General Relativity Verified from The Orientation of Galaxies Separated by Cosmological Distances:
A New Means to Explore The Universe
Released: 13th Apr., 2023, Academia Sinica, Institute of Astronomy & Astrophysics (ASIAA), Taiwan
A research team led by Teppei Okumura at Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) and Atsushi Taruya at Yukawa Institute for Theoretical Physics in Kyoto University, used data on 1.2 million galaxies obtained from the Sloan Digital Sky Survey to analyze the position of galaxies and the alignment of galaxy orientations. The researchers developed a theoretical model that the orientation of galaxies is determined by the gravity induced by the distribution of dark matter, and compared it with the measurements. Through newly combining galaxy orientations with the conventional information, the predictions of the Standard Model of Cosmology well explain the observations. This implies that gravitational forces acting between galaxies separated by tens to hundreds of millions of light years apart are well described by general relativity,
Cosmology, a research field revealing the origin, structure, and fate of the Universe, has been revolutionized thanks to a rapid progress of astronomical observations. One important milestone is the establishment of the Standard Model of Cosmology, which consistently describes various observed data. However, this model requires the existence of dark matter and dark energy whose entities are completely unknown. To unravel these mysteries, it is necessary to conduct more precise observations of the universe to verify the premises of the Standard Model of Cosmology. Such observations are currently ongoing over the world to establish a new view of the universe that goes beyond the Standard Model.
The three-dimensional map of the universe, traced by the galaxy distribution over vast scales of the universe, provides a powerful tool to test cosmological models because the galaxy distribution is formed via the gravity of dark matter. The position of galaxies from the three-dimensional map also contains information on their intrinsic motions which again caused by the effect of gravitational attraction between galaxies. These properties can be used to measure the speed of the growth of structure and test the laws of gravity that operate between galaxies, validating general relativity, the basis of the Standard Model of Cosmology. So far, most of the observational tests using the maps of galaxies have treated galaxies as points. However, galaxies have finite sizes and thus unique shapes. While such information has been greatly utilized in studies of galaxy formation and evolution, there has been no active application in studies of cosmology until now. Since dynamics of galaxy formation is mostly dominated by the gravity of dark matter, the shape and orientation of galaxies are also affected by tidal forces due to dark matter. The orientations of galaxies separated by cosmological distances are thus expected to be related via gravity. If so, we can use such information of galaxies to further investigate the origin and evolution of the universe.
This study used data on 1.2 million galaxies obtained from the Sloan Digital Sky Survey to analyze not only the position of galaxies but also the alignment of galaxy orientations. The 1.2 million galaxies were divided into three samples based on distance from the observer and galaxy brightness. For each sample, the researchers measured the magnitude of the alignment of the galaxies relative to the distant galaxies, and found a clear signal in all cases (Fig. 1). This means that even galaxies tens to hundreds of millions of light years away from each other are related in their orientations via gravity.
To explain this result, the researchers developed a theoretical model that the orientation of galaxies is determined by the gravity induced by the distribution of dark matter, and compared it with the measurements. The comparison showed that, by newly combining galaxy orientations with the conventional information, the predictions of the Standard Model of Cosmology well explain the observations, and no deviation from the model was found (Fig. 2). This implies that gravitational forces acting between galaxies separated by tens to hundreds of millions of light years apart are well described by general relativity. In the past, the growth rate due to gravity has been measured using various galaxy observation data using their three-dimensional positions. The present study succeeded in determining it more precisely by newly combining information on the orientations of galaxies, demonstrating that the use of galaxy orientation information provides a stronger test of general relativity.
"The measurement results that the orientation of galaxies, which are separated by tens to hundreds of millions of light-years, is related are amazing from the perspective of daily life. What is even more surprising is that the measurement results are in excellent agreement with theoretical calculations based on general relativity. While a further scrutiny is still necessary due to potential flaws in general relativity, this achievement represents a significant gain as we have pioneered a new method based on the current results. In the future, I hope to advance this research further by utilizing new dataset to uncover various mysteries of the universe," Atsushi Taruya explained.
Although galaxy orientation has been used to measure gravitational lensing effects in the past, galaxies' own orientation is an obstacle in capturing the slight distortions caused by gravitational lensing. Therefore, the research focus was how to remove it in order to capture a clear gravitational lensing signal. In this study, we have considered the galaxy’s own orientation as a cosmological probe. Although the archival data set was used in the research, observations are currently underway around the world, and it is expected that the availability of a vast amount of observation data will advance the cosmological analysis with high precision. In particular, the Subaru Telescope project in Japan will provide extremely high-quality observational data, thanks to the Hyper Suprime-Cam (HSC) and Prime Focus Spectrograph (PFS) mounted on the telescope, for which Taiwan has been contributing significantly. An important point to stress is that the anticipated achievement from future data with galaxy orientations are not only restricted to a precision test of general relativity. The alignment and shape of galaxies are closely related to the formation and evolution of the universe, and a further benefit to enhance scientific outcome is expected if we can elucidate their relation to unknown phenomena that have occurred in the early universe.
"The possibility that the shape of galaxies can be used to investigate the physical laws governing the universe has been discussed from the theoretical point of view. It’s a privilege to be able to demonstrate this observationally for the first time. In the future, I would like to extend this analysis to larger galaxy maps such as the Subaru PFS, which is an international collaboration that Japan is conducting with Taiwan and other countries, for more detailed analysis," said the lead author Teppei Okumura.
Figure1: Galaxy orientations aligned toward the filamentary dark matter distribution via gravity. The top-right panel shows the magnitude of the measured alignment at each distance between galaxies, and shows how the measured values agree well with the model. (Based on images from TNG simulations and the results of this study). Image Credit: Okumura & Taruya
Figure2: (Upper panel) The speed at which the structure of the universe grows. The higher the value, the faster it grows. The horizontal axis is the comoving distance to the sample of galaxies from the observer. The orange band is the expected range of the growth rate predicted by the Standard Model of Cosmology based on general relativity. The three red circles with error bars are the measurement results for the three samples of galaxies obtained in this study. All are consistent with general relativity. (Lower panel) The measurement error in the growth rate of the structure. The measurement error obtained in the upper panel (red circles) is compared to the error obtained from the conventional method (blue circles). Image Credit: Okumura & Taruya
This research presented in a paper “First Constraints on Growth Rate from Redshift-Space Ellipticity Correlations of SDSS Galaxies at 0.16 < z < 0.70,” by Okumura & Taruya, has appeared in the Astrophysical Journal Letters on Mar. 13th., 2023.
Dr. Teppei Okumura Email: email@example.com Tel: +886 2 2366 5401