With Laniakia, Hubble's tension rises

Laniakia supercluster. Credit: Andrew Z. Colvin

One of the biggest mysteries of modern cosmology is the so-called Hubble tension, which arises from our apparent inability to accurately and unambiguously determine the cosmic expansion rate. There are several ways to calculate this speed, from observing distant supernovae to measuring the anisotropy of the cosmic microwave background. Obviously it would be desirable to obtain the same result, using different methods, but unfortunately this is not the case: different measures give slightly different results. We may not fully understand the structure of the universe, or our view of the sky may be distorted, for example due to the fact that we are in a universe. Supercluster of galaxiesknown as sky. One studio Just posted on Journal of Cosmology and Astroparticle Physics, in an attempt to highlight the actual impact of aniakia, concludes that the problem could actually be much more serious. The first author of the study in question is Leonardo Gianni DaleUniversity of Queenslandin Australia, which we reached out to ask them to tell us the details of this interesting research.

Gianni, what is the Laniakea supercluster?

“Laniakea is what it is technically defined as Extensive structure Or large-scale cosmic structure. The name has Hawaiian origins, and can be translated as “immense paradise”. Since this structure contains milky way, often called our “home in the universe” or our “supercluster home.” Average density Dark matter In this region, it is about one percent larger than the rest of the universe, and therefore contains a large number of galaxies trapped by its gravitational field (about 100 thousand). To get an idea of ​​its size, it occupies the same volume as a sphere with a radius of 100 to 150 megaparsecs (Mpc, or hundreds of millions of light-years). The exact number depends on the value of the Hubble constant, whose exact value is one of the greatest mysteries of modern astrophysics.

How do you recognize him? And recognize its limits?

“To learn about them, a team of scientists as part of the “Cosmic Flows” project measured thousands of strange velocities of galaxies and built a map of their distribution in the local universe. the name Strange speeds It comes from the fact that these are velocities obtained after subtracting those resulting from the expansion of the universe. For example, if we see an individual running in a boat moving away from us, his own speed will be the speed at which he ran in the boat, not the total speed of the boat plus his running speed. Simply put, these speeds can be negative, that is, an individual can run towards us, even if the boat moves away. By summing these velocities, it is possible to distinguish between regions of the universe where gravity is holding back galaxies, in technical terms Gravity tubs. Laniakea is specifically Gravity basin Which contains the Milky Way.”

Leonardo Gianni was born in Ragusa and studied physics at the University of Bologna, where in 2016 he obtained a master's degree in theoretical physics. He later obtained a doctorate in astrophysics and cosmology from Vitoria, Brazil. He is currently a postdoctoral fellow at the University of Queensland, in Brisbane (Australia), where he studies the nature of energy and dark matter. Credits: L. Gianni

How might Laniakia affect the expansion rate of the universe?

“To measure the speed of the universe's expansion, we use light sources whose intrinsic brightness we know, in jargon Standard spark plugsFor example Type Ia supernova. Due to the expansion of the universe, these candles move away from us and their light changes Doppler effect Which makes its apparent color redder. The idea behind our work is just that The expansion rate of the universe inside Laniakia is slightly different from its expansion rate outside due to the different distribution of dark matter. In general relativity curvature Free time (What causes the expansion of the universe) depends on the amount of matter and energy in it, so it is natural for it to be affected (locally) by a giant mass the size of Laniakia. By not accounting for this different rate of expansion inside Laniakia, the expansion rate measured by a supernova outside it becomes a weighted average of the two. “Our work provides a recipe for calculating this weighted average appropriately.”

And what did you discover?

«The shape of Laniakea is very complex, therefore in principle it is not easy to obtain “corrections” for the brightness of all standard candles. In our study we suggest that as a first approximation Laniakea can be described as Homogenous ellipsoid (Hence the term Effective model) whose expansion rate along two major axes is slightly smaller than the expansion rate of the rest of the universe, and slightly larger along the third axis. Although simple, the approximation describes the statistical distribution of velocities within Laniakia remarkably well. Due to the simplicity of the elliptical model, it is easy to calculate the brightness correction for each source (e.g. each supernova) which will be different depending on the direction of the sky in which it is located (e.g. positive along a line). The axis that is expanding faster than the rest of the universe). When applied to the most important supernova catalog used to measure the expansion rate, we find that these corrections lead to a change in the Hubble constant by 0.5 km/s/Mpa. It is applied to another catalog of standard spark plugs, in terminology Sbf (abbreviation for Surface brightness fluctuations), the corrected Hubble constant is 1.1 km/s/Mbps».

Did you expect this result?

“Personally, no. The main motivation behind this analysis is the so-called Hubble tensor, one of the most important mysteries in modern cosmology. In summary, measurements of the Hubble constant obtained by analyzing supernovae (and other local sources) and those obtained by fluctuations in cosmic background radiation (and other early universe sources) differ greatly. Since the former are not distributed uniformly across the sky, our hope was that Laniakia anisotropy (a technical term for spatial asymmetry) was at least partly responsible. Simply put, if supernovae are mostly distributed in directions where La Niacia is expanding faster than the rest of the universe, the induced corrections will tend to dampen the Hubble strain. However, the majority of these regions are located in directions in which Laniakia is expanding more slowly, and the result obtained is exactly the opposite. “In short, our study makes the Hubble strain (slightly) worse.”

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