We can see to the edge of the universe.

[Spyman]

The Cosmic Microwave Background Radiation is not from some area constantly emitting it, it was emitted from every region in the universe during a very short event in the past. From the closer regions from us, the CMBR has already passed us and from more remote distances it is still on it's way towards us. It has to be this way, because otherwise it should already have been long gone and no longer be observable.

 

"The CMBR is well explained by the Big Bang model – when the universe was young, before the formation of stars and planets, it was smaller, much hotter, and filled with a uniform glow from its white-hot fog of hydrogen plasma. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled enough, stable atoms could form. These atoms could no longer absorb the thermal radiation, and the universe became transparent instead of being an opaque fog. The photons that existed at that time have been propagating ever since, though growing fainter and less energetic, since the exact same photons fill a larger and larger universe."

http://en.wikipedia.org/wiki/CMBR

 

We don't know if the universe is infinite or not, but even if it's finite, it's very possible that it's much larger than what we are able to observe. The observable universe is everything we are able to see and observe, hence it's also the same as the visible universe, and it makes no difference if there is or not, a greater finite or infinite universe outside of it, it would still be called "the observable universe".

 

We are due to large distances able to se stars the way they where billions of years ago, we know from other stars closer that they have evolved and changed by now, but they are still included in the observable universe since we can observe them.

 

We are due to expanding space able to observe objects that at that time had a very high recessional velocity and as such we can calculate how much farther out they should be today, and they are also included in the observable universe since we can observe them.

 

Even without the concept of an expanding space, we would still not be able to observe distant stars as they are or where they are today, the difference with an expanding space is that the travel time for light is larger than what the travel distance was when the light was emitted.

 

"The observable universe contains about 3 to 7 × 1022 stars (30 to 70 sextillion stars), organized in more than 80 billion galaxies, which themselves form clusters and superclusters."

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

 

Those 80 billion galaxies are inside the region that we can observe today, but the light from some of them could be very old and as such they might be in a different stage and place today, than what they where in when they emitted that light. Just like a good hunter estimates the speed of the prey and then aims and fires his rifle a little further forward, we can from our observations calculate where the objects should be today. Whether or not you prefer to use a calculated model of how our observable universe is thought to be today, a sphere with a radius of 46.5 billion lightyears, or how it's observed today, a sphere consisting of recent to 13.7 billion years old light, the estimate of 80 billion galaxies is still within there.

 

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I am not able to follow you, what 2% and what exactly do you mean when you say "visible" ?

[tar]

Spyman,

 

I had read that the observable universe is 2% larger than the visible universe by virture of the fact that gravity could be felt, earlier than light could been seen, in a time period just before the last scattering.

 

What do scientists think the expansion rate of the universe was, at 379,000 years old?

 

Do we have some guesses as to how the expansion rate changed from then to now? (to get us from that size, to our current size)

 

How big (degree, or minute or secondwise) would the Milkyway look at various distances, say 1 billion lys, 4 billion light years, 10 billion lys, 20 billion lys, and 50 billion lys?

 

If light travels for 13.73 billion years, through space, that in that time expands to 1200 times its original size, how much distance has that photon covered? How does that distance change according to the rate of expansion during different phases of the photon's trip?

 

Regards, TAR

[Spyman]

"Sometimes a distinction is made between the visible universe, which includes only signals emitted since the last scattering time, and the observable universe, which includes signals since the beginning of the cosmological expansion (the Big Bang in traditional cosmology, the end of the inflationary epoch in modern cosmology). The radius of the observable universe is about 2% larger than the radius of the visible universe by this definition.

 

The age of the universe is about 13.7 billion years, but due to the expansion of space we are now observing objects that are now considerably farther away than a static 13.7 billion light-years distance. The edge of the observable universe is now located about 46.5 billion light-years away.

http://en.wikipedia.org/wiki/Observable_Universe

 

I would calc the radius of the "Visible" to about 0.98 × 46.5 = 45.57 billion lightyears, from that text.

 

 

At the age ~400 000 years the Hubble Constant was ~1347787 km/sec/Mpc and matter at the distance of 40 million lightyears was receding from us with the speeed of ~57 times lightspeed.

(Cosmos Calculator values: Omega=0.27 Lambda=0.73 Hubble=71 and Redshift=1100 for the CMBR.)

