Map of the observable universe | Hacker News Reader

Map of the observable universe

mapoftheuniverse.net

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Nice visualisation, but the explanations contain a bunch of mistakes. The cosmic microwave background is not "the first flash of radiation" after the big bang (which would've been much earlier), it is the "wall" of last scattering before the universe became transparent enough. And its distance is also not limited by light travel distance within the age of the universe (13.7Gly) but by its rate of expansion, resulting in an actual distance closer to 40Gly.

Indeed, the Cosmic Microwave Background is the light that's still left over from a time when the Universe was much hotter and filled with plasma. At a certain point, the Universe cooled down enough to become neutral, which made it mostly transparent, and allowed the photons that had been bouncing around in the plasma to travel indefinitely.

> its distance is also not limited by light travel distance

At cosmological scales, there is no single, correct definition of "distance." There is coordinate distance, light travel distance, angular diameter distance, luminosity distance, and other measures.[0] All these distances are the same at very small scales, but when you start looking at larger scales, General Relativity starts to matter and the geometry of the Universe is no longer Euclidean.

0. https://en.wikipedia.org/wiki/Distance_measure

wow, your last paragraph filled a little empty gap in my brain, thank you!

On a global scale, the universe as a whole is actually remarkably close to euclidean [1] (which is a mystery of its own). So it's not curvature that's the problem, but expansion. To account for this, people usually quote the comoving distance [2] (which factors out expansion) when talking about very distant things like the microwave background. I.e. the distance light would have to travel today if you could freeze the evolution of the universe for the duration of the journey.

[1] https://en.wikipedia.org/wiki/Shape_of_the_universe

[2] see your own source

Only a constant-coordinate-time slice of the Universe is Euclidean. That's a 3D subspace of the 4D spacetime volume. The 4D metric of the Universe is not close to flat (or Minkowski, which is the closest thing you'll get to Euclidean in Relativity).

People quote different distances, depending on what they're using it for. Probably the most widely used distance measure, from Earth's perspective, is redshift. It's linear with distance nearby, and ask the other distances are relatively simple functions of it.

>The 4D metric of the Universe is not close to flat (or Minkowski, which is the closest thing you'll get to Euclidean in Relativity)

It is. See the source provided above. Technically the ΛCDM universe follows the FLRW metric, but if you look at its line element, you'll see it's usually just Minkowski in spherical coordinates times a scale factor (capturing the expansion) for spacelike coordinates:

ds^2 = -dt^2 + a(t)^2 dΣ^2.

Note that a scale factor that affects all spacial coordinates the same does not induce curvature. The generalised form of the FLRW metric includes a 1/(1-k) factor for the radial coordinate, but the curvature k for our actual universe is measured to be 0 within experimental precision, which, again, is a mystery. There's no good reason why a universe containing matter/energy should be globally flat (*cough* inflation). Factor out a(t) in a line integral and you'll get comoving distances over more or less euclidean space, i.e. the common format in cosmology literature. Things like luminosity distances are more interesting when you e.g. want to know the absolute magnitude of a far away object, but that doesn't really make sense for the CMB.

>Probably the most widely used distance measure, from Earth's perspective, is redshift.

Redshift is used to measure these distances sans any reasonable alternatives. But when you want to make e.g. a map, you can't use redshift (or proper distance for that matter), unless you want your map to be distorted.

I'm well aware of what the FLRW metric looks like. That a(t) factor means that the geometry of spacetime is not flat. For a full explanation, see this StackExchange answer: [0].

You're confusing the flatness of a particular 3D hypersurface (of constant coordinate time) with flatness of 4D spacetime. The k parameter describes the curvature of that hypersurface, not the curvature of spacetime.

This is all to say that we don't live in a flat Minkowski spacetime. We live in a curved spacetime, which means that things like distance measures are much less straightforward. The curvature of 3D slices of spacetime is not the primary complication - the overall curvature of 4D spacetime (which involves the scale factor a(t)) is the primary complication.

