The Last of the Universe’s Ordinary Matter Has Been Found | Hacker News Reader

The Last of the Universe’s Ordinary Matter Has Been Found

quantamagazine.org

160 pointsamaks61 comments

The hypothesis is they should see CMB distortion between galaxies, which is hidden by the distortion caused by the galaxies themselves. The solution was to assume they were removing the distortion caused by the galaxy halo, and what was left was the original distortion they were looking for.

sounds like the work to rule out false-positives would be huge. This is putting a lot of weight on a technique that is not fully described in the paper (i might have missed, just glanced at them for now, and i am an amateur that just like to fiddle with similar data).

There's more confidence in the result because two teams using very different methods have arrived at the same conclusion.

The three papers used the same 'filtering' methodology, to keep the simplification analogy.

I'm not really sure of what you mean by false positives.

After my understanding of the stacking technique as used in other contexts in astrophysics and without going into too much details :

The problem they were facing is that the signal to noise of the images of the filaments was too low to say that they had detected anything in any individual images. However by stacking (adding) images they were able to detect it because the signal grows roughly with N (N being the number of images) and the noise grows with sqrt(N). So by stacking enough images you'll get the signal to noise necessary to say you've detected sth.

Think like that: there are two bright lights in front of you. There will always be a halo between those two bright lights.

Now, my hypothesis is that "between all two bright lights, there is third, dimmer one, hidding". And then i prove it by filtering X from the two bright light halo, and prove that Y is left proving that the third light is there.

Now, how can i be sure Y is really a third dimmer light? and not just noise on the function i used to try to clean up the halo of the two bright light?

You are right that this could be the case, and it's mentioned in at least one of the papers [1] where they say that a better understanding of the physical state of this gas is needed to estimate its contributions to the baryonic mass.

[1] https://arxiv.org/abs/1805.04555

Now we can get to discuss the third paper ;)

I was wondering why stacking N different galaxies was any different than just taking one image and multiplying it by N, but your comment just provided the answer: noise.

You're right about the first two papers (which used the same approach). Ruling out a FP result would be difficult, which is why it wasn't fully embraced by the scientific community.

The third team took a different approach [1], with results that are both more accurate, less prone to FP, and generally agree with the other findings.

> Here we report observations of two absorbers of highly ionized oxygen (O VII) in the high-signal-to-noise-ratio X-ray spectrum of a quasar at a redshift higher than 0.4. These absorbers show no variability over a two-year timescale and have no associated cold absorption, making the assumption that they originate from the quasar’s intrinsic outflow or the host galaxy’s interstellar medium implausible.

[1] https://www.nature.com/articles/s41586-018-0204-1

Slightly related, I wonder if anyone has figured out the density of interstellar comet or asteroid like objects.

I notice that Oumuamua happened to pass within some 20 million km of earth within a decade or so of having systems in place to spot it. Wouldn't this imply there are an awful lot of them?

I'm not qualified to answer your question, but the observation of Oumuamua alone doesn't give a large enough sample size to estimate how often large interstellar comets or asteroids pass through our solar system.

> I'm not qualified to answer your question, but the observation of Oumuamua alone doesn't give a large enough sample size to estimate how often large interstellar comets or asteroids pass through our solar system.

It's an observation, so it sets some level of constraints on the rate. Though it's true that an estimate of that rate would have large uncertainties.

>I notice that Oumuamua happened to pass within some 20 million km of earth within a decade or so of having systems in place to spot it. Wouldn't this imply there are an awful lot of them?

Finding one in a decade's span within 20 million km would imply there are "an awful lot of them"?

Practically instantaneous on the lengthy time scales the universe operates in, don't you think? And our observations have far from complete coverage of the sky.

That's a pretty small volume and a pretty short timespan, all things considered.

I've wondered the same thing. Also consider that

  [t]he energy released by a cosmic collision increases as the 
  square of the incoming object's speed, so a comet could pack
  nine times more destructive power than an asteroid of the
  same mass. (https://www.space.com/26264-asteroids-comets-earth-impact-risks.html)

ʻOumuamua reached a barycentric speed of 87.71 km/s. The tables on Wikipedia's Impact event article (https://en.wikipedia.org/wiki/Impact_event) assume a speed of 17 km/s relative to Earth. The energy of objects local to our solar system is limited in a way that interstellar objects are not.

