APOD: The Observable Universe (2018 May 08)

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Expand view Topic review: APOD: The Observable Universe (2018 May 08)

Re: APOD: The Observable Universe (2018 May 08)

by JohnD » Fri May 11, 2018 11:35 am

Thank you, Anne, for the quote of Russell's words! He puts it so much better, but then he got the Nobel and I haven't!
John

Re: APOD: The Observable Universe (2018 May 08)

by Ann » Fri May 11, 2018 10:16 am

JohnD wrote: ↑Fri May 11, 2018 9:12 am Which is why I suggested some reading and study, rather than relying on previously acquired ideas.

Chris, and others here including myself, follow Russell and his teapot. Bertrand Russell, philosopher and another Nobel prize winner, pointed out that it was up to someone who proposed that a teapot was in orbit around the Sun to provide the evidence, not for others to disprove it. We have evidence on the Big Bang that started this Universe. Speculation on prior BBs is just teapots.
John

I had forgotten about the teapot! That's a really good one! :D

Ann

Re: APOD: The Observable Universe (2018 May 08)

by JohnD » Fri May 11, 2018 9:12 am

Which is why I suggested some reading and study, rather than relying on previously acquired ideas.

Chris, and others here including myself, follow Russell and his teapot. Bertrand Russell, philosopher and another Nobel prize winner, pointed out that it was up to someone who proposed that a teapot was in orbit around the Sun to provide the evidence, not for others to disprove it. We have evidence on the Big Bang that started this Universe. Speculation on prior BBs is just teapots.
John

Re: APOD: The Observable Universe (2018 May 08)

by Chris Peterson » Fri May 11, 2018 5:57 am

Guest wrote: ↑Fri May 11, 2018 12:52 am
Chris Peterson wrote: ↑Wed May 09, 2018 2:01 am
All of our physics tells us that time wasn't a property that existed before the Big Bang (which certainly can't have a probability of greater than one!). So time may well be infinite going forward from that event, but seems not to be infinite looking backwards.
Well, since you believe that there was no 'time and space' before the 'big bang', then it is logical to extend that postulate that there could have been no 'big bang' because there was no time or space in which to have a 'big bang' in.
It's not a question of what I "believe". It's simply that we have very good theory which is extremely well supported by a large amount of independent evidence. In other words, it's the best explanation we have.
Just because the models and theories need to have a 'zero' point (a frame of reference) on which to base everything we 'see', does not mean that infinite space and time going in all directions does not exist.
As previously noted, the understanding that time and space came into existence together is unrelated to frames of reference. And in fact, the excellent theory that we have very much does argue for there not being any time or space until the formation of the Universe.
The fact that we are here indicates that the probability of a 'big bang' is greater than one.
How can the probability of anything be greater than one? What does that even mean?
I know that it is hard to overcome how you were educated, but try and overcome the limiting ideas that were forced on you...
I believe it is your inability to get past the biases imposed by your simple worldview that reflects a lack of imagination and a limitation.

Re: APOD: The Observable Universe (2018 May 08)

by Ann » Fri May 11, 2018 4:47 am

Guest wrote: ↑Fri May 11, 2018 12:52 am
Chris Peterson wrote: ↑Wed May 09, 2018 2:01 am
All of our physics tells us that time wasn't a property that existed before the Big Bang (which certainly can't have a probability of greater than one!). So time may well be infinite going forward from that event, but seems not to be infinite looking backwards.
Well, since you believe that there was no 'time and space' before the 'big bang', then it is logical to extend that postulate that there could have been no 'big bang' because there was no time or space in which to have a 'big bang' in. Just because the models and theories need to have a 'zero' point (a frame of reference) on which to base everything we 'see', does not mean that infinite space and time going in all directions does not exist. The fact that we are here indicates that the probability of a 'big bang' is greater than one. So I suggest that you consider that a second other big bang had happened perhaps a hundred trillion years ago, in which case time and space would be in existence from that time forward. Or you could consider that time and space are infinite (in all directions) and rethink your perspective of time, the universe and everything... I know that it is hard to overcome how you were educated, but try and overcome the limiting ideas that were forced on you...
Could you spare a moment and consider how extremely tiny and short-lived we humans are compared with the size and scope of the Universe in space and time? And still, we have managed to find out as much about our Universe as we actually have. Personally, I think that humanity's feat of uncovering many of the aspects of the grand Universe that we are a tiny, tiny part of is absolutely amazing and truly awe-inspiring.

