# Size and age of the universe

• Physics-Learner
I think this analogy illustrates what I am trying to say. In summary, scientists think that the observable universe (that we can see and measure) is only a small fraction of the universe, which exists beyond our current technology and understanding. We don't know if there is an 'end' to this expansion or not, or if it will continue indefinitely.f

#### Physics-Learner

none of the forums seemed to be an exact match for this question, so i placed it in the most general forum.

i don't understand why scientists think they have an accurate answer to these questions. i understand that they are using the doppler effect with regards to light waves.

but according to their results the diameter of the universe is already more than double the speed of light, in light years.

so it would seem that we would have to conclude that space itself is expanding faster than the speed of light. or at least was at some point in time.

plus we know that the rate of this expansion is still increasing.

and we would no doubt already have areas of space that are receding from us at faster than the speed of light, which means that we will never have access to those areas, assuming that we can't receive any information faster than the speed of light.

so it seems to me that we may have very accurate numbers regarding the "observable universe", but we don't have even the slightest clue as to what percentage of this observable universe makes up the entire universe, and never will.

Without reading all you post, I suggest that you take note that scientists estimate the size of the visible universe, the word visible is always there.

What up Physics Learner,

You are for sure correct in saying that the Universe is expanding (which has been verified in support of various evidences) and that it is expanding faster than the speed of light, which is expected, according to General Theory of Relativity. Now, while you might very well be correct in stating that we can’t experimentally verify the diameter of the entire visible universe, we can extrapolate mathematics from current theories in conjunction with empirical observation to derive what we believe to be, a fairly accurate number (given the current parameters of technology and our understanding of physics and mathematics). We have peered back roughly, 13.6 billion light years into time, using the Hubble Deep Field Survey in accordance with a few other orbiting satellite telecsopes. I am not sure if you are having trouble visualizing this expansion or not, as your post seemed vague. However, if you are, perhaps I can describe a very simply analogy that might help you to understand. Keep in mind, this is not technical, merely a conceptual way to imagine this process.

Imagine an infinite rubber sheet stretched to infinity, so that from your vantage point on the sheet, it extends into the horizon, in all directions. This can be visualized in similar fashion to being on a ship in the middle of the ocean, in all directions the ocean extends into where the sky meets the water. This is the fabric of the space-time background. Now, imagine that we place an infinite amount of people at fixed positions, so that when you look into the horizons, you see people distributed at fixed points. The people represent our galaxies. Now, we have constructed our very simple, ideal Universe.

Imagine that the rubber sheet is pulled with equal force, evenly, in all directions. From your vantage point, you see the individuals at fixed points, recede away from you equally, as if they were moving. We continue to pull the infinite rubber sheet until everyone has receded away from you and from your vantage point, you appear at the center of the Universe. The center is an illusion and from each individual’s vantage point, they appear to be at the center of the universe, however, there is no center.

We can continue to pull the infinite rubber sheet in all directions, causing a continuous recession between the individuals on fixed points. Theoretically, we could imagine this simulation run forever until everyone has receded infinitely away from one another. We have allowed for the movement of bodies faster than the speed of light, without violating the constant speed of light as the background itself expanded and everything ‘on it’, merely moved ‘with it’. If we add light into our thought simulation, we can see that as the rubber sheet expands, the light must also cover the distance of the expansion, whereas the bodies themselves, are fixed and move in reference to the background itself. This is how the background of space-time can expand faster than $$C$$ and why $$C$$ has to traverse this constant expansion and why bodies of matter do not.

We can then, rewind this simulation, so that the infinite rubber sheet contracts continuously, until each individual is next to each other. We can continue to contract the infinite rubber sheet until each individual is stacked one on top of the other and this represents our 'cosmological singularity'. I am not sure whether or not there really was a 'singularity' as that seems physically impossible (in the sense that the singularity has some infinite property), however, it might help to visualize the process, I don't know.

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hi hootenanny,

http://www.space.com/scienceastronomy/mystery_monday_040524.html

"visible" is mentioned rarely. people want to know how old the universe is and how big it is - not how old and big the visible part is to us.

especially since the visible part could be only the tiniest fraction of it. how interesting is that ?

