I get a certain question every so often, and it's one of the most difficult questions any cosmologist faces. Today, I try to tackle it. It goes something like this:
If the Universe is 13.7 billion years old, and nothing can go faster than the speed of light, how is it that we see things that are 46.5 billion light years away?First off -- and I want to clarify this -- everything in this question is legit.
1.) The Universe is 13.7 billion years old. There are small errors there -- no one would be surprised if it was 13.5 billion or 14.0 billion years old -- but it's definitely not 12 billion years or younger and definitely not 16 billion years or older.
2.) Nothing can move faster than the speed of light. If you've got a mass, whether you're a galaxy, a spaceship, a bullet or a neutrino, you're going to go slower than the speed of light. And if you don't have a mass, you're going to move exactly at the speed of light. No exceptions.
3.) The farthest things in the Universe -- the things that emitted their light 13.7 billion years ago -- are 46.5 billion light years away from us now.
So how did this happen? Two things, one simple and one not-so-simple. The simple thing is that the Universe has been expanding this entire time. Imagine you've got an ant on a deflated balloon, and the ant moves at a rate of 1 cm/second. When the balloon is totally deflated, the ant is only 2 cm away from the top of the balloon, her destination. But as she starts walking towards the top, something inflates the balloon. As she walks towards the top, she notices that the balloon around her is expanding.
How does this expansion work? Well, this is the not-so-simple part. Expansion isn't a velocity. It's a velocity-per-unit-distance. Let's say that it's 0.4 cm/second per centimeter. This means that if the ant is 1 cm away from something, it expands away from her at 0.4 cm/second. The top of the balloon, initially, since it's 2 cm away, expands away at 0.8 cm/second. And something that's 15 cm away would be expanding away at 6 cm/second.
So if I run through the math of this ant walking at 1 cm/second to a point 2 cm away on this expanding balloon, it doesn't take 2 seconds to get there. In fact -- doing the math correctly -- it takes just a shade over 3 seconds for the ant to reach her destination. Moreover, the balloon has continued to expand, so when she looks back at her starting point, do you know how far away it is? Over 6 cm away! When she looks back at her starting point, not only is it more than three times as far away as it was when she started, but the entire balloon is bigger than it was before.
And that's what our Universe is doing: expanding while the light is traveling towards us from distant sources. There is, of course, one more caveat in our Universe. The expansion rate is mind-bogglingly slow, 72 kilometers per second per Megaparsec. In the ant's terms, that's 2.3 x 10^-18 cm / second / cm. It's just that our Universe is so big that as you get far enough away -- just under 13 billion light years -- the expansion rate eventually becomes greater than the speed of light.
But this is okay. It's only that space (i.e., the balloon) is expanding; there's no matter that's moving. So, in principle, space can expand as quickly as it wants, even faster than the speed of light, because there's nothing moving. And that's why, even though the Universe is only 13.7 billion years old, we can see things that are 46.5 billion light years away.
Any questions?
Physical Science
Comments
So, young Woody Allen's bedroom IS getting larger over time and therefore he does not have to clean it.
Oh, can massless particles stay still at all?
Posted by: Greg Laden | July 31, 2009 12:37 PM
Greg, the expansion only works for objects that aren't bound. Woody Allen's bedroom (and many of our waists) may be expanding, but we can't blame that on the Universe.
Massless particles can only stay still if you have a medium where the speed of light in that medium is infinitesimally small. Theoretically possible, but not yet discovered.
Posted by: Ethan Siegel | July 31, 2009 12:42 PM
The hard part to wrap my head around was that we can only see objects less than 13.7 billion years old (time), but that might have "traveled" more than 13.7 billion light-years (distance) even though we all started extremely close together. Obviously that is partly because time and space are so intertwined, yet time and space appear to function on wholly distinct levels.
Brian Greene explains some portion of it as a limitation of movement. You can either move through time or through space, but the more you move through one, the less you can move through the other. Mathematically, does this mean that a massless particle doesn't experience time? Or is that past the point in which our maths cease working?
Posted by: sean hogge | July 31, 2009 1:01 PM
I'm a little confused. I was under the assumption that the visible horizon of the universe is about 13.7 billion years but the 46.5 billion number is the physical size. We can't actually see anything beyond 13 billion light years, right?
Posted by: Brando | July 31, 2009 1:01 PM
Dude, that shit hurts my brain. I prefer to think of it like this:
(1) The universe is fucking huge, and getting even fucking huger.
(2) The universe is fucking old, and getting even fucking older.
Posted by: Comrade PhysioProf | July 31, 2009 1:06 PM
I'd say that's a pretty good answer to a darn good question... because I seem to understand what you said.
Greg Laden said:
"...can massless particles stay still at all?" It is said that nothing is able to leave black holes. So are massless particles doing anything in there? Just standing around? Motionless? Wriggling at light speed?
I am ill-equiped to deal with such thoughts, and unable to stop thinking them. Ouch.
Posted by: John Swindle | July 31, 2009 1:12 PM
That's just about the most straightforward explanation for this concept I've seen, but my head still hurts thinking about it. Then again, I had a migraine when I started reading this post, anyways.
One more question: does inflation theory have anything to do with this?
Rt
Posted by: Roadtripper
| July 31, 2009 1:26 PM
Thanks for the very cogent response. So, am I correct in assuming that the famous red-shift is due to the 'stretching' caused by the 13.7 billion year journey covering the 46.5 billion light-year distance?
Related to the geometry question: does the density of galaxies drop off as the distance away increases?
Posted by: jdhuey | July 31, 2009 1:43 PM
Actually, what I've heard is that Teh Expanshun as it were affects all things, but bunches of matter have other forces that counter act it at scales of Woody Allen's bedroom.
