A quick cosmology question :: The Future of the Internet

A quick cosmology question

January 16th, 2010 | by jz | Published in Future of the Internet | 12 Comments

The amazing Hubble telescope has now shown us images of galaxies from 13.2 billion years ago. That’s because the light comes from 13.2 billion light years away, and took (by definition) that much time to get here:

“The deeper Hubble looks into space, the farther back in time it looks, because light takes billions of years to cross the observable universe,” the Space Telescope Science Institute said in a statement released Tuesday.

So that makes sense on one level. But here’s what I don’t get: the light only took that long to get here if the starting point for it was in fact 13.2 billion light years away. Since the universe is expanding, if one rewinds time, it shrinks. Indeed, I thought the Big Bang to mean that at one point the Universe was a singularity, both meaning in a condition for which our laws of physics can’t say anything, and that it was essentially compressed into a single point.

But if it was compressed into a single point — apparently about 5-600 million years further back from the 13.2 billion we’re now seeing — that means that 14 billion years ago everything was, well, extremely close to everything else. So unless the universe is expanding faster than the speed of light, how could anything be 13.2 billion light years away from us, 13.2 billion years ago? Maybe something is that far now, but if so its light would only just be starting its journey to us. The whole light year calculation presumes that something was that far away from us then – a time when the whole universe was much, much smaller in diameter. Maybe it has something to do with the universe’s expansion as a matter of dark energy, e.g., the fabric of the universe itself expanding, vs. the expansion found as all the galaxies speed away from one another (countered by the actions of gravity)? Something to do with the “inflationary period” catapulting everything really far away from everything else in one swoop?

I’m sure I’m missing something here. What is it?

Responses

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zittrain at 01/17/10 12:23:47 | Exectweets says:

January 16th, 2010 at 7:46 pm (#)

[...] anyone answer a quick cosmology question? http://futureoftheinternet.org/a-quick-cosmology-question zittrain – Sun 17 Jan 0:23 0 votes previous [...]
Amy Whitaker says:

January 16th, 2010 at 7:50 pm (#)

I am not even an amateur cosmologist so take with a grain of salt:

we could have been moving in opposite directions at first, and then it came toward us.

what if we all started out close together. We went east 7bn light years. It went west 7bn light years, then for whatever boomerang reason it started moving back at us.

That’s not it exactly, but it seems we (a) could have been close together but moved in different directions and (b) we wouldn’t have visibility on whether it was a straight line of 13bn light years away, or had taken an out and back or ricocheting path. It only looks straight from an “earth, a view of space” perspective.
rektide says:

January 16th, 2010 at 7:59 pm (#)

that galaxy wasnt “13.2 billion light years away 13.2 billion years ago”, that point in time is 13.2 billion years & light-years away from here/now. ((( assuming our universal constants are indeed universal. )))
yvette says:

January 16th, 2010 at 8:14 pm (#)

My understanding is that the photos are a snapshot of history. It was 13.2 bil LY away at some point in time, but not any more. We can’t calculate at what point in time it was 13.2 bil LY away unless we know the speed of the expansion of the universe and assume that the expansion is linear, which most likely is not.
joe says:

January 16th, 2010 at 8:32 pm (#)

Yes, inflation is the answer. Inflation is essentially a period of exceedingly fast growth that happened very shortly after the universe started (10^-36 s afterwards). The universe wasn’t even transparent to light (photons) until 377,000 years after the Big Bang. The light that comes to us from the earliest part of when the universe was transparent is not of galaxies; the universe was too hot and such for these to form. That earliest light is actually the glow of the early, newly transparent and hot universe, called the cosmic background. The light from that hot time has been stretched during travel to us and is now no longer heat (infrared) but microwave. The light from these very old galaxies Hubble sees did start from galaxies that were 13.2 billion light years away then. Due to the expansion of space, “now” these galaxies are much farther away (47 billion light years or so) and probably look very very different.
markoff at 01/17/10 01:20:23 | Exectweets says:

January 16th, 2010 at 8:36 pm (#)

[...] answer ! (before my head explodes) http://futureoftheinternet.org/a-quick-cosmology-question (via @zittrain) markoff – Sun 17 Jan 1:20 0 votes previous [...]
Walter says:

January 16th, 2010 at 8:52 pm (#)

http://en.wikipedia.org/wiki/Metric_expansion_of_space will probably do a better job than me in helping you out here. But I’ll try anyway.

