# How is it possible for astronomers to see something 13B light years away?

In a NPR News story from a few years back:

"A gamma-ray burst from about 13 billion light years away has become the most distant object in the known universe."

I'm a layman when it comes to physics, so cut me some slack if this is an ignorant question, but assuming the universe is around 14B years old, and has been expanding since the Big Bang, how is it that we can see events so far back in time?

I understand how it would work if you had a static universe and the GRB happened 13B years ago at 13B Light years away and the light just arrived. However, at the time of the burst wouldn't we (or at least the matter that we are made from) have been much closer to the source of that burst, and wouldn't the light have blown by us eons ago? How is it we are seeing it now?

If we were expanding away from it at close to light speeds it would seem to make sense for why it took so long for it to get here, except for that whole notion that light moves at the same speed relative the to the observer, which I think would blow that idea out of the water.

Perhaps gamma rays travel at sub-light speeds? But, I'd still think the math would require that they travel MUCH slower than light for this scenario to play out.

Another, possibility is that the light has wrapped around a finite universe a few times before reaching us. Of course if that were the leading theory, there wouldn't be any remaining controversy about the finite vs. infinite universe models.

What am I missing here?

There are at least two ideas involved.

First is that the expansion of the universe is not linear. While the Big Bang happened around 14B years ago, that does not mean that 13B years ago, the Universe is 1/14th of its present size. Current theory suggests that a large portion of the cosmological inflation (where the Universe increased by 26 or more orders of magnitude in linear dimensions) happened within much, much less than a second after the Big Bang. And as another example, the current theory estimates that at the time that the cosmic microwave background was emitted (which was about 0.5 million years after the birth of the Universe, placing it about 1/30000 the current age of the Universe), the universe is already about 1/1000 its current size (in length).

Second is that the apparent recession of far away objects from us is not so much objects flying apart from each other. Rather, it is space being added in between objects. Imagine you being the photon, and two turtles (moving slower than you) being the galaxies. Put turtle one in the first carriage of a train, and put turtle two on the 10th carriage of a train. And you start walking. Say it takes you 1 minute to traverse a carriage, and it take the turtles 10 minutes. Then in the case where the turtles walk away from each other, it will take you a bit under 12 mintues to get from the first turtle to the second (you walk 10 minutes to the tenth train, and the turtle has gotten to the 11th. You walk another minute to the 11th train. The turtle is just a few steps in front of you.)

But that's not how the universe expands. The expansion of the universe is more like the following: suppose every 6 minutes, all the carriages decouple, and between each pair of the original carriages plops one more car! So you walk for 6 minutes (having traversed 6 cars), and you look up, and see that the second turtle is 8 cars in front of you (and the first turtle is 12 cars behind). And you walke another 6 minutes. Plop comes the extra cars, and now you are 4 cars from the second turtle and 36 cars from the turtle behind. And finally after another 4 mintues you catch up to the second turtle.

From the point of view of the second turtle though, you would have travelled from a turtle that is now 40 cars away from him, while taking only 16 minutes! This ties back into the funny idea that light emitted from an object 13B lightyear away can take quite a bit less than 13B years to get here, due to the inflationary Universe.

This is why cosmologists and astronomers use red-shift to measure distance, because there is no reasonable intrinsic notion of distance that is free from ambiguity: should distance be described by how far away the turtles are when you started walking? or when you finished walking? or the number of carriages you (the photon) have traversed? Instead of that, they measure it using red-shifts, which can roughly fit into this turtle-you framework as how flushed your cheek is from all that walking when you reached turtle number two. Based on the redness of your cheeks, the turtles can calculate how much you exerted yourself, and thus for how long you've been traveling, and using known rules of the addition of new cars (the value of Hubble constant), the turtles can estimate the distances to other turtles. :-)

(I'm going to skip discussion of standard turtles, which are turtles from which you will always depart well rested and not flushed, nor how the turtle simiano-ferroequinologists found out about their rates of locomotive expansion.)

• The turtle analogy can also be used to illustrate the notion of cosmological horizons. Say initially the turtles are 12 cars apart. You start walking, and after 6 minutes, "expansion" happens, and you are now 12 cars from the original turtle, and 12 cars from the second turtle. You walk another 6 minutes, and you are still 12 cars from the second turtle. In other words, if initially two turtles are at least 12 cars apart, they will never know the existence of each other based on observing shuttling humans. Jul 2, 2011 at 17:53
• Haha, I like this description a lot, I think you should make it into a kids animated t.v. show! Jul 2, 2011 at 18:01
• These wouldn't be the same turtles stacked under the earth, would they? Jul 2, 2011 at 20:29
• There's only one turtle there. And four elephants. Jul 2, 2011 at 20:55
• And here, I thought that the reason we use redshifts is because it's far easier than searching for standard candles within galaxies and comparing for absolute magnitude. In more distant galaxies, it's next to impossible to find say, Wolf-Rayet variable stars. Then add to that the fact that finding absolute distance of an individual galaxy would require months of observation. Redshift on the other hand, can be measured instantaneously and works great as a reasonable estimate. Every once in a while, we'll catch a type 1b supernova and compare it to the redshift to re-calibrate our measurments. Dec 12, 2012 at 21:11

The answer to this question goes back to the idea of cosmological inflation. Often, particular in popular media, expansion is viewed as two galaxies going farther and farther away from one another. This is only sorta true. What is really happening is the space between them is expanding (the balloon analogy is often used, imagine placing two dots on a deflated balloon, and then blowing that balloon up, the dots are now farther from one another), and because of this, it is very hard to judge what distance "means" at such great scales. Even terms like relative velocity get extremely hard to define when looking billions of light years away.