However, this is not entirely clear to em. So I get that adhesion keeps the fly the same distance from the ceiling as the universe expands, and so on, but are the subatomic forces that keep atoms the same relative size unaffected? Is the volume of space within a block of solid metal the same before and after expansion? From what you are saying, it sounds like it is.
Also, is expansion a force?
Posted by: Greg Laden | July 31, 2009 1:54 PM
This is so simply stated that I can't help but want to make it even more complex, simply on sheer contrariness. So here are a couple of questions:
1) Aren't tachyons supposed to move faster than the speed of light? Or are they still only considered hypothetical?
2) Is there any evidence that an anti-matter universe exists, expanding at the same rate of our own universe? If it is also theoretical that matter and anti-matter could destroy each other, would it also mean that there be some requisite "neutral zone" between them? And if such, what would the size of such a zone have to be?
3) If we use string theory (m-theory) to hypothesize the existence of an infinite number of parallel dimensions, might we also conclude that some of these other dimensions might be older/larger than our own dimension?
Yeah--I know; I read too much science fiction and watch too many specials on the Discovery channel. ;-)
Posted by: hyphenate | July 31, 2009 2:39 PM
As I understand it, they are moving at the speed of light along geodesics just like they do outside the black hole. The geodesics all curve toward the center eventually, though.
Posted by: Naked Bunny with a Whip
| July 31, 2009 3:04 PM
Given that the existence (but next to nothing about the features) of ~90% of the "known" cosmos has only been discovered during my lifetime so far, please allow me a grain or two of salt concerning present-day statements about the absolute limits of that universe thingy out there.
Posted by: Pierce R. Butler | July 31, 2009 3:43 PM
If the universe has a beginning, does that imply that the 'whole' universe (not just the 'observable' is not infinite in size. If the universe is infinite in size does this make inflation obsolete? And is a infinite universe automatically a static universe?
Not entirely related to the post, but something that kept hanging in my mind from one of your earlier posts a few weeks ago :)
Posted by: Nick | July 31, 2009 3:52 PM
Oh, and the question(s) I'd meant to ask:
If what Einstein labeled the cosmological constant is a mere 72 meters per second per kiloparsec (funny, my unit-converter desktop widget doesn't do parsecs, or even lightyears - time for a system upgrade!), how much space is even theoretically in reach?
Just as there is some calculable radius beyond which an ant with a set walking speed can never reach on an endlessly expanding balloon, there must be some sizable fraction of visible space which even a hypothetical Leonora Christine scooting along at .999...c could not approach (as it would, eventually, in a static universe, if the crew brought along enough brown rice & vitamin E to make maximum use of their time dilation).
We might hope for our 999...x grandkids to explore the local galactic cluster, but how far out is the locus where the expansion rate = c? Is there a convenient spacemark?
Posted by: Pierce R. Butler | July 31, 2009 4:14 PM
Sorry to burst your balloon, but if there is an ant walking on it, it is, except in very unusual circumstances, a MALE.
And there are some good questions in the thread. I thought you were saying that the universe is currently 46.5 billion light-years across, but we can only see things 13 billion light years away. If that is not what you are saying ("we can see things that are 46.5 billion light years away") I have a serious misunderstanding of space-time, the universe, and everything.
Posted by: David Horton | July 31, 2009 4:25 PM
@#15: David, I'm assuming you're referring to the likelihood that the referenced ant is a worker ant. In which case, it IS a female. Workers ants are females. Not males.
Posted by: Jo | July 31, 2009 4:45 PM
what is outside the universe for it to expand into? If time stopped, how could you know? How can time stop because if it did stop you would have to ask for how long thereby inserting time back in.
Posted by: gerwood | July 31, 2009 6:12 PM
@David: The farthest possible light source, when it started its travel towards us 13B years ago, did so in a 'smaller' universe. 13B years later, when it reaches us, the universe has expanded, and that particular light source is now calculated to be 46B light years away from us. Yes?
Posted by: MichelP | July 31, 2009 6:32 PM
13.7 billion years ago, the entire universe was all in one place, so why would light from then just get to us now? The distance from everywhere was short so it would not take light long to get 'here'.
@17 - there may not be any 'outside' the universe. It may just wrap around on itself. Time could speed up or slow down and you would not be able to sense it - see info on relativity.
Posted by: NewEnglandBob | July 31, 2009 7:16 PM
Oops, on re-reading the post I see my question @ # 14 was answered already:
That this magic radius should be a number so close to the estimated age of everything I rather hope is coincidental, or I'll have to take up some serious anthropic-principle crackpottery. Either way, I'm still hoping there's a handy supercluster edge or the like to indicate the boundaries of our invisible plate glass window - or something striking and famous on the other side, as a handy metaphor for the ever-unreachable.
Posted by: Pierce R. Butler | July 31, 2009 7:21 PM
Bob, you know how it took the ant more than 2 seconds (at 1 cm/sec) to go 2 cm? Yeah... it takes light 13.7 billion years to go the tiny distance that the Universe was spread apart all the way back at the beginning.
Mind-numbing, but true.
It also looks like there are some things I need to clarify, including just what I mean by "position, motion and moving", since those terms mean something very different in cosmology than in our common usage.
Posted by: Ethan Siegel | July 31, 2009 7:23 PM
This post is now available in Portuguese:
http://scienceblogs.com.br/chivononpo/2009/07/o_tamanho_do_universo_uma_pegu.php
Posted by: Ethan Siegel | July 31, 2009 7:46 PM
It also looks like there are some things I need to clarify...
Among those might be your choice of units - why "72 kilometers per second per Megaparsec" instead of 72 meters/sec/kiloparsec or "2.3 x 10^-18 cm / second / cm" instead of 2.3 x 10^-16 m/second/m, except for the sheer astronomical joy of packing on a few more orders of magnitude?
