Talk:Universe
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Contradiction
I removed the paragraph
But some of the objects outside of the observable universe can, in principle, be observed indirectly. For example, it is theoretically possible to meet an observer located near the end of our observable universe, who in his past has observed some galaxies that left our observable universe because of expansion.
I'm a mathematician not a physicist but this seems to contradict the paragraph above about Causality.
Meaning of Universe
The Universe is the whole spacetime continuum in which we find ourselves, together with all the matter and energy within it.
Different words have been used throughout history to denote "all of space", including the equivalents in various languages of "heavens", "cosmos" and "world".
For a large fraction of the twentieth century, the word Universe, with an upper case "U", was used to mean the whole spacetime continuum in which we find ourselves, together with all the matter and energy within it.
However, since the standard Big bang model has become well established observationally during the last few decades of the twentieth century, theoretical cosmologists have come up with new ideas of "the whole spacetime continuum" which are much, much larger than the "Universe" corresponding to the Big bang model. For this reason, the word universe can now be used in the plural and with a lower case "u" when discussing theories about all of space-time. There is no clear consensus on what new word to use for the whole spacetime continuum (though some like the term multiverse), and as long as there is no conceivable method of measuring anything beyond the observable horizon (formally speaking, the particle horizon), this is a seen as a moot point (irrelevant) for empirical scientists, and is of interest only to philosophy.
infinite universe?
It is not known whether the Universe is finite or infinite in spatial extent and volume, although current theories favor an infinite Universe.
although the majority of theorists presently favor an infinite Universe.
- Rubbish. The majority of theorists currently favour an infinite Universe, but this is a matter of personal taste and sociology and historical fashion and nothing to do with physics.
- I certainly also sense theorists favoring an infinite Universe. I disagree that it is a matter of taste.
- http://www.sciam.com/article.cfm?articleID=000F1EDD-B48A-1E90-8EA5809EC5880000 "Parallel Universes" Not just a staple of science fiction, other universes are a direct implication of cosmological observations. By Max Tegmark.
- NealMcB 17:46, 2004 May 4 (UTC)
size of universe
48 billion (48 ? 109) light years.
50x10^9 light-yr = 5x 10^10 *0.3 pc = 1.7x 10^10 pc = 17 Gpc = 12 /h Gpc (where h=0.7 is the Hubble constant) OK :), this is approximately correct for Omega_m=0.3, Omega_Lambda =0.7, though 10/h Gpc is closer. In any case, i'll round to 50 since it's no more than 10% precise.
I'm glad the way this is improving - and i love the desire to avoid ambiguity. Wikipedia is definitely a good tool for spreading relatively wellunderstood information to outsiders without expecting them to waste hours and hours to sort through ambiguous jargon.
--boud
I see various estimates of the number of particles in the observable universe, e.g. the claim in Wikipedia talk:Size comparisons that It is accepted by astrophysicists that the number of particles in the observable universe? is currently in the 1085 range. This seems like the place to document that. Does anyone have some good references? NealMcB 18:27, 2004 May 4 (UTC)
"...it is estimated to be about 78 billion light years (7.4 × 1023 km)."
Interesting....just based on what did this 'estimated value' came from? At least a note should be provided for this kind of 'data'. LegolasGreenleaf 11:26, Nov 7, 2004 (UTC)
"...billion ..."
What kind of billion?, I meen, 10^9 or 10^12? I know that wikipedia uses 10^9 "if don't state otherwise" but I think that talking about science and to avoid ambiguity, is better 10^9 or the prefixes "giga" or "tera" see wikipedia, long scale alternative approaches (http://en.wikipedia.org/wiki/Long_scale#Alternative_approaches)
Seeing around the universe
For the time being, I am removing the following statement from the article, because I don't see how it can be true:
"Therefore, strictly speaking, we should call the stars and galaxies "images" of stars and galaxies, since it is possible that the Universe is finite and so small that we can see once or several times around it, and the real number of physically distinct stars and galaxies could be a little smaller. There are observations underway to determine whether this is true."
