Is that red shift in your pocket...
December 8, 2009 1:24 PM   Subscribe

Is that red shift in your pocket...

O.k., this bugs me every time I see an article that states something along the lines of "hubble looks way back in time!" or " [we are] spotting galaxies that existed just 600 million years or so after the big bang"?

Since I am assuming we can not observe our own matter, what is it we are looking at?

I have been operating under the assumption that the matter we observe, who's light is reaching us 600 million years later, is matter that is more or less moving in a direction that is -- again more or less -- opposite of ours. A lot of time is spent talking about the when of it, but not the what.

Or did I miss something in cosmology for dummies....
posted by sundri to Science & Nature (19 answers total) 3 users marked this as a favorite
 
Light takes time to cross space. Therefore, the further away something is, the longer it takes for the light it emits to reach us & the further in the past it was when it emitted that light in the first place.

The second part is that the expansion of the universe means that the further away something is, the more the universe has expanded in the time it takes for the light it emits to reach us, so the more the light is s..t..r..e..t..c..h..e..d. Longer wavelenghts means that the emission / absorption sectra is shifted towards the red end of the visible spectrum, hence the term redshift.
posted by pharm at 1:29 PM on December 8, 2009


on second thought, to rephrase, or clarify or make a bigger mess (though I put a lot of thought into the initial question, really):

There has to be a limit to how far in the past what we can observe, because at some point we will be observing our own matter (which I assume is impossible), and this is rarely talked about in soft-core science lit...can anyone shed light on this matter (cause this post is more about matter than light).
posted by sundri at 1:39 PM on December 8, 2009


Looking at a light cone diagram might help explain this.

However, to address your second question - We have two theoretical limits on how far back we can see in time.

First, until around 300,000 years after the big bang, the universe contained an opaque hydrogen plasma, so you can't see past that - In fact, the last gasp of that plasma we have just recently started looking at - the Cosmic Microwave Background.

Second, universal expansion (which pharm pointed out gives rise to red-shift) means that at some distance between two points, they move apart at greater than the speed of light. That doesn't affect us yet, but in another 40 billion years (someone correct me if I have that number badly off), it means pretty much we'll never again see the entire rest of the universe ourside our galaxy (and Andromeda, which the Milky Way will eventually collide with and absorb).
posted by pla at 1:50 PM on December 8, 2009


I think I understand what you mean, however, we are not moving away from ourselves so we will never observe our own matter in redshift. Does this go in the vein of your question?
posted by InsanePenguin at 1:52 PM on December 8, 2009


There has to be a limit to how far in the past what we can observe...

Cosmology does go into the concept of the observable universe, which is the area that's theoretically observable (practically observable being a subset of that, as limited by technology available, etc). The horizon involved is affected by such things as the likely age of the universe, and more brain-bending things like the probability that space itself (as considered separate from the objects with in it) is expanding.
posted by Drastic at 1:53 PM on December 8, 2009


1) We are not looking at matter directly, we are looking at the light which the galaxy (or SNeIa or whatever) emits. That light travels for a certain time to get to us. In that time, the expansion of the universe stretches the light. This is where redshift comes into play.

2) Your assumption that we are moving opposite to whatever we're looking at is not really complete (but it's easy to think of that). Nothing is really moving, the distance between us and them is getting larger. The analogy that several pop-physics people make of an expanding baloon with pennies attached on a grid is a really good one to explain the expansion of the universe.

3) There is a limit to how far back in time (and consequently, how far away) we can see. Theory predicts a finite age of the universe (~13 Gyrs). You can naively multiply that number by the speed of light to get an estimate of how far away our horizon is.
posted by chicago2penn at 1:55 PM on December 8, 2009


Yeah - trivially, if you look at something a billion light years away, you're also looking at something that happened a billion years ago. Not for another billion years will you be able to see what's happening in that location "now", and whatever matter was in that location a billion years ago will, by "now", be rather further away.

The overarching concept is one of those squirrelly ones that's tricky to hold in your mind; it's that the very hard speed-of-light barrier means that stuff outside your four-dimensional light cone may as well, as far as you're concerned, not exist at all. You cannot detect or affect it in any way, and it cannot see or affect you.

That's why I put quotes around "now" above; it's actually unproductive to think about what might be happening "now" in a place light-years away from you, because it really might as well not be happening, until light/gravity/whatever makes it to your location from there.