Posted by: Pierce R. Butler | July 31, 2009 8:13 PM
#16 D'oh, you are right. Old age creeps up on one, and then, undetected, whacks you over the head with a large rock.
Posted by: David Horton | July 31, 2009 8:21 PM
#18 doesn't that imply that all the light from all the bit between 13.7 and 46.5 billion light-years away from would arrive on Earth at the same time? Or do I have ants in my head?
Posted by: David Horton | July 31, 2009 8:25 PM
I get what you're saying...it's the space that's expanding, and it can do so faster than the speed of light. However, from what I understand (and I could be wrong), gravity waves, which are only ripples in space-time, move at the speed of light. Why are they limited and the expansion is not? Is the universe accelerating outwards? If it is, what could make it do so?
Posted by: Eric | July 31, 2009 9:17 PM
Because the gravity waves are moving through space. They are a form of energy moving from one place to another, just like a radio wave. It's motion through space that is limited by the speed of light, not the expansion of space itself.
Posted by: Naked Bunny with a Whip
| July 31, 2009 9:30 PM
Or, since the distance units on top and bottom cancel out, the expansion rate can be expressed as a frequency, namely 2.3 x 10^-18 Hertz!
More seriously, though, I just realized that 2.3 x 10^-18 Hertz is approximately equal to 1 / (13.7 billion years) -- whoa. I don't entirely see why that works out, but I assume that that's not a coincidence. Ethan?
Posted by: Brian | July 31, 2009 9:38 PM
It is kind of a coincidence, Brian. It depends on what's in your Universe.
For a Universe that was 100% radiation, a 13.7 billion year old Universe would have an expansion frequency of (1/2) / (13.7 billion years).
For a Universe that was 100% matter, a 13.7 billion year old Universe would have an expansion frequency of (2/3) / (13.7 billion years).
For a Universe that was 100% dark energy, a 13.7 billion year old Universe would have an expansion frequency of (infinity) / (13.7 billion years).
And for our Universe, of about 27% matter, 73% dark energy and a negligible amount of radiation, it works out to just about 1 / 13.7 billion years. It's neat.
Posted by: Ethan Siegel | July 31, 2009 10:02 PM
Most definitely. I was impressed Ethan included that little detail in the article. Nice. Haplodiploidy FTW.
Posted by: JD | July 31, 2009 10:31 PM
But isn't the expansion frequency increasing? Does that mean the proportion of dark energy is increasing?
I wonder, is it just about 1 or is it exactly 1/ApparantAgeOfUniverse.
Posted by: Boronx | July 31, 2009 10:32 PM
David #24,
If it's any consolation, I assumed you meant that the ant was a drone, because it wouldn't be on top of a balloon in the first place unless it could fly...
Posted by: Anton Mates | August 1, 2009 2:09 AM
Two questions:
1) The universe could easily have expanded fast enough so that there are still things at the outer part of the universe whose light has never reached us, right? We therefore can have no idea how big the universe really is, but only a lower limit.
2) Do any "contradictions" show up based on the fact that when we see something so many billions of light years away it is actually the billions-years-old version of the matter? This doesn't happen in daily life because as nothing can move faster than the speed of light, we don't get any contradictions in what we see. But with things moving outward and expanding faster than the speed of light, it seems like what we see could create some paradoxes.
Posted by: Alex | August 1, 2009 2:17 AM
Can I ask some more ripe amateur questions?
1. So if the oldest things in the universe are now 46.5 billion light years away, then why doesn't this blog mention the number 93 billion light years via d=2r? Or are we not at the center of the universe? Is one side of the universe closer to us?
2. Is the same thing that's beyond the edge of the universe the same thing that was outside of the singularity in the moment before the big bang?
3. Is it annoying to cosmologists when in science documentaries the big bang is animated as a really humongous explosion in space as seen from the outside? Is there any chance that the singularity existed as a point inside a pre-existing space/vacuum?
Posted by: eff | August 1, 2009 6:49 AM
This is interesting. I never thought about it before.
Now, I plugged these into a differential equation to see if I arrive at 46.5 billion light years. I'm arriving at 8.63 billion light years. I'm probably missing something.
Posted by: Change | August 1, 2009 7:18 AM
[To everybody: I'm the guy who commited the translation into Brazilian-Portuguese. I'd like to thank Ethan publicly]
And back to the questions: there are two other points I would like to see explained. The first one is the question gerwood asked: "what is outside the universe for it to expand into?" And the second is about biting one's own tail when it comes to define "distance" and "time".
Posted by: João Carlos | August 1, 2009 8:45 AM
Thanks for the extra info, Ethan. I've definitely had my U.S.RDA of unexpected coolness now.
Posted by: Brian | August 1, 2009 9:20 AM
"Massless particles can only stay still if you have a medium where the speed of light in that medium is infinitesimally small. Theoretically possible, but not yet discovered."
They were able to slow light down to 38 mph. They should be able to bring light to a stand still fairly soon, if they haven't already.
http://www.news.harvard.edu/gazette/1999/02.18/light.html
Posted by: Bryan | August 1, 2009 1:27 PM
Ethan @21:
I am still perplexed. If the expansion is 72 kilometers per second per Megaparsec, but 13.7 billion years ago the universe was (relatively) small, then why would the expansion then have beeen faster than light when the expansion accumulates over a large distance to become faster than light. Across the small distance it should have been relatively small.
I am not trying to argue with you, I am trying to understand.
Posted by: NewEnglandBob | August 1, 2009 2:04 PM
Nice explanation.
Posted by: Curdle1 | August 1, 2009 2:25 PM
NewEnglandBob,
The quick answer is that the expansion rate is related to the energy density of the Universe. Because the Universe was smaller in the past, it was *denser* in the past, and therefore expanded faster. 72 -- the current value -- is the smallest value it's ever had.
