Re: Human Telescopes?

Yup, alien telescopes are much more powerful. Aliens probably saw this galaxy years ago and are having a good laugh at us puny humans for having to make use of gravitational lensing...

Re: Human Telescopes?

As opposed to human canonballs.

Re: Space, and the enormity of it

"The Hubble telescope has detected starlight that comes from a galaxy 13.2 billion light-years away... the light shows the galaxy as it was 13.2 billion years ago, when the galaxy itself was about 200 million years old and the universe was only 500 million years old, 3.6% of its current 13.7 billion years, and emerging from a cosmic dark age"

At 13:34 'Bored_Stupid' said, "just makes me go 'Woooaaaaahhhh' at times. Getting your head round a gazillion transistors on a chip is peanuts compared to this stuff.!"

This might make your head spin a bit more... Don't think that this galaxy is, as we type, 13.2 billion light years distant. It's probably much closer to 42-45-ish billion light years away right now! (The current distance to the most distant object we can see is about 46 billion light-years). The Universe has a 'diameter' of about 92 billion light years (think 'expansion'), not the commonly held misconception that it's diameter is 2 * age i.e. ~26 billion light years :)

It might have been useful if the article author had delved a little deeper into this as the article's opening sentence could be read as to imply imply that the galaxy in question is currently 13.2 billion light years distant, when it isn't. That's just the age of the light we are seeing. The galaxy has since receded much further from us.

Re: Space, and the enormity of it

"So you are telling me the universe has expanded (or is expanding) at greater than the speed of light? If the universe is ~13 billion years old, surely it can't be more than 26 billion light years in diameter?"

This oldish article might help here. [Warning. PDF!]

"And we are RIGHT IN THE MIDDLE?"

Possible, but unlikely. We are however right in the middle of our own observable universe :)

Re: Space, and the enormity of it

> The universe is infinitely big

We don't know that. It may well be a 3-sphere (not embedded in 4-space, but having the corresponding topology and curvature), so have finite extent at any "time".

Re: Space, and the enormity of it

"Now, if anyone can explain why expansion is valid on cosmological scales only and does not stretch "thightly bound systems" as the galaxy, that would be nice."

That's actually extremely straightforward. The expansion is a feature of the metric on cosmological scales, which we're taking to be Robertson-Walker - that is, expanding sheets of flat (or spherical, or hyperboloidal) 3-space. That metric is apparently fine on large scales, although actually demonstrating that rigorously is currently impossible. "Tightly-bound" systems don't expand for the very simple reason that they're not even slightly governed by the Robertson-Walker metric. The sun generates something very close to Scwarzschild around us, and the galaxy is made up of a horrifically complicated mess of things that are close to, but not quite, Schwarzschild solutions. Overall we might suggest that a galaxy is made up of some kind of cylindrical, or perhaps spherical, metric. Clusters are built up of a complicated mess of these cylindrical/spherical/whatever metrics (or, properly, a disgusting hash of billions upon billions upon billions of stars each of which produce something close to Schwarzschild), and so on. It's only on very big scales when there are no more bound structures that we assume the Robertson-Walker metric holds.

I'd emphasise that even holding Robertson-Walker to hold at all is basically an ill-defined statement since we've got absolutely no idea how to average across distributions of spherical sources, but it's observationally very well-supported.

The hierarchy in current theory then goes something like this:

* Stellar scales: Schwarzschild metrics, ~AU to ~pc. These could be taken as the basic building blocks, although in principle they're not given planets etc.

* Galactic scales, ~kpc: Meshes of billions of roughly Schwarzschild metrics, "averaging" (in whatever way that can be achieved) out to something that's cylindrical, or spherical, or something similar.

* Cluster scales, ~Mpc: Meshes of hundreds or thousands of these galactic metrics, or more physically, meshes of many billions upon billions of roughly Schwarzschild metrics, "averaging" to something very roughly spherical

* Supercluster scales, ~150-300Mpc: Past the homogeneity scale, there are no large structures to add anything more, so the meshes of all the cluster metrics (or of course all the way down to untold, unimaginable numbers of stars) "average" in some manner to Robertson-Walker.

The "expansion" is an artifact of the Robertson-Walker metric. Since this metric does not apply to smaller scales, they don't feel expansion.

