To celebrate Halloween, Hubble brings you this ink image of the galaxy cluster Abell 611, located more than 1,000 megaparsecs, or about 3.2 billion light-years, from Earth. Like all galaxy clusters, the continued existence of Abell 611 poses a mystery to astronomers.
Specifically, there doesn’t appear to be enough mass contained within its network of rapidly rotating constituent galaxies to keep the cluster from flying apart. This is a well-established problem in astronomy with very massive structures, such as galaxies and galaxy clusters – they just don’t seem to contain enough combined mass to stay whole.
Interestingly, this problem does not arise on smaller cosmic scales. For example, the passage of the planets of the solar system around the sun can be calculated relatively easily using the masses and locations of the planets and the sun. No additional mass is needed to explain the integrity of the solar system or other planet-star systems. So why is this intuitive rule collapsing on a larger scale?
The popular theory is that the universe contains large amounts of a substance known as dark matter. While the name might sound ominous, “dark” simply refers to the fact that this unknown quantity doesn’t seem to interact with light like any other matter does – neither emitting, reflecting, or absorbing any part of the electromagnetic spectrum. This dark quality makes dark matter incredibly difficult to characterize, although various possibilities have been postulated.
Essentially, most dark matter candidates fall into one of two categories: a type of particle that exists in large quantities throughout the universe, but which for some reason does not interact with light as do other particles; or some type of massive object also exists in great abundance throughout the universe, but is not amenable to detection using current telescope technology.
Two of the most fanciful dark matter candidates belong to the first and second category, respectively. Weakly Interacting Massive Particles (WIMPs) are hypothetical subatomic particles that do not interact with photons, that is, they do not interact with light. Massive Astrophysical Compact Halo Objects (MACHO) are a hypothetical set of very massive objects made (unlike WIMPs) of a type of matter that we already know, but which are extremely difficult to observe because they emit so little light.
Despite considerable effort, however, no conclusive evidence has been found regarding WIMPs, MACHOs, or any other form of dark matter.
If dark matter remains stubbornly indefinable, it is fortunately easily quantifiable. In fact, clusters of galaxies such as Abell 611 are ideal laboratories for dark matter quantification, due to the abundant evidence of gravitational lensing visible in the cluster. An example of a lens is perhaps most clearly visible in the center of the image, to the left of the cluster’s luminous core, where a curve of light can be seen. This curve is light from a more distant source that has been bent and distorted (or “lensed”) by the vast mass of Abell 611.
The extent to which light has been bent by the cluster can be used to measure its true mass. This can then be compared to an estimate of its mass derived from all visible components of the cluster. The difference between the calculated mass and the observed mass is staggering. In fact, zooming out, astronomers currently estimate that around 85% of the matter in the universe is dark matter.
While the mystery of what holds the cosmic web of galaxies together in Abell 611 remains unsolved, we can still enjoy this image and the fascinating science – both well-established and theorized – that unfolds within it. .
How a group of stars excluded the MACHO
Quote: Hubble celebrates spooky season with Abell 611, a cobweb of galaxies held together by a dark secret (2022, October 27) Retrieved October 27, 2022 from https://phys.org/news/2022-10-hubble -celebrates-spooky-season-abell.html
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