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How can we see Big Bang?

by Erland
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Erland
#1
Dec23-12, 10:44 AM
P: 345
Cosmologists say that in theory, it should be possible to see Big Bang, with a sufficiently large telescope. The idea is that when we look at distant objects with a telescope, we see them, not as they are now, but how the were when the light from them was sent. If we could look sufficiently far away, say 13-15 billions light years, we would see Big Bang happen, since it took place 13-15 billions years ago.

But I don't understand how we can see Big Bang. Because if I see Big Bang, this means that photons sent from Big Bang hit my eye. At the same time, I am made of elementary particles of which at least some might have originated at Big Bang. We thus have two events: Big Bang and me looking at Big Bang today, connected by the paths of several particles, the photons and elemetary particles mentioned above, who follow different trajectories in spacetime but with the same initial and final points. But the photons have travelled by the speed of light, and the elementary particles with slower speeds. How can they then start from the same point and end up at the same point? Seems like a contradiction to me.

I once asked a well known astronomer this question, and he replied that my thinking was flawed, and that I, wrongly, believed that Big Bang took place at one particular place in space.
He was wrong about that. I knew then, and I know now, that at Big Bang, all of Universe was a single point. But I don't see how this resolves the paradox. Still, we have different trajectories in spacetime (no matter which shape spacetime has) with the same initial and end points, and some are trajectories of photons travelling with the speed of light and others are trajectories of slower particles.
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mfb
#2
Dec23-12, 12:27 PM
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"Same point" is a bit problematic - let's ignore the very first moments after the big bang for a moment. Photons come from the cosmic microwave background anyway, which was created about 380000 years after the big bang. They were emitted at a different location. Expansion of space (between them and "us") delayed their path, so they took 13.7 billion years to reach us even if they were just some million light years away originally (made-up number, I did not calculate it).

If the photons would have been emitted earlier, their original distance would be even smaller (with 0 as LIMIT for emission at the big bang). But the very moment of the big bang is a singularity - we cannot treat it properly with the current models of physics. So it is pointless to say "they come from the same point", this "point" is not well-defined at all.
skydivephil
#3
Dec23-12, 01:00 PM
P: 452
First off you have to define what you mean by "see" the big bang. You certainly can't see the big bang with visible light, no mater how big your telescope is. The radiation is red shifted into longer wavelengths i.e the microwave. But even this was emitted 380,000 years after the expansion started. This moment is known as the era of recombination:
http://www.astro.cornell.edu/academi...ombination.htm
before this the universe was an opaque plasma. So there is no chance of getting any light from before this era. There's a wall we cant cross. However from this light we can infer things from before the era of recombination. information can be extracted from temperature differences and polarisation patterns to test models of the early universe. But if we want to get some radiation in our detectors from before that era there is one chance but its likely far in the future.
Gravitational waves can travel through this era and in principle we can detect them.
We have indirect evidence of gravity waves from binary pulsars and the NObel prize was awarded for this:
http://www.nobelprize.org/nobel_priz...993/press.html
But we have no direct evidence of gravity waves. Detectors have been built but so far no joy. Even if they do find something these will be from astro physical sources such as merging black holes not from the big bang. Thats most likely going to need a space based gravity wave detector. This has been proposed in the form of LISA and it has not been funded. if it were to get funded its probably 10 years away from flight. But even LISA probably wont be strong enough and the nest generation space craft will be needed . Read more here:
http://trs-new.jpl.nasa.gov/dspace/b.../1/05-2157.pdf
My guess is the big bang observer is many decades away if ever flies at all. Best bet is to hope we can detect a polarisation pattern called the b MODE in the CMB, this could give us our best formation on the early universe , its not quite "seeing the big bang" though.

marcus
#4
Dec23-12, 04:13 PM
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How can we see Big Bang?

Quote Quote by mfb View Post
"Same point" is a bit problematic - let's ignore the very first moments after the big bang for a moment. Photons come from the cosmic microwave background anyway, which was created about 380000 years after the big bang. They were emitted at a different location. Expansion of space (between them and "us") delayed their path, so they took 13.7 billion years to reach us even if they were just some million light years away originally (made-up number, I did not calculate it).
...
I like your whole answer but just quote this part of it to fill in the missing detail. The usual estimate is that the hot gas whose ancient light we are now seeing was 41 million LY away when the CMB was emitted.

