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Comment by Archelaos | Hacker News Reader

Layperson's question: The article talks about "a possible galaxy that exists some 13.5 billion light-years from Earth". But isn't that just the light traveling distance, and the object should actually be much further away due to the expansion of the universe?

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It sound odd, but that depends on what you mean by "now"[1]. If you mean, where would the galaxy be if I traveled there at the speed of light, you would actually travel less distance due to Length Contraction [2].

[1] https://en.wikipedia.org/wiki/Relativity_of_simultaneity [2] https://en.wikipedia.org/wiki/Length_contraction

It sounds odd to interpret it like that and not as usually a layperson ignoring relativity&physical limitations would mean it?

"Now" as in if I could just teleport there!

Speaking as someone who used to teach introductory cosmology at a good university ... your question is not a layperson's question! (That's a compliment, BTW.)

What is the expansion of the universe? Or should I first ask, is there a center?

The best analogy I've ever gotten is to think of an expanding balloon. Where on the surface of the balloon would you call the center? And if you imagine yourself standing on the surface while it expands, you will be still while all other points of the surface move away from you.

It's far from a perfect analogy, but probably the best way to visualize in human terms.

What is the expansion of the universe?

The distance between galaxies (that aren't gravitationally bound to each other) increases over time.

Or should I first ask, is there a center?

Probably not. If you inflate a balloon, any point on its surface will move away from any other point on the surface. And yet, no surface point can claim to be central.

Note that in the balloon example, there is of course a central point within the volume of the balloon. This is where the analogy breaks down: In case of the universe, we do not assume the existence of an ambient higher-dimensional space.

A perfect geometric surface of a sphere that is expanding has no center, but a balloon has a center. That's obviously the nozzle. The analogy breaks down in both places.

But who knows? Maybe the analogy is correct. I don't think we can say. In fact I would even wager that a "center" is more likely... because can you even imagine a real world analogy where something expanding has no center?

If you can't then there probably is some sort of center.

There's nothing special about the nozzle; it's just another point. The nozzle isn't the center. But if you can't get past that, imagine a balloon without a nozzle, with a canister of compressed gas inside, and the compressed gas is leaking out of the canister.

I mentioned a perfect geometric sphere that is expanding has no center on that surface.

I also stated that no real world analog of a perfect expanding sphere exists. Because we can't imagine an analog it's much more likely that the universe DOES have a center similar to the nozzle.

Try to imagine a real world analog to the expanding geometric sphere without a point of discontinuity. You can't. And because you can't maybe it doesn't exist. Maybe it does. But not being able to imagine a real world analog is slight evidence that there there is a center.

As soon as you can imagine and reason about infinity, you might be able to reason about our universe. “Center” makes sense for most bounded things but might be completely meaningless in an unbounded universe.

Example: the number line. You might be tempted to say 0 is at the center but… there are an infinite number of things on either side of any number you choose.

Does that mean nothing is the center of the number line?

Or everywhere is the center of the number line. Some people argue 0 is the center based on symmetry. My original point is that it is non-trivial to reason about infinite spaces. Since we do not know if we live in a bounded space or not, we can’t assume there is one center. Furthermore, even if we do live in a bounded space, there are different ways of measuring center. Eg: center of mass vs geometric center of a volume. Knowing both and comparing them would be pretty interesting!

0 is the center of an infinite number line.

Additionally nobody knows if the universe is infinite.

From an observer's point of view all the oldest gallaxies are distributed in the "surface" of a sphere of radius 13.5BLY, that surface IS the center of the Big Bang, the past, and the center of "now" is the point of the observer. Isn't this mind-warping?

Specifically, Wisconsin is the center. ;-)

The way to reconcile this paradox is that every person experiences the same thing, wherever they are located in the universe, but each person sees a different surface.

Best explanation of "the center of the big bang is the past" I've seen, bravo.

To add my dumb question: since the universe expands faster than light, how is this considered a very far object and not just that we're looking at something in the past? Is this confusion of mine between distance and time what physicists mean by space-time?

If that's the case, why isn't the article titled "astronomers just discovered the oldest object in the known universe"?

Distances of this sort of thing are not measured directly - what's measured is its red shift, which tells us both how distant it is (very) and how old it is (very). That inference is indirect and depends on the model of cosmology, but the relevant bit of cosmology is relatively uncontentious.

(But if it really was only 13.5 billion light-years from Earth then you'd be right. In fact it's much further away than that.)

Well spotted yes, by now that object will be about 45 bn ly away. The light reaching us from it has only traversed 13.5 bn ly though.

Define "now"

https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity

While that’s a problem in theory, to moderate accuracy it’s not a practical issue. Most objects in the universe are roughly at rest relative to the cosmic background radiation (in other words they don’t see it as Doppler shifted in any particular direction), so we can use that as a common frame of reference. When we’re talking about ballpark integer billions of years, or even to a few decimal places, that’s easily good enough.

Accuracy? "Now" is an inaccurate (nonsensical, actually) concept except in extremely local conditions.

It's perfectly sensible given a common frame of reference, which we have thanks to the CMB. Frames of reference aren't local. We have clocks and can measure time on them just fine, and 'now' is just a time on a clock.

What are you basing the 45 bn ly on? A redshift of z~13 should correspond to a comoving distance of about 33 bn ly, whereas 45 bn ly would correspond to z~600.

You may be right, I've just seen ~45 bn ly given as the radius of the observable universe accounting for expansion. The discrepancy could be due to changes in the rate of expansion over time, as it hasn't been constant.

45.5 bn ly is the approximate comoving distance of the surface of last scattering (the source of the microwave background and the point where the universe becomes opaque). This is the boundary of the universe observable via electromagnetic radiation.

“Light-years” literally is light-traveling distance, though. :)

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