With a single observation we can't deduce much of anything concrete except to floor the incidence of these interstellar objects at greater than 0. I'm no astronomer, but I assume models of interstellar objects as they reflect actual risk to Earth wouldn't be very useful without more observations. Whatever the average density in galactic space, I'm betting they're not uniformly distributed. Our solar system is speeding through space that could be littered with clouds of objects.[1] Are we entering a cloud? Leaving a cloud? We can't know without more observations.

[1] There are theories that posit that the ~30- and ~225-million year cycles we see in extinction events are a function of our solar system's orbit in the galaxy, which takes about 200-250 million years. Shorter cycles could relate to the inclination of our orbit (and other stars' orbits) relative to the galactic plane.

One of the interesting things about the galactic orbit is that our system is not exactly in the galactic plane, so as it orbits it passes through the plane to the other side and then back again.

The kinetic energy of objects is proportional to the square of the velocity (Ke = (mv^2)/2), so an object going 4 times faster than a solar system object has 16 times the energy for the same mass. This makes it possible to have extinction level events from rocks that are 1/4 the size of planet killing asteroids.

Ok, here's an extremely rough back-of-the-envelope calculation. As you'll see, these numbers can be out by orders of magnitude, and it doesn't greatly change the conclusion.

Oumuamua interstellar asteroid. 230x35x35m, ~= 280000 m^3

Density assumption: 2 x water. => mass is ~500,000 metric tonnes.

Spotted only after passing the Sun. Assume we'd spot such objects only if they came within the orbit of mercury so are well illuminated. Assume one such object every 10 years (we've not been searching very long with automated telescopes), and we spot all of them.

Mean mercury orbit radius ~ 60,000,000 km

Area of mercury's orbit: 1.1 x 10^16 km^2

Mercury's orbital area x path length in 10 years = volume swept by one visible object in 10 years.

Asteroid velocity ~100,000 km/h

Path length in 10 years = 100,000 x 10 x 24x365. Swept volume ~ 10 x 10^25 km^3

Distance to Alpha Centauri: 4.37 light years = 4.37 x 9.5 x 10^12 km = 4.15 x 10^13km

Sol's "cube of influence" ~= 7 x 10^40 km^3

Cube of influence / swept volume = rough estimate of number of asteroids in cube of influence. Number of asteroids: 7 x 10^14

Mass of asteroids: 3.5 x 10^20 tonnes. Mass of sun: 2 x 10^27 tonnes.

Conclusion: dark interstellar asteroids like Oumuamua are a tiny fraction of the visible mass of the galaxy.

There must be regions of the universe where this "not very dense" gas is dense enough to use a ramscoop

Probably more in high-gravity locations, such as planets and stars. Thinking about the Juno satellite orbit, would it be better/easier/cheaper to hang a mining station in orbit around Jupiter, or have a satellite/ship dip into Jupiter's atmosphere every orbit?

Counterintuitively, the energy costs are basically the same. Whether you dive in to pick it up, or run into it out in space, you will still have to accelerate the gas to orbital velocities before you can cram it into a tank. Some of the gas in space may already be moving very fast, may even be in an orbit, but it is probably not aligned with the orbit of your tank.

Either way, the energy required to maintain operations anywhere near Jupiter means you probably want to find your hydrogen somewhere else.

Assuming there really is other intelligent life out there, "where you were born" seems to make a huge difference in what your species can accomplish in its lifetime.

Seems like we are somewhat validating Vernor Vinge's "zones of thought" idea

Likely only around jupiter-like planets, if any large region of gas was that dense it would quickly collapse into either a gas giant or star or even multiple stars.

I know and understand nothing about this topic but given the fact that the universe is infinite by definition makes the conclusive/definitive statement very surprising.

The universe is not infinite by definition and anyway they are only talking about the visible universe.

I really dislike when they say "universe" and really mean "visible universe" or when they say infinite but don't really mean it.

I'm not really good at physics at this level so it throws me off. It makes it very difficult to really understand what they are talking about.

You would think physicists would be very precise with their language, by I guess they mostly write for people who are know what they are talking about.

