Soundings: More in universe than we can see
As we are held to the Earth, stars are held within galaxies, by gravity.
By measuring how much light a galaxy emits, we can calculate how many stars it has, and, thereby, its mass – what it “weighs.”
Gravity is directly related to mass; knowing a galaxy’s mass tells us how strongly it holds onto its stars.
Problem: Many galaxies are spinning too fast to hold together – their stars should be flying off into intergalactic space.
Something invisible must be adding mass – and thus gravity – to the galaxies.
Our hands are not very good at picking up water – it tends to slip right through our fingers.
The matter hypothesized to be adding mass to galaxies doesn’t interact with light, nor any other form of electromagnetic radiation. No matter how hot it gets, it won’t glow. Nor will it reflect any light. Light slips right through it. It’s dark … dark matter.
Invisibility makes dark matter hard to find; we can only observe its gravitational effects – helping galaxies hold onto their stars, for instance.
Astronomy is not an experimental science. Astronomers don’t throw planets at each other to see what happens, nor rip gas off one star and feed it to another until the latter explodes.
We let the universe perform such experiments for us, and observe the results.
Stars are huge: The sun could balance a third of a million Earths, and it’s only a middleweight star.
Big as they are, stars are not massive enough for an experiment to, say, make dark matter show itself.
The Milky Way Galaxy contains a hundred, maybe two hundred billion stars. Galaxies are really huge.
But not huge enough for a certain dark matter experiment.
Marbles in sacks, stars in galaxies … galaxies are organized into galactic clusters.
Now we’re talking serious mass.
Sometime between one and two billion years ago, in the direction of Pisces, two galactic clusters smashed into each other.
Rub a plastic pen through your hair and it will pull electrons off your hair’s atoms, giving the pen a negative charge. No longer electrically neutral, the pen can now exert an electrical force on other objects – for instance, on a tiny piece of paper, which will cling to it.
Adhesives are sticky because their molecules carry electrical charges. Teflon is slippery because its molecules hide away their charge.
Unable to interact at all via electromagnetism, dark matter is very, very slippery.
When the two Piscean galaxy clusters collided, both ordinary matter and dark matter splashed outward – not unlike the way water splashes when you throw a rock into a pond.
Gravity immediately began hauling both types of matter back toward the site of the collision.
Ordinary matter, feeling both gravity and electromagnetism, rubbed against itself, slowing down its rush outward, making it easier for gravity to pull it back inward.
Dark matter, feeling gravity but not electromagnetism, felt no friction. Slicker ‘n Teflon, it kept flying outward.
We can’t see dark matter, but we can see its effects – the way its gravity bends space-time, which forces the light coming from behind it to twist and bend (much as the light from a spoon in a half-glass of water gets bent).
Analyzing its effects on light, astronomers last week released this image of the collision in Pisces. The yellow and white parts are galaxies within the clusters, seen in visible light by HST.
The blue ring surrounding the clusters show where they have calculated dark matter is still spreading, like a ripple in a pond, out from the collision site.
For information on this discovery, I thank Richard Massey (Caltech), and discovery team members M. James Jee (Johns Hopkins) and Richard White (Space Telescope Science Institute).
Alan Stahler trained as a biologist and is an amateur astronomer. He teaches private enrichment classes for students of all ages. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).
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