The long answer will need its own post. ;-)
Posted by: Ethan Siegel | August 1, 2009 3:49 PM
How do I explain this to my father-in-law who has a sixth grade education and thinks the universe is 6 thousand years old? Should I just forget it or what.
Posted by: Lewis Thomason | August 1, 2009 6:19 PM
I'm still troubled with a two-part question. (1) If we see something whose red shift indicates it was 13 BLY away 13 BY ago, how far away is it now? (2) How did it get to be 13 BLY away so soon after the Big Bang?
Posted by: MarkSeid | August 1, 2009 6:33 PM
Lewis @42, you might want to start smaller. Like with something that's 7 thousand years old.
Posted by: Sam L. | August 1, 2009 7:25 PM
Expansion of the Universe (FTL)
According to Hubble's Law, two galaxies that are a distance D apart are moving away from each other at a speed HD, where H is Hubble's constant. So this interpretation of Hubble's Law implies that two galaxies separated by a distance greater than c/H must be moving away from each other faster than the speed of light. Actually, the modern viewpoint describes this situation differently: general relativity takes the galaxies as being at rest relative to one another, while the space between them is expanding. In that sense, the galaxies are not moving away from each other faster than the speed of light; they are not moving away from each other at all! This change of viewpoint is not arbitrary; rather, it's in accord with the different but very fruitful view of the universe that general relativity provides. So the distance between two objects can be increasing faster than light because of the expansion of the universe, but this does not mean, in fact, that their relative speed is faster than light.
As was mentioned above, in special relativity it is possible for two objects to be moving apart by speeds up to twice the speed of light as measured by an observer in a third frame of reference. In general relativity even this limit can be surpassed, but it will not then be possible to observe both objects at the same time. Again, this is not real faster than light travel; it will not help anyone to travel across the galaxy faster than light. All that is happening is that the distance between two objects is increasing faster when taken in some cosmological reference frame.
Posted by: Shadow14l | August 1, 2009 9:33 PM
When I think of the size of the universe I do it in the sense of #33. We can only see things that were close enough 13Byears ago that the light is just reaching us, a little further away (initially), and we are still waiting for it to meet us (at higher and higher redshift I presume). But we have several meanings of size, one is how far away we can see things from the past (that is the 13e9 LY number, -or 46e9 if we consider how far away those things are now, but anaything currently 46e9 LY away is noe incapable of ever sending a signal to us? So another question, is if I send out a signal at the speed of light, how far (in terms of todays space-time) will it get, i.e. if I see light from a galaxy say 4BY ago, can I send a beam of light that will reach it? Or will it expand out of range before my signal arrives? So I suspect the size of the universe we could get to if we could travel at c, is a lot smaller than the universe we can currently see.
And then we have the more fundamental question, how big in total is the universe, including those parts we can never see (noting that there is much more beyond our "horizon" than within it)? I suspect we don't know how to answer that one.
Posted by: bigTom | August 1, 2009 9:55 PM
that was an amazing explanation. thanks!
Posted by: bk | August 1, 2009 11:28 PM
This is what i think: our capacity to understand the massive/tiny time & distance scales of the universe are too hard for a monkey brain to comprehend intuitively. "Seeing is believing" - and we have trouble grasping the fact that the image of an object is not the object itself.
An experiment is everyday living:
1. you place a fresh green apple 1 foot away, and take a polaroid photo of it, and place the photo in front of the apple. (Don't look at the apple or the photo).
2. throw the apple on the back of a truck as it speeds away, wait a year till it's completely rotten in some landfill somewhere.
3. now open your eyes- and look at the apple image.
4. Ask yourself: "where is the apple really?"
5. Ask yourself: "where is the image of the apple?"
That's what looking up at the night sky is like. Looking up at the stars is not at all like looking down at your feet. Actually, it's a lot like looking at a photosynth image- a composite image of a region of space, but with each photograph representing a different time.
shlog
http://shlogblog.blogspot.com/
Posted by: shlog | August 2, 2009 2:40 AM
Yes, you are right. Good job !
Posted by: opony szczecin | August 2, 2009 3:17 AM
Still doesn't answer the question of reference frame for measuring space-time to my satisfaction. Where do you stand when you measure the age or size of the universe?
A related question would be, how can we say something is happening simultaneously here and on a star 40 billion light years away? There is no reference frame in this universe to determine the answer. While your intuition might imagine standing outside and seeing events happening simultaneously, in fact space IS time. You measuring device must exist somewhere in space-time.
If you think, well, the observer can be half way between the two stars, then consider determining simultaneity between 100 dispersed stars. No observer can be equidistant between all of them.
Posted by: private | August 2, 2009 9:22 AM
Very interesting article and discussion. Just as an observation, no one has mentioned that the universe was opaque until several hundred thousand years after the big bang. Any observable radiation (light or otherwise) has to have been radiated after the universe went transparent. Also, no one has mentioned that most physicists (from what I've read) now adhere to the concept of hyper-expansion that seemingly indicates that the rate of inflation is not constant across our 13.7 BYs of existence.
Although I am not a physicist nor a cosmologist, I have pondered what the hell is going on since I was young just like most of us. I'd like to address the question, "what is outside the universe for it to expand into?" postulated above.
