Mass is not expanding in the universe, space is expanding between the galaxies. Look out at almost any galaxy in the Universe, and you'll find it's moving away from us. The farther away it is, the faster it appears to recede. As light travels through the Universe, it gets shifted to longer and redder wavelengths, because the fabric of space itself is being stretched. At the longest distances, galaxies are being pushed away so rapidly by this expansion that no signals we can send will ever reach them, even at the speed of light.
But even though the fabric of space is expanding throughout the Universe — everywhere and in all directions — we aren't. Our atoms remain the same size. So do the planets, moons, and stars, as well as the distances separating them. Even the galaxies in our Local Group aren't expanding away from one another; they're gravitating towards one another instead. Someday we'll, the Milky Way Galaxy, collide with the Andromeda Galaxy (M-31, NGC-224).
The Milky Way and all the local group galaxies will stay bound together, eventually merging together under their own gravity. Earth will revolve around the Sun at the same orbital distance, Earth itself will remain the same size, and the atoms making up everything on it will not expand. Because the expansion of the Universe only has any effect where another force — whether gravitational, electromagnetic or nuclear — hasn't yet overcome it. If some force can successfully hold an object together, even the expanding Universe won't affect a change.
The fabric of space itself is getting stretched over time, and all the objects within that space are being dragged apart from one another. The farther away an object is from another, the more "stretching" occurs, and so the faster they appear to recede from each other. If all you had was a Universe filled uniformly and evenly with matter, that matter would simply get less dense and would see everything expand away from everything else as time went on.
The superclusters of the Universe — these long, filamentary structures populated with galaxies and stretching for over a billion light years — are being stretched and pulled apart by the Universe's expansion. In the relatively short term, over the next few billion years, they will cease to exist. Even the Milky Way's nearest large galaxy grouping, the Virgo cluster, at just 50 million light years away, will never pull us into it. Despite a gravitational pull that's more than a thousand times as powerful as our own, the expansion of the Universe will drive all of this apart.
With an expanding Universe, we can then understand why distant galaxies recede from us as they do.
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C. FAUCHER-GIGUÈRE, A. LIDZ, AND L. HERNQUIST, SCIENCE 319, 5859 (47)
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On the largest scales, the Universe expands and galaxies recede from each other. But on smaller scales, gravitation overcomes the expansion, leading to the formation of stars, galaxies and clusters of galaxies.
NASA, ESA, AND A. FEILD (STSCI)
Rethinking cosmology: Universe expansion may not be uniform (Update)
by
European Space Agency
Astronomers have assumed for decades that the Universe is expanding at the same rate in all directions. A new study based on data from ESA's XMM-Newton, NASA's Chandra and the German-led ROSAT X-ray observatories suggests this key premise of cosmology might be wrong.
Konstantinos Migkas, a Ph.D. researcher in astronomy and astrophysics at the University of Bonn, Germany, and his supervisor Thomas Reiprich originally set out to verify a new method that would enable astronomers to test the so-called isotropy hypothesis. According to this assumption, the Universe has, despite some local differences, the same properties in each direction on the large, or, macro scale.
Widely accepted as a consequence of well-established fundamental physics, the hypothesis has been supported by observations of the cosmic microwave background (CMB). A direct remnant of the Big Bang, the CMB reflects the state of the Universe as it was in its infancy, at only 380 000 years of age. The CMB's uniform distribution in the sky suggests that in those early days the Universe must have been expanding rapidly and at the same rate in all directions. In today's Universe, however, this may no longer be true.
"Together with colleagues from the University of Bonn and Harvard University, we looked at the behaviour of over 800 galaxy clusters in the present Universe," says Konstantinos. "If the isotropy hypothesis was correct, the properties of the clusters would be uniform across the sky. But we actually saw significant differences."
The astronomers used X-ray temperature measurements of the extremely hot gas that pervades the clusters and compared the data with how bright the clusters appear in the sky. Clusters of the same temperature and located at a similar distance should appear similarly bright. But that is not what the astronomers observed.
"We saw that clusters with the same properties, with similar temperatures, appeared to be less bright than what we would expect in one direction of the sky, and brighter than expected in another direction," says Thomas. "The difference was quite significant, around 30 percent. These differences are not random but have a clear pattern depending on the direction in which we observed in the sky."
Before challenging the widely accepted cosmology model, which provides the basis for estimating the cluster distances, Konstantinos and colleagues first looked at other possible explanations. Perhaps, there could be undetected gas or dust clouds obscuring the view and making clusters in a certain area appear dimmer. The data, however, do not support this scenario.
K. Migkas et al. 2020; Milky Way map: ESA/Gaia/DPAC – CC BY-SA 3.0 IGO
In some regions of space the distribution of clusters could be affected by bulk flows, large-scale motions of matter caused by the gravitational pull of extremely massive structures such as large cluster groups. This hypothesis, however, also seems unlikely. Konstantinos adds that the findings took the team by surprise.
"If the Universe is truly anisotropic, even if only in the past few billion years, that would mean a huge paradigm shift because the direction of every object would have to be taken into account when we analyse their properties," he says. "For example, today, we estimate the distance of very distant objects in the Universe by applying a set of cosmological parameters and equations. We believe that these parameters are the same everywhere. But if our conclusions are right than that would not be the case and we would have to revisit all our previous conclusions."
"This is a hugely fascinating result," comments Norbert Schartel, XMM-Newton project scientist at ESA. "Previous studies have suggested that the present Universe might not be expanding evenly in all directions, but this result—the first time such a test has been performed with galaxy clusters in X-rays—has a much greater significance, and also reveals a great potential for future investigations."
The scientists speculate this possibly uneven effect on cosmic expansion might be caused by dark energy, the mysterious component of the cosmos which accounts for the majority—around 69% – of its overall energy. Very little is known about dark energy today, except that it appears to have been accelerating the expansion of the Universe in the past few billion years.
See:
https://phys.org/news/2020-04-basic-assumption-universe.html
To gain a better understanding of the early history of the study of the expansion of the universe, see:
This web site describes the Hubble Space Telescope and its operations, images, and results.
asd.gsfc.nasa.gov