APOD: The Observable Universe (2018 May 08)

[APOD Robot]

The Observable Universe

Explanation: How far can you see? Everything you can see, and everything you could possibly see, right now, assuming your eyes could detect all types of radiations around you -- is the observable universe. In visible light, the farthest we can see comes from the cosmic microwave background, a time 13.8 billion years ago when the universe was opaque like thick fog. Some neutrinos and gravitational waves that surround us come from even farther out, but humanity does not yet have the technology to detect them. The featured image illustrates the observable universe on an increasingly compact scale, with the Earth and Sun at the center surrounded by our Solar System, nearby stars, nearby galaxies, distant galaxies, filaments of early matter, and the cosmic microwave background. Cosmologists typically assume that our observable universe is just the nearby part of a greater entity known as "the universe" where the same physics applies. However, there are several lines of popular but speculative reasoning that assert that even our universe is part of a greater multiverse where either different physical constants occur, different physical laws apply, higher dimensions operate, or slightly different-by-chance versions of our standard universe exist.

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[Chris Peterson]

I think one misconception images like this give is that if we could travel to the edge of the observable universe, we'd see what the image is showing at its edge. But that's not true. If we were, at this moment, 46 billion light years away, at the outer horizon of today's APOD, we'd see a universe around us that looked pretty much the same as it does from here. It would be our solar system that would be not yet born, its future location buried in the filaments at the edge of an entirely different observably universe.

Every point in the entire universe lies at the very center of its own, unique observable universe, including points that are so far away we'll never be able to see them, or they us.

[Ann]

I think one misconception images like this give is that if we could travel to the edge of the observable universe, we'd see what the image is showing at its edge. But that's not true. If we were, at this moment, 46 billion light years away, at the outer horizon of today's APOD, we'd see a universe around us that looked pretty much the same as it does from here. It would be our solar system that would be not yet born, its future location buried in the filaments at the edge of an entirely different observably universe.

Every point in the entire universe lies at the very center of its own, unique observable universe, including points that are so far away we'll never be able to see them, or they us.

You have taught me that the center of the Universe is located in time, not in space. The center of the Universe is at t=0, or in other words, at the moment of the Big Bang. Every part of the Universe existing "at the same time" is at the same distance from the beginning of the Universe.

Ann

[JohnD]

Chris,
The CMB is isotropic, but does that prove that the Universe is homogeneous?
And, what about the "Axis of Evil", the anomaly reported by the ESA's Planck probe, that found a difference in CMB aligned with the plane of the solar system? Suggested to be an artefact, or a perception problem (In a space probe?) has it been resolved?

JOhn

[De58te]

And to think they say that all this observable universe, or baryonic universe (if that is the correct term), is only 4.6% of what is out there in the universe! The unseen 95% is dark matter and dark energy.

[orin stepanek]

Exploring the universe will never get done!

[Czerno O]

«If we were, at this moment, 46 billion light years away...» @Chris, could you make it more precise what you meant by the phrase, italicized in the above quote ? Does it even make sense, any sense at all - as it seems to refer to a notion of absolute, universal time and simultaneity that is incompatible to our conceptions of (general) relativity ?
Thank you.

[heehaw]

Galileo was right! The Sun IS the center of the universe!

[Chris Peterson]

«If we were, at this moment, 46 billion light years away...» @Chris, could you make it more precise what you meant by the phrase, italicized in the above quote ? Does it even make sense, any sense at all - as it seems to refer to a notion of absolute, universal time and simultaneity that is incompatible to our conceptions of (general) relativity ?

That's a very good question, because, as you suggest, the whole issue of simultaneity is complex according to (actually special) relativity. It does make sense, but only in context and with more information about how it's being used- normally expressed mathematically.

In this case, I was really just alluding the the idea that no matter where we are in the Universe, at any particular amount of time after its formation, we see the same structure. There's no edge we could reach that would look like the rim of today's APOD.

[Chris Peterson]

And to think they say that all this observable universe, or baryonic universe (if that is the correct term), is only 4.6% of what is out there in the universe! The unseen 95% is dark matter and dark energy.

The observable universe isn't defined by baryonic matter. It is defined by the region we can interact with because it is receding from us at less than c. Beyond the edge of the observable universe space is moving away faster than light, which makes it forever unobservable.

Our observable universe, like the entire Universe, has your stated energy ratios. Our observable universe may be 95% invisible, but it isn't 95% unobservable, given that we can observe both dark energy and dark matter.

[Chris Peterson]

Chris,
The CMB is isotropic, but does that prove that the Universe is homogeneous?

