Field of Science

Hubble Ultra Deep Field Part II

The Hubble Ultra Deep field realized in 3D:
and the scientific image:
The Hubble Ultra Deep field is a 3 dimensional snapshot of our Universe, like a deep ice core taken from Antarctica, it has secrets which are frozen in time by the finite travel speed of light. Each galaxy is literally a frame of the time when the photons from that galaxy began their journey to our detectors on the Hubble Space telescope. The most distant galaxies visible in the image are the small red galaxies which existed when the Universe was just some 400 to 800 million years old; the larger bluer spiral shaped galaxies are snapshots when the Universe was some 13 billion years old. I should point out that the 'interesting' objects in the HUDF are galaxies; the few objects which are point like (and diffracted) are foreground stars in the Milky Way (the patch of sky selected for the HUDF is towards the constellation Fornax and was selected precisely because it was so devoid of stars). These stars are extremely close compared to the galaxies, they are like smudges on an airplane window as you gaze upon the landscape below.

I am now going to attempt to describe the canonical history of a galaxy seen in the Hubble Ultra Deep field. I will discuss the cosmic epochs, describe how the galaxies in the the HUDF formed, and what they tell us about the Universe. The image below displays graphically the cosmic epochs.
This illustration shows the cosmic epochs of our Universe from the Big Bang to the Present. The position of galaxy A1689-zD1 is shown as an example of a particularly early forming and distant galaxy. Image credt NASA, ESA, and A. Feild (STScI).

I. Big Bang
It began with nothing (or did it... see video below). No time. No space. No concepts and no questions to ask. Then approximately 13.7 billion years ago the Universe began; time and space were created. During the process known as inflation the Universe expanded exponentially and random quantum mechanical fluctuations in space time seeded the Universe with over dense regions which would later be the starting point for the formation of galaxy clusters. The flood of free energy in the Universe led to the spontaneous creation of particles via Einsteins famous equation.
From pure energy we obtained the fundamental particles and the forces of nature. The Universe cooled and from a fundamental particle soup formed electrons, protons, neutrons, and other familiar particles. These particles fused together in the process of Big Bang nucleosynthesis to form mostly Hydrogen and Helium. This all happened within the first 3 minutes or so of the Universe. This video below briefly describes the Big Bang and other possibilities.

At this time the Universe was in the radiation epoch. Photons were constantly interacting with particles (the Hydrogen was ionized, that is electrons and protons) through Thomson scattering, but as the Universe cooled these interactions became less likely to occur. When the Universe was approximately 370,000 years old we can say in a statistical sense that the photons scattered for the last time. In a cosmological phase transition known as recombination protons captured electrons to form neutral hydrogen. Photons began to travel freely through space and these photons constitute what we now call the cosmic microwave background radiation.

II. Dark Ages
The Universe was suddenly a lonely place. There were no stars or galaxies at this time and nothing was emitting any light, only the glow of of the cosmic background radiation was present. There was lots of matter in the form of neutral hydrogen, but mostly there was dark matter which was gravitating. The dark matter formed clumps at the same sites where quantum mechanical fluctuations had occurred during inflation. The Hydrogen and other normal matter followed the Dark Matter closely. As more matter began to aggregate at certain locations in space it attracted even more matter to that location in a runaway process such that massive clumps of matter began to dominate. Some regions of space became very under dense and formed voids. Other regions became very over dense. The dark ages lasted for about 400 million years, and then the conditions were right for there to be light again.

III. First Stars and Galaxies
The dark ages may have been dark, but the first stars in the Universe were spawned from the over dense regions that were forming during the dark ages. That is what was happening during the dark ages ignited the first stars. As matter gravitationally bound itself together tighter and denser it heated up to the point at which it could sustain nuclear fusion. These first stars, known as Population III stars, were hundreds of times more massive than our sun and they burned up extremely quickly. In their brief life time they created (they started shinning using only the abundant fuels available after big bang nucleosynthesis: hydrogen and helium, but as they exhausted these fuels they 'burned' successively heavier and heavier elements in their cores) many of the elements found in the periodic table up to iron and then these stars extinguished themselves as supernovae. These stellar explosions seeded the surrounding medium with heavy elements and the cycle repeated forming later generations of stars. The first star ionized the neutral hydrogen in their vicinity; hydrogen was blasted apart into free electrons and protons as it had been before recombination. Each star formed an ionizing bubble around it that grew and merged with other ionizing bubbles to form galaxies composed of a single monolithic bubble of ionized Hydrogen. Hierarchically the ionized bubbles of galaxies merged with other galaxies in their local cluster and ultimately the ionized bubbles around each cluster merged to form an ionized intergalactic medium around super clusters and eventually the entire Universe was ionized. In the simulation video below we begin by seeing a cube of t the Universe in a completely unionized state and we witness the bright ionizing bubbles expanding and merging.

A numerical simulation of cosmic reionization in a box 100/h Mpc, beginning at redshift z=20 and ending at z=8. Produced by Marcelo Alvarez, Ralf Kaehler, and Tom Abel, Kavli Institute for Particle Astrophysics and Cosmology, Stanford University.

This process of reionization was a gradually process that heralded the epoch of the first galaxies. At the center of the ionizing bubbles were generations of stars in groups of billions forming the young galaxies in the HUDF. The galaxies that appear to be interacting in the image are likely the very young galaxies before much order and structure and time to form. Galaxies evolved and formed relatively quickly in the Universe such that most of the important formation had occurred within a billion years of the big bang.

IV. Galaxy Evolution to Present
Galaxies continued to mature, merge, and collide in the next 12 billion years, but the major formation of entirely new galaxies was largely complete. It is hard to trace the evolution of a single galaxy for the next 12 billion years up to the point at which it was imaged in the HUDF, but the story I have told holds true for every galaxy seen in the HUDF. And remember from the the 3D HUDF video that some galaxies are distant and are seen as they were when they were very young and so the whole story has been told for these galaxies. The HUDF shows us what galaxies have looked like during each stage of their evolution. I have discussed up to this point the evolution of the Universe as a whole; I should also point out that some where along the way, towards the end of this timeline, there was the evolution of man, the scientific revolution, and the construction of the Hubble Space Telescope. Then people began to ask where did we come from? Why are we here? And where are we going?

V. The Distant Distant Future
Nobody knows what the future holds, but if Dark Energy continues its domination of space time the Universe may end in either in Heat Death, the Big Rip, both, or something entirely unexpected. What would we see or perhaps aliens see in the distant future if they created a new HUDF? If the Universe continues its Dark Energy driven accelerating expansion then eventually galaxies will be beyond our visible horizon and the image would be incredibly stark. This may be reason to enjoy our image all the more.


  1. Wow, this is a really interesting post. Well done.

  2. Thanks a lot for this interesting article!

  3. Amazing stuff. I wonder, if the most recent 'Big Bang' is just one of the infinite 'Big Bang's' that happend. What caused the first one? Something had to have started the whole process...

  4. how does matter appear in singularity?


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