(Thinking about an overall "now" also fails because of curvature of space-time; time actually flows at different speeds in different places, depending on gravity wells, relative travel velocities, etc. This has no perceptible effect at the scales and speeds of normal human life, but we do have to take relativity into account when, for instance, designing the GPS system or predicting the movement of the planets over long periods of time.)
posted by dansdata at 1:59 PM on December 8, 2009


Since I am assuming we can not observe our own matter

I am not sure what you mean by this.

Theoretically, the observable universe has a radius limited by the time since the 'Big Bang', occurred and then only if you happened to be located at the location where the Bang happened. I suppose one is entitled to believe that you in the center of the observable universe are if you are egocentric enough (pun intended).

The chances are that we are at some offset from that position. Some reading:

New Scientist from 1992

Some Physics Lecture notes on the BB

Scientific American on the possibility of seeing further

Disclaimer: I am completely unqualified in any academic sense to lecture on this subject: I am simply a lay autodidact.
posted by pjern at 2:00 PM on December 8, 2009


What do you mean by "our own matter." Your usage here is unclear.
posted by mr_roboto at 2:31 PM on December 8, 2009


Why would there be a problem seeing your own matter necessarily? Think of a mirror.
posted by Emanuel at 2:34 PM on December 8, 2009


Actually I would say we do observe our "own matter" in the past, in some sense. See, in the Cosmic Microwave Background mentioned above we observe fluctuations. Those fluctuations are associated with the large structure in the Universe, that is, the clusters of galaxies and great voids in between. So the fluctuations roughly indicated where matter is going to clump and form galaxies --- therefore our own galaxy is probably represented by one of those fluctuations there.

But that only happens because the CMB is a "snapshot" of the Universe at the time of recombination. Other than more exotic hypothetical phenomena (worm holes, etc.), no, we cannot observe the Milky Way as it was billions of years ago.
posted by natalinha at 3:08 PM on December 8, 2009


I'm not exactly sure what the matter part of it has to do with it, but here is my stab as explaining it.

OK, so you know how you see a bolt of lightning and, depending on how far away you are, you hear the thunder some time later? Or, back in the days of satellite long distance telephones, where there was always awkward pauses in the conversation? That's what is happening, just on a bigger scale.

Look at it like this- we aren't so much looking back in time as we are just really far away. Everything we see is just light particles that came off an object. Light bounces off the wall in front of me and lands on my eyes and the pattern those particles makes in my eyeball makes me perceive a wall. It is a constant stream of light particles travelling from the object to our eyes. So then, when I look into the sun, I am seeing light particles that left the sun eight minutes ago. When I look at Saturn, I am seeing light particles that left it an hour ago.

If there was a guy on the planet with a really big digital watch that was perfectly synchronized to my own really big digital watch, when I looked in the telescope at his wrist, it would read an hour (ish) ago. Because the light from the watch took an hour to get to me. It is still keeping time, and if I sit there watching it, it would click off seconds at the same rate as my watch.

Now put the guy on a planet in the next solar system. That's what, a couple of light years away? When I looked at his watch in my telescope, it would click off seconds at the same rate, but it would say that the date is Dec 8, 2007. Because the light that I'm seeing right now took two years to get here. If he flipped me the bird, I wouldn't know it for two years.

So, when things are SO far away that it takes billions of years for the light particles to get to us, we are seeing things how they existed billions of years ago.

The whole red-shit thing is a bit more complicated. In the above, light behaved as particles. In redshift, it behaves as a wave. To overly simplify it, something looks yellow because it is vibrating at the yellow frequency. Tiny little pushes and pulls sending light out in waves. If something is travelling away from us, each of those little pushes and pulls is just slightly farther away from us as the previous one. So even though the light that's coming out is yellow, because it came out just a little bit further away, that makes the vibration seem like it is going a little slower and thus a little redder.

Or, picture someone rowing a boat away from you. He pushes the oars forward, and you stop time. Measure the distance between his hands and you. Start time back up, and he pulls the oars towards him. Stop time again and measure the distance between you and his hands. For the sake of argument, lets say the distance from all the way out to all the way in of his oaring motion is two feet. But the difference between your two measurements is three feet. This difference divided by the time difference is his speed. If he's going really fast and pops out a yellow photon on every rowing motion, because each particle pops out a little further away, the light wave he creates comes back at us looking a little reddish.

And, no, I only wear a red shift on weekends.
posted by gjc at 5:26 PM on December 8, 2009 [1 favorite]


Um, red-shiFt. What I wrote in paragraph 7 is a whole different story...
posted by gjc at 5:29 PM on December 8, 2009


Theoretically, the observable universe has a radius limited by the time since the 'Big Bang', occurred and then only if you happened to be located at the location where the Bang happened.