0. https://physics.stackexchange.com/a/689320

When looking at spatial distances, we usually want to look at spatial coordinates. Note that there are no cross terms with a(t) in the metric, so the expansion factor (which is divided out if you look at the definition of comoving distance) can easily be accounted for. No curvature shenanigans necessary. Ofc that also implies looking at a single cosmological time, that was never disputed as it is also literally part of the definition. This in turn simplifies the whole idea of distances across cosmological scales, which is why it is so commonly used. It might depend on the time you choose, with today's proper distance for example being equal to comoving distance if you could freeze the universe. But you can't really do that, so it makes more sense to talk about comoving than proper distances. All of this is explained pretty well on wikipedia, but if you want you can also take a look at Hartle's or Carroll's textbooks on General Relativity.

> When looking at spatial distances, we usually want to look at spatial coordinates.

That's not the case, because most objects we're measuring distances to are not at the same coordinate time as us. In fact, these objects are generally in the past.

If you're trying to measure the distance to an object that you've observed, that object is, in fact, along your past light cone. The only coordinate-invariant distance that one can actually define, the metric distance, is zero. That means you have to define an alternate type of distance. Some of those choices (like the comoving distance) don't depend on a(t), while some do (like the light travel distance).

> All of this is explained pretty well on wikipedia, but if you want you can also take a look at Hartle's or Carroll's textbooks on General Relativity.

I prefer Wald, which is much more mathematically rigorous than Carroll. I haven't read Hartle, so I can't comment on it.

"global" in your first sentence was a tough word choice

It's a common term in mathematics and when talking about field theories.

I know. But context matters. When talking about distance and scale and the physical universe, "global" will be read as "planetary" by many. (shrug, no big deal)

And it's not an "actual photograph" LOL.

Also, the dark ages explanation is mistaken. From time 0 to 370,000 years: there were a bunch of short lived photons, but the Universe wasn't transparent to light. After a while, recombination and decoupling ended and this source of light ceased to exist, making the Universe dark again.

Only after ~200 million years the first generation of stars appeared, which we cannot hope to see with a telescope. The first proto-galaxies (too faint) and galaxies took even longer, hence this billion years of darkness.

I'm an atheist, but in place of spiritualism I have the mysteries of the universe.

Is the universe infinite? It appears to be based on existing evidence. And if not infinite, then being geometrically flat makes it unthinkably vast multiplied by unthinkably vast.

How did the universe even start? And how can that question make any sense at all without time? How can there be "before the beginning of the universe" when "before" is not a concept that exists?

Fabulous questions to ponder.

A genuine atheist, or an agnostic? If the former, why deny yourself another fabulous mystery?

I’m a deep atheist.

Nothing could convince me there is a god.

I simply don’t believe in any sort of god at all. No heaven, no hell. When you’re dead, you're dead.

You are welcome to your own beliefs. I won’t impose mine on you and don’t impose yours on me.

What if we’re in a simulation? Couldn’t you reasonably call the creators gods?

This is the line of thought that got me to abandon strict atheism, as if we are in a simulation (something I allow to be plausible if not probable) surely the computer running the simulation would be God.

So perhaps universal god atheist, but local god agnostic?

The problem is then what created the local god? Another simulation?

Even if it's turtles all the way down you still have the problem why does the infinite turtle system exist at all?

Heaven, hell, life after death are all completely unrelated to whether or not there is a God. Conflating them makes the question more prosaic.

Perhaps for students of theology/philosophy. For an atheist, having to debate them all separately creates a situation where the burden of debating each individual point (regardless of its logical soundness) is too exhausting of an effort and is unlikely to change anyone’s mind.

As such, for claims that haven’t or can’t be proven by the scientific method, the burden of proof should probably lay with the one making the claim, not the one defending against it.

Of course, in my experience, the scientific method is fundamentally incompatible with most modern religions. Hence, there is really no need to discuss these things, as much as I’d like to. You’re just going to come across as imposing and/or tone deaf.

a belief in god is not synonymous with religion and the former is orthogonal to the scientific method. questions that can’t be proven by the scientific method have no concept of a burden of proof. thus the failing of logical positivism.

I said most modern religions, and in my experience.

Isn’t it possible that incredibly advanced beings exist somewhere? And if so, what if they colonized earth long ago? Wouldn’t we call them “gods”?

