General Question

krrazypassions's avatar

How do experts calculate and interpret large distances in the universe?

Asked by krrazypassions (1350points) May 16th, 2011

Lets say we are talking about this far-away galaxy A. We are say it is 13.2 billion light years away from us.
Are we saying that
the galaxy is 13.2billion light years away from us right now?
Or we are saying that
(2) the light from this galaxy as we see it now has traveled for 13.2 billion light years before reaching us.

If it is the latter case, that means we don’t know where that galaxy is right now(that is, of all things, we don’t know how far away it is right now, we only know it was 13.2billion light years away 13.2billion years ago, which then means that the universe was atleast 13.2 billion light years across in size. But that would mean that universe was 13.7billion light years across in size if we see something that we say is 13.7 billion light years away from us).

In any case, how did they (the experts) calculate this distance?

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22 Answers

Nullo's avatar

IIRC, a lot of astronomical range-finding works out to trigonometry and physics.

krrazypassions's avatar

Are we calculating distances by calculating how old the light from the galaxy is?

krrazypassions's avatar

I might seem confused and naive, but let me tell you the source of this confusion.
It is the expanding universe.
See, if the universe was static, then saying that a galaxy is 13 billion light years away would always be the same as saying that the light from it has traveled for 13 billion years before we could see it

But in an expanding universe, if we say that the light has traveled for 13 billion years, it only means that the light which was emitted 13 billion years ago has reached us today. That does not mean that the galaxy was 13 billion light years away 13 billion years ago. In fact it should have been much closer than 13 billion light years. The reason light from it took 13 billion years was that since space-time was continuously expanding, the light had to cover increasingly larger distances before it could reach us. The fact that this light could reach us implies that space-time was not expanding faster than light speed then. But, this tells us nothing about the present situation of the galaxy(whether it exists, if it does then where, is it moving away from us today or has it started coming closer?).

I guess many thoughts I had about this are still unexpressed… but are you getting why it is so tough to figure out?

krrazypassions's avatar

@all also, the line in the question details- “we only know it was 13.2billion light years away 13.2billion years ago” is wrong in case of expanding universe- its true only in case of static universe-
In case of expanding universe, the line is modified as “we only know it was less than 13.2billion light years away 13.2billion years ago”

Thammuz's avatar

@krrazypassions Ok, first and foremost: expanding universe =/= something transcendental where space-time is expanding physically. It means all astronomical bodies are moving away from the origin point of the universe, at least as far as my uni physics lessons cared to delve in the subject.

Secondly, we know what speed the galaxy is moving thanks to the doppler effect, that influences the wavelenght of the light that reaches us, in relation to the direction and speed of the body in question, so we can also assess pretty precisely what position the galaxy should now occupy.

And lastly thanks to trigonometry we can calculate its distance thanks to its relation to other bodies (though this particular method does not work with bodies as far away as the example you mentioned).

But yes, it’s not something as verifiable as the position of, say, Mars. It’s also not as relevant, so it’s really not a problem.

krrazypassions's avatar

@Thammuz Expanding universe is indeed expanding space-time! “It’s 72 kilometers per second from mega parsec of space. So grab yourself a mega parsec of the Universe, wait a second and it’s going to be 72 kilometers bigger in diameter.” its source

Now, there are 3 different kinds of red-shift
We see all galaxies outside our local cluster moving away from us. We have not found any blue-shift on this scale. This is called as the universal expansion of space-time accounted by Cosmological redshift. In the local galactic cluster, we see both blue-shifts and red-shifts as there are stars and near-by galaxies that move towards us, accounted by Relativistic Doppler

Also, there is no origin point from which all astronomical bodies are moving away from- see this
You can say that at the very beginning there was a point (singularity)and this point itself expanded.

flutherother's avatar

I think the difficulty is that there isn’t an absolute frame of reference for observing the Universe though we imagine that there is. From our point of view the galaxy is 13 billion light years distant and the light took 13 billion light years to reach us and there is no contradiction. In our now that is the reality. In the frame of reference of the distant galaxy that light was emitted 13 billion years ago and its now is quite different from ours. There isn’t a now that unites these two frames of reference.

krrazypassions's avatar

Hey! Is it true that expansion of space-time is expanding light waves, causing red-shifts in this manner? Then, even if the space-time expands faster than light speed, it would only result in larger red-shifts, while the time taken for light of a particular galaxy to reach us would still remain the same??

flutherother's avatar

Nothing can travel through space faster than light but space itself can expand and carry galaxies apart at a faster than light speed. If a galaxy was receding from us faster than light we wouldn’t be able to see it.

chocolatechip's avatar

@krrazypassions How can space-time expand? This implies that there is space and time outside of spacetime in which it can expand.

krrazypassions's avatar

@flutherother Yes. Space itself can expand and carry galaxies apart at a faster than light speed. That means galaxies are moving away from us faster than light since the space itself is carrying them away from us at that speeds. And we are still able to see them because their light is also expanding, causing large red-shifts. Their light is massively red-shifted to a value of 8. A galaxy receding from us in a static space at the speed of light would have a red-shift of 1.4
I understand that the receding in your statement “If a galaxy was receding from us faster than light we wouldn’t be able to see it.” is relativistic movement, and sure nothing can move faster than light, so no galaxies would relativistically recede faster than light and therefore we would always see them. Only, since the space would have expanded, we would see a larger red-shift. Right?