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hi complex,

i do not disagree with anything that you said. and i suspect that our measurements could be accurate with regards to the "visible" universe.

i was referring to the "entire" universe. i think we think we know a lot more than we do. LOL.

Perhaps we should make a distinction about what defines the 'visible' universe and what constitutes the 'entire' universe, so that we can remain consistent, homie!

PAYCCCCCCEEEEEEEEE!

i don't know that we can make any distinction. we know a lot about the "visible" universe. we don't know anything about whatever part of the universe is no longer visible to us. the entire universe is all that is in this universe. the visible part is that which we have access to, by means of light reaching us.

Why do you presuppose the existence of a 'non-visible' universe and what are the contents of this region of the Universe? Quantum entities, strings, loops, particles?

I understand that by 'visible' universe, you are referring to what we can directly observe by means of the electromagnetic spectrum (not necessarily just visible light, correct?), however, you assume the existence of some other aspect of the universe that we can not perceptually 'see' and treat it as more real than the region that we can see? As far as I know, a 'non-visible' universe exists only theoretically, to explain dark matter/dark energy and/or Quantum Mechanics, correct. I might be wrong about that though.

i certainly don't know anything for sure. but it seems likely that due to the rate at which space has already expanded, and that the rate is increasing, that most of the universe is expanding away from us at greater than C, so we can never have access to it again.

why does that part of the universe have to be any different than our part ? i did not mean to imply that it was. in fact, i am thinking that it is no different. it consists of whatever we consist of. we just can't verify its existence with certainty, because it is expanding away at faster than C.

but again, it seems likely - due to what we do know about the expansion of our visible universe. (yes, any sort em, not just light).

thanks hoot,

i know it is sometimes mentioned, but most of the time it is not. most people are interested in the entire universe, whatever that may be.

if we were told that an infinitesimal portion of the universe is 156 billion light years in diameter and 14 billion years old (or whatever the latest numbers are), it would not be interesting. it would be like talking about the size of a grain of sand, but not knowing anything about the size of the beach that it lays on.

i am having these same sorts of discussions with jesse over at relativity. from a personal desire, i want to know what is, not what we perceive things to be. i suspect that this is beyond our ability to ever do.

The link provided by hootenanny, describes the 'visible' Universe as the region around the Earth, that has had enough time for light and radiation emitted from objects, to reach our vantage point, e.g. the surface of last scattering. It also describes the notion that at each distinct vantage point, a different 'visible' Universe emerges.

So, what you are referring to then, is the ENTIRE universe, which we have no experimental verification of, other than indirectly. So, when we say the 'visible' universe is 90 billion light years in diameter (or whatever it is approximated at right now), we are only referring to objects that we can observe at the surface of last scattering.

If this is logically sound, then we are correct in our approximations of the 'visible' universe as it only refers to this particular vantage point, unique to the Earth. The 'non-visible' universe you are referring to, would not be directly seen by us, even if we had the technology because there hasn't been sufficient time for the light to scatter.

thanks hoot,

i know it is sometimes mentioned, but most of the time it is not. most people are interested in the entire universe, whatever that may be.

if we were told that an infinitesimal portion of the universe is 156 billion light years in diameter and 14 billion years old (or whatever the latest numbers are), it would not be interesting. it would be like talking about the size of a grain of sand, but not knowing anything about the size of the beach that it lays on.

That would be very interesting indeed, though. If we said an infinitesimal portion of the universe was 156 billion light years in diameter and we had the ability to distinguish our portion as a 'grain of sand' buried in a 'beach', then we know that we live in an extremely large Universe. That is VERY interesting and has implications on our existence.

The notion that our visible portion of the universe is approximated somewhere between 78 to 90 billion light years in diameter (those are the numbers I saw last), then we know our universe is already extremely large. Unimaginably large, in fact. This is interesting.

The fact that we don't have access to the 'other portions of the universe' or the 'non-visible' portions, is irrelevant since all we can do is speculate that it even exists.

That would be very interesting indeed, though. If we said an infinitesimal portion of the universe was 156 billion light years in diameter and we had the ability to distinguish our portion as a 'grain of sand' buried in a 'beach', then we know that we live in an extremely large Universe. That is VERY interesting and has implications on our existence.

