Measuring cosmic distances to understand the expanding universe

In October, the Physics of the Accelerating Universe Survey (PAUS) collaboration released its first catalog data. As a member of the PAUS collaboration, the Cosmology group of Professor Alexandre Refregier has contributed to this project and now looks forward to gaining more insights from the released data. We spoke with Refregier to understand the importance of surveys such as PAUS.

What was the role of your group in the preparation of the PAU Survey?

Our group joined the survey relatively early, and we participated in the analysis of the very first data from PAUS. One of our PhD students, Luca Tortorelli, developed a new method to analyse PAU Survey data. The method, called forward modelling and Bayesian approximate computation, produces simulated data he then used to interpret the real PAU data. The models describing the galaxies and the universe are very complex. Thus, we need to fit our models to real data to check their validity: it can be thought of as a calibration procedure. For example, we don't know the exact values of the parameters that describe dark energy and dark matter, but we know that they lie within a certain range. Luca created simulated PAU data assuming certain values for these parameters, and compared the resulting findings with the real data. If the model agreed with the measurements, it was considered consistent; if not, it was discarded. In astrophysics, various groups test different approaches on the same data. They then compare their results to find consistency among them.

Can you tell us about the PAU Survey and PAU instruments?

It’s a very new measurement approach. In astrophysics, we usually consider two approaches. One is called the photometric technique and relies on an instrument that takes images of the sky using different broadband frequency filters. Each measurement corresponds to a different wavelength, and the collected data can then be analysed by combining images taken with different filters. The second approach is a spectroscopic survey that measures the spectrum of galaxies. The two methods yield different and complementary information. The spectroscopic survey provides finely resolved information on the wavelength distribution of the light emitted by an object: this allows us to determine precisely its redshift, which we use as a measure of the distance of this object from us. However, the spectroscopic survey is time-intensive and so limits the number of galaxies that can be observed with this approach. In contrast, the imaging method allows us to detect many galaxies at a time, but its distance measurements are less precise.

The PAU instrument is a wide-field camera that can capture large sections of the sky in a single exposure; it uses a large set of narrowband filters with an advanced mechanism that makes it possible to switch between them. It's installed on the William Herschel Telescope in the Canary Islands. The PAU camera is cutting-edge technology: it allows us to measure narrowband images extensively for large areas of the sky. As this is a new way of taking data, we need new analysis methods. The PAUS project is now a large collaboration involving many groups.

What will the data analysis allow you to study?

The primary goal of the survey is to measure the three-dimensional distribution of galaxies in the universe. Why? Because galaxies trace the distribution of visible matter in the universe. By studying this distribution, we can infer information about dark matter and dark energy. When light travels from distant galaxies to us, it's bent due to the presence of dark matter: it's a phenomenon called gravitational lensing. Anything that has mass bends light to some degree. In our current model of cosmology, dark matter consists of unknown particles distributed throughout the universe.

Can you tell us a bit more about dark matter and dark energy?

While we cannot see dark matter directly, we observe its gravitational effects. That is how we know it's prevalent in the universe. In particular, dark matter bends light due to its mass. Improved distance estimates from experiments such as the PAU Survey help us to interpret gravitational lensing data and map the distribution of dark matter over time more accurately. This provides us with insights into the constituents of the universe, such as how much dark matter exists and what its properties are.. Another mysterious component is dark energy. Dark energy accelerates the expansion of the universe. The acceleration rate depends on the amount and properties of dark energy. This acceleration affects the way structures form in the universe. In fact, we know there are two opposite processes: gravitational attraction, which tends to make structures grow, and dark energy that pulls them apart. By studying large-scale structures in the universe, we gain insight into both dark matter and dark energy. This is the reason why these measurements are so important.

How is the PAU survey related to the 2011 Nobel Prize in Physics assigned to Saul Perlmutter, Brian Schmidt and Adrian Reiss (PSR)?

The findings of PSR revealed that the expansion of the universe is accelerating, and now we want to understand why. Dark energy is believed to be evenly distributed across space, and measuring the universe's accelerated expansion can help us refine our understanding of it. Albert Einstein introduced the concept of the cosmological constant, which can provide an explanation for dark energy. Initially, he added the constant to his equations of General Relativity to prevent the universe from expanding. The cosmological constant – with the opposite sign with respect to Einstein's proposal – can lead to an accelerated expansion of the universe. We now know the universe's expansion is accelerating, and measurements have so far been consistent with the presence of a cosmological constant. However, uncertainties still exist and we may need to adjust the Standard Model of Cosmology. That's why the PAU Survey is important – it provides better distance estimation and therefore helps refine our understanding of dark energy and dark matter.

What's the difference between PAUS and the GAIA Survey?

GAIA also uses photometric and spectroscopic techniques, but the measurements are performed with two separate instruments. The beauty of GAIA is that it measures positions very precisely and it focusses on stars, instead of galaxies, in the distance range of our galaxy. With PAUS we cover way larger distances. Our galaxy is, well, close in astrophysical terms.