 

 

"Huge strides in Big Bang cosmology have been made since the late 1990s as a result of major advances in telescope technology as well as the analysis of copious data from satellites such as COBE, the Hubble Space Telescope and WMAP. Cosmologists now have fairly precise measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating."

http://en.wikipedia.org/wiki/Big_Bang

 

It is not just some guesses, we can measure the brightness, spectrum and redshift of different supernovas and draw conclusions to build models. The Cosmos Calculator fits observed data and it's easy to use it to view different values for Hubble values and recession speed in the past by simply changing the redshift.

 

 

Martin made a good thread about different angular diameter distance here: http://www.scienceforums.net/forum/showthread.php?t=22200

 

 

The distance covered by a photon, through space, during 13.73 billion years is 13.73 billion lightyears. The distance changes like the rubber band analogy in post #47.

[tar]

Spyman,

 

Thanks. Read the links and portions of the links in the links. Martin's angular size thread, and so on. Was particularly taken by the diagrams that plotted space and our observation line.

 

Like the one at http://www.astro.ucla.edu/~wright/cosmo_02.htm#DT

"if we plot exactly the same space-time in the special relativistic x and t coordinates we get:" (found about half way through the article)

 

"The distance covered by a photon, through space, during 13.73 billion years is 13.73 billion lightyears. The distance changes like the rubber band analogy in post #47."

 

Well sounds right, but I think that only works if you are looking at it from the photons perspective.

 

When the photon reaches us, it tells us a different story, cause we know it lost energy during the trip, by measuring its redshift. Now light doesn't get tired so it has to be telling us something about the distance it has traveled, combined with the relative motion of the atom that emitted it, to an observer (not us now) located at our position in the universe then. Now the observer, then, does not actually observe the photon till 13.73 bilion years later, from the moment of the emission. And regardless of the recession speed of the atom, then, the photon is immediately on its own, travelling through an expanding space for 13.73 billion years to reach the observer. It therefore has to have traveled through more than 13.73 billion lys of space to reach the observer. Its energy has been stretched out over that distance. It has been calculated to a scale factor of 1200 or something like that. So we observe the scale factor and attribute it to the expansion of space. But I think it is important to note that the "velocity away" is a number contrived after the fact, that includes the effects that exanding space had on the state in which the photon would arrive at the observer. Hence I believe that the redshift observed is a combination of velocity away, then, and the effects of expanding space on the photon we observe. In any case, when considering observing the surface of the last scattering, one has to consider that they are seeing a photon emitted from an atom, that is in a region of space, that a milky way observer, has never before today received a photon from. It is the "first light" from that 379,000 year old region of space. Hence if that light is observed at Z=1000, there should be photons arriving from Z=999,998,997...each emitted from a successively older and closer shell of our visible universe. Each increasing smaller spherical shell representing a smaller and smaller percentage of the atoms in the visible universe. The blackbody spectrum of the sum of the photons coming from each shell, should be evident, at smaller and smaller wavelengths, as closer and older shells are observed and studied, the blackbody spectrum, peaking at higher and higher frequencies in the microwave...infrared, and closest and oldest shells in the visible wavelengths.

 

This picture, I believe is consistant with current theory and observation. What seems to me, to be missing from the liturature is observations of all the stuff in the shells farther away, and younger, than Z=5 or 6 or so. Consequences of the fact that we can see Z=6 and z=1000, are that we can also see Z=100 and Z=822 and so on. Z=822 being a spherical shell of our visible universe, much more massive and distant and younger, than the shell we see at Z=2, and also much less massive, and closer and older, than the shell we see at z=1000.

 

Regards, TAR

[bascule]

We can't quite see the 'edge of the universe'. As we look foruther and further away, we see further and further back in time because the light takes a long time to get to us (and was therefore emitted a long time ago).

 

If we look far enough away, you might think that eventually we will be back to the big bang (the 'edge of the universe', or at least the edge of our horizon). But that is not quite true. For a long time (though a short time on cosmological scales) the universe was opaque to photons, so light couldn't travel freely. Our satellites such as COBE, WMAP and Planck, see only back to the time when the universe became transparent to photons (the 'surface of last scattering').

 

The idea that the early universe was opaque is a simple yet profound concept which never dawned on me. Thanks for sharing.

