European
Southern
Observatory

Science with the ELT
Cosmology and Dark Matter

What we see around us — the planets, stars and dust in between them — makes up just 5% of the Universe.

The rest is invisible dark matter and a mysterious dark energy that is thought to be causing the Universe to expand faster and faster. When this accelerating expansion was discovered in the 1990s, it completely challenged what we thought we knew about the laws of the Universe. The ELT will finally give us a way to search for, identify and ultimately characterise the new physics that is waiting to be unearthed.

In a nutshell

The rest is invisible dark matter and a mysterious dark energy that is thought to be causing the Universe to expand faster and faster. When this accelerating expansion was discovered in the 1990s, it completely challenged what we thought we knew about the laws of the Universe. The ELT will finally give us a way to search for, identify and ultimately characterise the new physics that is waiting to be unearthed.

What we see around us — the planets, stars and dust in between them — makes up just 5% of the Universe.

The rest is invisible dark matter and a mysterious dark energy that is thought to be causing the Universe to expand faster and faster. When this accelerating expansion was discovered in the 1990s, it completely challenged what we thought we knew about the laws of the Universe. The ELT will finally give us a way to search for, identify and ultimately characterise the new physics that is waiting to be unearthed.

Galaxies rotate fast. So fast that the gravity generated by stars, dust and gas could not possibly hold them together. This led astronomers to believe that galaxies are surrounded by a halo of invisible dark matter, but despite being suggested almost 100 years ago, the nature of this dark matter remains enigmatic. The ELT will be able to peer at the dark matter halos around distant galaxies, to help us understand how much of each galaxy is made up of dark matter, and perhaps finally understand what exactly this strange substance is made of. 

Dark matter makes up around a quarter of the Universe, but the even more mysterious dark energy makes up about 70%. In the 1920s, astronomer Edwin Hubble made revolutionary observations that provided the first direct evidence that the Universe was expanding, or that other galaxies are moving away from us and away from each other. For a long time, this expansion was assumed to be slowing down due to the gravitational pull that all matter in the Universe exerts on all other matter. However, in the 1990s a team of astronomers made the shocking discovery that the expansion is actually accelerating. 

This discovery profoundly changed cosmology because it suggested that there is another component in the Universe that we cannot see, but that is acting against gravity and pushing space apart. We have named this component dark energy, but it remains a mystery. It suggests our theories of cosmology and particle physics are incomplete (or possibly incorrect) and that new physics is out there, waiting to be discovered. A key task for the next generation of astrophysical facilities is to search for, identify and ultimately characterise this new physics. 

Ironically, the simplest form of such a dark energy is the cosmological constant that was introduced by Einstein when he was trying to ensure that his theory of general relativity allowed for a non-expanding Universe, the accepted view at the time. As it happens, the theory of general relativity with the cosmological constant actually explains the accelerating expansion very well. Alternatively, it has been proposed that general relativity should be replaced with a modified theory of gravity, which would explain the accelerating expansion and the formation of structures in the early Universe by a different behaviour of gravity on the largest scales.

The ELT will provide unique contributions towards deciding whether general relativity or a modified theory of galaxy best describes the expansion of the Universe, including testing the behaviour of gravity in unexplored regimes, as well as mapping the expansion history of the universe.

Cosmology: watching the Universe expand

To understand our Universe we need to understand what’s driving its acceleration, which requires astronomers to determine the expansion history of the Universe. The ELT will revolutionise the way we measure the acceleration of the Universe allowing us to unravel the mysteries of dark energy.

Standard probes of the expansion of the Universe include weak gravitational lensing and the signature that light imprinted shortly after the Big Bang on today’s distribution of galaxies, and type Ia supernovas. The ELT will contribute to current efforts to measure the acceleration of the Universe by characterising high-redshift Type Ia supernovas identified by James Webb Space Telescope and other survey facilities.  

