# How can we see back to the beginning of the universe?March 29, 2011 11:21 AM   Subscribe

How is it that I can look through a telescope and see the first few seconds of the universe, and that I can also do the same thing next week? Why hasn't that light passed us yet?

They always say that looking further out is the same thing as looking back in time. You see a quasar, you're seeing what that quasar was doing 6 billion years ago.

How far back does this work for? Because they talk about looking very far out to see what was happening in the first few seconds of the universe.

If, in the first few seconds of the universe, someone started a TV playing an episode of Battlestar Galactica, then that show would be over in three hours an the TV would turn off.

If I had a powerful enough telescope, could I watch that episode of Battlestar Galactica? Next week, if I had an even more powerful telescope, could I see that episode AGAIN? That doesn't make any sense.

At one time, the universe was the size of a beach ball. How could I possibly see any of the light from that time? Wouldn't that light have traversed the universe in nanoseconds and passed where we would eventually be?

It seems that this "looking further out equals looking back in time" metaphor breaks down at some point. What is that point?
posted by Galaxor Nebulon to Science & Nature (22 answers total) 10 users marked this as a favorite

Well, you can look out at things at different distances. So, this year, you can look out at something 10 light-years away and see something that happened 10 years ago. Next year, you can look out at something 11 light-years away and see something that happened at the same time.
posted by Paquda at 11:31 AM on March 29, 2011

I think your problem is at "they talk about looking very far out to see what was happening in the first few seconds of the universe." We have theories about what happened in the first few seconds of the universe, but from doing experiments with particle accelerators and such, not from direct observation. The oldest thing we can actually see is currently a small galaxy which dates (as far as we can tell) to hundreds of millions of years after the Big Bang.
posted by theodolite at 11:32 AM on March 29, 2011

Response by poster: @theodoite - my argument doesn't depend on the first seconds of the universe. Take @Paquda's statement of the problem:

Let's say that I was looking through my telescope and I saw, 10 light-years away, a fat guy jazzercising in front of an open window. I think it's hilarious. One year later, I meet someone with a similar sense of humor and we get a bigger telescope and look 11 light-years away. Can we see that same fat guy jazzercising? How? Hasn't that light already passed us?
posted by Galaxor Nebulon at 11:38 AM on March 29, 2011 [1 favorite]

When you look at different distances you are looking at different objects, so no.

Also, that light drops off as 1/r^2 which means that there is no hope whatsoever of anyone seeing objects through a window or picking up I Love Lucy reruns. Past the edge of the solar system those signals have so little power they are indistinguishable from noise.
posted by Rhomboid at 11:42 AM on March 29, 2011

The fat guy would still be only 10 light years away. The only way your example would work is if fatty was moving away from you at the speed of light.
posted by theodolite at 11:42 AM on March 29, 2011 [1 favorite]

Response by poster: Oh! Right. I guess my argument does depend on the universe being only seconds old. I'm worried about light that should have passed us because it was generated when the universe was the size of a beach ball. But I'm hearing that we haven't, in fact, seen anything that far away.

But that just seems like an engineering problem. Is it impossible that we ever could see something that far away, even given improbably big telescopes? What would we see if we were able to look that far away?
posted by Galaxor Nebulon at 11:47 AM on March 29, 2011

Best answer: The early universe was opaque to light, the earliest thing we can "see" is the cosmic microwave background radiation which is believed to originate from the time when the universe had cooled to the point that electrons and protons (+ some other light proton/neutron combinations) ceased to be a plasma and combined to form electrically neutral atoms. At this point, the CMB and matter "decoupled" as the universe became transparent to light. This is believed to have occurred about 300,000 years after the origin of the universe.

The thing is that this decoupling happened everywhere at about the same time: the early universe was almost completely isotropic. (ie the same everywhere) The visible universe appears to be embedded in a larger universe that we can't yet see until enough time has elapsed for the light from it to have reached us. Hence the continuous CMB radiation. It may be that the universe loops back on itself, but I think that would have shown up in the CMB somehow, unless it does so on a scale much larger than the part we can see.
posted by pharm at 11:50 AM on March 29, 2011

Best answer: There's a lot of questions, I can answer a few easily, the others, I'd have to think about.

How far back does this work for? Because they talk about looking very far out to see what was happening in the first few seconds of the universe.

There are two limits, one is the basically the boundary of the observable universe, due to expansion is about 45 billion light years. The second limit is a time limit. At some point in the very, very early universe, 13 billion years ago or so, the entire universe was opaque, and photons couldn't pass through it. You can't see anything past that point, and that is basically the Cosmic Microwave Background radiation, which has cooled and red shifted to the point where it's only a few degrees kelvin.

We are seeing things now which are 46 billion light years away, but when the light was emitted from those objects, they were much closer, but the objects and the light have both been affected by the expanding universe, so you're seeing things now which are a LOT further away now then they were when the light left them.
posted by empath at 11:56 AM on March 29, 2011 [1 favorite]

Best answer: When the universe was all in one place, it was also extremely dense, so gravity was extremely high. Where gravity is extremely high, time moves extremely slowly, so light took a long time to travel even very short distances. The universe then expanded very rapidly – much faster than the speed of light – distributing the radiation from the Big Bang (not quite) uniformly throughout. That radiation is coming from everywhere, not from some distant place or horizon; therefore it is arriving here on Earth continuously. The inflationary period is when, as you put it, the "metaphor breaks down".
posted by nicwolff at 11:57 AM on March 29, 2011 [1 favorite]

Let's say that I was looking through my telescope and I saw, 10 light-years away, a fat guy jazzercising in front of an open window. I think it's hilarious. One year later, I meet someone with a similar sense of humor and we get a bigger telescope and look 11 light-years away. Can we see that same fat guy jazzercising? How? Hasn't that light already passed us?