Our tiny size compared with the size of the Universe, our short lifespans, the incomprehensible value of the speed of light, yet at the same time the relative "snail's pace" of the speed of light in relation to the vastness of the Universe, plus the fact that the earliest Universe was wrapped in an impenetrable fog completely hiding it from our view - all these factors make it terribly difficult for us to ascertain the properties of the Universe, let alone the factors that made the Universe come into existence.

All we can do is observe the Universe with better and better instruments and make better and better models of how the Universe works, based on our ever-improving observations. With more and more powerful computers, we can test our theories by running them through our most powerful computers and see if our computers appear to come up with a Universe like ours, based on the models we fed into them based on our observations that we used to make our models.

That's what we can do, really.

We can make many observations that clearly demonstrate that the Universe was much smaller and more compact in its infancy than it is now, and it was also hotter. From these observations, we can extrapolate backwards in time and conclude that there must have been a time when the Universe was really, really tiny, and perhaps - but who knows? - infinitely tiny, as a mathematical point in a non-existing space in a non-existing time frame. From this mathematical point our Universe burst forth. This is what our models suggest.

But we can make no observations that suggest that there were other Big Bangs before the one that gave birth to our own Universe. Therefore, the question of previous Big Bangs and other universes than our own are not scientific questions, but philosophical ones.

Ann

Re: APOD: The Observable Universe (2018 May 08)

by geckzilla » Fri May 11, 2018 4:08 am

Guest wrote: ↑Fri May 11, 2018 12:52 am
Chris Peterson wrote: ↑Wed May 09, 2018 2:01 am
All of our physics tells us that time wasn't a property that existed before the Big Bang (which certainly can't have a probability of greater than one!). So time may well be infinite going forward from that event, but seems not to be infinite looking backwards.
Well, since you believe that there was no 'time and space' before the 'big bang', then it is logical to extend that postulate that there could have been no 'big bang' because there was no time or space in which to have a 'big bang' in. Just because the models and theories need to have a 'zero' point (a frame of reference) on which to base everything we 'see', does not mean that infinite space and time going in all directions does not exist. The fact that we are here indicates that the probability of a 'big bang' is greater than one. So I suggest that you consider that a second other big bang had happened perhaps a hundred trillion years ago, in which case time and space would be in existence from that time forward. Or you could consider that time and space are infinite (in all directions) and rethink your perspective of time, the universe and everything... I know that it is hard to overcome how you were educated, but try and overcome the limiting ideas that were forced on you...
Hi Guest, most of the perceived strictness and rigidity of Chris's posts are due to the forum rules reinforcing mainstream concepts in science. We're really not about exploring alternative theories here in any capacity beyond idle speculation. So if someone asks a question, most responses will be a kind of by-the-book answer if not some kind of creative allusion from Art. It's not that any of us don't harbor feelings that well, maybe the Big Bang isn't all there is, or maybe we're wrong. It's just what we have to work with. Quite frankly, alternatives have been hashed and rehashed over, and it's boring at this point.

Kindly set aside your righteous indignation and take a gander at the rules.

Re: APOD: The Observable Universe (2018 May 08)

by Ann » Fri May 11, 2018 3:30 am

I need to rectify what I said about the Sun (and therefore the Earth) having moved more than a trillion kilometers in 5,000 years.

Clearly we have moved much farther than that from the 3D region of Ötzi's life span. Because it is not only the Sun and the Earth that have moved. The Milky Way has also moved! Our galaxy is experiencing a fairly strong pull from Andromeda, the other large galaxy of the Local Group.

But that's not all. The Virgo Cluster is also pulling on our galaxy and our Local Group. And there is the Great Attractor, which is pulling not only on the Local Group of galaxies, but also on the entire Virgo Cluster and a lot of other galactic groups as well.

And of course, as Art pointed out, there is the general expansion of the Universe pulling on us. So, can we really find the 3D region in space where the Earth was located 5,000 years ago?

Maybe we can. But with all these tidal pulls and flows in space, the 3D distance to the Earth's location in space some 5,000 years ago is getting larger and larger.

