Our universe is approximately 13.8 billion years old. During this vast period, small initial asymmetries have grown into the large structures we can see today through telescopes: galaxies like our Milky Way, clusters of galaxies, and larger groupings of matter and filaments of gas and dust. The rate of this growth depends on the competition of natural forces: can dark matter, which holds everything together with its gravity and attracts additional matter, maintain balance against dark energy, which pushes the universe further apart? Precise measurement of structures in the sky allows us to observe this struggle, emphasizes LMU astrophysicist Daniel Grün. This is where telescope observation projects come into play, which precisely capture large parts of the sky. For example, there is the Dark Energy Survey with the Blanco telescope in Chile and the recently launched Euclid satellite. LMU scientists have been involved in both projects for years.

The largest dataset analyzed so far
Although precisely determining the distance of individual structures and galaxies from us is not always simple, it is extremely important. The farther the galaxy, the longer the light has traveled to us, and the older the image of the universe that its observation reveals. The observed color of the galaxy, measured by telescopes like Blanco or satellites like Euclid, provides crucial information. A new study by a team led by Jamie McCullough and Daniel Grün, published in the journal MNRAS, analyzed the largest dataset to date and shed light on what the color of different galaxies actually tells about their real distance.

The distance of a galaxy can be precisely determined by spectroscopy, measuring the spectral lines of distant galaxies. As the universe expands, these lines appear at longer wavelengths, the farther the galaxy is from us. The light waves of distant galaxies stretch on their long journey to us, changing the apparent colors measured by the instruments. This effect, known as redshift, makes galaxies appear redder than they really are, similar to the Doppler effect with the sound of an ambulance siren.

No two galaxies are the same
Jamie McCullough, a doctoral student at LMU and Stanford University, used spectroscopic measurements from the Dark Energy Spectroscopic Instrument (DESI) in combination with the largest dataset so far for precise measurement of galaxy colors (KiDS-VIKING) for her analysis. The authors combined spectroscopic data from DESI for 230,000 galaxies with the colors of those galaxies in the KiDS-VIKING survey to determine the relationship between the distance of a galaxy from us and its observed color and brightness. No two galaxies are the same, but for each class of similar galaxies, there is a specific relationship between the observed color and redshift. "By combining distance information with measurements of galaxy shapes, we can derive large structures from light distortions," says Jamie McCullough. The study results enable the statistical determination of the true distance of each galaxy observed in images taken by Euclid or the Dark Energy Survey.

Interactive flight through millions of galaxies
By analyzing the observed distortions of galaxy images, scientists will be able to learn more about the behavior of cosmic structures today and billions of years ago and better understand them. This will provide insights into the evolutionary history of the universe. To observe the formation of structures over time, it is sufficient to measure structures at different distances from Earth. Just with images, this is almost impossible because the distance of a galaxy cannot be determined from its appearance in the image. Jamie McCullough's study provides the key to this problem by providing a model for what the apparent "color" of a galaxy tells us about its distance from us.

Observing the battle between dark matter and dark energy
The main goal of these precise observations and the distribution of galaxies at different distances is to gain insight into the great struggle of natural forces of dark matter and dark energy. "To really see what's happening, you have to be able to observe individual rounds of this fight," says Grün. Dark energy is poised to catch up and potentially stop the formation of larger mass groups in the universe. "Only then will we understand what dark matter and dark energy really are, and which will ultimately prevail."

New insights from the Euclid mission
The Euclid mission, led by ESA, recently released its first scientific images showing the telescope's ability to capture the entire Perseus galaxy cluster in a single image. Euclid will map an area 30,000 times larger than this image, enabling the exploration of dark energy, which accelerates the expansion of the universe. Dark matter, five times more common than ordinary matter, will be mapped to study its distribution and impact on dark energy through Euclid's 3D map. Scientists expect this mission to provide new insights into these major cosmological mysteries.

Advancements in understanding dark energy
Scientists from DES have used the supernova technique to determine the history of the universe's expansion. This method uses type Ia supernovae, which have almost the same true brightness, allowing precise determination of relative distances. DES results show that dark energy could vary over time, requiring additional data for final conclusions. These results are crucial for understanding the accelerated expansion of the universe and future research projects.

Source: National Aeronautics and Space Administration