# A part of the universe is opaque. Why? **URL:** \ **Category:** Factual Questions\ **Created:** [May 2, 2012, 2:58pm UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644 "2012-05-02T14:58:45Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Tom\_Tildrum](https://avatars.discourse-cdn.com/v4/letter/t/e95f7d/32.png) [@Tom\_Tildrum](https://boards.straightdope.com/u/Tom_Tildrum)\ **Post date:** [May 3, 2012, 2:22am UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/21 "2012-05-03T02:22:01Z")
> [@Chessic\_Sense](#): > > What I don’t get is how a mass of stuff packed to infinite density can emit light that hasn’t reached us yet. And if we’re observing things after 13.7 billion years, how far away was the particle when it emitted the photon? It’d have to be less than 13.7 billion years, right? > > I’m confused. Think of the “[Dolly Zoom](http://en.wikipedia.org/wiki/Dolly_zoom)” and a character in a horror movie running down a hallway that just keeps getting longer. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [May 3, 2012, 3:17am UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/22 "2012-05-03T03:17:05Z")
> [@blood63](#): > > If we can’t see beyond the 13.7 billion light years, how do we know what’s there? How do we know anything is there? Scientists reason it out, given the mathematics of what they know. > [@](#): > > BTW, in another post, someone wrote that the universe is infinite. I always thought it was finite. Did I read that wrong? There’s a lot of controversy today over whether the universe is infinite or finite. If it is infinite then it has always been infinite. Nothing can go from finite to infinite. You can, however, go from nothing to infinity. That’s all I’m capable of saying. There have been endless threads discussing this, so you should do a search. But they’re mostly on varying levels of incomprehensibility. Mere inflation and expansion are kindergarten stuff by comparison. ---
**Author:** ![Captain\_Amazing](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@Captain\_Amazing](https://boards.straightdope.com/u/Captain_Amazing)\ **Post date:** [May 3, 2012, 4:57am UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/23 "2012-05-03T04:57:37Z")
> [@The\_Hamster\_King](#): > > It starts to expand. It’s not getting any bigger (it’s already infinite), but things inside it are moving apart. It’s becoming less dense. How does something expand and not get bigger? Isn’t that what expansion is? Getting bigger? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 3, 2012, 5:04am UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/24 "2012-05-03T05:04:41Z")
> [@](#): > > If we can’t see beyond the 13.7 billion light years, how do we know what’s there? How do we know anything is there? > BTW, in another post, someone wrote that the universe is infinite. I always thought it was finite. Did I read that wrong? We don’t _know_ what’s out there. But everything that we can see seems to be pretty much the same everywhere (on a sufficiently large scale), and it’d be really weird if, as soon as we can’t see it, it starts behaving differently. It’s sort of like asking how you know that the refrigerator light goes out when you close the door: Fun as a silly question, but it can’t really be serious. And we don’t know whether the Universe is finite or infinite. It appears to be flat, and the simplest models of a flat universe imply an infinite one, but it could be positively curved on scales much larger than we can see (sort of like how the surface of the Earth appears flat, even though it’s not), or it could be a slightly more complicated situation that really is flat but is finite anyway. In a way, it’s not controversial at all, since everyone agrees that we don’t know. ---
**Author:** ![Chessic\_Sense](https://avatars.discourse-cdn.com/v4/letter/c/7c8e57/32.png) [@Chessic\_Sense](https://boards.straightdope.com/u/Chessic_Sense)\ **Post date:** [May 3, 2012, 1:01pm UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/25 "2012-05-03T13:01:43Z")
> [@Exapno\_Mapcase](#): > > Let’s try the old-fashioned analogy. > … The total of the starting distance plus the increased space is 13.7 billion light years. > [@Irishman](#): > > How far away was the particle that emitted the photon? Far enough away that the space could grow before the photon reached us. > [@Tom\_Tildrum](#): > > Think of the “[Dolly Zoom](http://en.wikipedia.org/wiki/Dolly_zoom)” and a character in a horror movie running down a hallway that just keeps getting longer. I want a number, though. How far was the fog that we’re seeing now? How far is that particle now? Was it, say, 13 billion light years away and expansion has added 0.7 billion years to the trip? And now those particles are 30 billion light years away, or what? ---
**Author:** ![Candyman74](https://avatars.discourse-cdn.com/v4/letter/c/ad7895/32.png) [@Candyman74](https://boards.straightdope.com/u/Candyman74)\ **Post date:** [May 3, 2012, 1:13pm UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/26 "2012-05-03T13:13:40Z")
> [@Captain\_Amazing](#): > > How does something expand and not get bigger? Isn’t that what expansion is? Getting bigger? What’s infinity +1? It’s still infinity. Basically, things are moving further apart within that infinite area, so it’s becoming less dense. But there is no “edge” that could be moving away from you, so there’s no way you can say the construct you call the universe is getting “bigger”. Assuming the universe is infinite, of couse. We don’t actually know that for sure. ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [May 3, 2012, 1:39pm UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/27 "2012-05-03T13:39:55Z")