Well, obviously isotropy varies with scale. But I think the general view is that at a larger scale the Universe is close enough to isotropic that we'd see about the same structure from anywhere. If any anomalies hold up to examination, they will have interesting consequences in terms of our understanding of the details of the Big Bang cosmologies, but are likely to be very subtle as far as appearances are concerned.

[Chris Peterson]

I think one misconception images like this give is that if we could travel to the edge of the observable universe, we'd see what the image is showing at its edge. But that's not true. If we were, at this moment, 46 billion light years away, at the outer horizon of today's APOD, we'd see a universe around us that looked pretty much the same as it does from here. It would be our solar system that would be not yet born, its future location buried in the filaments at the edge of an entirely different observably universe.

Every point in the entire universe lies at the very center of its own, unique observable universe, including points that are so far away we'll never be able to see them, or they us.

You have taught me that the center of the Universe is located in time, not in space. The center of the Universe is at t=0, or in other words, at the moment of the Big Bang. Every part of the Universe existing "at the same time" is at the same distance from the beginning of the Universe.

Yes, I think that's exactly it. So to be clear, when I say that every point lies at the center of its own observable universe, I'm talking about every 3D point, not every 4D point (which would be the full description of any location in spacetime). That is, every spatial point with the same value of t.

[tomatoherd]

I think one misconception images like this give is that if we could travel to the edge of the observable universe, we'd see what the image is showing at its edge. But that's not true. If we were, at this moment, 46 billion light years away, at the outer horizon of today's APOD, we'd see a universe around us that looked pretty much the same as it does from here. It would be our solar system that would be not yet born, its future location buried in the filaments at the edge of an entirely different observably universe.

Every point in the entire universe lies at the very center of its own, unique observable universe, including points that are so far away we'll never be able to see them, or they us.

But somewhere there should be a last galaxy that sees none other beyond itself in a least one direction. If every galaxy see more galaxies in all directions, then the universe if by definition infinite, which neither I nor Einstein believes/d. And if you haven't been there, you're by definition making pretty confident assumptions for a momentary flicker of protoplasm called a human.

[JohnD]

Perhaps you should book into the Infinity Hotel, tomatoherd!
Oh! They're fully booked?
No problem! Everyone moves into the next room up, and you get Room 1!

There can be nowhere that is the "last galaxy". What would it see beyond? In what whit did Einstein not believe this? Or you? As for not believing something if you haven't been there, the whole of science, geography, astronomy, physics involves believing something that someone else reports, because they have good reason to do so, that you respect, even if you haven't 'been there yourself'. Only in mathematics can you follow in the originator's footsteps, and be there too, and even then you are likely to sink into the morass and be left behind.
John

[Chris Peterson]

I think one misconception images like this give is that if we could travel to the edge of the observable universe, we'd see what the image is showing at its edge. But that's not true. If we were, at this moment, 46 billion light years away, at the outer horizon of today's APOD, we'd see a universe around us that looked pretty much the same as it does from here. It would be our solar system that would be not yet born, its future location buried in the filaments at the edge of an entirely different observably universe.

Every point in the entire universe lies at the very center of its own, unique observable universe, including points that are so far away we'll never be able to see them, or they us.

But somewhere there should be a last galaxy that sees none other beyond itself in a least one direction. If every galaxy see more galaxies in all directions, then the universe if by definition infinite, which neither I nor Einstein believes/d. And if you haven't been there, you're by definition making pretty confident assumptions for a momentary flicker of protoplasm called a human.

It is entirely possible that the Universe is infinite. But it does not need to be infinite in volume to not have any edges. I know of no serious cosmological theory that proposes a universe which has edges in 3D space. No matter where you are in the Universe, you see the same general structure around you. That is something we can say with a pretty high degree of confidence.

[tomatoherd]

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.

[Ann]

Perhaps you should book into the Infinity Hotel, tomatoherd!
Oh! They're fully booked?
No problem! Everyone moves into the next room up, and you get Room 1!
John

I love that! Is it Douglas Adams?

Ann

[Chris Peterson]

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.

The Universe is a 4D manifold of uncertain geometry. Consider the surface of a sphere. It is of finite area but is unbounded, without edges.

And sorry, but if the big bang began at a geometric point, it could not reach infinite size in 14 billion years.

That is uncertain. Suffice to say that our current cosmological models do not exclude the possibility of an infinite universe. The actual size of the Universe remains an unsettled question. (That it is without 3D bounds is largely seen as certain, however.)

[tomatoherd]

I think one misconception images like this give is that if we could travel to the edge of the observable universe, we'd see what the image is showing at its edge. But that's not true. If we were, at this moment, 46 billion light years away, at the outer horizon of today's APOD, we'd see a universe around us that looked pretty much the same as it does from here. It would be our solar system that would be not yet born, its future location buried in the filaments at the edge of an entirely different observably universe.