Image
Theseus with Ariadne's thread in the labyrinth
of the Minotaur. Source:
https://www.icompositions.com/music/song.php?sid=223531


















Also bear in mind that there are no "roads" or "road signs" in space that we could use to "follow our own footsteps" back to where we were 5,000 years ago. We don't have Ariadne's thread to help us find our way back to where we started. We don't even have pebbles or bread crumbs like Hansel and Gretel.

Finding our 3D location in space 5,000 years ago would be very hard indeed, and traveling there would be harder.

Ann

Re: APOD: The Observable Universe (2018 May 08)

by Guest » Fri May 11, 2018 12:52 am

Chris Peterson wrote: ↑Wed May 09, 2018 2:01 am
All of our physics tells us that time wasn't a property that existed before the Big Bang (which certainly can't have a probability of greater than one!). So time may well be infinite going forward from that event, but seems not to be infinite looking backwards.
Well, since you believe that there was no 'time and space' before the 'big bang', then it is logical to extend that postulate that there could have been no 'big bang' because there was no time or space in which to have a 'big bang' in. Just because the models and theories need to have a 'zero' point (a frame of reference) on which to base everything we 'see', does not mean that infinite space and time going in all directions does not exist. The fact that we are here indicates that the probability of a 'big bang' is greater than one. So I suggest that you consider that a second other big bang had happened perhaps a hundred trillion years ago, in which case time and space would be in existence from that time forward. Or you could consider that time and space are infinite (in all directions) and rethink your perspective of time, the universe and everything... I know that it is hard to overcome how you were educated, but try and overcome the limiting ideas that were forced on you...

Re: APOD: The Observable Universe (2018 May 08)

by neufer » Thu May 10, 2018 10:02 pm

Chris Peterson wrote: ↑Thu May 10, 2018 9:19 pm
Ann wrote: ↑Thu May 10, 2018 7:39 pm
Theoretically at least, we could indeed travel to the general region of space through which the Earth was moving when Ötzi was alive. But if we managed to locate that region in space and travel there, we would find that the Earth would not be there. It would have moved more than a trillion kilometers away from the 4D location in spacetime where Ötzi was alive.
Another way to think of it, since spacetime dimensions have different units, is that we would have moved a trillion kilometers along the Universe's XYZ axes, and 5000 years along its time axis. We have freedom of movement on the former, but are basically just being carried along on the latter.
The Sun moves ≈ 370 km/s = 0.00123 c
(relative to the cosmic microwave background).

Hence, Ötzi would have moved about 6.2 (light) years along the Universe's space axes, and ~5000 years (+33 hours) along its (imaginary?) time axis.

Theoretically, we have freedom of movement to any spacetime point lying between our own future light cone and Minkowski's hyperbola (where the unit of time is defined by our own remaining life span). :arrow:

Re: APOD: The Observable Universe (2018 May 08)

by Chris Peterson » Thu May 10, 2018 9:19 pm

Ann wrote: ↑Thu May 10, 2018 7:39 pm Theoretically at least, we could indeed travel to the general region of space through which the Earth was moving when Ötzi was alive. But if we managed to locate that region in space and travel there, we would find that the Earth would not be there. It would have moved more than a trillion kilometers away from the 4D location in spacetime where Ötzi was alive.
Another way to think of it, since spacetime dimensions have different units, is that we would have moved a trillion kilometers along the Universe's XYZ axes, and 5000 years along its time axis. We have freedom of movement on the former, but are basically just being carried along on the latter.

Re: APOD: The Observable Universe (2018 May 08)

by Ann » Thu May 10, 2018 7:39 pm

geckzilla wrote: ↑Thu May 10, 2018 6:38 pm
tomatoherd wrote: ↑Thu May 10, 2018 4:40 pm
bystander wrote: ↑Thu May 10, 2018 4:00 pm
Space-time is 4D. 3D Space plus time.
No bystander, that's not what is meant by 4D. Not the way Chris and others are herein using it. They mean 4D space alone, with or without time. "hyperspheres and tesseracts" and all that.
Space is time. Just stop trying to separate them, and it'll make more sense. Or less.
This is how I visualize it. It may be right, it may be wrong.

