How and when did galaxies form and evolve in the early Universe?
On a clear, moonless night, a hazy, luminous band stretches across the sky. The ancients devised many fanciful myths to account for this "milky way." Galileo was the first to look at this haze with a telescope and discover that it was composed of countless dim stars. Today, we realize that this hazy band is our view from the inside of a vast disk that is home to billions of stars, including our own Sun, and vast amounts of interstellar dust. This is our galaxy — the Milky Way.
Earlier in the 20th century, astronomer Edwin Hubble's observations led to the discovery that ours is only one of many billions of galaxies that dot the universe, each galaxy home to billions of stars. Some, like the Milky Way, are flat disks with arcing spiral arms and regions of dense interstellar gas, called nebulae, which are active sites of star formation. Yet others are ellipse-shaped agglomerations of mature stars, virtually devoid of interstellar gas or dust.
Galaxy Origins — Theories and Discoveries
Galaxies are not scattered randomly throughout the universe, but are often found in "clusters," which are in turn parts of larger groupings called "super-clusters." How did these structures of the Universe come about? Did the material that galaxies comprise come together first, giving birth to stars, or did stars form first, gravitating toward one another to form galaxies?
Astronomers in the latter half of the 20th century have made discoveries, expanding our understanding of the Universe and our vision beyond the visible portion of the electromagnetic spectrum. Humanity's knowledge of how the cosmos was born and how its many phenomena arise has grown exponentially in a period of time equivalent to just one human lifetime. Nevertheless, despite these great strides, some fundamental questions remain largely unanswered, chief among them, how did the first galaxies form?
Much as archaeologists do, astronomers must peel away the strata of time to unearth clues to the birth of galaxies. As larger telescopes are built to observe fainter and more distant galaxies, we can probe further back in time. In recent years, astronomers have achieved great progress, peering farther out in space to study objects that existed when the universe was still very young. But no one has yet seen an epoch when galaxies did not exist. For that, we need to look back to a time when the universe was only a few hundred million years old — and look at nascent galaxies.
The Big Bang
The Big Bang was the primeval explosion that brought all space and time, all matter and energy, into being. For several hundred thousand years immediately thereafter, the universe was very hot and consisted only of a mix of subatomic particles and radiation. As the universe cooled to the point where the matter became transparent to the radiation, the first hydrogen and helium atoms began to form. Images taken by the Wilkinson Microwave Anisotropy Probe (WMAP) and other satellites indicate that the featureless sea of cosmic particles and radiation now showed the first signs of structure. Were these subtle variations in an otherwise smooth universe the seeds that grew to form the first galaxies? We do not know; and for now, we can only hypothesize.
Some astronomers believe the universe was built with small pieces, such as gas clouds and star clusters, that merged over time to form galaxies and clusters of galaxies. Others theorize that the early universe broke first into colossal clumps that contained enough building materials to make structures on the grandest scale — great walls and sheets of millions of galaxies — that fragmented into increasingly smaller gas and clouds, ultimately resulting in individual galaxies.
The answers to these questions remain tantalizingly out of reach, so the mystery remains. Astronomers would like to understand how density fluctuations in a sea of subatomic particles could have formed the great variety of galaxy shapes and sizes that make up the universe as we see it today. And understanding galaxy evolution is necessary for addressing the even more fundamental questions about the expansion of space and the ultimate fate of the universe.
Astronomers using NRAO telescopes will soon observe galaxies in their formative phases, as they were 10 billion years ago, and will establish the star-forming history in near and distant galaxies without the uncertainties caused by dust extinction.