I have come to my own conclusion that there are multiple universes, each one a derivative of an earlier universe. New universes "foam" (for lack of a better word) out of earlier universes in an ongoing process. Each is a distinct space-time-gravimatric (my term) envelope separated by the quantum curtain (the set of weird events observed at the scale of the infinitesimally small). The process is analogous in my theory to the way matter accretes in our universe into suns that ultimately collapse via novas/supernovas into denser objects (neutron stars or black holes), that then will logically continue to capture any additional available matter due to to their extraordinary mass. Unlike Hawking who sees black holes magically evaporating, I foresee that a black hole could continue growing in mass to a point where, analogous to a nova/super-nova, it could no longer support its mass within the context of our universe, and collapse catastrophically through a singular quantum event (i.e. a big bang event), exit our universe, and come out the other side as a discrete new space-time-gravimatric universe similar to our own, but now outside the envelope of our own space-time-gravimatric universe. Whether any given black hole experiences this is dependent on collecting enough matter to reach the threshold of necessary mass. The mass involved in the event would exit our universe and form a new one, taking the gravity associated with it into the new space-time bubble. The loss of gravity from our universe would contribute toward an accelerated expansion (as widely theorized to occur) since that which holds back universal expansion is the gravity of our universal mass. Ultimately, the answer to the question posed above is, "It is the quantum matrix within which all the space-time-gravimatric envelopes independently exist."
OK, so I'm crazy to have my own theory - prove me wrong.
Posted by: Edward L. Scott | August 2, 2009 9:37 AM
I'd like to contribute a few clarifications to comments upthread.
Someone mentioned the 72km s-1 Mpc-1 as being the Cosmological constant. It is Hubble's constant. Further, it is the current value of a measure that's constant over space at each time, but which changes with time. The Cosmological constant relates to how the Hubble constant varies.
In another comment was asked "Does that mean that photons don't travel through time?" No. Photons move along what are called 'null geodesics'. These are sets of events (world lines) for which the invariant interval is zero. The invariant interval is the root of the difference between the squares of the time and space intervals between events. It may be a real number, if the events are close enough to be causally connected, and imaginary if they are not.
Posted by: eddie | August 2, 2009 10:43 AM
There is something Newtonianly incorrect about this. It seems to imply that because of the mechanics of expansion, that two objects can move apart from each other at greater than the speed of light. In fact, the very statement that objects are farther apart from each other than 13.7 billion years implies that in some way we have had two objects move apart from each other at a rate greater than the speed of light. However, special relativity shows that this is impossible: any two objects can only depart from each other at a maximum rate of the speed of light. I don't feel you've addressed this well, because you have used Newtonian mechanics to explain an issue with relativity.
Posted by: Fjordo | August 2, 2009 10:50 AM
If space has been expanding then why not the objects (matter) occupying that space?
We know that light waves lengthened as the space the occupied expanded. Have any other waves lengthened? Elementary particles are both particles and waves so why has the 'wave' aspect of particles not increased?
Thanks for the interesting article.
Posted by: Darren | August 2, 2009 10:58 AM
Wouldn't it be possible for a particle to travel _faster_ then light if it some how had negative mass? (yes i know, it may be silly physically but come on the math works doesn't it?), I'm pretty sure this would also mean it traveled in the reverse direction in time but i haven't played with the relativity equations enough yet to be sure.
Posted by: Arthur Ice | August 2, 2009 1:45 PM
Man, I've been out of the loop for too long. What's 46.5 billion light years away?
Posted by: Crux Australis | August 2, 2009 6:05 PM
I'll try a few of the unanswered questions, if anyone's still reading. First though, I want to say that it's important to remember that it's not really possible to really visualize how the expansion of the universe works. You can't do it, and the smartest man alive can't do it. God even throws up his hands. Some ways of conceptualizing it are better at describing different aspects than others, but all are limited in some way.
@Alex
The universe could easily have expanded fast enough so that there are still things at the outer part of the universe whose light has never reached us, right?
Yes. And there is light that will never reach us.
We therefore can have no idea how big the universe really is, but only a lower limit.
Correct.
Do any "contradictions" show up based on the fact that when we see something so many billions of light years away it is actually the billions-years-old version of the matter... with things moving outward and expanding faster than the speed of light, it seems like what we see could create some paradoxes
I think I understand what you're getting at, and the answer is no. Matter has never been moving away from us faster than the speed of light through space, so no laws of physics are being broken which would create paradoxes.
@eff
So if the oldest things in the universe are now 46.5 billion light years away, then why doesn't this blog mention the number 93 billion light years via d=2r? Or are we not at the center of the universe?
There is no center to the universe. If you look at the balloon analogy, you can't pick any one point on the surface of the balloon and call it the center.
@João Carlos
what is outside the universe for it to expand into?
This is another thing that's difficult to conceptualize. If you go back to the balloon analogy again, you might answer that the balloon is expanding into air, but you've just broken the analogy. For the analogy to work, you have to accept that the surface of the balloon is all there is. If you were living on the balloon, you couldn't even point to anything off the balloon. Now, scale the analogy up to the three dimensional world we live in and try to point to a direction where space doesn't exist. You can't. The question stops making sense.
Posted by: José | August 2, 2009 11:33 PM
Let me paint a picture here using the ant on the balloon analogy and please tell me if I have made the correct deductions. Assume that the ant has a telescope that he uses to peer at the galaxies far far away. Since this is a two dimensional analog what he sees is bright line segment that will subtend a certain angle depending on how far away and how big the galaxy is. Now with a static balloon, I don't see any problems: the light from one tip of the galaxy heads toward the ant at the same time that light from the other tip of the
galaxy heads toward the ant. The light meets at the ant forming a triangle. The angle at the apex of the triangle is determined by the distance away and by the size of the galaxy
(ignore the curvature aspect of the balloon). But now if we allow the balloon to expand, the space between the galaxy and the ant expands, spreading the two beams of light apart
- the affect would be that by the time the light reached the ant, the galaxy would appear to be bigger than it would have in a static balloon. This same spreading of the light
should also make the distances between distant galaxies appear larger - that is the population of distant galaxies should appear to be less dense. (Note that this distortion occurs just because of the effect of the expansion on the light beams and has is not affected by the fact that the expansion does not change the size of the galaxy.)
The picture I painted here seems to me to be logically required by an expanding universe but I can't ever recall reading about this size distortion. So, do I have something terribly wrong? Or, is this size distortion just something that astromoners keep to themselves?
