APOD: The Observable Universe (2018 May 08)

[Chris Peterson]

Well, I'll disregard "spiritual frame" as pseudoscientific nonsense.

Oh, you mean like "dark energy"?

Well, we observe dark energy. We simply lack a complete explanation for the observation.

[carlscheske]

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.

[Chris Peterson]

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.

[Guest]

Sorry, but i'm a simpleton.
I believe our physics applies across OUR entire universe, and so does Euclidean geometry. If there's no edge any college math major could do a proof that it is also infinite. And sorry, but if the big bang began at a geometric point, it could not reach infinite size in 14 billion years. So you can't have your cake and eat it too: if the universe is infinite, there's no big bang, just an accelerating expansion none of you can explain.
Just cause you can't see an edge doesn't mean there isn't one. An amoeba can't appreciate the "No Parking Sign" but that doesn't mean it doesn't exist. And you all and me are cosmological amoebae.

Consider, the probability of a 'big bang' is greater than one. We are here, right. So it is logical that at least one more 'big bang' has occurred in infinite space and infinite time. My apologies if you are not able to comprehend the possibilities of infinite space and time. That does not mean that infinitive space, time and 'big bangs' are not out there. You cannot say that just because you can't see something, that it does not exist. That would be pretty narrow minded thinking. Imagine a grizzly bear stalking you in the woods; would you say NO YOU CAN'T EAT ME just because you couldn't see him? Just because we can't see it, does not mean that it isn't there.

[Chris Peterson]

Consider, the probability of a 'big bang' is greater than one. We are here, right. So it is logical that at least one more 'big bang' has occurred in infinite space and infinite time.

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.

[Boomer12k]

One way to look at it.... but too much like a Mandala...

:---[===] *

[zmon21]

As the speed of light is limited, what we see the far side of the universe is billion years ago. Vise versa, those who live there would see here billions of years ago, too. The would see nothing, no earth, no sun. Though we’re not able to see the present status of the far side of the universe, we can speculate that place could have evolved like here, too. Then now there are no quasars like objects there. Then can we think earth like objects are spread all over the universe?

[Guest]

Consider, the probability of a 'big bang' is greater than one. We are here, right. So it is logical that at least one more 'big bang' has occurred in infinite space and infinite time.

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.

Don't fathom your thinking. Time is infinite going forward? But not infinite going backward? I get it that traditional thinking is that 'if we have no frame of reference' then there is no point talking about it. But that does not mean that space and time were not always there. We are just too narrow minded in our thinking to expand out a thought process to consider and appreciate infinite space and time.

[geckzilla]

Does anyone know what the circular features around nearby stars are in this image? They must be deliberate, but I am not sure what they represent. They seem too large to be the Oort clouds of those stars and too circular to be planetary nebulae.

They are mimicking optical artifacts that telescopes sometimes leave around stars. When they are perfectly centered and smooth like this though, it looks a lot more artistic and a lot less like an artifact.

[Chris Peterson]

Consider, the probability of a 'big bang' is greater than one. We are here, right. So it is logical that at least one more 'big bang' has occurred in infinite space and infinite time.

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.

Don't fathom your thinking. Time is infinite going forward? But not infinite going backward? I get it that traditional thinking is that 'if we have no frame of reference' then there is no point talking about it. But that does not mean that space and time were not always there. We are just too narrow minded in our thinking to expand out a thought process to consider and appreciate infinite space and time.

This has nothing to do with frames of reference. It has to do with very well supported theory that informs us that both space and time were created at the Big Bang. There was no "before" that. No space. No time.

[suicidejunkie]

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.

Don't fathom your thinking. Time is infinite going forward? But not infinite going backward? I get it that traditional thinking is that 'if we have no frame of reference' then there is no point talking about it. But that does not mean that space and time were not always there. We are just too narrow minded in our thinking to expand out a thought process to consider and appreciate infinite space and time.

This has nothing to do with frames of reference. It has to do with very well supported theory that informs us that both space and time were created at the Big Bang. There was no "before" that. No space. No time.

Much like if you consider the surface of the earth for the 2D example, with latitude being time and longitude being space. The north pole is t=0, and as you go south, you see space is expanding rapidly from a point. In this spherical case, everything comes together in a big crunch at the south pole, but if our surface curved outwards below the equator instead, it would look more like we see in the current universe with no limit on how far south (future) you can travel.

Asking what happened before the beginning of time is equivalent to asking what is north of the north pole. At the north pole, every direction is south and there is no north.

[Chris Peterson]

Don't fathom your thinking. Time is infinite going forward? But not infinite going backward? I get it that traditional thinking is that 'if we have no frame of reference' then there is no point talking about it. But that does not mean that space and time were not always there. We are just too narrow minded in our thinking to expand out a thought process to consider and appreciate infinite space and time.

This has nothing to do with frames of reference. It has to do with very well supported theory that informs us that both space and time were created at the Big Bang. There was no "before" that. No space. No time.

Much like if you consider the surface of the earth for the 2D example, with latitude being time and longitude being space. The north pole is t=0, and as you go south, you see space is expanding rapidly from a point. In this spherical case, everything comes together in a big crunch at the south pole, but if our surface curved outwards below the equator instead, it would look more like we see in the current universe with no limit on how far south (future) you can travel.

Asking what happened before the beginning of time is equivalent to asking what is north of the north pole. At the north pole, every direction is south and there is no north.

There's a much better way of comparing a 3D sphere to the 4D universe. The 2D surface of the sphere is analogous to the 3D volume of the Universe. The Big Bang, at t=0, is located at the center of the sphere. The sphere is expanding outward, which is the time direction. The surface of the sphere is "now".

[RJN]

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".

- RJN

[Chris Peterson]

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.

[carlscheske]

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

[alter-ego]

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.

[Chris Peterson]

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.

[Ann]

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?

Click to view full size image

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.

Click to view full size image

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.

Click to view full size image

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.

Click to view full size image

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

[Jon Alexandr]

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.

[tomatoherd]

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,

[Chris Peterson]

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.

[tomatoherd]

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?

[bystander]

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.

[JohnD]

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

[tomatoherd]

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.