This is Ötzi, the iceman. :arrow: He was found in the Ötzal Alps on the border between Austria and Italy in 1991. He is Europe's oldest natural mummy, and he was alive some 5,000 years ago.

I think of Ötzi as a man who has been traveling on "spaceship Earth" for about 5,000 years. He was alive for only about 45 of those years, but his death didn't prevent his body from continuing his journey on spaceship Earth.

The Earth is "dragged along" in space by the Sun, which is moving at 72,000 kilometers per hour (45,000 miles per hour). Let's see. There are 24 hours in a day, which means that the Sun moves 1,728,000 kilometers in a day. Then there are 365 days in a year, which means that the Sun moves 630,720,000 kilometers per year, or almost 631 million kilometers per year, if I did the math right. Allow the Sun to travel at that speed for five thousand years, and it will have moved some three trillion, one hundred and fifty-three billion, six hundred million kilometers through space. And dragged the Earth along with it.

My figures may be correct, or not. But it is true without a shadow of a doubt that the Sun has dragged the Earth along a vast distance since Ötzi was alive on the Earth.

Here is my point. We can't travel back in time and meet Ötzi when he was alive, or visit the Earth at the time when Ötzi was alive. But crucially, we also can't travel back in spacetime and visit the 4D region in the Universe in which Ötzi was alive. Theoretically at least, we could indeed travel to the general region of space through which the Earth was moving when Ötzi was alive. But if we managed to locate that region in space and travel there, we would find that the Earth would not be there. It would have moved more than a trillion kilometers away from the 4D location in spacetime where Ötzi was alive.

Ötzi's mummified body exists in the same geographic location and time frame as we do, but he was alive in a 4D spacetime that is inaccessible to us. The spacetime coordinates of the Earth that he lived on are out of reach for us forever.

Ann

Re: APOD: The Observable Universe (2018 May 08)

by geckzilla » Thu May 10, 2018 6:38 pm

tomatoherd wrote: ↑Thu May 10, 2018 4:40 pm
bystander wrote: ↑Thu May 10, 2018 4:00 pm
tomatoherd wrote: ↑Thu May 10, 2018 3:30 pm
Einstein needed the space-time continuum. Do we need 4D to explain what we observe? (Is there any astronomical or physics observations that can't be explained with solely 3D space and time). Or just to explain Minkowski's synaptic discharges?
Space-time is 4D. 3D Space plus time.
No bystander, that's not what is meant by 4D. Not the way Chris and others are herein using it. They mean 4D space alone, with or without time. "hyperspheres and tesseracts" and all that.
Space is time. Just stop trying to separate them, and it'll make more sense. Or less.

Re: APOD: The Observable Universe (2018 May 08)

by Chris Peterson » Thu May 10, 2018 6:37 pm

tomatoherd wrote: ↑Thu May 10, 2018 4:40 pm
bystander wrote: ↑Thu May 10, 2018 4:00 pm
tomatoherd wrote: ↑Thu May 10, 2018 3:30 pm
Einstein needed the space-time continuum. Do we need 4D to explain what we observe? (Is there any astronomical or physics observations that can't be explained with solely 3D space and time). Or just to explain Minkowski's synaptic discharges?
Space-time is 4D. 3D Space plus time.
No bystander, that's not what is meant by 4D. Not the way Chris and others are herein using it. They mean 4D space alone, with or without time. "hyperspheres and tesseracts" and all that.
Nope, that's definitely not what I'm talking about. The 4D universe is defined by three spatial dimensions and one time dimension. I don't even know what four spatial dimensions means, outside of pure mathematics.

The geometry of the Universe can be a hypersphere or some other 4D geometry without the need for a fourth "spatial" dimension.

Re: APOD: The Observable Universe (2018 May 08)

by tomatoherd » Thu May 10, 2018 4:40 pm

bystander wrote: ↑Thu May 10, 2018 4:00 pm
tomatoherd wrote: ↑Thu May 10, 2018 3:30 pm
Einstein needed the space-time continuum. Do we need 4D to explain what we observe? (Is there any astronomical or physics observations that can't be explained with solely 3D space and time). Or just to explain Minkowski's synaptic discharges?
Space-time is 4D. 3D Space plus time.
No bystander, that's not what is meant by 4D. Not the way Chris and others are herein using it. They mean 4D space alone, with or without time. "hyperspheres and tesseracts" and all that.