The notion that our visible portion of the universe is approximated somewhere between 78 to 90 billion light years in diameter (those are the numbers I saw last), then we know our universe is already extremely large. Unimaginably large, in fact. This is interesting.

The fact that we don't have access to the 'other portions of the universe' or the 'non-visible' portions, is irrelevant since all we can do is speculate that it even exists.

ok if all that's true
then there is far more mass outside our visible bit of the univerce
then inside
so could the mass outside pull us in the observed expansion
without dark energy being the cause or part of the effect anyway
could the so called dark matter be real matter too faraway to allow
the light to reach us
or how does real matter outside our vision limits effect the given percents of real matter vs dark matter and dark energy in the total univerce

ok if all that's true
then there is far more mass outside our visible bit of the univerce
then inside
so could the mass outside pull us in the observed expansion
without dark energy being the cause or part of the effect anyway
could the so called dark matter be real matter too faraway to allow
the light to reach us
or how does real matter outside our vision limits effect the given percents of real matter vs dark matter and dark energy in the total univerce

I am not sure I follow. I didn't make any truth statements regarding the validity of any theories, results, observations or experiments. The only number I quoted, was an approximation from memory regarding the diameter of the visible universe and the definition of 'visible universe'.

What you quoted was my perspective regarding a hypothetical universe, in which we can transcend the barriers of perception and abstractly construct all of the axioms of our ideal universe. He was generating a universe, in which we know for certain, that we don't know about very much of our universe.

Realistically, we are not certain if there exists anything beyond our visible range, other than what emerges mathematically or theoretically, although I am sure that will change very soon.

I am not qualified enough to discuss dark matter and dark energy (nor is that my primary field of interest), so I will reserve that for someone else.

hi complex,

let me restate something, because i don't think you are grasping what i am intending to communicate to you.

you said something about the light not having enough time to reach us.

what i am saying is that this light will never ever reach us. we are running away from it at faster than the speed of light. so every second, that light keeps getting further and further away, even though it is traveling at the unbelievable speed of C.

i say this seems likely because - from the article i posted, the diameter of the visible universe is 156 billion light years, while the age of it is only 14 billion years. space is expanding at greater than the speed of light, has been for 14 billion years, and the rate that matter is expanding from other matter is increasing. i didnt say that the distance from us is increasing, which it is. but i am EMPHATICALLY saying that even the rate is increasing. and that rate is some humongous number already, and has been for a very long time.

so it seems very likely that there is a part of the universe which is hidden from us now, and always, unless we are unlucky to see it collapse. but you and i will be dead by a few billion years if that were to happen. LOL.

so not only does it seem possible, it seems extremely likely that not only do we have a part of our universe outside the visible one, but that most of the entire universe may be outside of the visible one. and my example of a grain of sand may not be farfetched at all, when looking at all the other numbers.

most people are interested in this type of stuff on some sort of spiritual level. we want to know what sort of world we live in, our purpose, etc. if in fact we only have access to a very small portion of our universe, it is startling to say the least. especially when we look at the astonishly, overwhelmingly size of the visible universe. LOL. it makes us look awfully insignificant.

this is why i am interested in what is, and not what we perceive things to be.

but unless we prove einstein wrong, and we have some way of getting info faster than C, we aint ever ever going to know any more about what lies beyond the visible universe than we do currently - which is zero, today.

that is disappointing to me, but i don't see any way around it - at least not in my lifetime. LOL.

btw, i only saw the 7 physics forums when i made my original post. now i see the whole forum screen - very impressive. it reminds me of my college days when some of it made some sense - LOL.

Seeing objects receeding faster than the speed of light is not a problem - which includes all objects with a redshift [z] more than about 1.6. We routinely observe objects in visible wavelengths up to z~10. We also routinely detect photons in the microwave band that were emitted from the surface of last scattering [i.e., the CMB], which is receeding at z~1100. This is perfectly fine under the rules of GR. Take a look at:
http://www.astro.ucla.edu/~wright/cosmolog.htm

We can see as much of the universe now as we could ever have seen since one Planck tick of time after the big bang - always have, and always will. Distant objects do not disappear from view, they merely redshift and time dilate into frozen obscurity.

i looked at the site, but could not find it. is it in one of the faq's ?

but it does not make sense to me that we could see light from a star moving away from us at faster than C.

that does not mean that we can't see light from the star in the past, which is basically what we see when we look thru a distance. we are seeing light that was emitted sometime in the past.

but the star, if it is still alive today, and moving away from us at faster than C, i don't think the light from that star emitted today will ever be seen by us.

and if the universe is as large as i think it may be, then there would likely be stars that were newly formed in places where the space was already receding away from us at faster than C, and therefore never see the light from those stars, and have no idea whatsoever how large the entire universe actually is.

in fact, it seems to me that if we could see light from any distance at any rate of expansion, there would be no need to use the term "visible universe".