The concept of God in Abrahamic mythology isn't about ancient beings that colonized the Earth. It's about a being that purposefully constructed our celestial sphere for our benefit. And most polytheist religions used gods as a proxy for natural phenomena that they couldn't explain otherwise (such as lightning, seasons, day/night, crop yield, etc).

I know I should be amazed by this, but whenever something like this pops up I get just a little bit sombre. With everything humans have accomplished, it still seems absolutely miniscule when looking at the scale of everything out there. More so, there's a sense of loneliness. Yes we have each other, but beyond that? We're all we've got for now, maybe ever. I just wish I could live long enough to know.

Potentially adding some somberness:

Feelings like somberness and loneliness only exist in your mind, as a result of how evolution formed how our brains work. It doesn’t have any real meaning outside of our current restraints. In fact, meaningfulness itself is a concept that only exists within our minds.

That set aside, what I find more disappointing is how the speed of light, the accelerating expansion of the universe, and the inescapable increase of entropy place serious constraints on anyone’s future.

Explain it to me like I'm five:

Why can we only see 90° of the sky?

I look at the map at the bottom of the page and wonder why it isn't a circle. I'm obviously missing a key concept.

Not sure if this answers your question, but from the description tab, under "What is this map?"

The full map is actually a sphere. This visualization shows a thin slice of the Universe. Its thickness is about 10 degrees. More astronomical data is available but it is not possible to show all of it at once on a 2D map. The image would be completely saturated with dots.

It actually visualizes the slice depicted on the main page as a 3D rotation. I would struggle to explain the concept of arc slicing to a five-year-old though.

> Its thickness is about 10 degrees.

Does that mean their slice is only 1/36th of the whole picture?

It's 1/72nd of the celestial sphere I think, since a single rotation gives you the east and west views in full, but due to the 90° field of view, you need to repeat that rotation in the north-south direction to cover the entire sphere.

I always appreciate content like this which is easy to digest but still inspires awe in the universe around us.

Scrolling around the map and looking further and further back in time reminded me of this Kurzgesagt video [0]. It's crazy to think how much of the universe is lost to and unreachable by us.

Kurzgesagt has a lot of great content (even if it's a bit cartoonish). I'm sure they get things wrong, but I feel like they make an effort to research the topic first.

[0] https://youtu.be/uzkD5SeuwzM

"Map of the Observable Universe" (big difference, and the actual title on the page itself)

The biggest difference in existence.

Ok, we've made the universe observable in the title above. Thanks!

Next stop: the stars. All of them. That we can see, anyway O_O

Reminds me of https://hitchhikers.fandom.com/wiki/Total_Perspective_Vortex

I don't understand. Is that thing from fiction, real or both?

You should start here:

https://www.amazon.com/Complete-Hitchhikers-Guide-Galaxy-Box...

Until we have discovered how to travel instantly through time and space all of our models of the universe are surely off target. Why do we think that the physics of matter are the same everywhere as if our solar system represents the benchmark of everything that exists everywhere? The possibilities of interacting systems of physics could be present in the entirety of space? What we are able to perceive from our vantage point is indeed inestimably small in face of what actually exists out there.

> Why do we think that the physics of matter are the same everywhere

Because we don't have evidence that they aren't.

Hope there's similar map of universe of all algebraic structures.

https://ncatlab.org/nlab/show/HomePage maybe?

Why only 90 degrees?

Well, we can't get a full 360 degrees because of the zone of avoidance caused by the center of the milky way.

Also after a particular number of degrees it doesn't matter quite so much, the universe is isotropic enough that looking in any direction pretty much gives the same results.

I suppose we only have one point of reference to make observations from, but it feels a little strange how the graphics tend to give a false sense of us being at the center of the universe.

Would this graph essentially look the same if the observations were made elsewhere, like way over near the redshifted elliptical galaxies? Sometimes it's difficult to wrap my mind around the combination of distance and time represented at these scales.

