Q: How can we see the early universe and the Big Bang? Shouldn’t the light have already passed us?

Physicist: This is a very common question that’s generated (as best I can tell) by a misrepresentation of the Big Bang that you’ll often see repeated in popular media.  In the same documentary you may hear statements along the lines of:

“our telescopes can see the light from the earliest moments of the universe” and

“in the Big Bang, all of the energy and matter in the universe suddenly exploded out of a point smaller than the head of a pin”.

The first statement is pretty solid.  Technically, the oldest light we can see is from about 300,000 years after the Big Bang, but we can use it to infer some interesting things about what was happening within the first second (which is the next best thing to actually being able to see the first second).  That light is now the Microwave Background Radiation.

The second statement has a mess of holes in it.

First, when someone says that all of the matter and energy in the universe was in a region smaller than the head of a pin, what they’re actually talking about is the observable universe, which is just all of the galaxies and whatnot that we can see.  If you’re standing on the sidewalk somewhere you can talk meaningfully about the “observable Earth” (everything you can see around you), but it’s important to keep in mind that there’s very little you can say about the size and nature of the entire Earth from one tiny corner of it.

The second statement also implies that there’s time and space independent of the universe.  Phrases like “suddenly exploded out of a point” makes it sound like you could have been floating around, biding your time playing solitaire and checking email in a vast void, and then Boom! (pardon; “Bang!”) a whole lot of stuff suddenly appears nearby and expands.  If the Big Bang and the expansion of the universe were as straightforward as an explosion and things flying away from that explosion, then the earliest light would be on the front of our ever-expanding universe.  If that were the case (and it seems to be from the images and videos presented in, like, every documentary evar), then there’s no way you’d be able to see the light from the early universe.

The incorrect misrepresentation often shown (implied) in many science shows and books: the Big Bang as an explosion that happens at some point, and all of the resulting light (blue ring) and matter (stuff in the ring) in the universe spreading out from that point (red star).  However, this means that the light from the early universe would be long gone, and we would have no way to see it.

Just to be doubly clear, the idea of universe exploding out of one particular place, and then all of the matter flying apart into some kind of pre-existing space, is not what’s actually going on.  It’s just that getting art directors to be accurate in a science documentary is about as difficult as getting penguins to walk with decorum.

The view of the universe that physicists work with today involves space itself expanding, as opposed to things in space flying apart.  Think of the universe as an infinite rubber sheet*.  The early universe was very dense, and very hot, what with things being crammed together.  Hot things make lots of light, and the early universe was extremely hot everywhere, so there would have been plenty of light everywhere, shooting in every direction.

If you start with light everywhere, you’ll continue to have it everywhere.  The only thing that changes with time is how old the light you see is, and how far it’s traveled.  The expansion of the universe is independent of that.  Imagine standing in a huge (infinite) crowd of people.  If everyone yelled “woo!” (or something equally pithy) all at once, you wouldn’t hear it all at once, you’d hear it forever, from progressively farther and farther away.

Everyone in a crowd yells “woo!” at the same time. As time marches forward you (blue dot) will continue to hear the sound, but the sound you’re hearing is older and from farther away (red line). Light from the early universe works the same way.

As the universe expands (as the rubber sheet is stretched) everything cools off, and the universe becomes clear, as everything is given a chance to move apart.  That same light is still around, it’s still everywhere, and it’s still shooting in every direction.  Wait a few billion years (14 or so), and you’ve got galaxies, sweaters for dogs, hip-hop music; a thoroughly modern universe.  That old light will still be everywhere, shooting in every direction.  Certainly, there’s a little less because it’s constantly running into things, but the universe is, to a reasonable approximation, empty.  So most of the light is still around.

The expansion of the universe does have some important effects, of course.  The light that we see today as the cosmic microwave background started out as gamma rays, being radiated from the omnipresent, ultra-hot gases of the young universe, but they got stretch out, along with the space they’ve been moving through.  The longer the wavelength, the lower the energy.  The background energy is now so low that you can be exposed to the sky without being killed instantly.  In fact, the night sky today radiates energy at the same intensity as anything chilled to about -270 °C (That’s why it’s cold at night!  Mystery solved!).  Even more exciting, the expansion means that the sources of the light we see today are now farther away than they were when the light was emitted.  So, while the oldest light is only about 14 billion years old (and has traveled only 14 billion light years), the location from which it was emitted can be calculated to be about 46 billion light years away right now!

