If the universe is 13.8B years old, how can we see 46B light years away? (2018)

If space wouldn't have expanded sufficiently fast in the beginning of time, all matter would be still clumped together as in a giant black hole. The fact that space expanded "faster" than the speed of light allowed this black hole to become free matter.

But then that expansion ratio may of been different over time, but it is a bit like sampling the second hand on a clock in a photo and working out the entire motion and workings of the clock - at least on the scale of the data/measurements we have and the age of the universe. Currently we are still not 100% sure when midnight was, but as always moving in the right direction.

PBS space time has a good video on this - https://www.youtube.com/watch?v=msVuCEs8Ydo

Metric expansion does not result in causality issues because it's an isolating phenomenon (one that slows down interactions between things) rather than an interconnecting phenomenon (one that would speed up interactions between things). Therefore it's at least possible, and observations (which you can read a brief summary of in TFA) back it up.

> If what we’re seeing was 13.8B light years away 13.8B years ago and are now 46B light years away, wouldn’t that mean that those things traveled ~32.2B light years in 13.8B years.

No. What we're seeing was 13.8BLY away (in our reference frame) when the light was emitted. Those things are 46BLY away now (again, in our reference frame) but we can't see them now, we can only see them as they were 138B years ago.

It's like a blowing up a balloon. There's no limit to how big it gets or how fast it grows, so objects on opposing sides can move apart at greater than the speed of light even if they're not moving themselves.

This rate of expansion is accelerating too, driven by something called "dark" (as in unknown) energy which is overpowering all the gravity that should be pulling things together. It's fascinating to think that in 100s of billions of years, our local galaxy will be the only thing we can see in space. Any civilizations then would believe that there's nothing else out there.

That doesn't make sense if you say the universe is curved or infinite. There will be no edge.

> - Space could be infinite but at the same time with a boundary. Can you imagine that boundary without trying to imagine what it is bounded from?

No. That's what the definition of infinite is. There are no bounds.

> - What is space fabric actually made from? Is it discrete or not.

Probably numbers. But then I like Math, so...

> - Are there other universes outside ours?

I don't think we are any special. But I'd be happy to be proven otherwise!

> - Try to imagine nothing.

> - Try to imagine infinite.

You are having trouble with nothingness because you are linking it to your consciousness. Infinity is easy though. You keep pushing infinitely and there is no edge!

> - Is there free will?

No. But don't go and do stupid stuff because of that.

> - Can you even imagine a proper mechanism that would allow for free will to exist?

There isn't. You are the product of your environment and your parents.

> - What would prevent you from "cutting" the smallest possible particle in 2 parts?

Max Planck.

> - Are there several dimensions of time?

> - Is there a beginning of time?

There is no time. Try understanding the theory of general relativity.

> - Is math invented or discovered?

Math is very basic. It starts with set theory and then builds on top of that. It is invented since set theory is invented.

Is (curved, infinite) sine-like ripples an option?

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

This limits our ability to reach galaxies further away from us as they are speeding away faster than we could conceivably travel since our engines would of course be bound by the speed of light.

Some more info: https://www.forbes.com/sites/startswithabang/2016/06/10/can-...

"reach" is possibly the wrong phrase, since it is the corollary of the "observable universe", one might call it the "observing universe", which is the slice of the universe that light from our present day can ever reach due to the accelerating expansion of the universe and the limitation of the speed of light.

No, the evidence suggests that the rate is time-dependent; that is, the expansion is accelerating.

> how do all the parts in space coordinate to expand at about the same steady rate?

This is a good question!

The answer is: the coordination happens in the much earlier much denser universe in our past, just like the coordination that happened among the fields of the Standard Model that led to the earliest atoms and molecules.

Let's first take the case where there is no accelerated expansion (which is unlikely with current evidence). In this case, the mechanism can simply be inertia, with an enormous early impulse like those in the family of cosmic inflation theories. There must be a fine balance between the density of matter in the earlier universe and the impulse, because gravitational interactions will decelerate the expansion from the early impulse until the matter-density drops/dilutes-with-expansion to a critical point after which the rate of expansion will be constant.

Observations support an early deceleration and subsequent fixed-rate expansion, but about 8 billion years ago the expansion began to accelerate, as seen in images of galaxies with redshifts less than about z ~ 0.5.

> a simple underlying mechanism

This is what the study of dark energy is about: a simple explanation for the accelerating expansion of the universe.

Typically one starts with a field that fills the whole of spacetime, with enormous energy-density in the early universe, with that energy-density decaying at later times. In extremely high-quality vacuum and far from structures like stars and black holes, the decay of this field is faster than near those structures (too near and the field becomes locally stable; the solar system, for example, is not expanding). When the field decays, it gives a local increase to the inertial expansion rate described in the no-dark-energy case a couple paragraphs up. Because there is so much empty space outside galaxy clusters, there is a lot of local, and the amount of local increases over time, so larger regions of the field decay at faster rates as the unvierse gets older.