Not a physicist but that's how I understand it, anywhere you go in the visible universe should have a perspective that looks more or less just like ours, as if they were at the center. We can only see so far though, so someone a billion light years from here would have a visible universe that overlaps with ours like a Venn diagram, but would be seeing things we can't. Who knows, maybe there's something really interesting they could see that we cannot, like an edge of the actual universe with nothing beyond?

I think it said right on the map that it's not only essentially the same in all directions, it's pretty much the same from any vantage point, since the distance makes most of the difference in what would be seen.

We are the center of the Universe. Of our observable universe, at least, by definition. And the total Universe might be much bigger, potentially infinite [or even smaller than the visible universe!]. Even if finite, the Universe would have no center. It's different from a finite amount of matter forming a sphere within an infinite grid, but much like a spherical surface that has no center (the sphere does, the surface does not).

> Would this graph essentially look the same if the observations were made elsewhere

Yes, an alien in a very far away galaxy would see a similar picture. If they are within a few billion light-years, they would see a younger red-shifted Milky Way.

> This map shows a slice of our Universe. It was created from astronomical data taken night after night over a period of 15 years using a telescope in New Mexico, USA.

So, this is the slice that the telescope was observing.

> Beyond this distance, galaxies are harder to see. However, we can still see quasars. They are much brighter and bluer.

But don't we have pictures of galaxies going back to the earliest universe? I thought that JWST is specifically made to image them.

Sometimes I'm in wondrous awe at the unfathomable scale of it all, and other times I can understand why the Krikkiters were so upset.

Are there any sites that have an actual map (i.e. like google maps but for the universe)?

Where is the dark matter?

As I understand it, dark matter is loosely clumped around the bright matter, held by gravity but otherwise not interacting with it.

According to the main view, dark matter floats throughout the galaxy (and outside) but is much more concentrated near the center. A tiny amount is probably crossing your body right now, but there's no way to feel it except for the minuscule gravitational force coming out of nowhere.

Dark matter appears to be distributed like a web, with concentrated regions responsible for normal matter collapsing into galaxies.

You can roughly "see" the dark matter web by looking at the web like distribution of galaxies.

Awesome! Loved it! Simple and so fascinating.

Is there an actual 3d map like this?

Would definitely be cool to have a 4D map, in a sense.

Like a google sky type interface with a timeline scroll.

How complete is it?

Neat! Does someone know why there is a band of white/blue galaxies between the redshifted ones and the edge of the universe? I would've assumed it kept getting redder!

Those are quasars (which are why they are bluer). Beyond a certain distance galaxies become undetectable, but quasars are much brighter.

according to the map itself, about 90°

Actually we cannot see far (other galaxies, early galaxies, quasars) in some parts of the sky because we are in our own galaxy plane [1], so stars, but mostly gas and dust between them obscure significant portion of our field of view [2]. So it is not flat 90 degrees, but roughly two cones with 90 degrees openings in Milky way's plane perpendicular directions.

1. https://scitechdaily.com/gaia-spacecraft-discovers-parts-of-...

2. https://www.darkenergysurvey.org/supporting-science/large-sc...

In a few more millennia, our arm of the Milky Way will spin around so that we are able to the other side. We should be able to get some decent parallax measurements to distant objects at that time as well.

About 57,500 more millennia till we get that quarter turn of our galactic orbit

So that's what, 2 days on the Cosmic Calendar?

https://en.wikipedia.org/wiki/Cosmic_Calendar

When I click the arrow to go down, it does nothing.

Wonderful

photons get stretched?

Their wave lengths get stretched.

A roughly accurate analogy: think of a boomerang thrown in outer space, moving at a constant velocity, but slowing its rotation over time (for some reason - work with me). The result is that over time, each loop of the boomerang will happen over a longer and longer distance.

The boomerang isn't stretched, but its spiral looping is.

wow very interesting

COOOOOOOOL!

I am a bit disappointed that they didn't mark where the alien civilizations are currently living.

Also add-on feature would be nice to find some tribes on the map that have either "scrolls of ancient wisdom" or a good chance of a "an advanced tribe" I'm less interested in the "valuable metal deposits worth 50" but that's far better than "unleashed a horde of barbarians!" but I guess it's a gamble for every tribe you find.

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