The universe, unseen and revealed
Review: The Invisible Universe, Matthew Bothwell, Oneworld
A clear night sky full of stars might give the impression that the universe is busy. But space is incredibly empty. The distances between objects are huge. Add to that the emptiness at the quantum level, and we can say that the universe is mostly made up of nothing. Then again, it’s not quite as empty as that might suggest.
Because of conditions on Earth, our eyes are attuned to the major spectrum of light emitted by the sun; if we lived elsewhere, with different light conditions, it would be different. That part of the light spectrum we can see is, however, a tiny part of the total spectrum, Matthew Bothwell points out. So, when it comes to astronomy, regular telescopes only offer a sliver of a view. Galileo, say, was restricted not just by how powerful – in other words, how far he could see – but by what part of the light spectrum he could see. Just like how catfish have a different view of the world by ‘seeing’ electrical impulses, telescopes and measuring devices that operate beyond the visible light spectrum offer a different and wider view of what is out there.
Dark matter was discovered by inference, and scientists infer that it just happens to be the most plentiful ‘stuff’ in the universe. It produces nothing on the electromagnetic spectrum – hence we can’t view it as light – visible to humans or otherwise – but gravity still interacts with it, and its effect on distant galaxy clusters meant it could be estimated how much dark matter there is. So-called gravitational lensing – the distortion of images – shows us that there are unseen masses warping the light as it passes by. (Images from deep space telescopes which show streaks that are actually distant galaxies are quite weird.) After initial over-estimations, ten times normal matter was settled on.
Speaking of the quantum level, that is where scientists turned to explain dark matter. Maybe dark matter consists of tiny, non-everyday particles that outnumber – and hence outweigh – everything else. These particles have to be much heavier than protons, but otherwise undetectable. Except if detectors buried way underground work. So far, after decades of trying and waiting, they haven’t.
Dark matter, by the way, has nothing to do with dark energy, an even more mysterious thing that repels, creating our expanding universe, so it is thought. Weirdly, the balance between matter and dark energy is currently such that this ‘coincidence’ means the promotion of a multiverse theory, a counter to the ‘Goldilocks’ idea of our universe being fortuitously just right for us. Bothwell suggests that while the multiverse may be yet another thing we can’t see, it also currently stumps scientists looking for a way to investigate it beyond just speculation. Bothwell is not sold on the idea but says we should be open to it, especially since the history of science is one of continually expanding our horizons. The multiverse idea does seem to me, though, to be in the realm of metaphysics (perhaps literally).
Of course, matter is one thing, but what it is housed in – space – is another. Well, maybe not another ‘thing’. Or is it? Spacetime is not just an unchanging arena where the action takes place – spacetime can vary. It bends with gravity, leaving scientists to surmise that as masses like the stars move through space they leave ripples, like the wake of a boat. Colossal cosmic collisions should, the theory went, send waves large enough to detect on Earth. But only just. Picking up these ripples is very difficult and requires mind-boggling precision. Scientists used multiple light beams bouncing of mirrors to detect gravitational waves, in a device that was the most sensitive measuring device ever created. In 2015 they found gravitational waves. Subsequently, these waves are enlightening where supermassive black holes come from.
Black holes emit X-rays because they are massive and hot. The material that ‘accretes’ around the black hole emits energy, hence light. Quasars are a kind of lighthouse beam of this energy, alerting us to the presence of supermassive black holes. Black holes were more plentiful in the early stages of the universe; many we ‘see’ are billions of years old, the light waves coming from so far away. But, as the name suggests, we can’t see the holes themselves, only what comes to us from around them, from the material that has not yet been sucked deep enough into the holes.
Pulsars are similar, in that they shoot out beams of high energy light, but they are the effects of neutron stars, super-dense collapsed stars. Because pulsars pulse with such precision astronomers can use them as forms of measurement for all sorts of weird observations, including the proposed detection of gravitational waves from the beginning of the universe. Pulsars pulse because they spin, like a crazed lighthouse, and some spin at unbelievable speeds. One of the fastest detected spins at 600 times a second, making a screaming sound in radio telescopes (which simply pick up light waves at very long wavelengths). Other pulsars sound like propeller planes.
In contrast to black holes, there are failed stars out there, too small to ignite hydrogen fusion, ‘burning’ at temperatures below what on Earth we would call ‘freezing’, but still emitting feeble light in the infrared spectrum. The discovery of these mundane objects through infrared observation led to the surprising conclusion that these ‘brown’ stars may be as numerous as the regular stars in the universe.
Dust is another thing that’s pervasive but doesn’t show up well in regular telescopes. Molecular dust clouds are ‘dense’ according to cosmologists but are so thin they are thinner than the most extreme vacuum created here on Earth. (Bothwell says think of them like smoke.) Infrared telescopes show these clouds, and the images are then artificially coloured in reds and blues to indicate that we are looking at infrared light with different wavelengths. These clouds are nurseries for stars. The images also happen to be extraordinarily beautiful. This dust is great for making stars, but it obscures what is behind them, at least in the visible light spectrum. But we can see through them with infrared, just like how firefighters can see through smoke with infrared goggles.
Bothwell does a fine job of explaining some counterintuitive or just odd science in fairly simple terms. Additionally, his book reinforces the intriguing fact that what operates at the gigantic level of galaxies is related directly to what happens in the atomic and subatomic worlds. There is an inseparability between the huge and the tiny. What happens at a level too small to see illuminates what happens at a scale too large to see. Another striking thing is how our technological advancements, which allow further understanding, also then throw up further conundrums and complexities. The sheer size of the universe is not the only reason for humility; much of the universe operates in ways we are just not privy to.
Nick Mattiske blogs on books at coburgreviewofbooks.wordpress.com and is the illustrator of Thoughts That Feel So Big.
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