Status
Not open for further replies.
S

#### skippystars

##### Guest
Hey all,<br /><br />Been a while since I've posted, hope all is well with everyone. I have a question.<br /><br />I was just reading on wikipedia where the diameter of the observable universe is 96 billion light years. I thought the universe was about 13.7 billion light years in radius and thus 27.4 billion light years in diameter (observable). In order for the 96 billion figure to hold then obviously matter must have travelled faster then the speed of light at some point right? <br /><br />So is it 96 billion or 27.4?<br />They mention "comoving" as well.<br /><br />Here is the link:<br />http://en.wikipedia.org/wiki/Observable_universe<br /><br />Appreciate any input.<br />SK

L

#### lukman

##### Guest
It was 27.4bly, now it is 96bly, it was the space expansion, very fast, seems to move the matter FTL -) <div class="Discussion_UserSignature"> </div>

S

#### SpeedFreek

##### Guest
Firstly, if you look under "misconceptions" on that page you will see the figure you quoted, of 13.7 billion light years.<br /><br />It is actually all explained very well on the page, but I will have a go. <img src="/images/icons/smile.gif" /><br /><br />The universe is around 13.7 billion <b>years</b> old. So any light we see has only had that much time to travel, so the furthest that <b>any</b> light in this universe can have <i>travelled</i> is 13.7 billion light years. So you might think our observable universe is 13.7 billion light years in radius, but things are a little more complicated than that.<br /><br />When we look at the most distant galaxies which formed something under 700 million years after the big bang, and thus are around 13 billion years old by now, we see them as they were shortly after they formed. They actually look as if they are only 1-2 billion light years from us, if you go by their size! This is because they were only 1-2 billion light years from us when they emitted the light we see.<br /><br />So, around 13 billion years ago, the first galaxies emitted their light and that light started travelling through space. But that space was expanding as the light made it's journey. Early on it was expanding really fast but the expansion was decelerating. It took around 13 billion years for that light to reach us here, at a place that was only 1-2 billion light years away from the source. The expansion of space has put that galaxy well over 30 billion light years away from us by now (that is the comoving distance, where we estimate that galaxy will be by now).<br /><br />Space had expanded quite a lot before the first galaxies appeared, so the edge of our observable universe is <i>now</i> over 46 billion light years away (comoving distance). This puts the size of our observable universe <i>now</i> at over 92 billion light years in diameter.<br /><br />The figure of 13.7 billion <b>light</b> years, which is often quoted, is very misleading. It is only the ma <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

L

#### lukman

##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p> Early on it was expanding really fast but the expansion was decelerating <p><hr /></p></p></blockquote><br /><br />I thought universe is still expanding at accelerating speed" ? <div class="Discussion_UserSignature"> </div>

S

#### SpeedFreek

##### Guest
It is still expanding and the rate of expansion seems to be accelerating, yes.<br /><br />It started expanding at a much faster rate, and the rate of expansion spent a long time slowing down, but recently the expansion has started speeding up again.<br /><br />It has always been expanding, starting very fast and slowing down, then speeding up again recently. Whilst the expansion was slowing, it never came to a stop as far as I know, so it was always expanding. <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

R

#### robina_williams

##### Guest
How exactly are you defining 'space' in: Space had expanded quite a lot before the first galaxies appeared... <br /><br />What was space, and what was in it, before the galaxies appeared in it?<br />

S

#### SpeedFreek

##### Guest
Here I use the term space to describe the dimensions of the universe, the theatre within which all ineractions occur, the volume within which resides all the mass of our universe. At first, there was no "empty" space (and actually there still isn't! nowhere is space truly empty).<br /><br />According to current cosmology the universe was actually opaque for quite a long time after the big-bang, so the blackness of space didn't appear for quite a while, but everything still resided within a volume of expanding space, the expansion of which caused the contents to cool and go lumpy!<br /><br /> This wiki link covers it all pretty well.<br /><br />A very simplified description would be that at the event we call the big-bang, space and all the matter within it appeared. This rapidly expanding volume of space was filled with a super-heated plasma where all forces were unified at first, but gradually separated as it expanded and cooled. The universe at this time is going through many changes as different reactions take place throughout the subatomic level.<br /><br />After 380,000 years, there comes an important time called <i>recombination</i> where atoms begin to form (before this, everything is pretty much still in a plasma state, with only subatomic particles filling the universe and interacting) and when photons separated from other matter. This is the time when the Cosmic Microwave Background radiation was "stamped" across the universe, the oldest radiation we can detect so far.<br /><br />At this time, large scale structure started to appear, with different densities of matter interacting due to gravity, causing higher densities of matter to be drawn together from lower densities and eventually the earliest stars form out of these areas of high matter density, causing the first galaxies to form something around 750 million years after the big-bang.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

