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Q: Measuring size of the Universe ( Answered ,   14 Comments )
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 Subject: Measuring size of the Universe Category: Science > Astronomy Asked by: halejrb-ga List Price: \$5.00 Posted: 16 Jul 2002 04:50 PDT Expires: 15 Aug 2002 04:50 PDT Question ID: 40087
 ```Can the age of the universe be used to calculate the size of the universe? For example, assume the universe is 15 billion years old. Since the universe cannot expand faster than the speed of light, does this imply that the "radius" of the universe is 15 billion light years? And if so, can you use this measurement to calculate the volume of the universe, like you can if you know the radius of a sphere?```
 Subject: Re: Measuring size of the Universe Answered By: till-ga on 16 Jul 2002 07:09 PDT Rated:
 ```In an rough estimation your assumptions are true. If you want to find out precise details of the modern cosmological theory this would definitely go beyond the scale of an answer that can be given here. So that´s what I´ve found out for you: “ Astronomers believe that the Universe began with the Big Bang,some 10 - 20 billion years ago. So, the most distant objects that we can see are about 10 - 20 billion light-years (= 1027 cm) away.” form: The Scale of the Universe ( http://casswww.ucsd.edu/public/tutorial/scale.html ) “The visible universe has a radius of about 15 billion light years simply because the universe is about 15 billion years old. The light from more distant objects simply has not had time to reach us. For this reason everybody in the universe is at the middle of their own visible universe about 15 billion light years in radius. The true size of the universe has to be much larger than this though. How large depends on whether the universe is open or closed, that is whether there is enough matter in the universe to halt its expansion at some point in the far distant future. If there is then the universe is probably finite in size. The other possibility is that the universe will keep expanding forever - this means the universe is probably infinite in size. Whichever scenario is correct, it is clear that the visible universe is only a minute speck in a far larger totality.” from: The Visible Universe ( http://www.anzwers.org/free/universe/universe.html ) “Q: When I read about the estimant of the age of the universe I see a paradox. Reports on NASA's site estimate telescopes like the HST can see about 10 billion light years and the latest speed of the expansion of the universe is about 300 kps. For the universe to be 10 billion light years + in diameter it would take 10 trillion years to be that size at that speed. Am I misunderstanding something? A: The universe is not expanding at a uniform rate, but for a rough estimate we can assume that the most distant galaxies are rushing away from us at the speed of light. There are about 30 million seconds in a year, so at 300,000 km/sec the universe would expand by an amount (30,000,000 sec ) X 300,000 km/sec = 9,000,000,000,000 km = 9 trillion km = 1.5 light years. So in 10 billion years the universe would expand to about 15 billion light years. “ from: Chandra Ressources (http://chandra.harvard.edu/resources/faq/cosmology/cosmology-15.html ) “The simple answer is that the observable Universe is about 10 billion light years in radius. That number is obtained by multiplying how old we think the Universe is by the speed of light. The reasoning there is quite straightforward: we can only see out to that distance from which light can have reached us since the Universe began. (But see my note marked * below). We determine the age of the Universe in a number of ways. One is to estimate the age of the oldest stars we see. Our knowledge of how stars of a given size evolve with time is very good (based on what we know about atomic and nuclear physics) so the major uncertainty here is usually measuring how far away (and so how big) such stars are. The standard method is to look for very small changes in the apparent positions of the stars as the Earth moves around the Sun. (This effect is called parallax). A second way to get an age for the Universe is to try to figure out the time of the big bang itself. Here the method is to use a series of techniques (based on how bright things appear to be - like Cepheid variable stars - that we think we know the true brightness of) to determine first the distance of the nearby galaxies, then increasingly distant galaxies, until we have estimated distances for many galaxies for which relative velocity measurements have been made (using the Doppler red shift of features in their spectra). The relative velocities we observe for distant galaxies have been largely determined by the expansion of the Universe begun with the 'big bang'. So, once we've determined how expansion velocity correlates with distance for some range of distances, it's possible to extrapolate back (with some assumptions) to calculate the instant of the big bang, when all the matter in the Universe was at a single point.” from: Measuring the size of the Universe ( http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/971124x.html ) Interesting in tis context is that the “amazing idea, that the universe could have expanded from one tiny point, was not accepted by everyone. A radical group of cosmologists at Cambridge, led by Fred Hoyle, came up with the steady state theory. This said that there had been no Big Bang and that the universe had always been roughly the size it is now, but that new matter was spontaneously being created all the time and that this was the reason that the universe was expanding. As proof for their theory they had to find a place in the universe where all the elements were formed. The steady state scientists made the brilliant discovery that the intensely hot conditions in the inside of stars formed all the elements that now make up our planet and even ourselves. However, stars start with the basic fuel of hydrogen. It seemed as if the Big Bang would be the only place hot enough to have formed hydrogen, was this the first piece of evidence for how the universe formed?” form: Stephen Hawking´s Universe ( http://www.psyclops.com/hawking/shu/shu2.html ) You´ll find a detailed quantum mechanical explanation at: ( http://superstringtheory.com/cosmo/cosmo1a.html ) I have not cited this one because there too many formulas there. Search Strategy ( ://www.google.de/search?sourceid=navclient&hl=de&querytime=M&q=size+universe+light+speed )```
 halejrb-ga rated this answer: ```This was a pretty good answer given the difficulty of the subject matter. I also liked everyone's comments.```

 Subject: Re: Measuring size of the Universe From: tehuti-ga on 16 Jul 2002 05:04 PDT
 ```Hi halejrb, I wouldn't DARE to answer this, as it's way over my head. However, I thought these links might interest you: http://www.physlink.com/Education/AskExperts/ae220.cfm "By what method is the size (or volume) of the universe at the moment of the big bang (or shortly thereafter) determined? Related to this, how is the size of a black hole determined?" http://online.itp.ucsb.edu/online/plecture/kirshner/ Taking Measure of the Universe: How Big? How Old? How Do We Know? by Robert Kirshner, Harvard University http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/age.html How do we measure the size and the age of the Universe? Good luck!```
 Subject: Re: Measuring size of the Universe From: bobthedispatcher-ga on 16 Jul 2002 07:25 PDT
 ```Though this is very far from my areas of expertise, I noticed in several of the references above that the estimated size seems to be based on the speed of light and the farthest visible stars. Wouldn't that presume we are at the center of the universe??? If we were not, and if it is expanding, as it is presumed by some that, the distance to the edge would depend on which way we look?????```
 Subject: Re: Measuring size of the Universe From: thx1138-ga on 16 Jul 2002 07:26 PDT
 ```Good answer. I love these types of questions! Just another thought: we know that the universe is expanding, but what volume/space is it expanding into if there is no 'outside' to the universe?```
 Subject: Re: Measuring size of the Universe From: thx1138-ga on 16 Jul 2002 07:31 PDT
 ```bobthedispatcher this might shed a bit of light on your point: "How far away from the center of the universe is the Milky Way ? If it were not for modern cosmology, the observation by astronomers that distant galaxies are moving away from us at higher and higher speed would mean that we are located at the center of the universe. But thanks to Einstein's theory of general relativity, which is the basis for all of modern cosmology, we now know how to interpret the apparent motion of distant galaxies. The answer, now, is that there is no unique center to the universe at all. The best way to see this is to take a balloon and put a few dots on it. Blow the balloon up and watch as the dots move away from each other. Which ever dot you would choose to identify as the Milky Way, all of the other dots would seem to be moving away from it giving you the illusion that your galaxy is at the center of the universe. So, the answer to this question is that the Milky Way appears to be sitting right at the center of the expansion of the universe, but the universe itself has no center. " http://itss.raytheon.com/cafe/qadir/q82.html```
 Subject: Re: Measuring size of the Universe From: bobthedispatcher-ga on 16 Jul 2002 07:46 PDT
 ```Of course if there is no center, it would be difficult to determine the distance from that(non-existant) point, therefor we would have no real point of reference to measure from, except our relative position, which doesn't really answer the original question. ??? Sort of like being adrift in the ocean, in a heavy fog, without compass, or any electronic aids, and trying to determine where the nearest land was.```
 Subject: Re: Measuring size of the Universe From: thx1138-ga on 16 Jul 2002 08:00 PDT
 `Or indeed looking for a mute black cat in a coal cellar with the lights out!`
 Subject: Re: Measuring size of the Universe From: halejrb-ga on 16 Jul 2002 08:46 PDT