If he's moving at the speed of light, you wouldn't see time pass at all.
posted by empath at 11:58 AM on March 29, 2011

Hmm, I think pharm and empath understand this better than I do – without hijacking the question, guys, how right or wrong am I?
posted by nicwolff at 12:01 PM on March 29, 2011

Response by poster: The thing is that this decoupling happened everywhere at about the same time ... Hence the continuous CMB radiation

We are seeing things now which are 46 billion light years away, but when the light was emitted from those objects, they were much closer, but the objects and the light have both been affected by the expanding universe, so you're seeing things now which are a LOT further away now then they were when the light left them.

The universe then expanded very rapidly – much faster than the speed of light

I think that these things, taken together, answer my question.

Man. The universe shore is cool.
posted by Galaxor Nebulon at 12:03 PM on March 29, 2011

I think that these things, taken together, answer my question.

Honestly, not really. Relativity is deeply, deeply weird, and I don't understand it, either...

That radiation is coming from everywhere, not from some distant place or horizon; therefore it is arriving here on Earth continuously

The way I think about it is this. You have two 'light-cones', which stretch out in all directions from you, both into the future and the past, as far as light can travel. Your future light cone is all possible events in your future, and the past light cone includes every event which could theoretically have effected you in the present.

Think about a black hole. Once you pass the event horizon of the black hole, your entire future light-cone bends towards the singularity. Every possible event in your future will be inside the event horizon, bending you steadily toward the singularity, every photon which reflects off of you or is emitted by you goes toward the singularity.

Now, reverse time, your future light-cone is now your past light-cone, and every single photon that hits you came ultimately from the 'white hole'. If you look around you, in any direction, you'll be seeing light arriving to you from the singularity, but the singularity will exist at the boundary of your observable universe, smeared across the entire horizon. Every direction, every place you look you'll ultimately be seeing the the same thing. In the same way that the black hole was your inexorable future, the white hole, the big bang, is your past.
posted by empath at 12:16 PM on March 29, 2011 [2 favorites]

pharm has the right answer here. To put it in terms of the fat Jazzercizer, suppose the early Universe was filled with fat Jazzercizers. Each one has been Jazzercizing since the beginning of the Universe. After a certain amount of time has elapsed since the Big Bang, all of the fat Jazzercizers trip and fall over at (approximately) the same time.

If you looked through your telescope ten years after the fat Jazzercizers fell over, you would be able to watch the Jazzercizer that was ten light-years away fall over. If you came back with your friend a year later, you could then watch the Jazzercizer that was eleven light-years away fall over. A year late, you could watch the guy who was twelve light-years away fall over. And so forth.

I'm glossing over a large number of details here, but the basic idea still holds. The event that allowed the CMB radiation to propagate through the Universe occurred everywhere in the Universe at about the same time. As time goes on, the light we see coming from the CMB has come from farther and farther away to reach us.
posted by Johnny Assay at 12:21 PM on March 29, 2011 [3 favorites]

nicwolff, the problem with your gravitational hypothesis is that the inflationary period is over by about 10-33 seconds in current cosmology. The CMB dates from about 300,000-500,000 years later. (The decoupling took over 100,000 years to complete.)

empath, while that's true it's still the case that the visible universe is probably much smaller than the totality. There still more of the original universe coming into view every second as I understand things.
posted by pharm at 12:34 PM on March 29, 2011

empath, while that's true it's still the case that the visible universe is probably much smaller than the totality. There still more of the original universe coming into view every second as I understand things.

I don't honestly know. I mean we can already 'see' to the big bang, right? Or does the expanding universe allow us to keep seeing new things emerging from the big bang, new quasars, etc? I really don't know...
posted by empath at 12:43 PM on March 29, 2011

There are two limits, one is the basically the boundary of the observable universe, due to expansion is about 45 billion light years. The second limit is a time limit. At some point in the very, very early universe, 13 billion years ago or so

Wait, now I'm confused. How do things get to be 45e9 light years apart if they only have 13e9 years to get there? That brief inflationary phase couldn't have kicked things that far out right?
I am now running to the attic to see if I did not throw out my relativity and cosmology notes.
posted by gijsvs at 1:03 PM on March 29, 2011

They got that far apart because the metric of spacetime expanded. It's not so much that they moved, it's that more space came into existence in between them. This apparent motion can be faster than the speed of light, for it's not actual motion.
posted by Rhomboid at 1:18 PM on March 29, 2011 [1 favorite]

No, we can't see the big bang. The earliest thing we can see is the CMB: anything before that is lost.

gijsvs: The universe is expanding. Hence things that we see now are in reality further away than they actually appear since the space between has itself expanded over the time it took the light to get from there to here.
posted by pharm at 1:20 PM on March 29, 2011

(Or stated differently, the Big Bang was not an explosion and when we say the universe is expanding or inflating, we don't mean it's because an explosion set things moving off in all directions. We mean that the actual space is expanding.)
posted by Rhomboid at 1:21 PM on March 29, 2011

No, we can't see the big bang. The earliest thing we can see is the CMB: anything before that is lost.

Sorry, that's actually what I meant.
posted by empath at 1:30 PM on March 29, 2011

No thing (.) can go faster than the speed of light, space ( ) however can expand faster than the speed of light. As things go on, the distant galaxies and stars are going to wink out one by one, as the space between expands beyond light speed, enjoy the view while you can. Our Miserable Future
posted by hortense at 4:30 PM on March 29, 2011

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