Instead of a sheet why not think of a one dimensional object such as time expanding in all directions, I would picture it more in the shape of a sphere. Time has to be expanding at a speed greater than light because all motion is through time or if you will time contains all motion.

Instead of a sheet why not think of a one dimensional object such as time expanding in all directions, I would picture it more in the shape of a sphere. Time has to be expanding at a speed greater than light because all motion is through time or if you will time contains all motion.

You lost me dude.

Time is the sheet; Space is the movement within the sheet.

So time is the sheet, and then space would be the people fixed on the sheet? Am I confusing your analogy? It's pretty insightful but I am retarded, doood.

I was trying to illustrate the expansion of space, the movement of essentially, static bodies of matter in relation to the expansion of space and finally, how light would traverse this expansion.

I probably did a poor job though so could you expand because I am stupid.

I probably did a poor job though so could you expand because I am stupid.

I do not nor did not think of you as stupid, I am sorry if my words offended you. I seen your analogy and thought about time, it is always expanding like the rubber sheet. Space on the other hand is pure motion, we get most of our information through the movement of EMR, hence the reference to as being the motion through time. I understand that both space and time are tied at the hip but I still like to think of them in duality, both with different identities. Time is seamless, we can take a picture of a duration and plot out all motion in it, but it does not stop the flow of time in the same way that I can affect the flow of motion or light through my lens. As for some force pulling the sheet to get it to stretch I see it as all durations expanding at the same rate.

PhysicsLearner, You are making a physics mistake here, usually SpaceTiger or Hellfire corrects this kind of mistake. But I will this time because they don't seem to be around and I happened to notice.

Indeed according to standard cosmology (LambdaCDM) there are parts of universe which we will never get light from no matter how long we wait but that's not the same as what is receeding at > c
... is expanding away from us at greater than C, so we can never have access to it again.

...verify its existence with certainty, because it is expanding away at faster than C.
...

This is just a physics issue. It IS possible for us to get light from objects which are at present receeding >c due to expansion of space. This was explained in a SciAm article by Lineweaver and Davis and also in an article about common confusions and misconceptions people have about the expoansion of space, an article called "Expanding Confusion" which is online.

Lineweaver is one of the leaders of COBE (along with Smoot and Mather who just got Nobel)---what I am telling you is standard mainstream consensus cosmology.

As an illustration, consider this: most of the objects we are now getting light from are at redshift z > 1.5.
Light which arrives to us today with redshift > 1.5 was EMITTED WHEN THE object was already receding from us at > c. But it got here nevertheless.

That is, most of what astronomers can see is objects which were receding faster than light when the emitted the light that we are getting from them.

The reason it is physically possible is actually fairly simple. Even tho expansion is accelerating the Hubble parameter H is still decreasing. As it decreases the Hubble radius which is c/H INCREASES (as the reciprocal of a decreasing quantity). The Hubble radius is the current distance from us of points which are receeding at exactly c. And as it increases it encompasses more and more photons which have been "trying to get to us but not making it" like fish swimming against the current.

Once a photon is safely inside the Hubble radius it cannot fail to arrive here because inside the Hubble radius space is receeding <c. The closer it gets, the easier it is for the photon to make headway because it has less recession-speed to cope with.

So it is physically possible for us to eventually receive a signal sent TODAY from some galaxy which TODAY is receding at >c.
================

In a philosophical sense you are quite right to suppose that there is some horizon. it is simply not the horizon you mentioned.
There IS a distance such that if an object is today beyond that distance, and sends light to us today, that light will never reach us even if we could wait an unlimited time for it.
that is part of the "accelerated expansion" business---the positive cosmological constant Lambda people speak of in terms of "dark energy".
We have that cosmological horizon because Lambda is positive.