 ```I agree there is no "center" to the universe, at least not the 3 dimensional universe we can observe. That's why the "dots on an expanding balloon" is such a good analogy. That's also the reason I put the term "radius" in quotes in my question. If you pick any point on the balloon as the "center" of this two dimensinal balloon universe, then the point opposite it on the other side of the balloon becomes the farther observable location in this universe. The distance between these points is 1/2 the circumference of the balloon, or in the example of the real universe, around 15 billion light years. It should be possible to "do the math" to figure out the volume for a universe in which it is 15 billion light years between two farthest points. I realize though that the universe is not really like the surface of a balloon. It's much more complicated than that and I assume the math needed for answering the question is very complicated too. I'm also aware though of the "Stickman" map of the known universe. Presumably, you can measure distance on this map and determine the volume of the known universe.```
 Subject: Re: Measuring size of the Universe From: bobthedispatcher-ga on 16 Jul 2002 08:55 PDT
 ```re: halejrb-ga comment Question: If your analogy of a balloon surface, as a 2 dimesional space is used, then how can you have a volume (3 dimensional??) I have a feeling that this may expand the scope of the original question enormously, and probably way beyond what was desired. But it is an interesting subject.```
 Subject: Re: Measuring size of the Universe From: halejrb-ga on 16 Jul 2002 09:50 PDT
 ```Hello bobthedispatcher. You're right, I did switch from 2 dimensions to 3 dimensions. The balloon example is a good way of showing in two dimensions how every point on the balloon can move away from every other point as the balloon expands. This is similar to the way every galaxy is moving away from every other galaxy in the real 3 dimensional universe. But the comparison is obviously not perfect. That's what I meant when I said the math gets more complicated when you try to calculate the volume of a 3 D universe in which space itself may be curved. But even if the math is complex, given the fact that the size of the universe is proportionate to the age of the universe due to the speed of light, I think it should be possible to calculate the size of the universe for any given age. :-)```
 Subject: Re: Measuring size of the Universe From: thx1138-ga on 16 Jul 2002 10:34 PDT
 ```Have a look at this site which explains what the hubble constant is: v = Ho * d "Why such a heated debate over a single number? The Hubble Constant is one of the most important numbers in cosmology because it is needed to estimate the size and age of the universe. " http://www.astro.washington.edu/astro101v/labs/hubblelaw/hubblelaw.html```
 Subject: Cosmology From: ulu-ga on 16 Jul 2002 11:47 PDT
 ```Just last week, the shape of the universe was discussed on the radio program "Science Friday". "Then, we'll turn our attention to the Big Questions in the universe -- where'd it come from? Where's it going? How'd it get to be the way that it is? Guest host David Kestenbaum talks with cosmologist Janna Levin about her work, cosmology in general, and her book, "How the Universe Got Its Spots: Diary of a Finite Time in a Finite Space." " http://www.sciencefriday.com/pages/2002/Jul/hour2_071202.html http://www.amazon.com/exec/obidos/ASIN/0691096570/sciencefriday Related Links: "Age of the Universe There are at least 3 ways that the age of the Universe can be estimated. I will describe The age of the chemical elements. The age of the oldest star clusters. The age of the oldest white dwarf stars." http://www.astro.ucla.edu/~wright/age.html (very technical site) Cosmology: The Study of the Universe http://map.gsfc.nasa.gov/m_uni.html (this link was referenced by others) "An Age Crisis? If we compare the two age determinations, there is a potential crisis. If the universe is flat, and dominated by ordinary or dark matter, the age of the universe as inferred from the Hubble constant would be about 9 billion years. The age of the universe would be shorter than the age of oldest stars. This contradiction implies that either 1) our measurement of the Hubble constant is incorrect, 2) the Big Bang theory is incorrect or 3) that we need a form of matter like a cosmological constant that implies an older age for a given observed expansion rate." http://map.gsfc.nasa.gov/m_uni/uni_101age.html```
 Subject: Re: Measuring size of the Universe From: pinkfreud-ga on 16 Jul 2002 12:31 PDT
 ```Luke Skywalker to C-3PO: "If there is a bright center to the universe, you're on the planet it's farthest from." If only George Lucas would tell us where to locate Tatooine, many cosmological questions would be answered. ;-)```
 Subject: Re: Measuring size of the Universe From: bobthedispatcher-ga on 16 Jul 2002 12:51 PDT
 ```This is turning into a discussion group, not an answer! :} :} Could be there are no answers, just theories (and guesses)```
 Subject: Re: Measuring size of the Universe From: stockzguy-ga on 16 Jul 2002 13:32 PDT
 ```It isn't the size of the universe that matters, it's the interaction between the planets. :)```