Re: APOD: The Observable Universe (2018 May 08)

by JohnD » Thu May 10, 2018 4:25 pm

Tomatoherd,
Richard Feynman, theoretical physicist, Nobel prize winner and bongo player, was one of the best teachers of physics there has been, and he wrote that, " If you can’t explain something in simple terms, you don’t understand it."
But he also said, when a journalist pressed him to explain "in layman's terms" the work that won him a Nobel prize," Hell, if I could explain it to the average person, it wouldn't have been worth the Nobel prize!"

This is tough stuff! We're not Richard Feynman, and still no one understands it, fully, so it's difficult to explain in a paragraph, or even two. It needs study, reading, discussion and argument, to get anything like a partial understanding. If you're on that path that seeks understanding, welcome! It's long and hard, and none of us are there yet!

I'd recommend Hawking's "Brief History of Time", said to be one of the books that most people can't completely read, but that's not true. It's thoroughly readable and useful, even if it's twenty years old and contains some ideas no longer held. Or, to be right up to date, Carlo Rovelli's "Seven brief Lessons on Physics". Enjoy!
John

Re: APOD: The Observable Universe (2018 May 08)

by bystander » Thu May 10, 2018 4:00 pm

tomatoherd wrote: ↑Thu May 10, 2018 3:30 pm
Einstein needed the space-time continuum. Do we need 4D to explain what we observe? (Is there any astronomical or physics observations that can't be explained with solely 3D space and time). Or just to explain Minkowski's synaptic discharges?
Space-time is 4D. 3D Space plus time.

Re: APOD: The Observable Universe (2018 May 08)

by tomatoherd » Thu May 10, 2018 3:30 pm

Chris Peterson wrote: ↑Thu May 10, 2018 1:39 pm
tomatoherd wrote: ↑Thu May 10, 2018 12:48 pm Can somebody tell me how we KNOW the universe is 4D? We know things from Copernicus, from Newton, and lately from Einstein, but who proved the universe is 4D?
Well, in science we don't ever prove things, we find models that explain what we see and which are predictive. For the most part, the connection between space and time are a consequence of Einstein's development of special relativity. With that foundation, Einstein went on to construct general relativity (with a good deal of intellectual input from others, particularly Minkowski), which provides a geometrical model of the Universe.
Einstein needed the space-time continuum. Do we need 4D to explain what we observe? (Is there any astronomical or physics observations that can't be explained with solely 3D space and time). Or just to explain Minkowski's synaptic discharges?

Re: APOD: The Observable Universe (2018 May 08)

by Chris Peterson » Thu May 10, 2018 1:39 pm

tomatoherd wrote: ↑Thu May 10, 2018 12:48 pm Can somebody tell me how we KNOW the universe is 4D? We know things from Copernicus, from Newton, and lately from Einstein, but who proved the universe is 4D?
Well, in science we don't ever prove things, we find models that explain what we see and which are predictive. For the most part, the connection between space and time are a consequence of Einstein's development of special relativity. With that foundation, Einstein went on to construct general relativity (with a good deal of intellectual input from others, particularly Minkowski), which provides a geometrical model of the Universe.

Re: APOD: The Observable Universe (2018 May 08)

by tomatoherd » Thu May 10, 2018 12:48 pm

Ann wrote: ↑Thu May 10, 2018 6:29 am
Chris Peterson wrote: ↑Thu May 10, 2018 4:14 am
carlscheske wrote: ↑Thu May 10, 2018 1:39 am

The Universe is a four-dimensional volume of some sort, and this volume can have no edge in three dimensions, any more than a three-dimensional volume like the Earth can have an edge in two dimensions.



And if these astronomers "far, far away in time and space in our 4D spacetime"



Can somebody tell me how we KNOW the universe is 4D? We know things from Copernicus, from Newton, and lately from Einstein, but who proved the universe is 4D?
thanks,

Re: APOD: The Observable Universe (2018 May 08)

by Jon Alexandr » Thu May 10, 2018 8:31 am

No 2D representation of the universe can really be accurate. But the artist — Pablo Carlos Budassi — has done a wonderful job of illustrating some sophisticated cosmological concepts. His graphic has been used in many online publications, in print, and in videos. I used it for the cover of my wedding guest book, and I have a gorgeous metal print reproduction on my wall. The illustration is now available in a variety of formats from a site that provides some reimbursement to the artist. Look for it online.