> [@Chessic\_Sense](#): > > I want a number, though. How far was the fog that we’re seeing now? How far is that particle now? Was it, say, 13 billion light years away and expansion has added 0.7 billion years to the trip? And now those particles are 30 billion light years away, or what? The Wikipedia page on [the observable universe](http://en.wikipedia.org/wiki/Visible_universe) has the answers you seek. The fog we can see now was only \*\*42 million \*\*light years away when it emitted those photons. But it took **13.7 billion** years for those photons to reach us because they were moving “upstream” against the expansion of space. And because of that expansion, the area of the universe where that fog used to be is now **46 billion** light years away from us. ---
**Author:** ![Balance](https://avatars.discourse-cdn.com/v4/letter/b/ccd318/32.png) [@Balance](https://boards.straightdope.com/u/Balance)\ **Post date:** [May 3, 2012, 2:06pm UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/28 "2012-05-03T14:06:34Z")
> [@Exapno\_Mapcase](#): > > Most scientists believe that the universe went through a period of [inflation](http://en.wikipedia.org/wiki/Inflation_%28cosmology%29) in its first few instants. That expanded space much faster than the speed of light, forever driving some of its contents out of causal connection to our observable universe. This is just a nitpick, but might it be better to say that it expanded much faster than c? I believe there are some theories that suggest that the speed of light was substantially higher during the inflationary period, though I don’t know how well supported they are. ---
**Author:** ![Sister\_Vigilante](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@Sister\_Vigilante](https://boards.straightdope.com/u/Sister_Vigilante)\ **Post date:** [May 3, 2012, 5:15pm UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/29 "2012-05-03T17:15:11Z")
> [@Chronos](#): > > We don’t _know_ what’s out there. But everything that we can see seems to be pretty much the same everywhere (on a sufficiently large scale), and it’d be really weird if, as soon as we can’t see it, it starts behaving differently. It’s sort of like asking how you know that the refrigerator light goes out when you close the door: Fun as a silly question, but it can’t really be serious. > > And we don’t know whether the Universe is finite or infinite. It appears to be flat, and the simplest models of a flat universe imply an infinite one, but it could be positively curved on scales much larger than we can see (sort of like how the surface of the Earth appears flat, even though it’s not), or it could be a slightly more complicated situation that really is flat but is finite anyway. In a way, it’s not controversial at all, since everyone agrees that we don’t know. First of all, this thread is fascinating. I can barely understand it but I’m trying. Why do we think the universe is flat? That’s the first I’ve heard of this. ---
**Author:** ![Candyman74](https://avatars.discourse-cdn.com/v4/letter/c/ad7895/32.png) [@Candyman74](https://boards.straightdope.com/u/Candyman74)\ **Post date:** [May 3, 2012, 5:17pm UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/30 "2012-05-03T17:17:08Z")
> [@Sister\_Vigilante](#): > > First of all, this thread is fascinating. I can barely understand it but I’m trying. > > Why do we think the universe is flat? That’s the first I’ve heard of this. It doesn’t mean flat as in a 2D sheet of paper. It means flat in terms of the curvature of space time. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [May 3, 2012, 5:33pm UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/31 "2012-05-03T17:33:24Z")
> [@Sister\_Vigilante](#): > > Why do we think the universe is flat? That’s the first I’ve heard of this. The Wiki page on [Shape of the Universe](http://en.wikipedia.org/wiki/Shape_of_the_Universe) is a good place to start. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 3, 2012, 6:40pm UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/32 "2012-05-03T18:40:14Z")
Around this point, one should also caution that there are different ways to define distance on cosmological scales, which can yield different values. Not that I’m speaking not of different _ways of measuring_ distance, but different interpretations of what the concept of distance actually _is_. ---
**Author:** ![Moe](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/moe/32/2972_2.png) [@Moe](https://boards.straightdope.com/u/Moe)\ **Post date:** [May 3, 2012, 7:26pm UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/33 "2012-05-03T19:26:39Z")
> [@Sister\_Vigilante](#): > > First of all, this thread is fascinating. I can barely understand it but I’m trying. > > Why do we think the universe is flat? That’s the first I’ve heard of this. I’m with you on that. Fascinating, despite being largely over my head. Here’s [Lawrence Krauss](http://www.youtube.com/watch?v=veU6hK3jMH4) on why he claims the universe is flat. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [May 3, 2012, 8:05pm UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/34 "2012-05-03T20:05:21Z")
> [@Exapno\_Mapcase](#): > > Remember, though, that the universe is everything. The universe doesn’t expand into empty space or any other thing. It just expands. This is the kind of statement that is completely unintelligible. It’s like stating you have a round square. How can it be round and a square? The definition of square defies roundness. How can something expand and not expand into something? How can something expand without getting larger? Bdbdbdbdbdbd. > [@Candyman74](#): > > What’s infinity +1? It’s still infinity. But it’s a bigger infinity. > [@](#): > > Basically, things are moving further apart within that infinite area, so it’s becoming less dense. But there is no “edge” that could be moving away from you, so there’s no way you can say the construct you call the universe is getting “bigger”. It’s a round square. > [@Balance](#): > > This is just a nitpick, but might it be better to say that it expanded much faster than c? Isn’t “c” just an abbreviation for the speed of light in a vacuum? It’s the constant “c” because we believe it is a constant, but if the speed of light in a vacuum were very different in the past, that term would also be abbreviated “c”. The numerical value would be different, and we’d have to change our wording a little (c is no longer a _constant_), but it would still fit in the same formulas as c. We’d just mean a varying number instead of the constant value we currently assign. ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [May 3, 2012, 8:29pm UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/35 "2012-05-03T20:29:04Z")
