EXPANDING Space

Lately, I have read of astronomers viewing objects farther and farther away. The most recent is 13 billion light years distant. Light from this object, the article says, left 13 billion years ago and this is almost the age of the universe itself. To me, this means it was 13 billion light years from us 13 billion years ago when the light actually left the object. The question I have is, since the universe formed from a single point 14 billion years ago, did this object travel at least half the distance (with us going the other half) in only 1 billion years? This seems to break the speed-of-light speed limit. Or am I just missing something?  Richard, World

You are missing one thing: the expanding nature of the Universe. The object you mention was only 4 billion (not 13 billion) light years from us when it emitted the light we see 13 billion years later. We and the object are separating from each other (not flying together) at about 1.8 times the speed of light. Although this appears to break the speed-limit law, it doesn't.

Hubble got us into this quandary, back in 1929, when he noticed the Universe is expanding. Edwin Hubble discovered galaxies recede from each other at a velocity proportional to their distance from us. The Universe is expanding like taffy stretched in all directions.

The arrow in this image points to the most distant quasar ever observed, with a record-shattering redshift of 5.8. Sloan Digital Sky Survey (SDSS) astronomers identified this faint speck of light as a possible quasar based on its distinctive red color. A spectrum of this object, obtained with the 10-meter Keck telescope in Hawaii, showed that this was indeed a quasar with a most impressive redshift. Image credit: Stephen Kent, SDSS Collaboration

Einstein's general theory of relativity quantifies this process. We can crank in the numbers and find out how far away an object was when it emitted light, how far away it is now, how old is the Universe, and such. First, though, let's consider the expanding nature of the Universe.

Imagine a two-dimensional universe as a very elastic, dark-blue bed sheet with yellow polka dots as its galaxies about an inch apart. Picture yourself expanding the universe by uniformly stretching the bed sheet to fit a BIG king-size mattress. Envision how the yellow dots get farther apart as you stretch the bed sheet.

The dot-galaxies move apart because you stretch the universe. It's the same with the real Universe: galaxies move apart as some force, called dark energy or "funny energy", stretches the Universe.

Waves of galaxy light propagate through expanding space and lengthen as they go--stretched by the dark energy. This lengthening is called redshift from the fact that the longer wavelengths of light are at the red end of the spectrum.

In March 2000, Sloan Digital Sky Survey (SDSS) astronomers found the most distant object yet observed: a redshift 5.8 quasar so far away dark energy stretched the quasar light by a factor of 6.8 before it reached us. (Quasars are the most luminous objects known, fueled by gigantic black holes).

Light left the quasar when the Universe was less than a billion years old and when the quasar was 4.0 billion light years away, say Dr. Michael S. Turner, SDSS spokesman and chair of the Department of Astrophysics at the University of Chicago and Craig Wiegert, University of Chicago astrophysics graduate student.

Today, however, the quasar is 27 billion light years away. For each mile a photon moves towards us now, says Turner, the quasar light must travel an additional 5.8 miles due to expanding space. Light waves lengthen as they speed through expanding space and this means the total distance light travels from a galaxy to us lengthens. But, the light photons speeding from our quasar cannot exceed the velocity of light and neither can any other motion through the Universe.

"'How fast is the distance between us and this quasar increasing?,'" asks Turner and answers, "about 1.8 times the velocity of light." Does this exceed the speed-of-light speed limit? No. Einstein's theory does not limit the speed of space expansion--only motion through it.

Contemplating the speeds, distances, and times of light travel through the expanding Universe may appear paradoxical. Think of it this way, say Turner and Wiegert: The quasar's light from 4.0 billion light years away had to fight its way "upstream" against the stretching Universe to reach us in 13 billion years.

By the way, the Universe didn't start from a single point. "The big bang was an explosion of space (with matter carried along), says Turner, "not stuff exploding from a point into space."

Further Surfing:

Space, Expanding, revisted:  I believe you are mistaken when saying a quasar is 27 billion light years away from us, as there is nothing in the universe that is currently farther away than about 13.7 billion light years away as this is currently the approximate age of the universe.  WonderQuest.
 

Sloan Digital Sky Survey website, Question and Answer: "How can an object we see today be 27 billion light years away if the universe is only 14 billion years old?" by Dr. Michael S. Turner and Craig Wiegert.

"Sloan Digital Sky Survey finds most distant object ever observed"

Space Telescope Science Institute: Quasars: the light fantastic, explanation and history

It is known that the Universe is accelerating while its  expanding due to Dark Energy. At what speed is the universe expanding now? Gary, Sydney, Australia

Space is expanding at about 1.8 times the speed of light.  Please see above for a more complete answer.

(Answered Dec. 7, 2007)