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An illustration shows the universe expanding during the cosmic dawn, with the flip side, the dark universe dominated by dark photons and dark matter also evolving. | Credit: Robert Lea created by Canva
This article was originally published at The conversation. The publication contributed the article to Space.com Expert Voices: Op-Eds and Perspectives.
Form of the universe it’s not something we often think about. But my colleagues and I have published a new study that suggests it might be asymmetric or warped, meaning it’s not the same in all directions.
Should we care about it? Well, today’s “standard cosmological model” – which describes the dynamics and structure of the whole cosmos – is directly based on the assumption that it is isotropic (looks the same in all directions) and homogeneous when averaged over a large scale.
But several so-called “tensions” — or disagreements about the data — pose challenges to this idea of a uniform universe.
We only have published a paper regarding one of the most significant of these tensions, called the cosmic dipole anomaly. We conclude that the cosmic dipole anomaly represents a serious challenge to the most widely accepted description of the universe, the standard cosmological model (also called Lambda-CDM model).
So what is the cosmic dipole anomaly, and why is it such a problem for attempts to provide a detailed description of the cosmos?
Let’s start with cosmic microwave background (CMB)what is the relic radiation left over from big bang. The CMB is uniform across the sky to within one part in a hundred thousand.
So cosmologists feel confident in modeling the universe using the “maximum symmetric” description of spacetime in Einstein’s theory of general relativity. This symmetrical view of the universe, where it looks the same everywhere and in all directions, is known as the “FLRW description”.
This greatly simplifies the solution of Einstein’s equations and is the basis of the Lambda-CDM model.
But there are several important anomalies, including a widely debated one called Hubble tension. It is named after Edwin Hubble, who is credited with discovering in 1929 that the universe is expanding.
The strain began to emerge from various data sets in the 2000s, mainly from Hubble Space Telescope, as well as recent data from the Gaia satellite. This tension is a cosmological disagreement, where measurements of the expansion rate of the universe from its earliest days do not match measurements from the nearby (more recent) universe.
The cosmic dipole anomaly has received much less attention than the Hubble tension, but it is even more fundamental to our understanding of the cosmos. So what is it?
After it was established that the cosmic microwave background is symmetric on large scales, variations of this relic radiation from the Big Bang were found. One of the most significant is called the CMB dipole anisotropy. This is the largest temperature difference in the CMB, where one side of the sky is hotter and the other cooler – by about one part in a thousand.
A 2013 map of the background radiation left over from the Big Bang, taken by ESA’s Planck spacecraft, captured the oldest light in the universe. This information helps astronomers determine the age of the universe. | Credit: ESA and the Planck collaboration.
This variation of the CMB does not challenge the Lambda-CDM model of the universe. But we should find corresponding variations in other astronomical data.
In 1984, George Ellis and John Baldwin asked whether there was a similar variation, or “dipole anisotropy”, in the sky distribution of distant astronomical sources such as radio galaxies and quasars. The sources must be very far away, as nearby sources could create a false “bunch dipole”.
If the “symmetric universe” FLRW hypothesis is correct, then this variation in distant astronomical sources should be directly determined by the observed variation in the CMB. This is known as Ellis-Baldwin testaccording to astronomers.
Consistency between variations in CMB and matter would support the Lambda-CDM standard model. Discord would directly challenge it and indeed FLRW’s description. Because it is a very precise test, the data catalog needed to perform it has only recently become available.
The result is that the universe fails the Ellis-Baldwin test. The variation of matter does not match that of the CMB. Since the possible sources of error are quite different for telescopes and satellites and for different wavelengths in the spectrum, it is reassuring that the same result is obtained with ground-based radio telescopes and satellites observing at mid-infrared wavelengths.
The cosmic dipole anomaly thus imposed itself as a major challenge to the standard cosmological model, even though the astronomical community chose to largely ignore it.
This may be because there is no easy way to fix this problem. It requires abandoning not just the Lambda-CDM model, but the FLRW description itself, and going back to square one.
However, an avalanche of data is expected from new satellites such as Euclid and SPHEREx and telescopes like Vera Rubin Observatory and the square kilometer matrix. It is conceivable that we may soon receive bold new insights into how to build a new cosmological model by leveraging recent advances in a subset of artificial intelligence (AI) called machine learning.
The impact would be truly huge on fundamental physics and our understanding of the universe.