One of the fundamental ideas of cosmology is that everything looks the
same in all directions if you look over large enough distances. A new study
using data from NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton is
challenging that basic notion.
Astronomers
using X-ray data from these orbiting observatories studied hundreds of galaxy
clusters, the largest structures in the universe held together by gravity, and
how their apparent properties differ across the sky.
“One of the
pillars of cosmology – the study of the history and fate of the entire universe
– is that the universe is ‘isotropic,’ meaning the same in all directions,”
said Konstantinos Migkas of the University of Bonn in Germany, who led the new
study. “Our work shows there may be cracks in that pillar.”
Astronomers
generally agree that after the Big Bang, the cosmos has continuously expanded.
A commonly analogy is that this expansion is like a baking loaf of raisin
bread. As the bread bakes, the raisins (which represent cosmic objects like
galaxies and galaxy clusters) all move away from one another as the entire loaf
(representing space) expands. With an even mix the expansion should be uniform
in all directions, as it should be with an isotropic universe. But these new
results may not fit that picture.
“Based on our
cluster observations we may have found differences in how fast the universe is
expanding depending on which way we looked,” said co-author Gerrit
Schellenberger of the Center for Astrophysics | Harvard & Smithsonian (CfA)
in Cambridge, Massachusetts. “This would contradict one of the most basic
underlying assumptions we use in cosmology today.”
Scientists have
previously conducted many tests of whether the universe is the same in all
directions. These included using optical observations of exploded stars and
infrared studies of galaxies. Some of these previous efforts have produced
possible evidence that the universe is not isotropic, and some have not.
This latest test
uses a powerful, novel and independent technique. It capitalizes on the
relationship between the temperature of the hot gas pervading a galaxy cluster
and the amount of X-rays it produces, known as the cluster’s X-ray luminosity.
The higher the temperature of the gas in a cluster, the higher the X-ray
luminosity is. Once the temperature of the cluster gas is measured, the X-ray
luminosity can be estimated. This method is independent of cosmological
quantities, including the expansion speed of the universe.
Once they
estimated the X-ray luminosities of their clusters using this technique,
scientists then calculated luminosities using a different method that does
depend on cosmological quantities, including the universe’s expansion speed.
The results gave the researchers apparent expansion speeds across the whole sky
– revealing that the universe appears to be moving away from us faster in some
directions than others.
The team also
compared this work with studies from other groups that have found indications
of a lack of isotropy using different techniques. They found good agreement on
the direction of the lowest expansion rate.
The authors of
this new study came up with two possible explanations for their results that
involve cosmology. One of these explanations is that large groups of galaxy
clusters might be moving together, but not because of cosmic expansion. For
example, it is possible some nearby clusters are being pulled in the same
direction by the gravity of groups of other galaxy clusters. If the motion is
rapid enough it could lead to errors in estimating the luminosities of the
clusters.
These sorts of
correlated motions would give the appearance of different expansion rates in
different directions. Astronomers have seen similar effects with relatively
nearby galaxies, at distances typically less than 850 million light years,
where mutual gravitational attraction is known to control the motion of
objects. However, scientists expected the expansion of the universe to dominate
the motion of clusters across larger distances, up to the 5 billion light years
probed in this new study.
A second possible
explanation is that the universe is not actually the same in all directions.
One intriguing reason could be that dark energy – the mysterious force that
seems to be driving acceleration of the expansion of the universe – is itself
not uniform. In other words, the X-rays may reveal that dark energy is stronger
in some parts of the universe than others, causing different expansion rates.
“This would be
like if the yeast in the bread isn’t evenly mixed, causing it to expand faster
in some places than in others,” said co-author Thomas Reiprich, also of the
University of Bonn. “It would be remarkable if dark energy were found to have
different strengths in different parts of the universe. However, much more
evidence would be needed to rule out other explanations and make a convincing
case.”
Either of these
two cosmological explanations would have significant consequences. Many studies
in cosmology, including X-ray studies of galaxy clusters, assume that the
universe is isotropic and that correlated motions are negligible compared to
the cosmic expansion at the distances probed here.
The team used a
sample of 313 galaxy clusters for their analysis, containing 237 clusters
observed by Chandra with a total of 191 days of exposure, and 76 observed by
XMM-Newton, with a total of 35 days of exposure. They also combined their
sample of galaxy clusters with two other large X-ray samples, using data from
XMM-Newton and the Japan-US Advanced Satellite for Cosmology and Astrophysics
(ASCA), giving a total of 842 different galaxy clusters. They found a similar
result using the same technique.
A paper
describing these results will appear in the April 2020 issue of the journal
Astronomy and Astrophysics and is available online.
Source: https://myfusimotors.com/2020/04/12/universes-expansion-may-not-be-the-same-in-all-directions/
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