We don’t think much about the shape of the universe. But my colleagues and I have published new research that suggests it may be asymmetric or biased. This means it’s not the same in all directions.
Should we care about this? Now, today’s “standard cosmological model” that describes the dynamics and structure of the universe as a whole is based squarely on the assumption that it is isotropic (looks the same in all directions) and that it is uniform on average over large scales.
But several so-called tensions, or data inconsistencies, challenge this idea of a homogeneous universe.
We just published a paper investigating one of the most important of these tensions, called the cosmic dipole anomaly. We conclude that the cosmic dipole anomaly poses a serious challenge to the most widely accepted description of the universe, the Standard Cosmological Model (also known as the lambda CDM model).
So what is the dipole anomaly in the universe? And why is it so problematic for attempting to explain the universe in detail?
Let’s start with the Cosmic Microwave Background (CMB). This is a relic radiation left behind by the Big Bang. The CMB is uniform throughout the sky and is within 1 part in 100,000.
Cosmologists are therefore confident in modeling the universe using the “maximally symmetric” description of spacetime in Einstein’s general theory of relativity. This symmetrical vision of the universe, which looks the same everywhere and in all directions, is known as the “FLRW description.”
This greatly simplifies the solution of the Einstein equations and is the basis of the Lambda-CDM model.
However, several important anomalies exist, including a widely discussed anomaly called the Hubble tension. It is named after Edwin Hubble, who is credited with discovering that the universe is expanding in 1929.
This tension began to emerge in the 2000s with various data sets, primarily from the Hubble Space Telescope, and more recent data from the Gaia satellite. This tension is a cosmological mismatch, in which measurements of the expansion rate of the early Universe do not match measurements from the nearby (more recent) Universe.
The dipole anomaly in the universe has received less attention than the Hubble tension, but it is even more fundamental to our understanding of the universe. So what is it?
It has been established that the cosmic microwave background radiation is symmetric on large scales, and variations in this relic radiation from the Big Bang have been discovered. One of the most important is called CMB dipole anisotropy. This is the largest temperature difference in the CMB, with one side of the sky being hotter and the other side being cooler, by a factor of about 1000. This is thought to be due to the local movement of the solar system.
Therefore, it does not refute the Lambda-CDM model of the universe. However, we need to find corresponding variations in other astronomical data as well.
In 1984, George Ellis and John Baldwin asked whether a similar change, or “dipole anisotropy,” existed in the sky distribution of distant astronomical sources such as radio galaxies and quasars. The sources must be very far away, as nearby sources can produce false “clustered dipoles”.
If the “symmetric universe” FLRW assumption is correct, this variation in distant astronomical sources should be directly determined by the observed variation in the CMB. This is known as the Ellis-Baldwin test, named after the astronomer.
The consistency between CMB and material variations supports the standard Lambda-CDM model. Discord directly disputes this and actually disputes FLRW’s explanation. Because this is a very precise test, the data catalog required to perform the test has only recently become available.
As a result, the universe failed the Ellis-Baldwin test. Changes in matter do not match changes in CMB. Because the possible sources of error are quite different between telescopes and satellites, and at different wavelengths in the spectrum, it is reassuring to know that the same results can be obtained with ground-based radio telescopes and satellites observing at mid-infrared wavelengths.
The cosmic dipole anomaly has thus established itself as a major challenge to standard cosmological models, even if the astronomical community has largely chosen to ignore it.
This may be because there is no easy way to resolve this issue. We need to abandon not only the Lambda-CDM model but also the FLRW description itself and go back to square one.
But new satellites like Euclid and SPHEREx and telescopes like the Vera Rubin Observatory and the Square Kilometer Array are expected to generate an avalanche of data. It is conceivable that we may soon gain bold new insights into how to build new cosmological models using recent advances in a subset of artificial intelligence (AI) called machine learning.
The impact will have a truly profound impact on fundamental physics and our understanding of the universe.
