Jess Lewis
Our universe contains millions of galaxies of all different types, but what they all have in common is rotation. Galaxies spin around their centre of mass at hundreds of kilometres per second, but these speeds are not what scientists expected. These galaxies were rotating at speeds so great that with their measured masses they should have been flying apart. This is where astronomers first saw the need for the introduction of a new ‘invisible’ matter, dark matter.
there was no where near enough mass in each galaxy to cause the amount of gravity needed to stop the galaxy from flying apart
Dark matter is very different from the regular matter that we encounter with on a daily basis as it doesn’t interact with the electromagnetic force. Normal atoms of matter contain electrons that exist at specific energies (due to quantization); to move these electrons up and down their energies the atom absorbs and emits photons (particles of light). These photons are what we can detect and see. We can use telescopes to construct pictures of this type of mass even if the light it emits isn’t visible to humans, but this isn’t possible for dark matter. It remains elusive to all current method of detection so when we call it ‘dark’ what we really mean is ‘invisible’. The only way we can even theorise about the existence of dark matter is via its gravitational effects.
The term ‘dark matter’ was first coined in the 1933 at Caltech by astronomer Fritz Zwicky. By measuring the speed of galaxies spinning in the Coma cluster he concluded that there was no where near enough mass in each galaxy to cause the amount of gravity needed to stop the galaxy from flying apart. He measured only a 10th of the mass needed and in effect discovered that most of the mass was invisible.
scientists have begun to look at the way dark matter is causing light to bend
At this point dark matter remained a concept, a highly theoretical idea that still had very little proof, it wasn’t until Vera Rubin’s work in the 1970s that real progress was made. Rubin’s worked focused on measure the speed of galaxies with more precision then ever before. She concluded that the inside and the outside of the galaxies she was measuring where spinning at the same rate which shouldn’t have been possible with the distribution of mass. She began sketching the galaxies and what they should look like and concluded that there must be a ‘halo’ of dark matter around the galaxies.
More recently scientists have begun to look at the way dark matter is causing light to bend. Mass works in a way that bends the universe’s fabric creating little wells; a high concentration of dark matter like that found in a universe creates one of these wells. When light passes by these it bends to travel around it with the contours of the fabric. Scientists can measure this light’s path and deviation and predict the amount of mass that has caused it and then compare it to the mass observed by conventional telescopes. This has created a lot more data in favour of the existence dark matter.
Despite the consensus within the scientific community for the existence of this allusive ‘invisible’ dark matter it’s still a long way from being proven but with dark matter making up over 85% of all the matter in the universe it’s a field with lots of studies behind it.
Jess Lewis
Featured image courtesy of Jeremy Thomas via Unsplash. Image license found here. No changes were made to this image.
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