If the universe were so small that one could see even once around it, wouldn't the night (and day) sky be completely bright? If the universe were unbounded yet sufficiently finite, wouldn't the "image" of the sun exist in every direction that one looked?Johnstone 13:31, 8 May 2004 (UTC)
- I'd put it back in. You bring up Olbers'_paradox. See that article for reasons why the sky isn't all bright. But that doesn't depend on whether the universe is bent or flat. It's more about whether you can see an infinite distance. NealMcB 16:16, 2004 May 8 (UTC)
- I was aware of that paradox, though I didn't know it had a name. It was in the back of my mind when I decided to question the material I removed. As you say, that paradox is not about the idea of seeing "around" the universe, so it does not necessarily relate to my question. However, I was thinking of the universe as a "spherical space"; I have re-read the sentences in question, and now I realize that my objections do not apply for most non-spherical geometries. I've tweaked a few words to (hopefully) clarify their meaning. Johnstone 22:51, 10 May 2004 (UTC)
universe (fiction)
It might be nice to have an article on "Universe (fiction)" or something -- the sense of the word where somebody says "Many of the Marvel Comics series take place in the same universe" or "The SERRAted Edge novels are set in the same universe as the Bedlam Bards novels". Cwitty
http://www.cs.appstate.edu/~sjg/class/1010/wc/geom/finitespace.html "Is Space Finite?" by Jean-Pierre Luminet, Glenn D. Starkman and Jeffrey R. Weeks
Shape of the Universe
Hi, I'd like to question the analogue of the shape of the Universe and the shape of the Earth. There are no current experimental hints, that the Universe is not flat. This is stark contrast to our experience on Earth, where it is rather easy to find that it is not flat (ships under horizon etc.) We should change/improve that sentence. Awolf002 23:17, 10 May 2004 (UTC)
At least 156 billion light-years?
According to http://www.space.com/scienceastronomy/mystery_monday_040524.html , the universe is at least 156 billion light-years across.
- I changed the article accordingly. If someone thinks it was too early and we should wait until the new estimate becomes widely accepted, change it back.Paranoid 12:50, 28 May 2004 (UTC)
- while changing from 50 to 78 Bly in radius, it looks like you left unchanged the language The observable universe contains about 7 × 1022 stars, organized in about 1010 galaxies,. Anyone know where that estimate came from? Based on applying the local density to the total volume? Based on observations of deep fields? Something else? If based on density, these numbers would also need to be adjusted based on the new volume. And this all relates to my question above about the number of particles in the universe. --NealMcB 00:04, 2004 May 29 (UTC)
Age - forever uncertain
According to http://www.sciencenews.org/articles/20040522/fob1ref.asp , physicists from National Laboratories of Gran Sasso (http://www.lngs.infn.it/) found the age to be 14.7, not 13.7.
- Well, kind of... You need to understand, that the age of the Universe can (and should) be obtained by many independent methods. Each of these methods has its own assumptions and systematic errors. So it is very unlikely you get the exact same numbers from all of these measurements. If the theory about the Universe (on which these numbers are based on) is correct, then these numbers should agree within their uncertainties!
- The article seems inconsistent. We have one paragraph stating the age is 13.7 (or 14.7) +-.2, and another saying it is 15.556 +- 0.024 billon years. These numbers aren't agreeing within their uncertainties. We need to square this somehow, at least we should point out the inconsistency. Zeimusu 16:17, 2004 Jun 15 (UTC)
- I suspect the 15.556 one is less reliable. If we knew the age with such precision, nobody would be discussing it, doing research and publishing paper about it.Paranoid 18:00, 15 Jun 2004 (UTC)
Does anyone know who wrote that stuff about "the age is the inverse square of the temperature"? I know quite a bit about cosmology, and never heard of such a formula - and actually I think that this claim makes no sense. WRT the 14.7 billion years, I think that was misreported in the popular science media. What the people actually showed that the age of stars and globular clusters is 1 billion years more than previously thought - but adding that 1 billion years simply to the WMAP result makes no sense, since the WMAP result does not depend in any way on the age of the stars. There could possibly be problems now that the age of some stars seems to be older than the age of the universe, but AFAIK, this is not the case, despite this age correction for the stars.
