How big is Distance to the HDF? | Scale of the Universe

The Hubble Deep Field is a picture of a tiny, nearly empty patch of sky, taken by the Hubble Space Telescope in 1995. It shows about 3,000 galaxies at many different distances. The farthest of them with confirmed distances, as of 2000, are seen by light that set out about 12.8 billion years ago and has traveled about 12.8 billion light-years to reach Earth, where a light-year is the distance light travels in one year. Because the universe has expanded since then, those galaxies are now about 27 billion light-years away.

Distance

There is no single distance to the Hubble Deep Field. The picture looks down a narrow tube through space, and the galaxies in it lie at every distance along that tube. Astronomers find how far away a galaxy is from its redshift, a measure of how much the expansion of the universe has stretched its light. A larger redshift means the light set out longer ago.

By 2000, astronomers had measured about 150 redshifts in the main field, ranging from 0.089 to 5.60. The two most distant galaxies, at redshifts 5.34 and 5.60, were first picked out by their colors and then confirmed in 1998. Most galaxies in the picture are too faint for their redshifts to be measured this way.

Turning a redshift into a distance needs a model of how the universe has expanded. With the values measured by the Planck satellite, light from redshift 5.60 set out about 12.8 billion years ago, when the universe was about 1 billion years old. Space has stretched while that light traveled, so the galaxy that sent it is now about 27 billion light-years away. By the same model, the nearest galaxies in the field, at redshift 0.089, are about 1.3 billion light-years away.

Size comparisons

The Hubble Deep Field covers a patch of sky about as wide as a dime seen from 75 feet (23 meters) away.
That patch is about one-thirteenth the width of the full Moon. About 28 million pictures of the same size would be needed to cover the whole sky.
Light from the Hercules–Corona Borealis Great Wall, a proposed structure at redshifts 1.6–2.1, set out about 10 billion years ago. That is about 2–3 billion years later than the light from the farthest confirmed galaxies in the Deep Field.
The edge of the observable universe is about 46 billion light-years away today. A galaxy at redshift 5.60, now about 27 billion light-years away, is a little more than halfway to that edge.

How the picture was made

Robert Williams, then director of the Space Telescope Science Institute, used much of his own share of the telescope's time for the project. His team chose a spot near the handle of the Big Dipper, in the constellation Ursa Major, far from the crowded plane of the Milky Way. Hubble took 342 exposures of it over ten days, from December 18 to 28, 1995.

The exposures were taken in ultraviolet, blue, red and infrared light and combined into one color picture. Most of the galaxies are about four billion times fainter than the human eye can see. The field is considered representative of the whole sky, because the universe looks much the same in every direction.

Deeper fields

Hubble repeated the project in the southern sky in 1998, with the Hubble Deep Field South in the constellation Tucana. In 2004, the Hubble Ultra Deep Field used exposures totaling a million seconds and showed about 10,000 galaxies. Its images reached some of the first galaxies to form after the Big Bang.