Your mistake was that you put the horizon at the wrong distance. you confused it with the Hubble radius (the distance beyond which things receed >c). The true horizon distance is greater than the Hubble distance, for the reason I mentioned.

================
just an afterthought. assuming you know some calculus and know what the scalefactor a(t) is.
to say there is expansion is to say a'(t) is positive
to say expansion is accelerating is to say the second derivative is positive, a''(t) > 0.

but the Hubble parameter H(t) is, by definition, the ratio ( a'(t)/a(t))
This can DECREASE even while a'(t) is increasing, as long as the denominator of the fraction is increasing fast enough to make the fraction decrease.
So even if there is some gradual acceleration, the Hubble parameter can still be decreasing!

And, in fact, at the present epoch H(t) is decreasing---though not as rapidly as it did during the early universe.

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hi marcus,

thanks for the post. my calculus is is 30 years in the past. LOL.

i did read the article - can't say that i understood it all. but for our discussion, this limit is being referred to as the "particle horizon" ? that which we can't see beyond. is this correct ?

with regards to my suppose - even with current cosmology thinking (not to say that it or gr is totally correct), we still will never know if there is anything outside of this particle horizon ? if so, that still leads us to the same problem - having no idea whatsoever how large the universe actually is, just a good idea on how large the universe is up to the particle horizon.

... but for our discussion, this limit is being referred to as the "particle horizon" ? that which we can't see beyond. is this correct ?
...

there are two relevant terms

On one hand there is the particle horizon (which has not reached its maximum extent yet)
http://en.wikipedia.org/wiki/Particle_horizon

The particle horizon distance is constantly increasing as light from more places reaches us.

By contrast, the cosmic event horizon distance is a fixed distance (it is infinite in some models but finite in others. In a given model it doesn't change).
http://en.wikipedia.org/wiki/Event_horizon

here are some quotes from these articles

"...The particle horizon differs from the event horizon in that the particle horizon represents the largest comoving distance from which light could have reached us by now, while the event horizon is the largest comoving distance from which light can ever reach the observer at any time in the future."

"... For events beyond [the particle horizon], light hasn't had time to reach our location, even if it were emitted at the time the universe began. How the particle horizon changes with time depends on the nature of the expansion of the universe.

"...If the expansion has appropriate characteristics, there are parts of the universe that will never be observable, no matter how long the observer waits for light from those regions to arrive. The boundary past which events can't ever be observed is an event horizon, and represents the maximum extent of the particle horizon.

Examples of cosmological models without an event horizon are universes dominated by matter or by radiation. An example of a cosmological model with an event horizon is a universe dominated by the cosmological constant (a de Sitter universe)..."

I am not sure these Wikipedia articles are as good as they could be. Maybe I can think of an description that I like better later today. But this is approximately right.

We CAN see past the present particle horizon if we just wait patiently for more light to come in.

IIRC the particle horizon distance is currently around 45 billion LY
and the event horizon distance is estimated at 55 or 60 billion LY.
===============
have to go, back later

in your previous post, you mentioned dark matter. it seemed as if you were using that term to define that matter which is beyond the EVENT HORIZON. i hope i am using these terms correctly. so dark matter is just matter that is so far away that we will never be able to see it.

is this how you are defining dark matter ?

i never thought about it like that. for some reason, i was thinking that dark matter was thought to be spread evenly everywhere, but it was something that we had not learned to detect. we really don't know for sure that it exists. we just think it does because it helps us with some gravity issues.

but if it does exist, i guess i would have thought that it existed somewhat evenly spread, just as regular matter - so that some of it would be inside the event horizon, and some of it outside.

at this point, we can not definitively say there is any matter outside the event horizon, nor can we say that dark matter exists at all. the former seems likely to me. i don't have enough knowledge about dark matter to render a reasonably informed conclusion.

i don't have the same level of confidence in gr that most scientists have. my gut just tells me that einstein will be proven wrong about a lot of it. i do not think spacetime exists, as a real thing. and i do not think that einstein's explanation of gravity being paths of least resistance is correct, either.