Posted by: jdhuey | August 3, 2009 12:39 AM
It's crazy to think about what it would have been like to have been born early enough to see the big bang and then, like, remember all the stuff that happens until now and we didn't even realize this stuff until a few years ago. Not only is the universe expanding, but so are our minds. I bet some day we can think at the speed of light, and poof... we're there.
Posted by: planck | August 3, 2009 2:23 AM
Isn't it that space doesnt expand into anyting but rather it is space itselfs that expands (stretches?)
Posted by: Nick | August 3, 2009 3:35 AM
Actually in response to no.2:
http://www.newscientist.com/article/dn16796-fasterthanlight-tachyons-might-be-impossible-after-all.html
Einstein theory of relativity is not guaranteed to be correct.
Posted by: boburob | August 3, 2009 4:26 AM
@jdhuey:
What you are talking about is the so-called "angular size distance"; it is described e. g. here: http://www.astro.ucla.edu/~wright/cosmo_02.htm
And yes, this is essentially "something that astromoners keep to themselves"; I've never seen this discussed anywhere in popular-science accounts of the Big Bang theory...
Posted by: Bjoern | August 3, 2009 5:26 AM
@Pierce R. Butler:
The reason for the unit "kilometers per second per Megaparsec" is simply convenience. Galaxies typically have speeds on the order of a few hundred or thousands of kilometers per second, and distances on the order of a few Mega- to a few Gigaparsecs. Hence the Hubble constant is usually expressed in these units, since one usually talks about the relation between the speeds and the distances of galaxies.
But following up with what Brian wrote, for visualizing the expansion of the universe, it is much more convenient to cancel the distance units out and think of the Hubble constant as 1/(13.7 billion years), or, equivalently, as about 7 percent per billion years. In this way, you avoid the problem which Ethan mentioned ("Expansion isn't a velocity. It's a velocity-per-unit-distance.") and see that the Hubble constant simply tells you by which factor the universe expands per billion years.
Posted by: Bjoern | August 3, 2009 5:32 AM
So, how does that all influence the redshift? Does an object 13 by old and 40 bly away from us have a different redshift than a 13 by old object 13 bly away?
Posted by: Mu | August 3, 2009 8:01 AM
"The Universe is 13.7 billion years old. There are small errors there -- no one would be surprised if it was 13.5 billion or 14.0 billion years old -- but it's definitely not 12 billion years or younger and definitely not 16 billion years or older. "
Expansion or not, the age of the universe can never be proven. Some scientists believe it to be infinite. So, I fail to see the point of that statement.
Sorry if I come across harsh. But I grow tired of the circulation of information based on scientific theories that have yet to be proven.
Posted by: Bill R. | August 3, 2009 8:23 AM
@Bjoern
Thanks for the link and thanks for confirming the lack of discussion of this topic in the realm of popular science topics. It is very gratifying to have ones intuitions validated by a more rigorous analysis.
Posted by: jdhuey | August 3, 2009 8:58 AM
But isn't all of this too confusing for me to understand? Shouldn't I not think so hard? If I wanted to think I wouldn't be pressing the stumble button.
Posted by: tom | August 3, 2009 9:10 AM
Great article, Science Be Praised! Read more science like this at BomVom.com
Posted by: bomvom.com | August 3, 2009 10:37 AM
@Jose #57: Thanks for clearing that up!
Posted by: Alex | August 3, 2009 10:47 AM
Having trouble with distances, positions, and motion in the expanding Universe?
Yeah, you and everyone else. I've written a little follow-up to help you try and wrap your head around it:
http://scienceblogs.com/startswithabang/2009/08/distances_position_and_motion.php
Posted by: Ethan Siegel | August 3, 2009 11:18 AM
I'm an M.Sc.EE., so bear with me ...
Your '2.)' says that light has no mass.
I was under the impression that gravitational lenses were created when heavy objects bend the path of light using gravity.
What am i missing?
Posted by: Flemming Jacobsen | August 3, 2009 1:21 PM
So what is on the 'other side' of nothingness? If our definition of the universe is stuff from the big-bang, then what is on the other side of the stuff or nothingness? If the big-bang reaches a stopping point (assuming contraction theory), then like a stop sign what is on the other side of the street?
So if we were in a ship and 'ran the stop sign' while the rest of the universe contracts backward, is there an end? Your ant-on-a-balloon example is good. But put the balloon and ant inside a football stadium and you see what I mean.
Just thoughts . . . don't let it get in way of your good stuff.
Posted by: D. Marshall | August 3, 2009 2:00 PM
@Flemming
As I understand it there are two things to keep in mind: rest mass and thinking of gravity as a bending of space-time. The photon has a rest mass of zero (that is from the perspective of the photon it is indeed massless) but WE forever see the photon as moving (with the speed c when in a vacuum). So, when we look at a photon it carries with it a certain amount of energy and - because of E=mc^2 - momentum and mass. (It was based on this mass that Einstein predicted that light from a star would be bent the mass of the Sun but he predicted an amount of deflection based on just Newtonian Gravity that turned out to be much less than the actual deflection.)
When Einstein developed his General Theory of Relativity and deduced that gravity was best thought of as mass causing space-time to bend, he revised his calculations of the deflection and came up with a revised number that matched what was actually observed.
Posted by: jdhuey | August 3, 2009 3:30 PM
First- the center of the universe is wherever you happen to be. If you go in any direction, any speed you choose, the universe will put you in the center of it. Second- we are all centers of our own universes, separated on our scale by the speed of light. Yes, the center of the universe of the person sitting next to me is 2' (or the time it takes light to travel that distance) from mine. 2' further than my universe in his direction and 2' further than his in my direction. Third- there is discussion of the idea that time itself is slowing down, though we would never know it. This has implications concerning dark matter and inflation. My head hurts now.