[tar]

Bascule,

 

Do you figure the opacity is on this side of the surface of last scattering, or the far side?

[Spyman]

When someone use the wording "through space", I assume it's from the perspective of the object going through space. If the ant on the rubber band would have counted every step it took and measured how long each and everyone step was, it could calculate how far it had "walked" on the surface of the rubber band. But relative the ground of Earth it would have travelled a very different distance due to the streaching of the rubber band. Likewise if the photon carried a device to measure the distance it travels "through space", it would have showed 13.73 billion lightyears, but relative us the photon has traversed both "through space" and togheter with space, like how the ant also is brought away togheter with the rubber band when it streaches.

 

The redshift observed is a combination of the emitting objects velocity away "through space" and the effect of expanding space on the lightray, but "velocity away" does not include recession speed of the emitting object due to expansion, you can't double the effects of expansion and have it twice.

 

In theory we should be able to observe stuff with redshift between 10 to 1100 too, but it's not that easy in reality. When the distance to the emitter increases the photons gets more spread out witch causes the intensity to get weaker, our recivers need to be sensitive enough to pick up the signals or the signals needs to be strong enough. The CMBR was more lika a flash of light that then faded and the reason we still see it is because it was emitted from such a large area that light from the far end of the area has still not reached us, and younger light from the CMBR has already passed us and can't be viewed anymore. When the light from the end of Recombination had faded the Universe was dark until the first quasars and stars started to form from gravitational collapse. Quasars and Gamma Ray Bursts are very powerful lightsources and can easier be observed from larger distances than ordinary stars, and with more and more sensitive instruments we will probably be able to see further back in time. But even so, there will be a limit how far back we can see because there is no stellar objects there to see yet, at redshifts greater than 22 there is not thought to be any stars at all yet.

[tar]

Spyman,

 

Hadn't really thought of a "dark ages", although I had seen it mentioned, it didn't make any sense to me. Sure, no star light, before stars. But I figured if the photons from the cosmic background radiation could reach us now, then the material that was emitting the photons was hot enough to be releasing a black body spectrum peaking in the visible wave lengths, which have been stretched out to microwave frequencies. It probably didn't go cold right away, and would still have been releasing some photons, at some frequency. Even if the matter, as it cooled, gave off infrared, then microwave, then radio wave, black body spectrum, it is still "light", and as such, should still be "visible". Even if it was to get to us as radio waves, of longer and longer wavelengths as the matter cooled and expanded. But at some point, early on, the voids we notice in space, must have begun to grow, as the matter began to clump. The matter as it clumped and gathered would be hotter than the voids, the expanding space trading off larger voids for more matter filled areas. I would think that the matter clumps would continue to give off infrared light, as gravity pressurized the incoming atoms. In any case, although the nuclear reactions had not started up yet, giving of gamma and x-ray and ultraviolet and visible and infrared light, some light, of some wavelength should have been emitted by the gathering matter.

 

Now I know we think of things as dark, when they emit no visible wavelengths, but we have equipment that can sense well below visible. Perhaps what is coming into us, radiowave wise is too difused by our atmosphere and too mixed with all other sources, to have been noticed and studied much. But I think it should be there. Perhaps even the Oort cloud somehow disrupts radio waves and makes them hard to identify, in terms of the distance and nature of their source. But we are pretting clever, I would think we could find a way to see it, if we looked for it.

 

Regards, TAR

[Martin]

Hi Tar,

this is an interesting thread. Shows careful unopinionated hardworking imagination on your part. I'm impressed by the thoughtful response by Spyman, Severian, Sisyphus, Bascule...and others.

 

For several weeks running, I've been preoccupied by real-life demands and trying to learn some unrelated stuff, so missed all this good discussion!

 

Just now noticed a post you wrote recently in an astro thread by Purin..(spelling?) where you indicated you thought you had gained some understanding of the conventional cosmology picture---confusion reduced, but still some confusion.

 

If you want, you could summarize for me briefly in simple language one or two points where you are still uncertain or puzzled. This would save me having to sift through the posts and guess what still might be bothering you.

 

Also I see you have what I'd judge to be philosophy of science type sophistication. You realize that science is just a community of mere humans, with an ethic of behavior, a selfselecting aristocracy, a tradition that serves as rough guide. And the models change over time. In cosmology folks only recently began getting lots of good data to adapt their model to, it is a very exciting time in cosmology because it only recently became a truly observational science.

