How big is the universe?

[adam SA]

It is almost universally (lol managed to use the word universe already) accepted that the universe is about 13.7 billion years old.

Then why do i keep hearing sizes of 93 billion light years across (wikipedia), and 33 billion years radius from another documentary, and many more bizarre sizes.

Doesnt this suggest alot of things are travelling faster than the speed of light? away from the "centre" of the universe which occurred 13.7 billion years ago. Does this mean that light from some galaxies has not reached us yet, which to me doesnt make much sense since we originated from a singularity, which kinda suggests we have always been receiving light from them, unless they somehow were travelling faster than the speed of light.

I thought that since the universe was measured at 13.7 billion years old, this would imply that the radius could be no greater than 13.7 billion light years across, does this mean that our laws of physics took "time" years to work itself into a "reality"?

What is goin on here?

Edited February 5, 2010 by adam SA
mispellings

[mulreay]

Sorry I mis-understood the question.. deleted original post

Edited February 5, 2010 by mulreay

[Iggy]

It is almost universally (lol managed to use the word universe already) accepted that the universe is about 13.7 billion years old.

Then why do i keep hearing sizes of 93 billion light years across (wikipedia), and 33 billion years radius from another documentary, and many more bizarre sizes.

Different things are meant by distance,

http://www.astro.ucla.edu/~wright/cosmo_02.htm#MD

and universe,

http://en.wikipedia.org/wiki/Observable_universe#Size_of_the_observable_universe

Doesnt this suggest alot of things are travelling faster than the speed of light? away from the "centre" of the universe which occurred 13.7 billion years ago.

A center isn't an event, but yes things are moving faster than light relative to other things (namely, us). In special relativity no two things have a relative speed greater than c, but in the more general, general relativity, they can exceed a relative velocity of c.

Does this mean that light from some galaxies has not reached us yet

yes

unless they somehow were travelling faster than the speed of light.

yes

I thought that since the universe was measured at 13.7 billion years old, this would imply that the radius could be no greater than 13.7 billion light years across

The radius of the visible universe is less than 13.7 billion lightyears measured in light travel time.

[adam SA]

Cheers for the answer but im still a lil confused.

 

"Originally Posted by adam SA

It is almost universally (lol managed to use the word universe already) accepted that the universe is about 13.7 billion years old.

Then why do i keep hearing sizes of 93 billion light years across (wikipedia), and 33 billion years radius from another documentary, and many more bizarre sizes. "

 

By this i am interpreting "distance" in terms of light years, because i always thought the speed of light was a "universal constant" (by this i know ( i think i know) that the speed of light is changing as the "known universe" increases in "size", but i always assumed the speed of light was a constant relative to the size of the universe so it was a perfect measure of "distance", in terms of "space/time").

 

I also thought that relatively, still nothing could exceed the speed of light, and that light itself posed a paradox that we still cant explain yet (2 light particles travelling away from each other, relatively their combined speed is still only the speed of light? i know this kinda breaks down relativity, is there something i missed?) Do we know of anything actually exceeding this speed other than examples like entanglement etc. (dont want to go into the quantum world yet)

 

I am aware of the "Hyper Expansion" theory by A H Guth. But i thought after a certain amount of "time" (space/time, or whatever it used to be), our interpretations/laws of the physical universe came into place, such as light being the maximum speed. Wasnt this meant to explain why the universe background radiation was so "ordered" as well as explain why the universe has a radius larger than 13.7 billion light years, or has this already been debunked?

I need to do some more recent research, i must be living in the past. oh no!

Im going to read ur articles this weekend,

Thankyou so much for posting the links Iggy.

[StringJunky]

As far as I understand it, it's the distance between Galactic clusters that is increasing at superluminal rates not the galaxies moving themselves. Space can expand at any rate apparently.

 

Think of a balloon with raisins stuck on its surface and as it blows up the distance increases between the raisins (galaxies).

 

The individual galaxies don't expand because their individual gravitational forces overwhelm the force of expansion.

 

This is how you get those figures bigger than the age of the Universe would suggest.

[adam SA]

Thanks StringJunky, you have made a good poing and provided a good example, though:

 

still, it seems to me that one galaxy on one side of the universe would therefore be travelling faster than the speed of light relative to a galaxy on the opposite side of the universe? I understand that the "space" between galaxies is expanding, and so is the "space" inside individual atoms (between electrons and protons etc). This is evident from the amount of Neutrons needed inside an atom to keep it stable (http://algebralab.com/practice/practice.aspx?file=Reading_TheBandOfStability.xml), today more neutrons are "apparently" required to "stabalise" atoms (generally larger ones), and so in the future, elements which are unstable now such as darmstadtium may be able to stabalise, or perhaps even the reverse may be true (dont know how to measure this 1 without alot of "time" to study it).

