Deepest Optical Image Of The Universe To Date

fenimor writes "The deepest optical view of the universe, obtained by Hubble Space Telescope, may turn out to be some of the earliest star-forming galaxies. The telescope has looked 95 percent of the way back to the beginning of time, to glimpse whether the hottest stars in these early galaxies may have provided enough radiation to 'cool' the universe after the big bang."

Comments

A question

[T.Hobbes]

I was thinking about these photos, and came upon was seems to me to be a paradox. OK, so the Hubble takes this ultra-deep image of a point in space. This is said to be an image of the universe X billion years ago, and Y billion years after the big bang. All well and good.

Now, so far as I know, intersteller distances are measured by the light year; Alpha Centuri is ~4 light years away, etc.

I extrapolate from this that this ultra-deep and ultra-old image of the universe is both the _oldest_ and the _most distant_ image yet taken.

The problem is this: You can point the hubble in any direction, and get an equally old image. Further, if you take a deep enough image, you can (theoretically) take an image of the Big Bang itself (or X million years after it, whatever).

The paradox to me, is that this means the Big Bang can be conceptualized as a the outer edge of a sphere that surrounds us. You can, with the telescope, image in any direction in all three dimensions, and your limit wrt distance in any of those directions is the big bang. So the big bang is the edge.

Now, this seems absurd to me, so I obviously got something wrong somewhere. Does anyone know what I got wrong?

Re:A question

[zrail]

Technically, the furthest you can look back is about 300,000 years after the big bang, because thats approximately the point in time when the universe cooled down enough to become transparent. Before that, the universe basically consisted of a really hot soup of plasma.

Try this page for some background information on cosmology and cosmic background radiation.

Re:A question

[Bootsy Collins]

Now, so far as I know, intersteller distances are measured by the light year; Alpha Centuri is ~4 light years away, etc.

Well, actually, parsecs (and kiloparsecs and megaparsecs) are what tend to be used, for mainly traditional reasons. But it's a straightforward unit conversion.

The problem is this: You can point the hubble in any direction, and get an equally old image. Further, if you take a deep enough image, you can (theoretically) take an image of the Big Bang itself (or X million years after it, whatever).

In practical fact, you can't see back to the Big Bang, for a number of reasons. The first is that until a few hundred thousand years after the putative Big Bang, the Universe was opaque to radiation. Photons were simply too unlikely to pass much of any distance through the Universe without scattering off a charged particle of some sort. After that point, the Universe became transparent to photons. Consequently, that's as far back as you can see -- with photons, anyway. But you're right that you can see this change of state in the Universe (the so-called "surface of last scattering") in any direction you look, and in fact that's what astrophysicists are looking at when they map the cosmic microwave background radiation.

The other thing that prevents you from seeing all the way back to the Big Bang is that as the Universe expands, light is redshifted (basically, its wavelengths are stretched out with the expansion). That's why we have to look in the infrared band for these distant galaxies, and that's why the light we observe from the surface of last scattering is in the microwave band. Light emitted at times closer and closer to the putative Big Bang is redshifted by larger and larger degrees, approaching infinite redshifting at the Big Bang itself (when the scale factor of the Universe, describing the expansion of space relative to today, is 0). So even if the surface of last scattering wasn't there, there'd be a practical limit to just how far back one could see, based on just how low-energy (long-wavelength) of photons one could detect and interpret.

The paradox to me, is that this means the Big Bang can be conceptualized as a the outer edge of a sphere that surrounds us. You can, with the telescope, image in any direction in all three dimensions, and your limit wrt distance in any of those directions is the big bang. So the big bang is the edge.

The Big Bang occurred everywhere. It occurred where you're sitting, where I'm sitting, and where Zaphod is sitting.

Imagine some event -- say, the change of state that I described above, referred to as "decoupling", when the Universe became effectively transparent to photons, where before that it was opaque. This happened basically everywhere at once -- as the Universe expanded, densities and temperatures dropped until the scattering probabilities fell low enough. This happened everywhere, including right where you and I are now. But the photons that were around here then are long-gone now. They've been flying off in different directions since then. Similarly, the photons that were 10 light years away aren't around us either: the time they've had to fly, times the speed of light, is a long way from here. If you think about it, the photons you're going to see are ones that started out on a shell that's centered on us, with a radius equal to the distance light could travel in the time since decoupling (it's a little more complicated than that because of the expansion, but that's the basic idea). At any later time, from 1 second to 1 billion years later, that shell will be larger.

So that's why we see a given time in the history of the Universe as a shell around us. It's not because these things (like the surface of last scattering, or the Big Bang itself) really are shells, but simply because that's all we can see. The photons that started at points interior to that shell aren't anywhere near us now; they've had enough time to propagate further than the distance between us and their starting points. The photons that started at points outside that shell haven't had enough time to reach us yet.

Hope this clears things up some . . .

Seems correct, but no paradox ..

[RedLaggedTeut]

I think what you say is basically correct:
If you look out as far is possible, which should be either the point in time where the universal "balloon" expanded at the speed of light, or maybe so far that the Hubble constant times the distance is the speed of light, then you get to see the big bang.

Most of it is called the cosmic microwave background.

There are two reasons why there isn't as much of a paradox:
One is that spacetime might look like this: Space is 3D, but consider that it as 2D, then the universe would look like a balloon that gets inflated: every point on the balloon seems to be at the center of the explosion called big bang.

The second reason is that it gets harder to see the big bang itself, because Einsteins relativity theory predicts really big shifts in wavelength for stuff that moves away near the speed of light - so any electromagnetic waves and light from the big bang would be far below infrared and low in energy. And incrementally so as you get to look closer to the big bang.

Re:Aha.

[escher]

(Because the Earth isn't at the center, that most distant point might be farther in one direction than in another.)

A better way to look at it is that every point can be considered the center of the universe. No matter where you are, the egdes of the "big bang sphere border" will be equidistant from your location.

Actually, you are correct...

[j_cavera]

This is not a paradox, rather just a way of looking at it that is different than what you are used to. The universe at the beginning of time, existed as a point (more or less) that expanded (somehow) into what we see today. As you look out into the universe, you also look back in time. The farther back you go, the smaller the universe was.

By logic, if you could look all the way back to the big bang itself, you would see a point of light. And this is where your percieved paradox occurs. But this is actually the correct way of thinking about it, because time = distance. So where does that point lie? Everywhere, at a distance of 15 billion (give or take) light-years from us! So no matter where you look, you see a "part of that point" from 15 billion years ago.

OK, this is an oversimplification as the universe was opaque for some time after the big bang, but you get the idea. Here's a potentially useful (though not perfectly accurate) analogy. Go inside a large spherical room with white walls. Put a bright light bulb at the center (big-bang). The walls are evenly illuminated because no matter which way you look, your line of sight intersects with some of the rays of the bulb, that seem to come to you from all around you.

In fact, if you had a good enough detector, you could determine the shape of the bulb's filament by irregularities in the light from the walls. This is what the cosmic background explorer (COBE) missions are about.

BTW, yes IAAP (I am a physicist).