Wonders of the Universe

VISIBLE LIGHT

Standing among the dunes of the Namib Desert you become aware of the sheer scale of the landscape. It is a landscape sculpted by the Sun and coloured by it at all times.

Stretching along the west coast of southern Africa is the Namib Desert. It is the oldest desert in the world; its landscape is a shifting sea of sand of over 77,700 square kilometres (30,000 square miles) which changes every minute, a consistently arid wilderness that has stubbornly avoided moisture for over fifty million years. This is a world sculpted by the Sun; its energy drives the wind that shapes the tiny grains of sand into magnificent dunes, and the colours hidden in its light paint the landscape deep orange. Yet even when the Sun has set, the desert remains awash with light and colour, but the human eye can’t see it.

Visible light is a tiny fraction of the light in the Universe. Beyond the red, the electromagnetic spectrum extends to wavelengths too long for our eyes to detect. It’s still light; still the sloshing back and forth of the electric and magnetic fields driving forwards through the void at the special universal speed, it’s just we didn’t evolve to see it. In the Namib Desert you can feel this light, though, if you hold your palms towards the sand. The dunes are warm long after sunset, and this residual heat is nothing more than long-wavelength light. A scientist would call it infrared light; the only difference between infrared and visible light is the wavelength – infrared has a longer wavelength than visible light. Travel further along the spectrum, past infrared, and we arrive at microwaves, with wavelengths unsurprisingly about the size of a microwave oven. The spectrum then seamlessly slides into the radio region, with wavelengths the size of mountains.

Throughout most of human history we have been blind to these more unfamiliar forms of light, but to detect them you don’t need expensive, hi-tech kit, just a radio. When tuning a radio you are not tuning into a sound wave, you are picking up information encoded in a wave of light. Most of the radio waves we are familiar with are artificially created and used for communication and broadcasting, but just as there is plenty of visible light in the Universe that isn’t manmade, so there are naturally occurring microwaves and radio waves too. And just like the visible photons from the most distant galaxies, the microwave and radio photons are messengers, carrying detailed information about distant places and times across the Universe and into our technologically created artificial eyes.

Next time you are tuning a radio and can hear static, you are actually listening to a deeply profound sound – you are listening to the Big Bang.

Next time you tune a radio, listen to the static between the stations. About 1 per cent of this is music to the ears of a physicist because it is stretched light that has travelled from the beginning of time. Deep in the static is the echo of the Big Bang. These radio waves were once visible light, but light that originated 400,000 years after the Big Bang. Prior to that, the observable universe was far smaller and hotter than it is today. At 273 million degrees Celsius, this is an order of magnitude hotter than the centre of a star, so hot that the hydrogen and helium nuclei then present in the Universe couldn’t hold onto their electrons to form atoms. The Universe was a super-heated ball of naked atomic nuclei and electrons known as a plasma. Light cannot travel far in dense plasma because it bounces off the electrically charged subatomic particles. It was only when the Universe had expanded and cooled down enough for the electrons to combine with the hydrogen and helium nuclei to form atoms that light was free to roam. This point in the evolution of the Universe, known as recombination, occurred around 400,000 years after the Big Bang, when the Universe had cooled to about 3,000 degrees Celsius and was around a thousandth of its present size. That is close to the surface temperature of red giant stars, so the whole Universe would have been glowing with visible light like a vast star. The Universe has become cooler and more diffuse since, so this ancient light has been free to fly through space, and it is some of these wandering messengers that we collect with a detuned radio today. However, as the Universe has expanded, space has stretched and so too has the light – so much so that the light is no longer in the visible part of the spectrum. It has moved beyond even the infrared, and is now visible to us only in the microwave and radio parts of the spectrum. This faint, long, wavelength universal glow is known as the Cosmic Microwave Background, or CMB, and its discovery in 1964 by Arno Penzias and Robert Wilson was key evidence in proving that the Universe began in a Big Bang

Forget state-of-the-art kit, all you need to use to detect hidden forms of light is a simple radio. As you tune, it you will pick up information encoded in a wave of light.

Only a fraction of light is visible in the Universe. This infrared image shows the massive scale of the Universe and demonstrates how the electromagnetic spectrum extends to wavelengths that are too long for our eyes to detect. Here we can see hundreds of thousands of stars at the core of the Milky Way Galaxy, but so many are still hidden from our view.

NASA