But the ESO telescope will also map the expansion history of the Universe using a whole different method, by watching the Universe expand in real time. 

Extracting information about the Universe’s expansion from standard probes relies on assumptions about the curvature of space, depends on the adopted cosmological model, and can only estimate the average expansion history over long time periods. A model-independent approach that measures the expansion rate directly was proposed as early as the 1960s, but limitations in technology have meant that astronomers have not been able to make such a measurement in practice. This approach is known as redshift drift, and is a method that offers a truly independent and unique approach to exploring the expansion history of the Universe. The redshift of the spectra of distant objects is an indication of the expansion of the Universe, so the change in this redshift over time is a measure of the change in the rate of expansion. However, the estimated size of this redshift drift over a decade is only about 10 cm/s. Such a signal is about 10–20 times smaller than today’s large telescopes can measure in such distant galaxies. The huge light-collecting area of the ELT, coupled with new developments in quantum optics to record ultrastable spectra, means that this amazing measurement now lies within reach. 

Astronomers will use the redshift drift method with the ELT’s HIRES instrument to determine the accelerating expansion of the Universe directly, thereby allowing us to quantify the nature of the dark energy responsible for the acceleration. Once a first epoch of observations is made, the redshift drift signal grows linearly with time. Hence, in the very long run (on the timescale of many decades) the redshift drift may well overtake the ability of other methods to constrain the expansion history of the Universe. 

Apart from the fundamental conceptual importance of directly observing the Universe’s expansion, the redshift drift provides a new and crucial consistency test of the assumptions of our theories of cosmology. Two other such tests that will be significantly improved by the ELT are measurements of the temperature of the cosmic microwave background radiation and the primordial abundances of light elements. 

Illustrating the constraining power of redshift drift measurements by the ELT for various cosmological models. Credit: Carlos Martins

Dark matter

The ELT will also help us gather clues about the other component of the dark Universe, dark matter, by helping to measure the shape of dark matter halos.

Artist’s impression of the expected dark matter distribution around the Milky Way. Credit: ESO/L. Calçada
Artist’s impression of the expected dark matter distribution around the Milky Way. Credit: ESO/L. Calçada

Astrophysical evidence for dark matter halos around galaxies first emerged in the 1930s with studies of galaxy rotation curves, which plot the velocities of stars and gas against their distance from the centre of their host galaxy. Such studies still play an important role today. They require high-resolution observations of kinematics from the inner parts of galaxies to it to constrain the distribution of baryonic  or visible  mass. At higher redshifts, rotation curves are not currently resolved enough to constrain the fraction of dark matter. However, using data from the SINFONI and KMOS instruments on ESO’s Very Large Telescope, two recent studies tried to do this in the inner regions of distant galaxies using both seeing limited and adaptive optics-assisted observations, as well as stacking several images together to reduce noise. Their results point towards a low fraction of dark matter, a finding that is supported by the observation of decreasing rotation curves. However, these results remain controversial because the signal-to-noise ratio is low, and the spatial resolution is rather coarse. 

The ELT’s HARMONI will be the first instrument able to reach the spatial resolution needed to disentangle the visible matter distribution from the dark matter distribution for high redshift galaxies. Mass model distributions will provide the measurements of dark halo central density and core radius for galaxies in a redshift range that is not reachable with 8–10-metre class telescopes. Astronomers will be able to use the ELT to measure the shape of dark matter halos as a function of galaxy mass assembly history, cosmology, and environments. Detailed simulations show that HARMONI will be able to study spatially resolved kinematics with sufficient details and signal to noise ratio to perform mass models and recover the shape of dark matter halos down to stellar masses of 109 solar masses at redshift 1.4 and 109.5 solar masses at redshift 2.7 using the Hα line in just 2 hours of exposure. A multi-object spectrograph like the ELT’s MOSAIC instrument will allow us to expand the statistical sample of galaxies, as well as extending studies of rotation curves and dark matter profiles.