How deep into the universe have we looked? As of today, this deep:
This is the Hubble eXtreme Deep Field, (XDF), and it’s a masterwork ten years in the making*. What you’re seeing is what you get when you take a very long exposure with two of Hubble’s best cameras of a region of the sky that contains no known stars – an ocean of 5,000 galaxies! And it’s a very deep ocean, indeed. More than 5,500 galaxies are crammed into a field of view just a fraction of the size of the full moon.
The galaxies are arranged at varying distances from us. Some are relatively bright and even have spiral arms as seen in nearby spiral and elliptical galaxies:
But others, way, way, waaaay in the background, don’t appear to have any structure at all. Instead, they just look like little blobs of stars and gas:
So what’s going on here? It turns out that these fainter galaxies are so far away, their light took billions of years to reach us. In other words, we’re seeing these galaxies as they were several billion years ago when the universe was only a few hundred million years old!
To put that into perspective, it helps to think of the XDF as a kind of “core sample” of the cosmos; the deeper into the field we look, the farther back into the universe’s past we can probe:
Our universe is 13.7 billion years old. Thanks to Hubble, we can see what galaxies looked like in the current era, what they looked like in its earlier years, and what they looked like a relatively short time after the Big Bang.
And so, in just one image, we can trace the evolution of galaxies over time – from small embryonic building blocks of fluff to beautiful spirals, to giant ellipticals that are the relic of collisions of multiple galaxies. It’s the story of the universe, writ in a single image.
I’ll never tire of looking at this image, and marveling at just how far we’ve come in our understanding of the universe in so short a time.
But what really gives me goosebumps is what’s left to discover.
* I realize in retrospect I didn’t explain this elsewhere in the post. XDF is actually part of the Hubble Ultra Deep Field, which was made with Hubble’s Advanced Camera for Surveys (ACS) from September 2003 through January 2004. But this new image was made with additional ACS images taken since then, as well as Hubble’s new Wide Field Camera 3 (WFC3) which was installed in 2009. WFC3 is sensitive to near-infrared, allowing even fainter, more distant proto galaxies to be imaged. Hence my comments about this image being ten years in the making, as well as the deepest view ever!