- Accordingly, I have deleted this paragraph. If anyone feels it must be reinstated, lets discuss it here first. Zeimusu 12:27, 2004 Jul 23 (UTC)
well, I can claim that the universe is 20 billion years old and you CANNOT prove me wrong...=) while we may have a vague figure of the visible universe, what is beyond what our vision could reach is anyone's guess...who's to say that the next second the Hubble telescope will not see a 'boundary' somewhere beyond the deep field... I think i'm a lil bit drunk... but this stuff is fun to think about — LegolasGreenleaf 11:34, Nov 7, 2004 (UTC)
Age of Universe - Observation and Theory
The often quoted age of 13.7+/-0.2 Gyr for the age of the universe comes from the first year WMAP results: This measurement is made by using the location of the first acoustic peak in the microwave background power spectrum to determine the size of the decoupling surface (size of universe at the time of recombination). The light travel time to this surface (depending on the geometry used) yields a pretty good age for the universe. Assuming all the various models used are valid in getting to this number, the accuracy of actual data allows a margin of error around 1%.
However, this age is only accurate if the assumptions built into the various models being used are also accurate. This is referred to as “strong priors” and essentially involves stripping the potential errors in other parts of the model to render the accuracy of actual observational data directly into the concluded result. Although this is not a totally invalid procedure in certain contexts, it should be noted that the caveat, “based on the fact we have assumed the underlying model we used is correct”, then the age given is thus accurate to the specified error (since this error represents the error in the instrument used to gather the raw data input into the model).
The age of the universe based on the “best fit” to WMAP data “only” is 13.4+/-0.3 Gyr (the slightly higher number of 13.7 includes some other data mixed in). This number represents the first accurate “direct” measurement of the age of the universe (other methods typically involve Hubble law and maximum age of stars, etc). There is a sense of triumphantism in the scientific community surrounding results like this, and therefore a more careful analysis of the methods and assumptions used, tend to be overlooked.
This, of course, is a classic example of how different methods for determining the same parameter (in this case – the age of the universe) can give different answers with no overlap in the “errors”. It is quite common to see two sets of uncertainties, one related to the measurement and other the related to the systematic errors of the model. In some cases, this can not be done (in theoretical a prediction), but it is not evident why WMAP were not able to do this?
Worth checking out is Science 299 (2003) 1532-1533, available here http://arxiv.org/abs/astro-ph/0303180
There is a purely theoretical approach to calculating the age of the universe which I can outline in more detail here. This comes from a very recent development and hasn't been published yet. Even after publication, it can take some years before a new result like this makes its way into the mainstream (so don't be surprised if you have not heard about this yet.) It is probably best to leave this development out of the main page until such time as it gains greater acceptance. For now this discussion forum should suffice for a preview however:
The redshift of an object in a dynamic universe is related to a scale factor of that universe by the relation R=Ro/(1+z). Where R represents the “scale” of the universe as seen at the redshift z, where the current scale is Ro. The “scale” is just a device to measure the size of the universe, it can be thought of as the radius, but most people use the “scale factor” a=R/Ro, which would be dimensionless regardless of how you represented R.
The temperature of the universe is inversely proportional to its scale; somewhat analogous to a gas that would cool down if expanded, or heat up if compressed, the temperature of the universe is thus related to redshift as T=To(1+z). We can do a quick test by using the current temperature of 2.7K and the redshift of CMB as 1089 to calculate the temperature of the decoupling surface T= 2.7*1090 = 2943K (this is the temperature of the universe when the CMB was emitted - around the dull red glow of a hot poker.)
One of the most important cosmological models, is based on the Friedmann equations. This allows you to describe how the universe has evolved over time using an equation like this: t=to(1+z)^-3(1+w)/2. As you can see, things are starting to get a bit more tricky, but this equation simply relates the age of the universe to the redshift. This particular example has an additional term w, which comes from something called the equation of state, relating the pressure and density of the universe (p=wdc^2, where p is pressure, d is density and c^2 is the speed of light squared).
In a universe like our own, most of the contents is in the form of stuff that does not exert much pressure on its surroundings (clouds of hydrogen gas, stars etc). In this model, w=0 and is known as a pressureless, or “dust” model. Here t=to(1+z)^-(3/2), and throwing in our redshift of 1089 and a current age of the universe to=13.7 Gyr gives us around 380,000 years for the age of the universe when the CMB was emitted. This may not seem so tricky after all, but unfortunately, it is not quite that simple.
Embedded in these models is an assumption about time and an interpretation of metric distance which is not entirely correct. That is not to say that they are entirely wrong either: The metric distance defined between two points in an expanding universe increases over time. However, the General Theory of Relativity does not explicitly state how that change in distance should be interpreted. It is entirely valid to consider this change as a fundamental change in the underlying “concept” of distance (and the same situation would also apply to the concept of time).