As sophisticated observer and questioner, you realize that we cant say the current standard cosmo model is TRUE, heh heh. Folks just do the best they can and stumble on, improving as they go.

But frankly it is extremely interesting and worth understanding, the current fit to the data is impressive. There are subtleties that take some effort to understand.

It seems to me like you want to learn what the current picture is, while retaining mental reservations about believing it. That seems to me like the perfect way to approach it. I may be misjudging. I didn't have enough time to read your posts at all carefully.

Edited October 2, 2009 by Martin

[tar]

Martin,

 

Thanks so much for your reply.

As well as the others who put me on to facts I was not aware of.

I think you have me, exactly right.

 

I came on this board, back in July, because the pictures of the background microwave radiation, made me think about what it really was we were seeing.

 

I thought that the universe might be all within our view. I am still holding that open as a possibility.

 

I did not know at the time that the hubble constant was not constant, nor thought that the universe was expanding so fast at the time of the last scattering, that regions of space could be receeding from each other faster than the speed of light. I thought that was only possible now, as the sizes have increased so. As such, I had maintained that if we were ever close enough to a region of space, for its light to reach us, it will still be reaching us now.

 

Not to go into all the stages of my education since then, and which parts of my ideas were discarded and reborn and so on. I will cut to my current state.

 

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

 

The fourth diagram on the linked page, shows space plotted using known observations and fact. It is not unlike a crude diagram I drew and posted at some point back, in the sense that it fullfills some of my imagined view. That is, that when we look out into space we are looking back in time, along the red lines. The further away we see, the longer the light took to get to us. Now we can't see any further back than the 13.7 billion years between the last scattering and now, but that is time, not distance.(note that the end of the red line is at the edge of the universe.)

 

The distance is hard to pin down, because people talk about it in so many different ways, and the distance itself, that the photons have traveled has constantly been changing along the way, stretching them out, so to speak.

 

But let me try to get to some point. And once I do, you might be able to see it as well, or better yet, see where I am getting it wrong, and help me cut away the misconceptions, or point me toward the facts I am leaving out.

 

We have a tendency to build a model in our heads, and then look at the model, all at once. (at least I do, and I am giving other people the same kind of facility, rightly or wrongly.) But the universe does not lend itself to this kind of all at once observation. It is too big, too old AND in constant motion, on scales we can't quite imagine. And most importantly, CANNOT be looked at all at once. Because the speed of light is slow compared to the distances involved, and there is no way to look at the model, all at once, and be imagining what it really looks like, now. What it Really looks like now, is what we see with our equipment, when we look at it.

 

What I am referring to, is the kind of picture that is painted in computer animations, that zoom around, into, and out of an infinite mesh of galaxy strings and clusters, as if there is an observation point, that could see this now. Reality is, if we would model what we would see now, if we were to transport our imaginations to a region of space, we now see as microwave background radiation, and look in the direction of the Milky Way, the Milkyway would look like microwave background radiation. In the universe now,(with all regions being 13.73 billion years old) the photons from the background radiation are just reaching the milkyway, and the milkyway's photons from when we were 300,000 years old are just reaching that region of space.

 

Take a region in space which we are now seeing as microwave background radiation. It cannot be both a region of space 300,000 years old, a flux of hot hydrogen atoms, AND a region of space, 13.73 billion years old, which looks like a galaxy. Its image is what we see and what is real to us, its gravity and photons are reaching us now. All regions of space should be able to be considered, from the same perspective. Our perspective.

 

Well I failed to state my questions in a few sentences as requested. I'll pause here, and try to ask something.

 

Shouldn't a square arc second of space, one light year deep, that is 5 billion light years distant from us contain less matter and space than a square arc second of space, one light year deep that is 6 billion light years distant?

 

If a square arc second of space, one light year deep, is 13.73 billion light years distant from us, shouldn't it contain an incredibly large amount more of matter and space than any square arc second of space, one light year deep, closer to us?

 

Regards, TAR

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If a 13.7 billion year old observer, located in a region of space that we see as the cosmic background microwave radiation, sees the Milkyway region as cosmic background microwave radiation, how do they see the microwave background radiation that we are currently seeing when we look in the opposite direction?