The 13.2 billion light-year number is an oversimplification. During the period light traveled from there to here, the space between expanded. When you’re dealing with expanding space, the word “distance” has to be (re-)defined before you can even have this discussion. (See “comoving distance” in the Wikipedia article.) You touch on a slightly-less-simplified version when you say that _by definition_ if it took 13.2 billion years for the light to get here, it’s 13.2 billion light-years away.

BTW, the universe _is_ expanding faster than the speed of light, if you measure by the change in distance between two sufficiently-distant objects. The objects’ velocities as measured in space aren’t faster than light, but space itself is expanding between them. This means there are parts of the universe we will never see, because space is expanding too fast for their light ever to get here. The boundary is called the “observable horizon”.
Steve Bryan says:

January 16th, 2010 at 9:43 pm (#)

Joe provides a nice explanation which can be summarized as the big bang is accompanied by truly incredible inflation. Also take with a grain of salt the statement that light took 13.2 billion years to get here. Along the path of a photon from emission there to absorption here the elapsed time is zero seconds because the interval is a light-like interval (rather than space-like or time-like). A non-zero mass spacecraft that left at the same instant as a photon that managed somehow to accelerate constantly at 1 g (10 m/s^2) for half the journey and decelerate at the same rate for the other half would measure elapsed time at well less than a normal human life span.

Of course we have no such design that could sustain constant acceleration and if we did the blue shift of cosmic background radiation would be a daunting issue. But the main point is that elapsed time depends on the details of your path and it is zero for photons. That is related to why we know neutrinos have non-zero mass: they could not oscillate among three states otherwise which is observed when measuring solar neutrinos. Sorry for the detour but spacetime is much more interesting than it is usually portrayed.
Brian Frank says:

January 16th, 2010 at 11:42 pm (#)

Given all the conditions and unknowns that come packaged with any explanation, I’m inclined to think that we’re just missing some key insight that change the way we look at all the data and will seem obvious 500 years from now.

Think of the poor pre-Copernicans and their idea that the universe moved around the Earth’s fixed position. How do we know that our assumptions, metaphors, and models aren’t just as flawed?

As a very casual observer, to me it looks like we might be at an analogous stage: instead of having a theory that helps us make sense of new information, new information seems to make our theory make less sense… (This is just the impression I get as an outsider.)

This line from the NASA article (via @laurenweinstein and @markoff) is telling:

“By definition, the universe encompasses all of space and time as we know it, so it is beyond the realm of the Big Bang model to postulate what the universe is expanding into.”

So — I’m just waiting for the proverbial apple to fall on someone’s head.
David Berry says:

January 17th, 2010 at 5:38 am (#)

Two things..

Dark Matter and Dark Energy..

You should read..

TROTTA, ROBERTO (2007) Dark Matter: Facing the Arche-Fossil. Collapse Vol. II: Speculative Realism. March 2007

But this is also interesting:

http://astro.ic.ac.uk/rtrotta/public-lectures-courses-and-exhibits
Scott MacLeod says:

January 17th, 2010 at 8:59 pm (#)

I wonder how quantum physics’ questions would rewrite ‘linearity of time’ assumptions. Might there have been (quantum) ‘rewrites’ in time and space (e.g. colliding universes) that render such Hubble data subject to significant, further interpretation?
Dale Sheldon says:

January 18th, 2010 at 4:40 pm (#)

I’ll refer you to a great blog, Starts With a Bang, which covers cosmology and has answered this exact same question:

http://scienceblogs.com/startswithabang/2008/02/q-a-the-age-and-size-of-the-universe.php

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