I was reading this article this morning:


Scientists have discovered a gamma-ray burst whose light has taken 13.14 billion years to reach Earth. This much is cool and interesting. However, the assumption is then stated that this is "the most distant single object yet spied by a telescope".

But hold on a minute. It is also known that galaxies are moving away from each other at incredible speeds, most faster than the speed of light, because the space/time between them is expanding (or something like that... I'm no scientist!)

So these so-called amazingly distant objects, well for starters, they don't exist any more... They are things that happened billions of years ago. But not only that, they are probably objects that were only a few million light years away from us when they actually took place. So surely then, the objects themselves aren't the most distant ones, but the light from them has been distorted such that the light has taken that long to reach us?

Furthermore, if the galaxies are spreading out faster than the speed of light, who is to say this explosion actually happened 13.1 billion years ago? Isn't it possible that the light was created say 5 billion years ago, but has taken much longer to reach us because of the expanding space between the galaxies?

I'm sure this stuff has already been considered by scientists, but I find it weird the way news articles always assume that just because light travels at a specific speed, that it's always going to take the same amount of time to reach us.

Or am I getting it wrong? I'd love to know!

EDIT: For people discussing the whole faster-than-light-speed thing, I came across this article:

As you look at galaxies further and further away, they appear to be moving faster and faster away from us. And it is possible that they could eventually appear to be moving away from us faster than light. At that point, light leaving the distant galaxy would never reach us. When that happens, the distant galaxy would just fade away as the last of the photons reached Earth, and then we would never know it was ever there.

And this one:

That mysterious dark energy force, which is accelerating the expansion of the Universe is making the most distant galaxies move faster and faster away from us. Eventually, they will cross an event horizon and appear to be moving away from us faster than the speed of light. At this point, any light emitted by the galaxy will cease to reach us. Any galaxy that crosses this horizon will fade away from view, until its last photon reaches us. All galaxies will disappear from view forever.

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    $\begingroup$ agreed, this is a pure physics question $\endgroup$
    – Anonymous
    Commented May 26, 2011 at 11:48
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    $\begingroup$ and as far as we have measured, no galaxy is moving faster than the speed of light, relative to any other galaxy. Where did you got this strange idea? $\endgroup$
    – Jader Dias
    Commented May 26, 2011 at 12:31
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    $\begingroup$ @Jader Dias--this isn't true--it's possible to view objects on different sides of the sky that would be outside of each others' light cones. $\endgroup$ Commented May 26, 2011 at 16:17
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    $\begingroup$ "It is also known that galaxies are moving away from each other at incredible speeds, most faster than the speed of light" Moving apart, yes. Faster than $c$, no. Though in the standard model of cosmology there was a time (inflation) when the expansion outpaced light, and there will come such a time again as dark energy dominates the cosmology. $\endgroup$ Commented May 26, 2011 at 16:38
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    $\begingroup$ @Jader Dias: something being outside of your lightcone is equivalent to that thing having a speed, relative to you, that is greater than the speed of light. $\endgroup$ Commented May 31, 2011 at 14:21

4 Answers 4


Of course the expansion of space is being considered by astronomers. In fact, it's pretty much the only thing they are considering. The redshift due to expansion of space is the way that astronomers know that it came from 13.14 billion years ago. What you do is look at the lightwaves very carefully. They will be stretched out (which looks like redshifting) due to the expansion of the universe. The longer they have been flying along, the more stretched out/redshifted they will be. The group cited in the article measured a redshift (a measure of this stretching) of 9.4, which is the largest we've ever observed; we conclude that this light has been traveling longer than any other light we've observed from a single source (the cosmic background radiation is way more redshifted).

A number of very clever methods allow us to identify just how long the light must have been traveling for it to received a particular amounts of redshift. This is an application of the relation known as Hubble's Law. If you use it, you find that light with a redshift of 9.4 has been traveling for about 13.14 billion years.

This method is used so routinely that you'll hear cosmologists talk about time in terms of redshift, like "Ionization occurred at redshift 17" rather than "Ionization occured 13.5 billion years ago" (those numbers are made up).

