Abell 2199, RXCJ1504.1-0248, Abell 85, Abell 3667
Credit: NASA/CXC/Univ. of Bonn/K. Migkas et al.
This graphic contains a map of the full sky and shows four of the hundreds of galaxy clusters that were analyzed to test whether the Universe is the same in all directions over large scales, as described in our latest press release.
Galaxy clusters are the largest objects in the Universe bound by
gravity and astronomers can use them to measure important cosmological
properties. This latest study uses data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton to investigate whether or not the Universe is "isotropic."
The sky map in this schematic is in "galactic coordinates," with the plane of the Milky Way
running along the middle (instead of the equator like is used for
Earth). Galactic longitude runs in the horizontal, or "x" direction,
and galactic latitude runs in the vertical, or "y" direction. The dark
points show the location in the sky map of the 313 galaxy clusters
observed with Chandra and XMM-Newton and included in this study. The
four Chandra images of galaxy clusters from the new study are, in a
clockwise direction from the top left, Abell 2199, RXCJ1504.1-0248,
Abell 3667 and Abell 85. Galaxy clusters with galactic latitudes less
than 20 degrees were not included in the survey to avoid obscuration
from the Galaxy itself, which has most of its stars, gas and dust along a
thin plane. Similarly, galaxy clusters behind two nearby galaxies, the
Small Magellanic Cloud and the Large Magellanic Cloud, and behind the
Virgo galaxy cluster were not included to avoid obscuration.
A2199, RXCJ1504.1-0248, A85, A3667
Credit: NASA/CXC/Univ. of Bonn/K. Migkas et al.
Astronomers generally agree that after the Big Bang,
the cosmos has continuously expanded 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.
This latest test uses a powerful, novel and independent technique and
suggests the concept of an isotropic Universe may not entirely fit. The
study 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 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.
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.
Either of these two cosmological explanations would have significant
consequences. The astronomical community must perform other scrutinized
tests obtaining consistent results every time to truly know if the
concept of an isotropic Universe should be reconsidered.
A paper describing these results will appear in the April 2020 issue of the journal Astronomy and Astrophysics and is available online.
The authors are Konstantinos Migkas (University of Bonn, Germany),
Gerrit Schellenberger (Center for Astrophysics | Harvard &
Smithsonian), Thomas Reiprich, Florian Pacaud and Miriam Elizabeth
Ramos-Ceja (University of Bonn), and Lorenzo Lovisari (CfA).
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages
the Chandra program for NASA's Science Mission Directorate in
Washington. The Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, controls Chandra's science and flight operations.
Fast Facts for Abell 85:
Scale: Image is about 15 arcminutes (3 million light years) across.
Category: Cosmology/Deep Fields/X-ray Background, Groups & Clusters of Galaxies
Coordinates (J2000): RA 00h 42m 50.7s | Dec -09° 38´ 45"
Constellation: Cetus
Observation Date: 12 pointings between September 2004 through August 2013
Observation Time: 65 hours (2 days 17 hours)
Obs. ID: 4881-4888, 15173, 16264, 15174, 16263
Instrument: ACIS
References: Migkas, K. et al., 2020, A&A; arXiv:2004.03305
Color Code: X-ray: Magenta
Distance Estimate: About 760 million light years (z=0.056)
Source: NASA’s Chandra X-ray Observatory
