Recently I attended a lecture at Jodrell Bank, home to the iconic Lovell telescope, on the subject of the continuing search for the first light in the Universe. The talk, from Professor James Dunlop of Edinburgh University, was very well delivered and interesting throughout. Here is a condensed version of the talk, in my own words:
Humanity has always been fascinated by the stars. Our ancient ancestors looked up at the night sky and doubtless marvelled at its brilliance. They saw patterns and figures; the constellations, and gave great weight to the predictions based around their movements. Thousands of years later, we still look up into the sky, but now we are searching for something; the very first light in the Universe.
A crucial part of this search is the Doppler effect. This is the same effect that causes the noise from an ambulance siren to change pitch after it passes you. It happens because the source of the sound waves, the ambulance, is travelling away from you. As it emits a sound wave, it then travels further away from you before emitting the next one.
This means that fewer waves reach you in a given time; the frequency of the sound, the pitch, has decreased.
The same effect happens with light. But, instead of the pitch increasing, it is the wavelength of the light that changes. If the light source is travelling away from you, the light will be stretched, its wavelength will increase, shifting the light to the red end of the spectrum; redshift. If the source is travelling toward you, the light waves will be compressed, the wavelength will decrease, shifting toward the blue end of the spectrum; blueshift. Physicists always seem to come up with the most inventive names.
Knowing that stars emit specific wavelengths of light, depending on the elements they contain, it is possible to calculate how fast a star is moving away from the Earth by how redshifted its light is. By doing just this for stars varying distances from Earth, Edwin Hubble was able to conclude that redshift increased with distance. The further away a star was, the faster it was moving away! This was hugely significant as it indicated that, if the clock were wound back, all the stars in the Universe would have been at a single point in space. This is one of many pieces of evidence that supports the famed Big Bang Theory.
The Universe is known now to be almost 14 billion years old. Thanks to experiments at CERN and other similar facilities around the world, we have a good understanding of the very early stages of the Universe, up to a few hundred thousand years. And, because we can see it, we have a good understanding of the more stable Universe, a Universe filled with trillions of stars in billions of galaxies. However, we do not (yet) have a brilliant picture of the Universe in the infancy of the stars – around 400,000 years after the Big Bang to a few million years later.
This is the hunt for the first light in the Universe, a search for the very first of the stars so that we may better understand how they formed out of the early dark Universe and paved the way for the next generation of star, and ultimately, the Sun and the inhabitants of a small rocky planet nearby.
Key to this search has been the Hubble Space telescope, which has produced a number of stunning pictures over its many years in service, some of which have been featured as the Picture of the Week. By looking at one tiny patch of the sky hundreds of times over a matter a weeks, Hubble was able to produce a famous image; the Hubble Ultra Deep Field. Again, Physics covering itself in glory in the naming game.
Another important piece of information must be mentioned here. Light does not travel infinitely fast. It certainly seems it does in our daily lives though, but this is because it travels fast. Really, really fast. 299,792,458 metres in one second fast. That’s going around the world 7 times in just under a second, or to the moon and back in under 2.
But space is big. Really, really big. It takes light 8 minutes to arrive from the Sun, while on Pluto, the warmth of the Sun takes around 5 hours to arrive. It takes light 4 whole years to arrive from our nearest star, Proxima Centauri. So the distances involved are absolutely phenomenal, or, as my old Maths teacher would say in a distinct Yorkshire accent, proper big.
And the finite speed of light has profound consequences. It means that the light that arrives on Earth shows us an old picture of the star that sent it. As you look up into the sky, you see the Sun as it was 8 minutes ago, if you catch a glimpse of Proxima Centauri, you are actually seeing our neighbour as it was 4 years ago. And if you look deep enough into the sky, you will be able to see the light from the earliest stars, ending its 13 billion year journey across the cosmos by hitting the eye of a creature on a planet that didn’t exist when the light set off.
And this is what Hubble did to produce the Ultra Deep Field image. It captured the light from the earliest stars and galaxies that we can see. But there is a problem. Remember that redshift effect? Well, for the light that originates from the earliest stars, its effect is massive. The stretching of the light toward the red end of the spectrum makes it harder for Hubble to detect the light, meaning that Hubble can only see starlight that has undergone less than a fixed amount of redshift. Any redshift greater than this and Hubble is unable to properly capture the light. This limits how far Hubble can see back in time, as it is the earliest stars have light that is redshifted the most.
Looking at what Hubble can see though, we know that we have not seen the very first stars yet. By analysing the spectrum of light from the oldest stars that Hubble is able to detect, it has been seen that they have some heavier elements, such as Carbon, Nitrogen and Oxygen present. These elements cannot be formed by the stars in the main stages of its life, but only as they are dying. Thus, these stars are not the first stars in the Universe, but only the children of stars that have gone before.
We must keep looking for first light then. And that is exactly what is happening, with NASA planning to launch the James Webb telescope in 2018 (although the launch has already been delayed several times). This brand new telescope will orbit the Earth much further than Hubble, meaning it will experience less interference from the Earth, will have a much bigger mirror to focus the light and, crucially, will be able to detect light much further along the spectrum than Hubble. The James Webb telescope then, may be the first time we have a real chance of seeing the very first light from the oldest of the stars and galaxies.
However, the James Webb telescope is hugely ambitious. With Hubble, the first mirror that the telescope was equipped with was not at all fit for purpose, rendering the telescope entirely useless until astronauts replaced it. In fact, almost the entirety of Hubble has been replaced and upgraded over the years, which is what has made the telescope so long lived. But with James Webb, no such repairs are possible; the telescope is planned to venture too far out into space for astronauts to be able to journey to for repair missions. If something breaks, the game is over. And that is only if the unfolding procedure more complex than IKEA furniture that must be undergone for the huge mirror to assemble is successful. There are serious doubts then, if the search for first light will be finally ended with the James Webb telescope.