Analyzing a million galaxies unravels cosmic structure origins

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Diving into the cosmic tapestry, a dedicated team of researchers meticulously scrutinized over a million galaxies, embarking on a quest to unravel the origins of the cosmic structures that grace our present-day universe. This recent exploration, detailed in a study published in Physical Review D and recognized as an Editors’ Suggestion, represents a leap forward in our understanding of the cosmos.

The backbone of our comprehension, until now, has rested on precise observations and analyses of the cosmic microwave background (CMB) and large-scale structure (LSS). These investigations have coalesced into the widely accepted ΛCDM model, where cold dark matter (CDM) and the enigmatic force known as dark energy (cosmological constant, Λ) play pivotal roles.

The ΛCDM model posits that primordial fluctuations, birthed in the infancy of the universe, served as catalysts, setting in motion the grand orchestration of celestial entities—from stars to galaxies and expansive galaxy clusters. Although minuscule at their genesis, these fluctuations burgeon over time under the gravitational embrace, culminating in the formation of dark matter-dense regions or halos. These halos, in a cosmic ballet, collide and coalesce, birthing the galaxies that adorn the vast cosmic expanse.

The spatial arrangement of galaxies, a testament to the nature of their primordial origins, has been a focal point of statistical analyses. Beyond the conventional examination of galaxies as mere points in space, a paradigm shift has occurred. Researchers, delving deeper into the cosmic symphony, now explore the shapes of galaxies. This nuanced approach not only offers additional insights but also provides a unique perspective into the intrinsic characteristics of the primordial fluctuations (see Figure 2).

Figure 2: Visualization of how the “different” primordial fluctuations of the universe lead to the different spatial distribution of dark matter. The central figure (common to both the upper and lower rows) shows the fluctuations in the reference Gaussian distribution. The color gradation (blue to yellow) corresponds to the value of the fluctuation at that location (low to high density regions). The left and right figures show fluctuations that deviate slightly from the Gaussian distribution, or are non-Gaussian. The sign in parentheses indicates the sign of the deviation from Gaussianity, corresponding to a negative (-) deviation on the left and a positive (+) deviation on the right. The top row is an example of isotropic non-Gaussianity. Compared to the central Gaussian fluctuation, the left figure shows an increase in large negative (dark blue) regions, while the right figure shows an increase in large positive (bright yellow) regions. It is known that we can search for such isotropic non-Gaussianity using the spatial distribution of observed galaxies. The lower panel shows an example of anisotropic non-Gaussianity. Compared to the isotropic case in the upper panel, the overall brightness and darkness is unchanged from the Gaussian fluctuation in the central panel, but the shape of each region has changed. We can search for this “anisotropic” non-Gaussianity from the spatial pattern of galaxy shapes. Credit: Kurita ＆ Takada

Led by Toshiki Kurita, a former graduate student at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) and currently a postdoctoral researcher at the Max Planck Institute for Astrophysics, along with Kavli IPMU Professor Masahiro Takada, a research team pioneered a groundbreaking approach to gauge the power spectrum of galaxy shapes. Their innovative method involved extracting crucial statistical insights from galaxy shape patterns by amalgamating spectroscopic data on the spatial distribution of galaxies with imaging data detailing individual galaxy shapes.

The team conducted a comprehensive analysis of both the spatial distribution and shape patterns of around 1 million galaxies sourced from the Sloan Digital Sky Survey (SDSS), the preeminent galaxy survey worldwide. Their efforts yielded a profound revelation – a statistically significant alignment in the orientations of two galaxies’ shapes, separated by over 100 million light years (see Figure 3). Strikingly, these correlations persisted among galaxies whose formation processes seemed entirely independent and causally unrelated.

Figure 3: The blue dots and error bars are the values of the galaxy shape power spectrum. The vertical axis corresponds to the strength of correlation between two galaxy shapes, i.e., the alignment of the galaxy shape orientations. The horizontal axis represents the distance between two galaxies, with the left (right) axis representing the correlation between more distant (closer) galaxies. The gray dots indicate non-physical apparent correlations. The fact that this value is zero within error, as expected, confirms that the blue measured points are indeed astrophysically origined signals. The black curve is the theoretical curve from the most standard inflationary model, and it is found to be in good agreement with the actual data points. Credit: Kurita & Takada

“In this research, we were able to impose constraints on the properties of the primordial fluctuations through statistical analysis of the ‘shapes’ of numerous galaxies obtained from the large-scale structure data. There are few precedents for research that uses galaxy shapes to explore the physics of the early universe, and the research process, from the construction of the idea and development of analysis methods to the actual data analysis, was a series of trial and error,” remarked Kurita, reflecting on the challenges overcome during his doctoral program.

Moreover, an in-depth examination of these correlations substantiated their consistency with predictions made by inflation, while notably lacking a non-Gaussian feature in the primordial fluctuation.

Takada lauded the research as a remarkable achievement, noting its threefold nature: theory development, measurement, and application. While the quest did not unearth groundbreaking revelations in detecting new physics of inflation, it has paved the way for future exploration. Takada expressed pride in the accomplishment and anticipates further research avenues using the Subaru Prime Focus Spectrograph.

The methodologies and outcomes of this study promise to be instrumental in future endeavors, offering researchers a means to rigorously test the foundations of inflation theory and potentially unlock new realms of understanding in cosmology.

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