[Martin]

If a 13.7 billion year old observer, located in a region of space that we see as the cosmic background microwave radiation, sees the Milkyway region as cosmic background microwave radiation, how do they see the microwave background radiation that we are currently seeing when we look in the opposite direction?

 

They will have to wait, won't they? The photons that are just now today reaching us from the east, most of them will pass on by us and will continue---many years later arriving at the telescopes of the people in the west.

 

Unless, there are always these awkward extra complications, unless...unless the expansion of distances is so rapid that it thwarts the photons and doesn't allow them to reach the people in the west.

 

It's 11 PM here and I'm getting sleepy earlier than usual. You are asking good questions all right. Just maybe faster than I can handle. If it gets more than I can cope with, someone else will help or I will tell you.

 

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

 

The fourth diagram on the linked page, shows space plotted using known observations and fact.

My favorite of those particular diagrams is the third, with the PEAR-SHAPE light cone. Horizontal distance in that picture corresponds to the actual distance at that time, if you could freeze expansion, and measure it by radar.

 

But I think we both realize that cosmological distances are inferred, using a model. One measures redshifts and infers distance. Have you used one of the calculators to convert redshift to distance? Ned Wright has a good one. Google "Wright calculator".

 

 

...Take a region in space which we are now seeing as microwave background radiation. It cannot be both a region of space 300,000 years old, a flux of hot hydrogen atoms, AND a region of space, 13.73 billion years old, which looks like a galaxy. Its image is what we see and what is real to us, its gravity and photons are reaching us now. All regions of space should be able to be considered, from the same perspective. Our perspective.

 

That sounds like a philosophical point of view. The universe is the photons etc that are arriving to us today. One could argue for it philosophically. But as a practical matter, cosmologists don't think that way. They construct a model of the way the universe IS today and the way it was at each moment in the past. They typically use the "frozen expansion" distance or socalled proper distance, to locate things in the model.

 

It is a collegial way of thinking. Imagine that there are observers scattered all over. Observers in every galaxy. Construct the common reality that we are all looking at, and could most likely agree on. Admittedly our friends the other observers are imagined. They may not exist and even if they did we might never be able to communicate with them. But the construct is as if it were to be shared by all observers. It is not just about the photons arriving here today at Earth observatories.

 

Do you know about universe time? Cosmologists have a criterion of rest, being at rest with respect to the ancient light, the expansion process itself.

And there is a corresponding notion of now. Of synchronicity. In pure relativity you don't get that, but in cosmology you do, because you have matter in the picture, not just pure geometry. So all the observers can have synchronized clocks and agree on a common figure for the age of expansion, 13.7 billion years or so.

 

Well, almost 12. Time for bed.

Edited October 3, 2009 by Martin

[tar]

Martin,

 

Thank you. You've hit all the points I'm having touble with. Not that I have been able to put my hand together, but all the cards are at least on the table.

 

I am going to have to think a few things though.

 

Regards, TAR

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Martin,

 

I can't get my head around the diagram you prefer. I guess that is the Hubble formula one. I like the special relativity one better.

 

The infinite universe, starting from a finite density and I would suppose, volume, just doesn't sit right with me. I try and imagine it and it, and I just can't make it add up.

 

On the other hand, if you allow for an edge, even if we can't see it yet, or ever, it is somehow more imaginable. And then if you put most of what we can observe, way at the edge of our observable universe, too far and redshifted to get a good resolution picture of, with our current tools, it seems to make good logical Euclidean sense, with some relativity time slowing and forshortening thrown in to see how things should add up.

 

Regards TAR

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Martin,

 

Can you help me with the angular distance thing?

 

I'm considering a sphere of stuff (even though its just a bunch of hot dense hydrogen, hasn't gone nuclear yet) 48 million lys from our (stuff that will be the Milkyway) region of space, in the year 380,000 (time of last scattering), that measures 1 degree in diameter, (would cover that portion of our sky). Now we can't see it yet in the year 380,000 cause the universe just went transparent. It's first light won't reach us, for 13.73billion years, cause the space between us is expanding very rapidly.

 

When the image of that region of space reaches us now, for the first time, will it cover exactly a one degree in diameter area of our sky? (maybe I should not consider a 3 dimensional sphere, but a disc facing us to form a circle, slightly concave, so all points on the disc were the same distance from us at the year 380,000)

 

Regards, TAR