Quicker precis for people who can't be arsed wading through all of that: the "force" exerted by the cosmological expansion is utterly, absurdly weak. The very slightest gravitational pull between two bodies more than overwhelms it. It is only on extremely large scales, when all the matter around a point has basically averaged out to be totally uniform and therefore exerts no attraction, that the cosmological "force" can actually be felt. This is a bit similar to how a magnet doesn't give a shit about gravity -- even the pull of the Earth is pretty insignificant compared to the strength in an electromagnetic bond.

Re: Space, and the enormity of it

So long as it made sense...

Re: Space, and the enormity of it

Oh just a quick comment on your earlier post about the universe being spherical and finite, I know some people who've spent about the last ten or fifteen years entertaining themselves investigating what happens if the topology of the universe is more complicated than that. The simplest is a torus - the universe is flat (rather than spherical), but still has a finite volume. It goes all the way up to some truly barking multiply-connected spaces, and the beauty is that general relativity tells us precisely nothing about any of it, so we're left with the hope that any non-trivial topology has a small enough scale that we can see it. So far, nah. I'd love there to be something properly crazy though.

Re: Space, and the enormity of it

I have no degree in physics so please be kind.

Before expansion kicked in, everything in the universe was relatively close together. Presumably this means everything in the universe was affected by the gravity of everything else in the universe to a much greater degree. How is it then that the force behind expansion ever overcame gravity?

Re: @ HolyFreakinGhost

Thank you - I'm genuinely flattered.

I also pride myself on using words like "shit" in semi-technical posts.

@AC

Structure grows over time. Imagine that at a single time the universe was like a stretched sheet. (It's a remarkably close analogy if you pretend the universe is 2D instead of 3D). Then at early times, the universe would be described by a sheet utterly smooth to more than 1 part in 10,000! That's very, very smooth. If you then imagine putting some kind of gravity meter in the middle of it, it won't feel any force -- the pull from each direction will cancel out. So at early times the attraction between different bodies was extremely small.

(It's the same as a high school/early university physics problem - showing that there is no gravitational field inside a spherical shell. The contributions from all sides cancel out and you end up floating helpless.)

Later in the universe those tiny, 1 in 10,000 ripples have grown. Gravity *is* certainly pulling them together but it's got a long time to go before they grow large. (And at very early times it's additionally countered by radiation pressure; when the universe was very young, less than about 300,000 years old, the average photon had more than enough energy to reionise any hydrogen atom that formed: the universe was a plasma of protons, electrons and photons. Then whenever clumps of protons attempted to fall together they were pushed apart again by radiation pressure. This set up waves that rang through the universe and which we see even today, 13bn years later, as ripples on the cosmic microwave background and as literal waves observable in the large scale structure of galaxies. The latter always astounds me. Even now you can see the imprint of sound waves ringing through the young universe.) But it isn't until relatively recently - cosmologically speaking, of course; I'm talking perhaps 5bn or 6bn years, more than the age of the Earth... - that those ripples have finally grown from around 1 in 10,000 to greater than 1 in more than small patches.

Re: Space, and the enormity of it

In reply to both sniperpaddy and Tinker Tailer Soldier, I'd say both ways of looking at it are perfectly reasonable - actually good ways of looking at it.

Re: Space, and the enormity of it

And we are RIGHT IN THE MIDDLE?

Unlikely. That's just an effect of isotropy. If everything is receding at an equal pace from everything else then every place seems to be the centre of the universe. No doubt many other planets have astronomers that are still struggling with heliocentricity, so I wouldn't feel too bad about the mistake you made :)

Re: Space, and the enormity of it

Are you talking to me? If not, feel free to ignore this post :)

If you are, obviously I agree with you; no-one worth listening to would ever say we're right in the middle - indeed, you'll find every professional will say exactly the opposite.* We are very most definitely not in the middle of the universe. The Copernican principle is the underlying foundation of cosmology; Robertson-Walker follows from combining the Copernican principle (encapsulated in large-scale homogeneity) with the observed isotropy of the CMB.

* Caveat: there are certainly isotropic but inhomogeneous universes under consideration. But anyone working on them will acknowledge that first these are toy models and not to be taken as serious claims that the universe *is* like this, and secondly will try and find enough wiggle room to let us be some distance from the centre. No-one, absolutely no-one, likes the idea of centring any cosmological model on the Earth.