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    $\begingroup$ Also it may be worth noting that the object from which the light originated is currently much more distant than 13.14B ly away, if it still exists. $\endgroup$
    – JYelton
    Commented May 27, 2011 at 15:42
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    $\begingroup$ @JYelton not sure I agree entirely - simultaneity is determined by the speed of light. There isn't really a good notion of "currently" that should go faster than the speed of light. $\endgroup$ Commented Jun 2, 2011 at 17:36
  • $\begingroup$ I think my brain just imploded. :) $\endgroup$
    – Joe
    Commented Jun 7, 2011 at 9:54
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    $\begingroup$ Actually, saying that any galaxy is X billion lightyears away is a misnomer. Because, due to the expansion of the universe, you can't translate its age to its actual distance (as in metres). Heck, it doesn't even make sense to speak of an actual distance because, as pointed by Spencer Nelson, there is no more simultaneity at these scales. You can't figure out where the object "currently" is. The only thing you can say is that it is so far that its light has taken X billion years to arrive. $\endgroup$
    – KPM
    Commented Jan 16, 2012 at 23:58
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    $\begingroup$ Well, we do operate with a "proper" distance, which is "the distance as we would measure it with a very long yardstick", so @KPM 's answer is not exactly true. Of course you can get philosophical about whether such a thing is really warranted, but I'd say it is - we have a pretty good idea about the expansion history of the Universe, and we have defined a set of "co-moving" coordinates (that is, coordinates as they would be if the Universe was frozen at its current size), and then scaling is a relatively simple exercise. $\endgroup$
    – Thriveth
    Commented Jun 6, 2013 at 13:56

We have a pretty good idea of the expension history of the Universe from numerous measurements etc., and we also know with great precision how fast light travels through empty Space, so if we assume our understanding of the Universe's expansion history is not-too-wrong, we can simply calculate the light travel time, current distance and original distance of the object once we have established, how much the light from it is redshifted.

The distance given in the article is the distance at the current time to the point from which the light was emitted 13.1 billion years ago. Yes, it was much closer at that time, but it isn't anymore.

It has been mentioned, that the "distance" concept isn't very well defined. Well, it is, but there is more than one definition. The distance we mostly give is the one called the proper distance which, roughly speaking, is the one we'd get if we froze the Universe at the current point of expansion and set out to measure the distance with a giant yard stick. It is true what people say, that what "now" means is also not totally clear, but assuming that the expansion of the Universe is uniform, we can define "now" not in terms of absolute time (which, it can be argued, doesn't exist), but in terms of how much the Universe has expanded since a given event, like e.g. the Cosmic Microwave Background - which is indeed how we do it.

The expansion of the Universe is a pretty difficult concept to grasp, and there are many misconceptions flowing about, even among research scientists in the field. This paper does a pretty good job of weeding out some of these misconceptions:

Tamara Davis, Charles M. Lineweaver. Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe. Publications of the Astronomical Society of Australia, Volume 21, Issue 1, pp. 97-109. (PASA Homepage) arXiv:astro-ph/0310808

It is relatively easy to read, and it has some very cool diagrams explaining how things like proper distance vs. comoving distance, horizon distance and superluminal expansion etc. work.


One of the reasons why scientists like to took at distant objects is because they know that, by doing so, they're looking back in time: a distant object is an object from the past. A very distant object (like the one you cited) is an object from near the beginning of the universe (which is interesting to people who want to know about the beginning of the universe).

  • $\begingroup$ So is it just the media who are speculating "furthest distance" when in fact that is in impossible concept given the warping of spacetime? $\endgroup$
    – Anonymous
    Commented May 26, 2011 at 11:47
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    $\begingroup$ @Joe - As far as I know, your sentence "galaxies are spreading out faster than the speed of light" is an 'impossible concept': the theory of 'special relativity' implies that the relative speed of two masses can't reach the speed of light. $\endgroup$
    – Anonymous
    Commented May 26, 2011 at 11:53
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    $\begingroup$ @Anonymous: That's not quite right. It's not that two masses can't move away from each other faster than c, but rather that a massive particle can't move relative to space faster than c. That means that you cannot accelerate a massive particle to the speed of light, but, for example, you can accelerate two particles to 99% of c, and shoot them in opposite directions. At that point, they are moving away from each other at a rate faster than c, but they themselves can never reach c. $\endgroup$
    – voithos
    Commented May 26, 2011 at 17:10
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    $\begingroup$ @Colin: I may be misunderstanding something, but I don't think my original point disagrees with yours: that galaxies, given enough space between them, can in fact appear to move faster than the speed of light. $\endgroup$
    – voithos
    Commented May 27, 2011 at 17:58
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    $\begingroup$ My understanding was that the bubbles of spacetime between the galaxies are expanding faster and faster and eventually will be faster than the speed of light: universetoday.com/13808/… $\endgroup$
    – Joe
    Commented Jun 7, 2011 at 10:04

Looking at a distant object, billions of light years distant, has many connotations.

By being so incredibly far away, you're seeing past events. But this does not necessarily mean the object no longer exists. It may still be around, but certainly could be very different.

I recommend checking out some of the starter Astronomy videos like this one at Khan Academy, it helps me get a handle on the scale of things and the physics behind it.


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