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A Bright Future for Dark Energy Physics

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Dark Energy Physics gets a Nobel Prize and a dedicated Space Mission in one day.

The Nobel Prize in Physics was awarded on Tuesday to Saul Perlmutter, Brian P. Schmidt and Adam G. Riess for their leadership of the teams that discovered the apparent "accelerating expansion of the Universe through observations of distant supernovae" in 1998.

At the time, the teams were seeking to measure whether the Universe was expanding at a continuous rate or slowing down, and the measurement of the acceleration was a complete surprise. The only way to explain the observed acceleration within the framework of general relativity was to hypothesise a mysterious field called 'dark energy' which acts as a form of anti-gravity pushing galaxies apart.

Just a few hours after the Nobel news, the European Space Agency (ESA) announced it had selected two new space missions for its Cosmic Vision programme. The Cosmic Vision programme started as a competition in 2004 and after several rounds the original 50 projects were whittled down to the final two yesterday. One of the two selected missions, called Euclid and scheduled to launch in 2019, is coincidently designed to investigate the nature of dark energy and further understand the observed accelerated expansion of the Universe.

Why do we still need to study dark energy and the Universe's expansion? Simply because more than 10 years after the Nobel-worthy discovery, the nature of dark energy is still completely unknown.

Cosmologists have used several methods along with the supernovae observations to measure the amount of dark energy in the Universe. These show the Universe is composed of roughly 73% dark energy, meaning nearly three quarters of the Universe is a bizarre form of energy. The rest is 22% dark matter (an exotic form of matter which doesn't emit or absorb light), and only about 5% of the Universe is the 'normal' matter that makes up the stars and other things like planets, DNA, and the computer you're reading this from. This distribution of 'stuff' in the Universe is called the Standard Model of Cosmology.

Knowing how much dark energy there is out there is not enough. Scientists are aiming to discover the actual nature of the dark energy, without excluding that it may turn out to be a fluke that could be explained away by rewriting the laws of gravitation.

It turns out one of the best ways to study dark energy is to look at gravitational lensing, an effect predicted by Einstein's theory of general relativity. General relativity predicts that space-time is curved by matter. Since light-rays are forced to follow the curvature of space-time, distance objects can appear distorted or lensed if there is a lot matter between the source and the observer.

If the distortion is very big, the effect is called strong lensing - this has been beautifully observed by the Hubble Space telescope, which shows distant galaxies completely warped into elongated arcs. In the majority of cases though the distortion is faint, just enough to lens a round galaxy into a slightly elliptical one. This is called weak lensing and affects almost all galaxies we see in the sky.

Observing weak lensing can tell you a lot about the matter that's acting as a gravitational lens, incidentally permitting us to study the otherwise invisible dark matter. Since the dark matter is being pushed away by dark energy, weak lensing is also a powerful tool to study dark energy and general relativity on very large scales. The effect is small though, and only useful if you can measure it on a large number of galaxies over a large area of the sky. The shape measurements also need to be very high resolution, which is very difficult from earth due to atmospheric distortions: this makes a space mission ideal.

This is exactly what Euclid will do. It will measure from space the shapes of over a billion galaxies over nearly half of the sky (the other half is mostly obscured by the Milky Way), giving us an amazing new tool with which to test Einstein's theory of general relativity and understand the mysterious dark energy. It will also make 3D maps of 50 million galaxies, which is another powerful way to investigate the Universe's expansion.

With funding for the Euclid mission secured, dark energy physics has a bright future ahead, perhaps paving the way towards the next Nobel prize.


The original articles by the High-z Supernova Search and the Supernova Cosmology Project are available here:
Perlmutter et al 1998: Supernovae Cosmology Project:
Riess et al 1998 (including Brian P. Schmidt), High Redshift Supernovae Search Team

The other space mission selected by ESA for the Cosmic Vision programme is Solar Orbiter, whose aim is to study the Sun's magnetic fields.