I Just cant help but to see a paradox here, without saying the laws of physics as we know it took time to exist in the form we perceive today (the speed of light has been exceeded in a relative form)?

because even if you say it is only the "space" expanding, the space between galaxies is therefore expanding (using light years as a measure of distance).

 

Are some galaxies therefore "dissapearing" from our sight?

 

P.S. I am not trying suggesting i know the answer, though i thankyou all for your help in understanding this

[swaha]

is it truely possible to measure the size of universe? for even light dont reach us from some parts.

[mickmeister]

I regularly listen to the Naked Scientists Podcast and this topic came up a few weeks ago. They estimated roughly 160 billion light years wide. I can't recall where I read it, but I read that the universe is expanding a billion miles every hour in all directions.

[swaha]

yah but how did theymeasure it? what's the method?

[adam SA]

I liked this representation of the universe, mainly because they mentioned that we are still redefining our theories, dont take it all for face value, but it is still quiet enlightening.

 

http://www.youtube.com/watch?v=D6CXsvNGkhg&feature=PlayList&p=16F91CAC49800483&index=8

 

The internet is almost the reason for the information revolution, i recommend these 3 movies to anyone interested in the universe, the future or quantum mechanics.

 

 

And this one is good for its easy explanations and its representation of the electron, in that its "minute mass means it can occupy a larger space" (please ignore the its almost funny its so bad animations (althought i am currently doing computer graphics design and cant replicate this yet));

 

 

Again, these are only good representations, take it with a pinch of salt, but worth a watch

[mickmeister]

Read this article on Space.com: Universe Measured: We're 156 Billion Light-years Wide!

[Iggy]

Cheers for the answer

 

Quite welcome.

 

By this i am interpreting "distance" in terms of light years

 

In any units there is more than one kind of cosmological distance. A galaxy moving away from you might send you a beacon of light when it's some distance away. By the time the light reaches you the galaxy is no longer where it was when the light was sent. How far is the galaxy really? The commoving distance is not the light travel time.

 

, because i always thought the speed of light was a "universal constant"

 

Einstein is often quoted on the matter:

In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity.

The speed of light is not globally equal to c in gtr. And, to continue from the previous subject--if there is more than one way to define distance then shouldn’t it follow that there is more than one answer for the speed of a ray of light. Speed is distance / time.

 

I also thought that relatively, still nothing could exceed the speed of light

 

If two rays of light are traveling different speeds then the idea of exceeding the speed of light gets a little more complicated. Nothing in the distant galaxy exceeds its own local speed of light. The link I gave explains very well: http://www.astro.ucla.edu/~wright/cosmo_02.htm#MD

 

and that light itself posed a paradox that we still cant explain yet (2 light particles travelling away from each other, relatively their combined speed is still only the speed of light?

 

That is only paradoxical if you add velocities u+v=w. That’s not how you add velocity in special relativity so it’s not paradoxical.

 

i know this kinda breaks down relativity

 

No, you’re just not doing it right. Read: http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html

 

Are some galaxies therefore "dissapearing" from our sight?

 

Yes. In the distant future the only visible galaxy will be the one you are on.

[swaha]

thanks brother mickmeister.

[Martin]

I regularly listen to the Naked Scientists Podcast and this topic came up a few weeks ago. They estimated roughly 160 billion light years wide. I can't recall where I read it, but I read that the universe is expanding a billion miles every hour in all directions.

 

yah but how did theymeasure it? what's the method?

 

Read this article on Space.com: Universe Measured: We're 156 Billion Light-years Wide!

 

thanks brother mickmeister.

 

That Space.com article was dated 2004. I remember the article when it came out---Cornish, Spergel, and somebody else.

That is not current. I wouldn't go by it. The number is not correct if you are talking about the OBSERVABLE portion of the universe. And if you are talking about the size of the whole universe according to the standard model then there is now a different size estimate as of 2008-2009.

And a different method is being used to measure, from the one in the 2004 paper that got the 156 number.

I have to go, but I think Iggy knows this stuff. Urge reading. I'll get back to this later.

[StringJunky]

 

Yes. In the distant future the only visible galaxy will be the one you are on.

 

Are the forces of expansion sufficient to separate our Local Group..I thought it was between clusters or superclusters?