So your "made-up" number is very close to standard model. A few million LY is hardly anything, so million and 41 million are in the same ballpark.

You can get these distances from, for instance, Prof. Morgan's calculator (google "cosmos calculator") or from Jorrie's calculator---link in my signature. His current model is called "A27" You just have to put in the REDSHIFT of the CMB which is roughly z = 1090.
Or, equivalently put in the corresponding stretch factor z+1 = 1091.
this will tell you things like when (e.g. 380 thousand years) and distance then and distance now. You probably know all about this but may prefer to calculate all this stuff yourself.

Good to know about the online cosmology calculators though.

I wonder if Erland is satisfied with your and Phil's answers though. He may still be puzzled.
Maybe he will post again and say how he sees the issue now.
Erland
#5
Dec23-12, 05:44 PM
P: 345
So what you guys are saying is that we can never see Big Bang? Those who say we can are simply wrong?

(As I understand it, those who say that we, in principle, can see it, talk about seeing it with visisble light in ordinary, but big, telescopes. They don't talk about cosmic background radiation or something like that...)
marcus
#6
Dec23-12, 07:33 PM
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Quote Quote by Erland View Post
So what you guys are saying is that we can never see Big Bang? Those who say we can are simply wrong?

(As I understand it, those who say that we, in principle, can see it, talk about seeing it with visisble light in ordinary, but big, telescopes. They don't talk about cosmic background radiation or something like that...)
Hi Erland, I am glad you come back and keep on asking question. The treatment in the media, in magazines or on "discovery channel" television is usually very bad and gives people such wrong ideas that it takes them a long time to recover. Popular books are not much better, and can also be misleading. So keep coming back and asking.

Try to forget what impression you got from commercial mass media, or from talking to people who only watch television.

Here is the beginnings of the usual professional cosmologist view:

Hot hydrogen gas is NOT TRANSPARENT. Like the 3000 K gas at the surface of a star, it scatters light and glows with a dazzling light. The hot atoms are always absorbing and re-radiating and re-absorbing and re-radiating light, so nothing can get straight thru. Everything that tries to get thru is scattered in crazy directions by interaction with the hot atoms.

It took 380,000 years for the gas to expand and cool enough to get below 3000 K and become transparent.

It is impossible that we will ever see virgin unscattered light from before that time.

Hey! you have a mathematics degree! You probably understand this very well!

And maybe in Sweden the television is better than it is here in Usa. So maybe there is some good popular science coverage. But here it is so bad it is worse than no coverage at all.

Even though you have a math degree, I will try to say this very simply.

Since year 380,000 when the ancient light originated, up to present, distances and wavelengths have expanded about 1100 fold, or more exactly 1090 fold. That means that the "temperature" of the light (the shortness of its mix of wavelengths) used to be 3000 K and it is now 3000/1100 ≈ 2.7 K. that is, it used to be an orange-ish mix of infrared and visible and all the wavelengths are now longer by 1100 factor and it is now a rather mild bath of microwave.

This is the most ancient light that we SEE.

It was everywhere, so now the CMB microwaves come from every direction. But the waves from nearby have already come and passed us by. So at present we are getting the CMB from hot gas which was at a distance of 41 million LY from us at the moment when things became transparent. Notice that 41 million LY is a comparatively short distance in cosmic terms.

It took so long, 13.7 billion years, for the light to get here because expansion of distances makes it hard for light to get to us.