Posted by: vince p | August 3, 2009 5:23 PM
@Mu:
"So, how does that all influence the redshift? Does an object 13 by old and 40 bly away from us have a different redshift than a 13 by old object 13 bly away?"
We can't see the light of objects which are 13 by old now (and hence we can't sensibly talk about its redshift); that light will reach us only in the future. The light which we receive now from these objects left them when they were younger than 13 by.
So, please try to rephrase your question; I'm not sure what you wanted to know exactly.
Posted by: Bjoern | August 4, 2009 4:59 AM
@Bill R.:
"Expansion or not, the age of the universe can never be proven."
No theory in science can ever be "proven". However, we have very good evidence for the Big Bang theory.
"Some scientists believe it to be infinite."
I assume you talk about people with ideas of a cyclic universe. If yes, then please consider that even these people agree that the Big Bang theory describes the universe right for the last 13.7 billion years, and only disagree about what happened in the first fractions of a second. I. e. to those people, the 13.7 billion years figure is essentially also right - it only does not describe the age of the universe per se, but only the age of the current cycle.
Posted by: Bj | August 4, 2009 5:02 AM
scientists just made the discovery and did the test that objects can go faster then light
http://en.wikipedia.org/wiki/Faster-than-light
Posted by: armtrack | August 4, 2009 5:04 AM
"Expansion or not, the age of the universe can never be proven..."
You're wrong. According with the WMAP measurements the age of the universe has been proven with a 99% of accuracy. See wikipedia.
"..Some scientists believe it to be infinite."
You're wrong again. Size and age are different things.
"So, I fail to see the point of that statement."
Yep, you're right. You failed.
Posted by: Floyd | August 4, 2009 5:30 AM
@jdhuey
I think i see it now. Thanks!
Posted by: Flemming Jacobsen | August 4, 2009 7:36 AM
Bjoern, rephrased: Is redshift related to age, distance, or both? As an example, if a 5 billion year old object is 5 billion light years from us and the light reaches us now, does it have the same redshift as a 5 billion year old object that seems to be 10 billion light years away due to expansion? Or can that not happen because all 5 billion year old objects will have the same virtual distance from us when the light reaches us now.
Posted by: Mu | August 5, 2009 7:51 AM
How do we know we're looking in the right direction?
Maybe there is more stuff to be seen a little to the left?
My question sort of fits with the ballon scenario.
Posted by: Ninjarabbi | August 5, 2009 3:26 PM
Arg... so then, what is space?
Posted by: Riya | August 7, 2009 1:47 AM
So if the universe is a balloon expanding and we understand that perfectly, then--why is there no equation or explanation for the girl? If one area is expanding then another should be contracting. I'm sure there must be an explanation for the girls lungs. Also, with virtually everything we observe in the universe expanding in a non-linear, but spiraling manner....are you certain that the universe itself is not spinning as it expands also?
Posted by: Rocko | August 7, 2009 9:37 AM
There were opinions that Apollo 11 has never been on Moon . One said that the Eagle , the module that " land " on the Moon didn`t make print of its landing foots in the dust of the Moon . I saw a picture of the footprint of an astronaut in the dust of the Moon . The dust seems to have a height of several centimetres . When Eagle was getting close to the surface of the Moon , at about 3-4 metres , the reactive jet of its engine which was directed downwards spread the dust on a surface of several square metres , and let the rock of the Moon uncovered by dust . So , considering that the rock of the Moon has a medium toughness and also the landing foots of Eagle has a certain , not very small , surface , it is absolute normal that the landing foots of Eagle didn`t let any trace .
Spanu Dumitru Viorel
Posted by: Spanu Dumitru Viorel | August 7, 2009 1:14 PM
How long will we be making up dates and lengths of time before we just go with what God has already said what he did to create the universe, just beleive it. He said, he created it all in 6 days and rested for one, then has been working ever since to keep his creation on track according to what he wants done. You can't change the fact that your going to die some day and meet your creator whether you like it or not. If you reject him in this world, then Hell is where you will go because thats what you wanted, you just have to live with your choice for all eternity. If you chose not to beleive in God then you will be kicking yourself for all eternity knowing that Jesus was right and you were wrong. I wouldn't wish hell on my worst enemy. Reason is the enemy of faith. It takes just as much faith to beleive in no God as it does to beleive in one. Only one path leads to life after death and that is Jesus Christ, all others are deceptions from the devil who is the liar who is the destroyer of life.
Posted by: Mark | August 7, 2009 2:25 PM
Mark, this is what is assumed God said in the Bible, however since the actual ancient text does not exist any more in the original language, we could say this is an inaccurate translation: 6 days in no way means 6 days, rather 6 time frames.
In the Qur'an it is all explained way better, and in the original language.
PS1 Oh yeah, in the Qur'an there is a statement that God created the universe and has been EXPANDING it since! Now this is something we did not know until some mid 19th century, cannot remember the exact date.
PS2 Since when God needs rest? ;)
Posted by: Jibreel | August 8, 2009 5:35 PM
Each observer is at the center of it's own universe. All of which you describe seems reasonable, but it is an illusion. The light from the edge of the observable universe does not limit the actuality of an infinity of universes larger than the observable one. The redshift of light is due to its approach to the observer, and all observers anywhere in your description of the universe or even in the unobservable beyond will see and experience the same properties you describe, happily so. My question? You say it is like a balloon. If one expands a balloon enough one gets a "BIG BANG". and that should put an end to all of the conjecture!