[Martin]

Are the forces of expansion sufficient to separate our Local Group..I thought it was between clusters or superclusters?

 

No, according to the projections I've seen. Our local group will stay together but gradually merge (on a very long timescale).

 

So "our galaxy" will consist of both the present Milkyway and Andromeda and other stuff merged into a large elliptical (not so pretty as a spiral).

Iggy is right, all we will see is "our galaxy".

 

But the local group will not be pulled apart.

 

Since I'm not an expert in this , I will just point you to Larry Krauss' article on the longrange future. A lot of it is written in clear non-math english. He's a recognized expert in cosmology:

http://arxiv.org/abs/0704.0221

Just go there and click on PDF to download the article.

=========================

 

mickmeister and swaha,

the headline was misleading "Universe measured: we're 156....wide"

the result was a LOWER BOUND.

Like saying "it is at least 300 miles from San Francisco to Paris, France."

If someone determines that the distance is at least X they haven't measured it, in the usual sense.

The 2004 article did not give a handle on the actual size. I think you both realize this, but in 2008 we got a NASA report from the WMAP mission that presented evidence that the U was either INFINITE in spatial extent, or, if space was slightly positive curved, then it was spatially finite but the circumference was AT LEAST 600 BILLION lightyears.

 

I understate the figure the 2008 WMAP report gave for lower bound was somewhat over 600.

 

Now there is a new 2010 WMAP report from which one can derive a larger lower bound. With 95% confidence the circumf is in excess of 800 billion lightyears (assuming it is positive curved so space closes on itself analogously to a sphere.)

And also the evidence continues to be consistent with infinite extent---in other words still much much larger than that.

 

From the 2010 WMAP report one can also derive a 68% confidence lower bound on the circumference, in that spatially closed case. 2pi*13.7/sqrt (.0079) billion lightyears. That comes to 968 billion lightyears. The circumference has to be at least that.

Picture there is no center, no radius, it is the 3D analog of the surface of a 2D sphere with all existence on that 2D surface (so no center exists). That is why I give the circumference.

 

Since it's so approximate, just a 68% likely lower bound, 968 is too precise looking. Let's say circumf at least 900 billion lightyears.

 

If you want the professional article it is Komatsu et al. Ask if you want a link. Currently the trend on size estimates is UP as they accumulate more and more data.

Edited February 9, 2010 by Martin

[Airbrush]

Interesting stuff above, thanks for that. In your estimation, if you traveled in a straight line at the speed of light in one direction, how long would it take before you arrived at your starting point, IF space is slightly curved? Since the universe is expanding, it must be a lot longer than 900 Billion years, or not?

 

If space is not curved, then you will travel towards infinity forever in one direction, right?

[Martin]

If space is not curved, then you will travel towards infinity forever in one direction, right?

 

The simple straightforward answer is yes, definitely! Very few cosmologists bother with weird exceptions to that in the form of flat "pacman" models.

Almost everybody, when they say flat (zero curvature at least on average, after smoothing out local bumps) thinks infinite. Very much the usual idea of infinite space.

 

But there are models that are flat (in the sense of triangles adding to 180 degrees) but have some odd topology. Example take a square and identify opposite edges, so pacman running off to the right reappears coming in from the left.

 

Or take a cube and identify opposite sides, so you get a 3D analog. I think you know those examples already---we've discussed that kind of thing.

 

But basically, ignoring these examples with peculiar topology, the answer is yes.

 

In your estimation, if you traveled in a straight line at the speed of light in one direction, how long would it take before you arrived at your starting point, IF space is slightly curved? Since the universe is expanding, it must be a lot longer than 900 Billion years, or not?

 

You are focusing on the case where space has a slight curvature and is a hypersphere (3D analog of the balloon surface). In that case the circumference COULD be a lot larger than 900. The 900 figure is just a current estimated lower bound.

 

To make the question more definite, let's assume this finite (slightly curved) hypersphere case and moreover assume that the circumference is 900, right at the estimated lower bound.

 

It's a good question. As I understand it you are asking could you ever get around? (And if you could, how long would it take?) Traveling at the speed of light.

 

Intuitively, I would say the answer is no, you could never circumnavigate, because of expansion. The point to emphasize is that we don't know the future rate of expansion, the Hubble rate H(t) is 71 and according to the standard model it is slated to decline to around 61 (km/s per megaparsec) and what 71 means is that a distance increases one c for every 13.7 billion ly. The distance 13.7 is increasing at rate c. The distance 27.4 is increasing at rate 2c. The distance 41.1 is increasing at rate 3c.