D

#### dragon04

##### Guest
After all, we ARE talking about the "observable" Universe. I can't say with any surety that the Observable Universe is the <b>entire</b> Universe; all I can say is that the most we can see is roughly 13.7 billion years old.<br /><br /> <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>

S

#### SpeedFreek

##### Guest
Indeed. That is where we derive the age of the universe from.<br /><br />One final distinction here, as the "observable" universe is a term that can be confusing in itself. It really means "all we can know about, due to the age of the universe", not "all we can see right now". It also includes the comoving distance, the distance away we think those objects are by now.<br /><br />The "distance" we can see is defined by the speed of light combined with the age of the universe, through space that has been expanding at different rates. Theoretically, our observable universe is only the size it is, due to the time that light has had to travel (or propagate) through that expanding space. Light cannot have travelled more than 13.7 billion light years at it has only had 13.7 billion years to do any travelling in.<br /><br />So what we have is a situation where the most distant objects we see were only a few billion light years away when they emitted the light we see today, around 13 billion years later (they appeared around .75 billion years after it all started). The light from any objects that might have been already more distant at the time <b>has not had time to reach us yet.</b><br /><br />So our <i>observable</i> universe was initially only a few billion light years in radius, and is now up to 46 billion light years in radius, but light (or radiation) has only had 13.7 billion years in which to propagate. So, as time goes on, will we be able to view objects that were initially further away than a few billion light years, as their light finally reaches us? That all depends on whether there <i>were</i> any objects more distant, and whether the early fast rate of expansion will preclude their light from ever reaching us in any measurable way.<br /><br />The picture we have of the expansion comes from our measurements of redshift from different distances. The further we look, the more the redshift increases, but at the furthest distances it increases sharply showing a much faster r <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

D

#### dragon04

##### Guest
IF we are correct in our assessment of the accelerating expansion of the Universe, we have to accept that relative superluminal velocities might be in play.<br /><br />IOW, we may NEVER be able to observe the "real" breadth and age of the Universe.<br /><br />If the expansion of the Universe was subject to an "ultimate speed limit" that was less than superluminal, it would be no surprise that the Universe gets bigger and older based on improved ability to resolve the borders, or time alone itself.<br /><br />This puts our current theories at risk. We work from the assumption that some primordial "force" is "pushing" matter away from some "center" in a way that violates Einsteinian physics.<br /><br />Yet, we haven't identified such a massive energy source that would account for that.<br /><br />We assume that the Universe is being "pushed" apart because we have no other observable explanation.<br /><br />But what if it's being "pulled apart"?<br /><br />If we inflate a balloon at a specific pressure here on Earth at one Atmosphere, we can expect to predict how large the balloon will get, and how rapidly it will do so.<br /><br />However, if we inflate the balloon at the same inflation pressure in a vacuum, it will do so more rapidly. Why? Because the environment outside the balloon, being a vacuum will tend to draw inflation pressure more rapidly to seek equilibrium.<br /><br />IOW, we can inflate a balloon at a given pressure more rapidly in a vacuum than we can at one Atmosphere of pressure.<br /><br />If this principle applies "locally" in regards to our balloon and a vacuum, why couldn't there be an analogous explanation that explains the accelerating expansion of our "Universe Balloon"? Especially in terms of dimensions beyond the 4 (including time) that we prefer to exist in?<br /><br /> <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>

S

#### SpeedFreek

##### Guest
I don't disagree with the idea that the expansion might be caused by something more analogous to a pulling apart rather than a pushing apart. That is simply the model I was using to illustrate the process as it is a little simpler to visualise the universe this way.<br /><br /><font color="yellow"> IF we are correct in our assessment of the accelerating expansion of the Universe, we have to accept that relative superluminal velocities might be in play. </font><br /><br />Yes. There might be objects that we cannot see yet, that were already receding from this point in space at <i>apparently</i> superluminal velocities when they formed and started to emit light. And objects that we see now, that will at some point (either already, or in the future) be receding from us superluminally whilst they emit light.<br /><br /><font color="yellow">IOW, we may NEVER be able to observe the "real" breadth and age of the Universe.</font><br /><br />True, but it is important to remember that the superluminal velocity is only <b>apparent,</b> they aren't really moving faster than light. Indeed, the space closest to them (within the system that is emitting the light) is not expanding at all. Also, outside that gravity bound system, the expansion is always incredibly small at the local scale ( 77(km/s) / Megaparsec right now). So the light from an object receding superluminally can easily start making it's way towards us through it's local space, at the speed of light. The expansion of space only adds distance to the journey at very large scales. Wherever the light is on that journey, it is travelling at the speed of light through local space. The expansion just means it takes longer to get here.<br /><br />The threshold of the distance we can see, the hubble distance, is defined by the age of the universe and the rate of expansion. For the foreseeable future this distance will be increasing as the light from more distant objects (if they existed) creeps within our radius. <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

