# Factoring 'Time' into Astronomic ObservationsApril 9, 2006 11:17 PM   Subscribe

AskMeFi Physics folk: How do astronomers account for the temporal distinctiveness of their galactic subjects in their calculations? I understand that observations of the red shift of quasars delinates a speed increase in the expansion of the universe - yet my brain explodes when I try to understand how the enormous expanse of time is factored into these models.

The hardest concepts for me to conceive are ones that factor the enormous AGE of the universe into their workings. I understand that by observing quasar red shift and comparing it to the shift of 'nearby' galaxies astronomers have determined that universe expansion is actually increasing. Surely though the fact that these quasar entities exist 'back in time' alters the nature of the data streaming from them? The photons of light astronomers gather in their observations have not just travelled great distances of space, but also great expanses of time, yet when the light was first emitted from these 'distant' objects their distinction in space was not as great as it is now (i.e. when the universe was smaller) a weird conflict indeed... This is where I reach the event horizon of my understanding.

Why doesn't the time aspect completely alter the nature of evidence gathered? Working with data that comes from billions of years hence must make calculations incredibly obscure.

In what ways is this temporality a help and a hinderance? How are the factors of time, space and motion / change plotted to form the model?
posted by 0bvious to Science & Nature (12 answers total)

This past AskMeFi post has some related links, but none connected with my question specifically.
posted by 0bvious at 11:20 PM on April 9, 2006

You pose a lot of questions and I expect many will chime in to help answer them, but here's a little bit.

An independent and more straightforward measure of the age of the universe is given by the observation of the cosmic microwave background.

Towards your other question: time doesn't do much to light in a vacuum. Properties such as the masslessness of the photon and most aspects of relativity and gravity have been rigorously tested in laboratories. Models of stellar radiation seem very well understood. Some assumptions must be made: that the laws of physics are invariant with time for instance and that old stars were made from the same kind of matter that's around today (well substantiated from emission/absorption spectra).
posted by fatllama at 12:00 AM on April 10, 2006

yet when the light was first emitted from these 'distant' objects their distinction in space was not as great as it is now (i.e. when the universe was smaller) a weird conflict indeed...

I was typing a longer response then my browser crashed...

Anyways, to address this part: There is no conflict. It just means that the Universe has a distinct toplogical shape in space-time. This is all well-accounted for in General Relativity. (see FLRW)

The mistake, when you look up at the stars, is to think you are looking out into a big sphere. A better analogy is that you are at the wide end of a big trumpet looking back toward objects at the narrower end. Two galaxies that are 180 degrees apart in our night sky may actually have been much closer at the time when their light was emitted - because, as you point out, they were part of a smaller universe. Anyways, working this out isnt hard - its just high-level geometry. The question, investigated by probes such as WMAP, is what geometry to use.

Also, I dont know if people understand this - but the point of the Cosmic Microwave Background is that since the universe was smaller in every direction, if we look back far enough in every direction we are seeing the same thing, the same early universe and/or remnants from that early universe. And so the CMB, which covers the sky is a "map" of the early hot Universe.
posted by vacapinta at 12:41 AM on April 10, 2006

fatllama : "old stars were made from the same kind of matter that's around today (well substantiated from emission/absorption spectra)."

Those are the spectras you are receiving today, here on Earth. At best, you can deduce that uniform change occured to the information that indicates the old distant stars and the nearby "recent" stars. Now, the first of your assumptions bears greater weight.
posted by Gyan at 1:18 AM on April 10, 2006

Actually, the CMB doesn't directly measure the age of the universe, as I understand it. You need some prior knowledge of the value of the Hubble parameter. I'll check up on that though. I could be wrong there.

I think the brief answer to the original question is that you can build mathematically quite simple models of distance, time and redshift relations which let you 'do things right' and allow for all these factors properly, all deriving from General Relativity. This lets you do things like work out how big an object appears at cosmological distances, and as you are getting at when you talk about the distant universe being smaller, this 'angular diameter distance' isn't monotonic, so more distant things start looking bigger rather than smaller as they get further away. wikipedia link.
posted by edd at 3:48 AM on April 10, 2006

Clarification on WMAP, the CMB and the age of the universe: 'The CMB data do not directly measure H0 ; however, by measuring ΩmH02 through the height of the peaks and the conformal distance to the surface of last scatter through the peak positions (Page et al. 2003b), the CMB data produces a determination of H0 if we assume the simple flat ΛCDM model.'. From Spergel et al's recent paper.

