Each galaxy lives within its own three-dimensional halo of dark matter whose gravitational field corrals the stars within it. Without the stars, the halo would still be there, albeit invisible to our eyes; but if the halo vanished, the stars would scatter into the depths of the universe – just as a Christmas tree remains a tree with or without the pretty lights. Whereas without the tree, the lights would merely be a puddle of colour on the snowy ground.
Growing up in the Southern Hemisphere, the traditional trappings of Christmas were always out of step with the onset of summer around us: Santa in his cozy suit, imagery of roaring fireplaces, snowy scenes on Christmas cards, a heavy meal we consumed on a hot day before a swim.
But after I’d experienced my first Northern Hemisphere Christmas in Ithaca, New York — which, with several feet of snow on the ground, was something close to Narnia — the childhood strangeness of that transplanted holiday melted away and the seasonal symbolism at last made sense. A festival of light in the darkness of midwinter; gathering around a warm hearth while it snowed outside.
Fast forward a few years and I was living in New York City, where the Rockefeller Christmas tree (and the ice skaters beneath it) stands as a marker of the turning seasons. Another few years further on, my family and I were living in Connecticut, where the town of New Haven marks the season by selecting an enormous local evergreen to make the ultimate sacrifice in exchange for the chance to stand, dramatically lit, in the New Haven Green through the Yuletide season. We lived close by, and it became an annual ritual to walk to see the tree with the kids, always in winter jackets, often with snow on the ground.
Once, as we approached the illuminated tree, my wife Jolisa – a literature person, never not searching for metaphors to help make sense of science – asked: “You know this dark matter stuff that you talk about, is it something like a Christmas tree at night — we can see the bright twinkling lights, but we can only make sense of why they’re hanging in the air in that shape if we know about the tree that holds them up?”
And she was exactly right.
Our sun is one of roughly 100 billion stars in the Milky Way Galaxy, and the Milky Way is itself one of roughly a trillion galaxies in the visible universe. For over 100 years, astronomers and physicists have been trying to understand how galaxies, the giant islands of stars that are the large-scale buildings blocks of the universe, hold themselves together. If galaxies are made entirely of stars – in other words, if what we see is all we’ve got – the stars would be moving too fast for their mutual gravitational attraction to hold a galaxy together.
A selection of galaxies; each image contains over a hundred billion stars.
To make sense of this, the vast majority of astrophysicists and astronomers have come to believe that the cosmos is now contains far more than our eyes can see. As we now see the universe, each galaxy lives within its own three-dimensional halo of dark matter, whose gravitational field corrals the stars within it. Without the stars, the halo would still be there, albeit invisible to our eyes; but if the halo vanished, the stars would scatter into the depths of the universe – just as a Christmas tree remains a tree with or without the pretty lights. Whereas without the tree, the lights would merely be a puddle of colour on the snowy ground.
So if you are seeking a secular interpretation of the iconography of Christmas, you could do worse than seeing a well-trimmed Christmas tree, illuminated with lights and bedecked with tinsel, as a metaphor for the cosmos.
Dark matter – by definition – neither emits nor absorbs light, and cannot thus be made of atoms, or indeed any of the fundamental particles known to physicists; it must be something entirely novel. So likewise, let the spectacle of a galaxy serve as a reminder that there is literally more to the physical world than meets the eye, and that there are deep mysteries for us to solve in the years and decades to come.
Happy holidays and compliments of the season.
Footnote: Actually this sort of was my TED talk. And, full disclosure, we can’t (and shouldn’t) be sure about dark matter until we have a better idea of its properties and composition.