S

#### skippystars

##### Guest
So in order for this to occur then either the matter at 1-2 billion light years away at the time must have travelled faster then the speed of light or that they were instantly there right after big bang right?<br /><br />I understand the comoving thing, but I want to know how did this happen?<br /><br />SK<br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>When we look at the most distant galaxies which formed something under 700 million years after the big bang, and thus are around 13 billion years old by now, we see them as they were shortly after they formed. They actually look as if they are only 1-2 billion light years from us, if you go by their size! This is because they were only 1-2 billion light years from us when they emitted the light we see. <br /><br />So, around 13 billion years ago, the first galaxies emitted their light and that light started travelling through space. But that space was expanding as the light made it's journey. Early on it was expanding really fast but the expansion was decelerating. It took around 13 billion years for that light to reach us here, at a place that was only 1-2 billion light years away from the source. The expansion of space has put that galaxy well over 30 billion light years away from us by now (that is the comoving distance, where we estimate that galaxy will be by now). <p><hr /></p></p></blockquote>

S

#### SpeedFreek

##### Guest
It happened because of the nature of the expansion during that time. It was a lot faster before the galaxies formed than after. Remember, the redshift of the Cosmic Microwave Background radiation (which signifies how much space has expanded since that radiation was emitted) is z=1089 (at 380,000 years) whereas the most distant galaxies show a redshift around z=9 or so (at something under 1 billion years). That's an incredible difference in the rate of expansion over a period of less than a billion years, for z=1.5 is recession at the speed of light!<br /><br />Also, the figure of z=1089 only represents the rate of expansion after around 380,000 years. If the rate of change was exponential, then it is entirely possible that when working backwards from our data points, an instant after the big-bang, <i>all</i> points in space were receding from each other at just under the speed of light, but this slowed immediately and sharply.<br /><br />Let's say (as a simple but arbitrary example) that after 1 second, the expansion rate was making distances double every second. And that after an hour the rate was making distances double every hour and so on until after a year, distances are only doubling every year. To do the maths properly requires the use of very complicated equations but the upshot of all this is that objects can end up far further apart than light can travel in the same time period. <br /><br />This is exactly the same reason as how the most distant objects can <b>now</b> be more than 3 times distant than light could have travelled in that time. But they aren't themselves moving faster than light, the space in between them and us is increasing in size. This effect was almost immeasurably larger between the big-bang and the time the first galaxies formed, putting them further apart than light could have travelled, even then.<br /><br />So it <i>is</i> actually about the comoving distance, as it was between the galaxies that formed in the first place, when space had exp <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

N

#### nimbus

##### Guest
The previous posts were somehow too difficult to understand, but this last post cleared everything up for me. <br />Thanks, speedfreek. <div class="Discussion_UserSignature"> </div>

M

#### MeteorWayne

##### Guest
It is a difficult subject to get your head around, and often will make your brain hurt.<br /><br />One of the advantages of SDC we have many people who understand things better than most, and have the eloquence to help the rest of us understand these "mental rocks" <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

N

#### nimbus

##### Guest
Hehe, today speedfreek, tomorrow borman! <img src="/images/icons/laugh.gif" /> <div class="Discussion_UserSignature"> </div>

K

#### kelvinzero

##### Guest
I hadnt got this point before. If Im understanding this correctly, space.com missed it too.<br /><br />http://www.space.com/scienceastronomy/070710_distant_galaxies.html<br /><blockquote><font class="small">In reply to:</font><hr /><p>The universe is estimated to be 13.7 billion years old, so that puts the newfound galaxies at 13.2 billion light-years away. A light-year is the distance light travels in a year, about 6 trillion miles (10 trillion kilometers).<p><hr /></p></p></blockquote>

S

#### SpeedFreek

##### Guest
That's the problem really. In order to make the subject palatable to the masses, populist websites (which includes SDC) tend to simplify their explanations. For the true meaning you have to trawl through the scientific papers themselves, which aren't generally palatable to the masses.<br /><br />It's not <i>really</i> wrong to say that the most distant galaxies we see are 13.2 billion light years away, as their light has indeed travelled 13.2 billion light years to get here. But, in truth, the only fact is that the light has travelled 13.2 billion light years. Those galaxies were a lot closer than 13.2 billion light years away when they emitted that light, and are now a lot further than 13.2 billion light years away as we are seeing that light. <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

Status
Not open for further replies.