescope is not going to do anything else," says Joyce.
Of course, the wealth of spectroscopic data that DESI is going to yield will benefit many research topics beyond the mapping of baryon acoustic oscillations.
Galaxy redshifts also provide information on proper motions, and thus on the distribution of dark matter in groups and clusters.
Knowing precise distances to galaxies makes it possible to better interpret their observed characteristics.
Quasar spectra contain information on intervening clouds of intergalactic hydrogen. Evolutionary models of galaxies – and of the whole Universe – will be put to test.
Finally, DESI will also study individual stars in our own Galaxy, the Milky Way.
However, the main goal of the new survey is to solve the riddle of dark energy – that mysterious ingredient that constitutes over 70% of the total mass-energy content of the Universe.
Discovering exactly when and how the expansion of the Universe started to accelerate, and whether or not dark energy is evolving over time, may help physicists to understand its true nature.
It’s an answer science has been chasing for over two decades. Hopefully, DESI will be able to provide it.
Does dark energy really exist?
Ever since it was first reported, the idea of an accelerating universe – the main motivation for believing in the existence of dark energy – has encountered scepticism and disbelief among a minority of astronomers.
They reasoned, for instance, that intervening dust would make a supernova appear dimmer than it really is.
Subsequent research has allowed astronomers to work around these issues but not everyone is convinced.
Last December, a team of astronomers led by Young-Wook Lee of Yonsei University in Seoul, Korea, claimed that dark energy does not exist at all.
According to their observations of a small sample of distant Type Ia supernovae, these stellar explosions are fainter in younger galaxies than in older ones.
If you take that effect into account, all evidence for an accelerating expansion of the Universe vanishes, according to the team in a paper to be published in The Astrophysical Journal.
Most astrophysicists and cosmologists are unimpressed by the arguments of Lee’s team.
Other studies, using larger samples, have failed to show up this relation between supernova luminosities and galaxy ages.
Moreover, there are other indications for the existence of dark energy apart from supernovae. It looks like dark energy is here to stay.
Hubble and the search for dark energy
The accelerating expansion of the Universe was discovered in 1998 by studying supernovae in distant galaxies. The initial supernova observations were carried out by telescopes on the ground.
However, to learn more about the expansion history of the Universe and the evolving role of dark energy, we want to study supernovas at a wide range of distances, and the most remote ones are hard to observe with ground-based telescopes.
That’s where the Hubble Space Telescope played a decisive role.
As soon as a distant supernova was discovered from the ground, Hubble would observe it in much more detail.
Moreover, through repeated observations of a large swathe of sky (known as the GOODS field, for Great Observatories Origins Deep Survey), Hubble discovered dozens of extremely distant supernovas on its own.
Thanks to these high-precision observations from Hubble, astronomers became convinced that the original 1998 claim of an accelerating expansion was real.
Apparently, some mysterious force is pushing empty space away from itself. The true nature of this dark energy is still very much a matter of debate.
Hubble helped frame the question; whether or not it will also help to provide the answer, only time will tell.
Govert Schilling is the author of Ripples in Spacetime: Einstein, Gravitational Waves and the Future of Astronomy. This article originally appeared in the April 2020 issue of BBC Sky at Night Magazine.