> [@Irishman](#): > > How can something expand and not expand into something? What we actually observe is a decrease in density. Things in the universe used to be more densely packed and now they’re less densely packed. This is commonly represented as “the universe is expanding”. But we can’t see any edge. We don’t even know if there IS an edge. All we know is that overall the density of the universe is decreasing. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [May 3, 2012, 8:43pm UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/36 "2012-05-03T20:43:34Z")
> [@Irishman](#): > > But it’s a bigger infinity. No, it’s not. Infinity + 1 is exactly the same size as infinity, because infinity is not a number. It’s more of a process. The even numbers are an infinity. The odd numbers are an infinity of the same degree. The even plus odd numbers are an infinity of the same degree. The even and odd numbers plus 0 are an infinity of the same degree. The even and odd numbers plus 0 plus the negative numbers are an infinity of the same degree. This is completely settled math, not physics. > [@](#): > > Isn’t “c” just an abbreviation for the speed of light in a vacuum? It’s the constant “c” because we believe it is a constant, but if the speed of light in a vacuum were very different in the past, that term would also be abbreviated “c”. The numerical value would be different, and we’d have to change our wording a little (c is no longer a _constant_), but it would still fit in the same formulas as c. We’d just mean a varying number instead of the constant value we currently assign. C is used in both ways in normal speak, as the actual speed of light in a vacuum and as the constant that is plugged into equations to make the units come out properly. That’s normally proper shorthand because in most cases they can be treated interchangeably. This would be an exception in which they can’t. If the constant we think of as c is actually a variable then a lot changes. Fortunately, I don’t think that many scientists believe that c was actually a variable even during inflation. ---
**Author:** ![romusz](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@romusz](https://boards.straightdope.com/u/romusz)\ **Post date:** [May 5, 2012, 3:12am UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/37 "2012-05-05T03:12:49Z")
Regarding the speed of light…wouldnt that be proof that “ghosts” exist? If youre just now seeing an object that has been “dead” for millions of years…thats proof enough, youd think? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 5, 2012, 3:41am UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/38 "2012-05-05T03:41:38Z")
Only if you’re defining “ghost” completely differently than most folks do. ---
**Author:** ![Autolycus](https://avatars.discourse-cdn.com/v4/letter/a/22d042/32.png) [@Autolycus](https://boards.straightdope.com/u/Autolycus)\ **Post date:** [May 5, 2012, 8:26am UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/39 "2012-05-05T08:26:55Z")
Yea that reminds me of the episode of Scooby Doo where they pull off the mask of the guy whose been pulling all the stunts in the haunted circus and it turns out the perpetrator was the Crab Nebula all along. ---
**Author:** ![Candyman74](https://avatars.discourse-cdn.com/v4/letter/c/ad7895/32.png) [@Candyman74](https://boards.straightdope.com/u/Candyman74)\ **Post date:** [May 5, 2012, 1:05pm UTC](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644/40 "2012-05-05T13:05:17Z")
> [@Irishman](#): > > This is the kind of statement that is completely unintelligible. It’s like stating you have a round square. How can it be round and a square? The definition of square defies roundness. How can something expand and not expand into something? How can something expand without getting larger? Bdbdbdbdbdbd. Because - as I’ve said many times on these boards - the fact that someone (in this case the worldwide physics community) chooses an imprecise non-mathematical term to attempt to describe something in the poorly-equipped English language doesn’t then mean that you can take that term and use it as a basis for reasoning. It’s just a word chosen to try and describe something which the English language can’t describe well, and the further implications of that word don’t apply. The word itself can’t be the basis of a logical train of thought, because the word itself isn’t accurate. The correct description, unfortunately, is a bunch of math. If you want to apply reasoning to the implications or consequences of a description, _that’s_ the description you have to first fully understand and then work with. Which isn’t something that’s gonna happen on a messageboard thread - or anywhere outside of a very long research paper. It sucks that the universe is so complicated and that we have to use such imprecise language and poor analogies to describe it outside of the math, but there it is. > [@](#): > > But it’s a bigger infinity. I’m afraid it’s not. Infinity doesn’t work that way. If it can get bigger, then it wasn’t infinite in the first place. [Previous page](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644.md?page=1) [Next page](https://boards.straightdope.com/t/a-part-of-the-universe-is-opaque-why/620644.md?page=3)