The problem is that there's no generic way to describe the universe without contextualizing the characteristic you're interested in. But as a layman (not an astronomer, not a physicist, not a topologist), I'd argue that it's fair to say that the universe is infinite. I say that not only because of where the evidence regarding expansion and geometry of the universe points, but because even in a discussion of the visible universe you have to address the fact that the visible universe is shrinking as objects at the edge disappear due to expansion. Those objects don't cease to exist (at least not unless you make some highly contentious metaphysical arguments), which means there's no avoiding the inference that there can be (and likely are) an infinite number of objects which exist but which are not visible.

Objects becoming invisible does not infer infinite objects. When you observe shopping carts leaving the store do you assume there are infinite shopping carts outside of the store?

True, but my point was that you already have to contend with the reality of objects that exist but which are beyond the visible universe. Where are all those shopping carts going? You can't hide the ball there without eliciting even more questions than you answer.

Limiting discussion to the visible universe (to a "finite" universe by eliding messy details) can mislead by creating seeming contradictions. It's sort of like saying that evolution doesn't exist (as a first order approximation) because the lineage from ape to man is just too complex and doesn't really matter; let's simplify things by eliding that lineage so we have an easier time analogizing human morphology and genetics as it relates to practical questions. It can work superficially but even laymen will have a sense that things don't add up, not to mention that it doesn't help resolve the more important "big" questions often implicit in any discussion.

The fact that the universe is likely infinite stems from experimental results confirming topological characteristics that reflect infinite space. Fortunately, when you try to conceptualize phenomena like the Big Bang, a flat, open infinite universe actually makes things simpler, IMO.

I'm probably missing something, but my understanding of the "visible universe" is that the light from further out has yet to reach us, and this the visible portion should be constantly expanding (at the speed of light). Then, how would objects at the edge disappear unless the expansion is faster than light?

The expansion is FTL.

Objects don't outright disappear: As with other event horizons, they freeze in time and become redshifted as they approach the cosmological one (they also become fainter due to an increase in proper distance).

The cosmological event horizon is the light cone at future infinity and the asymptotic boundary of the observable universe: Light emitted within the horizon will take a finite time to reach us, whereas light emitted right at the horizon would take an infinte amount of time to arrive; in a way, light emitted beyond the horizon still moves towards us in the sense that the comoving distance decreases, but we'd have to wait a longer-than-infinte amount of time for it to arrive...

Because they are beginning to move faster than c, when these objects are at the very edge, due to the space between us and the edge expanding faster than c. This is called the cosmic horizon of the universe - sort of like an inside out event horizon of a black hole. A particle inside the horizon with a speed of c can still reach us (albeit very strongly redshifted), a particle outside the horizon would have to travel faster than c to reach us. The thing is, our own universe is still not old enough, that it can have a proper event horizon. It is estimeted, according to a model that assumes dark energy is a cosmological constant, that the universe must be at least 16GY old for a cosmic horizon to develop.

Oh, and by the way, an expansion faster than the speed of light is consistent with relativity. Special relativity only describes local laws of physics -- you and the edge are not "local". And general relativity doesn't have a constraint on a maximum velocity between 2 arbitrary points in spacetime.

Huh? Whether the universe is infinite or finite is an open question. See https://en.wikipedia.org/wiki/Shape_of_the_universe#Infinite...

I guess the phrase could be parsed two ways but in context it should be clear - the universe is not "infinite by definition". The definition doesn't mention size.

This is why we should all speak like Yoda. There is no ambiguity between:

By definition, infinite the universe is not.

And

Infinite by definition, the universe is not.

If the universe were "infinite by definition", it wouldn't be an open question.

Who is telling you that the universe is infinite? Astronomers/cosmologists generally all agree that our current universe (the matter-containing 4 dimensions bit we live in) had a defined beginning from which is has grown. There is an outer edge,. defined by the rate of expansion starting from the big bang.

Then what do physicists call what is beyond the edge of the universe?

edit: excuse my french ;)

It may mean beyond the observable universe.

The universe may or may not be infinite, for some particular definition of infinity. Our light cone, which essentially defines our universe from our point of view, is distinct and measurably finite.

I don't think my physician calls the edge of the universe anything unless he's a hobbyist physicist as well.