There are other approaches too, but the effect is the same: the amount of space outside matter structures increases at an accelerating rate in the late universe, and this corresponds in a frame-dependent way to an energy-density that remains constant as the energy-density of the fields of the Standard Model decreases. That is, in such a frame of reference, matter and radiation dilute away with the expansion MUCH faster than dark energy does.

Another approach is literally the cosmological constant, which if zero gives the inertial picture above, but if it takes on a small positive value, it produces a late-time acceleration of the expansion. No known fields behave like this: matter-density varies by location in spacetime (clumpier in space, tending to diffuse apart (especially light) or collapse gravitationally (especially baryons) over time), gravitational fields vary with position in spacetime and typically with the distribution of matter. By contrast, the cosmological constant is literally identically everywhere-and-everywhen. More likely we will find evidence for something which closely approximates the cosmological constant at large scales, with differences in the density of the "cosmological not-quite-constant field" lining up very closely with observed differences in the density of matter.

> clever nature of this mechanism

Cleverness shouldn't be expected of the mechanism itself, although it will probably take some cleverness to describe the mechanism in a tractable/scalable-yet-accurate-in-detail manner.

WRT your final sentence, General Relativity works just fine in the total absence of dark energy, and dark energy as the cosmological constant has been captured as the factor \Lambda in the Einstein Field Equations. That's like saying that all matter has been captured as a factor in the Einstein Field Equations, though: it has taken a lot of work including things like studying https://en.wikipedia.org/wiki/Deep_inelastic_scattering to work out many of the finer details of the stress-energy tensor. A comparable program likely is needed to work out the finer details of \Lambda, which might lead to a formal explanation along the lines of the informal one a few paragraphs above.

If space wasn’t expanding, the distance to a stationary object (in light years) would be exactly equal to the travel time of light from it (in years).

The answer is, if you can't even conceivably test for it, it's mostly irrelevant to think too much about.

I don't think it's just you. Maybe I can help; unfortunately developing intuition without going through the mathematics even at an introductory level [as in [1]] is a hard road.

The tl;dr is that the objects are certainly moving apart; the question is whether or not the distance between them is increasing, and that depends on the coordinates in which one measures distance.

In the cosmological case (and there are others in other areas of physics) the distant astrophysical objects aren't moving spatially in one particular set of coordinates, and that set is deliberately constructed to be comoving with the moving objects. Instead, the coordinates' spatial labels themselves are time-dependent, and so absorb the expansion of space. Schematically, an astrophysical object that is at (t=5,a,b,c) in the past will be at (t=4,a,b,c), (t=3,a,b,c) and so on, and in the future will be at (t=6,a,b,c), etc., where a=const,b=const,c=const. The bookkeeping of keeping a,b,c constant for all a,b,c is rolled into the metric, a factor in the Einstein Field Equations, and so one typically hears about the "metric expansion" of the universe in cosmological contexts.

(The astrophysical objects themselves are an idealization of the largest known gravitationally-bound objects: galaxy clusters. The idealization represents them as particles of an ideal dust, to underline that the metric expansion is adiabatic).

One is perfectly free to choose non-comoving coordinates; a change of coordinates does not change the physics of the expanding universe, merely how the physics are represented in calculations. For instance, one can apply spherical coordinates with the origin on you (or some idealization thereof) at all times. Jumping up and down moves everything else in the universe in that set of coordinates. The movement of the Earth, solar system, etc., all results in distant stars moving in that set of coordinates. In those coordinates, the metric expansion is a tiny movement of distant objects in those coordinates compared to you just turning 90 degrees to your left. Distant galaxy clusters can be held still in those coordinates by flying in an aeroplane.

I say an idealization of yourself, because what are you from moment to moment? Several kilograms of gas and liquid pass through you over the course of a day. Tissue turnover happens at scales of weeks to months. You weren't alive at all a couple hundred years ago, much less several billion, so where is the coordinate origin supposed to be then?

Likewise, galaxy clusters have internal motions, radiate, have gas outflows; some collide and merge. The earliest galaxies weren't around at the time the cosmic microwave background became the surface of last scattering, but idealizing on a dust (which is a type of fluid) lets one use the comoving cosmological coordinates sensibly even that long ago. And also into the far future when the universe is so large that there will be on average only one "dust" particle per Hubble volume, and galaxies may have disintegrated into radiation or collapsed into black holes or both.