This is getting quite technical, but Ωm is the density of matter in the universe, and H0 is the Hubble parameter which will tells you how fast the universe is currently expanding and through that its age. So you basically need a little more information than you get from the CMB.
posted by edd at 3:56 AM on April 10, 2006

Thanks for the input so far...

I definitely get the most confused when thinking about the speed of light, the speed the universe is expanding (and the way this has changed through time), the distance between objects now compared to when the light was first emitted. on top of this, surely we have been moving away from all the light sources also? The red shift is not just a factor of an object's motion in relation to us, but in both our motions in relation to each other.

And yet this motion means nothing at any one point on the time axis because the radiation (light etc.) we gather the information from was emitted when the universe was arranged completely differently.

All this together causes severe confusion.

Is there a good analogy to the 4 dimensional model of our universe (time + 3 space)? If I could visualise us now in relation to the motion, the distance, the time etc. I might have a better grasp of all this (the trumpet idea mentioned thus far is the kind of thing I mean).
posted by 0bvious at 4:10 AM on April 10, 2006

You're right to worry about the distance between objects now and that when the light was first emitted. To work around this, cosmologists work in 'comoving' distance, which basically factors out the universe's expansion, and they can then work out from that the distance between the objects then, the distance now, the distance the light travelled and how far away it actually looks - all of which are generally different. The important thing is that our models of how the universe expands tells us quantitatively how they are different, so we can handle this all fine. It does generally cause severe confusion when one tries to think about it, but you can with practice get to grips with it, and most importantly as I've said, we've got equations that tell us exactly what happens.

What I think you're getting at with regards to 'our motions in relation to each other' is if we've got some velocity through space towards an object, or if the object has some velocity through space towards us, ignoring the expanding space bit. That's 'peculiar velocity', and is impossible to measure for most galaxies, and we have to satisfy ourselves with the fact that this effect is small compared to cosmological redshift, and that it statistically speaking can be averaged out and accounted for in other ways quite often.
posted by edd at 4:20 AM on April 10, 2006

i don't understand what you're asking, but will throw in my 2c anyway:

it is hard, and there's no easy answer. astronomers use mathematical models of how the universe evolves to help understand their observations, but these models themselves are uncertain.

so you end up with astronomers doing two conflicting things:

1: looking at distant objects because they want to understand the early universe (how stars and galaxies first formed, for example). in research like this, you use the models to calculate how far back in time you are observing etc etc and you are hoping to find things that are different to how you see them today (eg "baby galaxies")

2: trying to find distant objects that are the same as those nearby so that they can use the apparent difference to calibrate the models. in other words, if you take something far away, and correct your observations using maths based on the models, you should end up with something that looks like things nearby (and if you don't, you can learn something about the models for how the universe evolves). in research like this you are hoping to find things that are the same (intrinsically) to those you find today (nearby).

so there is a conflict/balancing act because both (a) what the early univese was like and (b) how the universe has evolved are both uncertain, and given an observation you typically assume one to find the other.

in fact, current ideas suggest that objects in the universe changed quite a lot right at the beginning, but then didn't do much more. so for well over half the age of the universe, things have been pretty much the same. you can probably see how that helps - models of the universe can be calibrated by looking back "not too far", and then are extrapolated to the very early times.

disclaimer - this is a bit simplified, but i think it's basically right.
posted by andrew cooke at 5:01 AM on April 10, 2006

I think I have a pretty good lamen's knowledge of physics / astronomy, but the perceptual block on this time / distance / size problem still lingers.

I am starting to see that maybe some aspects of physics cannot be even abstractly grasped without a well grounded knowledge in mathematics. This is quite an annoyance for me because my brain is not well equiped for such tasks, but the desire to understand is high.

Is there a 'Flatland' equivalent for modern cosmology?
posted by 0bvious at 6:08 AM on April 10, 2006

trouble is, cosmology just changed :o)

poking around on the net turned up this - is that any good?

this is pretty hard (especially angular size) in my opinon. just take your time...
posted by andrew cooke at 6:39 AM on April 10, 2006

Is there a 'Flatland' equivalent for modern cosmology?

Cosmos, by Carl Sagan. Sadly, it's somewhat dated now, but it frames the big questions - and the approaches to answering them - as eloquently as has ever been done.
posted by ikkyu2 at 1:45 PM on April 10, 2006

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