Isn’t that weird?


*The universe may be “closed”, in which case it’s curved and finite (like the surface of a balloon),  or “open”, in which case it’s flat and infinite in all directions (like an infinite rubber sheet).  “Curved” and “flat” are a little hard to picture when you’re talking about 3 dimensional space, but there’s a little help here.  So far, all indications are that the universe is flat, so it’s either infinite or so big that the curvature can’t be detected by our equipment (kinda like how the curvature of the Earth can’t be detected by just looking around, because the Earth is so big).  In either case, the expansion works the same way.  However, in the open case it’s a touch more difficult to picture how the Big Bang worked.  The universe would have started infinite, and then gotten bigger.  The math behind that is pretty easy to deal with, but it’s still harder to imagine.

This entry was posted in -- By the Physicist, Astronomy, Physics. Bookmark the permalink.

31 Responses to Q: How can we see the early universe and the Big Bang? Shouldn’t the light have already passed us?

  1. Will says:

    ‘The universe would have started infinite, and then gotten bigger.’

    I love physics.

  2. A.J says:

    I think there is some problema with an infinite universe, such as making hilberts hotel possible.

  3. GAO says:

    I can understand the general idea that the space and time fabric of the observable universe started out as a region smaller than the head of a pin and then expanded to its present size, much like a printed image on a party balloon starting out small and then getting larger as the balloon is inflated and the rubber fabric expands.

    What I don’t understand, however, is the conceptual distance measurement unit or reference that is being tacitly implied when it is said that the fabric of the universe has expanded: presumably any physical unit or reference of measurement will itself have expanded at the same rate, along with the fabric of the universe, making the universe in fact seem exactly the same size when measured by observers within the universe, using an internal measurement reference.

    I could understand the idea of the fabric of the universe expanding from the hypothetical point of view of an observer outside the universe, watching it inflate; but for any beings within the universe like us, surely observing the expansion of the fabric of the universe is not possible, in that us internal observers and our measuring instruments will expand equally with the universe, making the expansion undetectable from within.

    Put another way: if the space and time fabric universe and everything in it were suddenly halved in size, how would those beings inside and part of the universe even know this shrinking in half had taken place?

    If the reference of distance measurement tacitly employed in the statement “the universe started out smaller than a pinhead” can be explained to me, I’d be most grateful. I have always been curious about this.

  4. Madhu says:

    Gao’s question makes sense. We can say something is expanding only if that ‘something’ keeps expanding and our measuring instruments (scales) remain unchanged. it means in a smaller universe (close to big bang) the measuring scales may be much smaller than plank length and the living beings (if any) would still conceive the universe as being vast! but, this would happen only if the rate of the expansion of the universe is uniform and constant at all places. but we detect that the farther a galaxy, it’s moving faster away from us. it may be that our measuring scales are ALSO expanding, but no so much as compared to the receding away of farther galaxies.

  5. Delta1212 says:

    The answer is that space is expanding, but the stuff inside it is not. Since our references to distance are all based on that stuff, and not on space itself, there is no expansion of distances.

    For example, if two stars are one mightiest apart, it tales light one year to travel between them. If the space expands so that they are two lightyears apart, it will take light two years to travel between them. The speed of light does not increase to compensate for the expansion. Since we actually now define a meter in terms of lightyears, our reference to space has remained static while the space has expanded.

    Additionally, matter is not expanding with space. Atoms remain the same size, they just get pushed apart. The reason we don’t notice this/blow apart is that in very small spaces, the attraction between the atoms is many times more powerful than the rate of expansion.

    Imagine you have two large treadmills pushed together end to end and moving in opposite directions. If you put put two magnets in the center of each treadmill, they will move away from each other. if you put the magnets right at the edge of each treadmill so that they are touching, they will stick together and stay put instead of getting pulled along by their respective treadmills.

    So yes, units of measurement are required to measure the change in distance, but no, these units do not change themselves because they are not based on percentages of the total amount of spaces but ratios of forces that do not change in synch with the expansion of the universe.