Re: APOD: The Observable Universe (2018 May 08)

by Ann » Thu May 10, 2018 6:29 am

Chris Peterson wrote: ↑Thu May 10, 2018 4:14 am
carlscheske wrote: ↑Thu May 10, 2018 1:39 am Hi Chris
I am new to this site and having difficulty in trying to reply.
I am not talking about distance. I am talking about the time the Universe has been expanding. The Earth is supposedly 13.8 years (time wise) from the Big Bang. Our observable universe is a sphere with a radius of 13.8 Billion years around the Earth. The Big Bang is on the circumference of that sphere.
If you were located on the circumference of that sphere (that is, 43 billion light years from Earth), you'd see yourself in the center of a sphere, with the light from the Big Bang at the outer edges of your own observable universe, 43 billion light years, 13.8 billion light travel years, away.
As Chris has pointed out before, if the Universe is depicted as a round disk at all (as in the APOD), then the Big Bang should not be at the circumference, but at the center. Because the Universe has been expanding outwards for as long as it has existed.

So are we "at the edge", then? And why can't we see the edge, if we are close to it?

Where is the edge of the Earth?
Sorry, it has none.
Again, as Chris (and others) have pointed out, the Universe has no edge, certainly not an edge in three dimensions. The Universe is a four-dimensional volume of some sort, and this volume can have no edge in three dimensions, any more than a three-dimensional volume like the Earth can have an edge in two dimensions.

Think of it like this. On the Earth, if you live in the Himalayas, you will se mountains all around you. And if you had some neighbors not too far from you in different directions, they would also see mountains all around them.

If you lived in the Sahara desert, and the Sahara desert was dotted with tiny oases, and a tiny settlement of people lived at every such oasis, then every settlement at (almost) every oasis would see themselves being surrounded by a huge expanse of sand dotted with little patches of green where other people lived.

When do we come to the next village?





My point is that if your "neighborhood" mostly looks the same in every direction, and if it is sufficiently large, and if you are unable to travel beyond it, you won't be able to see beyond it. You will most likely feel that you are at the center of your very own world, which looks much the same to you in every direction.






The Hubble Ultra Deep Field.NASA, ESA, H.Teplitz and M.Rafelski (IPAC/Caltech),
A. Koekemoer (STScI), R. Windhorst(ASU), Z. Levay (STScI)




Consider the Hubble Ultra Deep Field, which shows us galaxies at different distances from us. Some of the galaxies look relatively large, and they are the ones that are the closest to us (although they are still very far away). Astronomers looking out from these galaxies will see other galaxies not too far from their own, and at least many of these nearby galaxies will be broadly similar to the galaxy they inhabit themselves.

Other galaxies in the picture are much smaller and much, much farther away. But they, too, are "encapsulated" in their own "bubble" in time and space. So If there are astronomers in them, so close to the earliest epochs of the Universe, who are studying their own galactic neighborhood, they, too, will see numerous galaxies in every direction that are broadly similar to the one they live in.


And if these astronomers "far, far away in time and space in our 4D spacetime" are unable to study the appearance of their own galaxy, then at least they might be able to see that most nearby galaxies appear to have similar sizes, morphologies and colors. So far away, most galaxies would be small, blue and intensely starforming.

Yup... it's the same all over.
Photo: David M. Doody.

The way I understand it, at almost every epoch of the Universe (with the exception of the earliest epochs) there were large galactic "neighborhoods" that looked broadly similar "all over". When the Universe was much younger than today, it is perhaps unlikely that there existed any astronomers or observers of any kind that were able to contemplate the cosmos that they lived in. But if there had been any such observers with the capability of turning powerful telescopes onto the deep sky above them, they too would see a skyscape that looked much the same in every direction.

That is why we have every reason to believe that most observers in most galaxies that have ever existed might easily conclude that they themselves are at the center of the Universe, because the Universe really looks the same all around them.