i was having an excellent conversation with jesse in the relativity forums. i think relativity is a good tool for us to use. but what the general population does not understand is that it is all about measurements. we measure things differently, based upon our velocity, even if the velocity is constant.

what interests me the most is what is, not how we measure it to be.

some things come to mind as being extremely important clues, but they may always be beyond us.

our measurements change as our velocity changes. all of our measurements come to us at a velocity, which is thought to be the limiting velocity of matter. as we come closer and closer to moving at the velocity that information is sent, we get stranger and stranger results. time and space approach zero, in the direction of the velocity (from physics, i recall that velocity is actually a vector, in the sense that it contains both a speed and a direction).

so time seems to be intricately connected to speed. and then our measurement of time disappears altogether if we could actually travel at the same speed that information travels.

so as i told jesse, i think there are some fairly solid suspicions that time is not a real thing, in and of itself. it may exist because motion exists.

and spacetime is a tool that may work, but only because information is not instantaneous. if info was instantaneous, then the whole universe would always be visible, and our view and understanding of it would be considerably different, imo. now, when we look at a picture of the cosmos, we see it invarious time stages. if we could see a picture of how all of it looked at any particular moment, we would more than likely have a lot different opinion about many things that i think we are overly-sure of today.

Newton had equations that were pretty accurate in prediction the results of gravity. but today's physics no longer thinks of gravity as an innate attraction of matter, acting at a distance. i think we will find the same thing about most aspects of relativity. but it may be hundreds of years, if ever, before we make a really humongous leap in our understanding (a leap like einstein over Newton, say.)

it is my opinion that there are some things about the universe that are forever beyond our understanding, no matter what tools or knowledge we have.

What about a star at a distance of 8 billion light years, that was formed 4 billion years ago? Is that dark matter?

We can't see it, because the light hasn't gotten the time yet to reach us. Is this star part of the "visible" universe?

Is the visible universe merely limited by distance or is it literally that what we can actually see?

Seeing objects receeding faster than the speed of light is not a problem - which includes all objects with a redshift [z] more than about 1.6. We routinely observe objects in visible wavelengths up to z~10. We also routinely detect photons in the microwave band that were emitted from the surface of last scattering [i.e., the CMB], which is receeding at z~1100. This is perfectly fine under the rules of GR. Take a look at:
http://www.astro.ucla.edu/~wright/cosmolog.htm

We can see as much of the universe now as we could ever have seen since one Planck tick of time after the big bang - always have, and always will. Distant objects do not disappear from view, they merely redshift and time dilate into frozen obscurity.

I am curious what objects you are referring to that we routinely observe in visible wavelengths of up to z~10. As far as I know, the highest redshift observed of a galaxy is a bit under z=7. Did Ned Wright say this or where did you get this information?

Correct. Around 6.8 is still the reigning champion. I merely wished to emphasize the point we can [and forever will] continue to see objects receeding at velocities >c.

What about a star at a distance of 8 billion light years, that was formed 4 billion years ago? Is that dark matter?
It is not, because stars usually form in giant molecular clouds that are part of the interstellar medium within galaxies. The density of the interstellar medium is known and is assumed to comprise about 5% of the mass of the visible stars in a galaxy. Simulations and some observations suggest that most of the baryonic dark matter is located in the intergalactic medium. Moreover, most of the dark matter that contributes to the rotation curves of galaxies is assumed to be non-baryonic.

We can't see it, because the light hasn't gotten the time yet to reach us. Is this star part of the "visible" universe?

Is the visible universe merely limited by distance or is it literally that what we can actually see?
The term visible universe or observable universe is independent of our current tecnology and relates to the current distance from which the most distant photons we should be able to detect today were sent.

PhysicsLearner, I didn't mean to suggest that dark matter is matter that is beyond the event horizon (whatever that is estimated to be, something like 60 billion LY IIRC)

dark matter is a "different matter"-----our own galaxy preumably has lots of it, if that turns out the be the correct explanaition of how galaxies manage to whirl so fast and yet hang together

now hellfire (who is the guru) has checked in so you should address your questions to him and the others. I'll migrate to another forum where I can be of more use

hi marcus,

i wasnt sure if you were suggesting that or not. but i guess our understanding is the same, in that if it exists, then it is everywhere. this is what i had read and seen it to be.