  6. Gareth says:

    Hi
    When astronomers say they have looked at early galaxies just hundreds of millions of years after the Big Bang, what direction do they look? Would they find the same early galaxies if they looked in any direction?

  7. The Physicist The Physicist says:

    Yup! They’re all over the place.

  8. mark says:

    If a black hole had enough time to consume every galaxy in the universe until it was so massive does the math make it possible to create a big bang? Before the big bang there was no time and in a black hole physics says time ultimitely stops so im trying to see a connection between the two. Could our Universe be some kind of “white hole” or the remains of space, time and matter that a black hole has gobbled up and threw back out in another dimension. It sure boggles my brain just thinking about it let alone trying to explain it to you. I hope you can understand what i am trying to ask you.

  9. Pingback: Long Ago

  10. The Physicist The Physicist says:

    There’s been some “professional pondering” along those lines. One of the theories that describes what you’re talking about is the “fecund universe” theory.

  11. mark says:

    Hi, inside an atom is protons, neutrons, electrons etc whizzing about in huge swathes of space. What stops these particles from flying off out of the atom? Is it elecro magnetism or quantum gravity or both as when you see diagrams it looks as though there is some form of shell containing everything somewhat like an egg. Do you think the origin of the Universe what caused the big bang will ever be explained? I know Stephen Hawking has explained it in terms similar to quantum mechanics where particles can just pop into existence from nothing but there is always something else these particles are related too when we observe them in our space and time. I suppose the next biggest puzzle is what is spacetime actually expanding into. I find it near impossible to imagine something expanding into nothing. Surely there must be an edge or something and if so what is beyond that! They appear to me like questions that can never be fully explained but hey, what would i know, i am just an unqualified interested person seeking answers. One thing for sure that i am totally convinced of is that there is definately no God or sentient creator of our universe. Maybe it is so complex that we are not meant to find out, but as human beings it is not in our nature to not to try to answer questions like this. Maybe we are all an illusion and nothing is actually real! It sure is fascinating but totally head banging as well.

  12. The Physicist The Physicist says:

    The electromagnetic force keeps electrons bound to atoms.
    I personally have no idea whether or not the beginning of the universe will/can be explained.
    Spacetime isn’t really “expanding into anything”, it’s just expanding (for what that’s worth)! There’s even a post about it.

  13. mark says:

    Is sentience a force of nature. It baffles me because i cant describe it. Is it matter, photons? Its gotta be made of something? Is this the part where people then say God? Theres gotta be a better explanation than that hasnt there. Or is explained through cause and effect. Its a toughie.

  14. Dark energy and expansion logic says:

    I think people here forget to mention the so-called “dark energy” that most objects and all particle clouds and forms are made of: It plays a very vital role in the expansion of the universe. Only a tiny fraction of even a rather dense mass is actually matter. The distance between the tiniest particles within atomic structures is relatively huge. Matter itself doesnt expand, but the dark energy in the long run”forces” the matter to lose its “infinite” process of exchanging energy sufficient for being matter. (See the effect of entropy)

  15. I would like to know whether both Light and Sound emerged during the process of big bang. Or these have nothing to do with big bang ?

  16. Daimer says:

    Hi first of all i haveto say my english is not so good coz im From Finland. But i want to say that if we can see big bang is it possible in theory that we can allso see “ancient” earth in somewhere in the universe? And if that is possible in theory can you please explain.

    Thanks.

  17. Christian says:

    I read the whole article, but from a lay man point of view it still hasn’t been explained clearly how light from the early universe is still reaching us and we measure the age of the universe by it. I do understand the concept of a baloon expanding and the distance between the dots on it becoming longer. The article’s author could not explain the idea beyond any doubt, so I doubt he understands the implications of this idea. I will only agree to it if the following were true: firstly, the dot on that baloon 13 billions ago that is now that most distant galaxy detected and the other dot 13 billions years ago that now became our galaxy were at the extreme opposite ends of the then universe, especially since astronomers measure the age of our universe by it. Secondly, the universe is expanding at slightly lower than the speed of light, so it took so long for those early lights to catch up with us. In other words, you are I are standing on two trains moving in opposite directions at slightly lower than the speed of sound, and you say “hi’ to me but I don’t hear the “hi” until a very long time from now.