 

So the distance 900 is increasing at rate 900/13.7 c. That is 65.7 c. In that case you would have to travel faster, say 66 times the speed of light, in order to make any progress at all, and the net progress towards the goal would be fairly slow.

 

The Hubble rate is slated to decrease to an asymptotic value around 61, and what that 61 means is that instead of using the figure of 13.7 we get to use a figure of around 16, so the break-even speed is 900/16 or around 56 c.

Edited February 9, 2010 by Martin

[adam SA]

Still havnt been ble to read all your posts, doing it now, but doesnt relativity imply you make one the "constant", and measure the other velocity relative to the "constant"?

Can someone pleacse explain to me three things.

 

If one light particle were measured in velocity relative to another with an equal and opposite velocity, why is it not therefore travelling faster than the speed of light (please explain so i can understand )?

 

Does infinity actually exist in our universe anywhere that we actually know, because we know blackholes radiate off energy and slowly die (i think in gravity waves), Is the universe really infinitely big, or is this just a theory (because apparenlty we can measure it)?'

 

And i dont really differentiate infinity from zero, so does nothing really exist anywhere (how do u measure nothing, space/time and Higgs felds always seem present)?

---

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Consecutive posts merged

intuitively, I would say the answer is no, you could never circumnavigate, because of expansion. The point to emphasize is that we don't know the future rate of expansion, the Hubble rate H(t) is 71 and according to the standard model it is slated to decline to around 61 (km/s per megaparsec) and what 71 means is that a distance increases one c for every 13.7 billion ly. The distance 13.7 is increasing at rate c. The distance 27.4 is increasing at rate 2c. The distance 41.1 is increasing at rate 3c.

 

Cheers for that Martin, put it in a way i understand, very enlightening.

---

Merged post follows:

Consecutive posts merged

Just watched this, was pretty enlightning in the way it showed light travelling three dimensionally not just "forward or backward", and it makes sense since it is vibrating.

[Iggy]

Are the forces of expansion sufficient to separate our Local Group..I thought it was between clusters or superclusters?

 

No, according to the projections I've seen. Our local group will stay together but gradually merge (on a very long timescale).

 

I agree. Sorry it took me so long to respond, although I'm glad I waited--excellent explanations Martin

 

If one light particle were measured in velocity relative to another with an equal and opposite velocity, why is it not therefore travelling faster than the speed of light (please explain so i can understand )?

 

Think of velocity as being measured with a measuring tape and clock. For example, a baseball might go around 30 meters along a measuring tape while a clock advances 1 second. The thing about relativity is that clocks and measuring tapes change the rate they run and the distance they measure as they change velocity. So while the person throwing the baseball might measure it at 30 m/s someone passing the field in a rocket might measure the baseball at 10 m/s relative to the pitcher with their tape and clock. The distance and time are relative to the velocity of the person measuring them.

 

Your thought experiment has two things moving away from a central observer,

 

A <----- B -----> C

 

There are three frames of reference. Intuition would say that A measures the velocity of C the same as B measures the velocity of A plus the velocity of C. In other words, A measures V_C the same as B measures |V_A| + |V_C|. If B measures V_A at -1c (negative meaning it’s moving to the left) and V_C at 1c like your thought experiment then that would be:

 

V_C' = |-1c| + |1c| = 2c

 

But, that would ignore time dilation and length contraction which are key to special relativity. Just because B says that C is moving at some speed that does not mean C is moving that speed relative to B in A’s reference frame where clocks and measuring tapes work differently.

 

The correct relativistic formula which accounts for relative time and distance is:

 

[math]{V_{C}}'=\frac{V_{C}-V_{A}}{1-\dfrac{V_{C} \times V_{A}}{c^2}}[/math]

 

where [math]{V_{C}}'[/math] is the velocity of C relative to and measured from A. Both [math]V_{C}[/math] and [math]V_{A}[/math] are relative to and measured from B.

 

If A and C are photons like you say where c=1 (in light units) you’ll get:

 

[math]{V_{C}}'=\frac{1-(-1)}{1-\dfrac{1 \times -1}{1}}[/math]

 

[math]{V_{C}}'=\frac{2}{2}=1[/math]

 

So it is not paradoxical if you add velocities using the relativistic velocity addition formula. The formula works because space and time are relative to velocity. In other words, taking into account that two observers in different reference frames will not measure distance and time equally.