Hence, when talking about "distances" in astronomy, one uses the redshift of the light, which is usually a more functional description. For example this object had a redshift of z=10, which has more of a redshift than any other astronomical object, and we are looking at an image of how it would have looked like if we were right next to it 485 million years after the big bang. However our proto-galaxy was probably some distance from it to begin with and as the light was traveling, space itself expanded considerably, making the light only reach us now, a dozen or so billion years later.

As far as "looping around a finite universe," this is very unlikely and there is no evidence for it. Gamma rays will travel at the speed of light through a vacuum.

That quasar's light started its travel to us about 13B years ago, but its distance is NOT 13B light-years. The distance is more something like 50 billion light-years (for afficionados, I am talking about the comoving distance). The reason is that the universe itself expands while the light travels to us, so that the distance is larger than the speed of light multiplied by 13B years (i.e. larger than if the universe were static).

To answer the reader's question, it is quite true that 'we' were close to the burst when it happened, but the space between us and the burst expanded since then, and it takes light time to cover that separation.

Some of the answers imply say that the reason is in cosmological inflation, but this is not correct. Inflation happens very early in the history of the universe (approx 10^{-35} seconds after the Big Bang), and cannot be verified just by looking at the quasars and such since it does not affect distances in the 'recent' history of the universe. Instead, verifying inflation involves sophisticated measurements of fluctuations of matter and light in the universe (cosmic microwave background radiation, etc) which were laid out well before the existence of any quasars.

• If I understand this correctly it has some depressing ramifications on the potential for interstellar travel. It sounds like, even if we could live a really long time, and travel at C, it would take significantly longer than X years to travel X light years. Sort of like running up the down escalator. Jul 7, 2011 at 15:52
• In principle yes, but in practice (even science-fiction practice), no. The expansion of the universe becomes important to take into account only for objects that are billions of light-years away; thus speeding away from us with speeds anywhere comparable to c, the speed of light. But the recession velocity of a nearby galaxy, for example, is only in the hundreds of kilometers/sec, which is 0.1% of the speed of light (and your spaceship is presumably moving with something close to c). For nearby stars, it's even easier, since they are gravitationally bound by our Galaxy and not receding at all. Jul 10, 2011 at 0:22

I believe the quasar you're refering to was observed to have a z, or redshift of 10. This implies it is receding from us at a velocity of 0.984c, very close to light speed. With a Hubble constant of 74 km/s/Mpc, its distance is indeed about 13 billion light years. There's no reason to believe we were close to the event when it occurred. For example, the CMB's redshift is about 1100, implying a much higher recession velocity and greater distance, and that was initiated about 400,000 years after the Big Bang.

• Of course the quasars radiation is very faint because of distance, but also because at z of 10, photon energy is reduced by an additional factor of z+1 = 11. Jul 2, 2011 at 17:20

All of these answers are very interesting and informative, but I dare say that none of them answers the original question. The original question has to do more with: How is it possible to see (light) so far back in time if we, ourselves, have not traveled away from said light at many, many times the speed of light in order to have been able to have gotten ahead of said light to observe it? And even if we did get ahead of said light, how is it possible that it is visible in every direction, as opposed to only in the direction from which we traveled so expeditiously?

It really makes no sense at all. If everything is expanding from a singularity, there should be varying amounts of information about the universe that are observable from our vantage point. The density of light emitting objects in any one direction should be defined by our coordinates within the expanding universe, the direction & speed at which we are traveling through the universe with respect to any light being observed in that particular direction, and the direction & speed at which all other light sources that reside in that particular direction within the universe are moving relative to both us and the singularity. (i.e. if our coordinates within the expanding universe were closer to a "shock front" of the expanding universe, there should be much less observable information about the contents of the universe that direction, as we would be located closer to the advancing edge of the universe where much less matter would occupy the more limited space between us and the advancing shock front of universal expansion.

This is one major reason why I just can't take the "big bang" theory seriously. Obviously the universe is an ever-changing thing that is very different today than it has been in the past and will be in the future, but to say that everything started as a singularity in a "bang" is sort of absurd. The mathematical evidence may show it, but the observable universe contradicts the math. So there must be something wrong with the math.

Considering that about 90% of the universe has yet to be understood or explained (dark matter/energy), I'd say it's pretty much the biggest stretch science has ever made to be stating that we know where the universe came from. Especially when the answer to that question can only be explained in the ether where physics and mathematics diverge and break down. And where is it that we know, without a doubt, that mathematics and physics break down? Black holes, of course. I believe black holes are the key to understanding both the history behind and the fabric of our observable universe. All that matter has to go somewhere, and quasars & cosmic jets radiate only a tiny, miniscule fraction of a fraction of that matter.

• You are assuming that the big bang was from a point in space and we're all zooming away from that point. That's NOT what the big bang theory says. Instead, the big bang was everywhere and we don't know of any edge or expanding shock front. In the current big bang model ($\text{\Lambda} \text{CDM}$) everything around us is zooming away from us as though we're at the center. Every other point also has this property. Oct 22, 2013 at 6:57
• ...which make perfect sense. Not. Still, the question remains unanswered. Oct 22, 2013 at 10:07
• Willie Wong's answer with the turtle analogy does answer the question and is a fantastic description of it. Oct 22, 2013 at 15:40