Stan says, "Let's say the expansion is uneven in a defined part of the universe."
Konstantinos Migkas, of the University of Bonn in Germany, says that extrapolating that isotropic property to the present day might be relying on too many assumptions.
The current model of the universe holds that the rate of expansion is forever accelerating due to an ever-mysterious force called “
dark energy” that is not well understood at all.
In a statement, Migkas says:
"[Dark energy’s] baffling nature has not yet allowed astrophysicists to understand it properly. Therefore, assuming it to be isotropic is almost a leap of faith for now. This highlights the urgent need to investigate if today’s universe is isotropic or not.”
In the study, Migkas and colleagues looked 842 galaxy clusters, using data from three space telescopes: NASA’s Chandra X-ray Observatory, Europe’s XMM-Newton and the Advanced Satellite for Cosmology and Astrophysics which was launched as a partnership between the US and Japan. The researchers analyzed the temperature data from these
galaxy clusters and used the data to calculate the X-ray luminosity of each galaxy in a manner that does not require knowledge of outside factors like the universe’s expansion rate.
Then, the researchers calculated X-ray luminosity for each cluster in a way that did require knowledge of the universe’s expansion. These two calculations side by side revealed the universe’s expansion rate across the sky, and it wasn’t the same everywhere. Migkas explains:
“We managed to pinpoint a region that seems to expand slower than the rest of the universe, and one that seems to expand faster! Interestingly, our results agree with several previous studies that used other methods, with the difference that we identified this ‘anisotropy’ in the sky with a much higher confidence and using objects covering the whole sky more uniformly.”
But Migkas says this doesn’t prove that the expansion rate isn’t uniform. He says that this is simply proof that it’s something cosmologists shouldn’t take for granted. The team says there are other possible explanations, such as gravity between separate galaxy clusters giving the illusion of different expansion rates.
But if it turns out that the expansion rates change throughout the universe it would reveal intriguing new mysteries into the structure and nature of the universe. It just goes to show that it might be a bit silly for us to have any assumptions at all about the true nature of the universe.
The researchers determined the temperature of each cluster by analyzing the X-ray emissions coming from huge fields of hot gas within them. They used this temperature information to estimate each cluster’s inherent X-ray luminosity, without needing to take into account cosmological variables such as the universe’s expansion rate.
The researchers then calculated X-ray luminosity for each cluster in a different way, one that did require knowledge of the universe’s expansion. Doing so revealed apparent expansion rates across the entire sky — and these rates didn’t match up everywhere.
“We managed to pinpoint a region that seems to expand slower than the rest of the universe, and one that seems to expand faster!” Migkas wrote in the blog post.
“Interestingly, our results agree with several
previous studies that used other methods, with the difference that we identified this ‘anisotropy’ in the sky with a much higher confidence and using objects covering the whole sky more uniformly.”
In a nutshell, the gas temperature, the flux and the redshift of a galaxy cluster do not require any cosmological assumptions in order to be measured. Using the flux and the redshift together with the luminosity distance, through which the cosmological parameters come into play, one can obtain the luminosity of a cluster. The luminosity however can also be predicted (within an uncertainty range) based on the cluster gas temperature. Hence, adjusting the cosmological parameters, one can make the two luminosity estimations match. This can be repeatedly applied to different sky patches in order to test the consistency of the obtained values as a function of the direction. The full detailed physical motivation behind this is discussed in M18. There, it is shown that the directional behavior of the normalization of the
LX–
T relation* strictly follows the directional behavior of the cosmological parameter values. This newly introduced method to test the Cosmological Principle (CP) could potentially prove very effective due to the very homogeneous sky coverage of many galaxy cluster samples (in contrast to SNIa samples**), the plethora of available data as well as large upcoming surveys such as eROSITA*** (
Predehl et al. 2016) which will allows us to measure thousands of cluster temperatures homogeneously (
Borm et al. 2014). For studying the isotropy of the Universe with the future surveys, it is of crucial importance that any existing systematic biases that could potentially affect the
LX and
T measurements of galaxy clusters would have been identified and taken into account by then.
Such effects are seen at smaller spatial scales in the universe, the researchers said. But the new study probes clusters up to 5 billion light-years away, and it’s unclear if gravitational tugs could overwhelm expansion forces over such vast distances, they added.
If the observed expansion-rate differences are indeed real, they could reveal intriguing new details about how the universe works. For instance, maybe dark energy itself varies from place to place throughout the cosmos.
“It would be remarkable if dark energy were found to have different strengths in different parts of the universe,” study co-author Thomas Reiprich, also of the University of Bonn, said in the same statement. “However, much more evidence would be needed to rule out other explanations and make a convincing case.”
See:
https://www.aanda.org/articles/aa/abs/2021/05/aa40296-21/aa40296-21.html
See:
https://www.aanda.org/articles/aa/full_html/2020/04/aa36602-19/aa36602-19.html
See:
https://india.timesofnews.com/enter...ansion-rate-may-vary-from-place-to-place.html
* The X-ray luminosity-temperature (LX −T) relation of galaxy clusters is one of the most fundamental parameter correlations, established from previous X-ray observations (e.g. Edge et al. 1990). Since the X-ray luminosity reflects temperature and density profiles of hot intracluster medium (ICM), the LX − T relation should contain information on physical status and evolution of the ICM.
See: The Astrophysical Journal · April 2006 DOI: 10.1086/500294 · Source: arXiv
** SNIa samples: The current sample of high-redshift Type la supernovae (SNela), which combines results from two teams, the High-z Supernova Search Team and the Supernova Cosmology Project, is analysed for the effects of weak lensing. After correcting supernovae (SN) magnitudes for cosmological distances, assuming recently published, homogeneous distance and error estimates, we find that brighter SNe are preferentially found behind regions (5-15 arcmin radius) that are overdense in foreground, z ∼ 0.1 galaxies. This is consistent with the interpretation that SN fluxes are magnified by foreground galaxy excess and demagnified by foreground galaxy deficit, compared with a smooth universe case. The difference between most magnified and most demagnified SNe is approximately 0.3-0.4 mag. The effect is significant at the >99 per cent level. Simple modelling reveals that the slope of the relation between supernova magnitude and foreground galaxy density depends on the amount and distribution of matter along the line of sight to the sources, but does not depend on the specifics of the galaxy biasing scheme.
See:
https://experts.umn.edu/en/publications/weak-lensing-of-the-high-redshift-snia-sample
*** eROSITA: This catalog contains the 542 galaxy cluster and group candidates detected in the eROSITA Final Equatorial-Depth Survey (eFEDS) presented in Liu et al. 2021. The main X-ray properties of these sources including temperature, luminosity, and flux, are measured with the eROSITA data, and are presented.
See:
https://erosita.mpe.mpg.de/edr/eROSITAObservations/Catalogues/
If the observed expansion-rate differences are indeed real, they could reveal intriguing new details about how the universe works. Perhaps dark energy itself varies from place to place throughout the cosmos.
Hartmann352