  18. Kenneth Kristiansen says:

    Hi! I just stumbled upon this very informative article through som link at wired…

    Read it, digested it but still agree With above poster Christian.

    This: So, while the oldest light is only about 14 billion years old (and has traveled only 14 billion light years), the location from which it was emitted can be calculated to be about 46 billion light years away right now!

    whaat? This has been a childhood thought of mine, I just could not grasp that, yes, the light we see is old, like 14 billion years old, but the objects emitting the light is not 14 billion ligth years away.. Where can I find more information on this topic? I am 44 years old, I believed in the “big Boom” :-) But now you tell me it should be called ” a rapid rubber sheet stretch theory” … I feel uneducated.

    Love the site though:-)

  19. James says:

    I think a way for me to understand maybe how we can still be able to see the faint light or images from the very early universe can be easily explained by the explanation of the universe as a Negative Curvature or Hyperbolic Space…. explained here
    http://science.howstuffworks.com/dictionary/astronomy-terms/space-shape2.htm

    light can then ultimately travel indefinitely basically in a straight line till it reaches its original point of origin, this can explain (well to me though) how it possible to capture these images… as far as I know the first few seconds after the big bang everything expanded faster than the speed of light or that photons did not exist yet, might be why we cannot see the actual very beginning…

  20. Pingback: A Guide to Reality, Part 7 | Whereof One Can Speak

  21. Zaron says:

    @A.J – What’s wrong with Hilbert’s hotel being possible?

  22. Nikki Ty says:

    I had trouble with the concept of expanding Universe because I’m not arrogant enough to assume we’re at the center of it. “Made in the image of God” and all that. I’m quite comfortable with being a speck of sand. The immensity of the Cosmos is absolutely wonderful and just thinkiing about it propels me into an almost ecstatic state.

    It all became clear with a very simple image suggested by a fellow blogger. If one thinks of the Cosmos as being a huge lump of bread dough filled with raisins … and rising slowly as the baker’s yeast goes to work …. well it’s easy to grasp. There’s no real “center” of action …. it’s expanding in all directions. Lovely.

  23. Nicholas Lee says:

    If we conclude the universe is expanding because the distances between separate lumps of matter are getting smaller, then couldn’t we equally assume that the distances are staying the same whilst the lumps of matter (and our measuring sticks) are shrinking?
    Is there any empirical way to tell which scenario is happening?
    What would the implications be for our world view if it was the size of the matter particles and the constant ‘C’ that was shrinking rather than the universe expanding?

  24. JimS says:

    If we can we see light from the early universe, does that mean we can see earlier versions of our own galaxy out there somewhere? Or am I completely missing the point?

  25. Xerenarcy says:

    the universe doesn’t need to be centered around us for it to have expanded ‘from a point’. due to relativity, from everywhere you look you will think you’re in the center.

    the sound analogy is most apt to explain why we can see this light…

    first, don’t think of the universe as having an edge or boundary; forget about that entirely because we cannot prove this either way with current theory (or whether it loops back on itself). instead, the correct way to think of the ‘boundary of the universe’ is ‘as far as you can see’.

    in other words, at the instant of the big bang, there would have been nothing coherent to look at because light had not yet moved at all anywhere in the universe to be observed at a distance.

    (ignoring the physics of light just after the big bang for convenience) a short while later the light from everywhere in the universe could have moved X distance. therefore, X distance in every direction around you would have been visible, and anything further had not yet had enough time for the light to reach you.

    for example – out at night on a field somewhere away from echo’s and such, have a large group of people clap on cue from a light source of some sort indicating the ‘start’ of the universe. if standing in the middle, you will hear those closer to you first, and those further from you afterwards.

    now suppose that the universe is filled with these ‘claps’ that all happened, everywhere, at the exact same time. since the universe is (for all we know) infinite, there is a constant stream of claps from the big bang (or the earliest moment when light could decouple and fly away unhindered) from further and further in the universe, yet all of them happened at the same time.

