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).
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.
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.
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?
[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.
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.
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.
>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"?
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.
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.
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?
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.
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.
gcbw2|7 years ago
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).
simsla|7 years ago
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
nfc|7 years ago
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.
macintux|7 years ago
TomMckenny|7 years ago
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?
wahern|7 years ago
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.
mhandley|7 years ago
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.
coldtea|7 years ago
Finding one in a decade's span within 20 million km would imply there are "an awful lot of them"?
Firerouge|7 years ago
gmailsyncer|7 years ago
zaarn|7 years ago
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analog31|7 years ago
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