Ann

Re: APOD: The Observable Universe (2018 May 08)

by Chris Peterson » Thu May 10, 2018 4:14 am

carlscheske wrote: ↑Thu May 10, 2018 1:39 am Hi Chris
I am new to this site and having difficulty in trying to reply.
I am not talking about distance. I am talking about the time the Universe has been expanding. The Earth is supposedly 13.8 years (time wise) from the Big Bang. Our observable universe is a sphere with a radius of 13.8 Billion years around the Earth. The Big Bang is on the circumference of that sphere.
If you were located on the circumference of that sphere (that is, 43 billion light years from Earth), you'd see yourself in the center of a sphere, with the light from the Big Bang at the outer edges of your own observable universe, 43 billion light years, 13.8 billion light travel years, away.

Re: APOD: The Observable Universe (2018 May 08)

by alter-ego » Thu May 10, 2018 3:15 am

carlscheske wrote: ↑Thu May 10, 2018 1:39 am Hi Chris
I am new to this site and having difficulty in trying to reply.
I am not talking about distance. I am talking about the time the Universe has been expanding. The Earth is supposedly 13.8 years (time wise) from the Big Bang. Our observable universe is a sphere with a radius of 13.8 Billion years around the Earth. The Big Bang is on the circumference of that sphere.
If we can see cosmic radiation as old as 13.8 Billion years to the Big Bang and can see cosmic radiation in the opposite direction from the Big Bang as old as 13.8 Billion years, the Big Bang has to have been expanding for at least 27.6 Billion years, which says the Universe must be at least 27.6 Billion years old.
Carl
The point your missing is no direction is opposite the Big Bang.
The shell of cosmic radiation that surrounds us is 13.8 Gyr old, but this light from all directions is reaching us at the same time. I.e. 13.8 billion years is the maximum time these photons spent traveling to us which equals the expansion time. Light emission from the observed shell occurred at the nominally same time, and if the time it takes the light to reach us from the recombination region is 13.8 billion years, then a reasonable model to visualize expansion is about us (and every other point in this Universe) There is no factor of 2 in the expansion time.

Re: APOD: The Observable Universe (2018 May 08)

by carlscheske » Thu May 10, 2018 1:39 am

Chris Peterson wrote: ↑Tue May 08, 2018 10:37 pm
carlscheske wrote: ↑Tue May 08, 2018 10:17 pm Subject: The Universe must be at least 27.6 Billion Years Old
If the Big Bang Theory is a true description of the beginning of the Universe, the Universe must be at least 27.6 billion years old.
If you consider our Observable Universe is a radius of 13.8 Billion years of cosmic radiation. Our position is 13.8 billion years from the Big Bang and the Big Bang is on the edge of that sphere of our observable Universe. So if we can see cosmic radiation all the way to the Big Bang and see cosmic radiation another 13.8 Billion years beyond us away from the Big Bang, the Universe must have been expanding for at least 27.6 Billion years.
The observable universe is about 93 billion light years across- 46 billion light years to any edge we can see. Because the Universe has been expanding since the Big Bang, this represents 13.8 billion years light travel time. That's the age of the Universe.
Hi Chris
I am new to this site and having difficulty in trying to reply.
I am not talking about distance. I am talking about the time the Universe has been expanding. The Earth is supposedly 13.8 years (time wise) from the Big Bang. Our observable universe is a sphere with a radius of 13.8 Billion years around the Earth. The Big Bang is on the circumference of that sphere.
If we can see cosmic radiation as old as 13.8 Billion years to the Big Bang and can see cosmic radiation in the opposite direction from the Big Bang as old as 13.8 Billion years, the Big Bang has to have been expanding for at least 27.6 Billion years, which says the Universe must be at least 27.6 Billion years old.
Carl

Re: APOD: The Observable Universe (2018 May 08)

by Chris Peterson » Wed May 09, 2018 6:47 pm

RJN wrote: ↑Wed May 09, 2018 6:24 pm In response to several inquiries, the words "visible light" have been changed just to "light" in the third sentence on the NASA APOD. This is because, in part, microwaves are light but not visible to the human eye. I apologize for the "oversight".
That's reasonable, but given that the usage of "light" to mean "electromagnetic energy" is somewhat specialized within the scientific community, I think this might still lead to confusion in much of the APOD audience. Maybe something like "In the light of electromagnetic radiation, the farthest we can see..." would be clearer to more people.

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