Every point in the entire universe lies at the very center of its own, unique observable universe, including points that are so far away we'll never be able to see them, or they us.

But somewhere there should be a last galaxy that sees none other beyond itself in a least one direction. If every galaxy see more galaxies in all directions, then the universe if by definition infinite, which neither I nor Einstein believes/d. And if you haven't been there, you're by definition making pretty confident assumptions for a momentary flicker of protoplasm called a human.

It is entirely possible that the Universe is infinite. But it does not need to be infinite in volume to not have any edges. I know of no serious cosmological theory that proposes a universe which has edges in 3D space. No matter where you are in the Universe, you see the same general structure around you. That is something we can say with a pretty high degree of confidence.

Of course I accept that. For the universe is defined by space-time, not matter or vacuum. There's no measurement beyond space itself.
But if the universe is not infinite, then it does have a shape. Not in its own reference frame, of course, but in another. You/we just don't know what that reference frame is. Could be a multiverse. Or could be a spiritual frame.
And that shape, i will be adventurous to say (and risk mockery), since it's likely not a cube, an egg, or a rubber ducky, is more than likely spherical. A drop of water. And i won't be losing any bets on that.

[Chris Peterson]

But somewhere there should be a last galaxy that sees none other beyond itself in a least one direction. If every galaxy see more galaxies in all directions, then the universe if by definition infinite, which neither I nor Einstein believes/d. And if you haven't been there, you're by definition making pretty confident assumptions for a momentary flicker of protoplasm called a human.

It is entirely possible that the Universe is infinite. But it does not need to be infinite in volume to not have any edges. I know of no serious cosmological theory that proposes a universe which has edges in 3D space. No matter where you are in the Universe, you see the same general structure around you. That is something we can say with a pretty high degree of confidence.

Of course I accept that. For the universe is defined by space-time, not matter or vacuum. There's no measurement beyond space itself.
But if the universe is not infinite, then it does have a shape. Not in its own reference frame, of course, but in another. You/we just don't know what that reference frame is. Could be a multiverse. Or could be a spiritual frame.

Well, I'll disregard "spiritual frame" as pseudoscientific nonsense. But an unbounded universe with some higher dimensional shape does not require a physical reference frame. The fact that we describe it mathematically from a higher dimension or some other "outside" reference does not mean such a thing physically exists.

And that shape, i will be adventurous to say (and risk mockery), since it's likely not a cube, an egg, or a rubber ducky, is more than likely spherical. A drop of water. And i won't be losing any bets on that.

There are other shapes which remain distinct possibilities, not yet excluded by the precision of our measurements. The Universe almost certainly isn't spherical... but perhaps you just meant that by analogy. At its simplest it could be a hypersphere.

[othermoons]

Another work of art using scientific data. It's amazing how in all the chaos around us and from our point of observation there seems to be balance when observed from a larger perspective.

[tomatoherd]

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

Oh, you mean like "dark energy"?
(sorry, i couldn't resist.)

I'll stop being a troll now.
I appreciate the patience and the repartee, even though i am out of my league.

[ta152h0]

Where in this image is the recently discovered " contact binary " set to explode in the next 5 years ?

[japepper]

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.

[JohnD]

Anne,
The Infinity Hotel is not mine, or even Douglas Adams', but from David Hilbert (1962-1943) the German mathematician. Neat, isn't it?

tomatoherd,
Consider a 2 dimensional being - or read Edwin Abbott's "Flatland" . Does their plane have a edge? They may think, of course it must, but YOU can see that it lives on a sphere, with no edges, because you are aware of three dimensions. Then consider a square, which would be the only regular rectangular shape that our 2D friend could see, and then as a straight line. You can see a cube, the 3D shape with six square faces, the 3d eqivalent of a square. You can draw a cube on paper, in 2D, and envision that cube.
Now, if you were a FOUR dimensional creature, your world could have a 4D cube, a hypercube that, as the our 3D cube is composed of 2D squares would have eight 3D cubes, in the same way. As you can draw a 3D cube in 2D, we can draw a 4D cube in 3D in the same way. And so on, five, six, any number of dimensions. Prepare to have your mind blown:

https://www.youtube.com/watch?v=-x4P65EKjt0

More than the three spatial dimensions that we are familar with is not just possible, but necessary, for modern cosmologies. As many as Eleven - they tell me!

Hold those ideas about higher dimensions, think about the Shape of the Universe, and watch this. It's Carl Sagan, teaching as long ago as the 1980s, but he's such a good teacher!
https://www.youtube.com/watch?v=3WL_vtu4r1w
Enjoy!
John