Imagine you are in inside of a giant beach ball, walking on the surface. You will never find an edge, and it is meaningless to talk about such a thing. Yet the space is finite.

Conversely, the surface of the beach ball has no "center". It's also fun to realize if you blow that beach ball up, from the point of view of any point on the ball, all other points grow further away.

A beach ball has no "edge", but it definitely has a boundary surface, and everything on the other side of this surface is "not beach ball".

Now what lies beyond the universe's boundary surface? Is such a surface even present?

That is indeed where the analogy starts to crumble. The reason is that space itself is curved. It is curved by the matter inside it. Still the property holds that if you travelled far enough fast enough you’d come back to where you started.

The Big Bang supposedly encompassed all of space and had no edges.

Has no edges, and therefore unbounded, but still finite in size.

How can it be both finite in size, and have no edges?

To me, it would seem that were it finite, there would be a point at which one would look back, and see the galaxy and clusters that compose the universe; forward would be an expanse of nothingness. But if this isn't the case, then how I can keep progressing forward (presumably forever, as I can't hit an edge) through space, encountering galaxy after galaxy, but it is still finite?

Unless this is like RPG games where the edges wrap.

How can it be both finite in size, and have no edges?

Good question, but find me the edge of an idealized balloon. Where is the edge of a sphere? As to why you won’t come back, remember that spacetime is expanding faster an faster, and you can only travel below light speed. Mind you that’s just one possibility. The universe at large could be a lot of different things, but as humans were causally disconnected from anything beyond the shrinking observable universe. Shrinking from our perspective at least, because of the aforementioned expansion and speed limit.

It is exactly the case that the edges wrap (presuming that I understand correctly, of course).

It is a running theory that space time could be curved and wrap, or be more traditionally flat, or even be some kind of saddle that means it is still curved but never meets itself again like a sphere does. Recent studies point to it being flat though, if I remember correctly.

The surface of the Earth is finite in size and has no edges. You can keep progressing forward without hitting an edge.

There have been recent studies which show that universe is probably "flat" though.

Or very slightly curved - measurements come with error bars, so we can't be sure if it's exactly flat. Also note that flatness and finite size are compatible in case of non-trivial topology (think of the flat torus - pacman world - which, however, is not isotropic).

All "space" ie the matter-containing 4d part we live in. There are things before and perhaps outside our universe, brains and such, but that is outside "our universe".

But what if your Big Bang was only a local bang among an infinite number of bangs so far apart from one another that the light from any of them would take 100s of billions of your earth years to reach its nearest neighbor. Then, even now, they are all expanding toward each other with no way for any of their passengers to know it. You won't need to worry about a heat death or a cold death, you will have a death by collision and probably some kind of rebirth to follow.

What you're describing is the 'visible universe'. There may well be other parts of the cosmos outside of our light cone. Einstein's equations allow it, and we have no way of knowing.

No, visible universe is what we can see, it's defined as all the points close enough in spacetime that light from there had enough time to get to us. Visible universe is centered on Earth, much smaller than what GP was talking about, and decreasing steadily (because at the edge of it expansion of space pushes stuff outside faster than speed of light).

at the edge of it expansion of space pushes stuff outside faster than speed of light

Not quite: According to the cosmological standard model, the visible universe will continue to grow (ie new galaxies will continue to come into view) - but only asymptotically, ie until a maximum size given by the comological event horizon is reached. However, the parts of the universe that aren't gravitationally bound to us will become fainter and fainter and increasingly resdhifted, and eventually, we'll be unable to detect other theoretically visible galaxies due to technological limitations.

If they are moving away from us faster than speed of light because of expansion of space - wouldn't it mean at some point no new light from them reaches us? Even ignoring the limitation of equipment?

In my opinion, that whole 'moving away from us faster than c' business is not really a good way to think about this: For one, we can see to a redshift of about 10, corresponding to a comoving distance of about 30Gly, and a recession velocity of about 4c (four times the speed of light!) at time of emission.

There's a cosmological event horizon. Light emitted from within will reach us in finite time, light emitted from without won't. Similar to how a distant observer will never see on object falling into a (stationary) black hole cross the Schwarzschild horizon, we won't see galaxies crossing the cosmological horizon.

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