Posted by: Paul McBride | August 9, 2009 8:58 AM
Each observer is at the center of it's own universe. All of which you describe seems reasonable, but it is an illusion. The light from the edge of the observable universe does not limit the actuality of an infinity of universes larger than the observable one. The redshift of light is due to its approach to the observer, and all observers anywhere in your description of the universe or even in the unobservable beyond will see and experience the same properties you describe, happily so. My question? You say it is like a balloon. If one expands a balloon enough one gets a "BIG BANG". and that should put an end to all of the conjecture!
Posted by: Paul McBride | August 9, 2009 9:01 AM
I've always thought that the Primordal Point of energy was dimensionless as pure energy is assumed to be (by me). It was contained in "non-space", therefore had no space in which to coalese into matter (E=MC^2).
Gravity is a distortion of spacetime, therefore, is not a force at all but an observation and as such cannot be included in a Unified Field Theory or TOE (theory of everything). Lastly, The Big Bang is but one of many, meaning the true size of the universe is even bigger than comprehension. Just my crazy thoughts. What do you think?
Posted by: Burnerjack | August 9, 2009 2:54 PM
Suppose we were in a Newton-Hubble Universe (NHU) - no Michelson-Morley observations, no Fitzgerald-Lorenz equations, no Einstein special or general relativity; but data that showed increasing red-shifts from further stars.
How rapidly would this NHU diverge from what we see and calculate about the real universe? Or, alternatively, what could we deduce and explain without jellying our brains?
Posted by: MarkSeid | August 10, 2009 11:01 AM
What is the age of the Milkyway? Why can't we look that many light years away and see ourselves? or would that require traveling faster than the speed of light? I am so confused.
Posted by: Mike | August 10, 2009 4:21 PM
O.K. If you could go faster then the speed of light couldn't you get to your destination and turn around really quickly and see yourself coming?
Posted by: Mike | August 10, 2009 4:56 PM
Alright, let me see if I got it. Nothing is really moving, the universe is expanding so everything is moving relatively to everything else?
Posted by: Mike | August 10, 2009 5:11 PM
@Ethan: "Massless particles can only stay still if you have a medium where the speed of light in that medium is infinitesimally small. Theoretically possible, but not yet discovered."
Yes it has, its been done in the lab with condensates.
Posted by: Doc Merlin | August 10, 2009 6:20 PM
I fear the answer is very mundane and not what you asked for, as AFAIU fields as we know them couldn't exist. The discrepancy would be immediate, no light, no matter, nothing.
Posted by: Torbjörn Larsson, OM | August 15, 2009 7:38 AM
Funny idea, from someone who waves a made up text around.
Posted by: Torbjörn Larsson, OM | August 15, 2009 7:54 AM
Just wondering how the red shift tells us that the univierse is STILL expaning? Would there not be a red shift for some period of time after the universe began contracting?
Posted by: Erik P | August 21, 2009 1:57 PM
Ethan, really am trying to understand this. But when I read this I think that if the red shift was caused by expanding universe, wouldn't the red shift continue even if contraction started to occur, at least until the stretched light waves are back to original wavelength? Or what am I missing.
Posted by: Erik P | August 27, 2009 3:04 PM
@Ethan: If the universe were to start to contract, we would need for the light from the nearest universes to reach us to confirm this, since it would only be visible on this scale. And since contraction/expansion becomes more and more visible the farther a universe is away from us, we'd probably need to wait even longer to be sure. So unless these things happened hundreds of millions to billions years ago, we'll never know the answer. However, everything points to expansion increasing over time, so something drastic would have to happen, or our understanding of the universe to be drastically wrong, for it to change back to contraction.
Posted by: Tim Walker | September 3, 2009 3:19 PM
Erik and Tim,
If contraction started to occur, we would be able to observe it. The nice thing about having the whole Universe to look through is that when you see something nearby, the light only traveled for a short amount of (recent) time. When you see something farther away, the light traveled for a longer amount of time, including the recent time. So by probing objects at all distances, we can learn about how the entire Universe expanded over its entire history.
We find that it expanded the same in all directions, the expansion was faster in the past, and the expansion rate is leveling off, not dropping to zero and definitely not contracting.
I hope this helps clear things up.
Posted by: Ethan Siegel | September 3, 2009 4:18 PM
Well, I become to the same conclusions. I just finished my calculations and found this article. Everything match!
Posted by: Raimonds Cirulis | September 6, 2009 3:31 PM
Ethan, thanks. So I am understanding that you are saying that we can comfirm expansion from redshift even looking at closer objects, just a few light years away. We are not only depending on the shifts of objects from millions or billions of light years out. I think I understand now.
Posted by: Erik P | September 11, 2009 3:36 PM
Please answer on this question - Does the density of galaxies drop off as the distance away increases?
Posted by: Universe Man | October 14, 2009 4:49 AM
Ethan: My question is this (if you ever come back to this post)
Will the light that has been comming to us from "the other side" of the big bang eventually distort our view of the "recent" visible universe?
More specifically: Will the light we see from galaxies now (our side of the big bang) eventually be "caught up" to light from the other side of the big bang (assuming galaxy formation happened on the other side of inflataion too) and then instead of having mostly black light in space there will be so much to see that in effect we will see 2 evoutionary periods (our present (and older) evolution coupled with the evolution of the galaxies beyond the visible universe?) So in essence, after a certian amount of time (i guess the initial distance of the big bang) passes by us then we will see the "other side" of the universe evlove from "ground zero" again?? I hope that was not to oconfusing
J
Posted by: Jim Jones | March 10, 2010 12:47 PM
@Jim Jones: I'm not Ethan, but I'll try... ;-)
I'm not entirely sure what you mean with "the other side of the big bang". Probably you mean in the balloon analogy the point which is on the exactly opposite side from us, the point with the furthest possible distance to us? If you mean that, then the answer is simply: if the universe keeps expanding as described by the current model, the light from that point will never be able to reach us.
If you mean something different, please explain.
Posted by: Bjoern | March 10, 2010 2:24 PM