    and then, add to this that the universe was initially ‘point-sized’. all this means is that the distances between everywhere were, compared to today, infinitely small. it helps to think of the universe at that time as simply ‘infinitely dense’ but not strictly speaking a point, just very very very small such that today’s physics would even assume a point-like description for simplicity.

    what does this do for the claps? suppose the sound from the claps can travel 300 m/s, and you’re interested in when the sound will arrive from someone who was 300m away from you when they first clapped… lets say the distance between you and that person doubled spontaneously half a second after everyone clapped – the sound had been in-flight for half a second, and covered 150m; since the distance doubled the total distance is now 600m, and since the sound was half-way at the time, it is now located 300m from you and 300m from the source!

    to recap, it moved 150m to get half-way of the distance to 600m, leaving 300m to go which would take 1s. the total travel time is 1.5s for what should have taken 1s.

    the universe expanding is similar to this but with light, and it happens continuously throughout the trip of the light rather than spontaneously growing in increments, but for a simple analogy this is sufficient i think.

  26. Brandon says:

    GAO if space expands that doesn’t mean im expanding…lol we can see the expansion because of dark energy*(universe inflating) by the galaxy moving apart. U could easily argue the universe isn’t infinite if u were god and could break the laws of physics. But the only reason it is infinite is because its always getting bigger and faster then the speed of light only if u look at the universe as a whole. Example. Im going to escape the universe- fastest speed is the speed of light, well u start ur journey and notice ur not getting far in space because the universe is expanding as a whole, so then u get to the end of time and ur still here, pound ur head! But if u could go faster then the speed of light u go backwards in time so ur still stuck. Well i dunno but ud probably have to rip spacetime itself to leave the universe, anyway when space expands u dont expand with it. Thats y we can measure the universe getting bigger but where not

  27. GDL says:

    Way to weave a patchwork! Instead of saying that the Universe was infinite and expanded everywhere at once (where’s the “bang” now in all this?) which accounts for increasing wavelengths, why not just say that Universe is indeed infinite and there is no expansion of space and there never was, but that light just naturally weakens (although very slowly) over time because of some process I won’t speculate about in this post, and that this constant weakening is however probably too small to measure in a lab at our instant time scale? This would explain Hubble’s red shift as well. Perhaps many people are fond of the new age creationist view offered by big bang theory, but faced with the accumulation of contradictions in itself and with observations, isn’t it time for serious scientists to question their devotion to this obsolete theory and start considering alternatives? If a serious scientist is willing, I’d welcome the chance to discuss these problems and my cosmological ideas, which may not be complete because so much is unknown, but at least they seem more logical (to me at least). Oh, and thanks for finally answering that basic big bang question that puzzled me for a very long time, even though it exposes a fatal flaw of the theory.

  28. Brad Bell says:

    Something cannot be infinite if it has a starting point, although it could grow inifinitely. If the universe started from a point, it was not small or large. We look at that point as though we are outside it in empty space. There is no such thing as empty space, only a Highs bosun field.
    We forget that we are inside of an apparently expanding universe that we are measuring. In fact, we are made up of part of the universe, ourselves. The universe is studying itself, through us and posing theories as to what it is.

  29. Stewart says:

    Surely if the universe is expanding then there is no end, so the light from the beginning should be long past us, so maybe it’s the expanding light and not the beginning. Just a thought.

  30. Strings are noncalculable. Relativistic theory is calculable. How to harmonise the
    two? I don\t think strings could wiggle, could form loop and even multidimensional
    branes. Strings must be simple random bnary points with values vibrating between
    1 and 0. Then it becomes calculable. For example a random vibration of 100 times will generate an average 50 * 1 and 50*0. But points are dispersed throughout the
    Universe which travel at different speed from standstill to speed of light. Therefore
    frequencies of vibration must vary. This redefined string might generate values which relativistic theory could use.
    Huen Yeong Kong (dummy number theorist)

  31. Fugitive says:

    How does this make sense?

    “So, while the oldest light is only about 14 billion years old (and has traveled only 14 billion light years), the location from which it was emitted can be calculated to be about 46 billion light years away right now!”

    Even if us and the source were traveling at opposite directions at the speed of light, we would only be 14 billion light years away from the location in space which the light was emitted from and 28 billion light years away from the source of the light. Am I missing something?

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>