Humanity’s quest for knowledge has taken us back in time to the early stages of the universe — well before the first grains of dust clumped together to form our home planet, Earth.
To accomplish this, we use powerful instruments high above Earth’s atmosphere to collect particles of light that are more than 13 billion years old.
Throughout our quest, we have accumulated countless pictures of space. But there are three images that stand out above the rest.
These three images, taken by the Hubble Space Telescope, tell a story about the youngest galaxies to ever exist and what likely happened to them as the universe expanded, cooled, and aged.
These are just a few of the more than a dozen amazing scientific findings that have come from the Hubble.
How To See Light Billions Of Years Old
When we look up at the sky, even in the blackness of space, much of it seems dark and empty.
But in reality, scientists theorized, ancient galaxies should be lurking out there — even in the parts of the sky in which their light is too faint to see with the naked eye.
To figure out what’s really out there, scientists used the Hubble to watch a patch of black sky for hours and hours, to detect even the faintest bit of light emitted from the furthest, faintest galaxies.
These galaxies are hard to see because they are very far away from Earth. And the further a light source is, the longer its light will take to reach Earth and, the more likely that light is to hit something else along the way and scatter off track, making the light that reaches Earth fainter.
But it also means that the incredibly faint galaxies we are looking at are very young, because their light has been travelling for billions of years at the constant speed of light. By studying this light, astronomers can measure the distances to these objects and from that, calculate the ages of far-off galaxies.
This gives us an awesome look at the history of our universe’s evolution over its 13.7 billion-year lifetime. But, to see these far and faint galaxies, Hubble must spend a long time watching a dark patch of sky.
1. The Hubble Deep Field
In 1995 — five years after NASA launched Hubble into orbit — observers at the Space Telescope Science Institute (STSI) trained the Hubble at a patch of dark sky for 10 consecutive days.
Hubble’s main camera took 342 pictures of the extremely small section of sky — a square measuring a tenth the width of the moon as viewed from Earth. It’s located just above the Big Dipper constellation:
At the end of the ten-day stretch, Hubble had collected light from 3,000 objects in this tiny patch of dark sky — an incredibly large number that no one was expecting. Most of the objects were galaxies, and more important, was the shapes, colours, and types of galaxies in the image, called the Hubble Deep Field. Some of these galaxies included the youngest, most distant galaxies ever discovered at the time.
“The variety of galaxies we see is amazing,” STSI director Robert Williams said at the time. “In time these Hubble data could turn out to be the double helix of galaxy formation. We are clearly seeing some of the galaxies as they were more than ten billion years ago, in the process of formation.”
“The past ten days have been an unbelievable experience,” Williams said.
From this Hubble data, astronomers estimate that galaxies today are producing far fewer stars than they were 8 to 10 billion years ago. In fact, they seem to have been pumping out 10 times more stars when they were young, and this star-making has dropped precipitously as they age.
Astronomers wondered, with excitement, what a larger observing stretch might uncover.
2. The Hubble Ultra-Deep Field
To dive deeper into cosmic history, the Hubble team set their sights on a different portion of the sky — a darker splotch with few foreground stars that drown out the light from distant objects for which Hubble was seeking.
The image they got back — the Hubble Ultra Deep Field — shows objects in the universe as they appeared less than one billion years after the Big Bang. Seeing this far back in time is especially important for understanding a crucial stage in evolution of the early universe called reionization.
Reionization occurred from a few million to one billion years after the Big Bang. Certain objects present in the universe at that time, transformed the universe from a foggy haze into the transparent universe we know today.
To better understand what exactly triggered reionization, Massimo Stiavelli (mission head for the James Webb Space Telescope) and his team took 800 exposures over 11.3 days, during 2003 and 2004. The result is an image containing 10,000 objects (mostly galaxies) — more than three times the number in the original Hubble Deep Field.
One of the most important discoveries that came out of this image was the unexpectedly high rate of stars forming during the early universe — far higher than expected. They discovered this by measuring how many young stars are present at different epochs.
Young stars emit ultraviolet (UV) light, which astronomers detect. By measuring the amount of UV light from objects at different distances, astronomers learned that the early universe had more young stars than previously thought possible.
This discovery gave astronomers today’s leading theory of what triggered reionization: light from early stars blooming to life. Energy from these stars could have “ionized” neutral hydrogen into a state that doesn’t block trapped light, making the universe transparent.
To further investigate this theory, astronomers needed to see even further back in time.
3. The Hubble extreme Deep Field
In 2012, NASA released what remains the deepest view into our universe: the Hubble eXtreme Deep Field (XDF). For the image, Hubble’s instruments collected light from over 2000 exposures taken over 23 days of observations, over a period of 10 years.
The XDF contains 5,500 more galaxies than the Hubble Ultra Deep Field and looks even further back in time: showing galaxies as they were 13.2 billion years ago — only 500 million years after the Big Bang.
Some of the galaxies are so faint, they’re one ten billionth the brightness our human eyes can detect. These distant, young galaxies eventually grew and aged to become mature galaxies like our home, the Milky Way.
This image is a more detailed look at the same spot of sky as the Hubble Ultra Deep Field. Except an even smaller section. It’s about 80% the size of the Hubble Ultra Deep Field. The image below shows just how small this patch of sky, denoted XDF, is:
Astronomers are still not certain if early stars, or something else, triggered reionization in the early universe, but the hunt for a solution is far from over.
The next-generation Hubble, the James Webb Space Telescope, is scheduled to launch in 2018. Its powerful instruments will look at the XDF in even more detail. One of the mission’s primary science goals is to better understand the end of what experts call the “Dark Ages” of the universe, which includes studying the epoch of reionization.
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