Strictly speaking, the Big Bang model effectively says that the "location where the Bang happened" is right here for all possible observers.

In other words: "the Universe is expanding" doesn't mean that the Universe is a bubble of some kind whose "edge" of some kind is expanding into a void of some kind; it means that in this unbounded Universe, the distance between any two randomly-chosen objects will be increasing more often than not.
posted by flabdablet at 5:49 PM on December 8, 2009


I'm trying to make sense of the «our own matter» bit.

Our own atmosphere and our galaxy are major light sources in the sky, but the light comes from the very recent past.

Once you have accounted for the bright patches of light, you can look in between for smaller and dimmer patches. A few of them will come from old and very bright sources. We get the speed of these sources directly from their redshift, and the distance and age are approximated from that.

For our own old light to come back at us, it would have to be reflected somewhere (leaving aside a torus universe, there is none small enough), losing most of its intensity from both distance and reflection (and being redshifted btw). Even then, it cannot compete with numerous brighter sources within the very large sphere old light can come from.
posted by Tobu at 6:37 PM on December 8, 2009


Apologies for my the lack of clarity with this question and thanks for the answers. The various articles have given me some clarity, thanks.

As for "our own matter": I think I must be linking up matter and light in my head in a manner that was not useful when thinking about the scope of the universe.

My (lack of?) logic was, I read about someone seeing some really old light, which was at one point associated with some matter, we continue to improve our ability to see far away, which also means we see -- and please kick me for this -- a smaller, younger universe? So it raised the question, to observe a certain distance, a certain x "time ago" would be impossible and literally paradoxical since the matter which is local to us now (like my carbon atoms or their bits) would have been local to the then. But I believe this has been more or less answered.
posted by sundri at 1:34 AM on December 9, 2009


For us to see "our own matter" — the matter that makes up our own galaxy say — in the distant past would require there be a way for the light that was emitted a billion years ago to travel a billion light years to get to our eyes (or telescopes). There is no such path for the light emitted by our galaxy to travel: it all went streaming out into the rest of the universe. Hence there is no way to observe the distant past of our own galaxy: we can only observe the distant past of other galaxies as the light they emitted in their past become visible to us.
posted by pharm at 2:58 AM on December 9, 2009


"Second, universal expansion (which pharm pointed out gives rise to red-shift) means that at some distance between two points, they move apart at greater than the speed of light. That doesn't affect us yet, but in another 40 billion years (someone correct me if I have that number badly off), it means pretty much we'll never again see the entire rest of the universe ourside our galaxy (and Andromeda, which the Milky Way will eventually collide with and absorb)."
It's rather more complicated than that. Recessional velocities for objects above z=1.4 are already greater than the speed of light, and they're very much observable. Those latest Hubble UDF pics show galaxies at redshifts beyond 7, reportedly, and the CMB itself is at a redshift of about a thousand. To find out what becomes invisible, you have to calculate a cosmological event horizon distance.

"Actually I would say we do observe our "own matter" in the past, in some sense. See, in the Cosmic Microwave Background mentioned above we observe fluctuations. Those fluctuations are associated with the large structure in the Universe, that is, the clusters of galaxies and great voids in between. So the fluctuations roughly indicated where matter is going to clump and form galaxies --- therefore our own galaxy is probably represented by one of those fluctuations there."
No, the CMB is a spherical surface snapshot of the universe - and we're at the centre of that sphere. None of the fluctuations in our CMB are responsible for structure here - they're responsible for structure out there.

We're part of the CMB for observers on the edge of the observable universe for us.

"we continue to improve our ability to see far away, which also means we see -- and please kick me for this -- a smaller, younger universe?"
Yes, as you look further away you see a smaller younger universe (this is why cosmologically more distant objects start looking bigger as they get more distant, not smaller). I don't think being right warrants a kick!
posted by edd at 4:00 AM on December 9, 2009 [1 favorite]


to build off edd's comment: we do see a smaller, younger universe when we look very far away, but not the whole universe. We see the part of the universe that's visible in our light cone at that distance, and there's no giant mirror out there to reflect our light back to us with a billion-year delay. The light from this galaxy (the light from our matter) that we see is much closer and thus younger. You're always looking into the past: when I hold my hand out in front of me, I am seeing it as it existed millionths of a second ago. But for all practical purposes, you have to be really far away from something to see into its past... and we're never that far away from our own matter :)
posted by Chris4d at 11:39 AM on December 9, 2009


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