 

I hope this helps, and also this really is a great page: http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html

 

and also:

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel2.html#c2

 

Does infinity actually exist in our universe anywhere that we actually know, because we know blackholes radiate off energy and slowly die (i think in gravity waves), Is the universe really infinitely big, or is this just a theory (because apparenlty we can measure it)?'

 

I wouldn't know how to answer if infinity actually exists in the universe. I am philosophically of the opinion that if something is real and exists then it can be measured and I don't think infinity, as a quantity, can be measured. But, I really don't feel qualified to say.

 

As far as the universe being infinitely big I would definitely say that is not certain. It is implied by general relativity and standard cosmology if the universe is homogeneous and flat or negatively curved. But, we don't know and may never know (as far as I know ) the geometry of the universe outside of our visible neighborhood.

 

And i dont really differentiate infinity from zero, so does nothing really exist anywhere (how do u measure nothing, space/time and Higgs felds always seem present)?

 

If I understand then I agree. The ideas of actually measuring something infinitely large or infinitely small seem problematic. That's not to say that infinity isn't a very useful concept, just that actually measuring such a quantity seems kind of like a contradiction in term.

Edited February 11, 2010 by Iggy
typo

[StringJunky]

Iggy: The last two quotes are not attributable to me.

 

Martin: Thanks for the link in response to my question to Iggy...it was excellent.

[Iggy]

Edited with apologies

[StringJunky]

Edited with apologies

 

No prob.

[mulreay]

Still havnt been ble to read all your posts, doing it

Does infinity actually exist in our universe anywhere that we actually know, because we know blackholes radiate off energy and slowly die (i think in gravity waves), Is the universe really infinitely big, or is this just a theory (because apparenlty we can measure it)?'

 

This is a problem that faces many issues. Does infinity exist? The problems I have seen are as Iggy has stated..

My knowledge is limited so please bare with my rambling.

By it's very nature you cannot judge something as infinite as you would never come to a conclusion e.g It is not possible for us to travel infinity so what is to say that somewhere up ahead is not the end? If we could travel infinity we would always be held by the fact that the journey may meet an end, (may or may not), proving infinity does or does not exist. So you enter a sort of paradox I suppose.

Even examples people have tried to attribute to infinity are flawed by there very nature.

For example the saying, 'if you had an infinite amount of typewriters and an infinite amount of Monkeys, one would type out the complete works of Shakespeare'. This is not true, an infinite amount of monkeys would type out an infinite amount of works of Shakespeare.

That actual sums have been done for this event they would be along the lines of 10 to the power of 10million zeros or you winning the lottery every week for 29,000 consecutive years. That gives some aspect of the numbers involved.

Many people especially mathematicians find Infinity a sore point and generally discount it. But as we can never reach infinity (Fact) as it's just a name not a thing we will never know. Then again many people have battled the idea of infinity in the universe to suggest that the universe in essence completes a circle, (the universe curves back on itself) the circle being the ultimate idea of infinity well apart from the ∞ of course.

 

As always My amount is little, but my support is sincere.

[Sisyphus]

This is a problem that faces many issues. Does infinity exist? The problems I have seen are as Iggy has stated..

My knowledge is limited so please bare with my rambling.

By it's very nature you cannot judge something as infinite as you would never come to a conclusion e.g It is not possible for us to travel infinity so what is to say that somewhere up ahead is not the end? If we could travel infinity we would always be held by the fact that the journey may meet an end, (may or may not), proving infinity does or does not exist. So you enter a sort of paradox I suppose.

 

That's not a paradox, it's just a limitation of measurement. In other words, you can't show that something is infinite by measuring it, you can only show that it is larger than the scope of measurement. Similarly, you can't say something is instantaneous, only "at least x fast," where x is fastest you are capable of measuring. Or "at most x large," etc. Science is full of this, and sometimes the measurement capability expands. This happened with the speed of light, for example.

 

Of course, not everything depends purely on measurement. You can still deduce that something must be infinite, though I can't think of any examples outside of mathematics.

 

For example the saying, 'if you had an infinite amount of typewriters and an infinite amount of Monkeys, one would type out the complete works of Shakespeare'. This is not true, an infinite amount of monkeys would type out an infinite amount of works of Shakespeare.

That actual sums have been done for this event they would be along the lines of 10 to the power of 10million zeros or you winning the lottery every week for 29,000 consecutive years. That gives some aspect of the numbers involved.

 

What numbers are those? No finite number of monkeys will guarantee any particular result in a finite amount of time. And it's possible, just extremely unlikely, that a single monkey could type them all up in one afternoon. And a single, immortal monkey would suffice to guarantee success.