Keep on thinking about it and come back with more questions. People here will be glad to talk it over with you.
skydivephil
#7
Dec23-12, 07:51 PM
P: 452
If you define "seeing the big bang" with visible light i would say there's no way we can conceive of that being possible and if anyone suggested it is possible I would say they are wrong yes. But I don't think thats what they mean, I think if anyone did say that they would mean at the moment seeing the CMB. but as we've sad the CMB was emitted 380,000 year after the expansion. However there is not even an agree definition of the phrase "big bang". To "see" the CMB modern detectors use something called a bolometer, this is a metal strip than measure small temp fluctuations across the sky.
http://en.wikipedia.org/wiki/Bolometer
But the microwave radiation we "see" (not visible light) is not just from the CMb, it's from across the whole sky. Some of it is from the plane of the milky way galaxy. What cosmologists do is to estimate sources of the microwave radiation form these "foreground" sources and then subtract them, this gives the background and now you have a map of the CMB. Different models of events that happened before the release of the CMB make different predictions for the patterns we see in these maps. Cosmologists try and infer wich of these models is correct from CMB measurements.
if we ever detect gravity waves form the big bang (and as Ive said my best guess is that this is decades away), all we will actually see is a wave being distorted in a detector caused by a small oscillation in the fractional displacement between points in space.
If you think you could build a powerful enough telescope and watch the sort of animation of the big bang you see on tv science documentaries then thats just not going to happen, sorry.
Naty1
#8
Dec24-12, 03:49 PM
P: 5,632
I, wrongly, believed that Big Bang took place at one particular place in space. .......He was wrong about that. I knew then, and I know now, that at Big Bang, all of Universe was a single point.
He is right!

The BB did not occur at a 'point' as mentioned above already . When people say that, and they shouldn't, they mean the visible [not necessarily visual] universe......not the entire universe which might have even been infinite at that moment. Whatever it's size at the initial moment of the bang, it happened 'everywhere'.

That's why in the explanations already posted, references are made to a CMBR that appears virtually the same from all directions...reflecting the fact that such relic radiation is present everywhere.
Chronos
#9
Dec24-12, 05:57 PM
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A neutrino, or gravity wave telescope would be capable of viewing events even more ancient than the cmb. We've built a few of both thus far, but, they are still somewhat primitive.
julcab12
#10
Dec25-12, 05:28 AM
P: 153
Quote Quote by Erland View Post
How can they then start from the same point and end up at the same point? Seems like a contradiction to me.

I once asked a well known astronomer this question, and he replied that my thinking was flawed, and that I, wrongly, believed that Big Bang took place at one particular place in space.
He was wrong about that. I knew then, and I know now, that at Big Bang, all of Universe was a single point.
According to them. A point is where it breaks thus 'infinite'. When they mention 'point'. It has probably unknown to none specific bound/s meaning space is not taken into account ('Point' without the usual space as we know it)not until the event of BB where space is 'created'. It is safe to say that a point ends up the same point given that condition and model (which is BTW consistent with the data with 'slight' difference in some areas). Thus the saying: 'The space we inhabit is itself expanding. There was no center to this explosion; it happened everywhere'. Whether it expand(s)contract(s) to infinitely and/or finitely, without bounds. It will remain a point and valid IMO.
Whovian
#11
Dec30-12, 07:48 PM
P: 643
I think something that's been failed to be mentioned is if the Universe is finite and unbounded. At this point, at t=0 (or, taking the limit, we really don't care what happened at t=0, just near it,) the Universe had no size. So (again, limits,) the photons started from the same point.

And them ending at the same point (and not traveling along the same path, remaining in a straight line) we can say is false for euclidian geometries, but consider starting two photons on a sphere. No matter what, they're going to meet at the opposite side of the sphere.

Anyway, most of your confusion comes from assuming that there was a point in space which exploded, spewing out matter, while it's generally thought that the BB happened at every point in space (however large that may have been, finite or infinite) simultaneously. The best way to think of it is that any two points in the Universe were (and possibly still are) accelerating away from each other, thus expansion of space. All we can really say for sure is that the Universe was much denser in the past than it is now, so one could extrapolate to say that, at one point, its density was infinite. Which is thought to be nonsense, perhaps, again, we should be talking about limits here.
petm1
#12
Jan8-13, 11:10 PM
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To look back at big bang we use two directions, looking outward with a telescope we can detect the oldest of photons, and looking inward using microscope, or a collider, we detect the elementary particles. Both of these directions show a path to a common point in the past after all both have been traveling for the same amount of time; give or take.


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