This type of model immediately solves an important problem relating to our CMB calculation above. If the photons in the CMB went from being hot enough to fry a burger, how come those same photons can't even defrost one today? Where did all that energy go? Of course, this comes back to our idea of the change in the distance scale: These universe expands by a change in the unit system, so the temperature likewise changes with the unit system. In this context, the temperature/scale of the universe can be thought of as being constant over the history of the universe, with no loss of energy in the CMB.
Things do start to get technical here, but there is a nice confirmation of this model which actually validates it against recent observations. Coming back to the math, the change in the distance is related to time with the redshift relation t=to(1+z)^-2. However, there is an additional change in time related to redshift as t=to(1+z)^-(1/2), which (the product of both) brings us back to the original form for our “dust” w=0 universe. The idea of time-variable time probably sounds bizarre, but this is expected since there is no “absolute” concept of time in General Relativity (even though it seems people try and introduce this idea in most models.)
So this was a very round about way of saying that we can relate the temperature of the universe to the age of the universe. Since we can measure the current temperature and have a model to extrapolate back, all we need to know now is the origin of the graph and read off the age. The earliest valid point in the evolution of the universe if the Planck time. At this time, the universe had the Planck temperature at a state of essentially zero entropy. The Planck temperate is the maximum attainable temperate in the universe and can be thought of as the Hawking temperature of black hole with a radius of the Planck length.
The Planck temperature Tp comes out to around 4.5x10^30K, and we can state Tp=To(1+zmax), where To=2.725K and zmax=1.65x10^30 is the maximum redshift at the Planck time tp. We know that tp=to(1+zmax)^-2, so putting in the Planck time gives us a n age of the universe of 11.667 Gyr. This is not the end of the story however: If time was absolute and never changed, then this would be the correct value, but we need to take into consideration of the change in time over the age of the universe. This is a fairly simple integration and results in a age one third as much at 15.556 Gyr. The CMB temperature is known to a 2mK accuracy, and with some error in things like the Planck units (mainly from G), the accuracy of this age determination is around 24 Myr.
There is a simplification where if expressed in Planck units, the temperature is equal to the inverse square. Dividing To/Tp gives the current temperature expressed in the amount of the Planck temperature 6x10^-31. Taking the inverse square gives 2.72x10^60 which is the age in Planck units. Multiplying by the Planck time gives the 11.667 Gyr again. There is mainly other simple relations like this, including the critical density as the Planck temperature raised to the forth power. In Planck units, the density is 1.3x10^-121, which multiplied by the Planck density is 3.3x10^-30 g/cm^3.
This was a very stripped down and somewhat mangled explanation, but hopefully it has shed some light on the “age of the universe” question.
- I notice that you not only snipped my questions and chose to ignore them; you also left our discussion in sci.astro. I count that as a defeat. You call our discussion "fruitless" - you are right there, but you might consider *who* was the one who kept ignoring questions and arguments, and thereby made the discussion fruitless...Bjoern 14:52, 3 Dec 2004 (UTC)
- Not so much a defeat, as a waste of time; your questions and arguments didn't warrant the time required to address them. However, if you are still interested, I will be presenting this work at the annual meeting[1] (http://www.aaasmeeting.org) of the American Association for the Advancement of Science on February 20th 2005 (so I need to focus my attention on that presentation for the time being).
- Wow, more discussion than I was expecting! I wonder if that can be incorporated into this page or another, it was worth reading. The universe page should have 13.7 billion years as its value for the age of the universe, but I'm going to copy some of your caveats to the main page. Zeimusu 14:41, 2004 Jul 27 (UTC)
Capitalizaton
I asked this question on Talk:Kardashev scale but I'm still not confident. this article mizes and matches the use of Universe and universe. Is there a distinction between the two forms or is this a formatting error? Many articles have the word Universe capitalized but this article starts out by uncapitalizing it and then shifts to upper case. The title:
Size of Universe and observable universe
seems to be using both versions in one line. If it isn't an error there should be a note about it somewhere. Comments? [[User:BrokenSegue|BrokenSegue]] 03:27, 19 Nov 2004 (UTC)