If we were to switch to spherical coordinates on some well-chosen point within our own galaxy cluster, we'd note that distant galaxies' motions are overwhelmingly radially outwards. We'd still see the (least gravitationally lensed) distant galaxies in our sky holding to a pattern: angularly-smaller, redshifted quasar spectal lines, molecular gas spectral lines, and spectral lines of younger and younger-generation/lower-metallicity stars. It'd be reasonable to think about the implications for RADAR-like round trips, and the connection with RADAR-painting e.g. an aeroplane accelerating away from you during it's take-off and climb. But we also want to think about what two other galaxy clusters would see in their sky, and an expanding universe would give each of those a highly similar view. But they might not even be able to know about the spherical set of coordinates above, and certainly would not find them useful for describing their views of the cosmos.

One might pause here for a moment to consider how an Earth-centric astronomy, a Moon-centric and a Mars-centric astronomy, would each have epicycles: different ones for Mars and Earth with respect to the other planets, and very different ones for the Moon with respect to the other planets. Presumably independent civilizations throughout our solar system would eventually come up with a sun-centric astronomy.

The comoving coordinates seem very likely to be discovered by any comparable astronomers who discover and study in detail the cosmic microwave background, and eventually figure out how to remove the 10^-3 to 10^-6 local-motion multipoles from it [2]. That's at the root of the cosmological coordinates: in them the cosmic microwave background has only tiny deviations from an ideal blackbody radiator at a temperature that decreases with time according to an expanding ideal gas law (for massless gas -- a gas of light, or a photon gas if you like). And the coordinates are useful because later large scale structures (active galaxies and so on) also end up with only small deviations from an expanding ideal gas law for a massive gas, and those deviations are straightforwardly explained by the gas being self-sticky (it clumps, unlike the radiation gas).

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[1] "Many distances" http://www.astro.ucla.edu/~wright/cosmo_02.htm

[2] T.M. Davis et al., https://arxiv.org/abs/1907.12639 in section 4 has an excellent overview of approximately this process for our civilization.

First, note that relative velocity and growth of distance between objects are distinct concepts: Shoot two bullets at 0.9c each in opposite directions, and from your perspective, the distance between them will grow at a rate of 1.8c, whereas their relative velocity (as computed from the 'angle' between their 4-velocities) will only be 0.994c.

In special relativity, that relative velocity will correspond to the growth of distance between the bullets as measured by an observer comoving with either bullet.

In general relativity, things are more complicated: Curved spaces lack distance parallelism, so you can only compare the velocity vectors of the two bullets when they are located in the same place at the same time. Also, there's generally no longer a canonical way to decompose spacetime into spatial slices across which you can measure distances.

Now, you can still define a relative velocity through parallel transport along a path connecting the two objects, and in particular the path of a photon emitted by a distant galaxy and observed by us. This will in fact recover cosmological redshift as a generalized Doppler shift. If defined this way, the relative velocity goes to c as you approach the cosmic event horizon, hence you'll never observe relative velocities larger than c.

However, while there's no canonical spatial slicing in general relativity, in Friedmann cosmology specifically, a preferred spatial slicing does exist: The one where the universe looks homogeneous and isotropic and the same amount of time will have passed since the big bang (as measured by an observer following the Hubble flow). This allows us to define the proper distance (at constant cosmological time) between astronomical objects, and the growth of this distance is what's known as 'recession velocities'. These velocities go to c at the Hubble sphere, which doesn't really have any particular significance: We can see galaxies that were located outside the Hubble sphere just fine (though as time goes on, the Hubble sphere will approach the cosmic horizon asymptotically).

Space inside the edges has photons flying in it, while this photon travels, space is expanding in all of the photons directions.

This would mean that a photon from 5 billion years ago would take longer to get here, and its source is even further away than the speed between us and the source would make it be.

What I think about during all this: If space is expanding everywhere, then it is sort of pushing things away from each other, meaning that objects are moved, like domino's everything is moved a bit so that everything has more space in between.

Do I have to separate expansion of the universe vs expansion of space?

https://en.m.wikipedia.org/wiki/Expansion_of_the_universe#To...

Sure he goes for clickbaity articles now and then, but he's pretty good at them in my experience, and your summary is both incorrect (there is no "current" over that distance scale) and leaves all explanation out.

Your "correct" version on the other hand, leaves more questions than answers, not least of which being how do we ever get 46B ly distance in something 13.8B years old. Then you have to cover the expansion, relativity, and hey presto you've written the article. Just skimming the other comments here makes it clear it's not a topic that's well understood or intuitive.

Clickbait it ain't, and the title is a very fair question.

I'm really disappointed to see this the top voted HN answer right now...

This comment heavily reminds me of the mannerisms of professors that have the mentality of "I deeply understand this subject from years of past studies, so you should be able to understand it from my vague and half assed explanation alone".