krrazypassions's avatar

@chocolatechip @all
All this while since the big bang, when the singularity expanded on itself, the space-time has been expanding. The universal expansion and cosmological red-shift: Expansion of space are the continuation of the same phenomena. (Rather than cosmological redshifts being a consequence of relative velocities, the photons instead increase in wavelength and redshift because of a feature of the spacetime, through which they are traveling, that causes space to expand)
Go through this video- What is the universe expanding into
and this and this video-how can galaxies travel faster than light

Thammuz's avatar

@krrazypassions don’t take this the wrong way but, since you seem to know much more than most people, why the hell are you asking the question? I learned more form your posts alone than from my physics lessons…

krrazypassions's avatar

@Thammuz See, i am not an expert in cosmology or physics. In fact i am not an expert at anything. But i am very curious about a lot of things and very enthusiastic and interested in knowing more- as a result I went through a few things on the internet and books-
But they did not answer all of my old questions completely and gave rise to new questions too! Since I am not an expert, i find heavy mathematical material and long texts very scary and tough to understand. But i do want my answers! Hence, I asked on fluther, so that any expert who might know about this can answer in simple words. Also, i tried to share a few sources so that non-expert people like me could go through some basic facts so that they can employ their common sense,general knowledge and intuition to it and share their opinion about the same with me and the rest of the fluther community.

flutherother's avatar

@krrazypassions This Wikipedia article is pretty good and answers some of your questions. It is all pretty mind boggling however.

ETpro's avatar

@krrazypassions Trigonometric parallax produced by Earth’s orbit around the sun can be used to accurately measure objects out to about 300 light years. Of course, our own Milky Way is about 100,000 light years in diameter, so unless we were willing to wait for a half revolution of it, we wouldn’t be able to use trigonometric parallax to measure anything at a far greater distance than that. The Sun is about 26,400 light years from the galactic center, so one half rotation would let us measure the parallax from points 52.800 light years apart. However, the negative rotation period (time required for our Sun to make one complete rotation around the galactic center) is 250 million years. That means we’d need to wait 125 million years between measurements to get a highly accurate trigonometric parallax. Not a terribly practical idea.

Fortunately, we do have measurements made by a European satellite, Hipparcos, launched in 1989. Given the distance it has now traveled, it can rather accurately measure objects up to 10,000 light years away from us.

Beyond that, we have to extrapolate from known behaviors of certain unique celestial bodies that we can measure nearby, and then observe how ones of the same sort appear when very far away. Rather than post a wall of words explaining how this works, you can read all about it in this Scientific American article.

krrazypassions's avatar

@ETpro but of course! Standard Candles! I had heard that first in one of these nice lectures/videos of quantum to cosmos festival by Perimeter Institute, Canada (i think you might have already gone through these, but if not, then let me recommend these to you as great resources for non-experts to know more about quantum physics and cosmology) and thanks a ton for reminding me again with this article!

Quoting from the article posted by ETpro: “A certain kind of exploding star, or supernova (called a Type Ia supernova), always seems to have the same peak luminosity, so these supernovae can be used as ‘standard candles’ instead of Cepheids. Supernovae are billions of times brighter than Cepheids; as a result, they can be observed at far greater distances. A number of researchers are trying to exploit this advantage and get more accurate information about the size and age of the universe. The Hubble Space Telescope is assisting in this work as well. ”

So earlier, Cepheids were used as standard candles, but it was found they are not ‘standard’ after all
However, Type Ia supernovae were found to be the best standard candles

Rarebear's avatar

@krrazypassions Perlmutter is actually an acquaintence of mine and I’ve seen several of his talks discussing Type 1a supernovas as standard candles.

krrazypassions's avatar

Perlmutter? Are you speaking of someone at the Perimeter Institute, Canada or one of authors of this book-
The Cosmic Triangle: Revealing the State of the Universe, (N.A. Bahcall, J.P. Ostriker, S. Perlmutter, and P. Steinhardt), Science,(1999)

Rarebear's avatar

Sorry, my mistake. I was talking about Saul Perlmutter. He’s one of the discoverers of the accelerating universe—he did it by using Type 1a supernovas as standard candles.

krrazypassions's avatar

@Rarebear Thats wonderful and you are lucky to have seen his talks :)

krrazypassions's avatar

@all Something to thing about guys

I have something to say on expansion theory and Hubble’s evidence:
Go through this part of the Wikipedia article which is titled “Understanding_the_expansion_of_Universe”.

Now, this is the observational evidence:
“Hubble demonstrated that all galaxies and distant astronomical objects were moving away from us, as predicted by a universal expansion. Using the redshift of their electromagnetic spectra to determine the distance and speed of remote objects in space, he showed that all objects are moving away from us, and that their speed is proportional to their distance, a feature of metric expansion. ”

Now bear both these things in mind and have a look at my thoughts:
” The deeper we look into space, the deeper we are looking at the past. So, the speed of expansion being proportional to distance actually indicates that the expansion rate was higher in the past!”

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